Seismic Phases Identification And 3D Earthquake Location:Application To Tengchong

Tengchong volcanic field that sits at the southeastern end of the Tibetan plateau is one of the youngest volcanoes in mainland China.Precise earthquake locations in active volcanoes are important to understand the seismotectonic environments,the size and geometry of the magma chamber,and eruptive mechanisms.Nowadays plenty of seismic location studys depend mainly on manual determination of phase onset times,while manual onset time picking generally requires considerable amounts of both manpower and time with sometimes undesirable results.Previous studies show obvious lower velocity and resistivity anomalies,suggesting strong lateral variation of crust-mantle velocities beneath the Tengchong,therefore using simple 1D velocity model for earthquake location can introduce large location errors.In this paper,we present probabilistic earthquake location based on seismic data recorded by permanent and temporary broadband seismic stations located in Tengchong region.Significant progress has been achieved in hypocenter locations by the application of accurate automatic picks and 3D model.Accurate phase detection and picking of seismic wave arrivals are the foundation of subsequent seismic analyses.This paper presents an optimization method of seismic P and S phases arrival picking,puts forward a revised cost minimization approach to train three automatic pickers: the frequency-band picker,the Akaike information criteria function derivative picker and the kurtosis picker at 7 broadband stations located in Tengchong,Yunnan.We also propose a revised weighting scheme that used four independent factors of weight to assign a weight to 1533 automatic phase picks of local and regional earthquakes.Four independent factors are calculated to assess the quality of the automatic identification of P-phase and S-phase.The comparisons of automatic picks and manual picks show that 74.66% for P-picks and 70.98% for S-picks are less than 0.1 seconds.These automatic picks using the optimized approaches described here improve the accuracy of the automatic picks when compared to previous automatic pickers.We obtain accurate picks by employing optimized AICD automatic pickers and quality assessment scheme at temporary and permanent seismic networks surrounding Tengchong volcanic area.Based on those high-quality first-arrivals,we relocated 799 local earthquakes with magnitudes larger than 1.5,using 1D(Hypomat)and 3D(Probabilistic earthquake location)location algorithms.The comparison of relocations obtained by 1D method to those relocated in 3D crustal velocity model shows no systematic shifts in epicenter locations but sometimes large shifts in focal depth.The new 3D locations are more clustered than 1D relocations and most of events located based on the 1D model seem deeper than those located with the 3D model.Analysis of the travel-time residuals,typical seismic sequences and the location of ground blasts showed that more precise and reliable relocations were obtained with 3D location method.An absolute accuracy of ~1km and 2 km for the horizontal and vertical errors are achieved by the 3D method.The earthquakes cluster primarily in the first 10 km of the crust.The cluster of earthquakes,together with the presence of the shallow low velocity anomaly imaged by previous tomography imply that the magma reservoirs the upper crust is at ~10 km depth.