Downdip landward limit of Cascadia great earthquake rupture

Abstract

[1] This paper examines the constraints to the downdip landward limit of rupture for the Cascadia great earthquakes off western North America. This limit is a primary control for ground motion hazard at near-coastal cities. The studies also provide information on the physical controls of subduction thrust rupture globally. The constraints are (1) "locked/ transition" zones from geodetic deformation (GPS, repeated leveling, tide gauges); (2) rupture zone from paleoseismic coastal marsh subsidence, "paleogeodesy"; (3) temperature on the thrust for the seismic-aseismic transition; (4) change in thrust seismic reflection character downdip from thin seismic to thick ductile; (5) fore-arc mantle corner aseismic serpentinite and talc overlying the thrust; (6) updip limit of episodic tremor and slip (ETS) slow slip; (7) rupture area associations with shelf-slope basins; (8) depth limit for small events on the thrust; and (9) landward limit of earthquakes on the Nootka transform fault zone. The most reliable constraints for the limit of large rupture displacement, >10 m, are generally just offshore in agreement with thermal control for this hot subduction zone, but well-offshore central Oregon and near the coast of northern Washington. The limit for 1-2 m rupture that can still provide strong shaking is less well estimated 25-50 km farther landward. The fore-arc mantle corner and the updip extent of ETS slow slip are significantly landward from the other constraints. Surprisingly, there is a downdip gap between the best other estimates for the great earthquake rupture zone and the ETS slow slip. In this gap, plate convergence may occur as continuous slow creep.

The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones

“…Because the subducting Juan de Fuca Plate is young (5-10 million years old) and thus warm, the locked zone that fails in megathrust earthquakes is relatively shallow and lies mostly offshore (see Flück et al, 1997;Hyndman, 2013).…”

“…Assessing the seismic and tsunami hazards of the Cascadia megathrust requires knowledge of the downdip and updip limits of the locked zone that generates earthquakes and how these limits vary along strike (see Hyndman, 2013). Of particular interest for assessing earthquake hazards are the downdip limits of the seismic source zone.…”

“…Map of the Pacific Northwest showing plate boundaries, convergence rates across the Cascadia subduction zone, and the approximate extent of fully locked (black line) and transition (dashed line) zones inferred from leveling and tide gauge geodetic data. From Hyndman (2013), based on Flück et al (1997) an earthquake. Figure 1 shows an early model of the along-strike variation of the Cascadia seismic source zone.…”

Determination of Mmax from Background Seismicity and Moment Conservation

“…In our analysis, we use a convergence rate of 40 5 mm=yr (Wilson, 1993;Wech and Creager, 2011) and a dip of 10° (Savage et al, 1991;Rong et al, 2014). The vertical extent of the seismogenic zone has been debated, from 15 km (Dragert et al, 1994) to 20-30 km (Williams et al, 2011;Hyndman, 2013;Rong et al, 2014); here we take a value of 25 10 km, giving an average seismogenic width of 125-150 km. We select instrumental earthquakes on the down-dip side, and as with other subduction zones, we take the coupled fraction to be 45%.…”

Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

“…Although the background seismicity in the studied region before the Tohoku-Oki earthquake was extremely low (~5 events/year) based on the Japan Meteorological Agency (JMA) dataset, the notable change in stress regime from compression to extension can be ascribed to a <1 MPa static stress change caused by the Tohoku-Oki earthquake . However, it remains ambiguous as to why the extensional tectonics, exceeding the regional compressive stress regime, were limited to near the Pacific coast in the Southeast Tohoku district (Imanishi et al 2012;Hyndman 2013).…”