Estimate the regional stress fied in the northern Ibaraki prefecture by using commercially-supplied IC recorders

Abstract:

In case of estimate focal mechanism solutions by using P-wave first-motion polarity data, a dense seismic observation network is required. In this study we propose a new seismic observation system to record a P-wave first-motion polarity. The system consists of a seismometer with a vertical component that price is approximately ten thousand yen and a commercially-supplied IC recorder that price is approximately ten thousand yen.

In this study, we conducted a temporary observation of micro-earthquakes for one month from August to September 2012 in the northern Ibaraki prefecture where many normal-faulting type events occur, and we addressed the effectiveness of the seismic observation system. The 29 seismic stations were deployed along a road so that it allows a deployment of many stations for a short time. After collecting the temporary stations, based on the P-wave first motion polarity, we estimated the focal-mechanisms by using HASH program (Hardebeck and Shearer, 2002). As a result, we obtain the 87 focal-mechanisms for micro-earthquakes occurred in the study area.

To test the accuracy of the focal mechanisms obtained in this study, we compared those with focal-mechanisms determined by Earthquake Research Institute, The University of Tokyo (ERI) temporary stations. We compared focal-mechanisms determined by ERI and Hi-net stations and focal-mechanisms determined by using IC recorder and Hi-net stations. We compare P axis and T axis for focal-mechanisms determined by ERI and Hi-net stations and determined by using IC recorder and Hi-net stations. As a result, nothing is difference of accuracy about focal-mechanisms between determined by ERI and Hi-net stations and determined by using IC recorder and Hi-net stations, because of P axis T axis has almost same distribution on the focal sphere. We conclude that focal mechanisms determined by using IC recorder stations has almost same accuracy as those determined by a traditional three component seismometer.