Saturday, April 14, 2012

A Big Earthquake...

Map from USGS.

The other day I was sitting around at the "base camp" of the Kerinci Birdwatching Club, waiting to begin the English conversation class I teach there every Wednesday for the club's members.  As my friends and I were chatting, we noticed a subtle shaking, barely noticeable but confirmed by the shaking power lines just outside the building.  Since this area is pretty seismically active, we all immediately recognized the earthquake.  As the shaking continued, first for one minute, then another, I got more and more worried.  I've been in earthquakes before, but normally they only last a few seconds.  To me the continued shaking indicated a very large earthquake a good distance away.  I thought about the 2004 earthquake off Aceh in northern Sumatra which triggered a massive tsunami that killed more than a quarter of a million people and thought "oh no not again".  I started sending text messages to friends that might have internet or TV access and asked them to send me updates as quick as possible.  I wanted to know where and how big the quake was so I could anticipate what sorts of impacts we could expect at the regional level.  The quake, 8.5 to 8.7 in magnitude (depending on the source), occurred about 300 miles out to sea in the Indian Ocean, which means there is a high potential for a big tsunami.  The first news I got indicated the earthquake was near Aceh and Padang and that the focus was about 18 kilometers beneath the surface (1)(2).  But as we got more information, there was less and less to worry about.  As it turns out, although there were tsunami waves, none of them were particular large or damaging.

A Tectonically Active Archipelago

As I've mentioned in previous posts, there are a lot of volcanoes and earthquakes in Indonesia.  These are due to the fact that Indonesia sits at the junction of a couple of tectonic plates, which are huge chunks of the earth's crust that "float" around on the aesthenosphere, a layer of "plasticky" mantle immediately beneath the rigid shell of our planet (usually referred to as the "lithosphere").  These plates move around at about the same rate as your fingernails grow and bump into each other with incredible force, which is usually manifested as an earthquake.  The places where these plates come together are called "plate boundaries", and there are three general types of these.

  1. Spreading zones.  This is where the plates are moving apart.  New crust is created at spreading zones as molten rock is pulled up out of the mantle.  When the molten rock cools it becomes new crustal material.  Most of these spreading centers are in the middle of ocean basins, and their existence was only confirmed in the 1960s.  
  2. Transform zones.  This is where two plates are sliding laterally against each other.  The boundary between the Pacific and North American plates in the area of California is an example of this kind of boundary; there is another in the Caribbean Sea.  This makes these areas very vulnerable to earthquakes, but there aren't a whole lot of volcanoes at transform boundaries.
  3. Convergent zones.   This is where plates are coming together.  There are two basic types: subduction zones, where one plate (usually an oceanic plate because the rock that makes up oceanic crust is denser and hence "sinks") goes under another, and continental-collision boundaries, where continental crust from two plates is coming together.  In these places you generally see big mountain chains (the Alps and Himalayas are active examples) because the crust is piling up.  Think of two cars running head on into each other.  What happens?  Subduction zones generally have the biggest earthquakes and they also have a lot of volcanoes, as the oceanic crust that is pulled under and back into the mantle melts.  When this happens some material works its way up through the mantle and crust and comes out as lava.  It's kind of like exhaust; you might think of the volcano as a natural smokestack.  

The above generalization sometimes confuses students because this classification system hides the complexity of these "zones", but I'll explain more about this in a minute.  Everybody was worried about a tsunami because they are frequently generated at subduction zones.  The 2010 Japan tsunami and 2004 Indian Ocean tsunami are chilling reminders of the type of damage these big waves can do; they were both generated at subduction zones.  Thus when the quake happened Wednesday alarm bells went off in a number of countries and people started to implement evacuation plans to avoid a repeat of the colossal devastation of 2004, which was a wake-up call in this region.  After 2004 a lot of money and effort went into preparedness.  Why, then, was there no big tsunami?

Plate Boundaries vs. Faults

This vexed a lot of people.  I was among them until I read that the quake happened along a "strike-slip fault".  What does this mean?  First we need to understand the difference between a fault and a plate boundary (lots of people conflate the two), and that will help us work through the confusion and understand exactly what happened.  There is a fundamental difference between a plate boundary and a fault.  A fault is defined as "a fracture that has experienced movement along opposite sides of the fracture" (Keller and Pinter 2002), which means basically a crack in the earth's crust where the two sides are moving in different directions or at different rates of speed.  The three basic types of faults roughly mirror the 3 types of plate boundaries, and so I think this is where a lot of the confusion comes from.  These three types are

  1. Normal faults, where the two sides are moving apart
  2. Reverse (or thrust) faults, where the two sides are coming together
  3. Strike-slip (or lateral) faults, where the two sides are moving past one another.

Diagram from here.
You can get the idea from the diagram above.  Faults are different from plate boundaries because they describe a more local phenomenon.  There can be a lot of faults in a system, and many of them don't occur near plate boundaries.  These are simply places where some sort of pressure within the earth has caused a rupture.  Sometimes they aren't even clearly visible on the surface, as many faults don't affect all the layers of rock that make up the crust.

Diagram from here.  Site is worth a look.  
On the other hand, plate boundaries are huge systems that normally include dozens and dozens of different faults.  I usually rather forcefully ask my Geography 101 students to imagine a turkey pot pie crust.  When you cut it with a knife it usually crumbles a bit where the knife meets the crust.  There are a lot of little cracks.  This is kind of like a plate boundary.  Because of the complex movement of the tectonic plates (they don't exactly fit together like puzzle pieces; there is a bit of torqueing and twisting in their movement), there are usually different types of faults at the plate boundary.  As an example I've included the cartoon diagram of a spreading center plate boundary.  As you can see, the spreading isn't as smooth as the more general plate diagrams show.  Instead the crust is torn in different places, which results in movement in opposite directions at different points.  So even though this is a spreading center, there are strike-slip faults.  There are two basic types of strike-slip faults: right laterals and left laterals.  You can use an easy rule to remember the difference: if your friend is standing on the other side of the fault and (s)he's moving to the right relative to you, then it's a right lateral.

Normally at subduction zones one plate is sliding under the other.  This isn't a smooth process, though, and sometimes pieces get hung up on one another because there are a lot of rocks and edges involved.  The place where the rocks "snag" on one another is called an asperity.  Geomorphologists use the term stress to refer to the force applied on the rocks; the way the rocks react to the stress is called strain.  Think about when you apply force to two ends of a pencil (like you are going to break it).  The pencil bends a bit, but if you continue to apply force eventually you will exceed the pencil's shear strength and it will break.  You hear a pop and then the two broken ends rapidly snap away from each other.  The sound and the motion are both expressions of the energy that has been built up and is suddenly released.  An earthquake is something like this; when the stress at the asperity gets to be too much for the rocks they will break, and then the two sides of the fault quickly shift positions.  This is where the energy for the earthquake comes from.  At a subduction zone, if you think about it, the rapid adjustment happens in the downward direction, which strong up and down waves, and if there is water on top this will trigger a tsunami.

In this particular case, though, since the earthquake occurred at a strike-slip fault rather than a normal fault, the strong up-and-down movement didn't happen, and so there wasn't a major tsunami.  I tried to find a detailed diagram of all the fault traces around this particular part of the subduction zone, but unfortunately had no luck, so I can't describe the precise mechanism of the quake.  A number of geologists have also expressed surprise at the size of this quake, because strike-slip faults aren't normally this big (many are saying this is the largest strike-slip quake on record).  For example, the 1906 strike-slip quake that leveled San Francisco was "only" 7.6, and the 1989 quake that caused so much damage there was 6.9.  But as big as this quake was, the fact that it occurred 300 miles out in the ocean limited the damage on the land.  

As we got news updates the situation turned out not to be as critical as we expected.  Even though the 8.7 quake qualifies as a "great earthquake", there were no major reports of damages, and according to media reports most of the new evacuation procedures were implemented relatively smoothly.  So in the end everything turned out okay and everyone seemed to be doing fine.  If you want to help, though, we are currently experiencing a critical shortage of beer and tacos.  Any and all donations will be greatly appreciated, and I'll make sure they make it to those most in need.


(1)  Padang and Aceh are pretty far apart, and so it wouldn't be possible to have an earthquake near both of them.  Moreover, we are relatively close to Padang, and so I figured, given the length of the shaking, the big earthquake must have been farther away.

(2)  People often mistakenly use the term "epicenter" to refer to the "focus" of the earthquake.  The focus is where the quake actually originated; the epicenter is the place on the surface of the earth directly above the focus.  18 kilometers is pretty shallow, and the shallower the earthquake is the more damage it usually does.

References and For Further Reading

Keller, Edward, and Nicholas Pinter.  2002.  Active Tectonics: Earthquakes, Uplift, and Landscape.  Second Edition.  Upper Saddle River, NJ: Prentice Hall.  362pp.

This is an excellent intermediate-level text on tectonics and geomorpholigical processes.  I knew it would come in handy when I had it shipped over from home.


  1. Tectonic activity is slow and constant that means that these faults are inevitable. If the seismic activity is very low when these things occur, how does this change to cause earthquakes?

    1. Hi there. Tectonic activity is very very slow by our standards. But in geologic time it is pretty quick. And it isn't smooth...if you think about the huge plates moving against one another...they get "caught" on one another and pressure builds up until finally the strength of one of the plates is exceeded and something breaks and all the energy is released. So it is a build up of energy that is suddenly released.

  2. The diagrams and illustrations in this entry were helpful in understanding the differences between fault zones and plate boundaries in relation to how tidal waves are formed. I was always curious as to why the tsunamis that we have had here in Hawaii differed from those elsewhere. As a lifelong Hawaii resident, I have always been aware that an strong enough earthquake could start a tidal wave that could cause some real damage could occur in my lifetime. This brought about a couple questions though: Why are tsunamis portrayed as these huge mountainous waves (as many we have witnessed recently appear as a huge rush of a tide surge) and at what point is a wave classified as a tsunami wave?

    1. Hi Jade. I am not sure what you mean by the portrayal of tsunamis, unless you mean in movies. And if that is the case, they are probably shown as big mountains of water because that makes for good viewing. As for the classification, I think that any sort of abnormal wave triggered by an earthquake would be considered a tsunami. Even 6" waves that have made it to Hawaii have been described as tsunami waves. So if an earthquake happens and it makes a measurable variation in the tide, then it's a tsunami.

  3. To have experienced an earthquake in that area must have been an un-nerving experience. I can only imagine the anxiety that must have been going through everyone there. I would have expected a tsunami warning as well but, now after your explanation on plate boundaries and faults I can see why there wasn't. I guess I was under the impression that a fault line was the point at which two tectonic plates meet. I was wrong. However I don't think I'm all the way wrong. From what you described earlier a bunch of faults can make up a plate boundary. This was a very informative post and I thank you for the knowledge.

  4. Thanks for the information! I never distinguished well before the difference between faults and plate boundaries. In fact, I always thought they were the same. Having only experienced one earthquake before I can't say I particularly know the feeling well (especially since I was asleep..) but the experience you went through, seems far more dangerous and intimidating and I must say, you handled the situation pretty well. Would there ever be a possibility of a global scale of earthquakes at the same time? How disastrous would that be?

  5. It must have been a great experience during the earthquake. I find it interesting how there are so many different ways that earthquakes are created. It's odd how a strike-slip earthquake can be just as big as any other earthquake. I have a question, how do scientists identify an earthquake on land and under water?

    1. They use seismographs to figure out where the earthquake is located. By interpreting the signals on the seismogram (the record from the seismopraph) they can figure out how far away it is. So if you have three seismographs in three different locations, you can figure out where the earthquake is.

  6. So to clarify my understanding base on the reading of this blog plate boundaries are different to faults. But when a plate boundary occurs, it may also have faults in them? But when fault happens this does not necessary mean there is a plate boundary. Right? My first response to reading was wow that is a big number and I assumed it would do a lot of damage to the earth. But since it did not, what do these numbers really mean? And what exactly do they measure? Because it does not seem like they measure the severity of an earthquake or the damage they have.

    1. Yes that is basically how plates and faults work. Faults are generally parts of plate boundary systems, but you can have a fault far away from a plate boundary. The numbers refer to how much energy is released by the earthquake. Damage depends on a lot of factors, and the amount of energy is just one. This earthquake was a long way off shore, and so there wasn't much damage. So location is another key variable. The way the earth shifts (the kind of movement in the earthquake) is important too.

  7. Your incident reminded me of the earthquake that happened in 2011/3/11. I was in Japan experienced the similar thing. As you know Japan does have earthquake quiet frequent but most of them are small. But that incident in 2011 was different. I was walking on the road and I felt drastic shake and I couldn't even walk straight. All the bicycles were on the ground and I see neighbors coming out of the house to see what is going on. When I got back to my house it was disastrous because things were scattered all over the place. This was my first time experiencing the real huge earthquake. I was shell shocked when I saw the tsunami swallowing the entire city/cars/buildings on the News.
    Is there any kind of solutions to the earthquake? Can it even be avoided?

    1. Hi there. That earthquake that hit off the coast of Japan was huge. I remember when the tsunami hit Hawaii. I don't think that earthquakes can be avoided ever, but one thing I think I recall reading is that they actually knew that earthquake was going to occur a split second before it did because they have sensors near the fault. That really amazed me, and I think that's as good as it gets in terms of advanced warning. The best to deal with the earthquake hazard I think is preparedness for the event.

  8. I can see how having a knowledge base of geography helped immensely in this situation, being that you mentioned that you stopped worrying about a Tsunami when you heard that the quake happened at a "strike-slip fault." Funny I don’t seem to remember reports after earthquakes stating what kind of fault it generated from, I probably just wasn’t processing it since I didn’t know what it meant. Thanks for your clarification! Feeling that long of earthquake must have been freaky at first though, especially since you mentioned that strike-slip fault quakes aren’t usually that long!