1820 Bulletin of the Seismological Society of America, 91, 6, pp. 1820–1830, December 2001 Shallow Seismic Profiling of the Exhumed Punchbowl Fault Zone, Southern California by Yong-Gang Li, Frederick M. Chester, and John E. Vidale Abstract The relationship between seismic velocity and internal fault structure was investigated through a shallow seismic refraction experiment across the Punch- bowl fault, Devil’s 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 ar- kosic sandstone of the Punchbowl Formation. Seismic refraction profiles using ham- mer 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 consis- tent 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 alter- ation. Apparent crack densities calculated from measured seismic velocities using O’Connell 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 Poisson’s 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 ve- locity model determined by raytracing and inversion of travel times is admissible on the basis of structural data. Introduction A refined definition of the internal structure of fault zones in the seismogenic portion of the continental crust is necessary to further our understanding of the processes of earthquake rupture initiation, propagation, and termination. Images of seismogenic fault zones at depth have been pro- duced using seismic tomography (Aki and Lee, 1976; Thur- ber, 1983; Lees and Malin, 1990; Michelini and McEvilly, 1991; Eberhart-Phillips and Michael, 1993) and by obser- vations and modeling of fault-zone trapped waves (Li et al., 1990, 1994, 2000; Leary et al., 1991; Hough et al., 1994). These studies have shown that faults are marked by low- velocity zones of tabular geometry. Fault-zone trapped waves suggest that the large-displacement faults of the San Andreas system are characterized by zones on the order of 100 to 200 m thick with average seismic velocity of ap- proximately 30%–40% of the surrounding host rock. On the basis of geologic studies of fault internal structure, the fault- related low-velocity zones are thought to reflect mineralogic alteration and intense fracturing associated with faulting (Sibson, 1977; Mooney and Ginzburg, 1986). Geologic study of earthquake rupture zones (Sieh et al., 1993; Johnson et al., 1994) and of exhumed faults (Chester and Logan, 1986; Chester et al., 1993; Schulz and Evans, 1998) document that fault displacements often are localized to a narrow fault core, but that fault-related fractures and other deformation occur over a broader damage zone. In the Punchbowl and San Gabriel faults, which are exhumed faults of the San Andreas system in southern California, fracture