Strain Release Characteristics Before Strong Earthquake In Chinese Mainland

As the base of our study, the calculating method for the optimal model parameters of ASR or DSR has been proposed, in which the enumeration and the LSE has been combined. It is high accuracy, fast calculation and ergodic. The curvature parameters C has been used to judge acceleration or deceleration of the Benioff strain release. Further more, the Kolmogorov test also has been introduced to check the difference of the actual data with the fitting power-law curve.Take the 21 earthquakes with M~7 in Chinese mainland since 1970 as the samples, the accelerating or decelerating phenomenon of Benioff strain release around the epicenter of the mainshock has been detected systematically. The patterns of the strain release and the complexity, numerical distribution of parameter m, as well as the precursor features have been discussed in this paper. The pattern of the strain release before 21 earthquakes with M7 can be divided into five types. For type 1 (DA pattern), the strain release accelerates in larger region, but decelerates in smaller region near the epicenter before the mainshock. For type 2 (AD pattern), the strain release decelerates in larger region, and accelerates in smaller region near the epicenter before the earthquakes. For type 3 (A pattern), only accelerating strain release could be detected. For type 4 (D pattern), only the decelerating strain release could be detected. There is only one case belongs to type 5, which strain release is roughly uniform. There is no obvious difference for the duration of the patterns. For DA or AD, the duration of accelerating patterns is about 3 to 6 years longer than that of decelerating pattern. The durations of simplex ASR or DSR are about 10 years. For DA pattern, which have steady accelerating and decelerating strain release before target earthquakes, the critical region is about 260 to 400 km and 100 to 200 km respectively. The size of accelerating and decelerating regions is about 3 to 5 times and 1 to 2 times of seismic rupture size of about M7 earthquake respectively. It is opposite for AD pattern. The critical size of ASR or DSR for different earthquakes is about the same, it is about 150 to 420 km and 200 to 450 km for ASR and DSR respectively. The m value, parameter of model, has a good stability. It is from 0.3 to 0.6 for accelerating strain release, whatever DA, AD, or simply ASR. It indicates that the accelerating strain release is roughly the same whatever which pattern it is and what the value of strain release is. However for decelerating strain release the m value of DA model ranges from 1.8 to 2.8, 1.3 to 1.9 for AD model and 1.5 to 2.5 for single DSR model. It seems that strain release of DA model decelerates more obviously than the single DSR model, and the AD model decelerates the least.Utilizing the data of laboratorial rock rupture under high temperature and pressure, we discuss, with the accelerating model, the characteristics of strain release of micro-fracture in groups inside rock samples under different temperature and pressure for the first time. Combining with the environment and mechanical properties of medium underground, we focus on discussing the relations between the m value of acceleration model and environment of temperature and pressure, and physical meaning of m in concept. This work has some innovation. The characteristics that rock rupture processing does nothing with scale can give us some help to know the characteristics and physical mechanism of seismic strain release. The results show that the strain release of micro-fracture has some accelerating characteristics as the confining pressure increases under the normal condition, but m value doesn't have the trending change. So m value of AER model for actual earthquake may have no relation with mechanical environment and loading strength around the hypocenter. At confining pressure of 400MPa, the accumulating strain release of micro-fracture shows some tendency. At the low temperature pure brittleness breaks, strain release of micro-fracture presents representative characteristic of acceleration, and the m value is low; With the increasing temperature the strain release of micro-fracture becomes low, so is the characteristic of acceleration , but the m value improves to some extent ;As the temperature is improved further, m value is approaching to 1, the micro-fracture demonstrates homogeneous characteristic of strain release; because of higher temperature corresponding to the lower intensity of medium, the medium's intensity is one of the reasons controlling the patterns, such as acceleration, deceleration and even, of strain release for small and medium earthquake in the critical region. Under the conditions that the temperature and the pressure change with crust depth, which equal to the environment in lower crust where quasi-abrupt failure, there is some accelerating characteristics of strain release before sample was destroyed, and the m value is smaller than 1. In the depth range where incremental failure is the major characteristics, release of strain represents decelerating characteristics gradually, and m value is obviously larger than 1. For actual earthquakes happening under temperature and pressure in deep environment, systematic failure is the characteristics of the break, and before failure micro-fracture became dense, accumulating release of strain accelerated and the m value was obviously low. This phenomenon means that the pattern of strain's release (DA or AD and so on) maybe has something to do with the major depth of the foreshocks in different regions of different time's duration, and in the same region the changes of the pattern of strain release, to some extent, may be due to changes of the depth of used earthquakes'distribution. The results also show that the characteristics of accelerating strain release is more obvious, and m value is low before macroscopic failure of the whole rock. Since the rock broke, the accelerating characteristics of strain release for tiny stick-slip of different stages has gradually weakened, simultaneously m value becomes large and strain release becomes homogeneous. This indicates that the state of rock obviously influence on the m value of AER model. If rock has good completeness and high intensity, strain of micro-fracture has obvious characteristics of acceleration as the failure approaching. Since rock broke, the strength breaks down. Under deformation causing by further pressure, as failure of stick-slip approaching the characteristics of strain release for tiny stick-slip represents increasingly weak and gradually becomes homogeneous. In the period of quasiperiodic stick-slip, the m value becomes large with time, so the less m value indicates the intensity of medium for the hypocenter of the lower crust. With the additional load strengthening the failure will happen irreversibly when the differential stress conquers the strength of medium for high pressure underground. Similar phenomenon should be noticed in new area of seismic activity.