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)

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 extr act the fundamental modes of Love and Rayleigh waves from digital records. A joint inversion of their amplitude spectra and of P-wave first-motion pol arities is then performed to calculate the source parameters (focal mechani sms, depths and seismic moments) of these two M-s similar or equal to 5 ear thquakes. 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 ev ent and 35-70 a for the latter. The inversion of the Tianzhu earthquake yie lds nodal planes with strike, dip and slip of 282 degrees, 72 degrees and 3 degrees and 191 degrees, 87 degrees and 162 degrees, respectively, a focal depth around 12 km and a seismic moment of 0.56 x 10(17) N m, consistent w ith the Harvard CMT calculation, and the alignment and depths of the afters hocks recorded by a local network. We propose two possible tectonic interpr etations for this off-fault event. The solution for the Yongden earthquake is consistent with a thrust, with strike, dip and slip of 105 degrees, 45 d egrees and 75 degrees, respectively, a focal depth around 6 km and a seismi c moment of 2.4 x 10(17) N m, also in agreement with the Harvard CMT mechan ism, the distribution of the aftershocks recorded by a regional network, an d the general tectonic setting that we refine.