Shallow Seismic Profiling of the Exhumed Punchbowl Fault Zone, Southern California
Shallow Seismic Profiling of the Exhumed Punchbowl Fault Zone, Southern California
复制标题
南加州挖出的 Punchbowl 断层带的浅层地震剖面
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
J. Vidale
中科院分区：
文献类型：
--
作者：
Yong‐Gang Li;F. Chester;J. Vidale
通过在加州的Devil 9 s Punchbowl洛杉矶县公园的Punchbowl断层上进行的浅层地震折射实验，研究了地震速度与断层内部结构之间的关系。Punchbowl断层是圣安德烈亚斯系统的一个西北走向的大位移断层，被挖掘到几公里深，并将结晶基底置于Punchbowl地层的长石砂岩上。沿沿着300米的测线利用锤击和脉冲剪切波源进行的地震折射剖面揭示了冲积层薄存款下断层带的速度结构。我们确定了一个速度模型，假设冲积层的速度和厚度相当均匀，与地质观测和P波走时一致。射线追踪与衰减最小二乘反演的P波和S波的走时表明，Punchbowl断层最好是模拟为一个几十米宽的带，速度从围岩速度减少10%-25%（Vp = 3.2公里/秒的花岗岩基底和Vp = 2.9公里/秒的Punchbowl形成）。低速带的厚度和整个区域的地震速度变化在质量上与基于所观察到的断层相关断裂和蚀变分布的预期一致。利用O 9 Connell和Budiansky（1974）公式，根据测量的地震速度计算出的裂缝介质的视在裂缝密度范围从岩芯中的约0.4到主岩中的背景裂缝密度0.1。计算的裂缝密度在整个断层上的变化与观察到的Punchbowl地层砂岩中微裂缝密度的变化相似，沿着横穿断层。断层附近的泊松比估计约为0.25，表明潘奇碗断层带浅层的开放裂缝是干燥的，与地下水位的地质推断位置一致。虽然地震资料不能完全约束速度结构，但在构造资料的基础上，通过射线追踪和走时反演确定的地震速度模型是可以接受的。
The relationship between seismic velocity and internal fault structure was investigated through a shallow seismic refraction experiment across the Punchbowl fault, Devil9s Punchbowl Los Angeles County Park, California. The Punchbowl fault is a northwest-striking, large-displacement fault of the San Andreas system that is exhumed to several kilometers depth and places crystalline basement against arkosic sandstone of the Punchbowl Formation. Seismic refraction profiles using hammer and impulsive shear-wave sources along a 300-m-long line reveal the velocity structure of the fault zone beneath a thin deposit of alluvium. We determine a velocity model assuming the alluvial layer is fairly uniform in velocity and thickness consistent with geologic observations and P -wave travel times. Raytracing with damped least-squares inversion of travel times of P and S waves indicate that the Punchbowl fault is best modeled as a zone several tens of meters wide with velocities reduced by 10%–25% from wall-rock velocities ( V p = 3.2 km/sec for granitic basement and V p = 2.9 km/sec for Punchbowl Formation). Thickness of the low-velocity zone and the variation in seismic velocity across the zone are qualitatively consistent with expectations based on the observed distribution of fault-related fracturing and alteration. Apparent crack densities calculated from measured seismic velocities using O9Connell and Budiansky (1974) formulation for a cracked medium range from about 0.4 in the core to a background crack density of 0.1 in the host rock. The variation in calculated crack density across the fault is similar to observed variations in microfracture density in the Punchbowl Formation sandstone along traverses across the fault. An estimate of the Poisson9s ratio near the fault is about 0.25, suggesting that open cracks in the shallow part of the Punchbowl fault zone are dry, consistent with the geologically inferred location of the groundwater table. Although the seismic data do not completely constrain the velocity structure, the seismic velocity model determined by raytracing and inversion of travel times is admissible on the basis of structural data.