A Study on Tidal Force Triggering of the Wenchuan M_S8.0 Earthquake and Its Strong Aftershocks

By calculating the azimuth, phase angle and change rate of tidal force, and contrasting their rupture types, we find that in 6 of 11 reverse faulting earthquakes, the angles between the direction of the horizontal tidal force and compressive principal stress are within 33°, the vertical phase angles of all 8 strike-slip earthquakes are in the upward peak section and the vertical phase angles of two extensional normal faulting earthquakes are in the downward peak section. According to the above statistics, the mechanism of tidal force triggering of the Wenchnan Ms8. 0 Earthquake and its strong aftershocks is discussed.

Spatio-temporal characteristics of strong aftershocks of the M_S8.0 Wenchuan earthquake

Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（greater than or equal to M6.0） of the MS8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA.The number of strong aftershocks,the parameters of Gutenberg-Richter＇s relation and the modified form of Omori＇s law are validated based on the relocation data of aftershock sequence of the MS8.0 Wenchuan earthquake.Moreover,the spatio-temporal characteristics and wave energy release of the strong aftershocks（M≥6.0） are analyzed.The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve.

Comparison of the spatial distribution of ground motion between main shocks and strong aftershocks

Using the ground motion attenuation relation, we calculated and compared the effective peak acceleration （EPA） generated by main shocks and their strong aftershocks of 21 earthquake sequences with Ms≥7 occurred in Chinese mainland and offing of China during 1966-2002. The result shows that EPA of strong aftershocks usually exceed that of main shock for 76.2% earthquake sequences and EPA of more than 50% strong aftershocks are greatly larger than that of main shocks in large area, which suggests that it is necessary to take damage produced by strong aftershock into account in the probabilistic seismic hazard analysis and the seismic design.

The relationships between earthquakes and positions of the sun and moon（Ⅱ）——Sometemporalcharacteristicsoftheaftershocksequencesofstrongearthquakes

This paper deals with the distributive characteristics of the occurrence time of earthquakes with respect to the aftershock sequences of strong earthquakes. The distribution of lunar and solar local hour angles at the time of commencement of moderate and strong aftershocks indicates that the time of commencement of moderate and strong aftershocks is modulated by the positions of the sun and moon and then the earthquake restrained time zones exists also. In this paper the differences of earthquake restrained time zones between the preshock sequences and the aftershock sequences are compared, and the possible mechanism is analyzed preliminarily. And the possible maximum scope of accuracy in predicting the occurrence time of an earthquake is determined as well.

Spatio-temporal variation and focal mechanism of the Wenchuan M_S8.0 earthquake sequence

Based on abundant aftershock sequence data of the Wenchuan Ms8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 krn and the depth between several and 40 km. The depth profile （section of NW direction） vertical to the strike of aftershock zone （NE direction） shows anisomerous wedgy distribution characteristic of afiershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like ＇to be going to rupture＇ segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because afiershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.

Characteristics of Seismic Sequences and Early Tendency Judgment for Yutian M_S7.3 Earthquake in 2014

The basic parameters,seismogenic structure and seismic sequences characteristics of the Yutian MS7. 3 earthquake on February 12,2014 are introduced and compared to the Yutian MS7. 3 earthquake in 2008. The results show that the MS5. 4 earthquake is regarded as an immediate foreshock of the Yutian MS7. 3 main shock. The frequency of strong aftershock sequences was low and their number declined quickly,and the maximum aftershock was a MS5. 7 earthquake. According to analysis of the historical earthquake sequence type,and parameter of h-value,b-value and energy release ratio between main shock and sequence etc.,we found the preliminary conclusion that the Yutian MS7. 3 earthquake sequence in 2014 was a foreshock-main shock-aftershock type.