An Experimental Study On The Fine Structure Of The Crustal P-wave Velocity Of Dense Arrays In The Weifang Section Of The Tanlu Fault Zone

The Weifang segment of the Tanlu fault zone is located in the northernmost part of the Shandong province,which is evolved from four major faults into two major faults along the NNE direction from south to north.There are several active tectonic systems around it,the Bohai Bay to the north,the North China Basin to the northwest,the Jiaoliao uplift to the northeast,Luxi uplift to the southwest,and the Yellow sea to the southeast.Compared to other regions,seismic activity in the Weifang segment of the Tanlu fault zone is relatively weaker,and we called it an aseismic gap,which makes it hard to carry out local seismic tomography in the study.The seismic velocity structure in the Weifang segment of the Tanlu fault zone,especially the crustal structure,is of great scientific significance not only for potential seismic hazard but also for understanding the deep dynamic processes.The 3003 high-quality arrival-time data are obtained by applying the Multi-Channel Cross-Correlation technique to the teleseismic waveform data recorded at a dense(a station interval of-5 km)and short-period(5 s)seismic array consisting of 302 stations in the Weifang segment of the Tanlu fault zone.Using the FMTT(Fast Marching Teleseimic Tomography)technique,we invert teleseismic relative travel-time residuals for P-wave crustal velocity structure of the study region down to 30 km depth with spacing resolution of-30 km in the horizontal directions.Our results show that obvious high-velocity anomalies are imaged to the west of the fault zone from the surface down to 30 km depth,within the fault zone striking low-velocity anomalies appear except for the high-velocity areas of 36.5°N-36.7°N,whereas to the east the velocity anomalies exhibit a complicated pattern.Meanwhile,our tomographic images show a clear boundary between the fault zone and its surrounding areas.All these results suggest that the fault zone provides a good channel for continuous upwelling of hot and wet materials to the surface when it arrivals at the crust from the mantle.The hot and wet mantle upwelling could be related to some geodynamic processes,such as the stagnancy and dehydration of the subducted Pacific slab in the mantle transition zone,which provides important evidence for the mantle dynamic evolutional processes of the Tanlu fault zone.On the other hand,in this study we use the P-wave arrival-time data recorded at a short period seismic array to carry out a useful exploration experiment for the detailed velocity structure of the crust,which has of reference value for making full use of the short period data to conduct similar studies in the future.