Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates

Abstract

Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3‐D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along‐strike slip heterogeneity, with multiple patches of high‐moment release separated by areas of low‐moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high‐quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.

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).…”

The Effects on Tsunami Hazard Assessment in Chile of Assuming Earthquake Scenarios with Spatially Uniform Slip

“…This function allows the slip to decrease in both the up-dip and down-dip directions, as expected to occur at real megathrust earthquakes (WANG and HE 2008). After correcting some typographical errors found first by WANG et al (2013), the slip function is defined as follows:…”

The 2004 Sumatra Earthquake and Tsunami: Lessons Learned in Subduction Zone Science and Emergency Management for the Cascadia Subduction Zone

“…This is important for Cascadia, as it has been well documented that the 1700 M 9 earthquake generated a tsunami that caused damage as far away as Japan (ATWATER 2005;SATAKE et al 2003). Since the 2004 earthquake, great progress has been made in understanding both tsunami generation and tsunami hazard mapping in Cascadia; For example, studies estimating heterogeneous slip along Cascadia associated with the 1700 earthquake (WANG et al 2013) have applications to tsunami generation, detailed inundation mapping efforts that have been undertaken along Cascadia (e.g., PRIEST et al 2014;WITTER et al 2014), and a probabilistic tsunami hazard map for Canada (LEONARD et al 2014). Details of tsunami science and inundation studies associated with the 2004 Sumatra earthquake are presented in other articles in this special volume.…”

“…These observations provide a good overall match with the shaking intensity estimates for western North America byYOUNGS et al (1997) andWALD et al (1999) For Cascadia, uplift would occur exclusively along the offshore (submarine) regions, except for a small region in northern California. This is consistent with the latest models developed for Cascadia (WANG 2013; personal communication) and paleoseismological observations along the Cascadia coast showing subsidence of up to *2 m associated with the 1700 Cascadia earthquake (e.g.,LEONARD et al 2004, 2010 and references therein;WANG et al 2013 and references therein).2.4. Earthquake Rupture CharacteristicsTherupture details of the 2004 Sumatra earthquake have been analyzed by numerous researchers using a wide variety of techniques.…”