Estimation of 1-D velocity models beneath strong-motion observation sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes
Estimation of 1-D velocity models beneath strong-motion observation sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes
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DOI：
10.1186/s40623-017-0685-4
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发表时间：
2017-07-24
影响因子：
3
通讯作者：
Sugimura, Yokito
中科院分区：
文献类型：
--
作者：
Bijukchhen, Subeg M.;Takai, Nobuo;Sugimura, Yokito
喜马拉雅碰撞带地震活动频繁，大地震时有发生。由于快速的造山过程，原始的巴格马蒂河筑坝，在加德满都山谷形成了一个湖泊，最终干涸，留下了厚厚的松散的湖泊沉积物。先前的研究表明，沉积物在中心的厚度接近600米。由于地处地震活跃区，加上厚厚的沉积物可能会放大地震波，加德满都山谷很容易发生地震。过去曾多次遭受地震破坏。加德满都谷地发展成为尼泊尔最大的城市聚集区，使大量人口面临地震危险。这种脆弱性在2015年4月25日的廓尔喀地震（Mw7.8）中表现得很明显，当时主震和随后的余震夺去了1700多人的生命，山谷内近13%的建筑物完全受损。为减少地震风险而制定安全和最新的建筑规范，需要对地震动放大进行彻底的研究。表征地下速度结构是实现这一目标的一个步骤。利用北海道大学和Tribhuvan大学的强震加速度计阵列记录，通过低频S波的正演模拟，构建了台站场地的一维速度模型。在2013年8月30日的一次中等规模（mb4.9）地震和2015年廓尔喀地震的三次中等规模（Mw5.1，Mw5.1和Mw5.5）余震期间，来自安装在岩石现场的加速度计之一的过滤记录（0.1-0.5 Hz）被用作低频S波建模的输入运动。我们参考了现有的地质图、横截面和钻孔数据，作为沉积物场地初始模型的基础。这项研究表明，该盆地地形起伏，沉积地点的沉积物厚度不同，从155米到440米不等。这些模型还显示了基岩深度处的高速度对比度，这导致了显著的波放大。
The Himalayan collision zone experiences many seismic activities with large earthquakes occurring at certain time intervals. The damming of the proto-Bagmati River as a result of rapid mountain-building processes created a lake in the Kathmandu Valley that eventually dried out, leaving thick unconsolidated lacustrine deposits. Previous studies have shown that the sediments are similar to 600 m thick in the center. A location in a seismically active region, and the possible amplification of seismic waves due to thick sediments, have made Kathmandu Valley seismically vulnerable. It has suffered devastation due to earthquakes several times in the past. The development of the Kathmandu Valley into the largest urban agglomerate in Nepal has exposed a large population to seismic hazards. This vulnerability was apparent during the Gorkha Earthquake (Mw7.8) on April 25, 2015, when the main shock and ensuing aftershocks claimed more than 1700 lives and nearly 13% of buildings inside the valley were completely damaged. Preparing safe and up-to-date building codes to reduce seismic risk requires a thorough study of ground motion amplification. Characterizing subsurface velocity structure is a step toward achieving that goal. We used the records from an array of strong-motion accelerometers installed by Hokkaido University and Tribhuvan University to construct 1-D velocity models of station sites by forward modeling of low-frequency S-waves. Filtered records (0.1-0.5 Hz) from one of the accelerometers installed at a rock site during a moderate-sized (mb4.9) earthquake on August 30, 2013, and three moderate-sized (Mw5.1, Mw5.1, and Mw5.5) aftershocks of the 2015 Gorkha Earthquake were used as input motion for modeling of low-frequency S-waves. We consulted available geological maps, cross-sections, and borehole data as the basis for initial models for the sediment sites. This study shows that the basin has an undulating topography and sediment sites have deposits of varying thicknesses, from 155 to 440 m. These models also show high velocity contrast at the bedrock depth which results in significant wave amplification.