Study Of Seismicity In The Region Of Sparse Stations Using Matched Filter Method

Earthquake catalogue is the fundamental data for confirming seismogenic structure and seismicity analysis. However, due to the interference of coda waves, sparse distribution of stations and other reasons, earthquakes may be missing in catalogue. Based on the cross-correlation of waveforms, the matched filter technique (MFT) is an effective method to detect events with low SNR. In this paper, we research the following two problems with the MFT.1、The Ms6.5 Ludian earthquake occurred on 3 Aug 2014 in Yunnan province. After the earthquake, the relationship between the earthquake and the impoundment of the Xiluodu reservoir is still in debate. Analysis of seismicity around an earthquake could provide some evidences for the cause of an earthquake. While some small earthquakes may be missing in the network catalogue, due to the small number and sparse distribution of seismic stations, and overlapping between small earthquakes and coda waves of the large earthquake, which may influence the seismicity analysis. Here we apply the MFT for detecting missing events 33days before and 5 days after the Ludian earthquake and come to the following conclusions:(1)We obtain 991 missing events by using 541 events listed in the network catalogue as templates, and decrease the magnitude of completeness (Mc) of aftershock from 1.8 to 1.3. Based on the detected catalogue, we find the b value within 33 days before the main shock is 1.04±0.18, and the b value within 5 days after the main shock increases from 0.5 to 0.9 and remains stable;(2)The background b value is 0.93±0.04 from the network catalogue of 5 years before the mainshock. The character of b value doesn’t match the variation of b value associated with reservoir-induced earthquakes;(3) We find that the seismicity is almost the same before and after the impoundment of Xiluodu reservoir in this region by analyzing the network catalogue of 5 years before the mainshock. Based on these observations, we suggest that the 2014 Ludian Ms6.5 earthquake don’t have the characters of classic reservoir-induced earthquakes.2、The Ms5.8 Alashan earthquake occurred on 15 April 2015 in Inner Mongolia. Since the Network catalogue has less events, and the error of focal position may be large because of the sparse permanent stations, it is difficult to judge seismogenic structure. In this paper, we adopt matched filter method for detecting missing events in the period of 10days before and 10days after the Alashan earthquake, using the data recorded by several mobile stations near the epicenter and 55 events in the Network catalogue as templates. Combined Hypo2000 and HypoDD, we have a preliminary analysis of the seismogenic structure. The main conclusions are as follows:(1)We obtain 362 missing events by using 55 events listed in the network catalogue as templates;(2)55 events are relocated by using Hypo2000. After relocation, the vertical and horizontal average errors are 36.9km and 2.50km, the residual error of the travel time is 0.58s.(3)We extract the P、S arrive time of missed events by using cross-correlation technique and relocate the detected events by using HypoDD. After relocation, the aftershocks are mostly distributed along the EW direction, and we can find aftershock distribution has a features of high inclination. By reference to mechanism of different research institutions and scholars resulting, we suggest that the seismogenic structure of Alashan Ms5.8 earthquake is a fault which has high-angle and left-lateral strike-slip.