THE FAULT RUPTURES OF THE 1976 TANGSHAN EARTHQUAKE SEQUENCE INFERRED FROM COSEISMIC CRUSTAL DEFORMATION

摘要

The Tangshan, China, earthquake with magnitude M-s = 7.8 occurred on 25 July 1976, Two large aftershocks (called the Luanxian and Ningho earthquakes) followed within a few months. The three major events and other small aftershocks make up The complete Tangshan earthquake sequence. Trilateration measurements and precise levelings were made in the Tangshan area before and after these shocks. In this study, the coseismic surface deformation of the Tangshan area is modeled by the finite-element method to infer the fault geometries and slip distributions of the sequence. Results show that most of the crustal deformation of the Tangshan area was associated with right-lateral slip on a NNE-striking mainshock fault rupture plane together with the ruptures of the two major aftershocks, The fault associated with the mainshock rupture can be divided into southern and northern subfaults with different strikes, dips, and rakes, and inhomogeneous slip distributions, The rupture processes are reconstructed here by tile complement of source time functions determined by teleseismic waveform modeling. The Luanxian earthquake was associated with pure normal faulting on a plane dipping 53 degrees N, while the Ningho earthquake was associated with a left-lateral strike-slip faulting with a small normal component. The focal mechanisms of the Tangshan earthquake sequence are consistent with regional tectonics, that is, extensive right-lateral strike-slip fault movement in a NNE direction and associated NE-SW-directed compression. The rupture lengths are 48 Bm fur the mainshock, 18 km for the Luanxian earthquake, and 20 km for the Ningho earthquake. The averaged stress drops are 45 +/- 5.5 bar for the mainshock, 34 +/- 3.2 bar for the Luanxian earthquake, and 15 +/- 1.9 bar for the Ningho earthquake. The high stress drop of the Tangshan earthquake, as compared with other intraplate earthquakes, can be correlated to its long return period inferred from Chinese historical earthquake catalogs. [References: 33]