High - Resolution Seismicity Study of the Yakima Fold and Thrust Belt Region , Washington

This report presents the final results from this contract to analyze the seismicity of the Yakima Fold and Thrust Belt (YFTB) region, Washington (Figure 1). The contract work involved the following: Apply conventional seismic tomography, using the public domain code simul2000 (Thurber and Eberhart-Phillips, 1999), to the YFTB region (121.25°W to 117.5°W and 45.5°N to 47.5°N) dataset of approximately 5000 earthquakes using available P-wave catalog earthquake picks and refraction picks, carrying out a trade-off analysis to estimate optimal model smoothing parameter, and using four different starting velocity models. Apply double-difference (DD) seismic tomography, using the code tomoDD (Zhang and Thurber, BSSA, 2003), which requires a commercial use license, to the YFTB region dataset using available P-wave catalog earthquake picks and their differential times and refraction picks using the best final model from Phase 2, carrying out a trade-off analysis to estimate optimal model smoothing parameter. Evaluate earthquake location uncertainties using estimate from simplified bootstrap test. The work for the contract is complete. My efforts can be divided into the following stages: (1) conversion of thousands of files of picks for individual earthquakes from the Pacific Northwest Seismic Network (PNSN; provided by Alan Rohay) to a usable format; (2) compilation of first-P-arrival picks from several refraction experiments and other identified explosion sources in the YFTB region; (3) tomographic inversion of the arrival time data with the simul2000 code to determine a three-dimensional (3-D) model of the P-wave velocity (Vp) structure of the crust and earthquake hypocenters in the YFTB region, using four different starting velocity models and multiple subsets of data from stages (1) and (2), and using a trade-off analysis to define optimal inversion control parameters for the preferred model; (4) plotting of the simul2000 3-D models and hypocenters, along with a set of digitized faults converted to the underlying Cartesian coordinate system used in the inversions; (5) assessment of earthquake location differences and uncertainties from simul2000; (5) tomographic inversion of the arrival time data with tomoDD to determine a 3-D model of the P-and S-wave velocities (Vp and Vs) of the crust and earthquake hypocenters in the YFTB region, using a trade-off analysis to define optimal inversion control parameters, with multiple sets and subsets of data and two different starting velocity models; (6) plotting of 3-D tomoDD models, hypocenters, and earthquake depth histogram; (7) assessment of the relationships among available focal mechanisms, the distribution of seismicity, and the 3-D …