Evidence for large earthquakes at the Cascadia Subduction Zone

Abstract

Abstract. Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with geodetic measurements and the results of geophysical modeling, which indicate that part of the plate boundary is locked and accumulating elastic strain that will be released during a future large earthquake. Arguments based on potential amounts of seismic slip and likely rupture areas suggest that most or all of the plate boundary earthquakes were magnitude 8 or larger events. The last earthquake or series of earthquakes, about 300 years ago, ruptured the entire 1000-km length of the subduction zone; if it was a single quake, it probably exceeded magnitude 9. Other earthquakes may have ruptured one or more segments of the subduction zone Or may have occurred on faults in the North American plate. Recurrence intervals are uncertain because of difficulties in identifying and dating earthquakes. In southwestern Washington state, intervals for the seven most recent earthquakes average about 500 years but range from less than 200 years to 700-1300 years. Future research on Cascadian plate boundary earthquakes will probably focus on (1) the relation between plate boundary and crustal earthquakes, (2) earthquake magnitude, (3) the areal extent and severity of seismic ground motions, (4) ages and number of past plate boundary earthquakes, and (5) land level changes preceding earthquakes.

Investigating subduction earthquake geology along the southern Hikurangi margin using palaeoenvironmental histories of intertidal inlets

“…Nelson et al 1996;Clague 1997;Cisternas et al 2005). Supporting evidence may include liquefaction of sediments and landsliding.…”

Strain accumulation along the Cascadia Subduction Zone

“…Geodetic measurements of present-day deformation show that the Cascadia subduction zone (CSZ) (Figure 1) is accumulating strain, and geologic evidence of abrupt episodic coastal subsidence in the past suggests that this strain is periodically released in large earthquakes [Clague, 1997]. Tsunami records suggest that the most recent event was a great M•9 earthquake that ruptured much of the megathrust in January 1700 [Satake et al, 1996], although evidence that the overlying plate is weak and rapidly deforming is more consistent with the megathrust rupturing in a sequence of smaller M•8 events [McCaffrey and Goldfinger, 1995].…”

Evaluating the coastal deformation mechanisms of the Raukumara Peninsula, northern Hikurangi subduction margin, New Zealand and insights into forearc uplift processes

“…Nelson et al 1996;Clague 1997), Chile (Cisternas et al 2005), central Hawke's Bay and Bay of Plenty, New Zealand (Hayward et al 2004Cochran et al 2005Cochran et al , 2006, we define two further characteristics of intermittent, coseismic coastal deformation that are typically related to subsidence: microfossil evidence of sudden palaeoenvironmental change and the presence of associated palaeotsunami deposits (Table 1). The methodology of using microfossils for relative sea-level studies is well-established and micropalaeontological detection of coastal earthquakes has also been used at several locations in New Zealand (e.g.…”