Research On Seismogenic Structure And Fracture Characteristics Of The 2003 Delingha, Qinghai, M6.7 Earthquake Sequence

An ML6.7 earthquake occurred on April 17, 2003, in Huaitoutala village (37.56°N,96.52°E), Delingha County, Qinghai Province. The epicentral region is in a mountainarea on northeastern Qingzang plateau. Then followed a series of aftershocks. Theseearthquakes are referred to as the Delingha earthquake sequence. Investigating thisearthquake sequence will be helpful for understanding the present-day tectonic movementof the northeastern Qingzang plateau and for estimating the future earthquake activity inthis region. The author did the research on space-time distribute, seismogenic structureand source fracture characteristics of the Delingha earthquake sequence. The data usedare the digital seismic records from 8 temporary seismic stations installed by the ChinaGeological Survey scientific research project (20010201) and the digital records from 28seismic stations of the China National Seismic Network (CNSN). The main works andresults of this study are as follows.1) The author conducted the preliminary location of the mainshock and 117 ML>1.0aftershocks occurred up to October 25, 2003, using the hypocenter distance trajectorycrossing approach, and then relocated the 117 events with the Double-DifferenceAlgorithm (Waldhauser and E11sworth, 2000). Finally 95 events were relocated. Theresult indicates that the aftershock epicenters distribute in a NWW-SEE elongated region,and the focal depths are dominantly in the range of 7-18 km. Vertically the aftershockregion is a rather steep zone.2) Focal mechanisms of the mainshock and 60 ML≥2.4 aftershocks were determinedusing Snoke's algorithm (Snoke et al., 1984). The data used are the polarity readings ofthe first P, SH and SV wave, as well as their amplitude ratio observation. The algorithmperforms an efficient, systematic grid search over the variables space of the 3 parametersof a double-couple source model, and finds acceptable solutions based on the selectedcriteria on the inconsistent polarity readings and errors in amplitude ratios. The resultshows that the mainshock and most aftershocks have a reverse faulting mechanism withthe possible fault striking in NWW-SEE and the maximum source compression inNNE-SSW direction.3) Though in a few years before the mainshock there were some middle sizeearthquakes in the source region, no earthquakes have been recorded in the 40 days justbefore the mainshock. The Delingha earthquake series is an earthquake sequence of themainshock-aftershock type. The daily aftershocks within first 10 days after the mainshockdecayed normally following the Omori law, with the decaying index p=1.2. Afterwardsthe aftershocks decayed slowly. More than one year later there successively occurred 6rather strong aftershocks with M beyond 5. The b value of the whole aftershock sequencetakes a low value of 0.65.4) Investigation on aftershock region expanding reveals that the source fractured in aprocess of unsymmetrical bilateral expanding, with NWW branch being longer than theSEE one. Average fault dislocation of the Delingha earthquake sequence is about 0.39 mand the average global stress drop is estimated to be 4.6 MPa, indicating this is a reversefaulting earthquake with high stress drop.5) An integrated analysis on the spatial distribution of aftershocks and the focalmechanisms given by both this study and Harvard university, in combination with theconsideration on relevant major geologic faults, results in the judgment that theseismogenic structure of the Delingha earthquake sequence is the NWW-SEE striking DaQaidam-Zongwulongshan active fault, which is a boundary fault between the Qaidamfaulted depression and the Qilian fold uplift. The earthquake sequence is an exhibition ofthe activity in deep part of the fault.6) Based on the characteristics of the aftershock spatial distribution and focalmechanisms, in reference to the study result of Neo-tectonic movement and present-daytectonic stress in northeastern part of the Qingzang plateau, we can see that theoccurrence of the Delingha earthquake sequence is a manifestation of the present-dayuplifting movement of the Qingzang plateau under the NNE-SSW compression. And thepresent-day uplift movement of the northeastern Qingzang plateau serves as theseismogenic origin of the Delingha earthquake sequence.