Mexico’s M7.3 Earthquake: Why It Struck Here

[00:00:01]Hey there, crew geology professor Sean Wily here. Today is Friday, July 16th, and we're focusing on a significant earthquake that took place this morning off the coast of southern Mexico. This is a magnitude 7.3. Happened at about 8:48 a.m. local time. You can see the location here of the large earthquake, that 7.3 just off the coast of southern Mexico near the border with Guatemala.

[00:00:25]The depth of the earthquake was about 12 miles or 19 km. And luckily so far we we have no reports of any serious damage or fatalities. So that's the good news and we'll hope that continues. But again it's still quite early. Also there was no any no sort of uh any significant tsunami generated from this as well. So so far it seems to be a fairly substantial earthquake but luckily none of the um effects of that earthquake detrimental effects anyway have taken place. So let's focus on the science a little bit here with this earthquake.

[00:00:58]You can see the earthquake's location off the coast. You can see the number of aftershocks that have taken place uh since that earthquake began. It does look like this earthquake might have been triggered uh with a 4.74 shock. So there was there was a small forshock that occurred a little more than an hour before that quake in pretty close to the same area. So similar depth, similar location, possibly related there. And now you can see all the aftershocks that have shown up on the USGS site since that 7.3 began. Uh let's go ahead and look at some of the data associated with the earthquakes. First of all, just sort of sticking with the human component right now. Again, no significant damage or injuries that we know of right now. Uh but we do have 245 responses uh to the USGS uh did you feel it uh data site and um we look looks like we have sort of our maximum intensity there about maybe magnitude uh seven on the Merkelli intensity scale. So looking at pretty strong shaking from this earthquake but again uh and mostly along the coast but again doesn't seem to be any damage reported thus far but again it's quite early. Um other things to look at with this earthquake. Um if we look at the the pager uh so this is the automated system from the USGS. Basically it looks at the size of the earthquake, the depth of the earthquake, all the data associated with the earthquake and also looks at proximity to the populations.

[00:02:26]Um looks at some of the other variables that affect how damaging an earthquake can be. So you do see that there is you know a chance that there could be some fatalities here. Again, just based on the models, uh, looking at maybe a 38% chance of 10 to 100 fatalities. Let's hope it it doesn't go there. Uh, and then you can see the estimates of the economic loss as well from structural damage associated with the earthquake.

[00:02:48]So again, it takes the earthquake's location, looks at population centers.

[00:02:51]So over here on the right, we have a number of urban areas in the this coastal area of southern Mexico and Guatemala. looks at their population sizes, looks at the level of shaking taking place there, and then just estimates what the effects might be. So, again, it's not a hard and fast um you know, it's not absolutely correct data.

[00:03:10]It's just a way to quickly model and estimate the effects of the earthquake just in, you know, the the few minutes to hours after the earthquake when we're obviously still trying to figure out uh what's gone on on the ground and what the effects of the earthquake actually are. Uh so let's also look at the uh beach ball for this earthquake. So no surprise there. Of course, if you know anything about this portion of uh southern Mexico and Central America, this is a large plate boundary, a subduction zone which trends northwests southeast. And because this plate here, the Kokos plate, and I'll show you a better graphic here in a second, is being thrust under or shoved underneath this portion of Central America, uh that creates these large, what we call thrust faults, or sometimes these are also called reverse faults. And if we look at this uh beach ball here, that's a classic reverse fault. You can see the orientation of the fault there, right along that northwest southeast uh trend or strike that we talked about there.

[00:04:08]So, pretty much looks uh like what we would expect there. 27 degree dip.

[00:04:14]That's pretty, uh, typical for what we'd expect to see along that kind of subduction zone. So, um, let's look at why this occurred. So, what was the reason for this earthquake? And let's just start with the big picture here. If we look at just the the plate tectonic setting of this area. So, here we have southern Mexico going into uh, Central America. So, our earthquake is somewhere in here. You can see all the black circles on here are historic earthquakes. So, this area is no stranger to earthquakes.

[00:04:42]The red triangles are volcanoes. So there are some volcanoes inland of this subduction zone. But what we have here is the uh convergence of basically three major plates. The three plates that are playing a role in this area are the Kokos plate, this ocean plate that's being shoved beneath southern Mexico and Central America. Uh we have the North American plate which is up here and the quake today is pretty close to the interface between the North American and the Cocoos and then our third plate which is the Caribbean plates. We have the Caribbean plate moving to the west.

[00:05:14]We have the Cocoos plate moving to the the northeast if you will and then we have the the North American plate which is moving slower than these two but is moving a little bit to the west or southwest. And so it's the convergence of these three plates that has produced uh the earthquakes and the volcanoes and the topography and a lot of the geological features we see in this part of the world. Um let's flip over then to uh a different map uh USGS map. did pull up u this area and I looked at the last um you know I guess what 76 years of earthquake so from 1950 to today and picked earthquakes bigger than 6.5. So you can see the earthquake from today shown in orange but you can see all these other earthquakes here as well.

[00:06:01]What's interesting is um if you pick the big circles here you've got a 7.2 in 1993.

[00:06:07]You've got a another Whoops. another 7.3 in 1970 and then you have today's 7.3.

[00:06:15]So literally, you know, uh three earthquakes of similar magnitude uh in the same area again. First one 1970 and then 23 years later to 1993 and then a good uh what uh 33 years later uh for the earthquake we have today along with some smaller earthquakes here shown by the small circles. So you can see just the sort of at least somewhat recent historic seismicity along this area, but seems to be quite a lot of earthquakes right near the border area here between uh Mexico and Guatemala. There is a little bit of a gap over here in this section. We'll talk about that here in a second. Um so there's our USGS map kind of showing where things play out. And then the other map I have for you here uh that's hopefully helpful. This is a map showing um basically historic earthquakes and then the the blobs here are sort of the areas of rupture. So you can see all along this subduction zone plate boundary here off the coast of Mexico where we've seen significant earthquakes. It doesn't give us the magnitude here, but you can assume more or less that the larger the the ellipse or the circle or the blob, the the bigger the earthquake because it's rupturing a larger area. And I plotted up today's earthquake here uh down here near the bottom yellow corner in red.

[00:07:32]And if you check that out there, uh it's a little hard to read, but basically this earthquake today um is in a place that had a rupture 1903 and then that 1970 earthquake we talked about 1993 and then this is just in that same little zone there. So it seems to be an area of uh recurrence I suppose. I mean all of these are areas of recurrence to some degree. Uh but we do have this interesting gap here, the Taywan Tepic uh seismic gap. The way we look at these earthquake zones is we sort of think of these seismic gaps which are again this is a hypothesis. This isn't set in stone. But one way to analyze these these long uh fault areas and these are all broken up into segments. I think there's four or five segments that define this specific plate boundary. Um you can see they're all moving at slightly different rates. Uh the idea here is that there if they if they all accumulate stress at the same rate which is a huge assumption then as soon as you relieve stress along one segment of the plate boundary like we did today with this significant earthquake um that relieves stress here but it means that other areas which haven't had an earthquake in quite some time might be more loaded if you will um and they might be be most likely to produce the next earthquake. So, had you looked at this map yesterday, you might have said, "Well, if we have an earthquake anytime soon in southern Mexico, it's likely to be in this area. Uh, but lo and behold, the actual place we got the earthquake was here where there's been already three significant earthquakes in the last 100 plus years, which is interesting. So, it's not not a way to predict earthquakes or it's a way to sort of just try to understand them.

[00:09:10]What we think might be going on here is there's this this uh old fracture zone, the Taiwan ridge that comes in here and that might be somehow locking this segment of the the plate boundary and sort of somehow preventing these big earthquakes and these big ruptures from taking place. Again, I'm no expert, just uh kind of looked into this a little bit this morning. So there's also another gap over here called the Guerrero gap.

[00:09:36]Um, which is important because it it may affect, you know, Mexico City, which doesn't sit too far away. Um, okay. So that's kind of a look at uh seismic gap and just sort of the areas of rupture in the earthquakes we've had over the last 100 plus years. Uh, and then to end this thing here, I think the last thing I have is looking a little bit more closely at this area and just what's going on. So, like I said before, this is a um it's a triple junction or it's at least a triple junction region. So, it's a poorly understood area where the Caribbean plates boundary and the North American plates boundary and the Koko's plate boundary all sort of merge together. And again, it's it's pretty poorly understood. But today's earthquake sits pretty close to where those three plates uh bring come together. And if we come down here into this paper here, this is a little bit better map. Um, just showing the broad deformation in this area. So you can see I believe this is the Gulf of Honduras up here. Here's Bise. This is southern Mexico. This is Guatemala. And again, today's earthquake was, you know, somewhere right here. There's the trench that defines the subduction zone, the plate boundary. Today's earthquake somewhere in here. But you can see some of these faults they've mapped out here.

[00:10:47]Um, and this conflicting motion. And you can see some of the topography, some some of the linear features here which definitely are influenced by those that tectonic boundary there. But seems to be you know a poorly understood area in terms of exactly how those three plates kind of meet here. Uh some triple junctions out in the ocean are very somewhat simple um and we can see them quite clearly. Uh these types of plate boundaries are a little bit more murky.

[00:11:14]It's more of a diffuse zone rather than like a a pinpoint spot where those three plates are coming together. Uh but for now, just wanted to provide a little bit of analysis and commentary on today's earthquake 7.3. It'll be interesting to see what other data comes to light. Uh thankfully, this one was so far at least much less destructive than the one we saw in Venezuela a few weeks ago. So hopefully this was helpful. Thanks for your support of the channel and we'll see you next time, friends. Take care.