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Ishii, M, Shearer PM, Houston H, Vidale JE.  2005.  Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature. 435:933-936.   10.1038/nature03675   AbstractWebsite

The disastrous Sumatra-Andaman earthquake of 26 December 2004 was one of the largest ever recorded. The damage potential of such earthquakes depends on the extent and magnitude of fault slip. The first reliable moment magnitude estimate(1) of 9.0 was obtained several hours after the Sumatra-Andaman earthquake, but more recent, longer-period, normal-mode analyses have indicated that it had a moment magnitude of 9.3, about 2.5 times larger(2). Here we introduce a method for directly imaging earthquake rupture that uses the first-arriving compressional wave and is potentially able to produce detailed images within 30 min of rupture initiation. We used the Hi-Net seismic array in Japan as an antenna to map the progression of slip by monitoring the direction of high-frequency radiation. We find that the rupture spread over the entire 1,300-km-long aftershock zone by propagating northward at roughly 2.8 km s(-1) for approximately 8 minutes. Comparisons with the aftershock areas of other great earthquakes indicate that the Sumatra-Andaman earthquake did indeed have a moment magnitude of similar to 9.3. Its rupture, in both duration and extent, is the longest ever recorded.

Shearer, PM, Flanagan MP, Hedlin MAH.  1999.  Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure. Journal of Geophysical Research-Solid Earth. 104:7229-7242.   10.1029/1998jb900119   AbstractWebsite

Long-period SS precursors result from underside reflections off upper mantle discontinuities. By grouping and stacking global seismic data by SS bounce point location it is possible to map lateral variations in depths to the 410- and 660-km discontinuities, a process analogous to common midpoint (CMP) stacking in reflection seismology. Because this method assumes horizontal reflectors, energy arriving from dipping or intermittent reflectors may not be correctly imaged. To address this possibility, we experiment with techniques based on migration processing of shallow seismic reflection data. The problem is complicated by the uneven distribution of sources and receivers for the SS precursor observations, but the data are sufficiently dense beneath the northwest Pacific Ocean that reasonably good coverage can be obtained for this region. We parameterize the model as a grid of point scatterers in latitude, longitude, and depth (from the surface to 1000 km depth) and compute travel times from each grid point to the source and receiver locations. These times are used to construct a matrix equation that yields predicted SS precursor waveforms from the assumed scatterers. To recover the model, we experiment with both simple back projection and full inversions using a conjugate gradient method. Tests on noise-free synthetic data (generated using the same source-receiver distribution as the actual data) suggest that detailed resolution of discontinuity structure is possible, at horizontal scales much smaller than the Fresnel zone. However, the real data do not produce coherent results unless some degree of horizontal smoothing is imposed, at least partially defeating the purpose of this approach. Results for the northwest Pacific find structure on the 410- and 660-km discontinuities and hints of intermittent reflectors at other depths. Random resampling tests, however, suggest that most of these features are not reliably resolved, with the exception of a depression on the 660-km discontinuity seen in the northwest Pacific. Our experiments show that it is unlikely that small-scale structure on the 660-km discontinuity near subducting slabs causes significant bias in maps of the large-scale 660-km topography derived from long-period SS precursor observations.

Shearer, PM.  1998.  Evidence from a cluster of small earthquakes for a fault at 18 km depth beneath Oak Ridge, southern California. Bulletin of the Seismological Society of America. 88:1327-1336. AbstractWebsite

A swarm of about 50 small earthquakes (M similar to 1.5) occurred for a month during 1989 beneath Oak Ridge, southern California. Location accuracy using conventional analysis of arrival-time picks is limited for these events by the weak, emergent nature of arrivals on the available seismograms. However, waveform crosscorrelation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations. The relocated events form a small cluster about 1 km across at a depth of similar to 18 km and are aligned along a plane that dips 35 degrees to the northwest. Estimated standard errors for the locations are generally less than 50 m. The time evolution of the sequence shows a gradual migration of activity away from its initiation point. Three additional events occurred several months later; these align along the same plane but are displaced about 500 m to the southeast from the main swarm. Reliable fault-plane solutions are difficult to obtain for these events due to the small number of station records available, the limited range of takeoff angles, and the weak initial arrivals on many of the seismograms, Stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average P first motions. Analysis of these first-motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity. The seismicity alignment may represent the extension of the Simi fault, in which case some shallowing of the fault dip would be required to match the observed 35 degrees dip at 18 km.

Lin, GQ, Shearer PM.  2009.  Evidence for water-filled cracks in earthquake source regions. Geophysical Research Letters. 36   10.1029/2009gl039098   AbstractWebsite

We identify lowered Vp/Vs ratios near earthquake source regions in southern California using observations from a seismic tomography model and high-resolution local Vp/Vs estimates using waveform cross-correlation data from within similar event clusters. The median tomographic Vp/Vs ratio is 1.716 +/- 0.008 at all the relocated crustal earthquake locations, compared to the background median value of 1.729 +/- 0.007 for the tomography model, although the error estimates overlap slightly. The median in situ Vp/Vs ratio of 1.673 +/- 0.022 within the similar event clusters suggests that tomographic studies are overestimating Vp/Vs at source regions. Interpretation of Vp/Vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute Vp and Vs velocities in the tomography model. However, the low Vp/Vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water-filled cracks with several percent porosity in earthquake source regions in southern California, which likely has an effect on faulting and earthquake activity. Citation: Lin, G., and P. M. Shearer (2009), Evidence for water-filled cracks in earthquake source regions, Geophys. Res. Lett., 36, L17315, doi: 10.1029/2009GL039098.

Shearer, PM, Lin GQ.  2009.  Evidence for Mogi doughnut behavior in seismicity preceding small earthquakes in southern California. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005982   AbstractWebsite

We examine the average space-time behavior of seismicity preceding M 2-5 earthquakes in southern California from 1981 to 2005 using a high-resolution catalog and identify regions of enhanced activity in a 1-day period preceding larger earthquakes at distances comparable to their predicted source radii. The difference in precursory behavior between large and small earthquakes is subtle but statistically significant when averaged over many earthquakes, and it has similarities to the "Mogi doughnut'' seismicity pattern observed to occur prior to some M 6 and larger earthquakes. These results indicate that many standard earthquake triggering models do not account for all of the processes involved in earthquake occurrence.

Lin, GQ, Shearer P.  2007.  Estimating local V-P/V-s ratios within similar earthquake clusters. Bulletin of the Seismological Society of America. 97:379-388.   10.1785/0120060115   AbstractWebsite

We develop and test a method to estimate local VP/V, ratios for compact similar earthquake clusters using the precise P and S differential times obtained using waveform cross-correlation. We demonstrate how our technique works using synthetic data and evaluate likely errors arising from near-source takeoff angle differences between P and S waves. We use a robust misfit function method to compute VPA, ratios for both synthetic data sets and several similar event clusters in southern California, and use a bootstrap resampling approach to estimate standard errors for real data. Our technique has higher resolution for near-source VP/V, ratios than typical tomographic inversion methods and provides constraints on near-fault rock properties.

RichardsDinger, KB, Shearer PM.  1997.  Estimating crustal thickness in southern California by stacking PmP arrivals. Journal of Geophysical Research-Solid Earth. 102:15211-15224.   10.1029/97jb00883   AbstractWebsite

We use observations of the Moho-reflected phase PmP to constrain crustal thicknesses and upper mantle velocities in southern California. We stack normalized absolute values of seismograms from local events in time and range bins after aligning on the initial P arrival and applying a range correction to adjust the various source depths to the surface. Although most individual. seismograms do not allow accurate determination of a PmP arrival time, imaging the whole data set in this way shows clear PmP arrivals at ranges from about 90 km to over 250 km. PmP-Pg and PmP-Pn differential times can be measured from the image and used to estimate the Moho depth and upper mantle velocity. For southern California, we obtain an average crustal thickness of 28 km and an upper mantle velocity of 7.8 km/s. Next, we map lateral variations by repeating this procedure for stacks of subsets of the data in which the traces are grouped in caps by Moho reflection point. Estimates of Moho depth range from 18 km in the Salton Trough to 33 km beneath the eastern Transverse Ranges and 36 km beneath the southernmost Sierra Nevada. The upper mantle velocities generally increase from southwest to northeast across the region. We also map postcritical PmP/P-g amplitudes which vary by a factor of 4 with the highest amplitudes in the northwest Mojave Desert. Preliminary experiments with stacking SmS arrivals indicate strongly correlated SmS and PmP amplitude variations. These results provide a guide to source-receiver paths that may produce anomalously strong Moho-reflected phases during future earthquakes.

Shaw, JH, Shearer PM.  1999.  An elusive blind-thrust fault beneath metropolitan Los Angeles. Science. 283:1516-1518.   10.1126/science.283.5407.1516   AbstractWebsite

Seismic reflection profiles, petroleum wells, and relocated earthquakes reveal the presence of an active blind-thrust fault beneath metropolitan Los Angeles. A segment of this fault likely caused the 1987 Whittier Narrows (magnitude 6.0) earthquake. Mapped sizes of other fault segments suggest that the system is capable of much larger (magnitude 6.5 to 7) and more destructive earthquakes.

Prieto, GA, Shearer PM, Vernon FL, Kilb D.  2004.  Earthquake source scaling and self-similarity estimation from stacking P and S spectra. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003084   AbstractWebsite

[1] We study the scaling relationships of source parameters and the self-similarity of earthquake spectra by analyzing a cluster of over 400 small earthquakes (M-L = 0.5 to 3.4) recorded by the Anza seismic network in southern California. We compute P, S, and preevent noise spectra from each seismogram using a multitaper technique and approximate source and receiver terms by iteratively stacking the spectra. To estimate scaling relationships, we average the spectra in size bins based on their relative moment. We correct for attenuation by using the smallest moment bin as an empirical Green's function (EGF) for the stacked spectra in the larger moment bins. The shapes of the log spectra agree within their estimated uncertainties after shifting along the omega(-3) line expected for self-similarity of the source spectra. We also estimate corner frequencies and radiated energy from the relative source spectra using a simple source model. The ratio between radiated seismic energy and seismic moment ( proportional to apparent stress) is nearly constant with increasing moment over the magnitude range of our EGF-corrected data (M-L = 1.8 to 3.4). Corner frequencies vary inversely as the cube root of moment, as expected from the observed self- similarity in the spectra. The ratio between P and S corner frequencies is observed to be 1.6 +/- 0.2. We obtain values for absolute moment and energy by calibrating our results to local magnitudes for these earthquakes. This yields a S to P energy ratio of 9 +/- 1.5 and a value of apparent stress of about 1 MPa.

Astiz, L, Shearer PM.  2000.  Earthquake locations in the inner Continental Borderland, offshore southern California. Bulletin of the Seismological Society of America. 90:425-449.   10.1785/0119990022   AbstractWebsite

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-l norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island (M-L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside (M-L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth.

Richards-Dinger, KB, Shearer PM.  2000.  Earthquake locations in southern California obtained using source-specific station terms. Journal of Geophysical Research-Solid Earth. 105:10939-10960.   10.1029/2000jb900014   AbstractWebsite

We relocate 297,400 events recorded by the Southern California Seismic Network (SCSN) between 1975 and 1998 using spatially varying station terms to improve relative location accuracy. Our method uses existing SCSN P and S picks, a smooth one-dimensional velocity model, and an iterative grid search approach based on the L1 norm. We apply empirical corrections for three-dimensional structure by computing station timing corrections that continuously vary as a function of source position. Station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor (Delaunay) tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved. Our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions. Median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth. Our locations exhibit much less scatter, particularly in depth, than those of the SCSN catalog and a greater tendency to align into linear and planar features suggestive of fault structures. Our results appear comparable to, and in some cases better than, previous SCSN relocation studies using joint-hypocenter-velocity inversion techniques. Plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity We observe no simple relationship between the maximum depth of seismicity and surface geology.