Strong M7.2 Earthquake Shakes Acapulco, Mexico City: AIR Analysis

According to catastrophe modeling firm AIR Worldwide, on the morning of April 18, 2014, a M7.2 earthquake struck southwestern Mexico, 37 km [23 miles] north of the municipality of Tecpán de Galeana (approximate population 53,000).

The epicenter is located 273 km [170 miles] southwest of Mexico City and lies between the resort cities of Acapulco and Zihuatanejo, where many residents and tourists are staying during the run-up to Easter.

According to the United States Geological Survey (USGS), the earthquake occurred at 09:27:26 AM local time on April 18 (14:27:26 UTC) and struck at a depth of 24.0 kilometers [15 miles]. The U.S. Pacific Warning Center has issued a statement indicating that there is no tsunami threat.

AIR’s report noted that as of “11:41 AM EDT, at least one building in Mexico City was damaged and there are several reports of broken windows. However, it is still in the early aftermath of this event and additional reports of damage may well come in. Nearer the epicenter, several residents have reported damage to masonry walls on older buildings, and to some contents inside apartment buildings.

“Power outages are widespread and internet access is down in some areas. Strong shaking was felt for at least 30 seconds in Mexico City, which was less crowded than usual due to the holiday; however, high rise buildings were evacuated. The earthquake was felt across a large region, affecting at least six states.

Dr. Claire Pontbriand, scientist, seismology at AIR Worldwide, said: “Mexico, which lies along the boundaries of three tectonic plates, is very seismically active. Along the Middle American Trench, located along Mexico’s southern Pacific coast, the oceanic Cocos Plate subducts underneath the continental edge of the North American plate.

“Subduction zones can generate large earthquakes when the subducting plate locks with the overlying plate and builds stress. The depth, location, and thrust-faulting source mechanism of today’s M7.2 earthquake suggest that it may be related to this subduction zone. Metamorphic changes in the downward-traveling slab also produce volcanic activity along the Cordilla Neovolcánica.”

Dr. Arash Nasseri, senior research engineer at AIR Worldwide, noted: “Mexico City is quite vulnerable to ground shaking even from distant earthquake events due to the local soil conditions, which includes soft and muddy drained lake beds. In 1985, an M8.1 earthquake located 400 miles from Mexico City destroyed many of its buildings and killed over 9,500 people.”

According to AIR, “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.”

Dr. Nasseri added: “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, and some designers and contractors often do not fully apply building regulations. In some small municipalities structural design and construction quality is not well inspected. Thus the seismic performance of buildings is greatly influenced by local construction practice. In areas where construction supervision is not proper, building damageability is exacerbated by quality of workmanship and material.”

Source: AIR Worldwide