Periodic spectral characteristics of seismicity before strong earthquakes and their application

Periodic spectral characteristics of earthquake activity in the seismic strengthening areas of 24 ewthquakes withM≥ 6.0 are studied by the maximum entropy spectral method whose superiority is tested. Then the follow resultshave been obtained : ① The periodic spectra of seismic activity in seismic strengthening areas are different indifferent stage in earthquake-generating processes. Long periodic spectra and short ones coexist in normal stage,while only short ones (on average, 43% of long ones) exist and long ones disappear prior to ear’thquakes. ② Theappearing time of short period before earthquakes has some relations with magnitude. The result shows thatdecades or even one hundred years is the common value for a great earthquake of M=8.0, 30 years for one withmagnitude about 7 and 20-30 years for a strong quake of M=6.0. For the same magnitude earthquakes in differentregions the appearing time is also different. For example, it is longer in North China than that in the western pan ofChina. Then the characteristics are preliminarily explained applying the strong body earthquake-generating model.Applying the maximum entropy spectral method, the idea of tendency prediction for strong and great earthquakesis suggested and used into practice. for example. the tendency predictions of the Wuding earthquake with M=6.5and the Lijiang earthquake of M=7.0 in Yunnan Province got some positive effects. So a new method of tendencyprediction of M≥6.0 earthquakes is offered.

The heterogeneous characteristics of crust-mantle structures and the seismic activities in the northwest Beijing region

In this paper, the abnormal characteristics of the crustal structures in the seismic active region, Yanqing-Huailai and Zhangbei-Shangyi, are obtained by means of comprehensively interpreting and studying the data of deep seis- mic sounding profiles passing through the northwestern part of Zhangjiakou-Bohai seismic zone. The results show that the fluctuation of crystalline basement in the study region is obvious and that there exist considerable differ- ences in depth in different geological units. The locally abrupt variation of crystalline basement depths may be regarded as a mark of existence of crystalline basement faults. These crystalline basement faults and deep crustal faults provide a pass for the magma upwelling, resulting in the strong inhomogeneity of crustal structures. These phenomena of the complex seismic reflected waves and locally discontinuous reflection zones with different en- ergy indicate that the intensive squeeze and deformation of crust took place, which have led to the complex crustal structures and offered the dynamic source for the earthquake occurrence in this region. The low velocity bodies in different depths of crust and the local interface C1 in Zhangbei-Shangyi region may result from repeated magmatic activities. The certain stress accumulation in the brittle upper crust can cause the occurrence of earthquake under the action of local tectonic activity.

Paleoearthquake Investigation along the Chenghai Fault Zone since~500 ka,Southeast Margin of the Tibetan Plateau

The Chenghai fault zone is located in the Sichuan–Yunnan rhombus block,which is surrounded by the Honghe River,Xianshuihe–Xiaojiang,and Jinsha River fault zones.As a mid-continental active fault,it is one of the most important seismogenic fractures in the southeast margin of the Tibetan Plateau.Geological seismic study is an important supplement to the historical seismic record;therefore,identification of paleoearthquake events in this region is of great significance to reveal the pattern and mechanism of earthquake development.In this study,detailed investigation has been carried out on the earthquake traces that formed in the Quaternary sediments in the Jinsha River section of the Chenghai fault zone,and samples for dating chronology testing were also collected.Many paleoearthquake traces were discovered in the field,including earthquake fissions,dammed lake sediments and landslides,earthquake-generated rock falls,seismic faults,and sand liquefaction veins.The collected samples were tested using optically stimulated luminescence,electron-spin resonance,and U-series methods.A total of 68 chronological samples were dated,combined with the results of field investigations,and 10 large paleoearthquake events were discovered in this region since 500 ka,which are at approximately 450,400,345,300,250,190,155,105,75,and 25 ka.Ten tectonic activity periods that produced multiple paleoearthquake events were identified since 500 ka.This study identifies paleoearthquake events in longer scales,larger spaces,and more extensive sediments,which provides new perspectives and new ideas for paleoearthquake research.