Geophys. J. Int. 12001) 144, 206±220 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) C. Lasserre,1 B. Bukchin,2 P. Bernard,3 P. Tapponnier,1 Y. Gaudemer,1 A. Mostinsky2 and RongDailu 4 1 Laboratoire de Tectonique et MeÂcanique de la LithospheÁre, CNRS UMR 7578, Institut de Physique du Globe de Paris, Paris, France. E-mail:[email protected] 2 International Institute of Earthquake Prediction and Mathematical Geophysics, Moscow, Russia. E-mail: [email protected] 3 Laboratoire de Sismologie, CNRS UMR 7580, Institut de Physique du Globe de Paris, Paris, France 4 Seismological Institute of Lanzhou, China Seismological Bureau, Lanzhou, Gansu, China Accepted 2000 August 25. Received 2000 August 21; in original form 2000 February 22 SUMMARY The 1996 June 1 Tianzhu 1Mw=5.2, Ms=4.9) and the 1995 July 21 Yongden 1Mw=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 1FTAN) to extract the fundamental modes of Love and Rayleigh waves from digital records. A joint inversion of their amplitude spectra and of P-wave ®rst- motion polarities is then performed to calculate the source parameters 1focal mech- anisms, depths and seismic moments) of these two Msc5 earthquakes. Such a joint inversion is tested for the ®rst 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 282u,72u and 3u and 191u,87u and 162u, respectively, a focal depth around 12 km and a seismic moment of 0.56r1017 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 earth- quake is consistent with a thrust, with strike, dip and slip of 105u,45u and 75u, respectively, a focal depth around 6 km and a seismic moment of 2.4r1017 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 re®ne. Key words: earthquake source mechanism, NE Tibet, P waves, seismic gap, seismotectonics, surface waves. surface waves have recently been developed 1e.g. Braunmiller 1 INTRODUCTION et al. 1994; Arvidsson & EkstroÈm 1998, and references therein). The inversion of surface wave amplitude spectra, based on the Until now, the inversion of surface wave amplitude spectra has method described by Bukchin 11989, 1990, 1995), has already never been used for source studies of such small events. Using demonstrated its reliability in the source mechanism deter- surface wave amplitude spectra only, however, does not pro- mination of large earthquakes 1Msi6.5). In particular, three vide a unique focal mechanism solution 1Mendiguren 1977). major earthquakes of central Asia, the Ms=6.9 1988 Spitak Additional information is needed to constrain the uniqueness event 1Bukchin et al. 1994), the Ms=7.4 1992 Susamyr event of the solution; this was provided by long-period 1Ti100 s) 1Gomez et al. 1997a) and the Ms=6.5 1992 Barisakho event surface wave phase spectra in the aforementioned studies of 1Gomez et al. 1997b), have been studied with this method, which large earthquakes and by P-wave ®rst-motion polarities in proves to be a good complement to classical body wave modelling this study. We test, for the ®rst time, the feasibility of a joint 1Gomez 1998). To estimate the source parameters of relatively inversion of surface wave amplitude spectra and P-wave ®rst- weak earthquakes 1Mwc5), for which body wave inversion motion polarities to calculate the source parameters of two generally fails or reaches its limit, different methods using moderate-sized earthquakes 1Mwc5.5, Msc5). 206 # 2001 RAS Msc5 Tianzhu and Yongden events, China 207 The two earthquakes we investigate are located in north- Bukchin 1990). In the case of a ®nite source, eq. 11) describes eastern Tibet: the Tianzhu earthquake 11996 June 1, Mw=5.2, the spectrum of displacements for periods much longer than Ms=4.9) and the Yongden earthquake 11995 July 21, Mw=5.6, the source duration. The low-frequency displacement energy is Ms=5.4). Both of them occurred in regions adjacent to the mainly carried by surface waves. Only low-frequency displace- c1000 km longHaiyuan fault system, the principal left-lateral ments transported by the fundamental modes of the Love strike-slip fault accommodatingthe eastward component of and Rayleigh waves will be considered here. We assume that movement between Tibet and the Gobi-Ala Shan Platform due variations of medium properties in any horizontal direction are to India±Asia collision 1Fig. 1; Tapponnier & Molnar 1977; small over distances alongthe wave paths of the order of a Zhang et al. 1988a,b). In 1920 and 1927, this fault system wavelength, so that the structure of the medium can be well experienced two Mi8 earthquakes, 300 km apart 1Fig. 1; Deng approximated by a weak horizontally inhomogeneous model et al. 1986; Zhang et al. 1987; Gaudemer et al. 1995). The 1Woodhouse 1974; Babich et al. 1976). The scatteringof energy Tianzhu and Yongden events took place near the 220 km long at subvertical boundaries is considered negligible; this has been quiescent western segment of the Haiyuan faultÐknown as veri®ed for a large variety of models 1Levshin 1985). Under the `Tianzhu seismic gap', a site of signi®cant seismic hazard, these assumptions, the spectral parameters of the surface waves Mi7.5 1Gaudemer et al. 1995; Lasserre et al. 1999)Ðbetween can be determined locally. The spectrum of the Green's function the 1920 and 1927 ruptures. A better understandingof their for Love and Rayleigh waves in eq. 11) depends on 11) the mechanism, in relation to the current deformation of the area, velocity and density models in the source region and under the was thus required, which motivated this study. Between the seismic stations recordingthe earthquake, 12) the mean phase 1990 October 20 Ms=5.8 and the recent 2000 June 6 Ms=5.6 velocity of waves alongthe path between the source and the earthquakes, both at the eastern end of the gap 1Fig. 1), the receiver and 13) geometrical spreading 1Levshin 1985; Bukchin Tianzhu and Yongden events are in fact the largest ones 1990). If all of these characteristics are known, as well as the recorded near the gap. Furthermore, the Tianzhu earthquake is origin time of the earthquake and the hypocentre location, the largest event to occur less than 3 km from the Haiyuan fault recording ui1x, v) at different points x on the Earth's surface trace, almost at the centre of the gap, since 1920. Its epicentre for a set of frequencies v gives, from eq. 11), a system of linear lies next to a 10 km wide pull-apart step between two segments equations from which the elements of the seismic moment of the fault 1Tianzhu basin, Fig. 1) 1Gaudemer et al. 1995), which tensor Mij can be retrieved. The mean phase velocity of surface could act as a nucleation zone for a future large earthquake in waves, however, is usually poorly known. To calculate the the gap 1King & Nabelek 1985; Barka & Kadinsky-Cade 1988; source parameters, only the amplitude spectrum |u1x, v)|, that Nielsen & Knopoff 1998). The Yongden event is further away is, the absolute value of the displacement spectrum u1x, v) from the Haiyuan fault but located less than 50 km northeast described in eq. 11), is used, as its value does not depend on of Lanzhou, a city of three million people. The fault responsible this average phase velocity. It is important to note also that for this earthquake was previously unknown. the value of |u1x, v)| is not affected by any error in epicentre We retrieved the Love and Rayleigh fundamental modes location 1Bukchin 1990). from the IRIS and GEOSCOPE broad-band digital records. A We assume now that the velocity and density of the joint inversion of their amplitude spectrum and of P-wave ®rst- propagating medium are given near the source and under each motion polarities at worldwide stations or at Chinese regional station at the free surface recordingthe seismic event and that stations was then performed to calculate the focal mechanism all of the followinghypotheses are valid: an instant point source parameters 1nodal plane strike, dip and slip angles), the depths on an ideal plane fault, with known origin time and epicentre and the seismic moments of the Tianzhu and Yongden events. location, and a medium with weak lateral inhomogeneities. The The results are compared to available Harvard CMT solutions seismic source is then completely de®ned by its double-couple and, for the Tianzhu earthquake, to the Lanzhou Seismological depth, its focal mechanism parametersÐwhether expressed by Institute solution. The focal mechanisms of the Tianzhu and the strike, dip and slip angles of the nodal planes or by the two Yongden events are then discussed with regard to the distri- unit vectors P and T in the directions of principal compression bution of their aftershocks, recorded by a local network and by and the tension axes, respectivelyÐand its seismic moment M0. the Chinese regional network, respectively, and interpreted in To estimate these parameters, we construct a grid in the 4-D relation to the regional tectonics.