Jiu Hui Chen

Seismic noise correlations to image structural and mechanical changes associated with the Mw 7.9 2008 Wenchuan earthquake

We locate temporal changes of seismic wave speed and scattering properties of the crust associated with the 2008 Mw7.9 Wenchuan earthquake. To that end, we analyze ambient seismic noise records from 2007 until the end of 2008 in the 1–3 s period band and in the 12–20 s period band measured in a region that covers the southern two thirds of the fault activated during the earthquake. To locate the changes, we use a refined imaging procedure based on the sensitivity of scattered waves to weak perturbations. This inverse method uses the radiative transfer approximation to describe the intensity of the noise correlation coda. Our results show that the largest structural changes are observed northeast of the Wenchuan epicenter, around the fault zone where aftershocks are distributed. The spatial and temporal characteristics of the behavior of the crust at depth around the Wenchuan earthquake suggest a postseismic and geology-dependent signature of the middle crust. We also observe a clear seasonal signature within the Sichuan basin at depth that we relate to the heavy rainfalls during the monsoon season. This seasonality can also be observed at the surface in form of a waveform decoherence. Our study highlights that seismic velocity changes, and waveform decoherence are independent measurements that are sensitive to different parameters and can thus give complementary information. In our study we also point out the value of studying the evolution of the changes at different times in the coda.

Seismogenic Tectonic Environment of 1976 Great Tangshan Earthquake: Results from Dense Seismic Array Observations

Abstract On July 28, 1976, the great Tangshan earthquake that shook the whole world took place in the Tangshan area of the Hebei Province, China. A big incomprehensible question is why such a tremendous earthquake took place in the Paleo-craton area in North China? It would be worth considering whether a similar event will reoccur in the Tangshan region. In this study, using the receiver function inversion technique and teleseismic P waveform data recorded at the Capital Circle Seismic network and our movable seismic array, we investigated the 3-D S-wave velocity structure of the crust and upper mantle down to 60 km beneath Tangshan area. Our results manifest that (1) the media beneath the Tangshan block cut by active faults are very different from the adjacent area, and all of the active faults surrounding the Tangshan block was through the whole crust; (2) in the upper and middle crust, there exist obvious heterogeneous low-velocity media beneath the Tangshan earthquake region; the crust-mantle boundary has an obvious block uplift and, in comparison with both sides, the top anomalous uplift of the upper mantle beneath the Tangshan block reaches to 10 km, and the upper mantle beneath has an anomalous heterogeneous structure; (4) beneath the Tangshan earthquake region, there are probably massive intrusions derived from the upper mantle, which form the low-velocity body in the upper and middle crust. Because of our results having much higher resolution than previous results, some new features of the crust and upper mantle velocity structure could be shown in this study; (5) the locations of destructive earthquakes are not random and are related closely to their deep structure of the crust and upper mantle. This provides a possibility of correctly estimating the location of destructive earthquakes. On the basis of our results, we discuss the dynamic genesis of the Tangshan earthquake. We consider that the main dynamic source for the Tangshan earthquake is the vertical movement of the upper mantle, which leads to the exchange of material and energy between the crust and the upper mantle. The horizontal stress field plays only a subordinate role. This observation facilitates our understanding of why the Tangshan earthquake with the magnitude greater than 7.0 occurred within the Paleo-craton area of North China. A problem worth considering is whether there is a possibility that same magnitude events will reoccur in the Tangshan region.