Neotectonics and Paleoseismology of the North Frontal Thrust System, southern California

Seismic hazard assessment of intersecting fault systems, such as the strike-slip and reverse faults of the Los Angeles basin, is hindered by complex patterns of rupture that are currently difficult to predict. To improve this understanding, constraints on the previous rupture patterns of such systems are needed. The junction between the Transverse Ranges and the Eastern California shear zone in southern California provides a natural analog to the seismic setting of the Los Angeles basin. Along the northern flank of the San Bernardino Mountains, the east-west trending North Frontal thrust system is intersected by several northwest trending dextral faults of known Holocene and historical rupture activity. This structural setting, along with an apparent decay in uplift rate along the thrust (from a 3-Myr average of 0.5 mm/yr to a late Pleistocene rate estimated as slow as 0.05 mm/yr), suggests the thrust system may have been rendered inactive by the shear zone that dissects it. However, a clear cross-cutting relationship does not exist, raising the possibility that the two systems are coactive.

To test this, we have constrained the recent rupture history of one thrust fault segment with paleoseismic investigations. We have excavated an apparently young thrust fault scarp along the central portion of the thrust system, chosen as the most likely to have ruptured in the recent past. At this location, just west of the intersection of the Helendale fault, a 7-m-high thrust scarp in older fanglomerate is dissected and replaced by younger alluvium with a 1.5-m-high scarp. An excavation across the smaller scarp revealed a 3-m-thick sequence of coarse alluvium cut by a shallow, south-dipping thrust fault with 1.65 m of throw. The simple, smooth trace of the fault plane and the lack of evidence for repeated deformation suggest the offset was produced by one event. A maximum age for this event is provided by disagregated detrital charcoal sampled from a sand lens in the lowermost gravel of the hangingwall, which yielded a calibrated radiocarbon age of 9220 BC (11220 yr BP). Subsequent to this inferred depositional age, an additional 2-m of gravel was deposited prior to fault rupture. Although a minimum age is not constrained, the event may thus have been as young as mid- to late-Holocene, consistent with the poor degree of soil development in several buried soil horizons in the alluvium. This indicates that at least part of the thrust system is coactive with the strike-slip strands that intersect it and implies that such intersections do not require either fault system to be extinct.

However, it is crucial to obtain a minimum age in order to constrain the recent rupture history. This is inherently difficult because where the required onlapping relationships are present, scarps associated with the most recent event have been buried or eroded. A second site does occur several km from our original site, yet without knowing the exact location or depth of the fault an excavation would be risky. To increase the likelihood of finding the fault with an excavation, we employed geophysical exploration techniques to image the fault at depth. Ground Penetrating Radar (GPR) is a technique that can be used for shallow high-resolution imaging by recording the propagation of radio waves. To calibrate this technique to locating a shallow fault in the conditions of the study area, we returned to the site of our original excavation. We observed reflections from subhorizontal strata and the fault plane extending to a depth ~10 meters. This was identical to our initial trench observations. Using the same technique at our candidate minimum-age site, we resolved the exact location of a dipping fault plane covered by several meters of young alluvium. Now that the fault has been located, excavation of the site can be undertaken with a good chance of success. This result shows the value of GPR being used as an innovative predictive tool in paleoseismology.