Seismicity and seismic structure at Okmok Volcano, Alaska
Seismicity and seismic structure at Okmok Volcano, Alaska
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DOI：
10.1016/j.jvolgeores.2014.04.002
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发表时间：
2014-05-15
影响因子：
2.9
通讯作者：
Prejean, Stephanie G.
中科院分区：
文献类型：
--
作者：
Ohlendorf, Summer J.;Thurber, Clifford H.;Prejean, Stephanie G.
奥克莫克火山（Okmok volcano）是一座位于阿留申岛弧Umnak岛东北部的活火山口，最近于1997年和2008年爆发。奥克莫克地区在2003年至2008年6月期间发生了近似900次可定位的地震，从2008年火山爆发开始到2009年中期又发生了近似600次地震，为地震层析成像提供了足够的数据集。为了对Okmok的地震速度结构进行成像，我们对这些地震的一个子集应用了双谱验证和双差层析成像的波形互相关。我们还使用频谱衰减技术进行P波衰减层析成像。我们研究了2008年火山喷发的开放序列中地震活动的时空特征，以探讨在建立新的喷发口过程中岩浆迁移的路径。我们还结合了新的地震位移和三维（3D）速度模型与第一运动极性计算震源机制选定的事件在2008年爆发前和爆发periods.Through这些技术，我们获得了精确的位移，一个约束良好的三维P波速度模型，和一个边缘解决S波速度模型。我们的图像的主要低Vp和Vs异常直接下的破火山口组成的浅区在0-2公里的深度连接到一个更大的更深的区域，延伸到约6公里的深度。我们发现，低Qp的地区集中在中央到西南部的破火山口和对应的低Vp的地区相当不错。我们解释的低速异常（4-6公里深度）的火山口下方的岩浆体的更深的部分。这与环境噪声层析成像的结果一致，并表明之前仅基于大地测量数据对奥克莫克岩浆房深度的估计可能太浅。2008年爆发前的事件分布表明，岩浆房周围区域的超压和从下面引入新物质的组合共同导致了爆炸性喷发。从主岩浆房顶部逸出的岩浆可能在上升到锥D区域下方的过程中与较小的浅岩浆和地下水发生反应。在2008年爆发前和爆发期间的地震被发现有一个混合的走滑，斜法线，斜逆冲机制，与一个占主导地位的P轴方向，几乎垂直于区域构造应力。这可能表明，与岩浆活动有关的应力在这一时期局部主导了区域构造力。(c)2014爱思唯尔有限公司版权所有。
Okmok volcano is an active volcanic caldera located on the northeastern portion of Umnak Island in the Aleutian arc, with recent eruptions in 1997 and 2008. The Okmok area had similar to 900 locatable earthquakes between 2003 and June 2008, and an additional similar to 600 earthquakes from the beginning of the 2008 eruption to mid 2009, providing an adequate dataset for seismic tomography. To image the seismic velocity structure of Okmok, we apply waveform cross-correlation using bispectrum verification and double-difference tomography to a subset of these earthquakes. We also perform P-wave attenuation tomography using a spectral decay technique. We examine the spatio-temporal characteristics of seismicity in the opening sequence of the 2008 eruption to investigate the path of magma migration during the establishment of a new eruptive vent. We also incorporate the new earthquake relocations and three-dimensional (3D) velocity model with first-motion polarities to compute focal mechanisms for selected events in the 2008 pre-eruptive and eruptive periods.Through these techniques we obtain precise relocations, a well-constrained 3D P-wave velocity model, and a marginally resolved S-wave velocity model. We image a main low Vp and Vs anomaly directly under the caldera consisting of a shallow zone at 0-2 km depth connected to a larger deeper zone that extends to about 6 km depth. We find that areas of low Qp are concentrated in the central to southwestern portion of the caldera and correspond fairly well with areas of low Vp. We interpret the deeper part of the low velocity anomaly (4-6 km depth) beneath the caldera as a magma body. This is consistent with results from ambient noise tomography and suggests that previous estimates of depth to Okmok's magma chamber based only on geodetic data may be too shallow. The distribution of events preceding the 2008 eruption suggest that a combination of overpressure in the zone surrounding the magma chamber and the introduction of new material from below were jointly responsible for the explosive eruption. Magma escaping from the top of the main magma chamber likely reacted with both a smaller shallow pod of magma and groundwater on its way up below the Cone D area. The earthquakes in the 2008 pre-eruptive and eruptive periods are found to have a mixture of strike-slip, oblique normal, and oblique thrust mechanisms, with a dominant P-axis orientation that is nearly perpendicular to the regional tectonic stress. This may indicate that the stresses related to magmatic activity locally dominated regional tectonic forces during this time period. (c) 2014 Elsevier B.V. All rights reserved.