2015年4月25日尼泊尔地震波场传播及烈度初步模拟分析

2015年4月25日尼泊尔发生Mw7.9地震.本文采用三维曲线坐标网格有限差分方法,依据USGS给出的震源运动学反演结果和震源区域地形数据,模拟了尼泊尔地震波场传播过程,得到震源区域烈度分布模拟结果,与实际观测的地表峰值速度(PGV)大体吻合.结果表明:地震烈度的空间分布整体上受控于震源的单边破裂特征,高烈度区域主要分布在破裂传播方向上,即震源东半部.震源南侧到东南侧近场,由于受到震源和地形的双重影响,形成最大烈度分布区域,最大烈度约为IX.南侧平原受低速沉积层影响形成高烈度区域.震源西侧及盆地内烈度相对较低.

Preliminary simulation of seismic wave propagation and the intensity map for the 25 April 2015 Nepal earthquake

An M_w7.9 earthquake occurred in Nepal on April 25, 2015. The epicenter is 28.147°N,84.708°E, 77 km NW of Kathmandu, Nepal. It caused mass casualty and heavy economic loss. The topography varies from ~100 m elevation in the southern plain to 5000~7000 m at the mountains. Such large geo-morphological change can significantly affect the wave field and must be considered when synthetically modeling the seismic intensity. As a quick investigation into this event, this work simulates the ground motion and analyzes the hazard in the affected region.We use the 3D non-staggered boundary-fitted grid finite difference method to simulate the wave propagation with topography. The curved free surface condition is applied by the Traction Mirror method. Our wave-speed model comes from CRUST1.0. The elevation data is GTOPO30, and the finite fault model is provided by USGS. The grid size is 500 m×500 m, and time step is 0.012 s. The highest frequency in our simulation is about 0.35 Hz.Our simulated wave field shows: (1)The eastern part from the epicenter has a stronger wave field than the western part, which is controlled by the Doppler effect of unilateral finite fault. (2) The wave field is enhanced in the mountain between two basins. (3) Wave fields in the southern plain are stronger, and have more high frequency components than those in the northern mountain.We get seismic intensity map from the horizontal Peek Ground Velocity (PGV-h). The highest seismic intensity zone is distributed along mountains in southeast direction, which is controlled mainly by the finite unilateral rupture source and topography. The highest intensity reaches IX. The plain in the south also appears to be of high seismic intensity due to the low velocity sediments and source radiation pattern. The seismic intensity in basins and west of the fault are relatively low.We compare our simulated Peek Ground Velocity (PGV) with records from 4 observational stations. Of these, 2 stations located in the southern plain are well consistent with the observation; the station located in the eastern basin is generally consistent and the station located in the west is mismatched. The reasons for these differences include : (1) The low resolution finite fault model lacks high frequency features beyond the unilateral rupture. (2) Low resolution wave-speed model and topography data are used. (3) Our cluster cannot meet the need for simulating wave field with frequency higher than 0.35 Hz. And (4) the lack of station site response.With the three-dimensional velocity model, topography data and preliminary kinematic source model from USGS, we simulate the seismic wave field of the Nepal earthquake by using the 3D curved grids finite-difference method, and further calculate the distribution of seismic intensity. Most of the results are generally consistent with the observed PGV. The result shows that the high seismic intensity zone trends in southeast direction. The highest intensity reaches IX. The plain in the south also appears to be high seismic intensity due to the low velocity sediment and source radiation pattern. The seismic intensity in basins and west of the fault are relatively low.