Geology topics

D.P. Hill

Publications and source records attributed to D.P. Hill.

At least 19 recordsLinked to original sources

USGS2014

The U. S. Geological Survey's gravity program in Washington, Idaho, Montana, and Wyoming

The following summary of the U . S . Geological Survey gravity program in Washington , Idaho , Montana , and Wyoming is one of a series of short papers that outline Geological Survey gravity projects in the western United States. The substance of this summary is a list of references of published papers on U . S . Geological Survey gravity projects in the northwestern states together with an index map (Figure 1) showing the location of the project, the approximate a real coverage reported in the publication, and the contour map given in the publication. Areas in Figure 1 are related to the References (part 2) by a number code. In addition, the gravity projects in progress are listed in Table 1, indicating the area in which the work is being done and the person in charge of the project. Capital letter relate the locations of the projects in Figure 1 to the list of projects in Table 1. No attempt has been made to indicate the areal coverage or contour interval of the projects in progress.

Washington, Idaho, Montana, Wyoming↗

USGS2012

An episode of reinflation of the Long Valley Caldera, eastern California: 1989-1991

Following the episodes of inflation of the resurgent dome associated with the May 1980 earthquake sequence (four M 6 earthquakes) and the January 1983 earthquake swarm (two M 5.2 events), 7 years of frequently repeated two-color geodimeter measurements spanning the Long Valley caldera document gradually decreasing extensional strain rates from 5 ppm/yr in mid-1983, when the measurements began, to near zero in mid-1989. The corresponding seismic activity within the caldera persisted at a low rate of fewer than 10 M ≥ 1.2 earthquakes per week from 1985 through November 1989 with no events exceeding M 3.0. Early October 1989 marked a change in activity when measurements of the two-color geodimeter network showed a significant increase in extensional strain rate (9 ppm/yr) across the caldera. The seismic activity began exceeding 10 M ≥ 1.2 per week in early December 1989 and rapidly increased to a sustained level of tens of M ≥ 1.2 per week with bursts having hundreds of events per day. Many events exceeded M 3.0 and the largest event was M ≈ 4. The 1989–1991 inflation episode is the first time that we have sufficient geodetic measurements in Long Valley to define the temporal relation between onset of an inflation episode and onset of brittle failure (earthquake swarm within the caldera). Here, the onset of deformation preceded the onset of increased earthquake activity by more than 2 months. The seismicity rate began to decrease in mid-July 1990, consistent with a gradually slowing of extension across the caldera as measured by the two-color geodimeter. The recent episode of inflation can be modeled by a single Mogi point source located about 7 km beneath the center of the resurgent dome. In contrast, the deformation pattern observed between mid-1983 and mid-1989 is best reproduced by fault slip in the south moat, inflation at 6.5 km depth near Casa Diablo Hot Springs and inflation beneath the resurgent dome. It appears that the 7-km source beneath the resurgent dome that was active for the earlier episodes is the primary source for the more recent episode. The model used to satisfy the line length observations predicts 7.5 cm of uplift along leveling route along highway 395 from mid-1983 to mid-1989 and an additional 11 cm through the end of 1991. To comp are with the energy release from seismicity, the modeled inflation from late 1989 through the end of 1991 has a moment that is a factor of 40 more than the cumulative seismic moment from earthquakes located within the caldera from the same period. Thus the recent inflation episode represents a significant portion of the observed geodetic deformation with only little seismic energy release.

Journal of Geophysical Research↗

USGS2012

Contemporary block tectonics: California and Nevada

Well-determined fault plane solution and the gross pattern of late-Cenozoic faulting in California and Nevada show a systematic relation between the orientation of fault planes and slip directions. In general, normal faults have northerly strikes, reverse faults have easterly strikes, and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes, respectively. Kinematically, this relation is consistent with the response of clusters of fault-bounded crustal blocks to a regional stress field generated by the relative motion between the Pacific and North American plates. In this stress field, the greatest and least principal (compressive) stresses are restricted to northerly and easterly striking vertical planes, respectively. Clusters of crustal blocks bound by faults having the above attitudes form three basic regimes: (1) a spreading regime with a combination of normal and conjugate, strike slip faults, (2) a converging regime with a combination of reverse and conjugate, strike slip faults, and (3) a wrench regime with sets of subparallel, throughgoing, northwest striking (dextral) faults or northeast striking (sinstral) faults. These three regimes are typified by faulting patterns in the Basin and Range province (spreading), the Transverse Ranges (converging), and the San Andreas-Garlock fault systems (wrench), respectively. The gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north-south shortening and east-west extension with the ratio of extensional to shortening strains (and the areal dilitation) decreasing systematically from spreading to wrench to compressional regimes. The wrench regime involves a component of net rotational deformation (clockwise for dextral slip and counter-clockwise for sinstral slip), while deformation of the spreading and converging regimes is irrotational. Local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes. Maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength (Byerlee's law) along preexisting fractures and that pore pressure in the brittle crust is laterally uniform. A minimum strength difference between active, block-bounding faults and block interiors is 15–30%. Simple arrangements of such block clusters mimic the gross kinematic pattern of Quaternary faulting in California and Nevada. Some implications for contemporary tectonics emphasized by this model involve the westward displacement of the Sierra Nevada block.with respect to the stable interior of the North American plates, oblique thrusting of the Salinian block over the Pacific plate, and a progressive increase in the offset of the San Andreas fault represented by the ‘big bend’ through the Transverse Ranges.

Journal of Geophysical Research Solid Earth↗

USGS2012

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.

Journal of Geophysical Research Solid Earth↗

USGS2011

Evidence for the contemporary magmatic system beneath Long Valley Caldera from local earthquake tomography and receiver function analysis

We present a new P wave and S wave velocity model for the upper crust beneath Long Valley Caldera obtained using local earthquake tomography and receiver function analysis. We computed the tomographic model using both a graded inversion scheme and a traditional approach. We complement the tomographic I/ P model with a teleseismic receiver function model based on data from broadband seismic stations (MLAC and MKV) located on the SE and SW margins of the resurgent dome inside the caldera. The inversions resolve (1) a shallow, high-velocity P wave anomaly associated with the structural uplift of a resurgent dome; (2) an elongated, WNW striking low-velocity anomaly (8%–10 % reduction in I/ P ) at a depth of 6 km (4 km below mean sea level) beneath the southern section of the resurgent dome; and (3) a broad, low-velocity volume (–5% reduction in I/ P and as much as 40% reduction in I/ S ) in the depth interval 8–14 km (6–12 km below mean sea level) beneath the central section of the caldera. The two low-velocity volumes partially overlap the geodetically inferred inflation sources that drove uplift of the resurgent dome associated with caldera unrest between 1980 and 2000, and they likely reflect the ascent path for magma or magmatic fluids into the upper crust beneath the caldera.

California↗

USGS2011

Migrating swarms of brittle-failure earthquakes in the lower crust beneath Mammoth Mountain, California

Brittle-failure earthquakes in the lower crust, where high pressures and temperatures would typically promote ductile deformation, are relatively rare but occasionally observed beneath active volcanic centers. Where they occur, these earthquakes provide a rare opportunity to observe volcanic processes in the lower crust, such as fluid injection and migration, which may induce brittle faulting under these conditions. Here, we examine recent short-duration earthquake swarms deep beneath the southwestern margin of Long Valley Caldera, near Mammoth Mountain. We focus in particular on a swarm that occurred September 29-30, 2009. To maximally illuminate the spatial-temporal progression, we supplement catalog events by detecting additional small events with similar waveforms in the continuous data, achieving up to a 10-fold increase in the number of locatable events. We then relocate all events, using cross-correlation and a double-difference algorithm. We find that the 2009 swarm exhibits systematically decelerating upward migration, with hypocenters shallowing from 21 to 19 km depth over approximately 12 hours. This relatively high migration rate, combined with a modest maximum magnitude of 1.4 in this swarm, suggests the trigger might be ascending CO 2 released from underlying magma.

California↗

USGS2010

Surface-wave potential for triggering tectonic (nonvolcanic) tremor

Source processes commonly posed to explain instances of remote dynamic triggering of tectonic (nonvolcanic) tremor by surface waves include frictional failure and various modes of fluid activation. The relative potential for Love- and Rayleigh-wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike-slip faults under the Coulomb-Griffith failure criteria as a function of incidence angle is anticorrelated over the 15- to 30-km-depth range that hosts tectonic tremor. Love-wave potential is high for strike-parallel incidence on low-angle reverse faults and null for strike-normal incidence; the opposite holds for Rayleigh waves. Love-wave potential is high for both strike-parallel and strike-normal incidence on vertical, strike-slip faults and minimal for ~45?? incidence angles. The opposite holds for Rayleigh waves. This pattern is consistent with documented instances of tremor triggered by Love waves incident on the Cascadia mega-thrust and the San Andreas fault (SAF) in central California resulting from shear failure on weak faults (apparent friction, ????? 0.2). However, documented instances of tremor triggered by surface waves with strike-parallel incidence along the Nankai megathrust beneath Shikoku, Japan, is associated primarily with Rayleigh waves. This is consistent with the tremor bursts resulting from mixed-mode failure (crack opening and shear failure) facilitated by near-lithostatic ambient pore pressure, low differential stress, with a moderate friction coefficient (?? ~ 0.6) on the Nankai subduction interface. Rayleigh-wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process, except perhaps for an indirect role on the SAF in sustaining tremor into the Rayleigh-wave coda that was initially triggered by Love waves.

Bulletin of the Seismological Society of America↗

USGS2009

Non-double-couple mechanisms of microearthquakes induced by hydraulic fracturing

We have inverted polarity and amplitude information of representative microearthquakes to investigate source mechanisms of seismicity induced by hydraulic fracturing in the Carthage Cotton Valley, east Texas, gas field. With vertical arrays of four and eight three-component geophones in two monitoring wells, respectively, we were able to reliably determine source mechanisms of the strongest events with the best signal-to-noise ratio. Our analysis indicates predominantly non-double-couple source mechanisms with positive volumetric component consistent with opening cracks oriented close to expected hydraulic fracture orientation. Our observations suggest the induced events are directly the result of opening cracks by fluid injection, in contrast to many previous studies where the seismicity is interpreted to be primarily shearing caused by pore pressure diffusion into the surrounding rock or associated with shear stresses created at the hydraulic fracture tip.

Journal of Geophysical Research B: Solid Earth↗

USGS2009

Analytical modeling of gravity changes and crustal deformation at volcanoes: The Long Valley caldera, California, case study

Joint measurements of ground deformation and micro-gravity changes are an indispensable component for any volcano monitoring strategy. A number of analytical mathematical models are available in the literature that can be used to fit geodetic data and infer source location, depth and density. Bootstrap statistical methods allow estimations of the range of the inferred parameters. Although analytical models often assume that the crust is elastic, homogenous and isotropic, they can take into account different source geometries, the influence of topography, and gravity background noise. The careful use of analytical models, together with high quality data sets, can produce valuable insights into the nature of the deformation/gravity source. Here we present a review of various modeling methods, and use the historical unrest at Long Valley caldera (California) from 1982 to 1999 to illustrate the practical application of analytical modeling and bootstrap to constrain the source of unrest. A key question is whether the unrest at Long Valley since the late 1970s can be explained without calling upon an intrusion of magma. The answer, apparently, is no. Our modeling indicates that the inflation source is a slightly tilted prolate ellipsoid (dip angle between 91° and 105°) at a depth of 6.5 to 7.9 km beneath the caldera resurgent dome with an aspect ratio between 0.44 and 0.60, a volume change from 0.161 to 0.173 km 3 and a density of 1241 to 2093 kg/m 3 . The larger uncertainty of the density estimate reflects the higher noise of gravity measurements. These results are consistent with the intrusion of silicic magma with a significant amount of volatiles beneath the caldera resurgent dome.

California↗

USGS2007

Dynamic triggering

A growing body of evidence demonstrates that dynamic stresses propagating as seismic waves from large earthquakes are capable of triggering additional earthquakes ranging from aftershocks in the near-field (within one or two source dimensions of the mainshock epicenter) to remotely triggered earthquakes at distances exceeding 10 000 km. Most of the triggered earthquakes are small (generally M ≤ 3) except within the near field, where dynamic stresses may trigger slip on subadjacent fault segments leading to complex rupture comprising of several large earthquakes of comparable magnitude. Crustal surface waves with periods of 15–30 s and peak dynamic stresses greater than ∼0.01 MPa seem to be most efficient in triggering remote seismicity. Current models for dynamic triggering fall under two broad groups: one appealing to Coulomb failure with various friction laws, and the other appealing to the activation of crustal fluids either hydrous or magmatic. No single model appears capable of accounting for the wide variation observed in the nature of triggered activity. Spatial sampling of dynamic triggering on a global scale is still woefully inadequate because of the limited distribution of adequate seismic networks. From the limited data currently available, it appears that extensional stress regimes hosting geothermal and volcanic activity are more susceptible to remote dynamic triggering than compressional stress regimes, although remote triggering is not limited to extensional regimes. Instances of remote triggering in the few areas with continuous, high-resolution deformation instrumentation (all volcanic or geothermal areas) include distinctive deformation transients, suggesting that the locally triggered seismicity in these areas may be a secondary response to a more fundamental aseismic process that likely involves some form of fluid transport or phase change. Recent evidence for triggering by solid Earth tides and ocean loading in convergent plate margins provides a low-frequency, low-amplitude reference point for the spectrum of stresses capable of dynamic triggering. Remaining challenges include establishing better sampling of the distribution of triggered seismicity and better constraints on physical models for the triggering process.

Book chapter↗

USGS2005

Magmatic unrest beneath Mammoth Mountain, California

Mammoth Mountain, which stands on the southwest rim of Long Valley caldera in eastern California, last erupted ∼57,000 years BP. Episodic volcanic unrest detected beneath the mountain since late 1979, however, emphasizes that the underlying volcanic system is still active and capable of producing future volcanic eruptions. The unrest symptoms include swarms of small ( M ≤ 3) earthquakes, spasmodic bursts (rapid-fire sequences of brittle-failure earthquakes with overlapping coda), long-period (LP) and very-long-period (VLP) volcanic earthquakes, ground deformation, diffuse emission of magmatic CO 2 , and fumarole gases with elevated 3 He/ 4 He ratios. Spatial-temporal relations defined by the multi-parameter monitoring data together with earthquake source mechanisms suggest that this Mammoth Mountain unrest is driven by the episodic release of a volume of CO 2 -rich hydrous magmatic fluid derived from the upper reaches of a plexus of basaltic dikes and sills at mid-crustal depths (10–20 km). As the mobilized fluid ascends through the brittle–plastic transition zone and into overlying brittle crust, it triggers earthquake swarm activity and, in the case of the prolonged, 11-month-long earthquake swarm of 1989, crustal deformation and the onset of diffuse CO 2 emissions. Future volcanic activity from this system would most likely involve steam explosions or small-volume, basaltic, strombolian or Hawaiian style eruptions. The impact of such an event would depend critically on vent location and season.

California↗

USGS2004

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

Most of 26 small (0.4≲ M ≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P - and S -wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO 2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a ‘spasmodic burst’ at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown.

California↗

USGS2004

Triggered deformation and seismic activity under Mammoth Mountain in Long Valley caldera by the 3 November 2002 Mw 7.9 Denali fault earthquake

The 3 November 2002 Mw 7.9 Denali fault earthquake triggered deformational offsets and microseismicity under Mammoth Mountain (MM) on the rim of Long Valley caldera, California, some 3460 km from the earthquake. Such strain offsets and microseismicity were not recorded at other borehole strain sites along the San Andreas fault system in California. The Long Valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes Mammoth Mountain - a young volcano on the southwestern rim of the caldera. The largest recorded strain offsets were -0.1 microstrain at PO on the west side of MM, 0.05 microstrain at MX to the southeast of MM, and -0.025 microstrain at BS to the northeast of MM with negative strain extensional. High sample rate strain data show initial triggering of the offsets began at 22:30 UTC during the arrival of the first Rayleigh waves from the Alaskan earthquake with peak-to-peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0.06 MPa. The strain offsets grew to their final values in the next 10 min. The associated triggered seismicity occurred beneath the south flank of MM and also began at 22:30 UTC and died away over the next 15 min. This relatively weak seismicity burst included some 60 small events with magnitude all less than M = 1. While poorly constrained, these strain observations are consistent with triggered slip and intrusive opening on a north-striking normal fault centered at a depth of 8 km with a moment of l016 N m, or the equivalent of a M 4.3 earthquake. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. These observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of MM following the 16 October 1999 M 7.1 Hector Mine, California, earthquake. However, in this case, we see little post-event slip decay reflected in the strain data after the Rayleigh-wave arrivals from the Denali fault earthquake and onset of triggered seismicity did not lag the triggered deformation by 20 min. These observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 M 7.3 Landers earthquake in the Long Valley caldera. Thus, each of the three instances of remotely triggered unrest in Long Valley caldera recorded to date differ. In each case, however, the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity.

California↗

USGS2004

Remotely triggered seismicity on the United States west coast following the Mw 7.9 Denali fault earthquake

The M w 7.9 Denali fault earthquake in central Alaska of 3 November 2002 triggered earthquakes across western North America at epicentral distances of up to at least 3660 km. We describe the spatial and temporal development of triggered activity in California and the Pacific Northwest, focusing on Mount Rainier, the Geysers geothermal field, the Long Valley caldera, and the Coso geothermal field. The onset of triggered seismicity at each of these areas began during the Love and Raleigh waves of the M w 7.9 wave train, which had dominant periods of 15 to 40 sec, indicating that earthquakes were triggered locally by dynamic stress changes due to low-frequency surface wave arrivals. Swarms during the wave train continued for ∼4 min (Mount Rainier) to ∼40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small ( M ≤ 2.5) earthquakes. Dynamic stresses within the surface wave train at the time of the first triggered earthquakes ranged from 0.01 MPa (Coso) to 0.09 MPa (Mount Rainier). In addition to the swarms that began during the surface wave arrivals, Long Valley caldera and Mount Rainier experienced unusually large seismic swarms hours to days after the Denali fault earthquake. These swarms seem to represent a delayed response to the Denali fault earthquake. The occurrence of spatially and temporally distinct swarms of triggered seismicity at the same site suggests that earthquakes may be triggered by more than one physical process.

North America↗

USGS2003

Relations between seismicity and deformation during unrest in Long Valley Caldera, California, from 1995 through 1999

Unrest in Long Valley Caldera and the adjacent Sierra Nevada from 1995 through 2000 was dominated by three major episodes: (1) the March-April 1996 earthquake swarm in the east lobe of the south moat; (2) the July 1997-January 1998 caldera-wide unrest; and (3) a sequence of three M>5 earthquakes (9 June 1998, 13 July 1998, and 15 May 1999 UT) located in the Sierra Nevada block immediately south of the caldera. These three unrest episodes each had distinct characteristics with distinct implications for associated hazards. Seismicity developed as earthquake swarms for the 1996 and 1997-98 episodes, both of which were within the caldera. In contrast, the series of three M>5 earthquakes south of the caldera in 1998-99 each developed as a mainshock-aftershock sequence. Marginal deformation within the caldera associated with the 1996 swarm and the 1998-99 M>5 earthquakes is consistent with the cumulative seismic moments for the respective sequences. Deformation associated with the 1997-98 episode, however, was roughly five times larger than can be accounted for by the cumulative seismic moment of the associated earthquake swarm. We conclude that the 1997-98 episode was associated with mass transport (local intrusion of magma or magmatic brine) and that the associated earthquake swarm activity, which had a relatively high b -value of 1.2, was largely driven by the intrusive process. In contrast, the 1996 earthquake swarm and the 1998-99 M>5 mainshock-aftershock sequences, both with 'normal' b -values of ???0.9, represent brittle relaxation to previously accumulated stresses associated with little or no mass transport. These relations emphasize the importance of simultaneous, real-time monitoring of both seismicity and deformation as a basis for judging whether an evolving unrest episode has the potential for culminating in a volcanic eruption. ?? 2003 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗

USGS2003

The 2002 Denali fault earthquake, Alaska: A large magnitude, slip-partitioned event

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

Science↗

USGS2003

The Long Valley Caldera GIS database

This database provides an overview of the studies being conducted by the Long Valley Observatory in eastern California from 1975 to 2001. The database includes geologic, monitoring, and topographic datasets related to Long Valley caldera. The CD-ROM contains a scan of the original geologic map of the Long Valley region by R. Bailey. Real-time data of the current activity of the caldera (including earthquakes, ground deformation and the release of volcanic gas), information about volcanic hazards and the USGS response plan are available online at the Long Valley observatory web page (http://lvo.wr.usgs.gov). If you have any comments or questions about this database, please contact the Scientist in Charge of the Long Valley observatory.

Data Series↗

USGS2003

Three-dimensional crustal structure of Long Valley caldera, California, and evidence for the migration of CO2 under Mammoth Mountain

A temporary network of 69 three-component seismic stations captured a major seismic sequence in Long Valley caldera in 1997. We performed a tomographic inversion for crustal structure beneath a 28 km ?? 16 km area encompassing part of the resurgent dome, the south moat, and Mammoth Mountain. Resolution of crustal structure beneath the center of the study volume was good down to ???3 km below sea level (???5 km below the surface). Relatively high wave speeds are associated with the Bishop Tuff and lower wave speeds characterize debris in the surrounding moat. A low-Vp/Vs anomaly extending from near the surface to ???1 km below sea level beneath Mammoth Mountain may represent a CO2 reservoir that is supplying CO2-rich springs, venting at the surface, and killing trees. We investigated temporal variations in structure beneath Mammoth Mountain by differencing our results with tomographic images obtained using data from 1989/1990. Significant changes in both Vp and Vs were consistent with the migration of CO2 into the upper 2 km or so beneath Mammoth Mountain and its depletion in peripheral volumes that correlate with surface venting areas. Repeat tomography is capable of detecting the migration of gas beneath active silicic volcanoes and may thus provide a useful volcano monitoring tool.

Journal of Geophysical Research B: Solid Earth↗