Chile, Mid-Atlantic and California: Earthquakes 17-23 September 2015


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This week's (17-23 September) earthquake map is dominated by events in Chile

This week’s (17-23 September) earthquake map is dominated by events in Chile. Image by USGS.

The week of 17-23 September 2015 saw the biggest earthquake of the year to date, a magnitude 8.3 (M8.3) event of the coast of Chile.

In a sense, a tremor as large as this renders the statistical part of the weekly digest pretty much useless as a comparator to previous weeks, because this one earthquake and its many aftershocks (almost 200 in excess of M4.0 at the time of writing, with more to come) completely skew the numbers.

We can look at the stats anyway. In a typical week we might expect a few (say, three or four) earthquakes of at least M6.0. This week there were 15, of which 13 were in the Chilean earthquake mainshock-aftershock series.

We might expect around 20 of at least M5.0 but we saw 74 (55 in Chile); and there are usually around 100 in excess of M4.0 but this week there were 281, with 196 in Chile.

You get the picture. If you take out the Chilean earthquakes it would have been a pretty normal week — but M8.3 is anything but normal.

The Week’s Biggest Earthquake: M8.3, Chile

The M8.3 earthquake off Chile was the largest earthquake of 2015 to date.

The M8.3 earthquake off Chile was the mainshock in an ongoing series of tremors and the largest earthquake of 2015 to date. Image by USGS

The margin between the Nazca and South American plates, which runs along much of western South America is a classic — perhaps THE classic — subduction zone. The subduction of the dense oceanic crust of the former beneath the buoyant continental crust of the latter along the Peru-Chile Trench raised the Andes mountains. It’s earthquake country; and when it comes to earthquakes, Chile has produced the daddy of them all.

The United States Geological Survey’s real time earthquake archive shows 83 earthquakes of at least M8.0 in the past century. Of those, 17 occurred along the Peru-Chile trench; they include the largest earthquake recorded, which occurred in 1960 and measured M9.6. There have been five of these large earthquakes in the 21st century, the most recent of which, in 2010, killed over 500 people.

The earthquake of 17 September (UTC; 16 September local time) caused extensive damage but killed relatively few people (early estimates indicated five but the number may have increased). Like many such events, the earthquake generated a tsunami, which reached heights of 4.75m locally and was also measured (at much lower heights) on the other side of the Pacific. Over 10 million people were affected by moderate to severe shaking.

What saved the lives of hundreds, if not thousands, of Chileans (over 1,500 died following the 1960 quake) was the realisation that such tremors are a fact of life. Chile’s disaster planning, which involved the evacuation of over a million people, was effective and timely. It will be needed again: the seismic history, and ongoing studies, suggest that big quakes on vulnerable coasts will just keep happening.

M6.0 Earthquake: Mid-Atlantic Ridge

An earthquake of M6.0 occurred in the mid-Atlantic

Constructive margins shake, too – an earthquake of M6.0 occurred in the mid-Atlantic. Image by USGS.

How do you follow an M8.3? With the dominance of the Chilean margin, it’s hard to find anything that’s interesting by comparison. In another week we might have been focusing a little more closely on the M6.0 which occurred in the middle of the Atlantic, along the spreading ridge which separates the African and South American plates.

M6.0 earthquakes occur on a weekly basis but are usually along subduction zones. This week, what’s interesting is perspective.

To the west of the South American plate, a tremor of M8.3, to the east, one of M6.0. Just for the record, the former, 2.3 times larger in terms of magnitude, would show up on a seismogram as almost 200 times larger — and released over 2,800 times as much energy.

US Earthquakes: California

This week saw an earthquake of M4.0 along the San Andreas Fault Zone

This week saw an earthquake of M4.0 along the San Andreas Fault Zone. Image by USGS.

The Pacific margin is earthquake-prone along its entire length and the San Andreas Fault Zone is possibly the most famous (or notorious) part of it, even if it doesn’t produce the largest earthquakes.

This week there was an M4.0 in the San Bernadino mountains. It was a small tremor, and affected very few people — but it serves alongside the Chilean quake as a reminder that larger tremors are possible, and potentially dangerous, in the highly populated areas of the North American, as well as the South American, Pacific coasts.

Last Thoughts: Earthquake Preparedness

Chile has learned its lesson from the past, and the low casualty count this week shows that major earthquakes in populated areas don’t necessarily lead to high death tolls, as long at the risks and the hazards are understood.

We can’t prevent earthquakes, nor (in the foreseeable future) predict exactly where, when and with what magnitude they will occur — but we do know that planning for them can save thousands of lives.

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