Evidence of Coseismic Subsidence Along the Newport-Inglewood Fault Zone During the Late Holocene

Abstract:

The Seal Beach wetlands (SBW) are located along strike of the Newport-Inglewood fault zone (NIFZ), where fault geometry is poorly understood. However, the fault steps to the right in the SBW, and oil well and seismological studies indicate that the northeast segment of the fault is down-dropped as a result of oblique-normal slip. The location is significant because coseismic subsidence could alter tidal and/or fluvial processes within the SBW. Such a change should be recorded in the stratigraphy by sudden changes in grain size, magnetic susceptibility, and/or diatom assemblages. Based on field observations of 48 reconnaissance gouge cores and multi-proxy analyses of three vibracores and one piston core, we identify four stratigraphic units in the SBW: (1) very fine to fine sand from 426-350 cm; (2) organic-rich mud from 350-225 cm; (3) fine to coarse silt and clay from 225-100 cm; and (4) organic-rich mud with interbedded mud laminae from 100-0 cm. We interpret unit 2 to represent a relic marsh surface that subsided coseismically and was preserved under unit 3. We analyzed 38 sediment samples for diatoms from vibracore SB002VC to test this idea and characterize the depositional environment of stratigraphic units 4, 3, and 2. The diatom data suggest unit 4 is an intertidal deposit, unit 3 is a fresh/slightly brackish water deposit, and unit 2 is an intertidal deposit. Units 2 and 3 are separated by a sharp, irregular contact. Thus, the diatom data and depositional contact from vibracore SB002VC are consistent with a relic marsh surface subsiding abruptly during an earthquake on the NIFZ. As a result of the earthquake, the intertidal environment abruptly changed to an environment dominated by fresh to slightly brackish water. We hypothesize that the earthquake did not result in flooding by seawater because coseismic uplift of the southwest segment of the NIFZ temporarily isolated the SBW from tidal influence. Radiocarbon dates constrain this event to no more than 1957 cal yr BP. Future work will include Cs-137/Pb-210 dating of unit 4 and pollen analysis in units 3 and 2, additional radiocarbon dating and completing grain size analysis of vibracore SB002VC.