Evaluation of Iran Strong Motion Network (ISMN) recording and its effects on the improvement of earthquake location in Zanjan and its neighboring regions

By merging the data from the University of Tehran’s Iran Seismic Telemetry Network (ISTN) with those from Iran Strong Motion Network (ISMN), we investigate the improvement of earthquake location accuracy for the events which happened in the Zanjan province and its neighboring regions (the region bounded by 35º-37.5º N latitudes and 46.5º-50.5º E longitudes and for the period 1996-2006). Due to insufficient distribution of seismic stations in the study region, most of the events have poor
azimuthal coverage and location accuracy. The events in the Zanjan province are largely recorded by Tehran and Tabriz seismic sub-networks of ISTN in the east and west of the province, respectively. ISMN has a very good coverage within Zanjan and its neighboring provinces, and thus has a promising potential to improve the location accuracy of the events within the Zanjan province.
In this study, we assess the improvement of location accuracy by merging the ISMN data with the catalog of relocated events for Zanjan and its neighboring provinces (Askari and Ghods, 2007).  The catalog is consists of 304 events for the period 1996-2006, with local magnitudes larger than 3.1, and RMS of less than 0.7 s. The catalog complete for magnitudes larger than 3.5. ISMN is an offline network and does not use GPS for precise timing of its waveform records. We first associate ISMN waveforms based on their time tag but later check if the phase readings are consistent with the phase readings from ISTN. Due to the lack of precise timing of ISMN waveforms, only relative phase arrival of Sg-Pg could be participated in the location procedure. We were able to associate 403 ISMN records with 76 events of the catalog.
We find that the ISMN data cannot significantly improve earthquake location in the Zanjan region. Out of 76 events with the ISMN data, azimuthal coverage and epicentral accuracy of 7 events could be improved significantly. This is primarily related to the fact that most of the regional faults are more or less aligned with the direction of the spread of the weak-motion seismic networks in east and west of the province. We found that ISMN could detect all events with magnitude above 4 within the study region. According to our results, the ISMN stations have been maintained properly, and we could not detect significant data loss. However, we could detect several problems in archiving the ISMN data. We have also found that the ISMN instruments do not have enough resolution for accurate recording of seismic amplitudes. This implies that picking of the first arrivals on ISMN waveforms may have errors in the range of 0.2-0.4 seconds.