An attempt to detect temporal variations of crustal structure in the source area of the 2006 Wen-An earthquake in North China

Lei et al. (2008) revealed a low-velocity (low-V) anomaly in the lower crust under the source area of the 4 July 2006 Wen-An earthquake (M 5.1) in North China. In this work we tried to investigate the temporal variations of the crustal structure by using a number of P and PmP (Moho reflected) wave arrival times recorded by 107 digital seismic stations from earthquakes that occurred separately in 2002, 2003, 2004, and 2005-2006 to determine P-wave velocity structures in and around the source area of the Wen-An earthquake in different periods. Our results show that tomographic images inferred from the data sets in different years are all dominated by a low-V anomaly in the lower crust under the Wen-An source area. However, there exist some differences in the P-wave velocity image in the Wen-An source area inferred from the P and P. +. PmP data sets, suggesting that the PmP data have improved the tomographic images in the middle and lower crust, but the results from the 2005-2006 P and P. +. PmP data sets all show a relatively lager increase of the low-V anomaly under the Wen-An source area in the amplitude and extent as compared with those from the 2003 and 2004 P and P. +. PmP data sets. Incorporating the previous results, if this low-V anomaly may indicate the existence of fluids, then our results suggest that the occurrence of the Wen-An earthquake is not only related to the long-term influence of fluids that decrease the effective normal stress on the fault plane, but also closely associated with the drastic increase of such influence. However, this study is just an experimental work and the results are still preliminary because the resolution scale of the present tomographic model is much larger than the Wen-An source area and our extensive tests show that different samplings of seismic rays from different data sets have affected the details of the tomographic images, suggesting that the present sparse data coverage can hardly detect reliably any temporal variations of the velocity anomalies in the Wen-An source area. In future studies it is necessary to improve the resolution of crustal tomography to the size of the rupture zone and utilize identical seismic ray paths from the same pairs of sources and receivers in order to detect any temporal variations of the crust structure in the source area of a large earthquake.