利用噪声研究地下介质速度结构及其变化在中国大陆的应用

Application Research on Velocity Structure of Subsurface Medium and its Changes in the Chinese Mainland Using Ambient Noise

摘要:

背景噪声层析成像技术已被广泛应用于地壳和上地幔速度结构的研究,这种方法不依靠地震的发生和人工源爆破,只需记录连续的噪声信号而无需产生信号,因为噪声穿过地下介质时会携带信息,然后通过利用台站记录到的连续背景噪声数据进行互相关计算和叠加,即可得到台站间的经验格林函数,从而获取对地下结构的认识。这种方法已经很好地应用于中国的东北地区、华北克拉通、青藏高原以及华南地区,并成功地揭示了这些地区地壳与上地幔顶部的速度结构。此外近年来,一些学者还利用噪声互相关技术研究地下介质地震波速度随时间的变化,通过对汶川大地震前后连续噪声记录的研究发现,大震发生后呈现同震波速降低和震后波速逐渐恢复的特点,这表明可以通过观测地震波特性的变化来监测地下应力的变化,从而为大震的预测预防工作提供科学依据。本文主要综述了近些年来背景噪声技术及其在中国大陆地区的应用。

Abstract:

Ambient seismic noise tomography has been widely used to study the crustal and mantle structure in the world.This method does not depend on natural earthquakes or artificial explosions, and therefore, it has become a very popular method to image the structure of the crust and uppermost mantle.The subsurface structure beneath the two stations can be determined by extracting the empirical Green's functions by the cross correlation and stacking of the continuous seismic noise recorded at the two stations.Ambient seismic noise tomography technique can be regarded as sourceless tomography because it functions without source-receiver propagation.The energy of surface waves is stronger than other waveforms and therefore, most studies of ambient noise have focused on surface wave measurements.Surface wave tomography from ambient noise has been successfully used in the United States and the Pacific Northwest, China, New Zealand, South Korea, and Europe.It is obvious that the ambient noise method has several important advantages over traditional surface wave tomography based on earthquakes:(1)the method can be applied to areas that are well instrumented but have a very low level of seismic activity;(2) the ambient noise method is relatively unaffected by seismic source location;and (3) it can be used to make short-to-intermediate period dispersion measurements.However, one limitation of the method is that it is difficult to extract long-period dispersion measurements because most noise sources are limited to shallow depths.In this paper, we briefly introduce an improved method to extract long-period dispersion measurements from ambient seismic noise.Ambient noise data have also been applied to study crustal and mantle anisotropy.Radial anisotropy results based on ambient noise show that in eastern and southeastern parts of the Tibetan Plateau, mid-to-lower crustal low-velocity zones are closely associated with positive radial anisotropy (VSH > VSV).Azimuthal anisotropy in the southeastern Tibetan Plateau revealed that the azimuthal anisotropy pattern in the crust differs to that in the upper mantle, which suggests that the crust and mantle might deform differently.Some researchers have used ambient noise correlation techniques to study temporal changes of seismic velocity.An analysis of the continuous ambient noise data, both before and after the Wenchuan Earthquake, revealed that the coseismic reduction of seismic velocity and postseismic recovery are prominent.The observed temporal changes likely reflect damage and heating processes in the crust associated with the Wenchuan mainshock.With development of the ambient noise method, the resolution of seismic images of the crustal and upper mantle structure will be enhanced.This paper presents the technique of ambient noise tomography and its application to the velocity structure and studies of temporal velocity change on the Chinese mainland.