An earth tremor of magnitude 3.5 (M3.5) rattled the outskirts of the city of Seattle, Washington in the early hours of 28 July (local time).
No damage or injuries from this earthquake have been reported at the time of writing – what’s behind the Washington quake?
The Tectonic Setting of Seattle
The Pacific coast of the US, from Cape Ferndale in California northwards, is characterised by a major subduction zone, the Cascadia subduction zone.
Here, the Juan de Fuca tectonic plate (thought to be the remnant of the much larger Farallon plate) is descending beneath the North American continent at rates of around 36mm per year.
The subduction zone is partly responsible for the uplift of the the Rocky Mountains and for the chain of volcanoes which brood above Washington state and which includes Mounts Rainier, St Helen’s and Hood among others.
These volcanoes, which present considerable hazards to the local population, are the result of melting of the earth’s crust at high pressures and temperatures as it descends; the buoyant molten rock then rises to the surface.
Cascadia is a major subduction zone, and one which is capable of generating major earthquakes. The largest, in 1700, predated seismographs but is estimated to have had a magnitude of around M9 — around ten times larger than the San Francisco earthquake of 1906.
The complicated tectonic setting of the region means, however, that subduction is not the only source of earthquakes; several types occur in the area.
Subduction earthquakes, at the plate interface, are one type but are mercifully rare; the subduction zone also sees deeper tremors, which may have a magnitude of around M7.0.
At the western, northern, and southern margins of the Juan de Fuca plate, earthquakes occur along fracture zones and spreading centres. These are typically shallow and though they can reach relatively high magnitudes, they are not generally damaging because of their size and depth.
The United States Geological Survey’s earthquake database includes around 100 of these with a magnitude of M6.0 or more since 1900.
The most recent Seattle earthquake is, in the light of the available evidence, almost certainly a shallow crustal tremor which results from the stresses generated within the crust by convergence and uplift.
Such earthquakes are potentially damaging at high magnitudes — but those of M7.0 or more are rare, with probable recurrence intervals of hundreds of years.
Cascadia Earthquakes: How Often?
While small earthquakes aren’t hugely damaging, there’s no question that a major crustal earthquake can be destructive in the immediate area (Yeats notes that “the large, damaging historical earthquakes in Oregon and Vancouver Island have all struck crustal faults”) and a large subduction event, which has the potential to generate a major tsunami, can be devastating on a Pacific-wide scale.
These tremors are, however, rare. Cascadia is not a typical subduction zone, having had relatively low levels of seismic activity over the recent past. The reason for this low level of activity is unclear.
The Pacific Northwest Seismic Network (PNSN) which puts the likely recurrence interval for an M9 tremor at around 5-600 years, observes that it “is unique among the world’s subduction zones in that it produces very few (if any) earthquakes unambiguously on the plate interface;” while recently-updated seismicity maps of the US show low levels of seismic risk.
Seismologists are divided about the reason for this. Robert Yeats notes that until recently Cascadia was thought to be aseismic (that is, moving very little and so accumulating little strain) but that the view is now changing.
It’s uncertain, on the basis of the earthquake history, whether the subduction zone is locked or moving very slowly — or, indeed, whether it historically ruptures in single “megathrust” events or more frequently at slightly lower, but still significant, magnitudes.
Washington Earthquake: No Major Danger
In the context of the tectonic setting, the most recent Washington earthquake represents no major danger – but it is a reminder of how many ways an earthquake can be generated.