关键词: 青海玛多MS7.4地震, 随机有限断层, 地震动模拟, 地震应力降, 地震烈度

Abstract: An MS 7.4 earthquake occurred in Maduo county, Qinghai Province on May 22, 2021, which is the largest magnitude event following the 2008 Wenchuan MS 8.0 earthquake in China. Due to the sparse strong-motion observation stations in this region, only a few numbers of strong-motion recordings were collected at far field during this event. In order to evaluate the spatial distribution of ground motion intensity, we assumed 4461 virtual monitors around the epicenter of Maduo MS 7.4 earthquake. The three-component acceleration time history series of these 4461 virtual points were given, by using the stochastic finite-fault method for three-dimensional ground motion simulation based on various source rupture models, fault slip model, and source rupture stochastic models. In comparison with the peak ground acceleration and peak ground velocity (PGA, and PGV) from the far-field strong-motion observations and the ground motion prediction equations, the peak ground motions from the simulated method are in good agreement with the median predictions, also could well describe the attenuation trend against distance. However, the peak values from simulations are in general much smaller than those for the observation. Furthermore, this study provided the acceleration and velocity time histories, and correspondingly the 5%-damped pseudo spectra acceleration (PSA). It shows a similar rule that the ground motion and PSA values of simulation are both lower than the observations. Finally, the spatial distribution of seismic intensity is displayed according to the simulated records. Results indicated that: (1) the principle axis of seismic intensity is well consistent with the strike of the interested fault plane; (2) the distribution of meizoseismal area is just along the rupture fault; and (3) the isoseismal lines are irregular ellipses because of the special local site effects. Compared with the seismic intensity map released officially by the Ministry of Emergency Management of the People’s Republic of China, the general seismic intensity based on the simulated records is 1-2 degrees lower. We believe the influence factor is the stress drop parameter. The statistical average stress drop used in the simulation is associated with the rupture and fault size, which is a mean value that represent the average level of the stress drop. Thus, it may underestimate the real stress drop of this earthquake, lead the simulated ground motion to the generally lower than the actual intensity. Considering the uncertainty of the seismic stress drop, the uncertainty of the seismic intensity based on the simulated record is about ±1.2 degrees.

Key words: Maduo MS7.4 earthquake, Stochastic finite-fault method, Ground motion simulation, Earthquake stress drop, Seismic intensity