Another large quake strikes Mexico
At 5:39 p.m. local time, a M=7.2 earthquake struck Mexico’s Oaxaca region. According to the USGS, the quake occurred at a depth of 25 km, and caused very strong shaking near the epicenter. Fortunately, this quake was in a sparsely-populated region of Mexico, though Mexico City’s 21 million people would have felt light shaking. Based on the depth and focal mechanism (shallowly-dipping thrust), this earthquake likely occurred on the subduction zone where the Cocos plate slides beneath the North American plate at the Middle America Trench, at a rate of approximately 70 mm/yr.
Even though this earthquake was not in a densely-populated area, the USGS PAGER system estimates that there will be fatalities and moderate economic losses. Mexican authorities are yet to report any major damage though. While no damage was sustained in Mexico City, 225 miles from the epicenter, according to the New York Times, buildings in the city swayed for more than 2 minutes. As the quake struck, the city’s earthquake alarm sounded approximately one minute before shaking started, allowing people to move to safer areas, highlighting the success of the city’s earthquake early warning system. Two people that took advantage of this were Monica and Evan Freedman, Ross Stein’s daughter and son-in-law, who were in the city for a vacation weekend.
A first-hand experience: As told by Monica and Evan Freedman
We had literally just entered the Airbnb where we were staying for the weekend when I heard what sounded like a loud police siren. Since we lived in the mission in SF where there’s a lot of noise, I had initially not paid any attention. It was only our host, who immediately stopped in his tracks, looked at me and said “SISMO!” I asked him how much time we had, he said about a minute.
We ran down to the street and saw that literally everyone in Mexico City does the same thing, which is leave their buildings and head to the street. When we were there, we saw all ages from toddlers to the elderly waiting for the shaking to start. Ironically the first time I’ve ever felt serious shaking was this sismo. Trees and buildings swayed, and cars moved in a wave and luckily didn’t hit each other.
But the thing that hit me was that the September earthquake deeply affected the city. Nearly everyone said, “not another one like September!” After about 30 seconds of strong shaking, everything felt normal. After another minute the warning siren stopped. About 30 minutes later we felt an aftershock.
Fresh, painful memories
This M=7.2 earthquake comes less than five months after a deadly M=7.1 quake struck Puebla, causing building to collapse in Mexico City, killing over 200 people in the city. Therefore, today’s quake brings back fresh, painful memories. Additionally, a month before that, a M=8.1 earthquake occurred off the coast of Chiapas. So, Mexico has experienced three M=7+ earthquakes in the span of six months.
Based on the location of this earthquake, it should not be considered surprising. In the figure below, the Global Earthquake Activity Rate (GEAR) model is shown. 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. In the location of today’s event, a M=7.25+ is likely, which is exactly what today’s event was. Because an earthquake of this magnitude is not surprising, highlights how seismically at-risk Mexico is. This is a fact known all too well to the millions of residents impacted over the last six months.
Are the three M≥7 quakes related?
At this point, we can say that all three had more than about a half a percent (0.5%) chance of occurring per year independently at each of their locations. So, none can be considered very rare. Second, all were far enough apart from each other what is called the ‘static stress’ imparted by one to the other was negligible. Third, they are nevertheless close enough to each other to be strongly influenced by the dynamic stresses imparted by the seismic waves within the first tens of minutes after rupture. But, none ruptured immediately, so how could these dynamic stresses have changed the conditions for failure on faults for weeks (Chiapas M=8.2 to the Puebla M=7.2) to six months (Chiapas M=8.2 to the Oaxaca M=7.2)? There is one intermediate process that could, however, link at least the Chiapas M=8.2 to the Oaxaca M=7.2 quakes. The Earth’s crust behaves like a slab of rubber over honey–or Silly Putty. The Silly Putty flows after an earthquake because the overlay rubber slab has been deformed. This flow changes the stress on faults, acting over a longer distance than the purely elastic slab, and over 6 months these additional stresses could add up. So, the riddle is unsolved. It could open up a new understanding of how quakes interact over very long distances, or it could be a blind alley.
New York Times