M 7.1 EARTHQUAKE 5KM ENE OF RABOSO, MEXICO EXACT LOCATION: 18.584 N 98.399 W DEPTH: 51.0KM SEPTEMBER 19, 2017 @ 1:14 LOCAL TIME Photo: Eduardo Verdugo / AP
Photo: Alfredo Estrella/ Agence France-Presse/ Getty Images
THE 2017 RABOSO (CENTRAL MEXICO), MEXICO EARTHQUAKE This earthquake was a magnitude 7.1 event at a depth of 50 km that occurred at 1:14 pm local time on 19 September 2017, 5km from Raboso, Mexico and approximately 120 km from Mexico City. At time of this writing, more than 270 deaths (including 137 in Mexico City, 43 in the State of Puebla and 73 in Morelos) and over 1,900 injuries have been confirmed, however, there are still many people unaccounted and search-and-rescue operations continue. The USGS model estimates an economic loss of $US100M to $1000M for this type of event. At least 25 people including 19 schoolchildren lost their lives when the three-story Enrique Rebsámen school building in Mexico City collapsed. A large number of buildings have collapsed or sustained significant damage; this included a number of hospitals that had to be evacuated. Over half of the fatalities occurred in the Greater Mexico City, population 21.3 million, where at least 44 buildings collapsed. The M 7.1 earthquake occurred close to the subduction of the Cocos plate beneath Central America at the Middle America Trench about 300 km away. The location, depth and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event within the Cocos slab. In the past 100 years, 19 other M 6.5+ earthquakes have occurred within 250 km of the hypocenter of this earthquake, including the 1957 M 7.6 earthquake that resulted in over 100 fatalities. Mexico is one of the most earthquake-prone countries in the world and has experienced a number of large earthquakes; see Figure 1. The 1985 M 8.0 earthquake generated more than 5,000 fatalities and hundreds of collapsed buildings. Figure 1. Earthquakes in Mexico, last 100 years
The main shock of this earthquake is classified as having an intensity (MMI) of VII, which is considered very strong shaking; see Figure 2. Figure 2. Intensity map A number of cities with large populations were exposed to shaking intensity of MMI VI-VII; see Table 1. City Population MMI Ayutla 7,000 VII Izucar de Matamoros 43,000 VII Jiutepec 162,000 VII Puebla 1,590,000 VII Mexico City 12,290,000 VII Table 1. Population exposure comparison for the 2010 and 2011 events
This event had a preliminary peak ground acceleration (PGA) of 0.4-0.5g near the epicenter; see Figure 3. In Mexico, a large array of strong motion stations are employed to record earthquake data. At station 27 (59 km away), PGA was 0.19g; At station 21 (87 km away), PGA was 0.28g. Figure 3. Preliminary PGA data Figure 4. Shaking intensity for the earthquake At Mexico City, PGA was closer to 0.2g and the shaking was strong but not severe; see Figure 4. The large number of fatalities and damage here was greater than anticipated and can be attributed to several factors: A large portion of Mexico City is situated on an ancient sea bed and the underlying soil is very soft. This causes the amplification of ground motion to the built infrastructure. So although there are no faults in the city, events from far-away earthquakes are amplified and can generate large forces. This phenomenon was observed during the 1985 earthquake and was again evident with this earthquake. After the 1985 earthquake, the building codes were modified and include good ductile detailing. However, compliance with the code is not always enforced, especially in outlying areas, and, in many cases, residents make alterations and additions to the buildings that can after alter these structures earthquake resistance. Damage assessment data from this earthquake is yet not available. However, preliminary data indicates that a large number of buildings with significant damage used either unreinforced masonry (URM) bearing walls or reinforced concrete construction of older vintage. These building types (especially with a soft/weak story or torsion) are most susceptible to earthquake damage and collapse. This event had a long duration for strong motion, resulting in buildings being subjected to a large number of shaking cycles.
REPORTED DAMAGE In Mexico City, more than 40 buildings have either collapsed or suffered significant damage; see Figure 5. This included the three-story Enrique Rebsámen school (Figure 6), two multi-story apartments, multi-residential buildings, a supermarket and a factory. There was also significant damage in Puebla (city), population 1.5 million.
Figure 5. Arial view of collapsed buildings Figure 6. Collapsed Enrique Rebsámen school building in Mexico City Figure 7 and Figure 8 show examples of observed building damage. As shown in Figure 9 through Figure 11, damage was not limited to building framing, older buildings, or buildings alone.
Cracking of CMU wall building Out-of plane failure of URM bearing wall building Commercial store Multi-story building Figure 7. Examples of building damage
Figure 8. Damaged Jojulta Municipal Palace Figure 9. Nonstructural damage Figure 11. Collapsed bridge span Figure 10. Exterior damage to a modern mid-rise building Shortly after the earthquake, an engineering team from Miyamoto International was in contact with local authorities and people on the ground to assess the situation and provide assistance. The findings of the team will be provided in the future communiques. Members of the Miyamoto team are heading to Mexico as of this writing to assist clients and government entities, as requested.
Photo: Eduardo Verdugo / Associated Press
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