Crustal and upper-mantle seismic reflectors beneath the Three Gorges Reservoir region

Abstract

Seismic studies of the crustal structure beneath the Three Gorges Reservoir (TGR) region in Central China have been limited by the sparse and uneven distribution of seismic stations. To increase the station coverage, we made three deployments of a mobile seismologic array in the TGR region during the three summers from 2008 to 2010. Here we present interpretations along a west-east profile through the central TGR region based on new seismic waveform data and a velocity model constrained by regional earthquake data. Two strong mid-crustal reflection interfaces at depths around 10 and 20 km are seen under the TGR. The shallow reflector defines the bottom of the Zigui (秭归) basin. The new waveform data show that the amplitude of the Moho reflection is quite weak, and beneath theMoho, there is a strong reflector around 54-km depth. It is likely that in the TGR region, the Moho is a gradient rather than a sharp boundary. We speculate that the gradient Moho and the 54-km-deep reflector in the upper mantle in the TGR region may be by-products of the Qinling (秦岭)-Dabie (大别) orogen.

Using a Three-Component Distributed Array of Mixed, Horizontal, and Vertical Single-Component Seismometers to Produce a Receiver Function from a Short Deployment

“…3b) from Zou et al (2011), who modeled reflection and refractions recorded by the larger 45-km verticalcomponent array deployed that included our subarray (Fig. 1).…”

“…1). Figure 4c shows the synthetic RF computed with the Zou et al (2011) model, along with the TG-RF. For the most part, the time delays for the lower crust and Moho phases in the synthetic RF match those on the TG-RF, but the amplitude of the deeper P49s phase is too small and the P36s phase is too large.…”

“…For the most part, the time delays for the lower crust and Moho phases in the synthetic RF match those on the TG-RF, but the amplitude of the deeper P49s phase is too small and the P36s phase is too large. The Zou et al (2011) model is based on reflection and refraction data, but the offsets were not long enough to model deep crustal boundaries and Moho. As a result, Zou et al (2011) used velocities from other regional models (e.g., Zhang et al, 2009).…”

“…All 45 stations of the larger array had short-period verticalcomponent seismometers. Zou et al (2011) used recordings of local earthquakes by the larger 45 component array to produce a reflection/refraction model of the crustal structure beneath the array. This provides a model for comparison with the inferred Moho depth from the RF computed using data from the subarray.…”

An evaluation of reverse-time imaging of clustering earthquakes

“…The seismic velocity of the shallow crust in the TG region is relatively homogeneous beneath the area of the small earthquake (Zou et al, 2014(Zou et al, , 2011, hence a homogeneous velocity model (V P =5.6 km/s, V S =3.1 km/s) is used here. With such a homogeneous velocity model we can avoid the scattered noise generated on velocity boundaries during the waveform modeling.…”