7.4 Magnitude Earthquake Shakes Guatemala: AIR

November 8, 2012

A 7.4 magnitude earthquake occurred on Nov. 7, about 15 miles off the coastal town of Champerico in southwestern Guatemala, according to catastrophe modeling firm AIR Worldwide. The epicenter was 35 miles SSW of Retalhuleu, Retalhuleu and 101 miles WSW of Guatemala City.

The event was felt as far away as Mexico City and El Salvador. The U.S. Pacific Tsunami Warning Center noted the possibility of a local tsunami within 100 or 200 miles of the epicenter, but no warning was issued and no tsunami has been reported.

Initial reports indicate collapsed buildings and landslides. There are reports of falling buildings crushing cars and landslides blocking roads. Buildings were evacuated in Guatemala City, Chiapas and in parts of Mexico City. There are widespread phone and power outages.

According to AIR, Central America, an isthmus that extends from Guatemala to Panama, is a highly active, complex seismic environment. The region is sandwiched between the large North American and South American plates, connected to the Caribbean plate on the east and bordered by the Cocos plate on the west.

Seismic activity in Central America is dominated by the subduction of the Cocos plate beneath the Caribbean plate along the Middle American Trench. In this region Western Guatemala is one of the more at-risk locations. The area within 250 km of Retalhuleu has experienced 50 earthquakes with M 6 or greater during the past 40 years, of which two were larger than M 7.

The region around Chimaltenango and Guatemala City experienced a catastrophic magnitude 7.6 earthquake in 1976. Buildings of all heights, construction types and occupancy types suffered at least partial damage and 23,000 people were killed. At the time of this event Guatemala had no national building code and buildings were not built to similar standards. In 1986, the country enacted a national building code which accounted for the construction of new buildings and the repair and retrofitting of existing construction. Revisions were made to this code in 1996.

According to AIR, the predominant construction type for insured residential and commercial buildings in Guatemala is masonry, with wood being the second most insured construction type. Within Guatemala however, there are significant differences in the types of building stock and building performance, largely related to local economic levels. Urban construction tends to employ newer, less vulnerable materials than buildings in rural environments. The coastal area closest to the epicenter of this earthquake is rural and sparsely populated. Many buildings in rural locations are older; they typically do not adhere to the country’s building code and are characterized by poorer seismic performance than modern buildings. Such structures, while vulnerable to structural damage from an event of today’s magnitude, are less likely to be insured.

According to the USGS, during this event, the peak ground acceleration near was violent enough to collapse these poorly constructed structures, many of which are built of unreinforced masonry in this region.

Despite the magnitude of today’s quake, damage may not be significant given the depth of the event, as well as the remote location in which it occurred and the sparse population it impacted. Properties near the epicenter may experience damage if they are constructed of unreinforced masonry, which is typical of uninsured properties in this region. The full damage picture from this event is still taking shape.

Source: AIR Worldwide

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