Nepal, Canada and California: Earthquakes 24-30 April 2015


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This week's earthquake map is dominated by events in Nepal

This week’s earthquake map is dominated by events in Nepal. Image by USGS.

Numbers. We usually begin this earthquake digest with some comforting statistics, but this week there’s really only one number that matters.

That one, the death toll from the major earthquake which struck Nepal on 25 April, currently stands at an estimated 5,500 people with many more injured and unaccounted for. By the time this article is published the number will almost certainly have increased and we may never know the full casualty roll.

For the sake of consistency and scientific detachment, however, we present the summary of the week’s earthquakes recorded on the United States Geological Survey’s real time earthquake map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥4.0) elsewhere.

The map showed a total of 1,779 earthquakes in the week of 24-30 April, 294 of them ≥M2.5 and 144 ≥4.0. These were, unsurprisingly, heavily concentrated in Nepal where the earthquake and its aftershocks accounted for 60 of those ≥4.0 and 17 of the 40 tremors which registered ≥5.0.

As a footnote, after several quiet weeks there were earthquakes of ≥6.0 in Canada, New Zealand and Fiji, each of which would, in an other week, have been worthy of much closer attention.

The Week’s Biggest Earthquake: M7.8, Nepal

Thousands of lives were lost as a result of Nepal's M7.8 earthquake and its aftershocks

Thousands of lives were lost as a result of Nepal’s M7.8 earthquake and its aftershocks. Image by USGS.

Larger earthquakes than the M7.8 which struck between Kathmandhu and Pokhara on Saturday happen several times a year with relatively little damage and few deaths and injuries. This tremor, which was felt across a large area, has killed thousands. Why is that?

There are two key reasons.

  • The first is the rule of nature: in a high-energy environment such as the Himalayas, earthquakes can trigger landslides and avalanches which supplement the original hazard.
  • The second is human, summarised by the USGS earthquake page as follows: “Overall, the population in this region resides in structures that are highly vulnerable to earthquake shaking, though some resistant structures exist.” In other words, it bears out the old seismologists saying that earthquakes don’t kill people: buildings do.

The problems of the earthquake were made worse by the fact that Kathmandhu itself is built on sediments from a former lake bed. Such sediments amplify the shaking, and, combined with the building issues outlined above, lead to extensive collapse of buildings locally and will have contributed to the death toll.

M6.2 Quake, Bella Bella, Canada

The junction between three tectonic plates generated an M6.2 earthquake off Canada.

The junction between three tectonic plates generated an M6.2 earthquake off Canada. Image by USGS.

The western seaboard of the Pacific is a seismic zone capable of generating earthquakes at least as large as that in Nepal. This week an M6.2 occurred off the Canadian coast along a length of strike-slip fault, the Queen Charlotte Fault, which extends from just north of Vancouver Island to the eastern end of the Alaskan subduction zone.

At just 10km depth and with an epicentre almost at the triple junction between the Juan de Fuca, Pacific and North American plates, the tremor was probably the result of crustal stress rather than slippage along the Queen Charlotte Fault, although no detailed data are available to confirm this.

In the last century, the Queen Charlotte Fault has experienced around fifty earthquakes of at least M6.0, the largest of them (in 1949) registering M8.2 and nine of them ≥7.0. Because of the movement along the fault (lateral, rather than vertical) these earthquakes don’t generate tsunamis and the remoteness of the area means that damage is limited.

In 1949, for example, Natural Resources Canada notes that after Canada’s largest recorded earthquake, the M8.2 cited above: “The shaking was so severe on the Haida Gwaii that cows were knocked off their feet, and a geologist with the Geological Survey of Canada working on the north end of Graham Island could not stand up”.

Compare that with the events in Nepal this week.

US Earthquakes: California

This week's largest US earthquake occurred off the coast of California.

This week’s largest US earthquake occurred off the coast of California. Image by USGS.

In an elegant (but coincidental) piece of seismic bookending, the southern edge of the Juan de Fuca plate also experienced a moderate earthquake this week.

This time the tremor occurred along the Gorda escarpment, a fracture zone between the Juan de Fuca and Pacific plates, around 250km off the Californian coasts. Such earthquakes are regular, barely felt on shore, and almost never damaging.

Nepal Earthquake

It’s difficult to find a closing piece this week when the events in Nepal aren’t over, with rescue operations still ongoing and human stories still playing out… Some to happy endings but many to tragedy. The Nepal earthquake reminds us not just of the strength of nature but also of just how vulnerable human beings are when confronted by its power.

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