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

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At least 325 records · Page 18Linked to original sources
USGS2001

Stress drop with constant, scale independent seismic efficiency and overshoot

To model dissipated and radiated energy during earthquake stress drop, I calculate dynamic fault slip using a single degree of freedom spring-slider block and a laboratory-based static/kinetic fault strength relation with a dynamic stress drop proportional to effective normal stress. The model is scaled to earthquake size assuming a circular rupture; stiffness varies inversely with rupture radius, and rupture duration is proportional to radius. Calculated seismic efficiency, the ratio of radiated to total energy expended during stress drop, is in good agreement with laboratory and field observations. Predicted overshoot, a measure of how much the static stress drop exceeds the dynamic stress drop, is higher than previously published laboratory and seismic observations and fully elasto-dynamic calculations. Seismic efficiency and overshoot are constant, independent of normal stress and scale. Calculated variation of apparent stress with seismic moment resembles the observational constraints of McGarr [1999].

Geophysical Research Letters↗
USGS2002

Subsidence at Kiska volcano, Western Aleutians, detected by satellite radar interferometry

Sequential interferometric synthetic aperture radar images of Kiska, the westernmost historically active volcano in the Aleutian arc, show that a circular area about 3 km in diameter centered near the summit subsided by as much as 10 cm from 1995 to 2001, mostly during 1999 and 2000. An elastic Mogi-type deformation model suggests that the source is within 1 km of the surface. Based on the shallow source depth, the copious amounts of steam during recent eruptions, and recent field reports of vigorous steaming and persistent ground shaking near the summit area, we attribute the subsidence to decreased pore-fluid pressure within a shallow hydrothermal system beneath the summit area.

Geophysical Research Letters↗
USGS2003

High precision earthquake locations reveal seismogenic structure beneath Mammoth Mountain, California

In 1989, an unusual earthquake swarm occurred beneath Mammoth Mountain that was probably associated with magmatic intrusion. To improve our understanding of this swarm, we relocated Mammoth Mountain earthquakes using a double difference algorithm. Relocated hypocenters reveal that most earthquakes occurred on two structures, a near-vertical plane at 7–9 km depth that has been interpreted as an intruding dike, and a circular ring-like structure at ∼5.5 km depth, above the northern end of the inferred dike. Earthquakes on this newly discovered ring structure form a conical section that dips outward away from the aseismic interior. Fault-plane solutions indicate that in 1989 the seismicity ring was slipping as a ring-normal fault as the center of the mountain rose with respect to the surrounding crust. Seismicity migrated around the ring, away from the underlying dike at a rate of ∼0.4 km/month, suggesting that fluid movement triggered seismicity on the ring fault.

California↗
USGS2008

Hydrocarbon lakes on Titan: Distribution and interaction with a porous regolith

Synthetic Aperture Radar (SAR) images of Titan's north polar region reveal quasi‐circular to complex features which are interpreted to be liquid hydrocarbon lakes. We investigate methane transport in Titan's hydrologic cycle using the global distribution of lake features. As of May 2007, the SAR data set covers ∼22% of the surface and indicates multiple lake morphologies which are correlated across the polar region. Lakes are limited to latitudes above 55°N and vary from <10 to more than 100,000 km 2 . The size and location of lakes provide constraints on parameters associated with subsurface transport. Using porous media properties inferred from Huygens probe observations, timescales for flow into and out of observed lakes are shown to be in the tens of years, similar to seasonal cycles. Derived timescales are compared to the time between collocated SAR observations in order to consider the role of subsurface transport in Titan's hydrologic cycle.

Geophysical Research Letters↗
USGS2010

Time-averaged paleomagnetic field at the equator: Complete data and results from the Galapagos Islands, Ecuador

We present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the Galapagos Islands, Ecuador, collected by Allan Cox in 1964–1965 but nearly all previously unpublished. The islands are located in the eastern Pacific Ocean within 1.4° of latitude from the equator and range in age from historically erupted to 3 Ma, mostly determined by published K-Ar and 3He isotopic dating. The number of sites collected on each island ranges from 1 to 28, for a total of 186. After combining duplicate site mean directions, 149 are used for an overall mean direction and 8 represent excursions and one reversal path. Divided by geomagnetic polarity chron, 110 site means are Brunhes or Jaramillo (normal polarity), 27 are Matuyama (reversed polarity), and 12 are Gauss (both polarities). We have completed the magnetic cleaning that was commenced in the late 1960s. Secondary (mostly viscous) magnetizations were nearly all removed by alternating field demagnetization at 10 mT. We have used the so-called blanket cleaning method, generally at 10 mT. All sites were in basalt flows and gave good paleomagnetic results; none was rejected in toto, and only a few core specimens were magnetically unsatisfactory. Nearly all sites had eight independently oriented cores, and within-site angular standard deviations of directions range from 1° to 8°. We used both Fisher and Bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian, while the corresponding virtual pole (VGP) populations are essentially circularly distributed. The paleomagnetic poles, calculated as the means of VGPs, are as follows: Brunhes and Jaramillo, north latitude = 86.9°, east longitude = 245.1°, and 95% confidence radius A 95 = 1.9°; Matuyama, latitude = 87.2°, longitude = 158.2°, and A 95 = 3.8°; Gauss, latitude = 83.0°, longitude = 204.7°, and A 95 = 7.0°. These paleomagnetic poles do not differ significantly from one another, but the Brunhes and Jaramillo combined pole is significantly near-sided with respect to the Galapagos, as is the overall mean pole, which is at latitude = 87.1° and longitude = 227.6°, with A 95 = 1.7°. Omitting the excursion and reversal path data, the overall angular standard deviation of VGPs is 11.7° with lower and upper 95% confidence limits of 10.8° and 12.7°, respectively, in good agreement with previously published values for near-equatorial sampling latitudes.

Galapagos Islands↗
USGS2018

The Global food‐energy‐water nexus

Water availability is a major factor constraining humanity's ability to meet the future food and energy needs of a growing and increasingly affluent human population. Water plays an important role in the production of energy, including renewable energy sources and the extraction of unconventional fossil fuels that are expected to become important players in future energy security. The emergent competition for water between the food and energy systems is increasingly recognized in the concept of the “food‐energy‐water nexus.” The nexus between food and water is made even more complex by the globalization of agriculture and rapid growth in food trade, which results in a massive virtual transfer of water among regions and plays an important role in the food and water security of some regions. This review explores multiple components of the food‐energy‐water nexus and highlights possible approaches that could be used to meet food and energy security with the limited renewable water resources of the planet. Despite clear tensions inherent in meeting the growing and changing demand for food and energy in the 21st century, the inherent linkages among food, water, and energy systems can offer an opportunity for synergistic strategies aimed at resilient food, water, and energy security, such as the circular economy.

Reviews of Geophysics↗
USGS2019

Authigenic mineral texture in submarine 1979 basalt drill core, Surtsey volcano, Iceland

Micrometer-scale maps of authigenic microstructures in submarine basaltic tuff specimens from a 1979 Surtsey volcano, Iceland, drill core acquired 15 years after eruptions terminated provide fresh perspectives for deciphering the initial alteration of oceanic basalt in a low temperature hydrothermal system. A novel investigative approach integrates synchrotron source X-ray microdiffraction (µXRD), X-ray microfluoresence (µXRF), micro-computed tomography (µCT), and scanning transmission electron microscopy (S/TEM) coupled with Raman spectroscopy to create finely resolved spatial frameworks that record a continuum of alteration in glass and olivine. Micro-analytical maps of vesicular and fractured lapilli in specimens from 157.1, 137.9, and 102.6 m depth, and borehole temperatures of 83, 93.9 and 141.3 °C measured in 1980, respectively, record the production of nanocrystalline clay mineral, zeolites, and Al-tobermorite in diverse microenvironments. Nanocrystalline clay mineral (nontronite) and zeolite (amicite) texture in linear microstructures have concentrically-oriented crystallographic preferred orientation. Raman spectra indicating degraded organic carbonaceous matter are associated with nanocrystalline clay mineral in 10–25 nm, sub-circular nanoscale cavities in altered glass at 137.9 m depth and in a concentrically-layered, crystallographically-oriented linear microstructure in altered olivine at 102.6 m. These features have little resemblance to previously described alteration features in basalt. Irregular alteration fronts between fresh and altered glass at 157.1 depth, however, show a resemblance to microchannels in older basalts. The integrated analyses describe the complex organization of previously unrecognized mineral textures in very young basalt and provide a foundational mineralogical reference for longitudinal, time-lapse characterizations of palagonitized basalt in oceanic environments.

Geochemistry, Geophysics, Geosystems↗
USGS2023

Exploring the influence of input feature space on CNN-based geomorphic feature extraction from digital terrain data

Many studies of Earth surface processes and landscape evolution rely on having accurate and extensive data sets of surficial geologic units and landforms. Automated extraction of geomorphic features using deep learning provides an objective way to consistently map landforms over large spatial extents. However, there is no consensus on the optimal input feature space for such analyses. We explore the impact of input feature space for extracting geomorphic features from land surface parameters (LSPs) derived from digital terrain models (DTMs) using convolutional neural network (CNN)-based semantic segmentation deep learning. We compare four input feature space configurations: (a) a three-layer composite consisting of a topographic position index (TPI) calculated using a 50 m radius circular window, square root of topographic slope, and TPI calculated using an annulus with a 2 m inner radius and 10 m outer radius, (b) a single illuminating position hillshade, (c) a multidirectional hillshade, and (d) a slopeshade. We test each feature space input using three deep learning algorithms and four use cases: two with natural features and two with anthropogenic features. The three-layer composite generally provided lower overall losses for the training samples, a higher F1-score for the withheld validation data, and better performance for generalizing to withheld testing data from a new geographic extent. Results suggest that CNN-based deep learning for mapping geomorphic features or landforms from LSPs is sensitive to input feature space. Given the large number of LSPs that can be derived from DTM data and the variety of geomorphic mapping tasks that can be undertaken using CNN-based methods, we argue that additional research focused on feature space considerations is needed and suggest future research directions. We also suggest that the three-layer composite implemented here can offer better performance in comparison to using hillshades or other common terrain visualization surfaces and is, thus, worth considering for different mapping and feature extraction tasks.

Earth and Space Science↗
USGS2026

Blind spots in our understanding of the tropical nitrogen cycle

Understanding environmental controls on tropical nitrogen (N) cycling is critical for projecting flux rates and modeling ecosystem responses to rising CO 2 . Here, we ask whether geographic bias in field sampling may shape our understanding of N cycling by failing to capture the breadth of environmental heterogeneity within the tropics. We mapped empirical sampling locations of two major ecosystem N inputs and outputs—biological nitrogen fixation and denitrification—paired with a wide range of variables thought to influence N cycling. Both single and multi-variate analyses revealed that sampling density is greater in productive, wet, fertile, warmer tropical ecosystems with low seasonality—areas expected to have higher process rates—but biased away from spatially extensive biomes which may have high process rate variability and thus higher uncertainty. Even within tropical broadleaf forests, sampling is biased toward sites with unusually high soil fertility (N and phosphorus), potentially skewing biome-wide averages. Although some geographically undersampled regions share environmental characteristics with well-studied sites, others are unique, compromising inference from existing data. Finally, we find that while sampled field locations capture the range of N cycle process means/variances predicted by an ensemble of terrestrial biosphere models, this may be the result of circularity in model parameters and driving data rather than true coverage, including overrepresentation of forest plant functional types that may be parameterized from a small subset of sites. Data coverage expansion into areas with novel environmental conditions and higher uncertainty could be used to assess and improve model parameterization, as well as test general hypotheses.

Global Biogeochemical Cycles↗
USGS2026

Asynchronous landslide seasonality across the United States

Mid-range landslide outlooks can facilitate weather-related landslide preparedness and disaster response planning, but seasonal landslide activity remains poorly quantified at continental scales. Leveraging >55,000 reported landslides from across the United States (U.S.), we used circular statistics to quantify landslide seasonality in 67 National Weather Service County Warning Areas (CWAs). We found regional differences in landslide season timing and duration, with transitions between domains variably corresponding to climate class or river basin. We assessed differences in seasonality by movement type for slides, flows, and falls, detecting apparent, but uncertain, differences between slide and fall seasonalities in 27 of 35 (77%) of CWAs with both types reported. In the Pacific Northwest, where long records exist, we found a credible shift toward a later mean landslide season in western Washington from 1990 to 2020, but no trend in western Oregon. Our results can provide emergency planners a resource to assess seasonal landslide probability nationwide.

Geophysical Research Letters↗
USGS2012

Observations constraining near-source ground motion estimated from locally recorded seismograms

To estimate the seismic hazard to underground facilities or operations in the environs of a mining-induced tremor or a natural earthquake, it is useful to be able to relate locally recorded seismic waveforms to peak ground velocity and slip at the causative fault. For this purpose, far-field S wave pulses are analyzed to define the faulting slip D and near-fault peak ground velocity D /2 that give rise to the most significant ground motion. This most intense region of faulting, an assumed circular asperity, has radius r within a broader source zone of radius r 0 , which is traditionally calculated from the corner frequency of the S wave spectrum. In developing relationships between peak far-field velocity v and peak acceleration a, and the source processes of the asperity, D and D , as well as its radius r , the key model assumption is that r = k β/ω, where ω is the angular frequency of the sinusoidal velocity pulse of maximum amplitude, β is the sheaf wave speed, and k is a constant. Observations in deep-level gold mines of fault slip and slip velocity as well as laboratory observations of slip rate as a function of stress drop for stick-slip failure support a choice of about k = 2.34, the value commonly used for estimating r 0 using the Brune model. In particular, observations of fault slip up to 410 mm for mining-induced tremors in the moment magnitude range 4–5 are consistent with D = 8.1 R v/β, where R is hypocentral distance. Moreover, estimates based on underground damage of near-fault ground velocities ranging up to 3.5 m/s are in accord with D /2 = 1.28(β/μ) ρ R a, where μ is the modulus of rigidity and ρ is the density. Alternatively, the average slip velocity 〈 D 〉 can be expressed in terms of the stress drop Δσ a of the asperity as 〈 D 〉 = 0.51 β Δσ a /μ, and the agreement of this relationship with measurements made during stick-slip failure in the laboratory is good. To the extent that seismic slip exterior to the asperity is a consequence of preevent suppression of slip due to the asperity, the broader-scale( r 0 ) slip can be related to that of the asperity. Just as the asperity radius r can be estimated from r = 2.34 βv/a, an alternative estimate for r 0 is given by r 0 = ρ R a M 0 /[75.8ρμ( R v) 2 ], the results of which are generally in good agreement with estimates based on the spectral corner frequency method.

Journal of Geophysical Research↗
USGS1996

The July 12, 1993, Hokkaido-Nansei-Oki, Japan, earthquake: Coseismic slip pattern from strong-motion and teleseismic recordings

We employ a finite fault inversion scheme to infer the distribution of coseismic slip for the July 12, 1993 Hokkaido-Nansei-Oki earthquake using strong ground motions recorded by the Japan Meteorological Agency within 400 km of the epicenter and vertical P waveforms recorded by the Global Digital Seismograph Network at teleseismic distances. The assumed fault geometry is based on the location of the aftershock zone and comprises two fault segments with different orientations: a northern segment striking at N20°E with a 30° dip to the west and a southern segment with an N20°W strike. For the southern segment we use both westerly and easterly dip directions to test thrust orientations previously proposed for this portion of the fault. The variance reduction is greater using a shallow west dipping segment, suggesting that the direction of dip did not change as the rupture propagated south from the hypocenter. This indicates that the earthquake resulted from the shallow underthrusting of Hokkaido beneath the Sea of Japan. Static vertical movements predicted by the corresponding distribution of fault slip are consistent with the general pattern of surface deformation observed following the earthquake. Fault rupture in the northern segment accounts for about 60% of the total P wave seismic moment of 3.4 × 10 20 N m and includes a large circular slip zone (4-m peak) near the earthquake hypocenter at depths between 10 and 25 km. Slip in the southern segment is also predominantly shallower than 25 km, but the maximum coseismic displacements (2.0–2.5 m) are observed at a depth of about 5 km. This significant shallow slip in the southern portion of the rupture zone may have been responsible for the large tsunami that devastated the small offshore island of Okushiri. Localized shallow faulting near the island, however, may require a steep westerly dip to reconcile the measured values of ground subsidence.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Extension across Tempe Terra, Mars, from measurements of fault scarp widths and deformed craters

Two independent methods, with no common assumptions, have been used to estimate the extension across the heavily deformed Tempe Terra province of the Tharsis region of Mars. One method uses measurements of normal fault scarp width with average scarp slope data for simple grabens and rifts on Mars to estimate the fault throw, which, combined with sparse fault dip data, can be used to estimate extension. Formal uncertainties in this method are only slightly greater than those in other methods, given that the total uncertainty is dominated by the likely uncertainty in the fault dip (assumed to be 60° ± 15°). Measurement of normal fault scarp widths along two N25°–50°W directed traverses across Tempe Terra both yield about 22 ± 16 km of extension (or ∼2% strain across the northern traverse and nearly 3% across the southern one). About three quarters of the extension has occurred during the two main phases of Tharsis-related deformation from Middle/Late Noachian to Early Hesperian and from Late Hesperian to Early Amazonian, with more extension closer to the center of Tharsis during the first phase. Extension across the region was also determined by measuring the elongation and elongation direction of all ancient Noachian impact craters without ejecta blankets, which predate most of the deformation. Results have been corrected for initial non circularity of craters, established from similar measurements of young (post deformation) impact craters, yielding a statistically significant mean strain of 1.96 ± 0.35% in a N38° ± 10°W direction across Tempe Terra (extension of ∼20 ± 4, comparable in magnitude and direction to the average result from the scarp measurement method). Both methods indicate an average extension for single normal fault scarps (and shortening across wrinkle ridges for the crater method) of ∼100 m. The agreement between the results of the two independent methods in overall extension and average single normal fault extension argues that the average scarp slope and fault dip data in the fault scarp width method accurately represent the actual extension across the observed structures. This conclusion supports existing geometric and kinematic models for structural features on Mars. A preliminary estimate of the total circumferential extension around Tharsis (at a radius of ∼2500 km) is roughly 60 ± 42 km; total hoop strain is about 0.4% distributed heterogeneously (Tempe Terra is the most highly strained region on Mars).

Journal of Geophysical Research E: Planets↗
USGS1998

Asymmetric radiation of seismic waves from an atoll: nuclear tests in French Polynesia

Seismic records of nuclear tests detonated in the Mururoa Atoll in French Polynesia show large unpredicted arrivals 2.2 and 4.5 seconds (X1 and X2) after the P-wave at the Australian Warramunga Array. These arrivals are not observed at the Canadian Yellowknife Array. X1 and X2 are also absent on Warramunga Array recordings of tests carried out at the Fangataufa Atoll situated 40 km SSE of Mururoa. Array analysis shows that X1 and X2 are produced within the source area. The layered crustal structure of the atoll, significant local inhomogeneities, and focusing effects due to the elongated shape and the steep flanks of the Mururoa Atoll are most likely responsible for X1 and X2. The form of Mururoa (28 × 10 km) and its East-West orientation is due to its location on the Austral Fracture Zone (AFZ). The Fangataufa Atoll on the other hand is almost circular (10 km diameter) and is unaffected by the dynamics along the AFZ. Our observations demonstrate that complicated structures in the source area can significantly alter the wave field at teleseismic distances and produce a large magnitude (m b ) bias. A better understanding of the exact cause of these unusual seismic observations will only become possible, if the coordinates of the tests and information on the detailed 3-D structure of the atolls are released.

Geophysical Research Letters↗
USGS2012

Petrology and geochronology of metamorphosed volcanic rocks and a middle Cretaceous volcanic neck in the east-central Sierra Nevada, California

Metamorphosed Mesozoic volcanic rocks from the east-central Sierra Nevada range in composition from basalt to rhyolite and have ages, based on whole rock Rb-Sr and U-Pb zircon dating, of about 237–224, 185, 163, 134, and 100 m.y. The major plutons of the batholith in this area are of Triassic (215–200 m.y.) and Cretaceous (94–80 m.y.) ages. Initial 87 Sr/ 86 Sr values for the metamorphosed volcanic rocks of the area are in the range from 0.7042 to 0.7058 and are generally different from the values for the surrounding batholithic rocks (0.7056–0.7066). A circular, zoned granitic pluton, with an outcrop area of 2.5 km 2 , similar in appearance to a ring dike complex, was apparently a conduit for some or possibly all of the middle-Cretaceous metamorphosed volcanic rocks exposed about 5 km to the south in the western part of the Ritter Range. Samples from the metamorphosed volcanic rocks and the pluton yield a Rb-Sr whole rock isochron age of 99.9 ± 2.2 m.y. with an initial 87 Sr/ 86 Sr of 0.7048 ± 0.0001. Major element variation diagrams of the pluton and volcanic rocks define coincident compositional trends. The pluton, intruded into Early Jurassic metamorphosed volcanic rocks (Rb-Sr whole rock isochron age of 185 ± 6 m.y., initial 87 / 86 Sr = 0.70474 ± 0.00009), is composed of a central body of granite that is intruded into and almost completely surrounded by a crescent-shaped outer rim of quartz monzodiorite. Contact relations between the granitic rocks, intrusive in some places and completely gradational at other places, indicate that the quartz monzodiorite was not completely crystallized prior to the intrusion of the granite. Modal minerology within the pluton is also consistent with this inferred crystallization history. The granitic rock textures are cataclastic, characterized by a groundmass of feldspar and quartz that surrounds larger grains of feldspar, hornblende, and biotite. Widely spaced aplite dikes from the granite intrude the quartz monzodiorite along the contact between the granitic rocks. The aplite dikes and interstitial alkali feldspar associated with myrmekite in the pluton, being undeformed, indicate crystallization of a small amount of silicate liquid after cataclasis. Aplite compositions suggest a minimum crystallization pressure of 0.5 kb (1–2 km); this depth is consistent with crystallization near the base of a large stratovolcano. The pluton is characterized by steep, linear features, defined by mineral clots and inclusions, that probably formed during the upward movement of the nearly crystallized magma in the volcanic conduit. The ages of volcanic events relative to the ages of the major intrusive epochs and the major element and isotopic compositions of the volcanic rocks relative to the major plutons indicate that the volcanic rocks are not simply or directly related to the major plutons in the Sierra Nevada.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geophysical investigation of a Suture Zone: The Border Ranges Fault of southern Alaska

The Border Ranges fault separates structurally complex accreted Cretaceous rocks from less deformed middle or late Paleozoic and younger rocks in the Cook-Shelikof basin. Of the five types of geophysical data used to investigate this fault, gravity data give the clearest indication of its presence and crustal structure. For at least 400 km along the fault, gravity anomalies include a +20 to +30 mGal peak along the fault's upper plate and a −40 mGal trough along the lower plate. The paired anomaly can be modeled satisfactorily by a simple step, in a deep dense layer, that lies within 3 km of the projected offshore location of the fault. Relatively low-density rocks lie along the fault's lower plate to a depth of about 10 km, and the upper part of the fault dips within 20° of vertical. Satellite altimetry data show that two circular geoid lows lie along the Border Ranges fault and coincide with lows in free air gravity data. Seismic refraction and seismic reflection data suggest that the large-scale density anomalies that cause both types of lows must lie at depths greater than about 1 km within the margin. Three regional magnetic anomalies (Knik Arm, Seldovia, and Shelikof) terminate at the Border Ranges fault, suggesting that the fault truncates obliquely rocks that lie along its northwest side. Six seismic reflection lines cross the Border Ranges fault, but none of them shows reflections from it. The absence of such reflections probably results from the fault's steep dip and from the presence of strong water bottom multiples in the data. From the Late Jurassic until the early Late Cretaceous, the magmatic arc near the Cook-Shelikof basin was inactive, and we infer that the predominant motion along the Border Ranges fault was strike slip. Resurgent Late Cretaceous magmatism was contemporaneous with uplift of rocks along the northwest side of the Border Ranges fault and with deformation of turbidite sequences in the fault's lower plate. We propose that during the Late Cretaceous, motion along the Border Ranges changed from strike slip to reverse. Cenozoic rocks near the fault show no evidence for post-Cretaceous fault movement.

Journal of Geophysical Research Solid Earth↗
USGS2012

Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor

Recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so-called long-period or low-frequency events. Accordingly, we can view the long-period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor-generating system. We present a seismic model in which the source of tremor is the acoustic resonance of a fluid-filled volcanic pipe triggered by excess gas pressure. The model consists of three elements, namely, a triggering mechanism, a resonator, and a radiator. For simplicity, we assume a hemispherical trigger, cylindrial resonator, and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk-shaped radiator shutting off its bottom. Considering the simple case of a source buried in a homogeneous half space, we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances. The results demonstrate that the displacement attributed to the pipe dominates the near-field motion, while that due to the disk is representative of the intermediate and far fields. The trigger itself has a smaller contribution, mainly limited to the field in the proximity of the source. The characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well-developed harmonic wave train dominated by Rayleigh waves at larger distances. No clear shear arrival can be detected in the synthetic seismograms. The displacement spectrum reflects the organ-pipe modes of the conduit, and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid. In the case of the cylindrical magma column considered, the radiation loss is proportional to the square of the pipe radius, while the loss related to viscous damping is inversely proportional to the same factor, indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir. The relative importance of the pipe and disk elements, likewise, is a function of the conduit cross section. This suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near- and far-field observations.

Journal of Geophysical Research Solid Earth↗
USGS2014

The volcanic history of the San Juan Mountains, Colorado

The San Juan Mountains, which are made up chiefly of volcanic rocks, are located in southwestern Colorado and occupy a rudely circular area of over 10,000 square miles, or larger than the state of Massachusetts. They have been studied and mapped geologically by the United States Geological Survey, chiefly by field parties under Whitman Cross and Esper S. Larsen.

Colorado↗