Another sobering week, another big earthquake in Nepal — and another reminder of how vulnerable we humans are against the power of nature.
At M7.3, the tremor which struck i nNepal on 12 May was smaller than its predecessor of 25 April but still came in — by some margin — as the largest of the week.
The United States Geological Survey’s real time earthquake map (which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) was congested this week.
The Nepal earthquake produced (at the time of writing) 9 aftershocks and last week’s large earthquakes in the western Pacific produced 12 more out of a total of 42 with a magnitude of at least 5 (≥M5.0).
The Week’s Biggest Earthquake: M7.3, Nepal
The largest earthquake of the week piled increased misery on the people of Nepal, causing further deaths and injuries as already-weakened buildings collapsed and already-destabilised slopes failed, causing avalanches. And the bad news is that, with 33 tremors of ≥M5.0 and almost 70 of ≥M4.0 since the 28 April, more shaking — and more damage — can be expected.
Stepping back from the human cost of the quakes, there is a interesting seismological debate associated with the pair of large earthquakes.
Were they part of the same series, with the second being an aftershock of the first? Or were they two separate earthquakes, the one triggering the other?
At this stage we can’t say for certain. Although the USGS records the smaller as an aftershock, the fact that it had its epicentre (just) outside the part of the crust which ruptured during the M7.8 events suggests that it may in fact be the mainshock of a separate series.
In a sense this is immaterial in the context of the human misery which both Nepal quakes have caused; but any increase of understanding of earthquakes in such areas can surely only be positive in terms of future mitigation.
M6.8 Earthquake: Japan
The Japanese are accustomed to dealing with the threat of a major earthquake, living as they do on an archipelago which spans the junction between four tectonic plates and which has a record of large and frequent earthquakes.
This week’s second largest tremor, an M6.8 which occurred of the east coast of Honshu, was significant — almost large enough to generate a tsunami and so potentially reminiscent of the (very much larger) earthquake of 2011.
In fact the evidence — a location some way to the west of the Japan Trench and a depth of 38km — suggests that the tremor might be the result of crustal deformation rather than movement at or along the subduction zone which ruptured so violently and extensively four years ago.
However, as the USGS notes: “Subduction zones at the Japanese island arcs are geologically complex and produce numerous earthquakes from multiple sources,” meaning that we cannot be certain of the precise mechanism on the basis of the existing information.
US Earthquakes: Hawaii M4.5
This week the largest earthquake in the US (jointly with one in Alaska) was an M4.5 on the island of Hawaii. Located far from any plate boundaries, Hawaii is the product of a magmatic hot spot, where rising magma stretches and strains the crust before emerging as the volcanic island chain we see today.
This stretching fractures the crust and generates earthquakes — usually small but with the potential for significant magnitudes and with associated damage.
Many Different Mechanisms for Earthquakes
Continental collision, crustal deformation (or possibly subduction) and hot spot magmatism were the causes of our featured earthquakes this week. This illustrates how diverse seismic activity can be. Truly, the earth moves in a multitude of ways.