Puerto Madera Earthquake
7/7/2014 4:30:00 PM
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First Posting 7/7/2014 4:30:00 PM  
Posting Date: 7/7/2014 4:30:00 PM

M6.9 Earthquake Strikes Southern Mexico

At about 6:30 am local time today, a powerful M6.9 earthquake struck near the border of Mexico and Guatemala near the town of Puerto Madero, Mexico, at a depth of 37.3 miles (60 km). At least 30 houses have been damaged or destroyed and 50 houses were evacuated in the region of San Marcos, Guatemala. Two deaths and several injuries have been reported. Landslides, power outages, and disruptions to communications have also been reported in the area.

The quake was felt strongly in the Mexican states of Chiapas and Tabasco. Authorities in Chiapas said there were no immediate reports of major damage. In Mexico City, buildings and light posts swayed, but no damage has yet been reported. The quake was also felt in El Salvador. The U.S. Pacific Warning Center issued a statement indicating that there was no tsunami threat. It is still in the early aftermath of this event, however, and additional reports of damage may well come in.

Regional Seismicity

The Southern Mexico-Guatemala region is a tectonically active plate boundary region, where the Cocos plate subducts northeastward underneath the North American Plate and the Caribbean Plate along the Middle America Trench. The 73-80 mm/yr plate convergence in this region results in thrust earthquakes concentrated along the slab of the Cocos plate, among them large magnitude “megathrust” earthquakes, which are capable of producing tsunamis.

The M6.9 earthquake today, based on early USGS reports, is a moderately deep event (focal depth at 37 miles (60 km)) which occurred along the downgoing slab. Subduction-related earthquakes in this region occur down to about 62 miles (100 km). The nature of motion based on the focal mechanism data shows normal faulting, most likely related to in-slab normal faults and/or bending-unbending processes that occur along the moderately deep part of the Cocos slab.

This particular event occurred about 590 miles (950 km) northwest of a recent M7.6 Nicoya thrust earthquake in Costa Rica in September 2012, and about 310 miles (500 km) southeast of the March 2012 M7.4 Oaxaca, Mexico thrust earthquake. The destructive M8 September 1985 Earthquake, which severely damaged Mexico City and resulted in several thousands of casualties, occurred in that same region. The subduction process in this region is also responsible for volcanic activity along the long line of stratovolcanoes in Guatemala parallel to the Middle America trench.

The USGS ShakeMap below provides information about the intensity of shaking expected to have been felt in the epicentral region. It should be noted that the earthquake magnitude estimates provided by the USGS may change as additional information becomes available from seismic networks in the area.

puerto_madera.jpg USGS ShakeMap (Source: USGS)


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. Building codes in Mexico are among the most comprehensive in the world, but there are no national codes (each of the more than 2,400 municipalities in Mexico enacts and enforces its own regulations), and code enforcement can be weak and designers and contractors often do not fully apply building regulations.

The predominant construction type for insured residential and commercial buildings in Guatemala is masonry, with wood being the second most predominant 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.

Based on the limited amount of reported damage thus far and low earthquake insurance penetration in the region, AIR does not expect significant insured losses from this event. The AIR earthquake team will continue to monitor the situation and provide additional information as warranted.



Puerto Madera Earthquake
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