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As an important segment of the North China Craton, the Trans-North China Orogen (TNCO) has experienced strong tectonic deformation and magmatic activities since the Cenozoic and is characterized by significant seismicity. To understand the mechanism of the crustal deformation and seismic hazards, we determined the crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (the fast polarization direction φ and splitting time τ) beneath the TNCO and its adjacent areas by analyzing receiver function data recorded by a dense seismic array. The (H, κ) and (φ, τ) at a total of 309 stations were measured, respectively. The Moho depth varies from ∼30 km beneath the western margin of the Bohai bay basin to the maximum value of ∼48 km beneath the northern Lüliang Mountain, which shows the positive and negative correlations with the elevation and the Bouguer anomaly. The average φ is roughly parallel to the strikes of the faults, grabens and Mountains in this study area, whereas a rotating distribution is shown around the Datong-Hannuoba volcanic regions. Based on the φ measured from the Moho Ps and SKS/SKKS phases, we propose that the crustal deformation and seismic hazards beneath the TNCO could be due to the counterclockwise rotation of the Ordos block driven by the far-field effects of the India-Eurasian collision.

On July 20, 1995, an earthquake of ML=4.1 occurred in Huailai basin, northwest of Beijing, with epicenter coordinates 40.326°N, 115.448°E and focal depth 5.5 km. Following the main shock, seismicity sharply increased in the basin. This earthquake sequence was recorded by Sino-European Cooperative Huailai Digital Seismograph Network (HDSN) and the hypocentres were precisely located. About 2 hours after the occurrence of the main shock, a smaller event of ML=2.0 took place at 40.323°N, 115.447°E with a focal depth of 5.0 km, which is very close to the main shock. Using the ML=2.0 earthquake as an empirical Green's function, a regularization method was applied to retrieve the far-field source-time function (STF) of the main shock. Considering the records of HDSN are the type of velocity, to depress high frequency noise, we removed instrument response from the records of the two events, then integrated them to get displacement seismogram before applying the regularization method. From the 5 field stations, P phases in vertical direction which mostly are about 0.5 s in length were used. The STFs obtained from each seismic phases are in good agreement, showing that the ML=4.1 earthquake consisted of two events. STFs from each station demonstrate an obvious "seismic Doppler effect". Assuming the nodal plane striking 37 ° and dipping 40 °, determined by using P wave first motion data and afiershock distribution, is the fault plane, through a trial and error method, the following results were drawn: Both of the events lasted about 0.1 s, the rupture length of the first one is 0.5 km, longer than the second one which is 0.3 km, and the rupture velocity of the first event is 5.0 km/s, larger than that of the second one which is about 3.0 km/s; the second event took place 0.06 s later than the first one; on the fault plane, the first event ruptured in the direction ?'=140 ° measured clockwise from the strike of the fault, while the second event ruptured at Y =80°, the initial point of the second one locates at ?" = -100 ° and 0.52 km from the beginning point of the first one. Using far-field ground displacement spectrum measurement method, the following source parameters about the ML=4.1 earthquake were also reached: the scalar earthquake moment is 3.3x 10 ~3 N.m, stress drop 4.6 MPa, rupture radius 0.16 km. ACTA SEISMOLOGICA SINICA Vol. 12 of the ML=4.1 earthquake. Key words: empirical Green's function Huailai basin rupture process source-time function

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