Bucaramanga Earthquake
Monday, 18. February 2008, 20:00:56
Maps can be treacherous, and interpretations should be handled with care.
The first map shows that the epicentre of the earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 occurred very close to a transform fault. The depth was however an estimated 150 km. At transform faults, that is faults where two continents slide past each other, like at the San Andreas Fault, you can only get shallow earthquakes. The earth is too hot below the asthenosphere for there to be earthquakes. Temperature increases as you go deeper into the earth. Most earthquakes stop happening at depths of about 20 km because the rock is too hot. The asthenosphere begins at about 100 km into the earth.
The map also shows a subduction zone - far to the west in the Pacific. In a subduction zone, a cold piece of crust is being pulled underneath other crust. This piece of crust is relatively cold, so that earthquakes can occur much deeper.
The second map shows historic seismicity in the area. I would like to point to three distinct features:
The Bucaramanga earthquake nest is situated at a depth of about 160 km beneath Colombia at 6.8° N, 73° W, and it produces about eight earthquakes of M 4.7 or more each year from a source region having dimensions of about 10 km or less. The nesting of earthquakes beneath the Bucaramanga area may be due to a collision between two subducting slabs, a northern slab with a dip angle of about 25° and a southern slab with a 50° dip angle, while the dip in the Bucaramanga nest is about 29° (according to this abstract). The Bucaramanga earthquake nest is of course also discussed in other paper as well.
The earthquake earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 was NOT a transform fault earthquake, BUT a subduction earthquake.
All maps should be treated with care - two maps are often better than one.
A final note is that the shallow earthquakes are the dangerous ones. Deep earthquakes don’t give rise to much damage.
• http://news.trendaz.com/index.shtml?show=news&newsid=1137573&lang=EN
PS of 19 Feb. 2008: I ought to have made a "depth" diagram illustrating the principle. Well, here it comes. (not to scale and not in accordance with actual relief)
The first map shows that the epicentre of the earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 occurred very close to a transform fault. The depth was however an estimated 150 km. At transform faults, that is faults where two continents slide past each other, like at the San Andreas Fault, you can only get shallow earthquakes. The earth is too hot below the asthenosphere for there to be earthquakes. Temperature increases as you go deeper into the earth. Most earthquakes stop happening at depths of about 20 km because the rock is too hot. The asthenosphere begins at about 100 km into the earth.The map also shows a subduction zone - far to the west in the Pacific. In a subduction zone, a cold piece of crust is being pulled underneath other crust. This piece of crust is relatively cold, so that earthquakes can occur much deeper.
The second map shows historic seismicity in the area. I would like to point to three distinct features:- A lot of shallow earthquakes show up along the transform fault.
- From west to east other earthquakes occurred deeper and deeper (following the Benioff zone - the seismic zone on top of the subducting slab).
- A small, but very active zone of intermediate-deep seismicity (blue dots) catch the eye near Bucaramanga. That is the so-called Bucaramanga earthquake nest.
The Bucaramanga earthquake nest is situated at a depth of about 160 km beneath Colombia at 6.8° N, 73° W, and it produces about eight earthquakes of M 4.7 or more each year from a source region having dimensions of about 10 km or less. The nesting of earthquakes beneath the Bucaramanga area may be due to a collision between two subducting slabs, a northern slab with a dip angle of about 25° and a southern slab with a 50° dip angle, while the dip in the Bucaramanga nest is about 29° (according to this abstract). The Bucaramanga earthquake nest is of course also discussed in other paper as well.
The earthquake earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 was NOT a transform fault earthquake, BUT a subduction earthquake.
All maps should be treated with care - two maps are often better than one.
A final note is that the shallow earthquakes are the dangerous ones. Deep earthquakes don’t give rise to much damage.
• http://news.trendaz.com/index.shtml?show=news&newsid=1137573&lang=EN
PS of 19 Feb. 2008: I ought to have made a "depth" diagram illustrating the principle. Well, here it comes. (not to scale and not in accordance with actual relief)
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