Analysis of teleseismic P waves with a 5200‐station array in Long Beach, California: Evidence for an abrupt boundary to Inner Borderland rifting
Analysis of teleseismic P waves with a 5200‐station array in Long Beach, California: Evidence for an abrupt boundary to Inner Borderland rifting
复制标题
对加利福尼亚州长滩 5200 个站的远震 P 波进行分析：内边境裂谷突然边界的证据
DOI：
10.1002/jgrb.50370
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
R. Clayton
中科院分区：
文献类型：
--
作者：
B. Schmandt;R. Clayton
本文分析了加州长滩石油工业调查记录到的4次Mw ≥ 6.5地震的纵波资料。该调查使用了一个二维阵列，拥有多达5200个地震仪，平均间距为120米，孔径为7 - 10公里。在频率接近1 Hz时，P波走时和振幅在短至~400 m的尺度上表现出相干的横向变化，包括长滩背斜顶部的局部延迟走时和振幅增加。较深的非均质性由P波相速度表示，该速度偏离西南方位角事件的参考模型预测。我们假设，从内边界（IB）到大陆南部加州莫霍面深度的东北方向的急剧增加导致的异常相速度。弹性正演模拟发现，走时与莫霍面很好地拟合，莫霍面向东北倾斜65°，位于纽波特-英格伍德断层带西南约10 km处。将大陆地壳的长英质厚度限制在28 km需要一个8 km厚的层，其下的P速度为7 km/s，这可能是前法拉隆洋壳的基底增生或中新世火山作用期间的岩浆底侵作用的结果。莫霍面倾角为65°的正演模型预测P-to-s转换的相速度约为5 km/s。阵列的平均P波信号的去卷积分离出类似的稍后到达的相位。陡峭的地壳厚度过渡支持了IB裂谷的局部突变边界。我们的研究结果突出了密集短周期阵列在近地表至最上层地幔深度进行被动成像的实用性。
We analyze teleseismic P waves from four Mw ≥ 6.5 earthquakes recorded by a petroleum industry survey in Long Beach, California. The survey used a 2‐D array with up to 5200 seismometers, 120 m mean spacing, and 7 – 10 km aperture. At frequencies near 1 Hz, P wave travel times and amplitudes exhibit coherent lateral variations over scales as short as ~400 m, including locally delayed travel times and increased amplitudes at the crest of the Long Beach anticline. Deeper heterogeneity is indicated by P wave phase velocities that deviate from reference model predictions for events from southwestern azimuths. We postulate that a sharp northeastward increase in Moho depth from the Inner Borderland (IB) to mainland southern California causes the anomalous phase velocities. Elastic forward modeling finds the travel times are fit well by a Moho that dips 65° to the northeast and flattens ~10 km southwest of the Newport‐Inglewood fault zone. Constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a P‐velocity of 7 km/s beneath it, which could result from basal accretion of former Farallon ocean crust or magmatic underplating during Miocene volcanism. Forward models with a 65° Moho dip predict a P‐to‐s conversion with a phase velocity of ~5 km/s. Deconvolution of the array's mean P wave signal isolates a similar later arriving phase. The steep crust thickness transition supports a locally abrupt boundary to IB rifting. Our results highlight the utility of dense short‐period arrays for passive imaging at near surface to uppermost mantle depths.