|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.
- 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.
- 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.
- 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
- Normal faults, where the two sides are moving apart
- Reverse (or thrust) faults, where the two sides are coming together
- Strike-slip (or lateral) faults, where the two sides are moving past one another.
|Diagram from here.|
|Diagram from here. Site is worth a look.|
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.