Three - Dimensional Compressional Attenuation Model ( QP ) for the Salton Trough , Southern California by Guoqing

I present a frequency-independent 3D seismic attenuation model (indicated by Q−1 P ) for the crust of the Salton trough and the adjacent regions in southeastern southern California. The simul2000 tomographic algorithm was used to invert the frequency-independent attenuation operator t values measured from amplitude spectra of 23,378 P-wave arrivals of 1203 events through a recently developed 3D velocity model. The QP model has a uniform horizontal grid spacing of 5 km, and the vertical-node intervals range between 2 and 5 km down to 27 km depth. In general, the QP values increase with depth and agree with the surface geology in the shallow depth layers. LowQP values are observed in the Imperial Valley, California, which are consistent with the sedimentary deposits and may also reflect the presence of pore fluid in the active fault zones, whereas greatly elevated QP values are shown in the surrounding crystalline ranges. The new QP model provides an important complement to the existing velocity models for interpreting structural heterogeneity and fluid saturation of rocks in the study area. Online Material: Figures of checkerboard resolution tests, QP versus VP, and QP versus heat flow. Introduction Seismic attenuation can provide important independent constraints on rock composition, fluid content, and temperature that are distinct from those provided by compressional (P)and shear (S)-wave velocities (Jackson and Anderson, 1970). However, attenuation studies have generally fallen behind velocity inversions because of the much greater scatter exhibited by observed amplitudes. Tomography has been applied to determine the 3D seismic attenuation structure in a manner similar to velocity inversion. The simul2000 program (Eberhart-Phillips, 1990; Thurber, 1993; Thurber and Eberhart-Phillips, 1999) is one of the most widely used algorithms for attenuation tomography. During the inversion, the high-frequency decay rate of direct-wave amplitude spectra is used to determine the whole path attenuation, quantified by the frequency-independent attenuation operator t values. The t values for each source–receiver pair are then inverted for the 3D attenuation structure, indicated by the inverse of quality factor Q, by tracing the ray paths through a given velocity model. The Salton trough is located along the southernmost section of the San Andreas fault (SAF) and is surrounded by crystalline ranges (Sharp, 1982). It sits over several other major fault systems in southern California, including the Imperial fault and the San Jacinto fault zone (SJFZ) (Fig. 1a). In this paper, I present a frequency-independent 3D compressional-wave attenuation model (QP) of the crustal structure near the Salton trough and the adjacent fault zones by tracing rays through a newly developed 3D velocity model by Lin (2013). The QP model provides an important complement to the existing velocity structure for understanding the structural heterogeneity and evaluating the seismic hazards in the study area. Data and Processing The seismic data used in this study are the same as those for the velocity tomography inversion and earthquake relocation studies in Lin (2013), obtained from the Southern California Earthquake Data Center and recorded by the stations (triangles in Fig. 1b) of regional seismic networks, including the Southern California Seismic Network and the ANZA Seismic Network. I apply the simul2000 algorithm (Eberhart-Phillips, 1990; Thurber, 1993; Thurber and Eberhart-Phillips, 1999) for the inversion of the QP model by inverting the frequency-independent attenuation operator t values. Their relation can be expressed by the BSSA Early Edition / 1 Bulletin of the Seismological Society of America, Vol. 104, No. 5, pp. –, October 2014, doi: 10.1785/0120140049