Last week I introduced the new-look geoscience digest with the observation that writing about earthquakes had become repetitive and risked becoming dull. Inevitably, of course, this week sees a major, and fascinating earthquake that I have to write about.
But there’s a lot more going on this week, both on the Earth and off it. So this week we’re looking at earthquakes, at volcanoes, and at landslides. Oh, and a quick trip to the Moon.
Earthquake of the Week: M7.9, Alaska
Last week I looked at 2017’s earthquakes: there were, if you recall, just seven registering between M7.0 and M7.9, which is rather fewer than we would normally expect. But make no mistake — 2018 is making a strong start, with three within the magnitude range before we’ve even hit the end of January.
This week Alaska was shaken by a tremor which reached M7.9 (having initially been reported as M8.2) — an event which, because of its potential serious nature, caused a degree of alarm, with residents being warned to head for higher ground as the authorities issued a tsunami alert.
The warnings, it turned out, were precautionary and were soon cancelled. The earthquake, which was caused by downward movement of the Pacific plate beneath the North American plate, was large enough to trigger a significant tsunami, but magnitude is just one of the factors at play. In this case, the key element was the nature of movement. While we would normally expect an earthquake of this magnitude and in this location to be caused by vertical movement at the interface between the two plates, this week’s was caused by lateral, or strike-lip, motion.
Vertical movement is crucial because it displaces significant amounts of water — intimating a tsunami. With lateral movement, that displacement is very much less. Lateral movements can have additional impacts, however, most notably in triggering underwater landslides which cause locally significant tsunami events.
Major Earthquakes and the Moon
There’s no question that the Moon, which is an integral part of the Earth system, plays a significant part in both our cultural history and in the world around us (the most obvious example being tides). Taking that example to its limits, there has for some time been a theory around that the gravitational pull of the Moon upon the Earth has some relationship with the occurrence of large earthquakes.
The New York Times, reporting on a study into this relationship, cites the examples of three major earthquakes of recent times — Sumatra in 2004, Chile in 2010 and Alaska in 1964 — all of which occurred during a full moon. But, as the newspaper goes on to report, the statistical analysis underpinning the study clearly demonstrates that the relationship is a myth. The gravitational pull simply isn’t strong enough to affect solid Earth in the noted way that it affects tides.
For the record, this week’s Alaska earthquake occurred on 23 January, when the Moon was in its first quarter.
Landslide! Watching and Waiting
Landslides are a significant, and often deadly, natural hazards. They occur when a slope becomes unstable and collapses, with the ground collapsing downwards. Earthquakes are a regular feature following earthquakes, when the shaking can cause slope failure. As noted above, if they occur underwater they can cause tsunamis, while on land they can bury settlements or block roads preventing rescuers from reaching damaged areas and causing additional loss.
Earthquakes aren’t the only direct cause of landslides, as the recent landslides in California have shown. In this case, the instability results from heavy rain on slopes left bare following wildfires. And sometimes landslides are much less dramatic than that, but more a part of a slow erosional process as gravity eventually changes the profile of a slope and leads it to collapse.
On a hill above Interstate 82 in Washington State, this process is playing out in front of our very eyes. In October 2017, a crack developed adjacent to a quarry on Rattlesnake Ridge and that crack is growing. The Washington State Department of Natural Resources, which is monitoring the situation, provides three possible scenarios ranging from the most likely, and least dramatic (“the landslide will continue to slowly move to the south, where the landslide mass will fall into the quarry pit”) to the extremely unlikely (“the landslide will run out beyond I-82”).
The ensuing landslide, which is expected some time in March, is unlikely to be catastrophic because of its slow development and careful monitoring. And it may help to provide geologists with clues for future hazard management and mitigation.
And finally, to a quick look at one of my favourite natural phenomena — volcanoes. Truly large volcanic eruptions are rare, and generally offer some kind of warning, so that evacuation plans can be put in place and many lives safe. That’s not to say that major eruptions, especially in heavily populated areas, don’t have the potential to kill very many people — but it does mean that it’s a little easier to appreciate their incredible beauty.
There are many types of volcano and the classic version is the cone-shaped stratovolcano which typically occurs behind a subduction zone. This week, one of the Earth’s big beasts is in action. Mount Mayon, in the Philippines, began rumbling last week, allowing time for the evacuation of thousands of local villages, and it’s currently putting on a spectacular show.