There were a couple of significant earthquakes for us to consider in the week of 22-28 July 2015, one of magnitude 7.0 and one of M6.9.
These earthquakes were among almost 1,750 tremors recorded this week on the United States Geological Survey’s real time earthquake map, which includes all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere.
With the USGS map showing 32 earthquakes equal to or in excess of M5.0, it was a busy week for seismologists.
Interestingly, the number of intermediate-sized earthquakes (which we at Decoded think of as roughly between M5-M6) wasn’t inflated by aftershocks from the two larger events.
Instead, there were medium-sized tremors in Europe, the Himalayas, South America, Alaska, the western Pacific and at various points mid-ocean.
The Week’s Biggest Earthquake: M7.0, Papua New Guinea
It isn’t really surprising that the Decoded earthquake digest regularly lands up somewhere in the western Pacific in reviewing the largest earthquake of the week.
The region is tectonically highly complex, with several slivers of crust (microplates) caught between the larger tectonic plates of the Pacific and Australasia.
The map (for once showing plate boundaries only, rather than terrain) illustrates just how complex this area is.
The red lines show areas where the crust is either being created (extensional tectonics); destroyed (compressional tectonics); or sliding past in a lateral movement (which may be either depending on relative plate motions). And the map included here is a simplified one, as these tectonic zones are accompanied by bands of uplifted or subsiding land, with associated fault zones.
A closer look at fault maps suggest that this week’s M7.0 may have occurred along one of these — possibly on the Mamberambo Thrust Belt or very close to it.
This implies that the earthquake was the result of the collision between the Pacific and Australian plates and was therefore a product of collisional tectonics.
M6.9 Earthquake, Alaska
Collisional tectonics played a part in the second largest earthquake of the week, an M6.9 in Alaska’s Fox islands.
In this case, the picture is much clearer, with a relatively straightforward plate margin where the Pacific plate is subducting beneath the North American plate. Earthquakes regularly occur along the plate interface or as a result of deformation around it.
The location of the earthquake, midway between the plate margin itself and the volcanic island arc, along with the depth (27km) suggests that the tremor may have been caused by deformation rather than direct plate movement.
But even on so (apparently) simple a margin, the picture is complicated by the existence of broadly north-south trending shear zones which intersect the wider subduction zone.
At M7.0 in magnitude, the earthquake was large (if nothing like as large as the earthquakes which can occur along the Aleutian trench). But size isn’t everything; the remoteness of the area, along with the fact that the tremor, though offshore, was on the small side in terms of producing a tsunami, meant that it passed largely unnoticed.
US Earthquakes: California
Elsewhere in the US, there was minor activity in more populated areas and this, inevitably, will have attracted the attention of a few more people. At just M4.2, the earthquake which struck Fontana, California, was a fraction the size of that in Alaska — but the USGS shake-maps suggest that it could be felt by well over a million residents of large parts of the surrounding area. It doesn’t seem to have registered across most of Los Angeles, though. We’d all have been talking about it then.
Magnitude Doesn’t Matter
The estimated population exposed to the shaking of the Papua New Guinea M7.0 was (at a very broad estimate) somewhere in excess of 200,000. For Alaska’s M6.9 it was probably only a few hundred (there are no available USGS data for this). If an earthquake of that magnitude had struck California, we would be talking about many, many millions affected, in terms of death, injuries and losses to property.
It’s an over-simplification to say that magnitude doesn’t matter — but the impact of an earthquake depends to a greater extent on the density (and resilience) of the human population.
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