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Fan, WY, Bassett D, Jiang JL, Shearer PM, Ji C.  2017.  Rupture evolution of the 2006 Java tsunami earthquake and the possible role of splay faults. Tectonophysics. 721:143-150.   10.1016/j.tecto.2017.10.003   AbstractWebsite

The 2006 Mw 7.8 Java earthquake was a tsunami earthquake, exhibiting frequency-dependent seismic radiation along strike. High-frequency global back-projection results suggest two distinct rupture stages. The first stage lasted similar to 65 s with a rupture speed of similar to 1.2 km/s, while the second stage lasted from similar to 65 to 150 s with a rupture speed of similar to 2.7 km/s. High-frequency radiators resolved with back-projection during the second stage spatially correlate with splay fault traces mapped from residual free-air gravity anomalies. These splay faults also colocate with a major tsunami source associated with the earthquake inferred from tsunami first-crest back-propagation simulation. These correlations suggest that the splay faults may have been reactivated during the Java earthquake, as has been proposed for other tsunamigenic earthquakes, such as the 1944 Mw 8.1 Tonankai earthquake in the Nankai Trough.

Denolle, MA, Shearer PM.  2016.  New perspectives on self-similarity for shallow thrust earthquakes. Journal of Geophysical Research-Solid Earth. 121:6533-6565.   10.1002/2016jb013105   AbstractWebsite

Scaling of dynamic rupture processes from small to large earthquakes is critical to seismic hazard assessment. Large subduction earthquakes are typically remote, and we mostly rely on teleseismic body waves to extract information on their slip rate functions. We estimate the P wave source spectra of 942 thrust earthquakes of magnitude M-w 5.5 and above by carefully removing wave propagation effects (geometrical spreading, attenuation, and free surface effects). The conventional spectral model of a single-corner frequency and high-frequency falloff rate does not explain our data, and we instead introduce a double-corner-frequency model, modified from the Haskell propagating source model, with an intermediate falloff of f(-1). The first corner frequency f(1) relates closely to the source duration T-1, its scaling follows M0T13for M-w<7.5, and changes to M0T12 for larger earthquakes. An elliptical rupture geometry better explains the observed scaling than circular crack models. The second time scale T-2 varies more weakly with moment, M0T25, varies weakly with depth, and can be interpreted either as expressions of starting and stopping phases, as a pulse-like rupture, or a dynamic weakening process. Estimated stress drops and scaled energy (ratio of radiated energy over seismic moment) are both invariant with seismic moment. However, the observed earthquakes are not self-similar because their source geometry and spectral shapes vary with earthquake size. We find and map global variations of these source parameters.

Goebel, THW, Hauksson E, Shearer PM, Ampuero JP.  2015.  Stress-drop heterogeneity within tectonically complex regions: a case study of San Gorgonio Pass, southern California. Geophysical Journal International. 202:514-528.   10.1093/gji/ggv160   AbstractWebsite

In general, seismic slip along faults reduces the average shear stress within earthquake source regions, but stress drops of specific earthquakes are observed to vary widely in size. To advance our understanding of variations in stress drop, we analysed source parameters of small-magnitude events in the greater San Gorgonio area, southern California. In San Gorgonio, the regional tectonics are controlled by a restraining bend of the San Andreas fault system, which results in distributed crustal deformation, and heterogeneous slip along numerous strike-slip and thrust faults. Stress drops were estimated by fitting a Brune-type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra. The estimates have large scatter among individual events but the median of event populations shows systematic, statistically significant variations. We identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting, changes in average tectonic slip rates, mineralogical composition of the host rocks, as well as the hypocentral depths of seismic events. We observed anomalously high stress drops (>20 MPa) in a small region between the traces of the San Gorgonio and Mission Creek segments of the San Andreas fault. Furthermore, the estimated stress drops are higher below depths of similar to 10 km and along the San Gorgonio fault segment, but are lower both to the north and south away from San Gorgonio Pass, showing an approximate negative correlation with geologic slip rates. Documenting controlling parameters of stress-drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes.

Matoza, RS, Shearer PM, Okubo PG.  2014.  High-precision relocation of long-period events beneath the summit region of Kilauea Volcano, Hawai'i, from 1986 to 2009. Geophysical Research Letters. 41:3413-3421.   10.1002/2014gl059819   AbstractWebsite

Long-period (0.5-5 Hz, LP) seismicity has been recorded for decades in the summit region of Klauea Volcano, Hawaii, and is postulated as linked with the magma transport and shallow hydrothermal systems. To better characterize its spatiotemporal occurrence, we perform a systematic analysis of 49,030 seismic events occurring in the Klauea summit region from January 1986 to March 2009 recorded by the approximate to 50-station Hawaiian Volcano Observatory permanent network. We estimate 215,437 P wave spectra, considering all events on all stations, and use a station-averaged spectral metric to consistently classify LP and non-LP seismicity. We compute high-precision relative relocations for 5327 LP events (43% of all classified LP events) using waveform cross correlation and cluster analysis with 6.4million event pairs, combined with the source-specific station term method. The majority of intermediate-depth (5-15km) LPs collapse to a compact volume, with remarkable source location stability over 23 years indicating a source process controlled by geological or conduit structure.

Kaneko, Y, Shearer PM.  2014.  Seismic source spectra and estimated stress drop derived from cohesive-zone models of circular subshear rupture. Geophysical Journal International. 197:1002-1015.   10.1093/gji/ggu030   AbstractWebsite

Earthquake stress drops are often estimated from far-field body wave spectra using measurements of seismic moment, corner frequency and a specific theoretical model of rupture behaviour. The most widely used model is from Madariaga in 1976, who performed finite-difference calculations for a singular crack radially expanding at a constant speed and showed that (f) over bar (c) = k beta/a, where (f) over bar (c) is spherically averaged corner frequency, beta is the shear wave speed, a is the radius of the circular source and k = 0.32 and 0.21 for P and S waves, respectively, assuming the rupture speed V-r = 0.9 beta. Since stress in the Madariaga model is singular at the rupture front, the finite mesh size and smoothing procedures may have affected the resulting corner frequencies. Here, we investigate the behaviour of source spectra derived from dynamic models of a radially expanding rupture on a circular fault with a cohesive zone that prevents a stress singularity at the rupture front. We find that in the small-scale yielding limit where the cohesive-zone size becomes much smaller than the source dimension, P- and S-wave corner frequencies of far-field body wave spectra are systematically larger than those predicted by the Madariaga model. In particular, the model with rupture speed V-r = 0.9 beta shows that k = 0.38 for P waves and k = 0.26 for S waves, which are 19 and 24 per cent larger, respectively, than those of Madariaga. Thus for these ruptures, the application of the Madariaga model overestimates stress drops by a factor of 1.7. In addition, the large dependence of corner frequency on take-off angle relative to the source suggests that measurements from a small number of seismic stations are unlikely to produce unbiased estimates of spherically averaged corner frequency.

Yang, WZ, Hauksson E, Shearer PM.  2012.  Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981-2010): Temporal Stability of the Style of Faulting. Bulletin of the Seismological Society of America. 102:1179-1194.   10.1785/0120110311   AbstractWebsite

Using the method developed by Hardebeck and Shearer (2002, 2003) termed the HASH method, we calculate focal mechanisms for earthquakes that occurred in the southern California region from 1981 to 2010. When available, we use hypocenters refined with differential travel times from waveform cross correlation. Using both the P-wave first motion polarities and the S/P amplitude ratios computed from three-component seismograms, we determine mechanisms for more than 480,000 earthquakes and analyze the statistical features of the whole catalog. We filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high-quality catalog with approximately 179,000 focal mechanisms. As more S/P amplitude ratios become available after 2000, the average nodal plane uncertainty decreases significantly compared with mechanisms that include only P-wave polarities. In general the parameters of the focal mechanisms have been stable during the three decades. The dominant style of faulting is high angle strike-slip faulting with the most likely P axis centered at N5 degrees E. For earthquakes of M < 2.5, there are more normal-faulting events than reverse-faulting events, while the opposite holds for M > 2.5 events. Using the 210 moment-tensor solutions in Tape et al. (2010) as benchmarks, we compare the focal plane rotation angles of common events in the catalog. Seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty. The common events with relatively large rotation angles are either located around the edge of the (SCSN) network or poorly recorded.

Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.

Konter, JG, Staudigel H, Hart SR, Shearer PM.  2004.  Seafloor seismic monitoring of an active submarine volcano: Local seismicity at Vailulu'u Seamount, Samoa. Geochemistry Geophysics Geosystems. 5   10.1029/2004gc000702   AbstractWebsite

We deployed five ocean bottom hydrophones (OBHs) for a 1-year seismic monitoring study of Vailulu'u Seamount, the youngest and easternmost volcano in the Samoan Archipelago. Four instruments were placed on the summit crater rim at 600-700 m water depth, and one was placed inside the crater at 1000 m water depth. An analysis of the first 45 days of records shows a very large number of seismic events, 211 of them local. These events define a steady background activity of about four seismic events per day, increasing to about 10 events per day during a week of heightened seismic activity, which peaked at 40 events during 1 day. We identified 107 earthquakes, whose arrivals could be picked on all five stations and that are likely located within the seamount, based on their similar waveforms. Two linear trends are defined by 21 of these events. These are extremely well correlated and located, first downward then upward on a steeply inclined plane that is close to the axial plane of the southeast rift as it emerges from the main summit of Vailulu'u. These events resemble volcanotectonic earthquakes from subaerial volcanoes in displaying very coherent seismic waveforms and by showing systematic, narrowly defined progressions in hypocenter locations. We propose that these events reflect brittle rock failure due to magma redistribution in or near a central magma reservoir.

Wolfe, CJ, Okubo PG, Ekstrom G, Nettles M, Shearer PM.  2004.  Characteristics of deep (<= 13 km) Hawaiian earthquakes and Hawaiian earthquakes west of 155.55 degrees W. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000618   AbstractWebsite

[ 1] High precision relocation of earthquakes recorded by the Hawaiian Volcano Observatory (HVO) seismic network provides new information on the characteristics of seismic faulting at this oceanic hot spot. Using waveform cross correlation, we have measured correlation coefficients and travel time differences for a set of 14,605 deep (greater than or equal to 13 km) earthquakes recorded from 1988 to 1998. We find that about half of the analyzed earthquakes are in similar event clusters that delineate fault zones in the lower crust and upper mantle. We suggest that much of this deep seismicity reflects rupture in the brittle lithosphere away from the magma pathways, although at Kilauea the stresses from magma movement may additionally help trigger mantle earthquakes on preexisting faults in regions with high differential ambient stresses. Focal mechanisms of similar event clusters throughout Hawaii display characteristic patterns and appear consistent with the hypothesis that deep earthquakes on preexisting faults reflect the stresses due to volcano loading and flexure. We also present the results of applying cross correlation analyses and relocation to -7000 earthquakes at all depths located west of 155.55degreesW and recorded from 1988 to 1998. The pattern of relocated earthquakes at the Kealakekua fault zone is consistent with the presence of a lowangle detachment on the west flank of Mauna Loa.

Shearer, PM.  1994.  Global Seismic Event Detection Using a Matched-Filter on Long-Period Seismograms. Journal of Geophysical Research-Solid Earth. 99:13713-13725.   10.1029/94jb00498   AbstractWebsite

An image derived by stacking long-period seismograms is used as an empirical matched filter to detect and locate earthquakes. Records from 564 events (m(b) greater-than-or-equal-to 6) recorded at very long periods (T greater-than-or-equal-to 60 s) by the 15 to 20 stations of the International Deployment of Accelerometers (IDA) network are stacked using a method that emphasizes the surface wave arrivals. The first 3 hours of this time versus range image are used to construct a matched filter for continuous application to the IDA data. The output of this filter contains spatial and temporal peaks that define the location and origin time of probable seismic events. Implementation of this technique to 11 years of IDA data from 1981 to 1991 identifies 4061 events. These include 65% of cataloged events Of m(b) greater-than-or-equal-to 5.5. Earthquakes which appear anomalously strong relative to their surface wave magnitudes are mainly located on oceanic transform faults and probably represent unusually slow ruptures. The method successfully detects two eruptions of the El Chichon volcano in southern Mexico, from the anomalous low-frequency energy radiated during the eruptions. In addition, 32 earthquakes are detected which are not in the standard global catalogs. These events appear to be about M(s) = 5 and are mainly located in the southern oceans, where there are gaps in the coverage of the high-frequency networks. Although these earthquakes are probably ''slow'' since they occur mostly on oceanic transform faults, they are not ''silent'' as they also can be observed in higher-frequency data.

Aster, RC, Shearer PM.  1992.  Initial Shear-Wave Particle Motions and Stress Constraints at the Anza Seismic Network. Geophysical Journal International. 108:740-748.   10.1111/j.1365-246X.1992.tb03465.x   AbstractWebsite

We use focal plane solutions to constrain principal stress directions in the vicinity of six Anza Network stations which show evidence for shallow shear wave anisotropy in the vicinity of the Anza seismic gap region of the San Jacinto fault. Faulting near all stations is consistent with approximately N-S maximum compressive stress. Five of these stations show nearly N-S initial particle motion alignment, consistent with the anisotropy-stress relationship expected for stress-aligned microcracks. However, one station (KNW) has a well-defined preferred initial shear wave polarization direction of N40-degrees-W. Although this polarization direction differs dramatically from the local maximum compressive stress direction, it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals, particularly biotite. Thus, anisotropy observed at this station most likely reflects a fixed, palaeostrain alignment of anisotropic minerals and/or microcracks and does not require a dependence on the current stress field.