Velocity ratio variations in the source region of earthquake swarms in NW Bohemia obtained from arrival time double-differences
Velocity ratio variations in the source region of earthquake swarms in NW Bohemia obtained from arrival time double-differences
复制标题
波希米亚西北部地震群震源区波速比变化的到达时间双差
DOI：
10.1093/gji/ggt410
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发表时间：
2014
影响因子：
2.8
通讯作者：
T. Fischer
中科院分区：
文献类型：
--
作者：
T. Dahm;T. Fischer
地壳地震群是地壳小震源区在数天、数周或数月内发生强烈破裂和岩石破坏的表现。它们是由源区域更持久的应力变化引起的。通常，地壳的局部应力与流体或气体迁移有关，可能与预先存在的薄弱区域相结合。然而，验证和量化深层局部流体运动仍然很困难，因为受影响的区域很小，并且地球物理勘探方法通常无法达到所需的分辨率。
我们应用一种简单而稳健的方法来估计地震群震源区域中压缩波 (P) 和剪切波 (S) 之间的速度比（upsilon(P)/upsilon(S) 比）。如果该群与多孔或裂隙岩石中的气体有关，则 upsilon(P)/upsilon(S) 比率可能会非常小。该方法利用地面地震台观测到的 P 波和 S 波之间的到达时间差以及地震对之间的相关双差。优点是不需要地震位置，并且该方法似乎较少依赖于震源区域外地壳中未知的速度变化。因此，它适合用于监控目的。
应用包括 1997 年至 2008 年间来自西北波希米亚地震群地区的三个自然中地壳（8-10 公里）地震群。我们解决了在群体主要活动之前和期间 upsilon(P)/upsilon(S) 的强烈时间下降，以及在群体结束时 upsilon(P)/upsilon(S) 恢复到背景水平的问题。这些异常现象可以根据 Biot-Gassman 方程进行解释，假设在群体活动的开始阶段存在过饱和流体脱气。
Crustal earthquake swarms are an expression of intensive cracking and rock damaging over periods of days, weeks or month in a small source region in the crust. They are caused by longer lasting stress changes in the source region. Often, the localized stressing of the crust is associated with fluid or gas migration, possibly in combination with pre-existing zones of weaknesses. However, verifying and quantifying localized fluid movement at depth remains difficult since the area affected is small and geophysical prospecting methods often cannot reach the required resolution.
We apply a simple and robust method to estimate the velocity ratio between compressional (P) and shear (S) waves (upsilon(P)/upsilon(S)-ratio) in the source region of an earthquake swarm. The upsilon(P)/upsilon(S)-ratio may be unusual small if the swarm is related to gas in a porous or fractured rock. The method uses arrival time difference between P and S waves observed at surface seismic stations, and the associated double differences between pairs of earthquakes. An advantage is that earthquake locations are not required and the method seems lesser dependent on unknown velocity variations in the crust outside the source region. It is, thus, suited for monitoring purposes.
Applications comprise three natural, mid-crustal (8-10 km) earthquake swarms between 1997 and 2008 from the NW-Bohemia swarm region. We resolve a strong temporal decrease of upsilon(P)/upsilon(S) before and during the main activity of the swarm, and a recovery of upsilon(P)/upsilon(S) to background levels at the end of the swarms. The anomalies are interpreted in terms of the Biot-Gassman equations, assuming the presence of oversaturated fluids degassing during the beginning phase of the swarm activity.