The complexity feature of crust-mantle boundary in Zhangbei seismic region and its tectonic implication
LAI Xiao-ling1), (赖晓玲) ZHANG Xian-kang1) (张先康) SUN Yi2) (孙 译)
1) Geophysical Exploration Center, China Earthquake Administration, 帮写英语论文Zhengzhou 450002, China
2) North China Institute of Water Conservancy and Hydroelectric Power, Zhengzhou 450008, China
Abstract
The PmP waveform data obtained from a deep seismic sounding profile, which ran through Zhangbei seismic region,were processed by means of both seismic wave complexity coefficient and frequency spectrum analysis
methods, and the complexity characteristics of crust-mantle boundary beneath the studied area and its adjacentregion were determined. The results show that the place below epicenter can be taken as boundary, the northernside of which is Inner Mongolia axis with small complexity coefficient and the southern side of which is Huai′anbasin with large complexity coefficient. The different spectrum patterns at the two sides of the epicenter were inferredfrom spectrum analysis. In the epicentral area, there have been multi-period magmatic eruptions sinceMeso-Cenozoic and craters exist at the surface. From the velocity imaging of middle and upper crust in Zhangbeiseismic region it can be found that there are crustal low velocity bodies around the craters and also there are low
velocity zones, which went into deep crust. It is suggested that the distinct zones of crust-mantle boundary complexitymay be the margin, where the magma had intruded due to magma activity in Meso-Cenozoic. The southernside with large complexity coefficient is deep magmatic activity area and the northern side with small complexitycoefficient is stable crust-mantle tectonics. The difference of crust-mantle complexity provides deep backgroundfor the development of strong earthquake.
Key words: Zhangbei seismic region; crust-mantle boundary; complexity coefficient; frequency spectrum analysis; seismic tomography
Introduction
On January 10, 1998, an earthquake of MS=6.2 occurred on the boundary of the Zhangbeicounty and Shangyi county in Hebei Province, along with a serial of aftershock. And an earthquakeof MS=5.6 occurred on March 15, 1999 in this region. So far, many related researches have
been published (XU et al, 1998; ZHANG et al, 1999; SHI et al, 1999; WANG et al, 2000; LIUand DIAO, 2000; DIAO et al, 2004).The continental strong earthquakes mostly occurred in top fragile zones or transfer zones of
fragile and malleability, but their dynamic sources are closely related to deep structure environ-
∗ Received 2005-04-14; accepted in revised form 2006-03-07.
Foundation item: Joint Seismological Science Foundation of China (104027 and 102025).
Contribution No. RCEG200505, Geophysical Exploration Center, China Earthquake Administration.
ment. Therefore, in addition to research on seismic fault, it′s also necessary to research structureproperty of crust-mantle boundary in strongly seismic active area, which is significant for interpretoccurrence mechanism of the continental strong shock. In this paper we have processed the PmPwaveform data of deep seismic sounding profile cross Zhangbei seismic region and the complexitydistribution of crust-mantle boundary and feature of frequency spectrum are obtained. Then wecombine with 2-D velocity profile and upper crust seismic tomography to discuss the relationsbetween structure property of crust-mantle boundary and strong earthquakes occurrence.
1 Geological setting and profile position
The geological setting in Zhangbei seismic region and its adjacent regions are mainlyNW-striking faults including a series of NE-striking faults crossing and cutting (ZHANG et al,
1999). And in this region, there have been themulti-period magmations since Meso-Cenozoic(YANG et al, 1989; LIU et al, 1989), and Miocene
basalts cover large of the surface. Fromsurface there are not obvious active fault, butthere exist NE-striking and NNW-striking buried
faults developing in shallow crust (XU et
