Crustal Seismic Velocities of the Baikal, Amur, and Sakhalin Regions, Eastern Russia

Using combined regional network phase data from eastern Russia, we have developed a preliminary crustal seismic velocity model (Pg and Sg or Lg onset phases) for the Baikal, Chita, Amur, and Sakhalin regions in southeastern Russia. The velocities were calculated using a moving window grid search that determined the best-fit velocity which minimized the RMS sum of the residuals from a number of well located events in each grid square. The results yield fast velocities in the Aldan shield, slightly elevated velocities in the Baikal rift zone, and slow velocities in Sakhalin. The results of our method are supported by independent Russian long- and short-range refraction lines and other travel-time studies. The southern Aldan shield, a Precambrian platform, has Pg velocities of 6.2 to 6.3 km/sec and Sg velocities of 3.5-3.7 km/sec. Pg velocities along the axis of the Baikal rift zone are 6.22 km/sec, slightly greater than the 6.14 km/sec off-axis velocities to the southeast. Russian refraction studies have also indicated this trend. In Buryatia and the Chita region, a deformed Paleozoic to Mesozoic fold and accretionary belt, Pg velocities are about 6.08-6.15 km/sec, decreasing to 6.0 to 6.1 km/sec in Mongolia and northeast China. The Amur district, a complex of old platform and superposed fold belts, has slightly elevated velocities with Pg ranging from 6.0 to 6.2 km/sec. This is in dramatic contrast to Pg velocities along the axis of Sakhalin Island, generally considered a Mesozoic to present fold and accretionary belt, which range between 5.65 and 5.85 km/sec. Sg velocities are similarly reduced, 3.3-3.5 km/sec in Sakhalin as opposed to 3.5 to 3.6 km/sec in Amur. The boundary between these velocity regions appears to be very sharp. These detailed velocities can be used to further improve epicentral locations in eastern Russia and identify active faults and other seismic hazards.