Tuesday, May 26, 2015
6:38 pm
There are four general categories of earthquakes – tectonic, volcanic, explosion, and collapse. Tectonic earthquakes are associated with plates, volcanic earthquakes are associated with magma moving up from the mantle to the surface and causing earthquakes as it does so., explosion earthquakes are formed by, well, explosions, and collapse earthquakes are formed when something (say, an underground cave) collapses and makes a small earthquake The massive landslide that took off the north face of Mt. St. Helens was triggered by a volcanic earthquake The world had a lot explosion earthquakes during the 50s and 60s when atomic bombs were being tested, especially those underground. I’ll stick to tectonic earthquakes here, since they are the largest and most destructive ones.
In the previous blog post, I talked about the three plate boundaries: convergent (and the three subtypes: oceanic-oceanic, continental-oceanic, and continental-continental), divergent, and transform. Not all tectonic earthquakes occur on fault zones, but the vast majority do. As the pictures below show, most of the earthquakes are centered across convergent, divergent, or transform plate boundaries. These “interplate” earthquakes contrast with “intraplate” earthquakes, which occur in the interior of plates and are relatively rare. The concentrated zone of earthquakes in East Africa is due to the East African Rift, which is a divergent boundary but is not illustrated in the diagram of the world’s plates below. Additionally, a significant number of earthquakes are centered on “hot spots,” which are locations of volcanic activity not affiliated with plate tectonics (Hawaii and Yellowstone are two examples of hot spots in the United States).
These plate boundaries are manifested as faults, which are discontinuities or fractures in a volume of rock due to different plate movements that stretch, shear, or compress that rock. Not every single fault is a plate boundary, but all plate boundaries are faults, and it is these faults that are responsible for earthquakes. There are three main types of faults: normal, reverse, and strike-slip, and they each create different types of earthquakes. As you will see, these fault structures are very similar to the plate boundary structures I talked about in my previous post.
Normal Fault:
Normal faults occur when two masses are being pulled apart and one slides down the other. This is analogous to the divergent plate boundaries that form mid-ocean ridges. Earthquakes with these types of faults can be frequent, but they are weaker than the earthquakes caused by reverse or strike-slip faults because the diverging motion prevents large amounts of stress from building up.
Reverse Fault:
Reverse faults occur when two masses converge and one subducts under the other. These are analogous to the oceanic-oceanic, continental-oceanic, and continental-continental plate boundaries talked about in the last post, but remember, not all faults are plate boundaries. Extremely long reverse faults at subduction zones between plates can cause the largest earthquakes in the world due to the enormous stress that builds up as one plate is “thrust” under the other. Earthquakes occur when this stress is released. The largest of these earthquakes are called megathrust earthquakes. The one in Alaska in 1964 was a megathrust earthquake, and two more recent events were the 2004 Indian Ocean earthquake and tsunami and the 2011 Japan earthquake and tsunami. These megathrust earthquakes often cause tsunamis because subduction zones are generally located underwater, and these tsunamis can impact areas thousands of miles away from the epicenter of the earthquake. The marinas in Crescent City, California were heavily damaged from the Japan tsunami due to local bathymetry enhancing the effects of the tsunami. Most of the West Coast of the U.S. escaped unscathed though.
Strike-Slip Fault:
Strike slip faults are faults where masses of rock slide laterally past each other. The transform fault talked about in the previous blog is a special type of strike-slip fault in that the fault separates two continents. Fast moving strike-slip faults can cause frequent earthquakes, and the earthquakes that do occur are often very shallow and can therefore be extremely intense. The great San Francisco Earthquake of 1906 not only had a moment magnitude of 7.8 but occurred at a depth of 5 miles, and as a result, 80% of San Francisco was destroyed, with much of the damage due to fires throughout the city initiated by the earthquake that roared on for several days. This earthquake occurred along the most famous transform/strike-slip fault in the world, the San Andreas Fault.
In case you were sleeping too easily at night, Seattle lies directly under a reverse fault called the “Seattle Fault.” This fault last shook approximately 1,100 years ago and delivered an earthquake having a moment magnitude of at least 7. While the Cascadia Subduction Zone off our coast can deliver much larger earthquakes (magnitude 9 or greater), the fact that this fault is right under our city makes it a far more dangerous fault for us.
Everybody always talks about “the big one” waiting for us off the coast, but “the big one” is actually right below our feet!
Thanks for reading!
Charlie
