Seismic imaging of source region in the 1976 M_s 7.8 tangshan earthquake sequence and its implications for the seismogenesis of intraplate earthquakes

One of the most devastating earthquakes in the last 100 years is the M_s 7.8 Tangshan intraplate earthquake occurred on 28 July 1976, caused more than 240,000 deaths, and razed the city of Tangshan. To investigate the possible dynamic mechanisms of the Tangshan earthquake, we conducted high-resolution seismic imaging with an extensive dataset of arrival times of P and S waves from locally recorded microearthquakes at 56 stations. Detailed 3D images of V_P, V_S, and V_P=V_S show a sharp velocity contrast to about 25 km depth coinciding with a high rate of seismicity. This contrast likely corresponds to the high-angle strike-slip Tangshan fault that ruptured during the earthquake. The rupture zone is roughly defined by high-V_P, high-V_S, and low-V_P=V_S anomalies that suggest that the media surrounding the fault is strong enough to accumulate the strain energy necessary for the coseismic slip related to the Tangshan earthquake. Migration of magma from the mantle into the lower crust could be potentially a key factor in the dynamics of large intraplate earthquakes within the North China craton.