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

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

Jan 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.136...68z&link_type=abstract

Geophysical Journal International, Volume 136, Issue 1, pp. 68-82.

Astronomy and Astrophysics

Astronomy

1

Fault Movement, Inversion, Levelling Survey, Stress Change, Tangshan Earthquake

Scientific paper

Levelling before the 1976 Tangshan earthquake (M_L=7.8, China) shows significant height changes in the Tangshan region. Inversion analysis of the height changes observed during 1971-1972 and 1972-1973 reveals two active segments on the Jiyunhe fault which truncates the Tangshan earthquake fault to the southwest. The active fault segment of 1971-1972 is characterized by left-lateral strike-slip (1.2+/-0.2 m) with a length of 37+/-3 km, a (down-dip) width of 7+/-2 km, a dip angle of 84 deg+/-2 deg, a (bottom) depth of 17+/-2 km and a strike direction of 65 deg+/-2 deg (NW). The active fault segment of 1972-1973 is characterized by left-lateral 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 deg+/-2 deg, a (bottom) depth of 30+/-2 km and a strike direction of 47 deg+/-2 deg (NW). The geometric characteristics of these two active segments are in good agreement with those obtained from geological investigations and source-mechanism solutions of the 1976 Ninghe earthquake (M_L=6.9). No microearthquakes of M_L>=3.0 occurred during 1971-1973 in the Tangshan region, suggesting that the activity of the fault segments during this period was largely aseismic. The results demonstrate that aseismic slip at the Jiyunhe fault occurred before the 1976 Tangshan earthquake. Evidence for activity of the Jiyunhe fault also comes from the horizontal deformation observed by triangulation and trilateration between 1960 and 1971, which clearly indicates relative motion between two flanks of the fault. Stress analysis reveals that aseismic slip at the Jiyunhe fault during 1972-1973 induced significant shear stress concentration (at a depth of 20 km) within an area of about 40x40 km close to the conjunction of the Tangshan and Jiyunhe faults, and that the Tangshan earthquake hypocentre received a Coulomb failure stress of about 4.5 bar. Our evidence suggests that the 1976 Tangshan earthquake was triggered or induced by aseismic slip at the Jiyunhe fault. In addition, all of the large aftershocks of M_L>=6.0 occurred within areas where the calculated Coulomb failure stress is positive and concentrated, indicating that the aftershocks were possibly triggered by the 1976 Tangshan main shock. This study shows that aseismic fault movement could play an important role in controlling regional stress change and in triggering earthquakes on surrounding faults.

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