SOURCE PROCESS OF THE 1990 GONGHE, CHINA, EARTHQUAKE AND TECTONIC STRESS-FIELD IN THE NORTHEASTERN QINGHAI-XIZANG (TIBETAN) PLATEAU

The M(s) = 6.9 Gonghe, China, earthquake of April 26, 1990 is the larg est earthquake to have been documented historically as well as recorde d instrumentally in the northeastern Qinghai-Xizang (Tibetan) plateau. The source process of this earthquake and the tectonic stress field i n the northeastern Qinghai-Xizang plateau are investigated using geode tic and seismic data. The leveling data are used to invert the focal m echanism, the shape of the slipped region and the slip distribution on the fault plane. It is obtained through inversion of the leveling dat a that this earthquake was caused by a mainly reverse dip-slipping bur ied fault with strike 102 degrees, dip 46 degrees to SSW, rake 86 degr ees and a seismic moment of 9.4 x 10(18) Nm. The stress drop, strain a nd energy released for this earthquake are estimated to be 4.9 MPa, 7. 4 x 10(-5) and 7.0 x 10(14) J, respectively. The slip distributes in a region slightly deep from NWW to SEE, with two nuclei, i.e., knots wi th highly concentrated slip, located in a shallower depth in the NWW a nd a deeper depth in the SEE, respectively. Broadband body waves data recorded by the China Digital Seismograph Network (CDSN) for the Gongh e earthquake are used to retrieve the source process of the earthquake s. It is found through moment-tenser inversion that the M(s) = 6.9 mai n shock is a complex rupture process dominated by shear faulting with scalar seismic moment of the best double-couple of 9.4 x 10(18) Nm, wh ich is identical to the seismic moment determined from leveling data. The moment rate tensor functions reveal that this earthquake consists of three consecutive events. The first event, with a scalar seismic mo ment of 4.7 x 10(18) Nm, occurred between 0-12 s, and has a focal mech anism similar to that inverted from leveling data. The second event, w ith a smaller seismic moment of 2.1 x 10(18) Nm, occurred between 12-3 1 s, and has a variable focal mechanism. The third event, with a scala r seismic moment of 2.5 x 10(18) Nm, occurred between 31-41 s, and has a focal mechanism similar to that inverted from leveling data. The st rike of the 1990 Gonghe earthquake, and the significantly reverse dip- slip with minor left-lateral strike-slip motion suggest that the press ure axis of the tectonic stress field in the northeastern Qinghai-Xiza ng plateau is close to horizontal and oriented NNE to SSW, consistent with the relative collision motion between the Indian and Eurasian pla tes. The predominant thrust mechanism and the complexity in the tempo- spatial rupture process of the Gonghe earthquake, as revealed by the g eodetic and seismic data, is generally consistent with the overall dis tribution of isoseismals, aftershock seismicity and the geometry of in tersecting faults structure in the Gonghe basin of the northeastern Qi nghai-Xizang plateau.