摘要
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 WL=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 toretrieve the far-field source-time fonction (STF) of the main shock. Considering the records of HDSN are the typeof 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 fieldstations, P phases in vertical direction which mostly are about 0.5 s in length were used. The STFs obtained fromeach seismic phases are in good agreement, showing that the Mt=4.1 eedhquake consisted of two events. STFsfrom each station demonstrate an obvious 'seismic Doppler effect'. Assuming the nodal plane striking 37° anddipping 40°, determined by using P wave first motion data and aftershock distribution, is the fault plane, through atrial and error method, the following results were drawn：Both of the events lasted about 0. 1s, the ruptUre length ofthe 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 5.0km/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 thefirst one; on the fault plane, the first event ruptured in the direction γ=140° measured clockwise from the strike of the fault, while the second event ruptured at γ=80°, the initial point of the second one locates at γ= -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 3.3×1013 N·m, stress drop 4.6 MPa, rupture radius 0. 16 km.
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 WL=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 toretrieve the far-field source-time fonction (STF) of the main shock. Considering the records of HDSN are the typeof 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 fieldstations, P phases in vertical direction which mostly are about 0.5 s in length were used. The STFs obtained fromeach seismic phases are in good agreement, showing that the Mt=4.1 eedhquake consisted of two events. STFsfrom each station demonstrate an obvious 'seismic Doppler effect'. Assuming the nodal plane striking 37° anddipping 40°, determined by using P wave first motion data and aftershock distribution, is the fault plane, through atrial and error method, the following results were drawn：Both of the events lasted about 0. 1s, the ruptUre length ofthe 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 5.0km/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 thefirst one; on the fault plane, the first event ruptured in the direction γ=140° measured clockwise from the strike of the fault, while the second event ruptured at γ=80°, the initial point of the second one locates at γ= -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 3.3×1013 N·m, stress drop 4.6 MPa, rupture radius 0. 16 km.
