Measuring PmP travel times using teleseismic S-wave waveform data

Joint inversion for crustal velocity structure using only surface wave dispersion and receiver function data often suffers the non-unique solution problem due to lack of P wave velocity constraint in the data.Here we developed a method for measuring PmP travel time using teleseismic S wave waveform.The major improvement by this method over the previous one-layer-crust search method is its use of a more realistic multi-layer-crust(MLC)velocity model in the study region based on available information.Numerical tests show that compared with the previous search method the MLC search method is faster and more reliable and accurate.One limit of the MLC search method is the requirement of a multi-layer background model,thus this method might not be applicable in regions where the velocity structure is unknown.Nevertheless,our numerical tests show that the MLC search method is robust in the sense that small deviations of the background model from the true velocity model do not influence the results severely.We applied the MLC search method to data from a temporary linear array across the Wabash Valley Seismic Zone in the central USA.We obtained 157 PmP travel-time measurements that show lateral crustal structure variation in the region.The measurements provide additional constraints in joint inversion for crustal velocity structure in this seismically active region.

Seismological study on the crustal structure of Tengchong volcanic-geothermal area

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the LonglingRuili fault and Tengchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the RehaiRetian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to plate boundary volcanoes.