A Gravity Method For Identifying The Risk Of Earthquake Based On Lock In Shearing Model

Earthquake prediction is a recognized global scientific problem.In China,frequent earthquakes pose a great threat to people’s lives and property.Therefore,the continuous and in-depth study of earthquake prediction is the only way to reduce the loss of earthquake disasters and protect people’s lives and property.Earthquake predictions must be based on precursors that are observed before the earthquake to indicate that the earthquake will occur.The experience of earthquake prediction shows that mobile gravity observation is an important means for medium-term earthquake prediction in China with the ability to capture earthquake precursors to some extent.The earthquake cases summary shows that a typical gravity field change image(which may be called “gravity precursor”)often appears before a strong earthquake or large earthquake: a gravity change gradient or a four-quadrant image with alternating high and low values,and the future epicenter located at the transition between positive and negative gravity changes(eg,zero line of gravity changes)or relatively low value area.How to explain this phenomenon and use it to predict the earthquake with a physical basis is one of the key issues currently facing.Shen Chongyang et al.(2011),based on the four-quadrant distribution of gravity changes before the Yao’an earthquake,found that the pre-existing locked shear force of the “seismogenic body” in the process of seismogenesis and the “co-seismic dislocation” shear stress shown by the focal mechanism have physical consistency,and then put forward the "locking shear mode" to provide a theoretical basis for explaining the above characteristics of typical gravity changes.Then,how to quantify the “locking shear force mode” and establish the corresponding earthquake prediction theoretical method has become a key problem need to be solved urgently.Therefore,this paper mainly focuses on the characteristic features of typical gravity changes given by decades of gravity observations,and develops a seismic gravity prediction method based on the locked shear mode to provide quantitative practical techniques for earthquake gravity prediction.The main research work and contributions of the paper are as follows:(1)Based on the concept of "locking shear mode" and using the seismogenic dynamic theory of quantitative seismology,we established a mathematic physical model expression formula for the locking shear force mode.According to the double-couple mechanism of structural earthquakes,an imaginary "equivalent dislocation" can be used to describe the locking shear force stored in the seismogenic source.The "equivalent dislocation" of the seismogenic body can be calculated by using the theoretical formula of the coseismic dislocation,so as to further calculate the equivalent seismic moment(energy)of the seismogenic body.(2)Using the Okubo dislocation theory,we give a method to simulate and invert the parameters of the "equivalent dislocation" model.The model parameters can be divided into two categories—mechanical parameters and geometric parameters;the method of selecting geometric parameters based on the images of the gravity field change before the earthquake is given through theoretical simulation.And the mechanical parameters that cannot be obtained directly from the images because of the crude error in the gravity data,so we selected the Robust-Bayesian Least Squares method to invert it.(3)Based on the above studies,using the Yao’an 6.0 earthquake and the Wenchuan 8.0 earthquake as examples,we studied the earthquake risk prediction method based on the locking shear force mode:(1)To verify the rationality of the model and the parameter selection scheme,we take the 2008 Wenchuan Ms8.0 earthquake and the 2009 Yao’an Ms 6.0 earthquake as examples,we inverted the "equivalent dislocation" model parameters based on the typical gravity change image identified by the floating gravity data in the vicinity of the earthquake region for many years.(2)The forward simulation results of the model show that the dynamic changes of the gravity field before the Wenchuan earthquake and the Yao’an earthquake respectively correspond to the surface gravity changes generated by the “slip displacement” and “slip-slip” equivalent dislocations.The initial values of the geometric parameters of the model are determined by the features of the dynamic change image of the gravity field before the earthquake in these two regions;and the corresponding mechanical parameters are obtained by the robust-Bayesian least squares inversion.The seismogenic energy was calculated by the model parameters.(3)Comparison the seismogenic energy corresponding to “equivalent dislocation” with the actual seismogenic situation,the result shows that the seismogenic energy of the Wenchuan earthquake is equivalent to Mw 8.0,which is close to the actual magnitude of the earthquake(Mw7.9);Yao’an earthquake The seismogenic energy is equivalent to Mw5.9,which is close to the actual magnitude of the earthquake(Mw5.7).It shows that the parameter selection scheme in this paper has certain rationality and effectiveness.(4)The results of the earthquake cases show that the earthquake risk prediction method based on the locking shear force mode can predict the magnitude and source mechanical properties of the continental structure earthquakes,and can be used to delineate the locations of strong earthquakes in the future.After further improvement,it can be an effective way and a potentially fast numerical prediction scheme of earthquake prediction based on gravitational field images.