The aftershocks just keep on going… And will keep on going for a while.

When I got up Saturday morning and turned on my cell phone it immediately filled up with text messages and after clearing those there were a bunch more.  Yup, my day job caught up with me on the weekend and after an event Saturday morning I spent the afternoon studying the developments and looking at the tectonics.

The basic information: The magnitude 8.8 earthquake off the coast of Chile was a shallow earthquake in the Peru-Chile trench and appears to have broken about 400 miles of the fault.  The fault is the boundary between the South American Plate and the Nazca Plate.  The Nazca Plate is a small and young tectonic plate completely under the Pacific Ocean.  The Nazca Plate is going under South America at about 80 mm/year and is responsible for the Andes Mountain Range and the active volcanoes in it.  At the time of this writing there have been 119 aftershocks of magnitude 5 or larger with an additional one now every hour or so now.

This earthquake makes the top ten list of events since 1900 and released about 500 times more energy than the recent earthquake in Haiti.  With the official death toll in this event still a bit below 1000 (it will certainly pass that mark) it is interesting to note the difference that preparedness and economic development have on earthquake survivability.  There is substantial damage and I spent a lot of time studying the failure modes that I see in a great collection of pictures by the Boston Globe, but I don’t see the total collapse of neighborhoods full of unreinforced masonry structures like the pictures from Haiti showed.

The Pacific Coast of South America is no stranger to great earthquakes. (And for the record, a “great earthquake” is a technical term to distinguish an even of magnitude 8 or larger.  There is a magnitude 8 or larger earthquake somewhere on the earth about every year and a half on average.)  The largest recorded earthquake was the 1960 Chile earthquake (magnitude 9.5) on the section of the fault just to the south of this earthquake.  And Charles Darwin experienced a large earthquake here while visiting on the voyage of the Beagle in 1835.

For an idea of the size of this earthquake consider the fact that the point the earthquake began (the epicenter) was about in the middle of the section of fault that broke.  As the earthquake happened it broke about 300 km in each direction.  At a rupture speed of 3 km/sec that give a rupture time of 100 seconds.  That is how long the fault took to break, but it generates different waves that travel at different speeds so the local shaking is longer as all those waves go by.  Another point of comparison is that we would expect the largest aftershock to be about the same size or slightly larger than the Haiti earthquake.  At the present time the largest aftershock is magnitude 6.9 and Haiti was magnitude 7.0.

Now the reminder for my North American readers:  Many of you are probably aware that Alaska had a great earthquake like this one back in 1964 that devastated southern Alaska, especially the Anchorage area.  There is a lesser known earthquake back on January 26, 1700 along the coast of Northern California, Oregon, Washington and British Columbia.  While this event is detailed in the oral traditions of the indigenous peoples of the area we also have a written record from Japan of the devastation caused by the tsunami there.  And the geologic situation in the Pacific Northwest is very similar to South America with the volcanic mountain range (Cascades) and a small, young tectonic plate (the Juan de Fuca Plate) going underneath North America.  It is good to know that locally this risk is now understood and preparedness measures are being taken.