Global Seismological Services

Golden, CO, United States

Global Seismological Services

Golden, CO, United States
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Nealy J.L.,U.S. Geological Survey | Benz H.M.,U.S. Geological Survey | Hayes G.P.,U.S. Geological Survey | Bergman E.A.,Global Seismological Services | Barnhart W.D.,University of Iowa
Bulletin of the Seismological Society of America | Year: 2017

The 2008 Wells, Nevada, earthquake represents the largest domestic event in the contiguous United States outside of California since the October 1983 Borah Peak earthquake in southern Idaho. We present an improved catalog, magnitude complete to 1.6, of the foreshock-aftershock sequence, supplementing the current U.S. Geological Survey Preliminary Determination of Epicenters catalog with 1928 well-located events. To create this catalog, both subspace and kurtosis detectors are used to obtain an initial set of earthquakes and associated locations. The latter are then calibrated through the implementation of the hypocentroidal decomposition method and relocated using the Bayesloc relocation technique. We additionally perform a finite-fault slip analysis of the mainshock using Interferometric Synthetic Aperture Radar (InSAR) observations. By combining the relocated sequence with the finite-fault analysis, we show that the aftershocks occur primarily up-dip and along the southwestern edge of the zone of maximum slip. The aftershock locations illuminate areas of postmainshock strain increase; aftershock depths, ranging from 5 to 16 km, are consistent with InSAR imaging, which shows that the Wells earthquake was a buried source with no observable near-surface offset. © 2017, Seismological Society of America. All rights reserved.

Karasozen E.,Colorado School of Mines | Nissen E.,Colorado School of Mines | Bergman E.A.,Global Seismological Services | Johnson K.L.,Colorado School of Mines | Walters R.J.,Durham University
Journal of Geophysical Research: Solid Earth | Year: 2016

Western Turkey has a long history of large earthquakes, but the responsible faults are poorly characterized. Here we reassess the past half century of instrumental earthquakes in the Simav-Gediz region, starting with the 19 May 2011 Simav earthquake (Mw 5.9), which we image using interferometric synthetic aperture radar and regional and teleseismic waveforms. This event ruptured a steep, planar normal fault centered at 7–9 km depth but failed to break the surface. However, relocated main shock and aftershock hypocenters occurred beneath the main slip plane at 10–22 km depth, implying rupture initiation in areas of low coseismic slip. These calibrated modern earthquakes provide the impetus to relocate and reassess older instrumental events in the region. Aftershocks of the 1970 Gediz earthquake (Mw 7.1) form a narrow band, inconsistent with source models that invoke low-angle detachment faulting, and may include events triggered dynamically by the unilateral main shock rupture. Epicenters of the 1969 Demirci earthquakes (Mw 5.9, 6.0) are more consistent with slip on the south dipping Akdağ fault than the larger, north dipping Simav fault. A counterintuitive aspect of recent seismicity across our study area is that the largest event (Mw 7.1) occurred in an area of slower extension and indistinct surface faulting, yet ruptured the surface, while recent earthquakes in the well-defined and more rapidly extending Simav graben are smaller (Mw <6.0) and failed to produce surface breaks. Though our study area bounds a major metamorphic core complex, there is no evidence for involvement of low-angle normal faulting in any of the recent large earthquakes. ©2016. American Geophysical Union. All Rights Reserved.

Yeck W.L.,U.S. Geological Survey | Weingarten M.,Stanford University | Benz H.M.,U.S. Geological Survey | McNamara D.E.,U.S. Geological Survey | And 4 more authors.
Geophysical Research Letters | Year: 2016

The Mw 5.1 Fairview, Oklahoma, earthquake on 13 February 2016 and its associated seismicity produced the largest moment release in the central and eastern United States since the 2011 Mw 5.7 Prague, Oklahoma, earthquake sequence and is one of the largest earthquakes potentially linked to wastewater injection. This energetic sequence has produced five earthquakes with Mw 4.4 or larger. Almost all of these earthquakes occur in Precambrian basement on a partially unmapped 14 km long fault. Regional injection into the Arbuckle Group increased approximately sevenfold in the 36 months prior to the start of the sequence (January 2015). We suggest far-field pressurization from clustered, high-rate wells greater than 12 km from this sequence induced these earthquakes. As compared to the Fairview sequence, seismicity is diffuse near high-rate wells, where pressure changes are expected to be largest. This points to the critical role that preexisting faults play in the occurrence of large induced earthquakes. ©2016. American Geophysical Union. All Rights Reserved.

McNamara D.E.,U.S. Geological Survey | Benz H.M.,U.S. Geological Survey | Herrmann R.B.,Saint Louis University | Bergman E.A.,Global Seismological Services | And 4 more authors.
Geophysical Research Letters | Year: 2015

The sharp increase in seismicity over a broad region of central Oklahoma has raised concern regarding the source of the activity and its potential hazard to local communities and energy industry infrastructure. Since early 2010, numerous organizations have deployed temporary portable seismic stations in central Oklahoma in order to record the evolving seismicity. In this study, we apply a multiple-event relocation method to produce a catalog of 3639 central Oklahoma earthquakes from late 2009 through 2014. Regional moment tensor (RMT) source parameters were determined for 195 of the largest and best recorded earthquakes. Combining RMT results with relocated seismicity enabled us to determine the length, depth, and style of faulting occurring on reactivated subsurface fault systems. Results show that the majority of earthquakes occur on near-vertical, optimally oriented (NE-SW and NW-SE), strike-slip faults in the shallow crystalline basement. These are necessary first-order observations required to assess the potential hazards of individual faults in Oklahoma. ©2015. The Authors.

Ghods A.,Institute for Advanced Studies in Basic Sciences | Shabanian E.,Institute for Advanced Studies in Basic Sciences | Bergman E.,Global Seismological Services | Faridi M.,Geological Survey of Iran | And 4 more authors.
Geophysical Journal International | Year: 2015

On 2012 August 11, a pair of large, damaging earthquakes struck the Varzaghan-Ahar region in northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics of the source region we applied a combination of seismological methods (local aftershock network, calibrated multiple event relocation and focal mechanism studies), field observations (structural geology and geomorphological) and inversions for the regional stress field. The epicentral region is north of the North Tabriz Fault. The first main shock is characterized by right-lateral strike-slip motion on an almost E-W fault plane of about 23 km length extending from the surface to a depth of about 14 km. The second main shock occurred on an ENE-striking fault that dips at 60-70° to the NW. Independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with σ1 oriented N132E. For the region northeast of the North Tabriz Fault, the presence of rigid lithosphere of the South Caspian Basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections. Our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation. At a global scale, our results indicate that despite the basic assumption of 'rigid blocks' in geodetic plate modelling, internal deformation of block-like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Hayes G.P.,National Earthquake Information Center | Bergman E.,Global Seismological Services | Johnson K.L.,National Earthquake Information Center | Johnson K.L.,Colorado School of Mines | And 3 more authors.
Geophysical Journal International | Year: 2013

After the 2010 Mw 8.8 Maule earthquake, an international collaboration involving teams and instruments from Chile, the US, the UK, France and Germany established the International Maule Aftershock Deployment temporary network over the source region of the event to facilitate detailed, open-access studies of the aftershock sequence. Using data from the first 9-months of this deployment, we have analyzed the detailed spatial distribution of over 2500 well-recorded aftershocks. All earthquakes have been relocated using a hypocentral decomposition algorithm to study the details of and uncertainties in both their relative and absolute locations.We have computed regional moment tensor solutions for the largest of these events to produce a catalogue of 465 mechanisms, and have used all of these data to study the spatial distribution of the aftershock sequence with respect to the Chilean megathrust.We refine models of co-seismic slip distribution of the Maule earthquake, and show how small changes in fault geometries assumed in teleseismic finite fault modelling significantly improve fits to regional GPS data, implying that the accuracy of rapid teleseismic fault models can be substantially improved by consideration of existing fault geometry model databases. We interpret all of these data in an integrated seismotectonic framework for the Maule earthquake rupture and its aftershock sequence, and discuss the relationships between co-seismic rupture and aftershock distributions.While the majority of aftershocks are interplate thrust events located away from regions of maximum co-seismic slip, interesting clusters of aftershocks are identified in the lower plate at both ends of the main shock rupture, implying internal deformation of the slab in response to large slip on the plate boundary interface. We also perform Coulomb stress transfer calculations to compare aftershock locations and mechanisms to static stress changes following the Maule rupture. Without the incorporation of uncertainties in earthquake locations, just 55 per cent of aftershock nodal planes align with faults promoted towards failure by co-seismic slip. When epicentral uncertainties are considered (on the order of just ±2-3 km), 90 per cent of aftershocks are consistent with occurring along faults demonstrating positive stress transfer. These results imply large sensitivities of Coulomb stress transfer calculations to uncertainties in both earthquake locations and models of slip distributions, particularly when applied to aftershocks close to a heterogeneous fault rupture; such uncertainties should therefore be considered in similar studies used to argue for or against models of static stress triggering. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Copley A.,University of Cambridge | Hollingsworth J.,California Institute of Technology | Bergman E.,Global Seismological Services
Journal of Geophysical Research: Solid Earth | Year: 2012

The 2006 Mw7.0 Mozambique (Machaze) normal-faulting earthquake ruptured an unusually steeply dipping fault plane (∼75°). The amount of slip in the earthquake decreased from depths of ∼10 km toward the surface, and this shallow slip deficit was at least partly recovered by postseismic afterslip on the shallow part of the fault plane. An adjacent normal fault segment slipped postseismically (and possibly also co-seismically) at shallow depths with a large strike-slip component, in response to the stresses generated by slip on the main earthquake fault plane. Our observations suggest that the fault zone behaves in a stick-slip manner in the crystalline basement, and that where it cuts the sedimentary layer the coseismic rupture was partially arrested and there was significant postseismic creep. We discuss the effects of such behavior on the large-scale tectonics of continental regions, and on the assessment of seismic hazard on similar fault systems. The steep dip of the fault suggests the re-activation of a preexisting structure with a coefficient of friction at least ∼25-45% lower than that on optimally oriented planes, and analysis of the deformation following an aftershock indicates that the value of the parameter 'a' that describes the rate-dependence of fault friction lies in the range 1 × 10-3-2 × 10-2. The lack of long-wavelength postseismic relaxation suggests viscosities in the ductile lithosphere of greater than ∼2 × 1019 Pa s, and an examination of the tectonic geomorphology in the region identifies ways in which similar fault systems can be identified before they rupture in future earthquakes. Copyright 2012 by the American Geophysical Union.

Barnhart W.D.,U.S. Geological Survey | Benz H.M.,U.S. Geological Survey | Hayes G.P.,U.S. Geological Survey | Rubinstein J.L.,U.S. Geological Survey | Bergman E.,Global Seismological Services
Journal of Geophysical Research B: Solid Earth | Year: 2014

The Raton Basin of southern Colorado and northern New Mexico is an actively produced hydrocarbon basin that has experienced increased seismicity since 2001, including the August 2011 Mw5.3 Trinidad normal faulting event. Following the 2011 earthquake, regional seismic observations were used to relocate 21 events, including the 2011 main shock, two foreshocks, and 13 aftershocks. Additionally, interferometric synthetic aperture radar (InSAR) observations of both the 2011 event and preevent basin deformation place constraint on the spatial kinematics of the 2011 event and localized basin subsidence due to ground water or gas withdrawal. We find that the 2011 earthquake ruptured an 8-10 km long segment of a normal fault at depths of 1.5-6.0 km within the crystalline Precambrian basement underlying the Raton Basin sedimentary rocks. The earthquake also nucleated within the crystalline basement in the vicinity of an active wastewater disposal site. The ensuing aftershock sequence demonstrated statistical properties expected for intraplate earthquakes, though the length of the 2011 earthquake is unexpectedly long for an Mw5.3 event, suggesting that wastewater disposal may have triggered a low stress drop, otherwise natural earthquake. Additionally, preevent and postevent seismicity in the Raton Basin spatially correlates to regions of subsidence observed in InSAR time series analysis. While these observations cannot discern a causal link between hydrocarbon production and seismicity, they constrain spatial relationships between active basin deformation and geological and anthropogenic features. Furthermore, the InSAR observations highlight the utility of space-based geodetic observations for monitoring and assessing anthropogenically induced and triggered deformation. © 2014. American Geophysical Union.

McNamara D.E.,U.S. Geological Survey | Benz H.M.,U.S. Geological Survey | Herrmann R.B.,Saint Louis University | Bergman E.A.,Global Seismological Services | And 4 more authors.
Bulletin of the Seismological Society of America | Year: 2014

The Mw 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M0 5.7 × 1017 N·m) occurred near Mineral, Virginia, within the central Virginia seismic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeaststriking reverse fault that nucleated at a depth of approximately 7 ± 2 km. The distribution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Nearsource and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and academic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence magnitude-frequency and temporal distribution. The observed slope of the magnitude-frequency curve or b-value for the aftershock sequence is consistent with previous EUS studies (b=0.75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approximately 2-8 km in depth and 8-10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east-southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast-southwest striking southeastdipping fault plane.

PubMed | National Earthquake Information Center, Pennsylvania State University, Natural Resources Canada, University of Chile and Global Seismological Services
Type: Journal Article | Journal: Nature | Year: 2014

The seismic gap theory identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault. It has successfully explained past earthquakes (see, for example, ref.2) and is useful for qualitatively describing where large earthquakes might occur. A large earthquake had been expected in the subduction zone adjacent to northern Chile, which had not ruptured in a megathrust earthquake since a M8.8 event in 1877. On 1 April 2014 a M8.2 earthquake occurred within this seismic gap. Here we present an assessment of the seismotectonics of the March-April 2014 Iquique sequence, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations and cumulative Coulomb stress transfer. This ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and/or elevated hazard. Our results constrain the size and spatial extent of rupture, and indicate that this was not the earthquake that had been anticipated. Significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 Iquique sequence.

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