Tomographic imaging of late Quaternary faulting, Oquirrh Mountains, Utah

Abstract

[1] Seismic tomography can be used to image colluvial material in the subsurface by inverting first arrival travel times for velocity. Colluvial material deposited at the base of a fault-scarp free face often appears as a low-velocity zone (LVZ) on a tomogram because it is generally less compacted and cemented than the surrounding alluvium. A tomogram generated from a forward model of a synthetic velocity structure successfully images two LVZs stacked in the hanging wall of a normal fault. The Mercur fan, Oquirrh Mountains, Utah, provides an opportunity to look for stacked LVZs in a distributed fault zone. Three tomographic images across fault scarps on an intermediate age alluvial fan can be used to identify two stacked low-velocity zones. Interpretation of a fourth tomogram is less conclusive. These two low-velocity zones are interpreted as colluvial packages separated by higher-velocity alluvial material and suggest that tectonic activity is interspersed with pulses of fan building.

Outcrops and well logs as a practicum for calibrating the accuracy of traveltime tomograms

“…Based on synthetic modeling tests of faulted strata conducted for this and other studies (e.g., Mattson, 2004), we apply the two criteria mentioned earlier to identify the fault on a tomogram. Protrusion of the higher velocity contours into the low-velocity hanging-wall side of the tomogram is caused in part by the lower lateral than vertical resolution in the inversion (Mattson, 2004;Buddensiek et al, 2008).…”

“…Figure 6a presents the tomogram as contours of seismic velocity in depth along the profile, and Figure 6b displays the raypath density through each cell in the tomogram. Based on the synthetic tests, two criteria are used to identify a fault in the tomogram (Mattson, 2004): (1) focusing of rays in the raypath density image. The fault is not exactly located at the greatest raypath density area, but it is located at the low-density side near the plain.…”

“…Colluvial wedges are often characterized by 10%-20% slower velocities than the surrounding stratified deposits, making them attractive targets for seismic velocity tomography (Morey and Schuster, 1999;Sheley et al, 2003;Mattson, 2004;Buddensiek et al, 2008). On the other hand, high-resolution seismic reflection techniques are better suited for imaging details of wellstratified deposits that lie adjacent to the colluvial wedges (Morey and Schuster, 1999).…”

“…The colluvial wedge starts to form shortly after a surfacerupturing earthquake, and the contact of the wedge with the underlying soil forms an earthquake "event" horizon. Although simple to envision in concept, the internal structure and composition of colluvial wedges are often complex, and several colluvial wedges may coalesce to form a low-velocity body formed by more than one earthquake (Nelson, 1992;Mattson, 2004).…”

“…The background velocity range is 300-1300 m∕s with a gradient of 4.0 m∕s∕m. The velocity range for each of the LVZs is 250-350 m∕s, shallow zone, and 375-475 m∕s, deeper zone, with a gradient of 1.7 m∕s∕m (Mattson, 2004). …”