Testing the use of microfossils to reconstruct great earthquakes at Cascadia

Abstract

The recent magnitude-9 (M9) earthquakes at subduction zones in Sumatra and Japan are focusing efforts to learn the history of infrequent earthquakes that are commonly accompanied by devastating tsunamis. The Pacific Northwest coastline along the 2 Cascadia subduction zone contains a sedimentary archive of past megathrust earthquakes.Where coastal stratigraphy records sudden subsidence due to relaxation of the overriding plate during rupture of the subduction fault, transfer functions can convert microfossil data into estimates of the amount of coseismic subsidence. Subsidence is then used in 3D elastic models of subduction-zone deformation to calculate earthquake magnitude. We tested this method with a simulated earthquake by "transplanting" a bed of modern high salt-marsh sediment into the low marsh, a vertical elevation change equivalent to coseismic subsidence of 0.62m. The transplanted bed was quickly buried by sediment and sampled for microfossils after five years. Our reconstruction of this simulated "coseismic" subsidence using our foraminiferal transfer function was 0.61m, which yields the same magnitude in deformation models. Our analyses also showed that sediment mixing could explain some occurrences of the "pre-seismic" microfossil signal inferred from prehistoric records in Cascadia and presented further evidence for sediment mixing from a record of the AD 1700 earthquake at the Coquille River, Oregon.

Differences in coastal subsidence in southern Oregon (USA) during at least six prehistoric megathrust earthquakes

“…1C) to the previous regional data set of samples from six estuaries (Coquille River, Coos Bay, Siuslaw River, Siletz Bay, Salmon River and Nehalem River estuaries;Figs. 1A and 1B;Engelhart et al, 2013b) compiled by Engelhart et al (2013a). The new data increased the number of modern samples from 158 to 174 (Table A3).…”

“…At Cascadia and Alaska, the extent to which taphonomic processes, such as post-depositional downward mixing of microfossil species or reworking of species by tsunamis, influence the accuracy of coseismic subsidence estimates is little studied (e.g., Hemphill-Haley, 1995;Nelson et al, 1996b;2008;Hamilton et al, 2005, Graehl et al, 2014. For example, in a transplant experiment simulating sudden subsidence during an earthquake in southern Oregon, Engelhart et al (2013a) attributed an unexpected abundance of the typical tidal-flat foraminiferal species M. fusca in the underlying peat to infiltration of overlying tidal-flat mud into the peat.…”

Shale‐gas Potential of the Mid‐carboniferous Bowland‐hodder Unit in the Cleveland Basin (Yorkshire), Central Britain

“…The performance of the instrument was checked every 10 samples against the accepted values of the Institut Français du Pétrole (IFP) standard (IFP 160 000, S/N1 5-081840). Rock-Eval parameters were calculated by LQWHJUDWLRQ RI WKH DPRXQWV RI +& WKHUPDOO\YDSRUL]HG free hydrocarbons) expressed in mgHC/g rock (S 1 ) and hydrocarbons released from cracking of bound OM expressed in mgHC/g rock (S 2 ) (Engelhart et al, 2013). The Hydrogen Index (HI) was calculated from S 2 x 100/TOC and the Oxygen Index (OI) from S 3 x 100/TOC.…”

Annual and Seasonal Distribution of Intertidal Foraminifera and Stable Carbon Isotope Geochemistry, Bandon Marsh, Oregon, Usa

“…Distinct changes in foraminiferal assemblages across such abrupt contacts record the subsidence of coastal wetlands (causing a sudden relative sea-level rise) during great [moment magnitude (Mw) 8-9] earthquakes on the subduction-zone megathrust fault. Transfer function analysis of fossil faunas provides a means of mapping the amount of subsidence during past earthquakes and estimating earthquake magnitudes (e.g., Guilbault et al, 1995;Nelson et al, 2008;Engelhart et al, 2013a;Wang et al, 2013).…”