Multiscale seismic imaging of active fault zones for hazard assessment: A case study of the Santa Monica fault zone, Los Angeles, California

Abstract

High‐resolution seismic reflection profiles at two different scales were acquired across the transpressional Santa Monica Fault of north Los Angeles as part of an integrated hazard assessment of the fault. The seismic data confirm the location of the fault and related shallow faulting seen in a trench to deeper structures known from regional studies. The trench shows a series of near‐vertical strike‐slip faults beneath a topographic scarp inferred to be caused by thrusting on the Santa Monica fault. Analysis of the disruption of soil horizons in the trench indicates multiple earthquakes have occurred on these strike‐slip faults within the past 50 000 years, with the latest being 1000 to 3000 years ago. A 3.8-km-long, high‐resolution seismic reflection profile shows reflector truncations that constrain the shallow portion of the Santa Monica Fault (upper 300 m) to dip northward between 30° and 55°, most likely 30° to 35°, in contrast to the 60° to 70° dip interpreted for the deeper portion of the fault. Prominent, nearly continuous reflectors on the profile are interpreted to be the erosional unconformity between the 1.2 Ma and older Pico Formation and the base of alluvial fan deposits. The unconformity lies at depths of 30–60 m north of the fault and 110–130 m south of the fault, with about 100 m of vertical displacement (180 m of dip‐slip motion on a 30°–35° dipping fault) across the fault since deposition of the upper Pico Formation. The continuity of the uncomformity on the seismic profile constrains the fault to lie in a relatively narrow (50 m) zone, and to project to the surface beneath Ohio Avenue immediately south of the trench. A very high‐resolution seismic profile adjacent to the trench images reflectors in the 15 to 60 m depth range that are arched slightly by folding just north of the fault. A disrupted zone on the profile beneath the south end of the trench is interpreted as being caused by the deeper portions of the trenched strike‐slip faults where they merge with the thrust fault.

Processing and preliminary interpretation of noisy high-resolution seismic reflection/refraction data across the active Ostler Fault zone, South Island, New Zealand

“…Seismic imaging of active fault zones and shallow glacial deposits are separately demanding tasks (Büker et al, 1998;Pratt et al, 1998;Long et al, 2003;Spitzer et al, 2003;Improta and Bruno, 2007). The combination of these two settings in the Mackenzie Basin results in an extremely complex structural environment, requiring the careful application of innovative processing strategies to ensure the extraction of maximum information from the seismic data while avoiding the creation of artefacts (Steeples et al, 1997).…”

Ultrahigh‐resolution seismic reflection imaging of the Alpine Fault, New Zealand

“…[5] Imaging of shallow features presents general challenges for seismic processing that are amplified when the targets are complexly deformed structures associated with active fault zones [e.g., Pratt et al, 1998;McBride and Nelson, 2001;Improta and Bruno, 2007]. Specific problems related to fault zones within unconsolidated sediments and underlying basement include (1) high degrees of nearsurface heterogeneity (e.g., large lateral variations in nearsurface velocity and weathered layer thickness) that require the careful computation and application of static corrections; (2) the presence of substantial source-generated noise (e.g., ground roll, guided phases, and multiple reflections); and (3) difficulties in imaging steeply dipping and/or severely deformed structures within and about the fault zone.…”

Detection of hidden faults beneath Khlong Marui fault zone using seismic reflection and 2-D electrical imaging

“…Reflection seismic attempts to detect possible change in elastic properties and/or density within the subsurface by recording and analysing the reflected wave generated by seismic boundaries, after the elastic waves have propagated from the sources. It is generally a noninvasive tool for groundwater exploration, site investigation, fault detections and petroleum prospecting [15][16][17]26 . Electrical imaging is based on indirect measurements of the electrical properties of subsurface by injecting the current into the ground and measuring the voltage difference through a set of electrode configuration.…”

“…Hence the purposes of this study are to detect minor fault locations and to understand its characteristics using geophysical methods. It is known that geophysical techniques are effective tools used in numerous fault investigations [15][16][17][18] . This study utilized the seismic and resistivity methods in an attempt to establish a link between shallow and deeper faults and the associated tectonic in the area.…”