BSSA updated 05 Sept 2016
Long‐Term Afterslip of the 2004 M 6.0 Parkfield, California, Earthquake—Implications for Forecasting Amount and Duration of Afterslip on Other Major Creeping Faults
James J. Lienkaemper, Forrest S. McFarland
DOI: 10.1785/0120160321 Published on April 2017, First Published on April 18, 2017
Which Fault Segments Ruptured in the 2008 Wenchuan Earthquake and Which Did Not? New Evidence from Near‐Fault 3D Surface Displacements Derived from SAR Image Offsets
Guangcai Feng, Sigurjón Jónsson, Yann Klinger
DOI: 10.1785/0120160126 Published on March 2017, First Published on March 14, 2017
Characterizing Potentially Induced Earthquake Rate Changes in the Brawley Seismic Zone, Southern California
Andrea L. Llenos, Andrew J. Michael
DOI: 10.1785/0120150053 Published on October 2016, First Published on August 23, 2016
The Brawley seismic zone (BSZ), in the Salton trough of southern California, has a history of earthquake swarms and geothermal energy exploitation. Some earthquake rate changes may have been induced by fluid extraction and injection activity at local geothermal fields, particularly at the North Brawley Geothermal Field (NBGF) and at the Salton Sea Geothermal Field (SSGF). We explore this issue by examining earthquake rate changes and interevent distance distributions in these fields. In Oklahoma and Arkansas, where considerable wastewater injection occurs, increases in background seismicity rate and aftershock productivity and decreases in interevent distance were indicative of fluid‐injection‐induced seismicity. Here, we test if similar changes occur that may be associated with fluid injection and extraction in geothermal areas. We use stochastic epidemic‐type aftershock sequence models to detect changes in the underlying seismogenic processes, shown by statistically significant changes in the model parameters. The most robust model changes in the SSGF roughly occur when large changes in net fluid production occur, but a similar correlation is not seen in the NBGF. Also, although both background seismicity rate and aftershock productivity increased for fluid‐injection‐induced earthquake rate changes in Oklahoma and Arkansas, the background rate increases significantly in the BSZ only, roughly corresponding with net fluid production rate increases. Moreover, in both fields the interevent spacing does not change significantly during active energy projects. This suggests that, although geothermal field activities in a tectonically active region may not significantly change the physics of earthquake interactions, earthquake rates may still be driven by fluid injection or extraction rates, particularly in the SSGF.
Geophysical Evidence for a San Andreas Subparallel Transtensional Fault along the Northeastern Shore of the Salton Sea
Valerie Sahakian, Annie Kell, Alistair Harding, Neal Driscoll, and Graham Kent
DOI: 10.1785/0120150350 Published on October 2016
The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pacific and North American plates; thus, the fault represents a major geohazard to the populated areas of southern California, in particular the larger Los Angeles metropolitan area. Paleoseismic chronology of ruptures along the SSAF segment suggests this fault is near the end of its interseismic period (∼180 years), because it has not ruptured in historic times (∼320 years). A recent active‐source seismic experiment performed in the Salton Sea west of the SSAF provides evidence for extensional deformation along the northeastern shore of the Salton Sea. This study posits that the extensional deformation is due to a previously unmapped fault, here named the Salton trough fault (STF). The seismic reflection data image a divergent sediment package that dips toward the northeast with thicknesses up to at least 2 km. Refraction inversion produces a southwestward‐dipping velocity discontinuity that crops out east of the SSAF surface trace, consistent with the existence of a southwest to northeast gradient in lithology. If present, the existence of the STF has scientific and societal relevance. First, the STF appears to control the recent Salton trough architecture north of Bombay Beach. Second, from a seismological hazards perspective, the presence of this structure could alter the current understanding of stress transfer and rupture dynamics in the region, as well as community fault models and ground‐motion simulations on the SSAF.
You have access Investigating Triggering of the Aftershocks of the 2014 Napa Earthquake
Vincenzo Convertito, Raffaella De Matteis, Antonio Emolo
DOI: 10.1785/0120160011 Published on October 2016, First Published on August 23, 2016
The occurrence of the Mw 6.0 South Napa California earthquake, on 24 August 2014 at 03:20 a.m. local time, triggered discussion in the seismological community about the level of damage associated with such a moderate‐magnitude event and about the geometry and orientation of the causative fault. In addition, coulomb static stress change mapping does not seem to be able to fully explain near‐source aftershock distribution. Here, we find clear evidence of a north‐northwest source directivity from the analysis of the spatial distribution of peak ground motion. The area of the highest values of the estimated peak dynamic strain field, computed accounting for fault extent and source directivity, agrees with the near‐source aftershock distribution. This might suggest that, in addition to coulomb static stress change, dynamic strain also contributed to the triggering of near‐source Napa earthquake aftershocks. The approach used here might be useful to identify areas likely prone to aftershock occurrence.
Seismic Imaging beneath an InSAR Anomaly in Eastern Washington State: Shallow Faulting Associated with an Earthquake Swarm in a Low‐Hazard Area
W. J. Stephenson, J. K. Odum, C. W. Wicks, T. L. Pratt, R. J. Blakely
DOI: 10.1785/0120150295 Published on August 2016, First Published on June 21, 2016
In 2001, a rare swarm of small, shallow earthquakes beneath the city of Spokane, Washington, caused ground shaking as well as audible booms over a five‐month period. Subsequent Interferometric Synthetic Aperture Radar (InSAR) data analysis revealed an area of surface uplift in the vicinity of the earthquake swarm. To investigate the potential faults that may have caused both the earthquakes and the topographic uplift, we collected ∼3 km of high‐resolution seismic‐reflection profiles to image the upper‐source region of the swarm. The two profiles reveal a complex deformational pattern within Quaternary alluvial, fluvial, and flood deposits, underlain by Tertiary basalts and basin sediments. At least 100 m of arching on a basalt surface in the upper 500 m is interpreted from both the seismic profiles and magnetic modeling. Two west‐dipping faults deform Quaternary sediments and project to the surface near the location of the Spokane fault defined from modeling of the InSAR data.
Relating Transient Seismicity to Episodes of Deep Creep at Parkfield, California
Charles G. Sammis, Stewart W. Smith, Robert M. Nadeau, Rachel Lippoldt
DOI: 10.1785/0120150224 Published on August 2016, First Published on July 12, 2016
The 2004 Mw 6 Parkfield, California, earthquake was preceded by a 4‐year period of anomalously high seismicity adjacent to, but not on, the San Andreas fault. The rate of small events (Mw<3) at distances between 1.5 and 20 km from the fault plane and at depths >8 km, increased from 6 events per year prior to 2000 to 20 events per year between the 2000 and the 2004 earthquake. This increase in seismicity coincided with an increase in the rate of nonvolcanic tremor, which, if tremor is indicative of creep on the fault plane, suggests that creep may have driven the enhanced seismicity. Coulomb stress‐transfer calculations predict the observed spatial pattern of the seismicity, and thus support a causal relation between creep at the base of the fault zone and off‐fault seismicity. In particular, an observed southeast‐striking lineation of enhanced seismicity is shown to be a direct consequence of a deepening boundary between the crust and mantle southeast of Parkfield, as evidenced by a deepening of the tremor and low‐frequency earthquakes. Other evidence for a causal link between deep creep and off‐fault seismicity is the observation that off‐fault seismicity before and after the 2004 earthquake occurred in the same location. This is expected if the foreshocks are driven by an episode of deep creep and the aftershocks are driven by afterslip, both occurring on the same deep extension of the fault plane.
Aftershocks of the 2014 South Napa, California, Earthquake: Complex Faulting on Secondary Faults
Jeanne L. Hardebeck, David R. Shelly
DOI: 10.1785/0120150169 Published on June 2016, First Published on May 24, 2016
We investigate the aftershock sequence of the 2014 Mw 6.0 South Napa, California, earthquake. Low‐magnitude aftershocks missing from the network catalog are detected by applying a matched‐filter approach to continuous seismic data, with the catalog earthquakes serving as the waveform templates. We measure precise differential arrival times between events, which we use for double‐difference event relocation in a 3D seismic velocity model. Most aftershocks are deeper than the mainshock slip, and most occur on the west of the mapped surface rupture. Although the mainshock coseismic and postseismic slip appears to have occurred on the near‐vertical, strike‐slip West Napa fault, many of the aftershocks occur in a complex zone of secondary faulting. Earthquake locations in the main aftershock zone, near the mainshock hypocenter, delineate multiple dipping secondary faults. Composite focal mechanisms indicate strike‐slip and oblique‐reverse faulting on the secondary features. The secondary faults were moved toward failure by Coulomb stress changes from the mainshock slip. Clusters of aftershocks north and south of the main aftershock zone exhibit vertical strike‐slip faulting more consistent with the West Napa fault. The northern aftershocks correspond to the area of the largest mainshock coseismic slip, whereas the main aftershock zone is adjacent to the fault area that has primarily slipped postseismically. Unlike most creeping faults, the zone of postseismic slip does not appear to contain embedded stick‐slip patches that would have produced on‐fault aftershocks. The lack of stick‐slip patches along this portion of the fault may contribute to the low productivity of the South Napa aftershock sequence.
Discriminating Characteristics of Tectonic and Human‐Induced Seismicity
Ilya Zaliapin, Yehuda Ben‐Zion
DOI: 10.1785/0120150211 Published on June 2016, First Published on April 12, 2016
We analyze statistical features of background and clustered subpopulations of earthquakes in different regions in an effort to distinguish between human‐induced and natural seismicity. Analysis of end‐member areas known to be dominated by human‐induced earthquakes (The Geyser geothermal field in northern California and TauTona gold mine in South Africa) and regular tectonic activity (the San Jacinto fault zone in southern California and the Coso region, excluding the Coso geothermal field in eastern central California) reveals several distinguishing characteristics. Induced seismicity is shown to have (1) higher rate of background events (both absolute and relative to the total rate), (2) faster temporal offspring decay, (3) higher rate of repeating events, (4) larger proportion of small clusters, and (5) larger spatial separation between parent and offspring, compared to regular tectonic activity. These differences also successfully discriminate seismicity within the Coso and Salton Sea geothermal fields in California before and after the expansion of geothermal production during the 1980s.
A Time‐Domain Detection Approach to Identify Small Earthquakes within the Continental United States Recorded by the USArray and Regional Networks
Aaron A. Velasco, Richard Alfaro‐Diaz, Debi Kilb, Kristine L. Pankow
DOI: 10.1785/0120150156 Published on April 2016, First Published on April 06, 2016
Technological advances in combination with the onslaught of data availability allow for large seismic data streams to automatically and systematically be recorded, processed, and stored. Here, we develop an automated approach to identify small, local earthquakes within these large continuous seismic data records. Our aim is to automate the process of detecting small seismic events triggered by a distant large earthquake, recorded at a single station. Specifically, we apply time‐domain short‐term average (STA) to long‐term average (LTA) ratio algorithms to three‐component data to create a catalog of detections. We remove some of the false detections by requiring the detection be recorded on a minimum of two channels. To calibrate the algorithm, we compare our automatic detection catalog to a set of analyst‐derived P‐wave arrival times for a subset of small earthquakes occurring in the December 2008 Yellowstone swarm. Of the four STA/LTA algorithms we test (1 s/10 s; 4 s/40 s; 8 s/80 s; 16 s/160 s), the 1 s/10 s and 4 s/40 s detectors proved most effective at identifying the majority of events in the swarm. We apply these detectors to ±45 hrs and ±5 hrs of USArray data from the 2011 Japan M 9.0 and the 2010 Chile M 8.8 earthquakes, respectively. Using time‐of‐day versus number of detection relationships, we identify 38 of the 728 available stations that exhibit strong anthropogenic noise following the 2011 Japan earthquake. Our detection algorithm identified three regional earthquakes concurrent with the passage of the S‐ and surface waves of the Chile mainshock at USArray station R11A that locate in the Coso region of California, as well as events in Texas following the Japan earthquake.
Along‐Strike Variations in Fault Frictional Properties along the San Andreas Fault near Cholame, California, from Joint Earthquake and Low‐Frequency Earthquake Relocations
Rebecca M. Harrington, Elizabeth S. Cochran, Emily M. Griffiths, Xiangfang Zeng, Clifford H. Thurber
DOI: 10.1785/0120150171 Published on April 2016, First Published on January 26, 2016
Recent observations of low‐frequency earthquakes (LFEs) and tectonic tremor along the Parkfield–Cholame segment of the San Andreas fault suggest slow‐slip earthquakes occur in a transition zone between the shallow fault, which accommodates slip by a combination of aseismic creep and earthquakes (<15 km depth), and the deep fault, which accommodates slip by stable sliding (>35 km depth). However, the spatial relationship between shallow earthquakes and LFEs remains unclear. Here, we present precise relocations of 34 earthquakes and 34 LFEs recorded during a temporary deployment of 13 broadband seismic stations from May 2010 to July 2011. We use the temporary array waveform data, along with data from permanent seismic stations and a new high‐resolution 3D velocity model, to illuminate the fine‐scale details of the seismicity distribution near Cholame and the relation to the distribution of LFEs. The depth of the boundary between earthquakes and LFE hypocenters changes along strike and roughly follows the 350°C isotherm, suggesting frictional behavior may be, in part, thermally controlled. We observe no overlap in the depth of earthquakes and LFEs, with an ∼5 km separation between the deepest earthquakes and shallowest LFEs. In addition, clustering in the relocated seismicity near the 2004 Mw 6.0 Parkfield earthquake hypocenter and near the northern boundary of the 1857 Mw 7.8 Fort Tejon rupture may highlight areas of frictional heterogeneities on the fault where earthquakes tend to nucleate.
A Fault‐Based Model for Crustal Deformation, Fault Slip Rates, and Off‐Fault Strain Rate in California
Yuehua Zeng, Zheng‐Kang Shen
DOI: 10.1785/0120140250 Published on April 2016, First Published on March 15, 2016
We invert Global Positioning System (GPS) velocity data to estimate fault slip rates in California using a fault‐based crustal deformation model with geologic constraints. The model assumes buried elastic dislocations across the region using Uniform California Earthquake Rupture Forecast Version 3 (UCERF3) fault geometries. New GPS velocity and geologic slip‐rate data were compiled by the UCERF3 deformation working group. The result of least‐squares inversion shows that the San Andreas fault slips at 19–22 mm/yr along Santa Cruz to the North Coast, 25–28 mm/yr along the central California creeping segment to the Carrizo Plain, 20–22 mm/yr along the Mojave, and 20–24 mm/yr along the Coachella to the Imperial Valley. Modeled slip rates are 7–16 mm/yr lower than the preferred geologic rates from the central California creeping section to the San Bernardino North section. For the Bartlett Springs section, fault slip rates of 7–9 mm/yr fall within the geologic bounds but are twice the preferred geologic rates. For the central and eastern Garlock, inverted slip rates of 7.5 and 4.9 mm/yr, respectively, match closely with the geologic rates. For the western Garlock, however, our result suggests a low slip rate of 1.7 mm/yr. Along the eastern California shear zone and southern Walker Lane, our model shows a cumulative slip rate of 6.2–6.9 mm/yr across its east–west transects, which is ∼1 mm/yr increase of the geologic estimates. For the off‐coast faults of central California, from Hosgri to San Gregorio, fault slips are modeled at 1–5 mm/yr, similar to the lower geologic bounds. For the off‐fault deformation, the total moment rate amounts to 0.88×1019 N·m/yr, with fast straining regions found around the Mendocino triple junction, Transverse Ranges and Garlock fault zones, Landers and Brawley seismic zones, and farther south. The overall California moment rate is 2.76×1019 N·m/yr, which is a 16% increase compared with the UCERF2 model.
Long Valley Caldera and the UCERF Depiction of Sierra Nevada Range-Front Faults
David P. Hill, Emily Montgomery-Brown
DOI: 10.1785/0120150149 Published on December 2015, First Published on November 10, 2015
Long Valley caldera lies within a left-stepping offset in the north-northwest-striking Sierra Nevada range-front normal faults with the Hilton Creek fault to the south and Hartley Springs fault to the north. Both Uniform California Earthquake Rupture Forecast (UCERF) 2 and its update, UCERF3, depict slip on these major range-front normal faults as extending well into the caldera, with significant normal slip on overlapping, subparallel segments separated by ∼10 km. This depiction is countered by (1) geologic evidence that normal faulting within the caldera consists of a series of graben structures associated with postcaldera magmatism (intrusion and tumescence) and not systematic down-to-the-east displacements consistent with distributed range-front faulting and (2) the lack of kinematic evidence for an evolving, postcaldera relay ramp structure between overlapping strands of the two range-front normal faults. The modifications to the UCERF depiction described here reduce the predicted shaking intensity within the caldera, and they are in accord with the tectonic influence that underlapped offset range-front faults have on seismicity patterns within the caldera associated with ongoing volcanic unrest.
The 1868 Hayward Fault, California, Earthquake: Implications for Earthquake Scaling Relations on Partially Creeping Faults
Susan E. Hough, Stacey S. Martin
DOI: 10.1785/0120140372 Published on December 2015, First Published on November 30, 2015
The 21 October 1868 Hayward, California, earthquake is among the best-characterized historical earthquakes in California. In contrast to many other moderate-to-large historical events, the causative fault is clearly established. Published magnitude estimates have been fairly consistent, ranging from 6.8 to 7.2, with 95% confidence limits including values as low as 6.5. The magnitude is of particular importance for assessment of seismic hazard associated with the Hayward fault and, more generally, to develop appropriate magnitude–rupture length scaling relations for partially creeping faults. The recent reevaluation of archival accounts by Boatwright and Bundock (2008), together with the growing volume of well-calibrated intensity data from the U.S. Geological Survey “Did You Feel It?” (DYFI) system, provide an opportunity to revisit and refine the magnitude estimate. In this study, we estimate the magnitude using two different methods that use DYFI data as calibration. Both approaches yield preferred magnitude estimates of 6.3–6.6, assuming an average stress drop. A consideration of data limitations associated with settlement patterns increases the range to 6.3–6.7, with a preferred estimate of 6.5. Although magnitude estimates for historical earthquakes are inevitably uncertain, we conclude that, at a minimum, a lower-magnitude estimate represents a credible alternative interpretation of available data. We further discuss implications of our results for probabilistic seismic-hazard assessment from partially creeping faults.
Semiannual Earthquake Periodicity
Bulletin of the Seismological Society of America, October 2015, v. 105, p. 2736-2749, First published on August 18, 2015, doi:10.1785/0120140270
Delayed Dynamic Triggered Seismicity in Northern Baja California, México Caused by Large and Remote Earthquakes
Bulletin of the Seismological Society of America, August 2015, v. 105, p. 1825-1835, First published on July 21, 2015, doi:10.1785/0120140310
Detection and Location of Low‐Frequency Earthquakes Using Cross‐Station Correlation
Bulletin of the Seismological Society of America, August 2015, v. 105, p. 2128-2142, First published on June 30, 2015, doi:10.1785/0120140301
On the Sensitivity of Transtensional Versus Transpressional Tectonic Regimes to Remote Dynamic Triggering by Coulomb Failure
Bulletin of the Seismological Society of America, June 2015, v. 105, p. 1339-1348, First published on April 28, 2015, doi:10.1785/0120140292
Detecting Deep Tectonic Tremor in Taiwan with a Dense Array
Bulletin of the Seismological Society of America, June 2015, v. 105, p. 1349-1358, First published on May 20, 2015, doi:10.1785/0120140258
An Abrupt Transition in the Mechanical Response of the Upper Crust to Transpression along the Queen Charlotte Fault
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1114-1128, First published on March 3, 2015, doi:10.1785/0120140159
Coseismic and Early Postseismic Deformation of the 5 January 2013 Mw 7.5 Craig Earthquake from Static and Kinematic GPS Solutions
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1153-1164, First published on April 14, 2015, doi:10.1785/0120140172
Triggered Seismic Events along the Eastern Denali Fault in Northwest Canada Following the 2012 Mw 7.8 Haida Gwaii, 2013 Mw 7.5 Craig, and Two Mw>8.5 Teleseismic Earthquakes
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1165-1177, First published on April 14, 2015, doi:10.1785/0120140156
Spatiotemporal Distribution of Events during the First Week of the 2012 Haida Gwaii Aftershock Sequence
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1231-1240, First published on April 7, 2015, doi:10.1785/0120140173
GPS Observations of Crustal Deformation Associated with the 2012 Mw 7.8 Haida Gwaii Earthquake
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1241-1252, First published on April 7, 2015, doi:10.1785/0120140177
Coulomb Stress Changes Following the 2012 Mw 7.8 Haida Gwaii, Canada, Earthquake: Implications for Seismic Hazard
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1253-1264, First published on April 7, 2015, doi:10.1785/0120140158
Thermal Condition of the 27 October 2012 Mw 7.8 Haida Gwaii Subduction Earthquake at the Obliquely Convergent Queen Charlotte Margin
Bulletin of the Seismological Society of America, May 2015, v. 105, p. 1290-1300, First published on April 7, 2015, doi:10.1785/0120140183
Fault‐Slip Distribution of the 1999 Mw 7.1 Hector Mine Earthquake, California, Estimated from Postearthquake Airborne LiDAR Data
Bulletin of the Seismological Society of America, April 2015, v. 105, p. 776-790, First published on February 3, 2015, doi:10.1785/0120130108
Coseismic Strains on Plate Boundary Observatory Borehole Strainmeters in Southern California
Bulletin of the Seismological Society of America, February 2015, v. 105, p. 431-444, First published on December 2, 2014, doi:10.1785/0120140199
Shear‐Wave Velocity Structure of the Koyna–Warna Region in Western India Using Ambient Noise Correlation and Surface‐Wave Dispersion
Bulletin of the Seismological Society of America, February 2015, v. 105, p. 473-479, First published on January 13, 2015, doi:10.1785/0120140091
The Burst‐Like Behavior of Aseismic Slip on a Rough Fault: The Creeping Section of the Haiyuan Fault, China
Bulletin of the Seismological Society of America, February 2015, v. 105, p. 480-488, First published on December 30, 2014, doi:10.1785/0120140237
The Coulomb Stress Changes and Seismicity Rate due to the 1990 Mw 7.3 Rudbar Earthquake
Bulletin of the Seismological Society of America, December 2014, v. 104, p. 2943-2952, First published on October 21, 2014, doi:10.1785/0120130314
Bulletin of the Seismological Society of America December 2014 104:3094-3114; published ahead of print October 14, 2014, doi:10.1785/0120140117
Bulletin of the Seismological Society of America December 2014 104:2966-2984; published ahead of print October 21, 2014, doi:10.1785/0120140003
Bulletin of the Seismological Society of America February 2015 105:447-451; published ahead of print January 13, 2015, doi:10.1785/0120130127
Bulletin of the Seismological Society of America February 2015 105:452-458; published ahead of print January 13, 2015, doi:10.1785/0120140132
Bulletin of the Seismological Society of America February 2015 105:431-444; published ahead of print December 2, 2014, doi:10.1785/0120140199
Bulletin of the Seismological Society of America, October 2014, v. 104, p. 2529-2540, First published on September 16, 2014, doi:10.1785/0120140047
Bulletin of the Seismological Society of America, August 2014, v. 104, p. 1750-1762, First published on June 24, 2014, doi:10.1785/0120130201
Bulletin of the Seismological Society of America, August 2014, v. 104, p. 1846-1859, First published on July 22, 2014, doi:10.1785/0120140007
Bulletin of the Seismological Society of America, August 2014, v. 104, p. 2073-2090, First published on June 17, 2014, doi:10.1785/0120130234
Bulletin of the Seismological Society of America, June 2014, v. 104, p. 1299-1328, First published on May 20, 2014, doi:10.1785/0120120322
Bulletin of the Seismological Society of America, April 2014, v. 104, p. 972-984, First published on March 11, 2014, doi:10.1785/0120130078
Bulletin of the Seismological Society of America, December 2013, v. 103, p. 3104-3114, First published on October 8, 2013, doi:10.1785/0120130042
Modeling Earthquake Rate Changes in Oklahoma and Arkansas: Possible Signatures of Induced Seismicity
Andrea L. Llenos and Andrew J. Michael
The rate of ML≥3 earthquakes in the central and eastern United States increased beginning in 2009, particularly in Oklahoma and central Arkansas, where fluid injection has occurred. We find evidence that suggests these rate increases are man‐made by examining the rate changes in a catalog of ML≥3 earthquakes in Oklahoma, which had a low background seismicity rate before 2009, as well as rate changes in a catalog of ML≥2.2 earthquakes in central Arkansas, which had a history of earthquake swarms prior to the start of injection in 2009. In both cases, stochastic epidemic‐type aftershock sequence models and statistical tests demonstrate that the earthquake rate change is statistically significant, and both the background rate of independent earthquakes and the aftershock productivity must increase in 2009 to explain the observed increase in seismicity. This suggests that a significant change in the underlying triggering process occurred. Both parameters vary, even when comparing natural to potentially induced swarms in Arkansas, which suggests that changes in both the background rate and the aftershock productivity may provide a way to distinguish man‐made from natural earthquake rate changes. In Arkansas we also compare earthquake and injection well locations, finding that earthquakes within 6 km of an active injection well tend to occur closer together than those that occur before, after, or far from active injection. Thus, like a change in productivity, a change in interevent distance distribution may also be an indicator of induced seismicity.
The Greenville Fault: Preliminary Estimates of Its Long‐Term Creep Rate and Seismic Potential
James J. Lienkaemper, G. Robert Barry, Forrest E. Smith, Joseph D. Mello, and Forrest S. McFarland
Once assumed to be locked, we show that the northern third of the Greenville fault (GF) creeps at 2 mm/yr, based on 47 yr of trilateration net data. This northern GF creep rate equals its 11 ka slip rate, suggesting a low strain accumulation rate. In 1980, the GF, easternmost strand of the San Andreas fault system east of San Francisco Bay, produced an Mw 5.8 earthquake with a 6 km surface rupture and dextral slip growing to ≥2 cm on cracks over a few weeks. Trilateration shows a 10 cm post‐1980 transient slip ending in 1984. Analysis of 2000–2012 crustal velocities on continuous Global Positioning System stations, allows creep rates of ∼2 mm/yr on the northern GF, 0–1 mm/yr on the central GF, and ∼0 mm/yr on its southern third. Modeled depth ranges of creep along the GF allow 5%–25% aseismic release. Greater locking in the southern two‐thirds of the GF is consistent with paleoseismic evidence there for large late Holocene ruptures. Because the GF lacks large (>1 km) discontinuities likely to arrest higher (∼1 m) slip ruptures, we expect full‐length (54 km) ruptures to occur that include the northern creeping zone. We estimate sufficient strain accumulation on the entire GF to produce Mw 6.9 earthquakes with a mean recurrence of ∼575 yr. While the creeping 16 km northern part has the potential to produce an Mw 6.2 event in 240 yr, it may rupture in both moderate (1980) and large events. These two‐dimensional‐model estimates of creep rate along the southern GF need verification with small aperture surveys.
Infrequent Triggering of Tremor along the San Jacinto Fault near Anza, California
Tien‐Huei Wang, Elizabeth S. Cochran, Duncan Agnew, and David D. Oglesby
We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 Mw≥7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller‐magnitude, regional event, the 2009 Mw 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 Mw 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love‐wave arrival, when surface‐wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (∼35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20–40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 Mw 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient‐shear stress (17–35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well‐studied San Andreas fault.
A Record of Large Earthquakes during the Past Two Millennia on the Southern Green Valley Fault, California
James J. Lienkaemper, John N. Baldwin, Robert Turner, Robert R. Sickler, and Johnathan Brown
We document evidence for surface‐rupturing earthquakes (events) at two trench sites on the southern Green Valley fault, California (SGVF). The 75–80 km long dextral SGVF creeps ∼1–4 mm/yr. We identify stratigraphic horizons disrupted by upward‐flowering shears and infilled fissures unlikely to have formed from creep alone. The Mason Rd site exhibits four events from ∼1013 CE to the present. The Lopes Ranch site (LR, 12 km to the south) exhibits three events from 18 BCE to present including the most recent event (MRE), 1610±52 yr CE (1σ) and a two‐event interval (18 BCE–238 CE) isolated by a millennium of low deposition. Using OxCal to model the timing of the four‐event earthquake sequence from radiocarbon data and the LR MRE yields a mean recurrence interval (RI or μ) of 199±82 yr (1σ) and ±35 yr (standard error of the mean), the first based on geologic data. The time since the most recent earthquake (open window since MRE) is 402 yr±52 yr, well past μ∼200 yr. The shape of the probability density function (PDF) of the average RI from OxCal resembles a Brownian passage time (BPT) PDF (i.e., rather than normal) that permits rarer longer ruptures potentially involving the Berryessa and Hunting Creek sections of the northernmost GVF. The model coefficient of variation (cv, σ/μ) is 0.41, but a larger value (cv∼0.6) fits better when using BPT. A BPT PDF with μ of 250 and cv of 0.6 yields 30 yr rupture probabilities of 20%–25% versus a Poisson probability of 11%–17%.
Coseismic Slip Distribution of the 2010 M 7.0 Haiti Earthquake and Resulting Stress Changes on Regional Faults
Steeve J. Symithe, Eric Calais, Jennifer S. Haase, Andrew M. Freed, and Roby Douilly
The 12 January 2010 Mw 7.0 Haiti earthquake ruptured the previously unmapped Léogâne fault, a secondary transpressional structure located close to the Enriquillo fault, the major fault system assumed to be the primary source of seismic hazard for southern Haiti. In the absence of a precise aftershock catalog, previous estimations of coseismic slip had to infer the rupture geometry from geodetic and/or seismological data. Here we use a catalog of precisely relocated aftershocks beginning one month after the event and covering the following 5 months to constrain the rupture geometry, estimate a slip distribution from an inversion of Global Positional Systems (GPS), Interferometric Synthetic Aperture Radar (InSAR) and coastal uplift data, and calculate the resulting changes of Coulomb failure stress on neighboring faults. The relocated aftershocks confirm a north‐dipping structure consistent with the Léogâne fault, as inferred from previous slip inversions, but with two subfaults, each corresponding to a major slip patch. The rupture increased Coulomb stresses on the shallow Enriquillo fault parallel to the Léogâne rupture surface and to the west (Miragoâne area) and east (Port‐au‐Prince). Results show that the cluster of reverse faulting earthquakes observed further to the west, coincident with the offshore Trois Baies fault, are triggered by an increase in Coulomb stress. Other major regional faults did not experience a significant change in stress. The increase of stress on faults such as the Enriquillo are a concern, as this could advance the timing of future events on this fault, still capable of magnitude 7 or greater earthquakes.
Subparallel Dipping Faults that Ruptured during the 2008 Wenchuan Earthquake
Eiichi Fukuyama and Ken Xiansheng Hao
We investigated the kinematic rupture along a complicated fault system during the 2008 Wenchuan earthquake. Assuming a slip distribution estimated using Interferometric Synthetic Aperture Radar (InSAR) data by Hao et al. (2009), we tried to fit two near‐fault station seismograms, one of which was located between two subparallel dipping faults. Forward numerical simulation results suggest that two subparallel dipping faults ruptured coseismically during the 2008 Wenchuan earthquake. This result provides a constraint on the dynamic fault interaction of reverse‐fault earthquake, which is different from the branched strike‐slip faults where the free surface does not affect the rupture branching.
Straightening of the Northern San Jacinto Fault, California, as Seen in the Fault‐Structure Evolution of the San Jacinto Valley Stepover
Gayatri Indah Marliyani, Thomas K. Rockwell, Nathan W. Onderdonk, and Sally F. McGill
We investigate a releasing stepover between the Casa Loma and Claremont strands of the northern San Jacinto fault zone to evaluate the late Quaternary structural evolution of the fault zone, and to assess the likelihood of a rupture jumping across the stepover. Our new cone penetration test (CPT) and trench observations along the Claremont fault at Mystic Lake indicate that the main strand of the Claremont fault has jumped nearly a half kilometer westward into the San Jacinto releasing stepover during the late Quaternary. Multiple faults are inferred from the CPT data within a small sag at the northeast side of the stepover that cuts through younger stratigraphy to the west of the basin‐bounding fault near Mystic Lake. Previous seismic‐reflection data also suggest the presence of a young fault that cuts basin strata beneath the middle of Mystic Lake farther west of our study area. Numerous tectono‐geomorphic features observed in satellite and Light Detection and Ranging Digital Elevation Model (LiDAR DEM) imagery are interpreted to delineate the location of the currently active faults, including a zone of faults that cut across the basin from the northern end of the Casa Loma fault to the Claremont fault. Seismicity observations suggest the presence of many faults within the stepover zone. Finally, new paleoseismic data from the Mystic Lake site suggest that some late Holocene earthquakes may have jumped the stepover. All of these observations suggest that the San Jacinto stepover, which has been used as the primary basis for segmenting the northern San Jacinto fault zone, is being bypassed and that the fault zone may now be capable of larger earthquakes than previously expected.
Earthquakes Triggered by Hydraulic Fracturing in South‐Central Oklahoma
Austin A. Holland
In January 2011, a sequence of earthquakes occurred in close proximity to a well, which was being hydraulically fractured in south‐central Oklahoma. The hydraulic fracturing of the Picket Unit B Well 4–18 occurred from 16 January 2011 18:43 through 22 January 16:54 UTC. This vertical well penetrated into the mature Eola‐Robberson oil field. Earthquakes were identified by cross correlating template waveforms from manually identified earthquakes and cross correlating these templates through the entire operation period of the Earthscope USArray Transportable Array (TA) station X34A. This produced a series of 116 earthquakes, which occurred from 17 January 2011 19:06 through 23 January 3:13 UTC with no other similar earthquakes identified at other times prior to or post‐hydraulic fracturing. The identified earthquakes range in local magnitude (ML) from 0.6 to 2.9, with 16 earthquakes ML 2 or greater and a b‐value of 0.98. There is a strong temporal correlation between hydraulic fracturing and earthquakes. This correlation is strengthened because hydraulic fracturing operations ceased for ∼2 days due to bad weather, and earthquakes can be observed to cease during this period and resume after hydraulic fracturing had resumed. Earthquakes were relocated using cross‐correlated phase arrivals and bootstrap iterations of hypoDD. Locations were well constrained for 86 earthquakes. These earthquake locations clearly delineate a fault which strikes ∼166°, subparallel to the mapped minor fault sets in the area, and dips steeply to the west. The earthquakes appear to have occurred at shallow depths from ∼2 to 3 km and within ∼2.5 km horizontally of the well. The first earthquake occurred ∼24 hrs after hydraulic fracturing began at the well. This delay is consistent with the diffusion of pore pressure in the subsurface over a distance of ∼2 km.
Introduction to the Special Issue on the 2011 Tohoku Earthquake and Tsunami
T. Lay, Y. Fujii, E. Geist, K. Koketsu, J. Rubinstein, T. Sagiya, and M. Simons
May 2013 Complete TOC