Elasticity soltion of rupture model of shallow earthquake

Based on the theory of elastic mechanics,and using the typical rupture model of shallow earthquake,the authors considered the shallow earthquake as a plane mechanical problem,which was constructed the corresponding mechanical model.By the stress components' formulas of the semi-infinite model acted by the finite even shearing force,the main stress is deduced.It is clear that the sector on the right of the center section is squeezed zone,where the maximum principal stress points at the "source of stress",and that on the left is tensile zone,where the minimum principal stress points to the "source of stress".

Shallow destructive earthquakes

Depths of earthquake occurrence and large slip distribution are critical for seismic hazard assessment.Numerous examples show that earthquakes with similar magnitudes,however,can result in significantly different ground shaking and damage.One of the critical factors is that whether the large slip was generated near the ground surface.In this article,we reviewed two aspects that are important on this regard,shallow slip deficit and nucleation depth.Understanding how shallow future earthquakes may nucleate in particular regions,such as shale gas fields,is critical for hazard assessment.Whether or not a strong earthquake may slip significantly at shallow depths(less than 3 km)plays crucial rules in seismic hazard preparation and should be further investigated by integrating high-resolution fault zone observations,dynamic rupture simulation,and fault zone properties.Moreover,precisely resolving shallow depth and slip distribution of earthquakes demands InSAR and/or other image data that can better capture the near-fault deformation to constrain the source parameters of earthquakes.

An overview on shallow strong earthquake activity and earthquake live losses of Chinese mainland in the centenary from 1901 to 2001

This paper reviews and analyses briefly the general characteristics of shallow strong earthquakes (MS≥6.0, focal depth h≤70 km) in space, time and magnitude and earthquake fatalities of Chinese mainland in the centenary from 1901 to 2001. During the period from 1901 to 2001, there occurred about 420 strong shallow earthquakes with magnitude MS≥6.0, 88% of them occurred in the western part of the Chinese mainland, which might be related to the strong deformation and motion of the active blocks in the western part. The average focal depth (25 km) in the western part is deeper than that (16 km) in the eastern part, which might be related to obviously thicker crust in the western part. The inhomogeneous distribution of focal depths with the depth profile is related to the variation of frictional and rheologic characteristics with depth in the crust. The shallow strong earthquake activity of Chinese mainland shows a tempo-spatial clustering process. The relation between earthquake magnitude MS and cumulative frequency Nc is lgNc=8.64–0.99MS. About 600 000 people died in the earthquakes of Chinese mainland from 1901 to 2001, the most serious earthquake live losses occurred in Hebei Province (250 723 people died) and Ningxia Hui Autonomous Region (246 269 died). There is no a statistically linear relationship between earthquake live losses and magnitudes. The statistical relationship between the earthquake deaths D and cumulative frequency Nc is lgNc=2.40–0.39lgD, which shows a fractal distribution.

Ground motion prediction equations based on shallow crustal earthquakes in Georgia and the surrounding Caucasus

Strong ground motions caused by earthquakes with magnitudes ranging from 3.5 to 6.9 and hypocentral distances of up to 300km were recorded by local broadband stations and three-component accelerograms within Georgia’s enhanced digital seismic network.Such data mixing is particularly effective in areas where strong ground motion data are lacking.The data were used to produce models based on ground-motion prediction equations(GMPEs),one benefit of which is that they take into consideration information from waveforms across a wide range of frequencies.In this study,models were developed to predict ground motions for peak ground acceleration and 5%-damped pseudo-absolute-acceleration spectra for periods between 0.01 and 10 s.Short-period ground motions decayed faster than long-period motions,though decay was still in the order of approximately 1/r.Faulting mechanisms and local soil conditions greatly influence GMPEs.The spectral acceleration(SA)of thrust faults was higher than that for either strike-slip or normal faults but the influence of strike-slip faulting on SA was slightly greater than that for normal faults.Soft soils also caused significantly more amplification than rocky sites.

Geographical distribution of hypocentral depths of Chinese earthquakes

The hypocentral depths of more than 200 Chinese earthquakes, of magnitudes from M 8.6 to M 3.0, are calculated from macroseismic data carried on earthquake catalogs, by using the formula for macroseismic hypocentral depths and the formula for general solution of macroseismic hypocentral depths. The results are plotted on maps to show their geographical distribution. It can be seen that most Chinese earthquakes are shollow ones. Of the 200 earthquakes calculated, 162(81.0%) hypocenters are shallower than 9 km, of which 111 (55.5%) hypocenters are shallower than 5 km. Such shallow earthquakes are mostly distributed in the provinces near to the North South Earthquake Belt, while the rest are scattered in the other provinces(except Zhejiang province). Earthquakes of medium(between 10 and 20 km) depth are relatively few (32 in number, 15.0%); they are distributed along the North South Earthquake Belt, and the western part of Xinjiang Uygur Autonomous Region and in provinces Shaanxi, Shanxi and Shandong (along the Tanlu Fracture Zone, crossing the sea to northeast China). Deep earthquakes are rare, being scattered in south Yunnan and the east end of Inner Mongolia Uygur Autonomous Region.

Statistical Analysis on Depth Distribution Model of Earthquake Hypocenters

In order to improve reliability of probabilistic seismic hazard analysis, shallow earthquake (depth <70 km) data, recorded with orientation precision grades 1 and 2 by modern instrument and containing depth information after 1970, are selected as statistical samples, meanwhile, North China seismic region, Central China seismic region, South China seismic region, Xinjiang seismic region and Qinghai-Xizang Plateau seismic region are chosen as statistical units to study the depth distribution characteristics of shallow earthquakes. Considering the differences of depth distribution characteristics of earthquakes with different magnitudes, the following magnitude intervals are adopted to analyze earthquakes with different magnitude scales, respectively: M S=2.0～2.9, M S=3.0～3.9, M S=4.0～4.9, M S=5.0～5.9 and M S=6.0～6.9. The results show that hypocenter depths are normally distributed by and large around the mean depth of the corresponding seismic region. The probabilistic distribution curves of earthquake depth in West China are wider than those in East China. The probabilistic distribution deviation, σ, of West China is greater than those of East China, that is, earthquakes in West China have a wider range in terms of depth. There is also a tendency that the absolute value of mean hypocenter depth increases with the magnitude by and large.

Tectonic Mechanism of the Suining(Ms5.0) Earthquake,Center of Sichuan Basin,China

On Jan.31 of 2010,the Suining earthquake occurred at Suining City whch is located the center of Sichuan Basin.It is unusual for the strong earthquake to occur at the center of Sichuan Basin with a stable geotectonic environment and a low-level historical seismicity.The macro-epicenter of the earthquake is located at Moxi town of Suining city,Sichuan province,China.The earthquake intensity of the epicenter area is degree VII,and the long axis of the isoseismal line trends in NE orientation.The Suining earthquake caused the collapse or destruction of 460 family houses.The earthquake focal mechanism solution and records of the near-field seismographic stations showed the earthquake occurred at the reverse fault at a depth 34 km.Based on the waveform and focal mechanism,we consider the Suning earthquake is triggered by the reverse fault and not by the gravitational collapse or man-made explosive sources.Basing on seismic refraction profile and borehole,we consider that the earthquake is triggered by the backthrust fault of Moxi anticline rooted in detachments at a depth 3-4 km.Furthermore,we infer that tectonic mechanism of the Suining(Ms5.0) Earthquake is driven by the horizontal crustal shortening and stress adjustment on a shallow detachment after the Wenchuan(Ms 8.0) earthquake.