OpenACC 平行化語言用於網格式定位程式之開發及其在地震預警之應用

Abstract

對於地震預警系統來說，在地震發生後能快速且準確的估計震源位置和規模是相當重要的項目。在地震定位方面，使用簡化的一維速度構造模型對於地質構造較複雜之區域無法得到良好的震源位置。因此本研究之目的在於利用三維速度構造模型建立一套準確且計算過程簡單的地震定位程式。首先將台灣地區切成密集的三維空間格點，格點間距在經度、緯度和深度上皆為1公里，總格點數為6.88百萬，並假設各點皆為可能之震源位置，利用三維速度構造模型與三維空間的波線追跡法來計算理論P波走時，並建立各格點至中央氣象局地震速報觀測網 (Real-Time Data stream, RTD) 之地震測站的理論走時資料庫。當地震發生時，將接收到地震P波訊號的測站兩兩配對建立等時差 (Equal differential time, EDT) 曲面，再以逐點計算殘差 (residual) 總和，找出最小殘差值的格點即為震源位置。為了處理運算過程中產生的大量數據，本研究利用OpenACC平行化套件發揮圖形運算處理器 (GPU) 的運算優勢來減少計算過程需要花費的時間。利用TAIGER炸測資料及115個歷史地震事件對於定位程式的可靠度進行測試，誤差分析顯示本研究之定位方法能在不增加運算時間的情況下，有效提升地震定位的精度，因此能進一步改善地震預警系統之執行效率。相較於中央氣象局地震目錄之定位結果，震央平均差異為4.77公里，深度差異為5.11公里。

The rapid and reliable hypocenter and magnitude estimation are two of the most issue for earthquake early warning (EEW) system after an earthquake occurs. In earthquake location aspect, for early warning purpose, the simplified one-dimensional (1-D) velocity model cannot reflect the complex structures in certain area and result in large location uncertainties. Therefore, an accurate location program based on three-dimensional (3-D) velocity model with a simple-calculating process is developed for earthquake location. First, we divided Taiwan region into 3-D grids, 1 km interval in longitude, latitude and depth, which are assumed as possible hypocenters. Thus, the study area consists of 6,880,000 grids. P wave travel times between each grid and seismic station are stored in a pre-calculated database using the 3-D velocity model and 3-D ray tracing method. When an earthquake occurs, an equal differential time (EDT) surface is then conducted in each different triggered station pairs. By comparing the residuals between EDT surfaces and each grid, the grid with the minimum residual will be considered as the hypocenter. In order to deal with massive computation load during the comparing process, we apply an OpenACC parallel computing toolkit to GPU (Graphic Processing Unit) to reduce the processing time. Records of 115 earthquakes and 10 TAIGER explosion events are used to test our location program performance. Results show our location program can provide a reliable hypocenter location in short times and thus it could improve the efficiency of the EEW application. The average location difference between result of this study and Central Weather Bureau (CWB) earthquake catalog for epicenter and depth are 4.77 km and 5.11 km respectively.