Relocation of recent 20-years earthquakes occurred in northwest of Iran by JHD algorithm

In current study, earthquakes occurred in northwest of Iran in the past 20 years were relocated using JHD algorithm. A dense seismic network is usually required to produce a stable and precise earthquake location, i.e., improved quantitative and qualitative earthquake data are achieved due to various parameters including low amount of azimuthal gap. Combining data from different seismic networks in the study area can be effective and useful to improve earthquake locations. Thus, a list of earthquakes in the northwest region of Iran was developed using data available from different seismic networks. The recorded earthquakes data by the seismic network stations of Iran and neighboring countries were collected and a complete data set of earthquakes in the region was prepared. Then, the information from 30,000 recorded and reported earthquakes in the area was compiled. This information includes more than 400,000 seismic phases from databases of International Seismological Center (ISC), Iranian Seismological Center (IRSC), Broadband Iranian National Seismic Network Center (IIEES) and National Seismic Network of Turkey (DDA). This dataset was sorted and the outliers were taken off based on analyzing the travel-time residuals. In the next step, the recorded phases of an earthquake from different stations were combined and a homogeneous catalog was established. This catalog is more complete than the original catalog, e.g., because of increasing the number of phases, azimuthal gap of earthquakes has decreased. In the next step, after the preparation of the data, the hypocenter relocation was done using single event and the JHD algorithms in VELEST computer program. Comparison of RMS of earthquakes indicates a decrease in the RMS due to relocation process. The average value of RMS for the entire earthquakes reduced from 0.69 s to 0.41 s at the final stage, and this shows a reduction in difference between calculated and real earthquake location. The RMS value for more than 90% of earthquakes is less than one second. Thus, a homogeneous catalog of earthquakes in the region was prepared and new results were evaluated and compared with the initial information. Finally, the seismicity patterns in the region were investigated by drawing the hypocenter depth sections in the active seismic areas. Based on the obtained results in many sections, the removal of unrealistic structures due to systematic errors in earthquake location was evident, and in some sections, scattering of the earthquake hypocenter locations reduced and the existing structures were more obvious.