THE PALEOSEISMIC RECORD OF SUBAQUEOUS DEFORMATION ALONG NORMAL FAULTS IN CENTRAL OREGON AND CENTRAL MEXICO

While major advances in lacustrine paleoseismology have come from coring studies that use turbidites to identify strong shaking events in lakes, these studies do not identify the source faults that generate large seismic events. Continental interiors such as the Great Basin and Trans-Mexican Volcanic Belt of the North American Cordillera house moderate to large lake basins, including Bonneville, Lahontan, Chewaucan, and Acambay, that have dried through the Holocene exposing active faults and fault-related sedimentary records that can be interrogated from outcrop and paleoseismic trenches. These records include evidence of soft-sediment deformation (seismites) structures and subtle erosional features that are characteristic of saturated and underwater environments and provide clues of mechanisms of fault rupture and scarp behaviour for paleo-earthquakes that occurred when these lakes were full during the latest Pleistocene.

The Ana River Fault is an active normal fault in the central Oregon Basin and Range province. From trenches and exposures along Ana River, Langridge (1998) identified a set of specific subaqueous processes related to paleoseismic events on that fault. These include: footwall scarp bevels and attenuated lake section, low-angle structures that both remove and stack section , tilting and exhumation of older section, scarp-vergent folds and post-event fill, boudinage related to competent tephra beds in the section, lateral spreading on the footwall scarp, high-angle ‘breakaway’ faults, shallow ramp-and-flat style detachment faulting, basal glide planes leading to slumps and folds divergent from the scarp, and hangingwall thickening and bed rotation. Some of these features, like footwall scarp bevels, can be recognised from traditional and LiDAR profiling of fault scarp geomorphology exposed in these dry basins. Paleoseismic studies in the Acambay Graben in central Mexico also identified such features in dry lake basins there. Collectively, these structures inform us about how normal faults rupture in subaqueous settings and how the soft sediments near the top of lake sections behave during and after a large earthquake.