A better understanding of seismic hazard in Tehuantepec, Mexico, using amphibious MT studies

Steven Constable, Scripps,
Allen Husker, Caltech,
Carlos Mortera, UNAM,
Diego Ruiz Aguilar, CICESE

The subduction zone offshore southwest Mexico is divided into two very different geological regions by the Gulf of Tehuantepec. North of the gulf, the Cocos tectonic plate is shallow as it subducts beneath the North American plate, earthquakes are less frequent, and the ocean trench is close to shore. At Tehuantepec the coastline bends to form the gulf, and south of the gulf the plate is steeper, earthquakes are more frequent, and the trench is far offshore. Located in between, the Gulf of Tehuantepec is considered a ``seismic gap'', in that there have been no recorded large thrust earthquakes associated with the tectonic plates slipping past each other. There are two possible reasons for this. The first is that the rocks of the Tehuantepec Ridge, which is being subducted with the Cocos plate, may be releasing water that lubricates the fault zone, allowing the plates to slowly slide past each other without producing large earthquakes. The second is that the topography of the ridge is mechanically locking the two plates together, in which case a very large earthquake is inevitable when the stress finally builds up to breaking point. In 2017 a large normal earthquake broke across the down-going Cocos plate, a type of earthquake not normally seen in subduction zones and which could increase the stress and the chances of another large earthquake. Do the people of Mexico plan for a strong, damaging earthquake that will likely create a large tsunami, or rest assured that the plate is slipping benignly? This study plans to answer this question by making geophysical measurements (magnetotelluric soundings) both onshore and offshore. MT measurements are sensitive to the presence of water deep in the subduction zone.

This proposal was funded in July 2021 and the land data collection was finished in 2023. The plan for the marine data was to piggy-back a cruise on the Mexican research vessel El Puma that was scheduled to collect bathymetry data in the Gulf of Tehuantepec, but it has taken 5 efforts to sail on this project. The first effort to sail in May 2022 was shut down by Covid. Then the ship's multi-beam failed and took a year to fix. Then the cruise was delayed with the hope that construction of new port facilities for the vessel would be finished. This clearly wasn't going to happend on time, so the cruise was scheduled for early this year, only to be delayed again because paperwork needed to add the MT to the cruise wasn't in place. Finally, we are about to embark on the R/V El Puma to carry out the marine data collection for this project, with a sailing date of the 28th of May out of Mazatlan, Sinaloa Mexico.

Funded by the National Science Foundation – OCE 2105776

Here's an attempt to explain to a non-scientist what marine EM is all about (4.7 Mb pdf)

Steve's blog (latest entries at the bottom):

Monday May 26.

The equipment sitting in a warehouse ready to be taken for loading on the ship is truly a sight for sore eyes. Getting the gear cleared through customs has been a nail-biting roller coaster. First they found our work clothes and PPE and declared them contraband (textiles are a prohibited import in this part of Mexico) and thus were "abandoned". Then three stacks of concrete anchors got lost (about a cubic meter and over a tonne each). Then all the tie-down straps and some other stuff was deemed to be not on the manifest (it never is). But we managed to import the straps by paying duty, and the anchors were found, and today we were able to go to the warehouse to make sure everything is there. And it is. (Well, except our clothes.)

Tuesday May 27.

Today's job is hurry up and wait. We can't load our equipment onto the El Puma until tomorrow at 7 pm, so nothing to do except shop for some snack food to take on the ship and search for tonight's restaurant. So far we have done very well with restaurants. On Sunday we went to Engracia which was awesome, and last night we went to Totem Cafe, which had a lovely roof-top patio and the best shrimp tacos I have ever eaten.

Meanwhile, you might be amused by my "then and now" photographs. On the left is a shot I took at the end of the East Pacific Rise cruise in 2004, on the waterfront in Mazatlan. Kerry Key, Chet Weiss, and Graham Heinson can be seen among others. On the right is part of the current science team, Luis Gerardo Alvarado Izarraras, Jake Perez, Eliana Vargas Huitzil, Yasmin Hernandez Aburto, and Diego Ruiz Aguilar.

Wednesday May 28.

The latest news is that the first tropical storm of the 2025 hurricane season is expected to form between Mazatlan and Tehuantepec in the next few days. It will be called Alvin. The plan now is to wait while the storm develops and load Friday night, pushing off then or Saturday morning.

This gives us more time to explore the dining opportunities in Mazatlan. To provide a change from beer and Margaritas on Tuesday night we chased down Hector's Bistro, which has a wine list (and damned good risotto - Jake had pork and saffron and I had duck and mushroom). Lunch was at a place called Fonda Fina Cocina de Tulio. Its speciality is crispy bean tacos with amazing stuff piled on top. We have already decided we will return tomorrow.

Windy.com's prediction for Friday noontime:

Thursday May 29.

Killing time in Mazatlan waiting for something to happen ...

Friday May 30.

Finally! This evening we successfully loaded all the equipment on the vessel. Everything went as planned. We have pretty well filled the ship, but there is enough space to make things work. But, we began to realize that we will not be sailing tonight or tomorrow as planned. Stay tuned...

Saturday May 31.

It became clear this morning that the ship was not sailing any time soon. Both Jake and I have long established plans for summer vacations, but more to the point there are 2 more tropical storms developing after Alvin. So we have run out of time and good weather. We are preparing the instruments to be taken off the ship, with a plan to put them into storage until the cruise can be re-scheduled during a better weather season. Sixth time lucky as they say. Meanwhile we pushed off to an anchorage just outside the port, since dock space is a limited resource.

Sunday June 1.

Hanging out at anchorage, checking emails and playing with data from a previous cruise. The highlight of the day was that the crew cleaned up the deck, re-painted a table, and brought out the BBQ for a Sunday dinner of carne asada and pollo asada, quesadillas, etc.

Monday June 2.

Still at anchorage. The good news is that Diego was able to organize trucking and storage of the equipment, with a plan to offload first thing tomorrow morning. So, we headed back to dock late afternoon, this time in the fishing boat district. A LOT of fishing boats, some so big and sophisticated that the boats have heli-pads. The fish don't stand a chance...

On the way back to dock we were accompanied by a tender (boat). This hints at the real reason for the delays. Rumor is that we are down to one generator, and if that fails we loose steerage. Presumably the tender was there to make sure that wouldn't lead to another "ship hits bridge" event.

Tuesday June 3.

Our truck and forklift driver showed up on time and we had all the equipment off the boat and into the truck in about an hour. We are good at that sort of thing - we have done it before. One last lunch on the ship and then got a ride into town and went back to the hotel where we started this trip. We were able to change our flights a second time to fly out tomorrow (third time actually - I haven't talked about the saga of getting to Mexico, which took a couple of days).

Adios, El Puma.