Waveform Signatures of Earthquakes Located Close to the Subducted Gorda Plate Interface
Waveform Signatures of Earthquakes Located Close to the Subducted Gorda Plate Interface
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俯冲戈达板块界面附近地震的波形特征
DOI：
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发表时间：
2022
期刊：
影响因子：
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通讯作者：
J. McGuire
中科院分区：
文献类型：
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作者：
J. Gong;J. McGuire
震源附近复杂的地震速度结构会影响破裂动力学，并强烈地修改断层附近记录的地震波形。断裂带波通常在大陆地壳环境中观测到，但由于地震台站与板块边界断层之间的空间分离，在俯冲带中不太清楚。我们观察到异常长的持续时间的S波从地震群位于附近的界面的俯冲戈尔达板块北部的门多西诺三联点。相比之下，位于每个集群下方几公里处的地震显示出脉冲S波。在北方加州地震台网KCT附近进行了两个月的节块阵列试验，以研究复杂S波的起源。波束形成分析表明，S波包含三个具有不同水平慢度的到达，我们称之为S1，S2和S尾波。对P波的类似分析也显示了具有不同水平慢度的两个到达，我们称之为P1和P2。P1和S1分别比P2和S2具有更大的水平慢度，表明相位对是具有不同射线路径的体波。建立在地震折射剖面，我们构建了一维速度模型，并测试了不同的厚度和VP/VS比的俯冲洋壳。P1/P2和S1/S2震相的到时和相对慢度表明它们分别是直达震相和莫霍面反射震相。它们的性质与地壳厚度为1.6km和中等的VP/VS比（1.8）相一致。S尾波更难描述，但有一个明显的主频，可能反映了近源速度和衰减结构。我们的研究表明，从俯冲板的界面附近的地震波形可以用来推断详细的结构信息的板块边界带在孕震深度。
Complex seismic velocity structure near the earthquake source can affect rupture dynamics and strongly modify the seismic waveforms recorded near the fault. Fault-zone waves are commonly observed in continental crustal settings but are less clear in subduction zones due to the spatial separation between seismic stations and the plate boundary fault. We observed anomalously long duration S waves from earthquake clusters located near the interface of the subducted Gorda plate north of the Mendocino triple junction. In contrast, earthquakes located just a few kilometers below each cluster show impulsive S waves. A nodal array experiment was conducted around the Northern California Seismic Network station KCT for two months to investigate the origin of the complex S waves. Beamforming analysis shows that the S waves contain three arrivals that have different horizontal slownesses, which we term S1, S2, and S coda. Similar analysis on P waves also show two arrivals with different horizontal slownesses, which we term P1 and P2. P1 and S1 have larger horizontal slowness than P2 and S2, respectively, indicating that the phase pairs are body waves with different ray paths. Building upon a seismic refraction profile, we construct 1D velocity models and test different thicknesses and VP/VS ratios for the subducted oceanic crust. The arrival times and relative slownesses of P1/P2 and S1/S2 phases indicate that they are the direct and the Moho reflected phases, respectively. Their properties are consistent with a crustal thickness of ∼6 km and a moderate VP/VS ratio (∼1.8). The S coda is more difficult to characterize but has a clear dominant frequency that likely reflects the near-source velocity and attenuation structure. Our study indicates that waveforms from earthquakes near the interface of the subducted slab can be used to infer detailed structural information about the plate-boundary zone at seismogenic depths.