Aseismic fault movement before the 1976 Tangshan earthquake detected by levelling: implications for pre-seismic stress localization?

Levelling before the 1976 Tangshan earthquake (M-L = 7.8, China) shows sign ificant height changes in the Tangshan region. Inversion analysis of the he ight changes observed during 1971-1972 and 1972-1973 reveals two active seg ments on the Jiyunhe fault which truncates the Tangshan earthquake fault to the southwest. The active fault segment of 1971-1972 is characterized by l eft-lateral strike-slip (1.2 +/- 0.2 m) with a length of 37 +/- 3 km, a (do wn-dip) width of 7 +/- 2 km, a dip angle of 84 degrees +/- 2 degrees, a (bo ttom) depth of 17 +/- 2 km and a strike direction of 65 degrees +/- 2 degre es (NW. The active fault segment of 1972-1973 is characterized by left-late ral strike-slip (2.8 +/- 0.2 m) with a length of 75 +/- 5 km, a (down-dip)) width of 8 +/- 2 km, a dip angle of 88 degrees +/- 2 degrees, a (bottom) d epth of 30 +/- 2 km and a strike direction of 47 degrees +/- 2 degrees (NW) . The geometric characteristics of these two active segments are in good ag reement with those obtained from geological investigations and source-mecha nism solutions of the 1976 Ninghe earthquake (M-L = 6.9). No microearthquak es of M-L greater than or equal to 3.0 occurred during 1971-1973 in the Tan gshan region, suggesting that the activity of the fault segments during thi s period was largely aseismic. The results demonstrate that aseismic slip a t the Jiyunhe fault occurred before the 1976 Tangshan earthquake. Evidence for activity of the Jiyunhe fault also comes from the horizontal deformatio n observed by triangulation and trilateration between 1960 and 1971. which dearly indicates relative motion between two flanks of the fault. Stress an alysis reveals that aseismic slip at the Jiyunhe fault during 1972-1973 ind uced significant sheer stress concentration (at a depth of 20 km) within an area of about 40 x 40 km close to the conjunction of the Tangshan and Jiyu nhe faults, and that the Tangshan earthquake hypocentre received a Coulomb failure stress of about 4.5 bar. Our evidence suggests that the 1976 Tangsh an earthquake was triggered or induced by aseismic slip at the Jiyunhe faul t. In addition, all of the large aftershocks of M-L > 6.0 occurred within a reas where the calculated Coulomb failure stress is positive and concentrat ed, indicating that the aftershocks were possibly triggered by the 1976 Tan gshan main shock. This study shows that aseismic fault movement could play an important role in controlling regional stress change and in triggering e arthquakes on surrounding faults.