Proceedings of the 30 th Monitoring Research Review : Ground-Based Nuclear Explosion Monitoring Technologies , 23-25 Sep 2008 , Portsmouth , VA sponsored by the National Nuclear Security Administration ( NNSA ) and the Air Force Research Laboratory ( AFRL

Seismic waveform data of the Korean Meteorological Administration (KMA) have been analyzed to perform 3-D tomographic travel-time inversions to produce high-resolution 3-D crustal Pand S-wave velocity models for the South Korean peninsula. In a first step, waveform data from 2001 through May 2008 have been analyzed to map the Moho discontinuity below South Korea using refracted Pn travel times. Phase-arrival information from both velocity and accelerometer sensors was collected. The analysis included 270 events throughout the region producing 8,860 phase picks of Pg, Pn, Sg/Lg, and Sn phases. A total of 5,090 P-wave and 3,770 S-wave phases were identified. Using the combination of all available velocity and accelerometer data from the 119 KMA stations, it was possible to estimate depth locations for 226 KMA events. The hypocenters were subsequently used to derive travel-time distance curves based on 1-D velocity models to appraise the quality of the travel-time picks. The analysis produced, respectively, crustal Pand S-wave velocities of 6.13 km/s and 3.57 km/s and upper mantle velocities of 8.02 km/s and 4.48 km/s. The travel-time distance curves were used to determine static corrections for all station locations. After applying static corrections to all observed travel-times, refracted P-wave phases along the Moho boundary were selected from the dataset to estimate the depth and topography of the Moho discontinuity beneath South Korea. The resulting Moho topography reveals a relatively flat interface with depth variations from 28 to 34 km. The shallowest parts are below the Yellow Sea and below the Sea of Japan, while the deepest structure is located below the Yeongnam Massif. Joint inversion for hypocenters and velocity structure was performed to derive 3-D Pand S-wave velocity models for the crust. The inversion was constrained by the depth of the Moho interface derived in the previous step. Using analyst phase picks, it was possible to reduce the variance between observed and calculated travel times by 77%. Compared to preliminary event locations based on the 1-D velocity model, the relocated events indicate crustal seismicity deeper than 20 km beneath the peninsula. Additionally, a gradual decrease in depth of seismicity is observed beneath the Yellow Sea. The obtained 3-D velocity distribution is not correlated with the geologic terrains of the peninsula. The velocity distribution is relatively homogeneous corroborating the good fit of the 1-D velocity model. Upper crust velocity values are estimated at 6.2 km/s and 3.6 km/s for Pand S-waves respectively. These values increase beneath 25 km to 7 km/s for Pand 4.3 km/s for S-waves. Mantle velocities are encountered below 32 km depth reaching 8.5 km/s and 4.8 km/s for Pand S-waves, respectively, corroborating the results of the Moho topography study. Finally, 35 GT5 events were selected from the 270 relocated events using Bondár’s criteria (Bondár et al., 2004). The 35 events are well located within the dense KMA network as required by Bondár’s criteria. 2008 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies