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70
Southern California hypocenter relocation with waveform crosscorrelation, Part 1: Results using the doubledifference method
 Bull. Seism. Soc. Am. 95
, 2005
"... Abstract We obtain precise relative relocations for more than 340,000 southern California earthquakes between 1984 and 2002 by applying the sourcespecific stationterm (SSST) method to existing P and Sphase picks and a differential location method to about 208,000 events within similarevent clu ..."
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Cited by 47 (8 self)
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Abstract We obtain precise relative relocations for more than 340,000 southern California earthquakes between 1984 and 2002 by applying the sourcespecific stationterm (SSST) method to existing P and Sphase picks and a differential location method to about 208,000 events within similarevent clusters identified with waveform crosscorrelation. The entire catalog is first relocated by using existing phase picks, a reference 1D velocity model, and the SSST method of RichardsDinger and Shearer (2000). We also perform separate relocations of Imperial Valley events by using a velocity model more suited to this region. Next, we apply cluster analysis to the waveform crosscorrelation output to identify similarevent clusters. We relocate earthquakes within each similarevent cluster by using the differential times alone, keeping the cluster centroid fixed to its initial SSST location. We estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs; these errors are often as small as tens of meters. In many cases the relocated events within each similarevent cluster align in planar features suggestive of faults. We observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends. In general, the finescale details of the seismicity reveal a great deal of structural complexity in southern California fault systems.
Spectralelement moment tensor inversions for earthquakes in
 Southern California. Bull. Seismol. Soc. Am
, 2004
"... Abstract We have developed and implemented an automated moment tensor inversion procedure to determine source parameters for southern California earthquakes. The method is based upon spectralelement simulations of regional seismic wave propagation in an integrated 3D southern California velocity ..."
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Cited by 37 (15 self)
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Abstract We have developed and implemented an automated moment tensor inversion procedure to determine source parameters for southern California earthquakes. The method is based upon spectralelement simulations of regional seismic wave propagation in an integrated 3D southern California velocity model. Sensitivity to source parameters is determined by numerically calculating the Fréchet derivatives required for the moment tensor inversion. We minimize a waveform misfit function, and allow limited time shifts between data and corresponding synthetics to accommodate additional 3D heterogeneity not included in our model. The technique is applied to three recent southern California earthquakes: the 9 September 2001, ML 4.2 Hollywood event, the 22 February 2003, ML 5.4 Big Bear event, and the 14 December 2001, ML 4.0 Diamond Bar event. Using about half of the available threecomponent data at periods of 6 sec and longer, we obtain focal mechanisms, depths, and moment magnitudes that are generally in good agreement with estimates based upon traditional bodywave and surfacewave inversions.
Full 3D tomography for the crustal structure
 in the Los Angeles region, Bull. Seism. Soc. Am. submitted
, 2006
"... Abstract We apply full 3D tomography (F3DT) to a regional dataset in the Los Angeles area. In F3DT, the starting model as well as the model perturbation is threedimensional and the sensitivity (Fréchet) kernels are computed using numerical schemes that account for the full physics of 3D wave propag ..."
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Cited by 36 (10 self)
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Abstract We apply full 3D tomography (F3DT) to a regional dataset in the Los Angeles area. In F3DT, the starting model as well as the model perturbation is threedimensional and the sensitivity (Fréchet) kernels are computed using numerical schemes that account for the full physics of 3D wave propagation. F3DT can account for the nonlinearity of structural inverse problem through iteration, thus providing the most efficient means for assimilating seismic observations into dynamic groundmotion models. We have successfully applied a scatteringintegral (SI) formulation of F3DT to improve a 3D elastic structure model, Southern California Earthquake Center (SCEC) Community Velocity Model version 3.0 (CVM3.0), in the Los Angeles region. Our data are time and frequencylocalized measurements of phasedelay anomalies relative to synthetics computed from the 3D elastic starting model. The Fréchet kernels for our measurements were computed by convolving the earthquake wavefields generated by point earthquake sources with the receiver Green tensors (RGTs), which are the spatialtemporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations. We inverted 7364 phasedelay measurements of P and S bodywaves using the LSQR method. The revised 3D model, LAF3D, provides substantially better fit to the observed waveform data than the 3D starting model. To
Frechet kernels for imaging regional earth structure based on threedimensional reference models
, 2005
"... Abstract Highresolution images of threedimensional (3D) seismic structures are not only of scientific interest, but also of practical importance in predicting strong ground motion after large earthquakes. Given the source and station distributions, resolutions in current regional seismic tomograph ..."
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Cited by 31 (11 self)
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Abstract Highresolution images of threedimensional (3D) seismic structures are not only of scientific interest, but also of practical importance in predicting strong ground motion after large earthquakes. Given the source and station distributions, resolutions in current regional seismic tomography studies have been limited by two types of simplifying practices: the adoption of highfrequency approximations such as the ray theory and the use of onedimensional (1D) reference (starting) models. We have developed a new approach to compute accurate finitefrequency 3D Fréchet (sensitivity) kernels of observed travel time and amplitude anomalies relative to 3D reference models. In our approach, we use a fourthorder staggeredgrid finitedifference method to model the seismicwave propagation in 3D media, and the reciprocity property of the Green’s tensor to reduce the number of numerical simulations. This approach accounts for the perturbations in compressional and shearwave speeds in the same way, leading to a capability of inverting for the shearwave speed directly from seismic data. The algorithm is readily parallelized to allow for realistic regional highresolution 3D tomography inversions. We have implemented the algorithm for the Southern California Earthquake Center (SCEC) Community Velocity Model, SCEC CVM 3.0, a complex 3D model for Southern California including a number of sedimentary basins. By enabling the inversion of 3D structural perturbations to 3D reference models, our approach provides a practical means of iteratively solving the nonlinear regional tomography problems.
Block models of crustal motion in southern California constrained by GPS measurements,
 J. Geophys. Res.,
, 2005
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Strain Green’s tensors, reciprocity and their applications to seismic source and structure studies, Bull. seism
, 2006
"... Abstract Green’s function approach is widely used in modeling seismic waveform. The representation theorem expresses the wave field as the inner product of the moment tensor and the spatial gradients of the Green’s tensor. Standard practice in waveform calculations has been to compute the Green’s t ..."
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Cited by 28 (14 self)
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Abstract Green’s function approach is widely used in modeling seismic waveform. The representation theorem expresses the wave field as the inner product of the moment tensor and the spatial gradients of the Green’s tensor. Standard practice in waveform calculations has been to compute the Green’s tensors first and then obtain their gradients by numerical differentiation. The reciprocity of the Green’s tensor enables us to express the wave field explicitly in terms of the strain Green’s tensor, a thirdorder tensor composed of the spatial gradients of the Green’s tensor elements. We propose here to use the strain Green’s tensors rather than the Green’s tensors themselves in computing the waveforms. By bypassing the need for Green’s tensors and directly using the strain Green’s tensors, we can improve the computational efficiency in waveform modeling while eliminating the possible errors from numerical differentiation. The strain Green’s tensor elements are also directly related to the partial derivatives of the waveforms with respect to moment tensor elements and structural parameters. Through the inversion of the focal mechanisms of 27 small events in the Los Angeles region, we demonstrate the effectiveness of the strain Green’s tensor database approach in quickly recovering source parameters based on realistic 3D models. We show that the same database can also be used to improve the efficiency and accuracy in computing the Fréchet kernels for tomography inversions.
LOTOS code for local earthquake tomographic inversion: Benchmarks for testing tomographic algorithms
 Bull. Seismol. Soc. Am
, 2009
"... Abstract We present the LOTOS07 code for performing local earthquake tomographic inversion, which is freely available (see the Data and Resources section for the Web site). The initial data for the code are the arrival times from local seismicity and coordinates of the stations. It does not requir ..."
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Cited by 19 (11 self)
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Abstract We present the LOTOS07 code for performing local earthquake tomographic inversion, which is freely available (see the Data and Resources section for the Web site). The initial data for the code are the arrival times from local seismicity and coordinates of the stations. It does not require any information about the sources. The calculations start from absolute location of sources and estimates of an optimal 1Dvelocity model. Then the sources are relocated simultaneously with the 3Dvelocity distribution during iterative coupled tomographic inversions. The code allows results to be compared based on node or cell parameterizations. The synthetic dataset used for testing the code is based on source–receiver configurations from a real experiment in Costa Rica. The travel times for this dataset are computed by 3D tracing through a rather complicated synthetic model and are perturbed with realistic noise. We also present a series of synthetic datasets with unknown sources and velocity models (see the Data and Resources section for the Web site) that can be used as blind benchmarks for testing different tomographic algorithms. We encourage other users of tomography algorithms to join the program on creating benchmarks that can be used to check existing codes.
Can observations of earthquake scaling constrain slip weakening? Geophys
 J. Intl
, 2005
"... Our knowledge of the dynamics of earthquake rupture is based largely on measurements providing earthquake source parameters such as radiated seismic energy and stress drop. The scaling of these parameters over a wide range of earthquake sizes has been used to ..."
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Cited by 16 (4 self)
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Our knowledge of the dynamics of earthquake rupture is based largely on measurements providing earthquake source parameters such as radiated seismic energy and stress drop. The scaling of these parameters over a wide range of earthquake sizes has been used to
Testing community velocity models for southern California using the ambient seismic field
 Bulletin of the Seismological Society ofAmerica
, 2008
"... Abstract We correlate the vertical component of ambient seismic noise data recorded on 56 broadband stations with dense coverage in the greater Los Angeles area to determine stationtostation Green’s functions. These Green’s functions provide an important test of community velocity models (Souther ..."
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Cited by 14 (0 self)
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Abstract We correlate the vertical component of ambient seismic noise data recorded on 56 broadband stations with dense coverage in the greater Los Angeles area to determine stationtostation Green’s functions. These Green’s functions provide an important test of community velocity models (Southern California Earthquake Center [SCEC] CVM 4.0 and CVMH 5.2) used for strong groundmotion prediction for future scenario earthquakes in southern California. Comparisons of the ambientnoise Green’s functions for nearly 300 paths, with those calculated by the finiteelement method in the community velocity models, reveal a strong waveform similarity for the dominant surface waves between 0.1 and 0.2 Hz. We find a mean correlation coefficient between the ambientnoise and finiteelement Green’s functions of 0.62 for the CVM 4.0 and 0.49 for the CVMH 5.2, indicating stronger waveform similarity for CVM 4.0. We also find that for 77 % of the paths, the surface waves in the finiteelement Green’s functions for CVM 4.0 arrive early, suggesting that the CVM 4.0 has velocities in the upper 10 km that are too fast along these paths. The same bias is evident for CVMH 5.2, but is substantially smaller, with only 61 % of the paths too fast. For 67 % of the paths, CVM 4.0 has velocities faster than CVMH 5.2. The time lags we obtain between the ambientnoise and finiteelement Green’s functions provide key information for improving future community velocity models.