Modeling of the Stress–Strain State and Coseismic Effects of the Epicentral Zone of the Tangshan Earthquake (Northeast China)

The paper presents the results of numerical modeling and analysis of the stress–strain state of the epicentral zone of a strong earthquake in Northeast China that occurred on July 27, 1976, with M_s = 7.8. Many present-day works continue to discuss the reasons for such a strong earthquake, which occurred in tectonic conditions: far from interplate boundaries, inside the Tangshan tectonic block bounded by tectonic faults. However, new published geodynamic, seismological, geophysical, and geodetic data provide confidence in the determining role of fault tectonics in this region. Based on an analysis of the results of modeling of the stress–strain state preceding the Tangshan earthquake with coseismic geophysical and geodetic data, we propose an earthquake rupture formation model. The results of comparing independent shear stress estimates with the results of modeling in the sources of strong earthquakes suggest that areas of tectonic stress concentration are localized in the interfault rupture of the Tangshan fault, reaching maximum values at the termination of fault ruptures σ_i ≈ 50 MPa and τ_xy ≈ 20 MPa. The hypocenter of the main seismic event (taking into account the coordinate determination error) is located in the region of stress intensity 35‒50 MPa and a ratio of main stresses σ_xx/σ_yy ≈ 8–10. It should be expected that these zones are the rupture initiation site, the extent of which depends on the amount of accumulated elastic potential energy of tectonic stresses in the adjacent region. For the Tangshan earthquake, this area corresponds to high-intensity stresses in a band with a length exceeding 30 km and width reaching 4.5 km, exceeding 30 MPa.