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At least 1,369 records · Page 76Linked to original sources
USGS2023

Monitoring Avian Productivity and Survivorship (MAPS) 6-year summary, Naval Outlying Landing Field, Imperial Beach, southwestern San Diego County, California, 2014–20

Executive Summary From 2014 to 2020, a Monitoring Avian Productivity and Survivorship (MAPS) banding station (station) was operated at the Naval Outlying Landing Field (NOLF), Imperial Beach, in southwestern San Diego County, California. The station was established as part of a long-term monitoring program of Neotropical migratory bird populations on NOLF and helps Naval Base Coronado (NOLF is a component) meet the goals and objectives of the Department of Defense Partners in Flight program and the Birds and Migratory Birds Management Strategies of the Naval Base Coronado Integrated Natural Resources Management Plan. The station was established in 2009 and has been in operation during the spring and summer since 2009 except for 2016 when it was not funded. The station was operated by AMEC Earth and Environmental, Inc., from 2009 to 2011, by the U.S. Geological Survey from 2012 to 2015, the San Diego Natural History Museum in 2017, and the U.S. Geological Survey again from 2018 to 2023. This report synthesizes results from 2014 to 2020. A prior report presents summaries and analyses from 2009 to 2013. The banding station at NOLF was operated according to the standard MAPS protocol with some exceptions. Ten mist nets used to capture birds were erected in fixed locations that remained consistent between and within years, with few minor relocations. Nets were open for 6 hours per day, once every 10 days (a netting period) for 13 netting periods starting April 1 each year. Occasionally, poor weather conditions (for example, rain, wind, or excessive heat) prevented net operation or forced nets to be closed early (or, rarely, late). Nets were checked periodically throughout the day and birds were removed, processed (leg bands affixed, measurements recorded), and released. From 2014 to 2020, we had 3,543 captures (including initial captures and recaptures) of a maximum of 3,264 year-unique captures (543±143 year-unique captures [the total number of individual birds captured for the first time each year]). The count of year-unique captures included 2,702 newly banded birds, 258 individuals that were recaptured from previous years, and 304 birds that were released unbanded (218 hummingbirds and 86 other birds that were intentionally released unbanded [game birds, and so forth] or escaped before banding). Individuals of 68 species were captured, 39 of which breed at or in the immediate vicinity of the MAPS banding station. Bird capture rate averaged 43±30 captures per day (corrected to account for variation in effort) for all years (range 7–163 effort-corrected captures per day) and species richness per year averaged 43±4. Bushtit ( Psaltriparus minimus ) was the most abundant species captured, followed by Orange-crowned Warbler ( Leiothlypis celata ), Wilson’s Warbler ( Cardellina pusilla ), House Finch ( Haemorhous mexicanus ), Song Sparrow ( Melospiza melodia ), and Common Yellowthroat ( Geothlypis trichas ). The mean adult sex ratio of all species combined across all years was 54:46 male:female. Adults averaged 73±12 percent of known age captures per year (range 59–94 percent), and juveniles averaged 27±12 percent (range 6–41 percent). Nineteen sensitive species were detected at NOLF (12 captured and 7 observed only). During 2014–20, we captured one State and federally endangered species, Least Bell’s Vireo ( Vireo bellii pusillus ); one federally threatened species, California Gnatcatcher ( Polioptila californica ); one State endangered species, Willow Flycatcher ( Empidonax traillii ); and two State species of concern, Yellow-breasted Chat ( Icteria virens ) and Yellow Warbler ( Setophaga petechia ). One additional State species of concern, Northern Harrier ( Circus hudsonius ), was observed at the MAPS banding station but not captured. Peregrine Falcon ( Falco peregrinus ) and White-tailed Kite ( Elanus leucurus ), California State fully protected species, also were observed at the MAPS banding station. Seven federal bird species of conservation concern—Calliope Hummingbird ( Selasphorus calliope ), Rufous Hummingbird ( Selasphorus rufus ), Allen’s Hummingbird ( Selasphorus sasin ), Nuttall’s Woodpecker ( Dryobates nuttallii ), Wrentit ( Chamaea fasciata ), California Thrasher ( Toxostoma redivivum ), and Lawrence’s Goldfinch ( Spinus lawrencei )—also were captured, and four additional federal bird species of conservation concern—Willet ( Tringa semipalmata ), Western Gull ( Larus occidentalis ), California Gull ( Larus californicus ), and Bullock’s Oriole ( Icterus bullockii )—were observed but not captured. Local population trends varied among species and years. From 2012 to 2019, year-round residents Bushtit, Song Sparrow, and Common Yellowthroat significantly decreased, whereas the migrant Least Bell’s Vireo increased. The total number of captures for all species except Least Bell’s Vireo was lowest in 2017, corresponding to the habitat damage caused by Kuroshio shot hole borer beetle ( Euwallacea kuroshio ) in the Tijuana River Valley. Annual productivity and annual adult survival were calculated for seven breeding species based on criteria used by the Institute for Bird Populations (Least Bell’s Vireo, Bushtit, Wrentit, House Wren [ Troglodytes aedon ], Song Sparrow, Orange-crowned Warbler, and Common Yellowthroat). Productivity was highest for most species in 2010 and 2019, years with high precipitation, and lowest in 2014 and 2018, years with low precipitation. Song Sparrow demonstrated the highest productivity among species and Least Bell’s Vireo had the lowest productivity. Annual adult survival was generally high from 2011 to 2012 and from 2018 to 2019. Bushtit had higher annual survival with lower late winter precipitation. Either temperature or precipitation was associated with productivity for all species except Wrentit, and with survival for all species except Least Bell’s Vireo and Common Yellowthroat. For most species, productivity was positively associated with precipitation, and both productivity and survival were negatively associated with temperature. Other studies have found that higher temperatures led to increased predation by snakes and birds and also increased vector-borne disease transmission, such as West Nile virus. Predicted regional increases in temperature over the next 30 years will likely affect the demographics of these species. The Song Sparrow population increased with higher breeding productivity during the previous year, and the Bushtit population increased with higher annual survival and higher productivity during the previous year. Aside from a possible positive association between survivorship and Common Yellowthroat population growth, productivity and survival rates did not appear to influence population change for other focal species.

California↗
USGS1988

The development of the continental margin of eastern North America-conjugate continental margin to West Africa

The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins—only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper Cretaceous deeper water chalk and shale.

Journal of African Earth Sciences↗
USGS1983

Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He 4He(RA) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He 4He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 RA), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium. Crushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low 3He 4He) helium. 3He 4He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in 3He 4He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 RA) helium with some (variable) component of lithospheric contamination added during "breakthrough", while the later stages are characterized by a relaxation toward lithospheric 3He 4He ratios (??? 8 RA) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in 3He 4He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume. ?? 1983.

Hawaii↗
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↗
USGS2004

Assessing the record and causes of Late Triassic extinctions

Accelerated biotic turnover during the Late Triassic has led to the perception of an end-Triassic mass extinction event, now regarded as one of the "big five" extinctions. Close examination of the fossil record reveals that many groups thought to be affected severely by this event, such as ammonoids, bivalves and conodonts, instead were in decline throughout the Late Triassic, and that other groups were relatively unaffected or subject to only regional effects. Explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms. Regression during the Rhaetian, with consequent habitat loss, is compatible with the disappearance of some marine faunal groups, but may be regional, not global in scale, and cannot explain apparent synchronous decline in the terrestrial realm. Gradual, widespread aridification of the Pangaean supercontinent could explain a decline in terrestrial diversity during the Late Triassic. Although evidence for an impact precisely at the boundary is lacking, the presence of impact structures with Late Triassic ages suggests the possibility of bolide impact-induced environmental degradation prior to the end-Triassic. Widespread eruptions of flood basalts of the Central Atlantic Magmatic Province (CAMP) were synchronous with or slightly postdate the system boundary; emissions of CO2 and SO2 during these eruptions were substantial, but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved. A substantial excursion in the marine carbon-isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary. Release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion, although the triggering mechanism and climatic effects of such a release remain uncertain. ?? 2003 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗
USGS2011

Thermal modelling of stepwise anatexis in a thrust-thickened sialic crust

One-dimensional modelling of the thermal history of a sialic crust thickened by multiple overstack thrusting of upper crustal material shows that anatexis is likely. both the uplift rate and the length of the incubation period between end of tectonism and start of uplift are important controls on the amount and temperature of the melt. Heat of fusion does not significantly affect the long-term thermal structure of the crust if the melt is not extracted because only a small fraction of conductive heat is converted to latent heat, though short-term thermal effects of latent heat can be locally important. Model results show that commonly <15% of mantle heat flux is converted to latent heat; even during peak melting in the most productive models, less than half of incremental mantle flux is converted. The results have obvious implications on the acceptability of proposed heat sources for crustal anatexis. Fusion could retard crustal temperature rise by nearly 100??C, but the system would recover except for situations of very rapid uplift. Understanding of the thermal evolution of a burial-uplift system requires knowledge not only of the timing of anatexis but of the pooling and movement of the magma, as well as the duration and nature of the incubation period; we are poorly equipped to measure these events. The model predicts that the characteristic time for anatexis is a thickened sialic crust is several tens of millions of years, comparable to the time lapse between orogenies; in making geological interpretations of magmatism, this time lag must be considered. -Author

Transactions of the Royal Society of Edinburgh, Ea↗
USGS2009

On near-source earthquake triggering

[1] When one earthquake triggers others nearby, what connects them? Two processes are observed: static stress change from fault offset and dynamic stress changes from passing seismic waves. In the near-source region ( r ≤ 50 km for M ∼ 5 sources) both processes may be operating, and since both mechanisms are expected to raise earthquake rates, it is difficult to isolate them. We thus compare explosions with earthquakes because only earthquakes cause significant static stress changes. We find that large explosions at the Nevada Test Site do not trigger earthquakes at rates comparable to similar magnitude earthquakes. Surface waves are associated with regional and long-range dynamic triggering, but we note that surface waves with low enough frequency to penetrate to depths where most aftershocks of the 1992 M = 5.7 Little Skull Mountain main shock occurred (∼12 km) would not have developed significant amplitude within a 50-km radius. We therefore focus on the best candidate phases to cause local dynamic triggering, direct waves that pass through observed near-source aftershock clusters. We examine these phases, which arrived at the nearest (200–270 km) broadband station before the surface wave train and could thus be isolated for study. Direct comparison of spectral amplitudes of presurface wave arrivals shows that M ∼ 5 explosions and earthquakes deliver the same peak dynamic stresses into the near-source crust. We conclude that a static stress change model can readily explain observed aftershock patterns, whereas it is difficult to attribute near-source triggering to a dynamic process because of the dearth of aftershocks near large explosions.

Journal of Geophysical Research B: Solid Earth↗
USGS1985

Evaluating earthquake hazards in the Los Angeles region— An earth-science perspective

Potentially destructive earthquakes are inevitable in the Los Angeles region of California, but hazards prediction can provide a basis for reducing damage and loss. This volume identifies the principal geologically controlled earthquake hazards of the region (surface faulting, strong shaking, ground failure, and tsunamis), summarizes methods for characterizing their extent and severity, and suggests opportunities for their reduction. Two systems of active faults generate earthquakes in the Los Angeles region: northwest-trending, chiefly horizontal-slip faults, such as the San Andreas, and west-trending, chiefly vertical-slip faults, such as those of the Transverse Ranges. Faults in these two systems have produced more than 40 damaging earthquakes since 1800. Ninety-five faults have slipped in late Quaternary time (approximately the past 750,000 yr) and are judged capable of generating future moderate to large earthquakes and displacing the ground surface. Average rates of late Quaternary slip or separation along these faults provide an index of their relative activity. The San Andreas and San Jacinto faults have slip rates measured in tens of millimeters per year, but most other faults have rates of about 1 mm/yr or less. Intermediate rates of as much as 6 mm/yr characterize a belt of Transverse Ranges faults that extends from near Santa Barbara to near San Bernardino. The dimensions of late Quaternary faults provide a basis for estimating the maximum sizes of likely future earthquakes in the Los Angeles region: moment magnitude .(M) 8 for the San Andreas, M 7 for the other northwest-trending elements of that fault system, and M 7.5 for the Transverse Ranges faults. Geologic and seismologic evidence along these faults, however, suggests that, for planning and designing noncritical facilities, appropriate sizes would be M 8 for the San Andreas, M 7 for the San Jacinto, M 6.5 for other northwest-trending faults, and M 6.5 to 7 for the Transverse Ranges faults. The geologic and seismologic record indicates that parts of the San Andreas and San Jacinto faults have generated major earthquakes having recurrence intervals of several tens to a few hundred years. In contrast, the geologic evidence at points along other active faults suggests recurrence intervals measured in many hundreds to several thousands of years. The distribution and character of late Quaternary surface faulting permit estimation of the likely location, style, and amount of future surface displacements. An extensive body of geologic and geotechnical information is used to evaluate areal differences in future levels of shaking. Bedrock and alluvial deposits are differentiated according to the physical properties that control shaking response; maps of these properties are prepared by analyzing existing geologic and soils maps, the geomorphology of surficial units, and. geotechnical data obtained from boreholes. The shear-wave velocities of near-surface geologic units must be estimated for some methods of evaluating shaking potential. Regional-scale maps of highly generalized shearwave velocity groups, based on the age and texture of exposed geologic units and on a simple two-dimensional model of Quaternary sediment distribution, provide a first approximation of the areal variability in shaking response. More accurate depictions of near-surface shear-wave velocity useful for predicting ground-motion parameters take into account the thickness of the Quaternary deposits, vertical variations in sediment .type, and the correlation of shear-wave velocity with standard penetration resistance of different sediments. A map of the upper Santa Ana River basin showing shear-wave velocities to depths equal to one-quarter wavelength of a 1-s shear wave demonstrates the three-dimensional mapping procedure. Four methods for predicting the distribution and strength of shaking from future earthquakes are presented. These techniques use different measures of strong-motion

California↗
USGS2019

Slow-growing and extended-duration seismicity swarms: Reactivating joints or foliations in the Cahuilla Valley Pluton, Central Peninsular Ranges, Southern California

Three prolific earthquake swarms and numerous smaller ones have occurred since 1980 in the Mesozoic igneous plutonic rocks of the Perris block of the Peninsular Ranges, Southern California. The major swarms occurred in 1980–1981, 1983–1984, and 2016–2018, with the latest swarm still ongoing. These swarms have no clear mainshock, with the largest events of M L 3.6, M L 3.7, and M w 4.4. Each successive swarm had larger cumulative seismic moment release with about 314 and 411 events of M ≥ 1.5, while the third swarm has produced about 451 events of M ≥ 1.5 (as of September 2018). The concurrent strike‐slip faulting occurred on north to northwest striking planes but with no orthogonal northeast trending seismicity alignments. These shallow swarms are probably driven by intrablock Pacific‐North America plate boundary stress loading of the two bounding major late Quaternary strike‐slip faults, the Elsinore and San Jacinto faults. The state of stress within the Cahuilla Valley pluton has a ~40° angle between the maximum principal stress and the average trend of the swarms, suggesting that migrating pore fluid pressures aid in the formation and growth of zones of weakness. These swarms, which last more than 600 days each, exhibit clear bilateral spatial migration for distances of up to ~7–8 km and reach their full length in about 20 months. The slow spatial‐temporal development of the swarms corresponds to a fluid diffusivity of 0.006 to 0.01 m 2 /s, consistent with very low permeability rocks as expected for this block. There is no geodetic or other evidence for a slow slip event driving the swarms.

California↗
USGS2017

Evaluating the role of land cover and climate uncertainties in computing gross primary production in Hawaiian Island ecosystems

Gross primary production (GPP) is the Earth’s largest carbon flux into the terrestrial biosphere and plays a critical role in regulating atmospheric chemistry and global climate. The Moderate Resolution Imaging Spectrometer (MODIS)-MOD17 data product is a widely used remote sensing-based model that provides global estimates of spatiotemporal trends in GPP. When the MOD17 algorithm is applied to regional scale heterogeneous landscapes, input data from coarse resolution land cover and climate products may increase uncertainty in GPP estimates, especially in high productivity tropical ecosystems. We examined the influence of using locally specific land cover and high-resolution local climate input data on MOD17 estimates of GPP for the State of Hawaii, a heterogeneous and discontinuous tropical landscape. Replacing the global land cover data input product (MOD12Q1) with Hawaii-specific land cover data reduced statewide GPP estimates by ~8%, primarily because the Hawaii-specific land cover map had less vegetated land area compared to the global land cover product. Replacing coarse resolution GMAO climate data with Hawaii-specific high-resolution climate data also reduced statewide GPP estimates by ~8% because of the higher spatial variability of photosynthetically active radiation (PAR) in the Hawaii-specific climate data. The combined use of both Hawaii-specific land cover and high-resolution Hawaii climate data inputs reduced statewide GPP by ~16%, suggesting equal and independent influence on MOD17 GPP estimates. Our sensitivity analyses within a heterogeneous tropical landscape suggest that refined global land cover and climate data sets may contribute to an enhanced MOD17 product at a variety of spatial scales.

Hawaii↗
USGS2010

Coherence of Mach fronts during heterogeneous supershear earthquake rupture propagation: Simulations and comparison with observations

We study how heterogeneous rupture propagation affects the coherence of shear and Rayleigh Mach wavefronts radiated by supershear earthquakes. We address this question using numerical simulations of ruptures on a planar, vertical strike-slip fault embedded in a three-dimensional, homogeneous, linear elastic half-space. Ruptures propagate spontaneously in accordance with a linear slip-weakening friction law through both homogeneous and heterogeneous initial shear stress fields. In the 3-D homogeneous case, rupture fronts are curved owing to interactions with the free surface and the finite fault width; however, this curvature does not greatly diminish the coherence of Mach fronts relative to cases in which the rupture front is constrained to be straight, as studied by Dunham and Bhat (2008a). Introducing heterogeneity in the initial shear stress distribution causes ruptures to propagate at speeds that locally fluctuate above and below the shear wave speed. Calculations of the Fourier amplitude spectra (FAS) of ground velocity time histories corroborate the kinematic results of Bizzarri and Spudich (2008a): (1) The ground motion of a supershear rupture is richer in high frequency with respect to a subshear one. (2) When a Mach pulse is present, its high frequency content overwhelms that arising from stress heterogeneity. Present numerical experiments indicate that a Mach pulse causes approximately an ω −1.7 high frequency falloff in the FAS of ground displacement. Moreover, within the context of the employed representation of heterogeneities and over the range of parameter space that is accessible with current computational resources, our simulations suggest that while heterogeneities reduce peak ground velocity and diminish the coherence of the Mach fronts, ground motion at stations experiencing Mach pulses should be richer in high frequencies compared to stations without Mach pulses. In contrast to the foregoing theoretical results, we find no average elevation of 5%-damped absolute response spectral accelerations (SA) in the period band 0.05–0.4 s observed at stations that presumably experienced Mach pulses during the 1979 Imperial Valley, 1999 Kocaeli, and 2002 Denali Fault earthquakes compared to SA observed at non-Mach pulse stations in the same earthquakes. A 20% amplification of short period SA is seen only at a few of the Imperial Valley stations closest to the fault. This lack of elevated SA suggests that either Mach pulses in real earthquakes are even more incoherent that in our simulations or that Mach pulses are vulnerable to attenuation through nonlinear soil response. In any case, this result might imply that current engineering models of high frequency earthquake ground motions do not need to be modified by more than 20% close to the fault to account for Mach pulses, provided that the existing data are adequately representative of ground motions from supershear earthquakes.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Flood pulsing in the Sudd wetland: analysis of seasonal variations in 2 inundation and evapotranspiration in Southern Sudan

Located on the Bahr el Jebel in South Sudan, the Sudd is one of the largest floodplain wetlands in the world. Seasonal inundation drives the hydrologic, geomorphological, and ecological processes, and the annual flood pulse is essential to the functioning of the Sudd. Despite the importance of the flood pulse, various hydrological interventions are planned upstream of the Sudd to increase economic benefits and food security. These will not be without consequences, in particular for wetlands where the biological productivity, biodiversity, and human livelihoods are dependent on the flood pulse and both the costs and benefits need to be carefully evaluated. Many African countries still lack regional baseline information on the temporal extent, distribution, and characteristics of wetlands, making it hard to assess the consequences of development interventions. Because of political instability in Sudan and the inaccessible nature of the Sudd, recent measurements of flooding and seasonal dynamics are inadequate. Analyses of multitemporal and multisensor remote sensing datasets are presented in this paper, in order to investigate and characterize flood pulsing within the Sudd wetland over a 12-month period. Wetland area has been mapped along with dominant components of open water and flooded vegetation at five time periods over a single year. The total area of flooding (both rain and river fed) over the 12 months was 41 334 km2, with 9176 km2 of this constituting the permanent wetland. Mean annual total evaporation is shown to be higher and with narrower distribution of values from areas of open water (1718 mm) than from flooded vegetation (1641 mm). Although the exact figures require validation against ground-based measurements, the results highlight the relative differences in inundation patterns and evaporation across the Sudd.

Earth Interactions↗
USGS2012

Sea-level history during the Last Interglacial complex on San Nicolas Island, California: implications for glacial isostatic adjustment processes, paleozoogeography and tectonics

San Nicolas Island, California has one of the best records of fossiliferous Quaternary marine terraces in North America, with at least fourteen terraces rising to an elevation of ~270 m above present-day sea level. In our studies of the lowest terraces, we identified platforms at 38-36 m (terrace 2a), 33-28 m (terrace 2b), and 13-8 m (terrace 1). Uranium-series dating of solitary corals from these terraces yields three clusters of ages: ~120 ka on terrace 2a (marine isotope stage [MIS] 5.5), ~120 and ~100 ka on terrace 2b (MIS 5.5 and 5.3), and ~80 ka (MIS 5.1) on terrace 1. We conclude that corals on terrace 2b that date to ~120 ka were reworked from a formerly broader terrace 2a during the ~100 ka sea stand. Fossil faunas differ on the three terraces. Isolated fragments of terrace 2a have a fauna similar to that of modern waters surrounding San Nicolas Island. A mix of extralimital southern and extralimital northern species is found on terrace 2b, and extralimital northern species are on terrace 1. On terrace 2b, with its mixed faunas, extralimital southern species, indicating warmer than present waters, are interpreted to be from the ~120 ka high sea stand, reworked from terrace 2a. The extralimital northern species on terrace 2b, indicating cooler than present waters, are interpreted to be from the ~100 ka sea stand. The abundant extralimital northern species on terrace 1 indicate cooler than present waters at ~80 ka. Using the highest elevations of the ~120 ka platform of terrace 2a, and assuming a paleo-sea level of +6 m based on previous studies, San Nicolas Island has experienced late Quaternary uplift rates of ~0.25-0.27 m/ka. These uplift rates, along with shoreline angle elevations and ages of terrace 2b (~100 ka) and terrace 1 (~80 ka) yield relative (local) paleo-sea level elevations of +2 to +6 m for the ~100 ka sea stand and -11 to -12 m for the ~80 ka sea stand. These estimates are significantly higher than those reported for the ~100 ka and ~80 ka sea stands on New Guinea and Barbados. Numerical models of the glacial isostatic adjustment (GIA) process presented here demonstrate that these differences in the high stands are expected, given the variable geographic distances between the sites and the former Laurentide and Cordilleran ice sheets. Moreover, the numerical results show that the absolute and differential elevations of the observed high stands provide a potentially important constraint on ice volumes during this time interval and on Earth structure.

California↗
USGS2018

Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3–4 Ma pulse of basaltic magmatism.

Earth and Planetary Science Letters↗
USGS1999

Foreshock occurrence before large earthquakes

Rates of foreshock occurrence involving shallow M ≥ 6 and M ≥ 7 mainshocks and M ≥ 5 foreshocks were measured in two worldwide catalogs over ∼20-year intervals. The overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in California. The aftershock model was extended to the case of moderate foreshocks preceding large mainshocks. Overall, the observed worldwide foreshock rates exceed the extended California generic model by a factor of ∼2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike-slip events lower than the worldwide average. Among the thrust events, a large majority, composed of events located in shallow subduction zones, had a high foreshock rate, while a minority, located in continental thrust belts, had a low rate. These differences may explain why previous surveys have found low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggests the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock-rich. If this is so, then the California generic model may significantly underestimate the conditional probability for a very large ( M ≥ 8) earthquake following a potential ( M ≥ 7) foreshock in Cascadia. The magnitude differences among the identified foreshock-mainshock pairs in the Harvard catalog are consistent with a uniform distribution over the range of observation.

Journal of Geophysical Research B: Solid Earth↗
USGS1989

Review of magnetic and electric field effects near active faults and volcanoes in the U.S.A.

Synchronized measurements of geomagnetic field have been recorded along 800 km of the San Andreas fault and in the Long Valley caldera since 1974, and during eruptions on Mount St. Helens since 1980. For shorter periods of time, continuous measurements of geoelectric field measurements have been made on Mount St. Helens and near the San Andreas fault where moderate seismicity and fault slip frequently occurs. Significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include: (1) two moderate earthquakes (ML > 5.8) for which magnetometers were close enough to expect observable signals (about three source lengths), (2) one moderate earthquake (MS 7.3) for which magnetometers were installed as massive fluid outflow occurred during the post-seismic phase, (3) numerous fault creep events and moderate seismicity, (4) a major explosive volcanic eruption and numerous minor extrusive eruptions, and (5) an episode of aseismic uplift. For one of the two earthquakes with ML > 5.8, seismomagnetic effects of -1.3 and -0.3 nT were observed. For this event, magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event. Similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer. Precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a ML 5.2 earthquake. Longer-term changes were recorded in one region in southern California where a moderate ML 5.9 earthquake has since occurred. Surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision (0.25 nT and 0.01%, respectively) and are consistent with near-surface fault failure models of these events. Longer-term creep is sometimes associated with corresponding longer-term magnetic field perturbations. Correlated changes in gravity, magnetic field, areal strain, and uplift occurred during episodes of aseismic deformation in southern California primarily between 1979 and 1983. Because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. An unknown source of meteorologically generated noise in the strain, gravity, and uplift data and an unknown, but correlated, disturbance in the absolute magnetic data might also explain the data. No clear observations of seismoelectric or tectonoelectric effects have yet been reported. The eruption of Mount St. Helens generated large oscillatory fields and 9 ?? 2 nT offset on the only surviving magnetometer. A large-scale traveling magnetic disturbance passed through the San Andreas array from 1 to 2 h after the eruption. Subsequent extrusive eruptions generated small precursory magnetic changes in some cases. These data are consistent with a simple volcanomagnetic model, magneto-gas dynamic effects, and a blast excited traveling ionospheric disturbance. Traveling ionospheric disturbances (TIDs), also generated by earthquake-related atmospheric pressure waves, may explain many electromagnetic disturbances apparently associated with earthquakes. Local near-fault magnetic field transients rarely exceed a few nT at periods of a few minutes and longer. ?? 1989.

Physics of the Earth and Planetary Interiors↗
USGS2012

Water quality and landscape processes of four watersheds in eastern Puerto Rico

Humid tropical regions occupy about a quarter of Earth's land surface, yet they contribute a substantially higher fraction of the water, solutes, and sediment discharged to the world's oceans. Nearly half of Earth's population lives in the tropics, and development stresses can potentially harm soil resources, water quality, and water supply and in addition increase landslide and flood hazards. Owing to Puerto Rico's steep topography, low water storage capacity, and dependence on trade-wind precipitation, the island's people, ecosystems, and water supply are vulnerable to extreme weather such as hurricanes, floods, and droughts. Eastern Puerto Rico offers a natural laboratory for separating geologic and land-cover influences from regional- and global-scale influences because of its various bedrock types and the changing land cover surrounding intact, mature forest of the Luquillo Experimental Forest. Accordingly, a multiyear assessment of hydrological and biogeochemical processes was designed to develop an understanding of the effects of these differences on local climate, streamflow, water quality, and ecosystems, and to form the basis for a long-term and event-based program of climate and hydrologic monitoring. Because infrequent, large storms play a major role in this landscape, we focused on high-runoff events, sampling 263 storms, including all major hurricanes from 1991 through 2005. The largest storms have profound geomorphic consequences, such as landslides, debris flows, deep gullying on deforested lands, excavation and suspension of sediment in stream channels, and delivery of a substantial fraction of annual stream sediment load. Large storms sometimes entrain ocean foam and spray causing high concentrations of seasalt-derived constituents in stream waters during the storm. Past deforestation and agricultural activities in the Cayagu&aacute;s and Can&oacute;vanas watersheds accelerated erosion and soil loss, and this material continues to be remobilized during large storms. Nearly 5,000 routine and event samples were analyzed for parameters that allow determination of denudation rates based on suspended and dissolved loads; 860 of these samples were analyzed for a comprehensive suite of chemical constituents. The rivers studied are generally similar in water-quality characteristics, and windward or leeward aspect appears to exert a stronger influence on water quality than geology or land cover. Of samples analyzed for comprehensive chemistry and for sediment, 543 were collected at runoff rates greater than 1 millimeter per hour, 256 at rates exceeding 10 millimeters per hour, and 3 at rates exceeding 90 millimeters per hour. Streams have rarely been sampled during events with such high runoff rates. Rates of physical and chemical weathering are especially high, and physical denudation rates, forested watersheds included, are considerably greater than is expected for a steady-state system. The elevated physical erosion drives an increased particulate organic carbon flux, one that is large, important to the carbon cycle, and sustainable, because soil-carbon regeneration is rapid. The 15-year Water, Energy, and Biogeochemical Budget dataset, which includes discharge, field parameters, suspended sediment, major cations and anions, and nutrients, is available from the U.S. Geological Survey's National Water Information System (http://waterdata.usgs.gov/nwis). The dataset provides a baseline for characterizing future environmental change and will improve our understanding of the interdependencies of land, water, and biological resources and their responses to changes in climate and land use. Because eastern Puerto Rico resembles many tropical regions in terms of geology and patterns of development, implications from this study are transferable to other tropical regions facing deforestation, rapid land-use change, and climate change.

Professional Paper↗
USGS2009

Kilauea slow slip events: Identification, source inversions, and relation to seismicity

Several slow slip events beneath the south flank of Kilauea Volcano, Hawaii, have been inferred from transient displacements in daily GPS positions. To search for smaller events that may be close to the noise level in the GPS time series, we compare displacement fields on Kilauea's south flank with displacement patterns in previously identified slow slip events. Matching displacement patterns are found for several new candidate events, although displacements are much smaller than previously identified events. One of the candidates, 29 May 2000, is coincident with a microearthquake swarm, as are all of the previously identified slow slip events. The microearthquakes follow the onset of slow slip, implying that they are triggered by stress changes during slip. The new slow slip event brings the total number of events on Kilauea, between 1997 and 2007, to eight, the smallest having M W = 5.3, and the largest having M W = 6.0. While the recurrence time between the four largest events is 2.11 ± 0.01 years, the repeat time for all eight events is 0.9 ± 0.6 years. We invert for the fault geometry and distribution of slip during the slow slip events. The optimal source depths of 5 km, assuming uniform slip dislocations in an elastic half‐space, are considerably shallower than the accompanying swarm earthquakes (6.5–8.5 km), which would place the earthquakes in a zone of decreased Coulomb stress. Inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths, such that the earthquakes are located in a region of increased Coulomb stress. We also invert for time‐dependent fault slip directly from the 30 s GPS phase observations, constraining the source to the optimal uniform slip geometry. On the basis of these inversions, the larger events last between 1.5–2.2 days. The data are unable to resolve migration of slip along the fault. The temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate and state‐friction, making the swarm earthquakes coshocks and aftershocks of the slow slip events.

Hawaii↗