日本地區Mw ≥ . 地震之快速規模估計

Abstract

快速準確的提供規模估計是地震預警的關鍵技術，利用近震央有限測站的地震早期訊號 (P波)，快速並推估可靠的地震規模仍是現今地震預警研究中的重大挑戰。地震預警可分為現地型與區域型，現地型的優勢在於能針對觀測到的P波早期訊號快速計算後續破壞性震波強度，而區域型預警優點則是利用近震央的數個測站資訊，綜合推估地震規模、位置、以及各地震度，其準確性較高。為整合兩種預警系統的優勢，黃皓昀 (2021)於台灣地區地震研究中，探討P波到時後1至20秒每秒時間窗下，綜合分析五個初始地動參數: 累積絕對速度(Cumulative Absolute Velocity , CAV)、累積絕對位移(Cumulative Absolute Displacement, CAD)、累積絕對位移積分(Cumulative Absolute Absement, CAA)、位移峰值(Peak displacement, Pd) 及P波平均週期(Average period, τc)，估算最終累積絕對速度 (estimated Final Cumulative Absolute Velocity, eCAVF)，並應用於等當量地震矩規模 (equivalent moment magnitude, Mew)中。本研究參考相關指標參數，使用日本地區規模Mw大於5.5的95起地震事件之強地動紀錄，基於地震預警的理念精簡計算流程，探討各初始地動參數直接應用於規模預估上的效能。結果顯示五個參數組合的規模預估值，僅需利用距離震央最近的六個測站即可獲得良好的規模預估結果，其估計規模在一秒至二十秒時窗皆在標準差 ± 0.3範圍內，顯示本研究中利用五個地動參數組合的經驗關係式適用於日本的中大規模地震，並且可以有效地縮短前人於預估規模上所花費的時間。此外，於本研究中所使用的地動參數中顯示，CAA比起常見的地動參數Pd具有更佳的規模預估效能，建議於未來地震預警系統下可以加入CAA作為更好的規模估計參數。

Rapid and accurate magnitude determination is a key task for earthquake early warning (EEW). Using the initial signals (P wave) from a few stations near epicenter to obtain rapid and reliable magnitude is still a major challenge in recent EEW. Onsite EEW uses the initial P wave signals from a single station or a small array to calculate the destructive S wave intensity rapidly. Regional EEW uses the seismic recordings from several stations near the epicenter to obtain the intensity, location, and magnitude of earthquake. It has the advantage of higher accuracy compared to onsite EEW. Huang (2021) integrated the advantages of the two EEW type systems in Taiwan. First, they combine the five initial ground motion parameters: Cumulative Absolute Velocity (CAV)、Cumulative Absolute Displacement (CAD)、Cumulative Absolute Absement (CAA)、Pd and τc to obtain the estimated Final Cumulative Absolute Velocity (eCAVF), and then, apply eCAVF to obtain equivalent moment magnitude (Mew). Our study will choose the Mw greater than 5.5 seismic events. This study follows the same analysis in Huang (2021), and apply these parameters to the magnitude estimation for regional EEW in Japan, trying to reduce the bias caused by the eCAVF. The purpose of this study is establishing the attenuation relationships for moderate to great earthquakes in Japan at 1 to 20 second time windows after P arrival by using the integration of five ground motion parameters, and analyzing the ability of each initial ground motion parameter for magnitude estimation. Results show that the magnitude estimation based on the integration of the five parameters can obtain good magnitude estimation results by only using the six stations closest to the epicenter, and the standard deviation between the estimated and real Mw varies from 0.29 to 0.20 for the 1 to 20 seconds after the P arrival, which are within the magnitude estimation tolerance error range ± 0.3. This indicate that the attenuation relationships using the integration of five initial ground motion parameters are applicable to moderate to great earthquakes in Japan, and it can reduce the time spent on the magnitude determination effectively. Additionally, results show that the CAA used in this study has better magnitude determination ability than the common ground motion parameter - Pd. This study suggested CAA can be added as a better magnitude determination parameter for EEW system in the future.