Late Stage Interseismic Strain Interval, Cascadia Subduction Zone Margin, USA and Canada

Abstract

Modern horizontal strain (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) measured along 56 new and 108 previously published GPS station baselines are used to establish the length (800 km) and width (300 -400 km) of the central Cascadia convergent margin seismogenic structure. Across-margin (west-east) annual rates of shortening range from 10 −9 a −1 at the eastern (landward) limit of the central Cascadia seismogenic structure to 10 −7 a −1 along the western onshore portion of the interplate zone. Relatively high shortening strain rates (10 −8 a −1 to 10 −7 a −1) are also measured in western transects from the northern (Explorer plate) and southern (Gorda plate) segments of the convergent margin, demonstrating that the full length of the margin (1300 km length) is currently capable of sustaining and/or initiating a major great earthquake. Vertical GPS velocities are averaged over the last decade at 321 stations to map patterns of uplift (0 -5 mm yr −1 ) and subsidence (0 -9 mm yr −1 ) relative to the study area mean. Along-margin belts of relative uplift and subsidence, respectively, are approximately associated with Coast Ranges and the Cascade volcanic arc. However, the vertical velocity data are locally heterogeneous, demonstrating patchy "anomalies" within the larger along-margin belts. A large coastal subsidence anomaly occurs in southwest Washington where the modern shortterm trend is reversed from the long-term (~200 yr) tidal marsh record of coastal uplift since the last co-seismic subsidence event (AD1700). The modern vertical displacements represent a late stage of the current inter-seismic interval. If the horizontal strain is considered largely or fully elastic, extrapolating the modern strain rates over the last 100 years show the accumulated strains would be similar in magnitude to the observed co-seismic strains resulting from the Tōhoku, Japan, Mw 9.0 earthquake in 2011. We believe that the central Cascadia seismogenic structure has accumulated sufficient elastic strain energy, during the last 300 years, to yield a Mw 9.0 earthquake from a rupture of at least one-half (400 km) of its length.

Cascadia Convergent Zone: An Example of Primary Convergent Seismogenic Structure

“…GPS baseline strains, i.e., baseline shortening or lengthening, between 1200 paired adjacent GPS stations are well documented in the Cascadia margin [24].…”

“…Surface GPS stations can be used to look at the variation in the vertical component of GPS station motion [24]. Though not recording vertical strain, the GPS station velocity data do demonstrate vertical deformation at the regional scale.…”

“…It has a well-dated paleo-record of coseismic coastal subsidence and corresponding nearfield tsunami excitation from major mega-thrust ruptures [60] [61]. Of particular importance to the analyses presented here, the Open Journal of Earthquake Research Cascadia margin is well-instrumented for GPS station baselines [22] [24] and broadband seismograph stations [46]. However, the Cascadia margin has not experienced a great earthquake in historical time (pre-European contact), thus motivating studies of predicting potential seismic energy distributions from a future major mega-thrust rupture.…”

“…How far inland from the buried trench or deformation front can inter-seismic stress and accumulated strain be propagated and stored in the upper-plate? Such questions are increasingly relevant to safety hazards from under-reinforced infra-structure developed well away from the presumed "seismic sources" in offshore "locked zones" of subduction-zone margins [24]. In this article, we compile several previously reported datasets for the Cascadia convergent zone ( Figure 1) that indicate both the scale and geometry of the Cascadia primary seismogenic structure.…”

Tidal Flat Depositional Response to Neotectonic Cyclic Uplift and Subsidence (1–2 m) as Superimposed on Latest-Holocene Net Sea Level Rise (1.0 m/ka) in a Large Shallow Mesotidal Wave-Dominated Estuary, Willapa Bay, Washington, USA

“…However, the duration of the first stage of the interseismic strain interval, a period of interplate recoupling, might last only a few decades after coseismic subsidence. Tectonic uplift during the middle stage of the interseismic interval could emerge the tidal flats to pre-subsidence tidal elevations within another 100-200 years (Cruikshank & Peterson, 2017). The third stage of the present interseismic interval in Willapa Bay is characterized by slight subsidence (several millimeters per year), based on a decade of averaged GPS station vertical velocities.…”