Source parameters 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)

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