Crustal structure in southern Korea from joint analysis of regional broadband waveforms and travel times

Regional broadband waveforms and travel times of seismic phases are combined to model crustal structure in regions where the signal-to-noise ratio (SNR) of waveforms is low. The utilization of travel-time data enables us to overcome the problem occurring in waveform inversion with low SNR data: the distortion of deep velocity structure. The genetic algorithm (GA) adopted as a search algorithm is useful in regions where there is little a priori information about crustal structure. After verifying the robustness of the joint inversion technique by numerical tests, we applied the technique to southern Korea in order to get a simple one-dimensional average crustal model, using broadband waveforms bandpassed between 0.03 and 0.3 Hz and travel times of Pg, PmP, and Pn waves recorded at ten broadband and five short-period stations from the 2 June 1999 Gyeongju earthquake (M-L 3.4), Korea. The P-wave velocities in the resulting three-layered crustal model are 5.67 +/- 0.06, 6.05 +/- 0.05, 6.67 +/- 0.02, and 7.88 +/- 0.02 km/sec from the top layer to the half-space, respectively, and the depths of layers are 5.1 +/- 0.8, 16.7 +/- 1.0, and 31.9 +/- 1.0 km. Synthetic waveforms and travel-time curves corresponding to the resulting velocity model fit observed seismograms generally well. The synthetics are preceded by the observed waveforms in the Southwestern part of Korean Peninsula, inferring the presence of a high-velocity zone in that area.