Oaxaca Earthquake

First Posting | Summary
Posting Date: March 20, 2012, 5:00:00 PM

An earthquake with a preliminary magnitude of 7.4, according to the U.S. Geological Survey (USGS), struck a sparsely populated region in the state of Guerrero, Mexico today, just after noon (local time). The earthquake struck close to the border between Guerrero and the southern state of Oaxaca. The damage picture from this event is still emerging, but so far, no major damage (nor injury) has been reported. Today’s earthquake was followed by an aftershock that caused buildings in Mexico City, about 200 miles to the north, to sway for roughly sixty seconds. This sent office workers and residents fleeing to the streets; some were evacuated. According to Mexico City Mayor, Marcelo Ebrard, the event did not cause damage in Mexico City, nor to vital city infrastructure like the water system. The President confirmed this assessment. Mobile phone networks were disrupted, however. Tourists and residents also felt the earthquake in the resort city of Acapulco, nearly 200 kilometers east of the earthquake’s epicenter. No major damage was reported there either, however. In the epicentral region, southwest of the town of Ometepec (Guerrero), 500 houses were damaged. The earthquake was felt most strongly of all in the neighboring state of Oaxaca. Even there, however, damage reports have been minimal so far; residents noted the quake felt strong but they didn’t see items fall to the ground from their shelves, for example. Telephone lines are down in parts of Oaxaca, however, and residents did observe the telephone lines shaking side-to-side. A local official also reported cracks in school buildings and damaged roofs in one part of the state. The tectonic setting of western Oaxaca is shaped by the subduction of the Cocos plate beneath the North American plate along a zone known as the Middle American trench, which repeatedly generates large magnitude earthquake events. Indeed, since 1973, the region within 200 kilometers of the epicenter of today’s event has experienced 20 seismic events exceeding M 6.0, and seven events exceeding M 7.0. In 2010 a M6.3 earthquake occurred 52 km southeast of today’s quake, and in 1982 a M7.2 event occurred about 7 kilometers from today’s temblor. Below is a picture of the USGS event footprint for this earthquake. It should be noted that the earthquake magnitude estimates provided by the USGS are indeed estimates and may change as additional information becomes available from seismic networks in the area. MMI map for today’s event; the strongest shaking estimates are MMI VII (Source: USGS) Exposure Today, the majority of residential buildings in Mexico are of masonry construction. Masonry buildings typically fall into one of three classifications: reinforced masonry, confined masonry, and unreinforced masonry. Unreinforced masonry buildings are one of the construction types most vulnerable to shake damage. Commercial buildings in Mexico are primarily of engineered masonry or concrete construction, and are better able to withstand ground motion. In large urban areas, such as Mexico City, most middle- to upper-class families live in five- to fifteen-story tall reinforced concrete commercial dwellings. These buildings are generally well designed and built with high quality materials. Due to the repetitive and destructive nature of earthquakes in Mexico, building codes in this country are among the most comprehensive in the world. However, there are no national codes. Instead, each of the more than 2,400 municipalities in Mexico enacts and enforces its own regulations. Among these municipalities, building code enforcement is often lacking; furthermore, designers and contractors often do not fully understand, nor apply, building regulations. Finally, local offices, particularly in small municipalities, typically are without a technical group to review structural design and inspect quality of construction, and thus, the seismic performance of buildings in Mexico is greatly influenced by local construction practices. Building damageability is exacerbated by poor workmanship and inadequate materials. It is in the very early aftermath of this event, and source parameters may change as reports continue to come in from seismic networks in Mexico and around the world. The AIR earthquake team is gathering the available data on this event and will provide additional information if warranted by this event.

First Posting | Downloads
Posting Date: March 20, 2012, 5:00:00 PM