Figure: Exposure of near-surface sediments in a tidal marsh near Tofino, British Columbia. The marsh subsided during the most recent of the great Cascadia subduction earthquakes in AD 1700. This earthquake lowered the marsh surface, here labelled ‘pre-earthquake peat’. About 30 minutes later, a tsunami swept over the marsh and deposited the sand layer above the pre-earthquake peat. In the decades that followed, tidal mud slowly settled out on the down-dropped marsh. (Photo: J.J. Clague)

Subduction zone earthquakes off BC’s South Coast: were they all giants?

The motivation People living on the south coast of British Columbia are at risk from extremely large (magnitude 8-9) earthquakes sourced at the Cascadia subduction zone, which is located off the coast of North America from northern California to Vancouver Island. The Cascadia subduction zone marks the boundary between two of Earth's crustal plates - the oceanic Juan de Fuca plate and, to the east, the much larger North America plate over which it is riding. Geologists have shown that about 20 giant earthquakes have happened along the fault that marks this plate boundary in the past 10,000 years, most recently in AD 1700. Previous research suggests that all, or nearly all, of these earthquakes have involved the full rupture of the 1100-km long fault and thus would have magnitudes of 9 or larger. Such behaviour is unique among the world's subduction zones, all of which display rupture of sections or 'segments' of the plate-boundary fault, commonly with earthquakes in the magnitude 8-8.8 range.

In their recent paper, veteran Cascadia earthquake researchers Drs. Hutchinson and Clague test the full-rupture hypothesis with geological data from the northern (Canadian) portion of the Cascadia subduction zone. They tested the assumption that earthquake events in Northern Cascadia coincided with earthquake events documented farther south, while bearing in mind the uncertainties in the large body of radiocarbon ages that have been obtained to date these events. Specifically, they tested the hypothesis that the ages of the northern and southern events are statistically equivalent. If this hypothesis were shown to be invalid, at least some of the great earthquakes must have been the product of segmented rupture and thus were probably smaller than magnitude 9.

The discovery – Drs. Hutchinson and Clague conclude that most of the subduction zone events in Northern Cascadia satisfy the hypothesis of temporal equivalence, that is they cannot be statistically separated in age from events in central and southern Cascadia. Some of the northern events, however, have no matches to the south, suggesting that the Canadian portion of the subduction zone, on occasion, ruptures independently of the American portion.  

Its significance – Why is this important? A magnitude-9 earthquake releases about 30 times the energy of a magnitude-8 quake, and cities in south-coastal British Columbia would experience far more damage from the larger quakes. This study highlights the need to better characterize the extent of segmentation of the Cascadia subduction zone.   

Read the paper“Were they all giants? Perspectives on late Holocene plate-boundary earthquakes at the northern end of the Cascadia subduction zone” by Hutchinson, I. and Clague, J. Quaternary Science Reviews 169:29-49 (2017). DOI: 10.1016/j.quascirev.2017.05.015

Website article compiled by Jacqueline Watson with Theresa Kitos