Vp/Vs Ratio Determined from Local Seismicity along the Dead Sea Transform for the Period 2010-2016
An-Najah National University
The Dead Sea Transform fault system (DST) is the most important tectonic structure in the Levant region as well as in the Middle East. Historically, this system caused destructive earthquakes, causing human and property losses. The systematic monitoring of the local seismicity of the DST started on 1980’s, in which many different types of seismic stations are installed on both wings along the DST. These stations are operated by different local and global seismic agencies. These agencies use different parameters to locate the earthquakes. The most important ones are the Vp/Vs ratio and the velocity model, which directly has an effect on the earthquake location accuracy. More than 60 seismic stations are used to study the local seismicity of the DST and the surrounding area during 2010-2016, by using SEISAN software. While, these data are available in the global archive (GEOFON). Very broadband, broadband and short period stations are used. The Vp/Vs ratio is an important factor stressed in this study. About 190 earthquakes are used to estimate this ratio. These earthquakes have five clear of both P and S-arrival times at least. This parameter is 1.44 to 2.14 with an average 1.75 (± 0.08). Different statically analyses are used to prove the average fitness. While, most of these earthquakes have a correlation coefficient ≥ 0.9. Many 1-D velocity models are tested and compared together with the average of the RMS arrival time, horizontal-location error (ERH) and vertical-location error (ERZ). These models are used by different local agencies and one of them is considered as a model reference, created by using seismic refraction method. On the other hand, the reference model is modified (initiative model) to reduce the earthquake location errors. The average of RMS and ERH are 4.1 seconds and 45.59 km, which are the lowest value compared with the tested models. While, the ERZ is about 42.26 km. The epicenter map shows the tectonic structures that mainly cause the earthquakes in the study area. The epicenters, concentrated on the DST in the northern part in the study area, are mostly very shallow earthquake focal depths (≤ 10 km). While, the epicenters distributed on the southern part of the study area with a significant activity along the DST, increase the earthquake focal depth. They are mostly ≤ 40 km. Finally, there are additional factors that have an effect on the location accuracy. One of them is the picking error. More than 120 P-wave arrival times in our data are compared with the European-Mediterranean Seismological Centre (EMSC), which include national arrival time picking. Mostly, the difference is (-0.7 to 1) second. While, most of them are (-0.2 to 0.2) second. The second factor is the network(s) geometry. It shows the ERH and ERZ which sharply increase when the azimuth gap is above 180° and 160°, respectively.