利用远震地震波形畸变现象探测地幔低速体

地幔低速异常体的精确探测对于研究热点/地幔柱的成因至关重要.但是,由于观测手段以及波前愈合现象的影响,远震走时层析成像在探测地球深部的低速异常体时具有极大的限制.因此,越来越多的学者利用地震波穿过低速体后产生的绕射波和直达波.干涉造成的波形畸变信息来研究地幔低速体.本文通过三维地震数值模拟来拟合长白山、黄石公园地区宽频地震记录,分析绕射波和直达波相互干扰造成的波形畸变现象.模拟结果显示,绕射波/波形畸变对于探测地幔深处的低速体具有较高分辨率;目前常用的基于互相关测量的有限频层析成像方法对于深部低速体速度异常低估现象明显.如果将有限频层析成像结果在深部的速度异常值放大到原来的2倍,数值模拟可以很好的拟合观测到的地震波形畸变现象.通过对长白山地区、黄石公园地区记录的地震波形畸变现象的拟合,我们进一步确认了长白山、黄石地区下方分别在400~600 km、700~1000 km处存在地幔低速体,并对低速体速度异常值进行了矫正.

Detecting slow velocity anomalies in deep mantle from waveform distortion of teleseismic events

The imaging resolution of slow velocity anomalies in deep mantle plays a key role in studying the origin of hotspots/mantle plumes. However, improving the resolution in traveltime tomography is still a big challenge due to wavefront healing, which is caused by the interference between the transmitting waves and diffracted waves that were generated at the boundaries of slow velocity anomalies. More and more seismic studies focus on this wave distortion and use it to resolve low velocity anomalies in the deep mantle. In this study, we conducted 3-D forward waveform modeling based on spectral-element method (SEM) to investigate waveform distortions observed by dense broadband seismic arrays in the Changbaishan volcano in NE China, as well as in Yellowstone volcano in western US. Our results show that a proper use of diffraction induced waveform distortion could improve the resolution of slow anomalies in deep mantle. The usually used finite-frequency tomography based on cross-correlation measurements underestimate the anomaly significantly in imaging slow velocity anomalies. The wavefront distortions could be fitted much better if we inflate the velocity anomaly in deep mantle by a factor of 2. We also confirmed the presence of a strong low velocity anomaly in the uppermost part of the lower mantle beneath the two volcanoes.