Coseismic gravity and displacement changes of Japan Tohoku earthquake(Mw 9.0)

The greatest earthquake in the modern history of Japan and probably the fourth greatest in the last 100 years in the world occurred on March 11, 2011 off the Pacific coast of Tohoku.Large tsunami and ground motions caused severe damage in wide areas, particularly many towns along the Pacific coast. So far, gravity change caused by such a great earthquake has been reported for the 1964 Alaska and the 2010 Maule events. However, the spatial-temporal resolution of the gravity data for these cases is insufficient to depict a co-seismic gravity field variation in a spatial scale of a plate subduction zone. Here, we report an unequivocal co-seismic gravity change over the Japanese Island, obtained from a hybrid gravity observation（combined absolute and relative gravity measurements）. The time interval of the observation before and after the earthquake is within 1 year at almost all the observed sites, including 13 absolute and 16 relative measurement sites, which deduced tectonic and environmental contributions to the gravity change. The observed gravity agrees well with the result calculated by a dislocation theory based on a self-gravitating and layered spherical earth model. In this computation, a co-seismic slip distribution is determined by an inversion of Global Positioning System（GPS） data. Of particular interest is that the observed gravity change in some area is negative where a remarkable subsidence is observed by GPS, which can not be explained by simple vertical movement of the crust. This indicated that the mass redistribution in the underground affects the gravity change. This result supports the result that Gravity Recovery and Climate Experiment（GRACE） satellites detected a crustal dilatation due to the 2004 Sumatra earthquake by the terrestrial observation with a higher spatial and temporal resolution.

Integrated construction technology for natural gas gravity drive and underground gas storage

Based on the mechanisms of gravity displacement,miscibility,viscosity reduction,and imbibition in natural gas flooding,an integrated reservoir construction technology of oil displacement and underground gas storage(UGS)is proposed.This paper systemically describes the technical connotation,site selection principle and optimization process of operation parameters of the gas storage,and advantages of this technology.By making full use of the gravity displacement,miscibility,viscosity reduction,and imbibition features of natural gas flooding,the natural gas can be injected into oil reservoir to enhance oil recovery and build strategic gas storage at the same time,realizing the win-win situation of oil production and natural gas peak shaving.Compared with the gas reservoir storage,the integrated construction technology of gas storage has two profit models:increasing crude oil production and gas storage transfer fee,so it has better economic benefit.At the same time,in this kind of gas storage,gas is injected at high pressure in the initial stage of its construction,gas is injected and produced in small volume in the initial operation stage,and then in large volume in the middle and late operation stage.In this way,the gas storage wouldn’t have drastic changes in stress periodically,overcoming the shortcomings of large stress variations of gas reservoir storage during injection-production cycle due to large gas injection and production volume.The keys of this technology are site selection and evaluation of oil reservoir,and optimization of gravity displacement,displacement pressure,and gas storage operation parameters,etc.The pilot test shows that the technology has achieved initial success,which is a new idea for the rapid development of UGS construction in China.

Recent advances of computing coseismic deformations in theory and applications

This paper reviews the recent advances in computing coseismic deformations,and their contributions to seismology and geodesy. At first,an overview on the history of the dislocation theory development is given in the introduction section. Then,emphasis are given on some new developments through few examples in the following sections,such as the new dislocation theory for a 3D Earth model,a new computing scheme on coseismic deflection change of vertical,the relation of dislocation Love number and the conventional Love numbers,the application of dislocation theory applied in satellite gravity observations,the coseismic deformations observed by GRACE,and a new method to determine dislocation Love numbers by GRACE. Furthermore,some advanced theoretical and cases studies are introduced to illustrate how dislocation theory is important in interpret geodetic data,or invert seismic slip for co- and post-seismic processes,using seismic and geodetic data. Final remarks are given in the last section,with discussions,conclusions,comments on existing problems,and expected methods to solve them.