Biao Guo

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.

GPS/GLONASS Combined Precise Point Positioning With the Modeling of Highly Stable Receiver Clock in the Application of Monitoring Active Seismic Deformation

The high-rate kinematic Precise Point Positioning (PPP) of the Global Navigation Satellite System has become an effective method for monitoring crustal deformation caused by earthquakes. In this contribution, the method of GPS/GLONASS PPP with the receiver clock modeling is applied in active seismic deformation monitoring for the first time. With the modeling method, the short-term vertical positioning accuracy of 2–4 mm that usually cannot be obtained by standard PPP is achieved. Our PPP results confirm that the positioning accuracy is improved due to the increase of GLONASS observations compared to the GPS-only solution. Based on the external seismic data and the high-rate GPS/GLONASS data for the 2011 Japan earthquake and 2010 and 2015 Chile earthquakes, comparative analyses concerning receiver clock modeling are carried out. The results show that a high degree of decorrelation between the height position estimates and receiver clock offsets can be obtained by using the receiver clock modeling. The short-term accuracy of the GPS-based vertical displacements is improved to the level of about 4.4 mm, and the short-term accuracy of better than 4 mm for the GPS/GLONASS-combined vertical displacements is achievable. Furthermore, the weak vertical signals that are not detected by standard PPP can be captured with the modeling of highly stable receiver clock.