Yet another strong earthquake has occurred in Indonesia (as I mentioned in my previous post
) - in the same area as the boxing day quake (this time fortunately seemingly without any casualties!). This area is in many ways worth noticing. When we look at the earthquake location map we may notice a series of islands between the Sumatra coast and the Sunda Trench, here marked as a purple line indicating a subduction zone.
These islands are the top parts of an accretionary wedge, here made visible because they pop up above the sea level. Normally you won’t see anything of such an accretionary wedge, because it it hidden by the sea. They are sediments scraped off the top of the downgoing oceanic crustal plate (in this case the Australian Plate) during its subduction and appended to the edge of the continental plate (in this case the Sunda Plate).
Colour coding: Orange = volcanic arc and Asian continent. Light blue = shelf sea (Sunda/Eurasian plate). Purple = accretionary wedge. Green = islands within the accretion wedge area. Dark blue = Indian Ocean. Brown = oceanic crust of Australian plate. Yellow = oceanic lithosphere of Australian plate.
I have used the same colour code is my extremely simplified location map.
As the diagram of the accretionary wedge it is not to scale and out of any proportions.
The coldness of the subducting plate permits brittle failure, and thereby earthquakes, down to as much as about 700 km along the so called Benioff zone Earthquakes at depth between 500 and 800 km are marked as red dots on USGS earthquake maps). The Benioff Zone is defined as the active seismic zone in a subduction zone. Water escaping from the descending plate is probably the primary cause of volcanic activity at subduction zones, but whatever the reason melting of rocks take place and magma rises to form a magmatic or volcanic arc. Volcanoes at subduction zones can be extremely explosive like Krakatau. I have also marked the famous and in the past extremely destructive volcanoes Toba and Tambora in the volcanic arc formed by the subduction of the Australian plate beneath the Sunda Plate.
As the subducting plate slides beneath the upper plate, stress begins to build where the plates meet and the upper plate can deform to create a large structure called a forearc basin. With time this basin, a sort of a bowl-shaped depression, fills with sediment. It appears that the most severe subduction zone earthquakes occur in areas where such sediment-filled basins are found (like the earthquake that triggered the Boxing Day Tsunami in 2004).
Finally a couple of very instructive images from USGS:
First a historic seismicity map from Sumatra. (Red line = subduction zone) The star shows the 8.4 earthquake of 14 September 2007.
And then a cross section A-A'.
Here we can follow the Benioff zone as marked by historic earthquakes. Todays earthquake is in the “yellow zone” - at a depth between 35 and 70 km.
Obviously it is well worth keeping an eye on the extremely destructive forces at play at the Sunda Trench (also known as the Java Trench). The area is well studied, and it is an area where we can learn a lot about how subduction zones work. (Sorry if I have made it look too simple).PS:
According to a post at the Dongeng Geology
blog (in Indonesian) there is still a “seismic gap” to be filled off the coast of Sumatra - with an earthquake above the magnitude of 8.
I wish that either
a) I understood Indonesian
b) Machine translation worked better than they do (having worked as a translator for 30 years, I know why they don't!)
But for those who do understand Indonesian (or Malay may probably do) do read this interesting post.