Xu Zhen J, Song Xiaodong
Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Proc Natl Acad Sci U S A. 2009 Aug 25;106(34):14207-12. doi: 10.1073/pnas.0901164106. Epub 2009 Aug 10.
Detecting temporal changes of the medium associated with major earthquakes has implications for understanding earthquake genesis. Here we report temporal changes of surface wave velocity over a large area associated with 3 major Sumatra earthquakes in 2004, 2005, and 2007. We use ambient noise correlation to retrieve empirical Green's function (EGF) of surface waves between stations. Because the process is completely repeatable, the technique is powerful in detecting possible temporal change of medium. We find that 1 excellent station pair (PSI in Indonesia and CHTO in Thailand) shows significant time shifts (up to 1.44 s) after the 2004 and 2005 events in the Rayleigh waves at 10-20 s but not in the Love waves, suggesting that the Rayleigh time shifts are not from clock error. The time shifts are frequency dependent with the largest shifts at the period band of 11-16 s. We also observe an unusual excursion approximately 1 month before the 2004 event. We obtain a total of 17 pairs for June, 2007 to June, 2008, which allow us to examine the temporal and spatial variation of the time shifts. We observed strong anomalies (up to 0.68 s) near the epicenter after the 2007 event, but not in the region further away from the source or before the event or 3 months after the event. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture and the broad area near the fault zone.
探测与大地震相关的介质的时间变化，对于理解地震成因具有重要意义。在此，我们报告了与2004年、2005年和2007年苏门答腊三次大地震相关的大面积地表波速度的时间变化。我们利用环境噪声相关性来获取台站之间地表波的经验格林函数（EGF）。由于该过程完全可重复，这项技术在探测介质可能的时间变化方面很强大。我们发现，一对出色的台站（印度尼西亚的PSI和泰国的CHTO）在2004年和2005年事件之后，在10 - 20秒的瑞利波中显示出显著的时间偏移（高达1.44秒），但在勒夫波中未出现，这表明瑞利波的时间偏移并非由时钟误差导致。时间偏移与频率相关，在11 - 16秒的周期带中偏移最大。我们还在2004年事件前约1个月观测到一次异常偏移。我们获取了2007年6月至2008年6月期间总共17对数据，这使我们能够研究时间偏移的时空变化。我们在2007年事件之后，在震中附近观测到强烈异常（高达0.68秒），但在离震源更远的区域、事件之前或事件之后3个月均未观测到。这些观测结果被解释为破裂附近和断层带附近广大区域上地壳中部的应力变化及随后的松弛。
