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A Study of Seismic Wave Velocity Changes in the Wenchuan Earthquake Region Using the Seismic Ambient Noise Autocorre-lation Method

Author: ZhaoPanPan
Tutor: ChenJiuHui
School: Institute of Geology of China Earthquake Administration
Course: Solid Earth Physics
Keywords: Environmental noise Autocorrelation function Seismic velocity changes Wenchuan earthquake Kawanishi array
CLC: P315
Type: Master's thesis
Year: 2011
Downloads: 40
Quote: 2
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Abstract

May 12, 2008, the Longmen Shan fault zone in Sichuan Mw7.9 earthquake occurred, causing huge casualties and losses. After the earthquake surface geological study (Xu et al., 2009), the earthquake source process inversion (Zhang Yong, 2008; Ji and Hayes, 2008; antiscnse et al, 2008; Shen et al, 2009), the aftershocks positioning (Huang Yuan et al, 2008; Chen Jiuhui etc., 2009) and other aspects of the research results show that the Wenchuan earthquake so far in high-angle thrust faults on the intensity of the largest inland earthquake occurred (Zhang et al, 2010). Breeding and occurrence of earthquake coseismic stress field of underground fluid migration, destruction of the upper crust is usually accompanied by the occurrence of the event, leading to changes in the physical properties of the crust medium. The study will contribute to the understanding of the strong earthquakes that occurred after the earthquake recovery process, and contribute to the process of earthquake dynamics in strong earthquakes before and after the change in physical properties of crustal media. Regional media and changes in the stress field before and after the Wenchuan earthquake on special seismogenic structures Wenchuan earthquake is particularly important. In recent years, the use of passive source seismic ambient noise cross-correlation function of coda imaging interferometry (PII) a change in the nature of a new monitoring crustal media has the passive advantages and sustainable monitoring features, it has broad application space. In the earthquake (Wegler et al., 2009; Xu and Song, 2009; Chen et al, 2010; Cheng, et al, 2010; Liuzhi Kun and Huang Jinli, 2010), volcanoes (Brenguier et al., 2008a), fault with (Wegler et al, 2007; et al. Brenguier., 2008b) and other aspects of the study results show that the method for how to understand the strong earthquakes that occurred after the earthquake recovery process, and how to understand the process of the activity of the volcano internal magma, and thus to predict their time, strength outbreak has very important significance. Support 973 research projects since the start of October 2006, the Institute of Geology, China Earthquake Administration earthquake dynamics laid by the State Key Laboratory in western Sichuan, China (100 ° ~ 105 ° E, 26 ° ~ 32 ° N) by 297 broadband digital seismographs mobile observatory array (referred to as the the Kawanishi array), to obtain a complete seismic activity before and after the Wenchuan earthquake observation records, provided valuable information for the further study of the Wenchuan earthquake. In this paper, the area north of 29 ° N Kawanishi mobile seismic array 137 stations from January 2007 to October 2008 consecutive three-component seismic ambient noise recorded before and after the earthquake in Wenchuan earthquake crustal velocity variation. With the advantage of the the Kawanishi array density, self-correlation function between the different components of a single cross-correlation function for a single three-component noise using the moving window technique cross spectrum is measured within the frequency range 0.33 ~ 1HZ after the 50-day moving average correlation function with the long time average reference correlation function of the rate of change in the travel time, and then obtain the shallow crust velocity relative change over time, and its spatial distribution characteristics. The research results show that the: ⑴ Wenchuan Mw7.9 level earthquake caused the Longmen Shan fault zone and its surrounding area along the fault rupture direction exists significantly reduce with the speed of the shock wave, and along the rupture opposite direction Songpan - Ganzi block western Sichuan-Yunnan block speed change is relatively weak, but the the Sichuan Basin internally but also significantly speed relative reduction. ⑵ and Chen et al. (2010) using the cross-correlation function Shuangtai coda coseismic speed the rate of change in the region resulting comparison, the the coseismic velocity variations are measured in rates of larger magnitude, but both changes morphology The consistent of both coseismic velocity relative change before the earthquake disturbance amplitude is the ratio of 2 and 2.1, respectively. The difference should be the relative change in the sensitivity of the different methods used in this area at different depths crust speed, and therefore the the biliterate results there is no essential difference. ⑶ despite coseismic speed changes in the spatial distribution of coseismic strain compared, but the coseismic noise correlation function measurement speed the rate of change of Bi Wenchuan earthquake same shock body strain rate (about 10-6 middleweight ) 2-3 orders of magnitude, indicating that the the coseismic body strain is not caused by the coseismic crustal speed change main factors. For the the shallow crustal media, by coseismic stress changes caused by the medium porosity or medium fluid migration coseismic velocity changes. Strong ground shaking damage caused by the shallow media close to the focal region, may also affect the results of the noise correlation function measured velocity variations. ⑷ with higher spatial resolution, noise autocorrelation study further found to increase with the speed of the shock zone exists in Xianshuihe fault and the Longmenshan fracture intersection District Kangding, Luding near, its speed increased significantly above the background velocity perturbation amplitude. This area and coseismic Coulomb stress changes and surface deformation observation prediction peripheral fault Coulomb stress increase zone (Parsons et al., 2008; Wan Yongge, 2009; Single Bin et al, 2009). This is related to the speed of change in the measurement of the noise of the high density of stations can measure not only the average large-scale crustal velocity changes can also detect coseismic stress increase. Our study also found that the coseismic Coulomb stress increase the effect of duration of about 2 months or so, After this, the regional stress field is basically consistent with the surrounding area stress decreased gradually restored. Stress drops in the region may be related to the earthquake occurred in the region. ⑸ before the Wenchuan earthquake, the various regions, especially in the Longmen Mountain region and the Sichuan Basin region are showing similar crustal velocity variation over time (Figure 4-7), the time scale of about 200 days before the earthquake showed the varying degrees of crustal velocity increase. This speed change with the season there is no obvious relationship, and therefore is likely to be increased manifestation of crustal stress. However, due to the of Kawanishi array observations before the Wenchuan earthquake only about a year and a half, the velocity and stress field changes before the Wenchuan earthquake is still difficult to give definitive conclusion. About the change of the stress field before the Wenchuan earthquake and its relationship with the Wenchuan earthquake still need further study.

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