HR: 16:30h
AN: T54A-03 INVITED     [Abstracts]
TI: Strategies to Solve for a Better Vp/Vs Model Using P and S Arrival Times - Application to Japan
AU: * Zhang, H
EM: hjzhang@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Thurber, C H
EM: thurber@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AB: Knowing 3D Vp/Vs variations is valuable for providing a more complete characterization of the mechanical properties and geological identity of crust and upper mantle materials [Thurber, 1993]. Vp/Vs variations can be reliably determined directly from the Vp and Vs models if they have essentially identical data quality and quantity. In cases where S-wave arrival data are less numerous and of poorer quality than P-wave data, however, Vs will not be as well resolved as Vp, making the interpretation of Vp/Vs variations difficult [Eberhart-Phillips, 1990]. Alternatively, Vp/Vs variations can be determined by the inversion of S-P time differences [Walck, 1988; Thurber, 1993] by assuming the ray paths of P and S waves are identical. We face something of a dilemma concerning the S-wave structure. If we use S arrivals to solve for a Vs model, then we potentially face difficulties inferring Vp/Vs variations [Eberhart-Phillips, 1990]. Conversely, if we use S-P times to solve for a Vp/Vs model, we are somewhat restricted by the ray path assumption and the Vs structure cannot be inferred reliably [Wagner et al., 2005]. To solve this dilemma, we adopt the strategy of solving jointly for Vp, Vs and Vp/Vs using P, S and S-P times. To reduce the effect of the assumption of similar ray paths for P and S waves, we propose to use two strategies in our inversion algorithm: 1) at each iteration, we check ray paths for P and S waves and then remove them from the inversion if they are considerably different as measured by Hausdorff distance; 2) we use the differential S-P times from event pairs at common stations in the same way as double-difference tomography [Zhang and Thurber, 2003] to determine Vp/Vs ratios. By using the differential S-P times, we can remove the effect of different ray paths of P and S waves outside the source region. Also the P- and S-wave ray paths are generally closer to each other near the source region. We apply the new algorithm to the 2004 M6.6 Niigata-Chuetsu earthquake and the subduction zone beneath northern Honshu, Japan, for a comparison to the method of calculating the Vp/Vs ratios by directly dividing Vp by Vs.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Tectonophysics [T]
MN: Fall Meeting 2005