About Earthquakes– An informative email

I need to be able to move on and stop writing (and thinking) about earthquakes and what is going on in Chile.  I agree with Mandy that is is really hard to move on and and is frustrating that so many people around me have no idea what is going on.  But, I try to remind myself how lucky I am.  I have, after a week, been able to contact everyone. Sure, I have friends who lost everything.  Sure, I have heard about walls collapsing on my loved ones– and how they escaped.  But, I did contact everyone.  So, after this post, I am going to work on blogging about the other things on my list of what to blog about it.

I, as part of the Chile Spouse group, received an email about earthquakes that really helped me understand some facts. I thought the email was incredibly useful and asked the author if I could share it.  She agreed.  That said, these are the understandings and thoughts of one person— do not use this information to write your next dissertation or classroom paper.  Like Wikipedia, my blog should not be on your bibliography page.  Without further ado, a friend’s wisdom about earthquakes:

Since the massive 8.8 earthquake in Consititucion last Saturday morning, I have been bombarded with questions from friends relating to the recent earthquake, as have many others who like me, are geologists. I am a geologist working in mineral exploration but like most geologists I have general knowledge of many of the areas covered in geology. I also work with many Chilean geologists with much experience and interest in the Chilean subduction related earthquakes. Our team is also fortunate to be currently contracting one of the few academic structural geologists that work in Chile who has a PhD and conducted post doctoral work on Chilean seismicity. Only two weeks ago I asked him when Santiago’s earthquake was due. He said he was not so concerned about Santiago but that the Constitucion area was due a big earthquake first as that segment was overdue an earthquake. The coincidence of the timing of our conversation still leaves me flabbergasted and full of more questions. Poor guy. Over the past few days back at work the discussions have been incredibly interesting.

I thought I might share some general earthquake information and also some thoughts that have been circulating at my workplace with the hope it will help to explain things and quieten the fear that has gripped some of us in Chile.

Before going on to the specifics of the 8.8 last weekend I would like to go over the basics of why earthquakes happen.

Plate tectonics is the movement of the crust, a thin layer of solid, relatively cold rocks covering the Earth’s surface which lie on the warmer and very viscous rocks of the mantle,. This movement is caused by the formation of very slow moving convection cells in this underlying mantle. The currents of the cells are driven by the heat from the core of the Earth, allowing the warmed mantle to rise to under the crust where it cools over time. The currents then move laterally, cooling until their densities increase again and become cool enough to sink back into the depths of the mantle. For more information see wikipedia http://en.wikipedia.org/wiki/Plate_tectonics.

The actual movement of the crust is constant and the mantle pulls the crust above with it (basal drag). Also sinking oceanic plates at subduction zones pull the plates along. The Earth’s surface is made up of many sections of crust, called plates, and it is at the boundaries of these, where rocks move against each other, where earthquakes are generated. There are four types of plate boundary:

  1. Divergent plate boundary: Where the flow of the mantle rises up and separate outwards causing the overlying plates to separate from each other, forming new crust in the gap produced (like in the centre of the Atlantic which split Pangaea to form Africa and South America, about 175 million years ago).
  2. Convergent plate boundary (subduction zones): Where two mantle flows converge and sink taking the crust with it. This is called subduction and crust is destroyed here, by sinking below the Earth’s surface. This happens where an oceanic and continental crust converge. The denser (basaltic) oceanic crust sinks under the less dense (granitic) continental crust and after many millions of years is eventually assimilated into the mantle. The compression of the crust at the surface forms a mountain range in front of the sinking plate. The high water and volatile (carbon dioxide, methane etc.) content of the sinking oceanic plate form volcanoes at the surface, usually within the mountain range, since heating these reduces their density and they rise to the surface with other melted rock as lava in a series of magma chambers. When there is not too much compression of the continental plate, lava can escape and produce volcanoes. When the compression of the crust is too great the lava cannot escape and elements such as copper, gold, molybdenum, tin, tungsten and silver can concentrate in the magma chambers and form mineral deposits.
  3. Collision plate boundary: where two continental crusts meet due to converging mantle currents and neither plate sinks. This is typified by the Indian sub-continent smashing into Asia or the Africa continent into Europe where very big mountain ranges form.
  4. Transform/conservative plate boundary: where the plates move past each other and no crust is created or destroyed. This is the situation in the San Andreas fault of California and in Haiti.

The earthquakes in Chile are convergent and subduction-related as described above. In Chile the oceanic Nazca plate is pushed up against and sinks under the South American plate. They move together in a diagonal direction at about 70mm a year. The terrain in Chile is relatively young (some Palaeozoic and Mesozoic rocks but mainly Cenozoic rocks, i.e. post dinosaurs and younger than 65 million years old) and the majority of the formation of Chile can be accounted for by the presence of the subduction zone. The subduction zone in Chile manifests itself on the surface as a deep trench off the coast and a volcanic/collision mountain range inland which also contains rich mineral deposits. As the oceanic plate sinks, friction with the above continental plate prevents a constant smooth movement and the plates lock and get stuck together. The plates however continue to move towards each other but there is less movement near to the subduction zone. The strain between the two plates builds up until the stress overcomes the friction and movement suddenly occurs between the plates producing an earthquake. The size of the resulting earthquake depends upon how deep is the movement that takes place (the plates can lock from shallow to deep depths), how much tension had built up (how much movement had occurred since the plates became locked) and if the subduction zone is lubricated with volatiles (still very poorly understood). It is important to remember that although these processes described have formed the Andes Mountains they take a very long time when compared to our own lifetimes. The will be no Hollywood type rapid changes in topography such as mountains suddenly appearing or huge cracks in the ground separating continents. It is a very gradual process.

The processes at subduction zones in general are still only basically understood as we can’t drill holes 100 kilometres deep to check what is down there. However it has been possible in recent years to be able to use GPS data to see how quickly the plates are moving relative to each other and calculate the strain that has built up since the last occurring earthquake in the area under study. This, with much speculation, can be use to predict the size of the imminent earthquake. I have a paper from 2008 predicting a 7.2 earthquake in Haiti and another paper from 2009 speaking of a possible 8.0-8.5 earthquake in Constitucion. At the current time it is impossible to predict or forecast earthquakes with any accuracy.

Using this information and recently released data we can try to interpret what went on during the recent earthquake:

  • In the recent 8.8 in Chile the plates moved 8m against each other. This is might not sound a lot but it is huge!
  • The earthquake was the fifth biggest in recorded history (http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php). The biggest was the Valdivia earthquake which, when I read what happened and try to comprehend the amount of energy released, I find it almost inconceivable this occurred on planet Earth (details on wiki: http://en.wikipedia.org/wiki/1960_Valdivia_earthquake).
  • The force of the earthquake was so massive that the Earth’s axis has moved by 8cm and the days on Earth have been shortened by 1.26 microseconds.
  • There have been over 200 aftershocks since the earthquake. An aftershock represents the plates to each side of the initial movement finding their own equilibrium with each other again. The United States Geological Survey has some great sites detailing these:
  • Santiago region: http://earthquake.usgs.gov/earthquakes/recenteqsww/Maps/10/290_-40.php
  • A list of the aftershocks in the Santiago region: http://earthquake.usgs.gov/earthquakes/recenteqsww/Maps/10/290_-40_eqs.php
  • A world map: http://earthquake.usgs.gov/eqcenter/recenteqsww/
  • This earthquake was one of the less frequent ones which occur around every 100-150 years depending on the region. It was not one of the smaller earthquakes (these are still above 7.0 and are considered big by anyone’s standards) which occur every 10 to 40 years depending on the location.
  • It is unlikely that if there is another earthquake or big aftershock it will be of the same magnitude as the 8.8.
  • From what I have heard there were three tsunamis. One after 10-15 minutes, and two more in the subsequent hours.
  • Although hundreds of people have died as a result of the earthquake and the following tsunami the numbers of deaths are very low. If this happened in any other country the results would have been much worse.

This last point highlights a bee I have in my bonnet. Many people in Santiago since the earthquake who have spoken to national and international media outlets have expressed that they “are lucky to be alive”. I actually think that it is the deceased who have been very very unlucky as Chile is a country well prepared for earthquakes. I urge us not to forget the unsung heroes of this disaster: the engineers, building designers, construction code enforcers, building inspectors and school teachers who by doing their jobs saved most of our lives. In Santiago, where the earthquake had a magnitude of 8.0 there were 38 deaths. In a city of over 6 millions that is an unbelievably low figure.

There are also some other points of discussion which have been generated by the earthquake:

  • As with the 1985 Santiago earthquake, the structural geologists are saying that not enough tension was released by the following aftershocks to each side of the main earthquake. In 1985 some predicted another earthquake after the main shock and again rumours are saying there could be another now. In 1985 there was not another earthquake and it is impossible to say such things with conviction.
  • The strain is likely to have transferred to the Valdivia and Santiago sections of the subduction zone, the zones on each side of the recent rupture zone.
  • For those in the capital, an earthquake in Santiago will happen sometime in the future.
  • This earthquake could be tomorrow or in 50 years.
  • In recorded human history this Santiago section of the subduction zone has not had huge earthquakes like the one just past, since the flat slab subduction, where the oceanic plate sinks at a shallower angle (caused by the stiffening of the Nazca plate by the Juan Fernández ridge) generates different dynamics which are still not clearly understood.
  • Chile is now inundated with seismologists from all over the world, so hopefully in the future we will have a better understanding of this whole earthquake region.
  • New studies are shining a light on earthquake prediction (such as monitoring electrical currents in rocks prior to earthquakes) thanks to work in California and other regions. Maybe in the future we will be able to forecast earthquakes.

Unfortunately we live in a very seismically active region of the world and with current technology it is impossible to tell when and where an earthquake will happen. We just have to be prepared and trained. The following are some things we can to help prepare us for the next earthquake, but please note it is by no means an exhaustive list:

  • Although easy to say, remain calm during and after an earthquake and if possible try not be scared of earthquakes or tremors. They are part of life in Chile. I try and think of it as mother Earth reminding us of her immense power. It is best to keep your wits about you and keep those around you calm. For me remembering the science helps.
  • If you live at the coast train yourself, your kids, your neighbours and tourists to go to higher ground after an earthquake. The general rule is if you can’t easily stand in an earthquake there will probably be a resulting tsunami within 10-15 minutes at the Chile coast. There will also probably be more than one tsunami so don’t return immediately to the coastal area.
  • Keep exits to buildings clear and know where they are.
  • From what Chilean geologists here have told me: if in an apartment block do not go to the stairs during the earthquake. I have heard that a lot of the fatalities of the fallen building in Concepcion were those in the stairwell. It is best to wait and hold on in a door frame or near a supporting building pillar in the apartment.
  • Always sleep with shoes next to your bed (in case of broken glass) and have plastic glasses for water in the night.
  • Keep an earthquake survival kit. Mine was very useful on Saturday. I have candles, matches, a torch, batteries, leather gardening gloves and duck tape. I also keep a container with 20 litres of water for me and my boyfriend and some cans of tinned food. Since Santiago is so well prepared for earthquakes in general it is not necessary to keep days of food.
  • Make sure your kids know what to do in an earthquake. The schools should teach these things in case the earthquake happens during school hours.

As a side note, if you really want to have some kind of earthquake prediction I suggest you get a pet snake. It is believed they can feel the very low frequency vibrations which precede an earthquake. They go bonkers prior to earthquake and can actually kill themselves trying to escape any compound within which they are held. Studies in China have shown that this can be an efficient warning system when properly applied i.e. don’t use the lift in an apartment block if your snake is acting strangely. However, maybe it is cruel to own a snake if earthquakes and tremors are very stressful for them. It occurred to me that maybe this is why there are so few in Chile. Who knows?!

I hope this clarifies things. We can never avoid earthquakes in Chile. Just prepare for them.

3 comments

  1. Escriba, escriba… es terapetico, necesario y ayuda a muchas mas personas de las que siquiera te imaginas. Abrazos solidarios!!

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