Source parameters and tectonic origin of the 1996 June 1 Tianzhu ( M w =5.2) and 1995 July 21 Yongden ( M w =5.6) earthquakes near the Haiyuan fault (Gansu, China)

SUMMARY The 1996 June 1 Tianzhu ( M w =5.2, M s =4.9) and the 1995 July 21 Yongden ( M w =5.6, M s =5.4) earthquakes are the two largest events recorded in the last 10 years between the 1990 October 20 M s =5.8 and the recent 2000 June 6 M s =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 M s ≃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 0.56×10 17  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 2.4×10 17  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.