A study on the determination of cumulative absolute velocity (CAV) threshold levels for Vrancea earthquakes

Natural disasters from earthquakes can cause considerable damages, with potentially severe effects to urban environment. Last strong Vrancea earthquake on March 4, 1977 (M_w=7.4, depth=95km); tens of thousands of buildings damaged, many people injured and dead. Strong earthquakes in the Vrancea zone occur between 60-200 km depth within an almost vertical column. Bucharest Earthquake Early Warning (EEW) system detects earthquakes with a seismic network in the epicentral Vrancea region and issue a warning in Bucharest providing 20-25s warning time. In this study, we have investigated the relationships of the bracketed cumulative absolute velocity window (BCAV-W) approach versus epicentral distance and magnitude for Vrancea region. Firstly, a set of time histories contains 11 earthquakes, 111 strong ground motion records for intermediate depth earthquakes with different magnitudes (4.0≤M_w≤6.0) and epicentral distances (0-200 km) are analyzed to determine relations between the window size (W) and BCAV-W ratio. The BCAV-W computed for different windows time size 2s, 4s, 8s, 12s, 16s, 32s, 64s, 128s and 256s have the ratios 24%, 40%, 61%, 74%, 84%, 97%, 97%, 100% and 100% to the total BCAV respectively. The results show that there is no significant benefit to choose a window size more than 16s. Subsequently, rational threshold levels related to M_w=5.4+ earthquakes are determined as 0.28 m/s and 0.34 m/s related to 12-second and 16-second windows, respectively. The goal of the paper is to enhance EEW capability to decrease the effects of Vrancea earthquakes on urban and built environment.