The last two decades have seen 88 earthquakes of magnitude 5.0 or greater within 300 kilometers of where Tuesday’s temblor struck. That fact alone gives the region a long, hard memory of shaking ground. Tuesday’s event, a magnitude 5.3 at a depth of just 10 kilometers, is a shallow quake by any seismologist’s definition.
Anything less than 70 kilometers deep is considered shallow. At 10 kilometers, this one concentrated its energy close to the surface.
Shallow quakes rattle harder. That is the simple physics of it. The same magnitude event at 30 or 50 kilometers depth would have spread its force across more rock before reaching houses and roads.
A 10-kilometer depth means the rupture is practically underneath people’s feet. The USGS reported no tsunami warning.
At least one person has so far reported feeling the shaking. That number will likely rise. Compare this to the strongest event in the area’s recent history.
A magnitude 6.7 earthquake struck in 2015. That is more than 20 times the energy release of a 5.3, based on the logarithmic scale seismologists use. The 2015 quake was a big one.
Tuesday’s was moderate. Moderate still means minor to moderate damage near the epicenter.
The USGS recorded Tuesday’s quake at 21:01 UTC, 84 kilometers south-southwest of El Progreso, Mexico. Aftershocks are not a maybe here. They are a when.
Seismologists note that secondary shaking may continue for days or even weeks. The crust adjusts after a main shock.
Stress releases in fits and starts. People in the affected area are generally advised to be prepared for further shaking. That advice comes from experience.
The region has seen enough magnitude 5 events to know the pattern. The 88 quakes of magnitude 5.0 or greater in the past 20 years include a range of depths and locations. Some were deeper, some shallower.
Some hit closer to populated areas. Some did not.
Each one tells a piece of the story. A shallow 5.3 is not a rare event here. It is part of the background noise of living on active tectonic margins.
But rarity is not the point. Impact is.
And impact depends heavily on depth. A shallow earthquake at 10 kilometers depth can rattle buildings and infrastructure significantly more than a deeper quake of the same magnitude. That is the key difference.
The energy does not have to travel far. It arrives fast and strong. For people in the area, the difference between a 10-kilometer quake and a 40-kilometer quake is the difference between dishes rattling and dishes falling.
Between cracks in drywall and cracks in foundations. The USGS detected the event through its real-time global seismic monitoring network.
That network is always watching. It logged the location, the depth, the magnitude. It also logged the history.
The 88 previous events are in the database. The 6.7 from 2015 is in the database.
The data says this area shakes often. It says a magnitude 5.3 is not unusual. It also says shallow quakes demand attention.
For now, the advice stands. Be ready for aftershocks. They may come in hours or days or weeks.
The crust does not settle quickly. The 2015 quake had its own sequence of aftershocks.
The pattern repeats. Tuesday’s event is another entry in that long record.
