STUDY ON THE SEISMOGENIC STRUCTURE OF THE YUMEN, GANSU PROVINCE M_S 5.9 EARTHQUAKE OF DECEMBER 14, 2002

On December 14, 2002, an earthquake of M S 5.9 occurred in Yumen, Gansu Province. Field investigation revealed that both the macroscopic and measured epicenters of this event, the coordinate of which are 39.8°N,97 3°E and 39.7°N,97 3°E, respectively, are located on the northern mar￣ginal fault of the Qilian Mountains. The epicentral intensity was determined to be Ⅶ. The epicentral area appears as an elongated ellipse with N65°W trending long axis of 15km long and N25 E trending short axis of 12km long. The earthquake took place in the Yumen area, which is located on the northern margin of the Qinghai Tibet Plateau and the western section of the Qilian Mountains. There are several Holocene active faults, of which the most famous ones are the Altun and the Changma faults. Historically, several large earthquakes occurred on these two faults, such as the Changma ( M S=7 6) earthquake of 1932 and the earthquake ( M S=5.5) occurred on the eastern section of the Altun fault in 1933. The Yumen earthquake is a moderate strong one, which didnt produce distinct surface rupture zone. The event was the result of the most recent movement on the secondary fault of the northern marginal fault zone of the Qilian Mountains, namely the Hanxia Dahuanggou Fault. The evidence is as follows: Field investigation shows that the macroscopic epicenter of this earthquake is located in the area involving the Hanxia Coal Mine of Yumen, Yuerhong Village of Subei County and Yaomoshan. The trend of the epicentral area is just consistent with the Hanxia Dahuanggou Fault. Focal mechanism solution shows that the nodal plane A appears as thrust fault trending to 147° with a relatively small dip angle of about 26°, in accord with the general strike of the Hanxia Dahuanggou Fault. The P axis is oriented in 277° direction having an elevation angle of 27°. This may indicate that this earthquake was the result of stress concentration due to the action of nearly SN trending horizontal compressive stress. Aftershocks of this event were very frequent, and 113 aftershock of M S≥0.5 had been recorded before Feb. 20, 2003. Most aftershocks were distributed near the Hanxia Dahuanggou Fault and located to the south of the main shock, making a NE trending zone. This may indicate that the faulting propagated from south to north along the fault, and that the event was the result of thrusting along the reverse fault. Several lines of evidence shows that a low angle major detachment plane exists at a weak zone 6～9 km beneath the surface of Yumen area. All folding and thrusting deformation occurred in the strata above the detachment plane (Fig. 6). The northern marginal fault of the Qilian Mountains consists of several secondary reverse faults dipping southwest at an angle of 25°～70°. They all converge on the low angle detachment plane. The Hanxia Dahuanggou Fault is a part of the fault zone, and has a nappe structure. The focal depth of 31 recorded aftershocks coincides well with the depth of the detachment plane. Therefore, it is unlikely that a greater earthquake will occur on this fault in the near future, and this recognition may provide credible evidence for earthquake trend assessment after the Yumen earthquake.