USGS Science⌕ Search

SEARCH · USGS Science

Results for “Near Surface Geophysics”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 973 records · Page 54Linked to original sources
USGS2019

Multidecadal geomorphic evolution of a profoundly disturbed gravel-bed river system—a complex, nonlinear response and its impact on sediment delivery

A 2.5-km3 debris avalanche during the 1980 eruption of Mount St. Helens reset the fluvial landscape of upper North Fork Toutle River valley. Since then, a new drainage network has formed and evolved. Cross-section surveys repeated over nearly 40 years at 16 locations along a 20-km reach of river valley document channel evolution, geomorphic processes, and their impacts on sediment delivery. We analyzed spatial and temporal changes in channel morphology using two new metrics: 1) a shape index that defines the degree of U-shaped or V-shaped valley geometry; and 2) an alluvial phase-space diagram that relates bed degradation or aggradation between consecutive surveys to increases or decreases in cross-section area. Metric relations reveal more diverse channel evolution than originally described by a simple, linear-response model of sequential channel initiation and incision; aggradation and widening; and subsequent episodic scour and fill with little change in bed elevation. Instead, vertical and lateral adjustments have been crucial processes intertwined throughout channel evolution. Channel evolution has followed a distinctly nonlinear and non-sequential trajectory, migrating through several phase spaces and involving varied combinations of (1) degradation and aggradation with widening and narrowing, (2) bed-level fluctuations with little change in cross-section area, and (3) changes in cross-section area with little change of bed elevation. Persistent channel widening and reworking of the channel bed presently drive elevated sediment delivery from this basin. Elevated sediment delivery is likely to persist until valley-floor widths greatly exceed that of the channel-migration corridor, and/or channel banks and valley walls stabilize.

Washington↗
USGS2012

The growth of geological structures by repeated earthquakes: 2, Field examples of continental dip-slip faults

A strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault-bounded geological structures, such as basins and ranges, which are built by repeated cycles of deformation. Along strike-slip faults, the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign, resulting in little permanent deformation except for the fault offset. For dip-slip faults, portions of the crust are lifted and dropped, and so buoyancy forces are exerted. The seismic and interseismic deformations do not balance, and structures grow and become subject to erosion and deposition. We consider three examples for which the structure and fault geometry are well known: the White Wolf reverse fault in California, site of the 1952 Kern County M=7.3 earthquake, the Lost River normal fault in Idaho, site of the 1983 Borah Peak M=7.0 earthquake, and the Cricket Mountain normal fault in Utah, site of Quaternary slip events. Basin stratigraphy and seismic reflection records are used to profile the structure, and coseismic deformation measured by leveling surveys is used to estimate the fault geometry. To reproduce these structures, we add the deformation associated with the earthquake cycle (the coseismic slip and postseismic relaxation) to the flexure caused by the observed sediment load, treating the crust as a thin elastic plate overlying a fluid substrate. The cumulative deformation is principally dependent on the elastic plate thickness, modestly sensitive to the sediment-substrate density difference, and insensitive to the fluid viscosity for the 4- to 8-Ma structures. We deduce a longterm flexural rigidity of 2–15 × 10 19 Nm; this is equivalent to an elastic plate thickness of 2–4 km for a Young's modulus of 2.5 × 10 10 Nm −2 . This value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km, but where coseismic fault slip extends to a depth of 10–15 km. Thus much of the seismogenic crust must weaken substantially during the life of active faults, causing the fault-bounded basins to narrow over time.

Journal of Geophysical Research Solid Earth↗
USGS2012

Topography of Valles Marineris: Implications for erosional and structural history

Compilation of a simplified geologic/geomorphic map onto digital terrain models of the Valles Marineris permitted an evaluation of elevations in the vicinity of the troughs and the calculation of depth of troughs below surrounding plateaus, thickness of deposits inside the troughs, volumes of void spaces above geologic/geomorphic units, and volumes of deposits. The central troughs north Ophir, north and central Candor, and north Melas Chasmata lie as much as 11 km below the adjacent plateaus. In Ophir and Candor Chasmata, interior layered deposits reach 8 km in elevation. If the deposits are lacustrine and if all troughs were interconnected, lake waters standing 8 km high would have spilled out of Coprates Chasma onto the surrounding plateaus having surface elevations of only 4–5 km. In this case, interior deposits above about 4 km in the central troughs would not be lacustrine. They could be volcanic. On the other hand, the troughs may not have been interconnected at the time of interior‐deposit emplacement; they may have formed isolated ancestral basins. The existence of such basins is supported by independent structural and stratigraphic evidence. The ancestral basins may have eventually merged, perhaps through renewed faulting, to form northern subsidiary troughs in Ophir and Candor Chasmata and the Coprates/north Melas/Ius graben system. The peripheral troughs are only 2–5 km deep, shallower than the central troughs. They may have formed from a combination of erosional collapse and structural activity. Chaotic terrain is seen in the peripheral troughs near a common contour level of about 4 km on the adjacent plateaus, which supports the idea of release of water under artesian pressure from confined aquifers. The layered deposits in the peripheral troughs may have formed in isolated depressions that harbored lakes and predated the formation of the deep outflow channels. If these layered deposits are of volcanic origin, they may have been emplaced beneath ice in the manner of table mountains. Areal and volumetric computations show that erosion widened the troughs by about one‐third and that deposits occupy one‐sixth of the interior space. Even though the volume eroded is larger than the volume deposited, topographic and geologic considerations imply that material eroded from trough walls was probably part of the interior layered deposits but not their sole source. Additional material may have come from subterranean piping, from reworking of local disintegration products on the floors, such as chaotic materials, or from eolian influx. But overall it is likely that the additional material is volcanic and that it forms mostly the upper, more diversely bedded layers of the interior deposits.

Journal of Geophysical Research E: Planets↗
USGS2009

Brine migration from a flooded salt mine in the Genesee Valley, Livingston County, New York: Geochemical modeling and simulation of variable-density flow

The Retsof salt mine in upstate New York was flooded from 1994 to 1996 after two roof collapses created rubble chimneys in overlying bedrock that intersected a confined aquifer in glacial sediments. The mine now contains about 60 billion liters of saturated halite brine that is slowly being displaced as the weight of overlying sediments causes the mine cavity to close, a process that could last several hundred years. Saline water was detected in the confined aquifer in 2002, and a brine-mitigation project that includes pumping followed by onsite desalination was implemented in 2006 to prevent further migration of saline water from the collapse area. A study was conducted by the U.S. Geological Survey using geochemical and variable-density flow modeling to determine sources of salinity in the confined aquifer and to assess (1) processes that control movement and mixing of waters in the collapse area, (2) the effect of pumping on salinity, and (3) the potential for anhydrite dissolution and subsequent land subsidence resulting from mixing of waters induced by pumping. The primary source of salinity in the collapse area is halite brine that was displaced from the flooded mine and transported upward by advection and dispersion through the rubble chimneys and surrounding deformation zone. Geochemical and variable-density modeling indicate that salinity in the upper part of the collapse area is partly derived from inflow of saline water from bedrock fracture zones during water-level recovery (January 1996 through August 2006). The lateral diversion of brine into bedrock fracture zones promoted the upward migration of mine water through mixing with lower density waters. The relative contributions of mine water, bedrock water, and aquifer water to the observed salinity profile within the collapse area are controlled by the rates of flow to and from bedrock fracture zones. Variable-density simulations of water-level recovery indicate that saline water has probably not migrated beyond the collapse area, while simulations of pumping indicate that further upward migration of brine and saline water is now prevented by groundwater withdrawals under the brine-mitigation project. Geochemical modeling indicates that additional land subsidence as a result of anhydrite dissolution in the collapse area is not a concern, as long as the rate of brine pumping is less than the rate of upward flow of brine from the flooded mine. The collapse area above the flooded salt mine is within a glacially scoured bedrock valley that is filled with more than 150 meters of glacial drift. A confined aquifer at the bottom of the glacial sediments (referred to as the lower confined aquifer, or LCA) was the source of most of the water that flooded the mine. Two rubble chimneys that formed above the roof collapses in 1994 hydraulically connect the flooded mine to the LCA through 180 meters of sedimentary rock. From 1996 through 2006, water levels in the aquifer system recovered and the brine-displacement rate ranged from 4.4 to 1.6 liters per second, as estimated from land-surface subsidence above the mine. A zone of fracturing within the bedrock (the deformation zone) formed around the rubble chimneys as rock layers sagged toward the mine cavity after the roof collapses. Borehole geophysical surveys have identified three saline-water-bearing fracture zones in the bedrock: at stratigraphic contacts between the Onondaga and Bertie Limestones (O/B-FZ) and the Bertie Limestone and the Camillus Shale (B/C-FZ), and in the Syracuse Formation (Syr-FZ). The only outlets for brine displaced from the mine are through the rubble chimneys, but some of the brine could be diverted laterally into fracture zones in the rocks that lie between the mine and the LCA. Inverse geochemical models developed using PHREEQC indicate that halite brine in the flooded mine is derived from a mixture of freshwater from the LCA (81 percent), saline water from bedrock fracture zones (16 percent), and an hypothesized bromide-rich brine (3 percent) assumed to originate from salt-bearing rocks above the flooded mine. Geochemical modeling results also indicate that halite brine entering the rubble chimneys is diluted by both bedrock water and aquifer water, and that water from the mine has not reached the bedrock surface. Forward geochemical models indicate that additional land subsidence could occur if pumping from the brine-mitigation project were to introduce either freshwater or bedrock water that is undersaturated with respect to anhydrite into the lower part of the rubble chimneys. In this unlikely scenario, the maximum subsidence rates are predicted to range from 0.6 to 1.1 centimeters per year—subsidence rates would be lower (0.1 to 0.6 centimeters per year) if ion-exchange reactions affect the water chemistry. Variable-density, transient groundwater-flow models were constructed using SEAWAT to simulate the movement of saline water, aquifer water, bedrock water, and brine within the rubble chimneys and surrounding deformation zone during the 10.7-year period following flooding of the salt mine. Two three-dimensional models reproduced the profile of halite saturation with depth measured in September 2006 reasonably well, and neither model indicated that saline water had migrated beyond the collapse area. The models differed in the number of fracture zones represented: one zone in model A (O/B-FZ) and three zones in model B (O/B-FZ, B/C-FZ, and Syr-FZ). It is unknown whether model A or model B better represents current conditions because the lateral extents of the B/C-FZ and Syr-FZ have not been delineated beyond the collapse area. In model A, the salinity of water in the upper part of the rubble chimneys is derived mainly from the inflow of bedrock water from the O/B-FZ, as indicated by geochemical models. Bedrock water that was pushed upward by brine during the 10.7-year simulation period formed a diffuse front above a nearly horizontal brine level in both chimneys. In model B, some of the salinity in the upper part of the rubble chimneys is derived from mine water. The rate of bedrock-water inflow from the O/B-FZ was lower in model B than in model A, and mixing with waters from the Syr-FZ and B/C-FZ transported mine water higher in the water column than in model A. Simulated brine levels in both chimneys sloped northward, reflecting lateral diversion of brine into the B/C-FZ, and less aquifer water was displaced from the collapse area than in model A. Models A and B were used to simulate changes in water levels and salinity produced by pumping for the brine-mitigation project from September 2006 through February 2008. Both simulations indicated that current pumping rates are sufficient to offset upward migration of brine and saline water through the collapse area and, therefore, to further prevent contamination of the LCA. A greater decrease in salinity was simulated in model B, however, because the porosity of the rubble chimneys was lower (6 percent compared to 10 percent in model A), and some brine and saline waters were diverted through the B/C-FZ. Model B better simulates the influent saturation to the desalination plant, the amount of halite produced, and the observed declines in saturations than model A, which is more consistent with results of geochemical modeling. Sensitivity analyses indicate that the actual brine-displacement rate could be lower than estimated because simulated declines in saturations underpredict the observed decline from September 2006 through February 2008. Although halite saturations within the upper part of the collapse area are predicted to decrease with continued pumping, brine displacement from the flooded mine is expected to continue for hundreds of years. Simulations of a shutdown of the brine-mitigation project indicate southward migration of saline water through the LCA, extending 700 meters to the model boundary within 10 years. Continued migration of saline water would eventually form a pool in the LCA in a bedrock depression 8 kilometers south of the collapse area near Sonyea, but the large relative density of the saline water would likely prevent it from reaching overlying aquifers. Simulations also indicate that brine will migrate through bedrock fracture zones—some brine could possibly emerge updip to the north where the subcrop area of the Bertie Limestone intersects the bedrock surface near Avon, but the projected time of travel is unknown.

New York↗
USGS1998

Water flow through temperate glaciers

Understanding water movement through a glacier is fundamental to several critical issues in glaciology, including glacier dynamics, glacier-induced floods, and the prediction of runoff from glacierized drainage basins. to this end we have synthesized a conceptual model os water movement through a temperate glacier from the surface to the outlet stream. Processes that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology. Where a glacier is covered by a layer of porous, permeable firn (the accumulation zone), the flux of water to the glacier interior varies slowly because the firn temporarily stores water and thereby smooths out variations in the supply rate. In the firn-free ablation zone, in contrast, the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time. Water moves from the surface to the bed through an upward branching arborescent network consisting of both steeply inclined conduits, formed by the enlargement of intergranular veins, and gently inclined conduits, sprqwned by water flow along the bottoms of near-surface fractures (crevasses). Englacial drainage conduits deliver water to the glacier bed at a linited number of points, probably a long distance downglacier of where water enters the glacier. Englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn. their size adjusts so that they are usually full of water and flow is pressurized. In contrast, water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms; flow is otherwise in an open-channel configuration. The subglacial drainage system typically consists of several elements that are distinct both morpphologically and hydrologically. An up-glacier branching, arborescent network of channels incised into the basal ice conveys water rapidly. Much of the water flux to the bed probably enters directly into the arborescent channel network, which covers only a small fraction of the glacier bed. More extensive spatially is a nonarborescent network, which commonly includes cabities (gaps between the glacier sole and bed), channels incised into the bed, and a layer of permeable sediment. The nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system. The arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent net6work. The volume of water stored by a glacier varies diurnally and seasonally. Small, temperate alpine glaciers seem to attain a maximum seasonal water storage of ~200 mm of water averaged over the area of the glacier bed, with daily fluctuations of as much as 20-30 mm. The likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes, so most water storage may actually occur englacially. Sotred water may also be released abruptly and catastrophically in the form of outburst floods.

Reviews of Geophysics↗
USGS2012

Earthquake depths and the relation to strain accumulation and stress near strike-slip faults in southern California

In this paper I report and attempt to interpret several observations about the depth distribution of earthquakes in the region of southern California containing the southern San Andreas and San Jacinto fault zones. These observations are as follows: (1) Earthquakes in the major fault zones are predominantly deep (maximum, 22 km; characteristic, 11–18 km). (2) Earthquakes in the crustal blocks bounding the fault zones are predominantly shallow (maximum, 17 km; characteristic, 1–6 km). (3) In the San Jacinto fault zone, maximum earthquake depths correlate with surface heat flow. These relations together with focal mechanisms, geodetic strain measurements, and fault zone models are consistent with the following ideas: (1) Interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes. (2) The aseismic slip in a narrow fault zone in the brittle-plastic transition region concentrates strain at the base of the brittle fault zone. (3) Deep earthquakes occur in the lower part of the brittle fault zone due to stick-slip failure of highly stressed patches. (4) Background earthquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding (deep) and stick-slip (shallow) regions of the fault zone. This zone of mixed slip may fail predominantly by stable sliding, analogous to the central San Andreas and Calaveras faults of California. Seismic precursors to large earthquakes may occur in this zone, and large earthquakes may nucleate near the upper edge of this zone. Furthermore, the difference in seismicity between the San Jacinto and southern San Andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases. (5) Shear stress may be less in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks. (6) In the stress field produced by plate motion and slip in the deep fault zone, the upper brittle fault zone is not oriented favorably for shear failure. Lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation.

California↗
USGS2004

Hydrogeologic characterization of the Modesto Area, San Joaquin Valley, California

Hydrogeologic characterization was done to develop an understanding of the hydrogeologic setting near Modesto by maximizing the use of existing data and building on previous work in the region. A substantial amount of new lithologic and hydrologic data are available that allow a more complete and updated characterization of the aquifer system. In this report, geologic units are described, a database of well characteristics and lithology is developed and used to update the regional stratigraphy, a water budget is estimated for water year 2000, a three-dimensional spatial correlation map of aquifer texture is created, and recommendations for future data collection are summarized. The general physiography of the study area is reflected in the soils. The oldest soils, which have low permeability, exist in terrace deposits, in the interfan areas between the Stanislaus, Tuolumne, and Merced Rivers, at the distal end of the fans, and along the San Joaquin River floodplain. The youngest soils have high permeability and generally have been forming on the recently deposited alluvium along the major stream channels. Geologic materials exposed or penetrated by wells in the Modesto area range from pre-Cretaceous rocks to recent alluvium; however, water-bearing materials are mostly Late Tertiary and Quaternary in age. A database containing information from more than 3,500 drillers'logs was constructed to organize information on well characteristics and subsurface lithology in the study area. The database was used in conjunction with a limited number of geophysical logs and county soil maps to define the stratigraphic framework of the study area. Sequences of red paleosols were identified in the database and used as stratigraphic boundaries. Associated with these paleosols are very coarse grained incised valley-fill deposits. Some geophysical well logs and other sparse well information suggest the presence of one of these incised valley-fill deposits along and adjacent to the Tuolumne River east of Modesto, a feature that may have important implications for ground-water flow and transport in the region. Although extensive work has been done by earlier investigators to define the structure of the Modesto area aquifer system, this report has resulted in some modification to the lateral extent of the Corcoran Clay and the regional dip of the Mehrten Formation. Well logs in the database indicating the presence of the Corcoran Clay were used to revise the eastern extent of the Corcoran Clay, which lies approximately parallel to the axis of valley. The Mehrten Formation is distinguished in the well-log database by its characteristic black sands consisting of predominantly andesitic fragments. Black sands in wells listed in the database indicate that the formation may lie as shallow as 120 meters (400 feet) below land surface under Modesto, approximately 90 meters (300 feet) shallower than previously thought. The alluvial aquifer system in the Modesto area comprises an unconfined to semiconfined aquifer above and east of the Corcoran Clay confining unit and a confined aquifer beneath the Corcoran Clay. The unconfined aquifer is composed of alluvial sediments of the Modesto, Riverbank, and upper Turlock Lake formations. The unconfined aquifer east of the Corcoran Clay becomes semiconfined with depth due to the numerous discontinuous clay lenses and extensive paleosols throughout the aquifer thickness. The confined aquifer is composed primarily of alluvial sediments of the Turlock Lake and upper Mehrten Formations, extending from beneath the Corcoran Clay to the base of fresh water. Ground water in the unconfined to semiconfined aquifer flows to the west and southwest. The primary source of present-day recharge is percolating excess irrigation water. The primary ground-water discharge is extensive ground-water pumping in the unconfined to semiconfined aquifer, imposing a significant component of vertical flow in the system. A water budget was calculated for water year 2000 using a land-use approach. During water year 2000, the total water supply in the Modesto area was more than 2.5 billion m3 (cubic meter) (2 million acre-ft [acre-foot]). Surface-water deliveries accounted for 60 percent of the total water supply, whereas ground-water pumpage accounted for 40 percent. Ninety-four percent of the water supply was used to meet irrigation demand and approximately 6 percent was used to meet urban demand. The total recharge in the model area was estimated at 1.4 billion m3 (1,100,000 acre-ft). The largest component of recharge is from excess irrigation water (58 percent); precipitation in excess of crop requirements accounted for 41 percent of the recharge. Geostatistical methods were used to develop a spatial correlation model of the percentage of coarse-grained texture in the Modesto area. The mean percentage coarse-grained texture calculated for each depth increment indicates a regional trend of decreasing coarse-grained texture with increasing depth, which is consistent with increasingly consolidated sediments with depth in the study area. The three-dimensional kriged estimates of percentage coarse-grained texture show significant heterogeneity in the texture of the sedimentary deposits. Assuming the hydraulic conductivity is correlated to the texture, the kriged result implies significant heterogeneity in the hydrogeologic framework.

California↗
USGS2006

Geohydrology and water chemistry of the Alexander Valley, Sonoma County, California

This study of the geohydrology and water chemistry of the Alexander Valley, California, was done to provide an improved scientific basis for addressing emerging water-management issues, including potential increases in water demand and changes in flows in the Russian River. The study tasks included (1) evaluation of existing geohydrological, geophysical, and geochemical data; (2) collection and analysis of new geohydrologic data, including subsurface lithologic data, ground-water levels, and streamflow records; and (3) collection and analysis of new water-chemistry data. The estimated total water use for the Alexander Valley for 1999 was approximately 15,800 acre-feet. About 13,500 acre-feet of this amount was for agricultural use, primarily vineyards, and about 2,300 acre-feet was for municipal/industrial use. Ground water is the main source of water supply for this area. The main sources of ground water in the Alexander Valley are the Quaternary alluvial deposits, the Glen Ellen Formation, and the Sonoma Volcanics. The alluvial units, where sufficiently thick and saturated, comprise the best aquifer in the study area. Average recharge to the Alexander Valley is estimated from a simple, basinwide water budget. On the basis of an estimated annual average of 298,000 acre-feet of precipitation, 160,000 acre-feet of runoff, and 113,000 to 133,000 acre-feet of evapotranspiration, about 5,000 to 25,000 acre-feet per year is available for ground-water recharge. Because this estimate is based on differences between large numbers, there is significant uncertainty in this recharge estimate. Long-term changes in ground-water levels are evident in parts of the study area, but because of the sparse network and lack of data on well construction and lithology, it is uncertain if any significant changes have occurred in the northern part of the study area since 1980. In the southern half of the study area, ground-water levels generally were lower at the end of the 2002 irrigation season than at the end of the 1980 season, which suggests that a greater amount of ground water is being pumped in the southern half of the study area in recent years compared with that pumped in the early 1980s. Water-chemistry data for samples collected from 11 wells during 2002-04 indicate that water quality in the study area generally is acceptable for potable use. Two wells, however, each contained one constituent (241 ?g/L of manganese and 1,350 ?g/L of boron) in excess of the recommended standards for drinking water (50 ?g/L and 1,000 ?g/L, respectively). The chemical composition of water from most of the wells sampled for major ions plot as a mixed cation-bicarbonate, magnesium-bicarbonate, or calcium-bicarbonate type water. The ionic composition of the historical and recent samples from wells in the Alexander Valley is similar to that of the historical surface-water samples collected from the Russian River near Healdsburg. This suggests a similar source of water, particularly for wells that are less than 200 feet total depth and perforated in Quaternary alluvial deposits. Water from deeper, non-alluvial wells may contain slightly higher concentrations of sodium as a result of cation exchange. Water samples collected from several wells over an approximately 30-year period suggest a progressive change in water chemistry over time. Samples from the southern part of the valley show a trend towards higher ionic concentrations and increasing concentrations of particular constituents such as sulfate.

California↗
USGS1997

Geohydrology and water chemistry in the Rialto-Colton Basin, San Bernardino County, California

The 40-square-mile Rialto-Colton ground- water basin is in western San Bernardino County, California, about 60 miles east of Los Angeles.This basin was chosen for storage of imported water because of the good quality of native ground water, the known capacity for additional ground-water storage in the basin, and the availability of imported water. Because the movement and mixing of imported water needed to be determined, the San Bernardino Valley Municipal Water District entered into a cooperative program with the U.S.Geological Survey in 1991 to study the geohydrology and water chemistry in the Rialto- Colton basin. Ground-water flow and chemistry were investigated using existing data, borehole- geophysical and lithologic logs from newly drilled test holes, measurement of water levels, and chemical analyses of water samples. The Rialto-Colton basin is bounded on the northwest and southeast by the San Gabriel Mountains and the Badlands, respectively. The San Jacinto Fault and Barrier E form the northeastern boundary, and the Rialto-Colton Fault forms the southwestern boundary. Except in the southeastern part of the basin, the San Jacinto and Rialto-Colton Faults act as groundwater barriers that impede ground- water flow into and out of the basin.Barrier E generally does not impede ground- water flow into the basin. The ground-water system consists primarily of gravel, sand, silt, and clay. The maximum thickness is greater than 1,000 feet. The ground- water system is divided into four water-bearing units: river-channel deposits, and upper, middle, and lower water-bearing units. Relatively impermeable consolidated deposits underlie the lower water- bearing unit and form the lower boundary of the ground- water system. Ground water moves from east to west in the river-channel deposits and upper water-bearing unit in the southeastern part of the basin, and from northwest to southeast in the middle and lower water-bearing units. Two major internal faults, Barrier J and an unnamed fault, affect ground-water movement. Ground water moves across Barrier J in the unfaulted part of the ground-water system. The unnamed fault is a partial barrier to ground-water movement in the middle water- bearing unit and an effective barrier in the lower water-bearing unit.Imported water flows laterally across the unnamed fault above the saturated zone. Major sources of recharge to the ground- water system are underflow; precipitation that collects in small streams that drain the San Gabriel Mountains and the Badlands or runs off the mountain front as sheet flow, and sub-surface inflow; imported water; seepage loss from the Santa Ana River and Warm Creek; infiltration of rainfall; and irrigation return flow. The main component of discharge is pumpage. Long-term water levels in production wells reflect precipitation cycles. During a 194777 dry period, water levels in three wells declined almost continuously?as much as 100 feet in one well.Water levels in a well north of Barrier J are not affected by stresses on the groundwater system south of the barrier, indicating that these two parts of the ground-water system are not well connected. Water levels in cluster wells east of the unnamed fault north and south of the Linden Ponds artificial-recharge site rose as much as 70 feet during 1992-95. The rise in water levels in wells near the recharge ponds was observed within 2 months after the beginning of recharge. Water levels in most wells west of the unnamed fault changed very little during 1992-95. Water-chemistry data indicate that chemical characteristics vary within the groundwater system, and that dissolvedsolids concentrations are generally higher in the river-channel deposits, upper water- bearing unit, and the consolidated deposits than in the middle and lower water-bearing units. The chemical characteristics in water from the middle water-bearing unit were similar for most wells sampled west of the unnamed fault. In water from well

Water-Resources Investigations Report↗
USGS2012

The competition between thermal contraction and differentiation in the stress history of the Moon

The scarcity of both extension and compression features on the Moon strongly constrains the history of the lunar radius—to variations of less than ±1 km over the past 3.8 Gyr. This limit has traditionally been interpreted as requiring a delicate balance between thermal contraction of the near‐surface and expansion of a substantial cold interior region. Recent theories of lunar origin (e.g., giant impact), in contrast, favor a “hot” initial state. We propose that a reconciliation may be possible by taking account of the volume change Δ V / V | d due to differentiation. We calculate STP densities based on simplified normative mineralogies for a suite of estimates of the bulk lunar composition, of primary lunar basalt, and of the residuum left when the maximum amount of the latter is extracted from the former. Typically Δ V / V | d ≃ 2 to 5%—an expansion equivalent to heating by ∼10 3 K. Provided the timing of differentiation is correct, one might offset the cooling of a magma ocean as much as 630 km deep by differentiation of the remainder of the Moon (which need not start much below the solidus temperature). A large but not impossible amount of gabbroic melt production is implied: ∼100 times the volume of mare basalts known to have been extruded. We do not address the detailed genetic relationship of this melt to the basalts observed on the lunar surface but point out that it need not have reached the surface directly or even have entered the crust in order for the expansion to have occurred. To assess the timing of melt formation, we investigate a simple conductive lunar thermal model which takes account of both Δ V / V | d and thermal contraction. Our initial state is characterized by a central temperature T c and a depth Z 0 above which the material (derived from the magma ocean) is already at the solidus and is not suceptible to volume changes upon further differentiation. We find a range of models satisfying the limits on radius increase and decrease. The hottest has T c = 1210 K, Z 0 = 400 km; without Δ V / V | d , we would need a larger or colder (or both) core, e.g., T c ≲ 700 K for Z 0 = 200–400 km, in agreement with previous investigators. Our modeling thus lends credence to the idea that the Moon could have been initially ≳50% molten (with the remainder relatively close to the solidus) and yet experienced little volume change over the last 3.8 Gyr.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Cretaceous-Eocene (Laramide) landscape development and Oligocene- Pliocene drainage reorganization of transition zone and Colorado Plateau, Arizona

Landscape development of central and northern Arizona can no longer be ascribed mainly to events of Miocene and Pliocene age. New information on the age and distribution of older Cenozoic deposits has led to the recognition of a regional Cretaceous-Paleocene(?) surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion. These relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed. The timing of events postulated for development of drainage on the Colorado Plateau can now be compared and partly reconciled with events recognized in the adjacent closely related Mountain Region (Transition Zone) of central Arizona. As a consequence of Late Cretaceous-Paleocene (Laramide) compression, central and northern Arizona underwent at least 1200 m of uplift, documented by paleochannels cut into erosionally truncated Paleozoic strata on the Hualapai Plateau of the southwestern Colorado Plateau. During this time, a highly irregular erosion surface was developed on Proterozoic rocks across the Transition Zone south of the Mogollon Rim, the scarp of the Mogollon Rim was eroded to its present height (600–900 m), and an extensive stripped surface was developed on resistant upper Paleozoic strata north of the rim. Deposition of several hundred meters of Paleocene-Eocene “Rim gravels” derived from highlands south and west of the region followed, covering much of the Cretaceous-Paleocene erosion surface. Nearly complete burial of the rim is suggested by the distribution of remnants of the Rim gravels across the erosional scarps and on high plateau areas north of the rim. A second increment of uplift, apparently occurring in late Eocene time and apparently recorded by a series of fission track cooling ages from the Marble and Grand canyons, is inferred to have been responsible for ending deposition of the Rim gravels, for initiating differential uplift of contemporaneous deposits (Canaan Peak and Claron formations) to their positions in the high plateaus of central Utah, and for causing the drainage reorganization required to explain the extensive removal of Rim gravels from much of the region. A southerly flowing ancestral Verde River related to the drainage reorganization removed much of the older gravel cover from the Transition Zone of central Arizona, resulting in a younger regional erosion surface having 600–900 m of relief, a surface closely approximating the Cretaceous-Paleocene erosion surface. Late Oligocene and early Miocene rocks locally rest unconformably on remnants of Rim gravels in the Transition Zone, indicating that the second episode of regional erosion had been completed by late Oligocene time. North of the Mogollon Rim, a west flowing(?) ancestral Colorado River is inferred to have become established on the Rim gravels, draining the interior parts of the Colorado Plateau and transporting detritus off the plateau. Exhumation of the Mogollon Rim and development of 600–900 m of topographic relief in the Transition Zone by an ancestral Verde River system suggests the potential for a comparable, coeval entrenchment of an ancestral Colorado River in Paleozoic strata north of the Mogollon Rim. Regional extension and volcanic activity ensued in late Oligocene to Pliocene time. The Oligocene erosion surface in the extensional basins of central Arizona became largely concealed by Miocene and Pliocene deposits as the Neogene climate became drier. In late Miocene and Pliocene time, perennial streams appear to have been lacking, transport of detritus appears to have been principally by flash flooding, little or no detritus appears to have been removed from the region, and much of the precipitation presumably moved by groundwater flow through the deposits of aggradation. A coeval episode of aggradation in the Grand Canyon is suggested by deposits that appear to have once choked much of the canyon. If this event parallels the episode of late Miocene and Pliocene aggradation recorded east, south, and west of the Grand Canyon, the Colorado River could have been incised to its present level by late Miocene time. A return to wetter conditions in late Pliocene time presumably was responsible for renewed erosion and reexcavation of older drainages and basins. An understanding of this Tertiary structural, erosional, and depositional history can be important for the geological analysis of geophysical transects across the region.

Journal of Geophysical Research↗
USGS1998

Poroelastic rebound along the Landers 1992 earthquake surface rupture

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS-1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large-scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson-Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10–20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [ Peltzer et al ., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60-km-long transect across the Emerson fault [ Savage and Svarc , 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio V p /V s of 2.1, consistent with the observed low V s values of fault zone guided waves at shallow depth [ Li et al ., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post-seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Generation of a pseudo-2D shear-wave velocity section by inversion of a series of 1D dispersion curves

Multichannel Analysis of Surface Waves utilizes a multichannel recording system to estimate near-surface shear (S)-wave velocities from high-frequency Rayleigh waves. A pseudo-2D S-wave velocity (vS) section is constructed by aligning 1D models at the midpoint of each receiver spread and using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. The receiver spread length sets the theoretical lower limit and any vS structure with its lateral dimension smaller than this length will not be properly resolved in the final vS section. A source interval smaller than the spread length will not improve the horizontal resolution because spatial smearing has already been introduced by the receiver spread. In this paper, we first analyze the horizontal resolution of a pair of synthetic traces. Resolution analysis shows that (1) a pair of traces with a smaller receiver spacing achieves higher horizontal resolution of inverted S-wave velocities but results in a larger relative error; (2) the relative error of the phase velocity at a high frequency is smaller than at a low frequency; and (3) a relative error of the inverted S-wave velocity is affected by the signal-to-noise ratio of data. These results provide us with a guideline to balance the trade-off between receiver spacing (horizontal resolution) and accuracy of the inverted S-wave velocity. We then present a scheme to generate a pseudo-2D S-wave velocity section with high horizontal resolution using multichannel records by inverting high-frequency surface-wave dispersion curves calculated through cross-correlation combined with a phase-shift scanning method. This method chooses only a pair of consecutive traces within a shot gather to calculate a dispersion curve. We finally invert surface-wave dispersion curves of synthetic and real-world data. Inversion results of both synthetic and real-world data demonstrate that inverting high-frequency surface-wave dispersion curves - by a pair of traces through cross-correlation with phase-shift scanning method and with the damped least-square method and the singular-value decomposition technique - can feasibly achieve a reliable pseudo-2D S-wave velocity section with relatively high horizontal resolution. ?? 2008 Elsevier B.V. All rights reserved.

Journal of Applied Geophysics↗
USGS1996

Crustal structure of the Colorado Plateau, Arizona: Application of new long-offset seismic data analysis techniques

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ∼37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ∼15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Journal of Geophysical Research B: Solid Earth↗
USGS2019

Potential interaction of groundwater and surface water including autonomous underwater vehicle reconnaissance at Nolin River Lake, Kentucky, 2016

The U.S. Geological Survey collaborated with the U.S. Army Corps of Engineers, Louisville District, on a synoptic study of water quality at Nolin River Lake during August 2016. The purpose of the study was to develop a better understanding of the potential for interaction between groundwater and surface water at Nolin River Lake, Kentucky. Groundwater can have properties that are measurably different from those in adjacent surface water, and inflows and outflows can be an important component of water quality and quantity. An improved understanding of potential interaction of groundwater and surface water at Nolin River Lake may be used to refine lake-management strategies. This study (1) compiled and interpreted existing information to characterize the hydrogeological setting and implications for potential interaction of groundwater and surface water in the Nolin River Lake watershed; (2) collected transects of onsite water-quality parameters using an autonomous underwater vehicle (AUV) in areas with potential for interaction of groundwater and surface water, including five sites on Nolin River Lake and one site on the Nolin River; and (3) collected discrete water-quality and phytoplankton community data at the same six sites. A review of existing hydrogeologic information did not indicate the presence of karst features adjacent to or beneath Nolin River Lake that would facilitate groundwater interaction with the reservoir. Observations leading to this conclusion include (1) limestone that is adjacent to the shoreline and perhaps beneath the lake, is overlain with siliciclastic rocks and fine-grained sediment that inhibits infiltration and development of karst features that encourage rapid groundwater flow; (2) the geologic deposits surrounding the reservoir are described as having limited or no potential for development of karst features, some exceptions may exist in tributary valleys; (3) very few karst features were mapped within 1 mile of the reservoir or in the area currently occupied by the reservoir; and (4) faults that intersect the reservoir but may not possess hydraulic properties that cause the faults to be conduits for groundwater flow. Groundwater interaction with reservoir tributaries is likely more common in areas of the watershed upstream from Nolin River Lake where karst hydrogeology is prevalent. Results of water-quality surveys using an AUV from August 15 to 19, 2016, did not identify areas of anomalous values that might indicate groundwater inflows through preferential flow zones. Spatial distributions of water-quality parameters were generally uniform within each constant-depth layer. The constant-depth layers were selected to be above, within, and below the thermocline and ranged from the water surface to 25 feet. Surveys near the bottom of the reservoir that might have been more sensitive to groundwater inflows were not done because presurvey data were not available to indicate locations of obstacles that could ensnare the AUV. Water-quality data collected with the AUV did identify water-quality anomalies where stream tributaries were discharging to the reservoir. The discrete water-quality samples indicated uniformity among the five reservoir sites. The riverine site that is immediately upstream from Nolin River Lake, however, had some unique water-quality characteristics relative to sites on the reservoir. The highest concentrations of nitrate plus nitrite as nitrogen (0.145 milligrams per liter [mg/L]), total phosphorous (0.07 mg/L), chlorophyll a (36.1 micrograms per liter), and pheophytin a (10.2 micrograms per liter) were measured at the Nolin River Lake riverine site (site 2NRR20034). The concentrations of nutrients and chlorophyll a at the riverine site did exceed the 25th percentile of median concentrations measured by the U.S. Environmental Protection Agency (EPA) at other lakes and reservoirs in EPA level IV ecoregion 71a. Concentrations of most nutrients and chlorophyll a at the five reservoir sites also exceeded the 25th percentile of median concentrations in EPA level IV ecoregion 72h. The exception was the concentrations of total phosphorus as phosphorus at the reservoir sites that were at or below the 25th percentile of median concentrations measured by EPA (0.03 mg/L). Concentrations of orthophosphate as phosphorus were less than the method detection limit of 0.004 mg/L at all sites. The phytoplankton community in Nolin River Lake was almost exclusively (greater than 90 percent of total phytoplankton abundance) cyanobacteria, also known as blue-green algae. A species of Cylindrospermopsis dominated the cyanobacterial community at the five reservoir sites, while Chroococcus microscopicus was most abundant at the riverine site. Cyanobacterial cell densities ranged from 10,000 to 198,067,460 cells per liter in five areas in the reservoir and from 4,800 to 73,751,253 cells per liter at the riverine site. Multiple potential sources of water to Nolin River Lake include direct precipitation, overland flow, interflow, groundwater, and surface water. Understanding the exact contribution of each of these components to the water budget at Nolin River Lake may help the U.S. Army Corps of Engineers manage the water quality, water quantity, and biological communities in the reservoir. Additional hydrogeologic and water-quality data that builds on the results of this study may refine the inferences of this study; for example, deeper AUV surveys that target the largest fault zones might further the understanding of the potential for groundwater flow through those features. A complete understanding of the reservoir hydrology, however, may require the use of scientific methods intended for water bodies as large as Nolin River Lake, such as aerial infrared photography and imagery; water mass, chemical, and isotopic balance studies; geophysical measurements; and numerical simulations.

Kentucky↗
USGS2001

Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000

Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO 2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ∼1 μm. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO 2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High-resolution spectral observations of Emakong caldera show thermal emission and SO 2 within the same pixels, implying that patches of SO 2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO 2 distribution maps show that the highest concentration of SO 2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO 2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO 2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO 2 . The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.

Journal of Geophysical Research E: Planets↗
USGS2020

Peak ground motions and site response at Anza and Imperial Valley, California

Power spectra of shear-waves for eighteen earthquakes from the Anza-Imperial Valley region were inverted for source, mid-path Q, site attenuation and site response. The motivation was whether differences in site attenuation (parameterized as t*, r/cQ, where r is distance along ray path near the site, c is shear velocity and Q is the quality factor that parameterizes attenuation) and site response could be correlated with residuals in peak values of velocity or acceleration after removing the affect of distance-dependent attenuation. We decomposed spectra of S-waves from horizontal components of 18 earthquakes from 2010 to 2018 into a common source for each event with ω −2 spectral fall-off at high frequencies and then projected the residuals onto path and site terms following the methodology of Boatwright et al. (Bull Seismol Soc Am 81:1754–1782, 1991). The site terms were constrained to have an amplification at a particular frequency governed by V S30 at two of the sites which had downhole shear-wave logs. The 18 events, 3 < M < 4, had moments between approximately 10 20 and 10 22 dyne-cm, and stress drops between 1 and 100 bars. Average mid-crust attenuation had a Q of 844 reflecting the average path through the crystalline rock of the San Jacinto Mountains. t* for each station corresponded to the geologic environment such that stations on hard rock had low t* (e.g. stations KNW, PFO and RDM) a station in the San Jacinto fault zone (station SND) had a moderate t* of 0.035 s and stations in the Imperial Valley usually had higher t*s . Generally t* correlated with average amplification suggesting that sites characterized by low surface velocities and higher attenuation also have more amplification in the 1–6 Hz band. Residuals of peak values were determined by subtracting the prediction of Boore and Atkinson ( 2008 ). There is a correlation between average amplification and peak velocity, but not peak acceleration. Interestingly, there is less scatter at high values of amplification although there is also less data. Scatter in values of peak velocity and peak acceleration are higher at shorter compared to longer durations. When using a frequency-dependent form for Q , variances are higher, sometimes much higher; the dataset does not support frequency-dependent Q , which is not similar to results from the Imperial Valley and northeastern North America.

California↗
USGS2020

Towards understanding relationships between atmospheric pressure variations and long-period horizontal seismic data: A case study

Variations in atmospheric pressure have long been known to introduce noise in long-period (>10 s) seismic records. This noise can overwhelm signals of interest such as normal modes and surface waves. Generally, this noise is most pronounced on the horizontal components where it arises due to tilting of the seismometer in response to changes in atmospheric pressure. Several studies have suggested methodologies for correcting unwanted pressure-induced noise using collocated microbarograph records. However, how applicable these corrections are to varying geologic settings and installation types (e.g. vault versus post-hole) is unclear. Using coefficients obtained by solving for the residuals of these corrections, we can empirically determine the sensitivity of instruments in a specific location to the influences of pressure. To better understand how long-period, pressure-induced noise changes with time and emplacement, we examine horizontal seismic records along with barometric pressure at five different Global Seismographic Network stations, all with multiple broadband seismometers. We also analyse three Streckeisen STS-2 broadband seismometers, which are collocated with a microbarograph, at the Albuquerque Seismological Laboratory. We observe periods of high magnitude-squared-coherence (γ 2 -coherence; γ 2 > 0.8) between the seismic and pressure signals which fluctuate through time, frequency, and even between seismic instruments in the same vault. These observations suggest that these tilt-generated signals are highly sensitive to very local (<10 m) site effects. However, we find that in cases where instruments are not located at a large depth (<100 m), the pressure-induced noise is polarized in a nearly constant direction that is consistent with local topographic features or the geometry of the vault. We also find that borehole instruments at a large depth (>100 m) appear to be unaffected by pressure-loading mechanisms outlined by Sorrells (1971) but possibly by Newtonian attraction. Correlating the induced-noise polarization direction with times of high coherence, we work to identify sensors that are ultimately limited by pressure-induced horizontal noise as well as period bands that can benefit from pressure corrections. We find that while the situation is complex, each sensor appears to have its own unique response to pressure. Our findings suggest that we can determine empirical relationships between pressure and induced tilt on a case by case basis.

Geophysical Journal International↗