A Robust Real-Time S-Wave Detection Method with Low Computational Cost Based on Modified STA/LTA

Abstract

Abstract Since October 2007, the Japan Meteorological Agency has been operating an Earthquake Early Warning (EEW) System in Japan. To ensure the timely estimation of magnitude immediately after the detection of an earthquake, the EEW process utilizes two types of magnitude calculation formulas: one for the P-wave phase and another after the arrival of S-wave. The process dynamically switches between these two formulas, depending on the estimated arrival time of the S-wave at each observation point. Misidentifying the amplitude of S-wave as a P-wave may lead to an overestimation of the magnitude, which results in the issuance of a wide-area warning for a region with only weak ground motion. However, real-time picking of S-wave arrivals with low computational costs poses a challenge. In this study, we developed a novel robust method to determine the arrival time of S-waves in real time with a low computational cost while adhering to the basic algorithm of short-term average/long-term average (STA/LTA). This approach deliberately introduces additional noise prior to the identified P-wave to enhance the differentiation between the P- and S-waves. Subsequently, the STA/LTA method is reapplied to detect the S-wave. This method was effective in determining the S-wave arrival time, with the residuals (automatic reading minus manual reading) revealing that the percentage of correct decisions (residual < 1.5 s) for all earthquakes with all observation points was 67%. However, the percentage of correct decisions for earthquakes of magnitude 5.5–6.5, which may cause excessive alarms, with observation points at the epicentral distance less than 100 km was 81%. The error in automatic readings can be reduced by optimizing the parameters for each earthquake and possibly for each station. However, because this method is designed for real-time operation, the optimal parameters must be set after the occurrence of an earthquake. It may be feasible to analyze numerous earthquakes and empirically determine the parameters based on information such as STA/LTA and P-wave amplitude.