High-resolution tomography of P-wave velocity structures in the Haicheng region: Implications for the seismogenesis of the 1975 M_S7.3 Haicheng earthquake

The 1975 Haicheng earthquake represents the first successful prediction of a major earthquake in China and is the only earthquake forecast officially recognized by the United Nations. Understanding the crustal structure in the Haicheng region is critical for determining the seismogenic mechanisms of large earthquakes. Pg-wave tomography of the Haicheng region was used to obtain the upper crustal structure at depths of 5–10 km, in which lateral velocity variations indicate differences in tectonic activity within the seismogenic layer. A dataset comprising 62,610 Pg-wave arrival times was used to obtain high-resolution seismic velocity and anisotropy images of the upper crust in the Haicheng region. The tomography results indicate that a distinct high-velocity anomaly is located in the region that produced the Haicheng and Xiuyan earthquakes, as well as a few small earthquakes at the southern end of the Jinzhou Fault. This suggests that a high-velocity asperity beneath the Haichenghe Fault was able to accumulate stress due to long-term tectonic loading, eventually producing the Haicheng earthquake. The seismogenesis of Haicheng earthquake can also be used to explain other large earthquakes in the slowly deforming eastern region of China.