M6.1 Oaxaca Earthquake
9/25/2017 2:30:00 PM
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First Posting 9/25/2017 2:30:00 PM 
Posting Date: 9/25/2017 2:30:00 PM

An M6.1 earthquake struck the state of Oaxaca, Mexico, about 18.6 km (11.6) miles south-southeast of the town of Matías Romero (est. pop. 21,064) at 7:53 a.m. local time (12:53 UTC) on September 23, 2017. This is the third earthquake to impact Mexico in three weeks. The USGS placed the epicenter at 16.74°N, 94.95°W, in the same area hardest hit by the Chiapas quake earlier this month, at a shallow depth of 9 km. In addition to the town of Matías Romero, nearest population centers include the community of San Jerónimo in the city of Ixtepec (24.1 km northeast, est. pop. 23,150), the town of Unión Hidalgo (31.8 km north-northwest, est. pop. 12,736), and the city and municipality of Oaxaca de Juárez (192.8 km east, est. pop. 262,566). Shaking was reported as far west as Mexico City.

m61 matias romero eq newsalert_shakemap_20170923.jpgShakeMap for the M6.1 earthquake that struck near Matías Romero, Oaxaca, Mexico, on September 23, 2017. (Source: USGS)

The M8.1 quake on September 7 struck off the coast of the Mexican state of Chiapas close to midnight local time; it killed nearly 100 people and damaged thousands of buildings in the states of Oaxaca and Chiapas. On September 19, an M7.1 earthquake caused damage in Mexico City. Recovery and rescue efforts for both of these events are still underway.

Seismotectonic Background

Mexico sits on the western edge of the North American plate, which is being underthrust by the eastward-moving oceanic Cocos plate. The subduction of the Cocos plate is occurring at a rate of ~6 to 7 cm/year and is the source of many large earthquakes along the western coast of Mexico. While subduction earthquakes are quite common in Mexico, there are also crustal faults along which relatively large earthquakes can also occur.

m61 matias romero eq newsalert_plate map_20170925.png

Map of tectonics in the region affected by the M6.1 earthquake on September 23, 2017. (Source: USGS)

In general, aftershock determination is rather difficult; the proximity of this quake to the September 7 event strongly suggests that it is an aftershock caused by the overriding North American plate reacting to the stresses released in the subduction zone farther south earlier this month. The location of this M6.1 earthquake coincides with the northernmost extent of the M8.1 rupture. It is likely that the September 7 rupture along the subduction interface transmitted stresses to faults within the crustal layer. Review of the Coulomb stress change following the M8.1 event reveals that the location of the M6.1 earthquake is within a region of increased Coulomb stress, suggesting that the September 23 event was more likely to occur because of the M8.1 quake on September 7 and may be an aftershock.

The USGS has identified the M6.1 quake on September 23 as an aftershock from the M8.1 temblor that struck Chiapas, Mexico, earlier this month, making this the largest aftershock to date. Thousands of aftershocks have been recorded, most of which have had magnitudes between 4.0 and 5.0.

Exposure at Risk

Structures left standing by the M8.1 Chiapas earthquake can be more vulnerable to shaking from the M6.1 quake, so smaller jolts can cause damaged buildings to fail. The majority of residential buildings in Mexico are of masonry construction, falling 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 and are likely to be found in the more rural parts of Oaxaca, along with more informal construction, such as adobe. Commercial buildings in Mexico are primarily of engineered masonry or concrete construction, and are better able to withstand ground motion.

In large urban areas, most middle- to upper-class families live in multi-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.

Reported Damage

Buildings swayed and seismic alarms were set off in Oaxaca de Juárez, the capital of the state of Oaxaca. The Oaxacan government reported that some homes collapsed, and residents reported that homes left standing by the M8.1 quake on September 7 had fallen. Homes in the city of Juchitán, Oaxaca, damaged by the M8.1 event showed new cracks and collapsed in some cases. In the town of Asunción Ixtaltepec in the Juchitán District, where about 80% of homes have been uninhabitable since the September 7 Chiapas quake, some homes showed new fissures or collapsed. A bridge already damaged by the M8.1 event collapsed, according to photos posted by the Mexican federal police, and three hotels and two churches were damaged. A military base that has been central to recovery efforts for Oaxaca since the M8.1 earthquake reported damage, and a hospital in the Oaxacan port city of Salina Cruz was evacuated. The Federal Electrical Commission reported power outages to 327,000 homes and businesses in Oaxaca; service was restored to 72% of customers within a few hours.

At least four deaths have been reported—two in Oaxaca and two in Mexico City, where seismic alarms sounded and buildings swayed. Civil defense officials temporarily suspended rescue efforts in Mexico City in buildings collapsed by the M7.1 quake on September 19, under which people are still trapped. Mexico City Mayor Miguel Angel Mancera said there were no reports of new significant damage there.

M6.1 Oaxaca Earthquake
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