By David Jacobson, Temblor
Updated: 6 a.m. Pacific time
At 11:49 p.m. local time, a M=8.0 earthquake struck offshore Mexico at a depth of 69 km according to the USGS (The European-Mediterranean Seismological Centre assigned it a depth of 61 km). In the six hours following the mainshock, there have been 28 M=4.3+ aftershocks, which have caused further panic in Mexico. This quake occurred approximately 80 km offshore, and triggered a tsunami warning for Mexico, Guatemala, Panama, El Salvador, Costa Rica, Nicaragua, Honduras and Ecuador. As of 6 a.m. Pacific time, the largest waves recorded were 2.3 feet.
This massive earthquake took place on the Middle America Trench, where the Cocos Plate subducts beneath the North American Plate at a rate of roughly 80 mm/yr. Because of the magnitude of this quake, it was felt as far away as Mexico City and Guatemala City, which combined, are home to over 10 million people. Based on the USGS ShakeMap, and regional populations, over 40 million people would have felt this earthquake at varying degrees of severity. According to the New York Times, panicked residents in Mexico City rushed into the streets as their buildings swayed. Even though USGS PAGER system forecasts that fatalities will be between 1,000 and 10,000, so far, there are only 15 confirmed. However, because this earthquake happened at night, this number is sure to rise. The USGS PAGER system also predicts that economic losses will likely total more than $1 billion. Initial reports suggest that there are several collapsed buildings close to the epicenter.
Despite the fact that this earthquake occurred near the Middle America Trench, a compressional environment, the initial (as of 11 p.m. Pacific Time) USGS focal mechanism suggests that this quake was due to extensional motion. Based on the depth of the quake (69 km) it struck below the subduction zone, meaning the extensional nature could be due to a changing dip angle in the subducting slab. However, it is also possible that in the coming hours, new data will arrive, and if it does, we will be sure to update this post.
Based on the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, this M=8.0 earthquake can be considered relatively surprising. This model uses global strain rates and the last 40 years of seismicity to forecast the likely earthquake magnitude in your lifetime anywhere on earth. From the figure below, one can see that in this area off the coast of Mexico, the likely magnitude in your lifetime is 7.25. This highlights the unpredictability of earthquakes, and that in areas susceptible to large quakes, people need to be aware of the risks and protect themselves in any ways possible.
New York Times