9 January 2009

Magnitude Mw=6.1 earthquake in Costa Rica

Posted by Dave Petley

At 19:21 UTC (13:21 local time) on 8th January a Magnitude Mw=6.1 earthquake struck Costa Rica. Although this is not a particularly large earthquake, it is certainly substantial enough to have triggered many landslides, especially given that it occurred very close to the surface (the USGS depth estimate is just 4.5 km). Several reports have come in already about fatalities associated with landslides. Confirmed are the deaths of two girls near the city of Alajuela, NW of San Jose, whilst less well confirmed reports suggest that a family of five may also have perished in a landslide.

The USGS has provided a shake map:
This is an area of known high levels of seismic hazard:
This area is probably particularly prone to landslides as the soils consist of heavy weathered soils and those derived from volcanic products, both of which are potentially unstable when disturbed. I have come across two very interesting resources. First, a map of damage reports (in Spanish) is being put together, based on information from Twitter. This should be visible below:

Ver mapa más grande

Second, there is an interesting Youtube video of TV footage of the aftermath, much of it shot from a helicopter. The first part of the footage shows I think attempts to recover the remains of the two girls killed in a landslide, whilst the middle section shows landslides triggered in the epicentral region. Based upon this it might well be that further fatalities become apparent when daylight reaches this area. The video should be visible below, or can be viewed here.