Summary The dominance of P-wave reflectivity over S-wave reflectivity from crustal fault zones suggests that variations in Lamé param- eter λ control fault zone reflections. Reflections due to λ varia- tions radiate isotropically, unlike the backscattering from imped- ance contrasts or the forward scattering from velocity contrasts. We therefore suggest that crustal faults may be distinguishable from non-tectonic crustal structures on the basis of this isotropic P-wave scattering. We show crustal reflectivity images from the Los Angeles Basin of southern California that select for such isotropic reflectivity, preferentially showing crustal fault struc- ture. Our depth sections sum together backscattered and forward scattered arrivals from subsurface depth points, effectively fil- tering out simple impedance or velocity variations. Imaging be- neath the 1991 Sierra Madre aftershock zone from seismograms recorded on the southern Calif. Seismic Network shows the same “lower crustal reflective zone” below the San Gabriel Mts. im- aged by the 1994 Los Angeles Region Seismic Experiment. Introduction Variations in P-velocity anisotropy dominate the reflectivity of exhumed mylonitic fault zones, producing stronger P-wave than S-wave reflectivities. On the other hand, overall variations in density or rigidity dominate most other types of geologic con- trasts, such as pluton boundaries; resulting in stronger S-wave reflectivities. For instance, McCaffree and Christensen (1993) show that for a mylonite zone compressional reflectivity is stron- ger than shear reflectivity. Hence, mylonites have a wide V P /V S range and stronger P-wave reflection coefficients, making it pos- sible to devise an effective imaging technique to image ∆V P and ∆λ signatures that identify crustal fault zones. However, earthquake seismologists may be reluctant to believe that one can successfully image crustal faults using only P-P scattering characteristics and earthquake sources, as we do here. For instance, Aki (1992) shows that P-S conversion for earth- quake sources is much greater than S-P. Thus, the dominance of waves in the coda of seismograms raises concern about the va- lidity of dealing with only P-P scattering in imaging crustal re- flectors. In fact, the mode conversion problem is inherently troublesome in earthquake seismology, and this remains the case in exploration seismology. In spite of this, successful P-P scat- tering applications of this type abound in oil-related seismic im- aging for fracture detection and reservoir characterization. We believe this is the first time this is done for crustal targets using earthquake recordings. Crustal imaging technique We are interested in acoustic images or ∆λ-images. ∆λ contribu- tions are isotropic (Wu, 1989) and come from local P-wave en- ergy, mostly P-P reflections and transmitted P-wave arrivals (Fig. 1). This is clearly not the case when we try to obtain density images or rigidity images (∆μ-images). For instance, μ varia- tions lead to anisotropic scattering and one needs to take S-waves into account (Fig. 1) to correctly image such variations. Fig. 2 shows that in the presence of ∆λ, back and forward scattered events constructively interfere while for a ∆μ they tend to cancel out. Thus, the search for ∆λ structures becomes practical with an acoustic processing scheme. Clipped and saturated records are quite common in short-period earthquake data. We regard them as sign-bit recordings (O'Brien et al., 1982) that will acquire dynamic range through Kirchhoff summation. Based on these working assumptions, the crustal imaging technique proceeds as follows: a) the record sections we use comprise 200 km in epicentral distance and 30 s duration to include wide-angle reflections between first compressional, P g , and first shear, S g , arrivals, and we mute outside the window between P g and S g traveltime branches to extract only compres- sional arrivals, mostly P g , P m P and S-P converted energy. b) Pre- processing includes trace equalization for receiver amplitude balancing, i.e., the amplitudes are normalized so that the mean- Isotropic scattering and seismic imaging of crustal fault zones using earthquakes Sergio Chávez-Pérez * and John N. Louie, University of Nevada, Reno Forward scattering P-S Backscattering First arrival Reflection P P P-P P-P P-P P-P Fig. 1. Vertical-component elastic synthetics for a two-layer model showing the early P-wave coda for variations in Lamé parameters (∆λ and ∆μ) during back- and forward-scattering.