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Results for “Journal of Applied Geophysics”

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 145 records · Page 8Linked to original sources
USGS2014

Real-time inversions for finite fault slip models and rupture geometry based on high-rate GPS data

We present an inversion strategy capable of using real-time high-rate GPS data to simultaneously solve for a distributed slip model and fault geometry in real time as a rupture unfolds. We employ Bayesian inference to find the optimal fault geometry and the distribution of possible slip models for that geometry using a simple analytical solution. By adopting an analytical Bayesian approach, we can solve this complex inversion problem (including calculating the uncertainties on our results) in real time. Furthermore, since the joint inversion for distributed slip and fault geometry can be computed in real time, the time required to obtain a source model of the earthquake does not depend on the computational cost. Instead, the time required is controlled by the duration of the rupture and the time required for information to propagate from the source to the receivers. We apply our modeling approach, called Bayesian Evidence-based Fault Orientation and Real-time Earthquake Slip, to the 2011 Tohoku-oki earthquake, 2003 Tokachi-oki earthquake, and a simulated Hayward fault earthquake. In all three cases, the inversion recovers the magnitude, spatial distribution of slip, and fault geometry in real time. Since our inversion relies on static offsets estimated from real-time high-rate GPS data, we also present performance tests of various approaches to estimating quasi-static offsets in real time. We find that the raw high-rate time series are the best data to use for determining the moment magnitude of the event, but slightly smoothing the raw time series helps stabilize the inversion for fault geometry.

Journal of Geophysical Research↗
USGS2008

Modeling the evolution of channel shape: Balancing computational efficiency with hydraulic fidelity

The cross-sectional shape of a natural river channel controls the capacity of the system to carry water off a landscape, to convey sediment derived from hillslopes, and to erode its bed and banks. Numerical models that describe the response of a landscape to changes in climate or tectonics therefore require formulations that can accommodate evolution of channel cross-sectional geometry. However, fully two-dimensional (2-D) flow models are too computationally expensive to implement in large-scale landscape evolution models, while available simple empirical relationships between width and discharge do not adequately capture the dynamics of channel adjustment. We have developed a simplified 2-D numerical model of channel evolution in a cohesive, detachment-limited substrate subject to steady, unidirectional flow. Erosion is assumed to be proportional to boundary shear stress, which is calculated using an approximation of the flow field in which log-velocity profiles are assumed to apply along vectors that are perpendicular to the local channel bed. Model predictions of the velocity structure, peak boundary shear stress, and equilibrium channel shape compare well with predictions of a more sophisticated but more computationally demanding ray-isovel model. For example, the mean velocities computed by the two models are consistent to within ???3%, and the predicted peak shear stress is consistent to within ???7%. Furthermore, the shear stress distributions predicted by our model compare favorably with available laboratory measurements for prescribed channel shapes. A modification to our simplified code in which the flow includes a high-velocity core allows the model to be extended to estimate shear stress distributions in channels with large width-to-depth ratios. Our model is efficient enough to incorporate into large-scale landscape evolution codes and can be used to examine how channels adjust both cross-sectional shape and slope in response to tectonic and climatic forcing. Copyright 2008 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2023

Tree-ring derived avalanche frequency and climate associations in a high-latitude, maritime climate

Snow avalanches are a natural hazard in mountainous areas worldwide with severe impacts that include fatalities, damage to infrastructure, disruption to commerce, and landscape disturbance. Understanding long-term avalanche frequency patterns, and associated climate and weather influences, improves our understanding of how climate change may affect avalanche activity. We used dendrochronological techniques to evaluate the historical frequency of large magnitude avalanches (LMAs) in the high-latitude climate of southeast Alaska, United States. We collected 434 cross sections throughout six avalanche paths near Juneau, Alaska. This resulted in 2706 identified avalanche growth disturbances between 1720 and 2018, which allowed us to reconstruct 82 years with LMA activity across three sub-regions. By combining this tree-ring-derived avalanche data set with a suite of climate and atmospheric variables and applying a generalized linear model to fit a binomial regression, we found that February and March precipitation and the Oceanic Niño Index (ONI) were significant predictors of LMA activity in the study area. Specifically, LMA activity occurred during winters with substantial February and March precipitation and neutral or negative (cold) ONI values, while years not characterized by LMAs occur more frequently during warm winters (positive ONI values). Our examination of the climate-avalanche relationship in southeast Alaska sheds light on important climate variables and physical processes associated with LMA years. These results can be used to inform long-term infrastructure planning and avalanche mitigation operations in an urban area, such as Juneau, where critical infrastructure is subject to substantial avalanche hazard.

Alaska↗
USGS2014

The 2010 slow slip event and secular motion at Kilauea, Hawai`i inferred from TerraSAR-X InSAR data

We present here an Small BAseline Subset (SBAS) algorithm to extract both transient and secular ground deformations on the order of millimeters in the presence of tropospheric noise on the order of centimeters, when the transient is of short duration and known time, and the background deformation is smooth in time. We applied this algorithm to study the 2010 slow slip event as well as the secular motion of Kīlauea's south flank using 49 TerraSAR-X images. We also estimate the tropospheric delay variation relative to a given reference pixel using an InSAR SBAS approach. We compare the InSAR SBAS solution for both ground deformation and tropospheric delays with existing GPS measurements and confirm that the ground deformation signal andtropospheric noise in InSAR data are successfully separated. We observe that the coastal region on the south side of the Hilina Pali moves at a higher background rate than the region north side of the Pali. We also conclude that the 2010 SSE displacement is mainly horizontal and the maximum magnitude of the 2010 SSE vertical component is less than 5 mm.

Hawaii↗
USGS2018

Hydrodynamics of a tidally‐forced coral reef atoll

The hydrodynamics of a tidally forced semi‐enclosed coral reef atoll (North Scott) at the edge of the continental shelf of northwestern Australia were investigated by combining field observations and numerical modeling. The observations revealed that the spring tidal range outside the atoll reaches 4 m, and as the water level drops below mean sea level, the reef rim surrounding the shallow (~10–15 m) lagoon becomes exposed. During this time, the lagoon can only exchange with the open ocean through two narrow channels, resulting in highly asymmetric water levels and velocities that were most pronounced during spring tide. On average, the ebb tide duration was ~2 hr longer than the flood, with rapid flood velocities in the channel reaching 2 m/s. We applied an unstructured grid model Delft3D‐Flexible Mesh to simulate the atoll hydrodynamics and were able to replicate the asymmetric water levels and complex velocities in the lagoon. The results revealed that at higher tidal stages, a dominant momentum balance exists between the pressure gradient (established by the propagation of the tide on the shelf) and the local flow acceleration of water throughout the interior of the atoll. At lower tidal stages, which coincided with a reversal of the offshore tidal pressure gradient, the lagoon became isolated from offshore dynamics and all momentum terms were negligible. This resulted in a tidally averaged residual westward flow within the lagoon that drove an asymmetric flushing pattern within the atoll, which we propose would be a common flushing mechanism within other tide‐dominated atolls worldwide.

Journal of Geophysical Research C: Oceans↗
USGS2019

A combinatorial approach to determine earthquake magnitude distributions on a variable slip-rate fault

Combinatorial methods are used to determine the spatial distribution of earthquake magnitudes on a fault whose slip rate varies along strike. Input to the problem is a finite sample of earthquake magnitudes that span 5 kyr drawn from a truncated Pareto distribution. The primary constraints to the problem are maximum and minimum values around the target slip-rate function indicating where feasible solutions can occur. Two methods are used to determine the spatial distribution of earthquakes: integer programming and the greedy-sequential algorithm. For the integer-programming method, the binary decision vector includes all possible locations along the fault where each earthquake can occur. Once a set of solutions that satisfy the constraints is found, the cumulative slip misfit on the fault is globally minimized relative to the target slip-rate function. The greedy algorithm sequentially places earthquakes to locally optimize slip accumulation. As a case study, we calculate how earthquakes are distributed along the megathrust of the Nankai subduction zone, in which the slip rate varies significantly along strike. For both methods, the spatial distribution of magnitudes depends on slip rate, except for the largest magnitude earthquakes that span multiple sections of the fault. The greedy-sequential algorithm, previously applied to this fault (Parsons et al., 2012), tends to produce smoother spatial distributions and fewer lower magnitude earthquakes in the low slip-rate section of the fault compared to the integer-programming method. Differences in results from the two methods relate to how much emphasis is placed on minimizing the misfit to the target slip rate (integer programming) compared to finding a solution within the slip-rate constraints (greedy sequential). Specifics of the spatial distribution of magnitudes also depend on the shape of the target slip-rate function: i.e. stepped at the section boundaries versus a smooth function. This study isolates the effects of slip-rate variation along a single fault in determining the spatial distribution of earthquake magnitudes, helping to better interpret results from more complex, interconnected fault systems.

Geophysical Journal International↗
USGS2023

Assessment of resource potential from mine tailings using geostatistical modeling for compositions: A methodology and application to Katherine Mine site, Arizona, USA

The mining industry, in most cases, targets a specific valuable commodity that is present in small quantities within large volumes of extracted material. After milling and processing, most of the extracted material and the effluents are stored as waste (tailings) in impoundments, such as dams or waste dumps, or are backfilled into underground mines. In time, tailing materials may become an issue of environmental and health concern due to the hazardous elements, ions, and oxides contained within the waste material. In addition, handling and storage of such waste in dams may pose the risk of dam failure with catastrophic consequences to nature and nearby communities. On the other hand, tailings may offer potential as secondary sources of critical elements (CEs), including rare earth elements (REEs), which may have been overlooked during primary production and processing. Therefore, treating mine tailings as a resource has economic and environmental benefits by reducing the waste from new and historical mine sites through remining. One of the critical steps for taking advantage of these benefits is to spatially quantify the resources and the pollutants, which require the application of adequate data analysis and modeling methods, often to compositional geochemical data. Utilizing adequate methods is especially important for correctly quantifying resource potential, as the quantities will often be at low concentrations. This work presents quantification of resource potential (Au, Ag, Cu, Zn, Pb) and elements of environmental concern (Hg and As) from the tailings of a historic mine site, Katherine Mine, AZ, USA. Data reported by the U.S. Bureau of Mines (USBM) after extensive field campaigns in the 1990s, including sampling from tailing impoundment and surrounding areas for geochemical characterization and geophysical surveys, were used. First, compositional data (CoDa) analysis was employed to explore associations of sampling locations, geochemical parts, and the clustering of samples. Next, sequential Gaussian simulation (SGSIM) was applied to samples that showed a genetic link to tailing material after isometric log-ratio transformation (ilr) and mix/max autocorrelation factor (MAF) transformation for spatial modeling and uncertainty evaluation. Geostatistical results revealed spatial variability of concentrations within the tailing area. Uncertainty evaluation based on realizations indicated that Cu (14.27–20.01 t), Zn (44.23–76.23 t), and Pb (22.56–38.28 t) are the most abundant elements within a 5 %–95 % interval, followed by Ag and Au (~5.3 and 0.18 t, at 50th percentile), respectively. Of the elements of health concern, As was found to be ~4.8 t (50th percentile) in the tailing area. The work also showed that ~0.51 t As, 0.005 t Hg, 0.020 t of Au, and 0.62 t of Ag were carried to Lake Mohave by an ephemeral stream called Katherine Wash, which transects the tailings.

Arizona↗
USGS2012

The mechanics of ground deformation precursory to dome-building extrusions at Mount St. Helens 1981-1982

Detailed monitoring at Mount St. Helens since 1980 has enabled prediction of the intermittent eruptive activity (mostly dome growth) with unprecedented success. During 1981 and 1982, accelerating deformation of the crater floor around the vent (including radial cracks, thrust faults, and ground tilt) was the earliest indicator of impending activity. Numerical experiments using the finite element method to model the mechanics of the crater floor show that all deformation features can be explained by a uniform shear-stress boundary condition along the conduit wall. The magnitude of the shear stress required to match observed displacements (1–7 MPa) is inversely proportional to the conduit diameter (estimated to be 25–100 m). The most probable source of this shear stress is the flow of viscous magma up the conduit and into the lava dome. We propose a model in which the accelerating deformation, beginning as much as 4 weeks before extrusions, is caused by the increasing velocity of ascending magma in the conduit. This model is examined by using deformation data of the dome before four extrusions in 1981 and 1982 to estimate the volumetric flow rate through the conduit. This flow rate and an estimate of the effective viscosity of the magma enable calculation of an ascent velocity and an applied shear stress that, again, depend on the conduit diameter. The results of these calculations are consistent with the finite element experiments and show that the proposed model is feasible. In light of this model, events observed just before or near the time extrusions began, such as reversals of ground tilt direction from outward to inward and the sudden decrease in the number of shallow earthquakes, may indicate an abrupt decrease of shear stress in the conduit. This could be explained by a decrease in either the ascent velocity, or the effective viscosity of the magma ascending through the shallow conduit, or both, near the time of extrusion. Precursory deformation like that measured at Mount St. Helens should be observable at similar volcanoes elsewhere because it is caused by the fundamental process of magma ascent.

Journal of Geophysical Research Solid Earth↗
USGS2011

Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars

The presence of outcrops and soil (regolith) rich in opaline silica (∼65–92 wt % SiO 2 ) in association with volcanic materials adjacent to the “Home Plate” feature in Gusev crater is evidence for hydrothermal conditions. The Spirit rover has supplied a diverse set of observations that are used here to better understand the formation of silica and the activity, abundance, and fate of water in the first hydrothermal system to be explored in situ on Mars. We apply spectral, chemical, morphological, textural, and stratigraphic observations to assess whether the silica was produced by acid sulfate leaching of precursor rocks, by precipitation from silica‐rich solutions, or by some combination. The apparent lack of S enrichment and the relatively low oxidation state of the Home Plate silica‐rich materials appear inconsistent with the originally proposed Hawaiian analog for fumarolic acid sulfate leaching. The stratiform distribution of the silica‐rich outcrops and their porous and brecciated microtextures are consistent with sinter produced by silica precipitation. There is no evidence for crystalline quartz phases among the silica occurrences, an indication of the lack of diagenetic maturation following the production of the amorphous opaline phase.

Journal of Geophysical Research E: Planets↗
USGS2012

The seismic velocity structure of the Newfoundland Appalachian orogen

The deep structure of the Newfoundland Appalachian orogen is investigated by analyses of three intersecting seismic refraction/wide-angle reflection profiles which traverse the Gander and Dunnage zones or central mobile belt of Newfoundland. A simultaneous travel time inversion for velocity and interface was applied to the in-line seismic refraction/wide-angle reflection data and constrained by synthetic amplitude models. The results of the modeling procedure show a subhorizontally layered crust with upper crustal velocities ranging from 5.4 to 6.2 km/s, a midcrustal velocity of 6.25–6.35 km/s, and a lower crustal velocity of 6.7±0.2 km/s. The top of the lower crust is marked by a series of prominent reflections between 18 and 23 km depth which suggest a complex layered velocity interface. Strong laterally coherent Moho reflections indicate a sharp crust-mantle transition at 35 ± 3 km. The uppermost mantle has a velocity of 8.0±0.2 km/s, and a reflecting horizon at 55 km depth suggests an increase to velocities approaching 8.5 km/s. Normal moveout corrections applied to fan profiles provide constraining evidence for the reflecting horizon at the top of the lower crust and laterally continuous Moho reflections at 11–12 s two-way travel time. Comparisons with a coincident deep seismic reflection profile show that the refraction and reflection Mohos match to better than 2–3 km. Bulk Poisson's ratios of 0.23–0.24 for the whole crust calculated from P m P / S m S travel times suggest a crust dominated by quartzofeldspathic lithologies and a notable absence of voluminous mafic additions to the lower crust. The absence of a deep crustal root, coupled with the bulk intermediate composition inferred for the lower crust from the seismic refraction/wide-angle data, implies that the crust beneath central Newfoundland has undergone multiple periods of reactivation and equilibration following successive orogenic episodes.

Newfoundland and Labrador↗
USGS1988

Waveform modelling using locked-mode synthetic and differential seismograms: application to determination of the structure of Mexico

We have developed algorithms for modelling seismic waveforms to constrain regional Earth structure. The seismogram is represented as a sum of locked-mode travelling waves in a layered medium. This representation is convenient as it allows us to model structures with slowly varying heterogeneity and to construct differential seismograms. Describes the techniques we have implemented that enable us to compute synthetic and differential seismograms in an efficient and stable manner. The computational methods are sufficiently rapid that many modes can be included and in some cases the entire seismogram may be modified. These algorithms are applied to model a set of seismograms of southern Mexican earthquakes recorded in northern Mexico. The frequency bandwidth of these data is centred at 0.067 Hz and we demonstrate that even at these relatively high frequencies, many features of the seismogram can be successfully modelled. Our results suggest that the structure within the recording array in northern Mexico is resolvably different from that to the south. We find that the average shear velocity of the lower lithosphere of southern Mexico is very low, approximately 4.3 km s-1. If the low-velocity region is confined to the Trans Mexican Volcanic Belt, the shear velocities between 20-80 km depth are approximately 3.3 km s-1. This may be correlated with partial melt and is consistent with the active volcanism and high heat flow found in the region. -Authors

Geophysical Journal International↗
USGS2006

Spirit rover localization and topographic mapping at the landing site of Gusev crater, Mars

By sol 440, the Spirit rover has traversed a distance of 3.76 km (actual distance traveled instead of odometry). Localization of the lander and the rover along the traverse has been successfully performed at the Gusev crater landing site. We localized the lander in the Gusev crater using two-way Doppler radio positioning and cartographic triangulations through landmarks visible in both orbital and ground images. Additional high-resolution orbital images were used to verify the determined lander position. Visual odometry and bundle adjustment technologies were applied to compensate for wheel slippage, azimuthal angle drift, and other navigation errors (which were as large as 10.5% in the Husband Hill area). We generated topographic products, including 72 ortho maps and three-dimensional (3-D) digital terrain models, 11 horizontal and vertical traverse profiles, and one 3-D crater model (up to sol 440). Also discussed in this paper are uses of the data for science operations planning, geological traverse surveys, surveys of wind-related features, and other science applications.

Journal of Geophysical Research E: Planets↗
USGS2012

Improving the accuracy of S0 2 column densities and emission rates obtained from upward-looking UV-spectroscopic measurements of volcanic plumes by taking realistic radiative transfer into account

Sulfur dioxide (SO 2 ) is monitored using ultraviolet (UV) absorption spectroscopy at numerous volcanoes around the world due to its importance as a measure of volcanic activity and a tracer for other gaseous species. Recent studies have shown that failure to take realistic radiative transfer into account during the spectral retrieval of the collected data often leads to large errors in the calculated emission rates. Here, the framework for a new evaluation method which couples a radiative transfer model to the spectral retrieval is described. In it, absorption spectra are simulated, and atmospheric parameters are iteratively updated in the model until a best match to the measurement data is achieved. The evaluation algorithm is applied to two example Differential Optical Absorption Spectroscopy (DOAS) measurements conducted at Kilauea volcano (Hawaii). The resulting emission rates were 20 and 90% higher than those obtained with a conventional DOAS retrieval performed between 305 and 315 nm, respectively, depending on the different SO 2 and aerosol loads present in the volcanic plume. The internal consistency of the method was validated by measuring and modeling SO2 absorption features in a separate wavelength region around 375 nm and comparing the results. Although additional information about the measurement geometry and atmospheric conditions is needed in addition to the acquired spectral data, this method for the first time provides a means of taking realistic three-dimensional radiative transfer into account when analyzing UV-spectral absorption measurements of volcanic SO 2 plumes.

Journal of Geophysical Research D: Atmospheres↗
USGS2024

Imaging of seismic discontinuities using an adjoint method

For imaging of seismic discontinuities at depth, reverse time migration (RTM) is a powerful method to apply to recordings of seismic events. It is especially powerful when an extensive receiver array, numerous seismic sources, or both, permit adequate reconstruction of incident and scattered wavefields at depth. Reconstructing either the incident or scattered wavefield at depth becomes less accurate when relatively few recordings of seismic events are available. Here we explore an inverse scattering approach to imaging discontinuities based on an adjoint method, employing sensitivity kernels (Frechet derivatives) that represent jumps in material properties across seismic-discontinuity surfaces. When combined with ray-based requirements on scattering geometry, it constitutes a powerful approach to determining the locations and amplitudes of the discontinuities, recovering only those properties that can be resolved by a spatially limited source and/or receiver distribution. This is illustrated by synthetic examples with local sources followed by a field example in a subduction zone setting

Washington↗
USGS2012

Implications of regional gravity for state of stress in the earth's crust and upper mantle

Topography is maintained by stress differences within the earth. Depending on the distribution of the stress we classify the support as either local or regional compensation. In general, the stresses implied in a regional compensation scheme are an order of magnitude larger than those corresponding to local isostasy. Gravity anomalies, a measure of the earth's departure from hydrostatic equilibrium, can be used to distinguish between the two compensation mechanisms and thus to estimate the magnitude of deviatoric stress in the crust and upper mantle. Topography created at an oceanic ridge crest or in a major continental orogenic zone appears to be locally compensated. Such features were formed on weak crust incapable of maintaining stress differences much greater than the stress from the applied load. Oceanic volcanoes formed on an already cooled, thickened lithosphere are regionally supported with elastic stresses. Similarly, the broad topographic rise seaward of subduction zones is elastically supported as the lithosphere is bent near the plate margin. Although the implied stress is to some degree dependent on the rheological model assumed, the gravity anomalies and surface deformation produced by these features demonstrate that the upper 30–40 km of the oceanic lithosphere is capable of regionally supporting stress differences in the 100-MPa range. Given certain conditions of load emplacement, continental crust can also support loads regionally over 100-m.y. time scales, but the effects of erosion only allow an estimate of a lower bound on stress. Data from space probes indicate that the upper layers of other terrestrial planets also support topographic-induced stress differences in excess of 100 MPa.

Journal of Geophysical Research Solid Earth↗
USGS2017

Source spectral properties of small-to-moderate earthquakes in southern Kansas

The source spectral properties of injection-induced earthquakes give insight into their nucleation, rupture processes, and influence on ground motion. Here we apply a spectral decomposition approach to analyze P -wave spectra and estimate Brune-type stress drop for more than 2000 M L 1.5–5.2 earthquakes occurring in southern Kansas from 2014 to 2016. We find that these earthquakes are characterized by low stress drop values (median ∼0.4MPa) compared to natural seismicity in California. We observe a significant increase in stress drop as a function of depth, but the shallow depth distribution of these events is not by itself sufficient to explain their lower stress drop. Stress drop increases with magnitude from M1.5–M3.5, but this scaling trend may weaken above M4 and also depends on the assumed source model. Although we observe a nonstationary, sequence-specific temporal evolution in stress drop, we find no clear systematic relation with the activity of nearby injection wells.

Kansas↗
USGS1991

Single-station decomposition of seismograms for subevent time histories

We have adapted an iterative least-squares inversion with positivity constraints to the problem of deconvolving the recordings of a small earthquake from the recordings of a large composite earthquake. The inversion uses an F test to minimize the number of non-zero amplitudes or subevents in the solution set, by requiring each added subevent to reduce the variance significantly. We apply this inversion to the analysis of strong motion recordings of a set of moderate-sized (4.0 ≤ M L ≤ 6.4) aftershocks of the 1976 Friuli earthquake. The earthquake pairs are selected by comparing S trigger times and polarization diagrams. The deconvolved time series are interpreted as the time history of slip events or stress release in the composite earthquake, depending on whether the recordings of the small earthquake are unfiltered or filtered to simulate the waveforms radiated by subevents. Deconvolving with the filtered recordings reduces the variance more rapidly, as a function of the number of subevents, than deconvolving with the unfiltered recordings. The deconvolved time series for three of the four earthquakes analysed indicate that the rupture processes resemble the failure of asperities where the initial subevent has the largest stress drop and the later subevents have longer durations and weaker stress drops.

Geophysical Journal International↗
USGS1998

Three-dimensional seismic structure and moment tensors of non-double-couple earthquakes at the Hengill-Grensdalur volcanic complex, Iceland

The volcanic and geothermal areas of Iceland are rich sources of non-double-couple (non-DC) earthquakes. A state-of-the-art digital seismometer network deployed at the Hengill–Grensdalur volcanic complex in 1991 recorded 4000 small earthquakes. We used the best recorded of these to determine 3-D V P and V P /V S structure tomographically and accurate earthquake moment tensors. The V P field is dominated by high seismic wave speed bodies interpreted as solidified intrusions. A widespread negative (−4 per cent) V P /V S anomaly in the upper 4km correlates with the geothermal field, but is too strong to be caused solely by the effect of temperature upon liquid water or the presence of vapour, and requires in addition mineralogical or lithological differences between the geothermal reservoir and its surroundings. These may be caused by geothermal alteration. Well-constrained moment tensors were obtained for 70 of the best-recorded events by applying linear programming methods to P - and S -wave polarities and amplitude ratios. About 25 per cent of the mechanisms are, within observational error, consistent with DC mechanisms consistent with shear faulting. The other 75 per cent have significantly non-DC mechanisms. Many have substantial explosive components, one has a substantial implosive component, and the deviatoric component of many is strongly non-DC. Many of the non-DC mechanisms are consistent, within observational error, with simultaneous tensile and shear faulting. However, the mechanisms occupy a continuum in source-type parameter space and probably at least one additional source process is occurring. This may be fluid flow into newly formed cracks, causing partial compensation of the volumetric component. Studying non-shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general.

Geophysical Journal International↗