Major M7.3 Earthquake Strikes Mexico; Geologist Analysis

[00:00:00]At 8:48 am local time yesterday, a major magnitude  7.3 earthquake struck offshore of Mexico. Centered 80 kilometers west-southwest of the city of  Tapachula, seismic waves radiated outwards in all directions, causing back and forth ground  motion that in some spots on land was at least as strong as this simulation. This earthquake lasted  for 24.2 seconds, and was felt by tens of millions of people as far away as a part of Belize and  even by some people in Mexico City. Since this represented the 6th largest earthquake in terms of  magnitude to strike so far in 2026, you might have expected it to cause some localized heavy damage  and at least a few dozen fatalities. And yet, there were thankfully no fatalities despite my and  the U.S. Geological Survey model's expectations, and while some minor and moderate damage did  occur, the worst actual buildings seemed to have experienced in terms of damage was only minor. So,  in my opinion, there was more than 1 million but less than 10 million U.S. dollars in damage.  The initial model in my opinion seems to have overestimated the intensity of shaking by around  1 MMI intensity in some areas, which might be the reason for this disconnect. But, why did this July  17th 2026 earthquake occur in the first place?

[00:01:29]Simple. The fact that there is a tectonic plate  collision ongoing offshore of southeastern Mexico.

[00:01:36]There, the north-northeast moving Cocos Plate is  colliding with and subducting underneath the North American or Caribbean Plate depending on your  interpretation of the region. The reason I say this is that the boundary between the North  American and Caribbean Plates is sort of a jumbled mess; much of it does not seem to occur on  a singular fault, as much like Iceland's Reykjanes Peninsula, it seemingly occurs across a zone of  many faults that are up to interpretation. Thus, here are two possible interpretations I have  showing the possible boundary of these tectonic plates. There isn't exactly a specific fault line  at the spots I have labeled with a white line, as these white lines denote sort of a midway point  within a broad region where the motion of the tectonic plate changes. This isn't too important  for understanding why yesterday's earthquake happened, but I nevertheless thought it was  interesting. Regardless of which interpretation is correct, over lengthy periods of time, generally  decades to centuries, the pull of the downward moving subducting slab can create compressional  stresses on the overlying plate, also causing it to deform downwards. Eventually, this downwards  motion reaches a critical point, creating a major earthquake which causes the overlying  plate to all of a sudden snap back upwards, in what is known as a megathrust earthquake. Now,  I want to note that some scientific literature only considers an earthquake at a convergent plate  boundary with a subduction zone to be a megathrust if its magnitude was greater than or equal to 7.5  and occurred on the right area. Yet, the mechanism is the same for smaller events, just with a lesser  motion and more limited extent of what does slip.

[00:03:28]The reason why no tall tsunami was generated  is that the fault plane that slipped either did not reach the bottom of the ocean floor at all, or  moved there at such a tiny amount that the tsunami that was generated did not seem to achieve heights  of any more than 16 inches of 41 centimeters.

[00:03:47]In the time since the major earthquake,  an aftershock large enough to represent essentially the statistically most likely largest  aftershock has already struck. Specifically, a magnitude 6.0 at 9:20 am local time on July  17th 2026. I say this because the largest aftershock following an earthquake is on average  about 1.2 magnitudes lower than the mainshock.

[00:04:14]This is not always true, hence it being a mere  average, sometimes the largest aftershock can be notably lower or higher than this.  And, the occurrence of that aftershock does not prevent more similar or less likely,  larger magnitude aftershocks from occurring, I just thought it was an interesting fact.  Aftershocks should decrease in abundance as time since the magnitude 7.3 earthquake increases. As a final note, I would like to thank this channel's patrons on Patreon and channel  members on YouTube for supporting my work!