DOES EVIDENCE OF ABRUPT COSEISMIC SUBSIDENCE AND TSUNAMI DURING THE LATE HOLOCENE EXIST IN SEAL BEACH MARSH STRATIGRAPHY?

The Newport-Inglewood Fault Zone (NIFZ) is a major right-lateral strike-slip fault in southern California. The NIFZ is seismically active and structurally complex. The North Branch fault of the NIFZ strikes northwest through the Seal Beach marsh where fault geometry produces a localized depression. Sixteen gouge cores were taken during paleotsunami fieldwork at the Seal Beach marsh. The cores penetrated depths with a range of 150 and 240 cm below the land surface (bls) and recovered estuarine sediment interbedded with mud-capped organic layers. Three piston and three vibracores penetrated depths with a range of 350 and 425 cm bls and recovered estuarine sediment interbedded with mud-capped organic layers, and two shell-hash deposits. One piston core revealed a muddy sand layer at 325 cm depth that contained a 2-cm thick shell-hash. One vibracore exposed a sand layer at 400 cm depth containing a 10-cm thick shell-hash.

It is possible the mud-capped organic layers at depth are the result of abrupt coseismic subsidence. In marsh settings, abrupt subsidence would cause an immediate rise in relative sea level. Deposition during such an event would be characterized in marsh stratigraphy by an abrupt change from peaty (subaerial) sediment to mud, and a microfossil assemblage that changes from brackish to marine. We have cursory microfossil data from one core that show this pattern. Additionally, the shells in the hash could have originated in a lagoon or offshore; the spike in magnetic susceptibility across the shell-hash boundary suggests they originated offshore. We use a multi-proxy methodology to quantitatively characterize the stratigraphy. The methodology includes laboratory analyses such as loss on ignition at 550°C (%TOM) and 950°C (%TC), magnetic susceptibility (CHI), grain size distribution, identification of microfossils (diatoms and foraminifers), and radiocarbon geochronology. We also use a “criteria matrix” established during a similar paleoseismic study along the San Gregorio fault in northern California (Koehler, 2005). This multi-proxy approach may lead to an understanding of the possible paleoseismic and paleotsunami record within Seal Beach marsh stratigraphy, localized coseismic deformation, and seismic hazard associated with the North Branch fault of the Newport-Inglewood Fault Zone.