CHIN ESE JOU RNAL OF GEOP H YSICS Vol. 51, No. 6 Nov., 2008,..,2008,51 (6) : Hu X G, Hao X G. The short2term anomalies detected

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1 CHIN ESE JOU RNAL OF GEOP H YSICS Vol. 51, No. 6 Nov., 2008,..,2008,51 (6) : Hu X G, Hao X G. The short2term anomalies detected by broadband seismographs before the May 12 Wenchuan earthquake, Sichuan, China. Chinese J. Geophys. (in Chinese), 2008, 51 (6) : , 3,, , , : 1 3,,, 2 10 s..,,,. :,.,,,, (2008) P , The short2term anomalies detected by broadband seismographs before the May 12 Wenchuan earthquake, Sichuan, China HU Xiao2Gang, HAO Xiao2Guang 3 Key L aboratory of Dynamical Geodesy, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan , China Abstract We investigated broadband seismograp hic waveform data in 7 13 May 2008 f rom 40 seismic stations of t he National Digital Seismograp h Network of China (NDSN) after t he 12 May 2008 Wenchuan eart hquake, showing t hat o bvio us p recur so r wa s widely detected t hro ugho ut China. The p remonitory p henomena is t hat t remor amplit udes in t he period band of 2 10 s have increased steadily wit hin 1 3 days before t he main shock rupt ure. Anomaly tremors can also be detected by gravimeter s and tilt meter s. Tremo r signal s are widely detected in mainland China, but t hey are very strong in So ut heastern China and weak in Western China. The po ssible slow slip of Sout h China block before t he great Wenchuan eart hquake may be t he cause of t remors, which is related to some frictional p rocesses of t he slow slip. Keywords The great Wenchuan of 12 May 2008, Anomaly t remors, Broadband seismograp hs, Gravimeter, Tilt meter , :,Lacoste ET [ 1, 2 ] , ( , , ) (102215).,,,. 3,. E2mail ac. cn

6 : 1727 Lacoste ET ( 1050 km),, 4 8 s [3 ].,, : [4 ]., 40 5 7 13. 1 3.,. 2 2008 5 12 14 :28, 8. 0., 5 7 13 ( 1A),. :,, ( 1B).,,,. Lacoste ET ( 5 km).., 2 10 s, 4 8 s. 1C 4 8 s,. 5 9, 5 12.. 48.

2 6 : 1727 Lacoste ET ( 1050 km),, 4 8 s [3 ].,, : [4 ]., , :28, 8. 0., ( 1A),. :,, ( 1B).,,,. Lacoste ET ( 5 km).., 2 10 s, 4 8 s. 1C 4 8 s,. 5 9, ,, 500 km , (A),. (B) 4 8 s,. (C) , 4 8 s. Fig. 1 Seismic records of three2component broadband seismograph within 6 13 May 2008 from station W HN (A) Close view of the waveform showing that recording curve became obviously thick before the main rupture. (B) Record filtered on 4 to 8 s to highlight t he tremors before the Wenchuan earthquake. (C) FF T for observations in 9 11 May 2008 showing t hat tremors are enriched in t he period band of 4 8 s J CZ21 CTS21, 50 Hz. 10 STS21

3 1728 (Chinese J. Geophys. ) 51 STS22, 40 Hz., 24 75,40, 2 10 s, 4 8 s ,.,. 5.,.,,.,,,., ( > 1500 km),,,. ( < 600 km),,..,., 4 8 s ,, ,,. 3,.,,, 5.,, 2 5.,, 2.,, Table 1 Basic information and observed tremor intensity of 40 seismic stations (km) WZH CTS SSE STS QZH CTS SZN CTS CNS CTS NNC CTS NJ2 CTS GZH J CZ HEF CTS WHN J CZ QZN STS ENS STS GYA CTS L YN CTS HNS CTS CD2 J CZ SN Y J CZ XAN STS CN2 CTS BJ T STS KMI STS PZH CTS BNX CTS YCH CTS H HC J CZ MDJ STS HL R STS XL T CTS L ZH CTS WUS CTS TNC CTS L SA STS GTA J CZ HEH J CZ WMQ STS AXX CTS CAD CTS GOM J CZ NAQ CTS H T G CTS

6 : 1729 2 40 Fig. 2 Map showing locations of 40 seismic stations of National Digital Seismograph Network of China 3??.,. 40 mm/ a [5, 6 ],, 20 mm/ a [5, 6 ].

4 6 : Fig. 2 Map showing locations of 40 seismic stations of National Digital Seismograph Network of China 3??.,. 40 mm/ a [5, 6 ],, 20 mm/ a [5, 6 ].,, 500 km km,, [7, 8 ]. 2. [9 ]. [10 13 ],, (Nankai),, s,. (Non2volcanic tremor,nv T) [ 9, 10 ]. NV T, NV T [ ].,, [ ]..,,. ( ) [17 ].,., : 3,,.,,.

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7 1732 (Chinese J. Geophys. ) 51 3, 5 ( km), [24 ]. 2 ; 4 10 s.,. 4 6.,,,.,. 1. Fig. 6 The compass graph in which origin is the epicenter, and the dash line indicates the direction of Longmenshan reverse fault, and vectors are determined by azimuths of 40 stations in mainland China and tremor intensity observed in these stations, showing that premonitory anomalies are weak to the west of Longmenshan fault, but strong to the east,i. e. the South China block. The tremor intensity at each station is given at Table 1.. :,,,. GPS.. ( ). [18,19 ], [20 23 ],.,.,,, :,.,. GPS,,.. ( Ms 7. 0),, 7 [25 ]. : ,..,,,,.,,,,.,,..

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9 1734 (Chinese J. Geophys. ) 51 velocity anomalies in the Lhasa terrain : tomographic evidence for partial melting in the crust. A cta Geoscientia S inica (in Chinese), 2005,26 (5) : [ 24 ],, Ms 8. 1., 2003, 9 :1 4 Yang Y L, Zhao G M, Gao G Y, et al. The slow eart hquake event occurring before west to Kunlun Mountain Pass eart hquake of M s 8. 1 on November 14, Recent Developments in Worl d Seismology (in Chinese), 2003, 9 :1 4 [ 25 ],,.., 2001, 21 (1) : Zhao G M, Yang G S, Chen H R. Quiescent foreshock and eart hquake prediction. Earthquake (in Chinese), 2001, 21 (1) : ( )

14 17.,., Fig. 1 Bougeur gravity map of research region 2 Fig. 2 Magnetic map of research region

14 17.,., Fig. 1 Bougeur gravity map of research region 2 Fig. 2 Magnetic map of research region 1 7 1 Vol. 17 No. 1 2002 3 (13 23) PROGRESS IN GEOPHYSICS March 2002 (,,100101) [ ],, 39.,,. [ ] ; ; [] P738 [] A [ ]100422903 (2002) 0120013211 0,,.,,, NE [1 ],.., ( NE, EW,NE ),.,.,,. 1 20 20 km ( 10