Source parametrs and tectonic origin of the 1996 June 1 Tianzhu (Mw=5.2) and 1995 July 21 Yongden (Mw=5.6) earthquakes near the Haiyuan Fault (Gansu, China)

Abstract

The 1996 June 1 Tianzhu (MW=5.2, MS-4.9) and the 1995 July 21 Yongden (MW = 5.6, MS = 5.4) earthquakes are the two largest events recorded in the last 10 years between the 1990 October 20 MS = 5.8 and the recent 2000 June 6 MS = 5.6 earthquakes near the 'Tianzhu seismic gap' on the Haiyuan fault in northeastern Tibet. We use frequency-time analysis (FTAN) to extract the fundamental modes of Love and Rayleigh waves from digital records. A joint inversion of their amplitude spectra and of P-wave first-motion polarities is then performed to calculate the source parameters (focal mechanisms, depths and seismic moments) of these two MS ≃5 earthquakes. Such a joint inversion is tested for the first time. We use IRIS and GEOSCOPE network records for period ranges of 20-40 s for the former event and 35-70 s for the latter. The inversion of the Tianzhu earthquake yields nodal planes with strike, dip and slip of 282°, 72° and 3° and 191°, 87° and 162°, respectively, a focal depth around 12 km and a seismic moment of 0.56 x 1017 N m, consistent with the Harvard CMT calculation, and the alignment and depths of the aftershocks recorded by a local network. We propose two possible tectonic interpretations for this off-fault event. The solution for the Yongden earthquake is consistent with a thrust, with strike, dip and slip of 105°, 45° and 75°, respectively, a focal depth around 6 km and a seismic moment of 2.4 x 1017 N m, also in agreement with the Harvard CMT mechanism, the distribution of the aftershocks recorded by a regional network, and the general tectonic setting that we refine.