Earthquake location:
Shaking intensity:
Exposed population (PAGER):
At this point (c. 8:00 UT) the USGS is estimating that there are about 68,000 people living in areas that have suffered an earthquake intensity of VIII (severe shaking) or above, mostly in the town of L'Aquila. This is an area with a mix of old and new buildings built in a hilly area, as this image (from here) shows:
Although the earthquake is not huge (USGS estimates are Mw=6.3), the shallow depth (10 km) and fairly vulnerable buildings means that the impact could be quite substantial, albeit in a fairly limited area. Italy is well-prepared for earthquake response, which will help.So, what of landslides? Well, an earthquake of this size should be capable of triggering a fair number of slides. A good starting point is the Keefer (1984) relationship between earthquake magnitude and area affected by landslides:
This gives an area affected by landslides as about 2000 km2 (give or take quite a lot, though). The area is certainly landslide prone, as this Google Earth perspective view shows (I have marked the epicentre location as per the USGS. The town in the foreground is L'Aquila):
Incidentally, the mountain in the background is Gran Sasso, which houses an important particle physics laboratory in a deep tunnel. I would be interested to know how the experiments have fared during the earthquake.
Hi, Daves. I'm Paolo Cortopassi from Italy. I think you have been very fast to write about earthquake in L'Aquila, before many italian's web.I find interesting the Keefer relationship (1984), but i have two questions:which are the limitations for this representation? Which kind of areas are put inside the relationship? Thank you. Paolo Cortopassi
ReplyDeleteDave and Paolo.
ReplyDeleteThe Keefer plot is interesting. It seems to me however, that the boundary that encapsulates the data points represents a maxima ... that is, to the left of the line are conditions where landslides would be unlikely to occur. For example, one would not expect landslides 10 km from epicenter if the magnitude is only 6.0, but you MIGHT start to see landslides at magnitudes of 7.0 and higher, depending on the geographic and geological conditions.
The first point to note is that the data on Keefer's graph (it comes from his classic 1984 paper) are from real earthquake events - i.e. what you see is data from actual earthquake events. Second, note that the y-axis scale is logarithmic, so if you take any particular earthquake magnitude you see a scatter of points over an order of magnitude or so. The line that Keefer drew is indeed the apparent upper boundary (as he noted in his paper). The scatter below this represents the effects of multiple processes. I am not sure that geology is the most important actually as I think that the landscape is adapted to the strength of the rocks (i.e. harder rocks will, all things being equal, form steeper slopes). Geology is important, but the key factors for me are the depth of the earthquake, the configuration of the fault(s) that ruptured and the groundwater state at the time of the earthquake. Geology controls the type of failure for sure as well as the distribution of landslides to a certain extent.
ReplyDeleteFinally, it should also be noted that Rodriguez et al (1999) repeated Keefer's work using earthquakes for the period 1980-1997. They concluded that "Generally the results are very similar, though the presence of extreme outliers in some of the correlations emphasises the need to be aware of special cases, particularly those involving quick clay landslides." However, the graph between landslide area and earthquake magnitude was slightly amended - in fact they found that the upper boundary of the area of landsliding for any given magnitude was a little to the left / higher than Keefer found (i.e. they found landslide areas greater than Keefer's upper boundary line indicated). They also noted a greater scatter below the upper boundary as well, but this would be expected given that they included earthquakes in coastal areas.
In southwest British Columbia, Canada, where I live, it took many years to recognize that we are living above a subduction zone where very large earthquake with return periods of several hundred years are possible. This has led to a continuing effort to not only upgrade building codes, but also to retrofit old buildings.
ReplyDeleteWe now must design for an earthquake with a 2% in 50 year probability of exceedence. Initially, some asked "how can we afford to build and retrofit to such a standard?" Now when we hear of sad events such as the earthquake at L'Aquila, we ask ourselves "How can we afford not to?"
Hi Dave. Frank said on april 11, that in Canada (i think) they design for an earthquake with a 2% in 50 year probability of exceedence. The new italian laws of 2003 (i have said about in my answer), decided to design for an earthquake with a 10% in 50 year probabily of exceedence (Tr 475 year). What do you think about this? I don't know how this problem is organized in USA and Japan. Can you help me? Thank you Paolo.
ReplyDelete