Internal structure of the San Jacinto fault zone at Jackass Flat from data recorded by a dense linear array

The internal structure of the Clark fault in the trifurcation area of the San Jacinto fault zone is imaged using seismograms recorded by a dense linear array (Jackass Flat, JF) crossing the surface trace of the fault and an adjacent array (TR) to the SW. Delay times between phase arrivals associated with ∼3500 local earthquakes and nine teleseismic events are used to estimate velocity variations within the arrays. The teleseismic P waves travel faster beneath the TR than the JF array, in contrast to larger scale tomographic results. Statistical analysis of local P-wave delay times indicates that the entire JF array, with an aperture of ∼400 m, is inside a low-velocity damage zone. This low-velocity zone is bounded on the NE side by a shallow bimaterial interface generating fault zone head waves, and it contains an inner zone of more intense damage generating fault zone trapped waves. The P-wave velocity contrast across the local bounding bimaterial interface is 10–15 per cent. The trapping structure is associated with a width of ∼200 m, S-wave velocity reduction of ∼35 per cent with respect to the surrounding rock, Q-value of ∼20 and depth of ∼3.5 km. The imaging results suggest that the main seismogenic fault is near the SW end of the JF array, in agreement with a prominent geomorphologic feature. The existence of intense local damage on the crustal block with faster larger scale velocity at depth is consistent with common propagation of earthquake ruptures in the area to the NW.

利用穿过圣哈辛托断裂带三岔区克拉克断层地表迹线的密集线性阵列（JF，位于杰克驴平地）以及西南方向相邻阵列（TR）所记录的地震图，对克拉克断层内部结构进行了成像。利用约3500次本地地震和9次远震事件相关震相到达的延迟时间来估算阵列内的速度变化。与更大尺度的层析成像结果相反，远震P波在TR下方的传播速度比JF阵列快。对本地P波延迟时间的统计分析表明，孔径约为400米的整个JF阵列位于一个低速损伤带内。这个低速带在东北侧由一个产生断层带首波的浅双材料界面界定，并且包含一个产生断层带陷波的内部损伤更强烈区域。穿过局部界定双材料界面的P波速度差异为10% - 15%。陷波结构相关的宽度约为200米，相对于周围岩石S波速度降低约35%，Q值约为20，深度约为3.5千米。成像结果表明，主要的发震断层靠近JF阵列的西南端，这与一个显著的地貌特征相符。地壳块体在深部具有较大尺度的较快速度但存在强烈局部损伤，这与该地区向西北方向地震破裂的常见传播是一致的。