Hola! Welcome to the first installation of the Nicoya-MT blog, posted by me, Anna Redanz everyone’s favorite research assistant in the Electromagnetic Geophysics Lab (EGL) lab at Georgia Tech (there is only one, so I win by de-fault). This first post will serve as an introduction to the objectives of the project that will be running for the next month on the Nicoya Peninsula in Costa Rica.
The highlighted light blue line off the coast is a subduction zone where two tectonic plates are colliding; this one is the Middle America Trench! The denser of the two plates, the Cocos Plate, is pulled under the Caribbean Plate which creates subduction. Our study area is the Nicoya Peninsula, denoted by the pin. At this location, the distance to the trench is notably shorter than is typical, as indicated by the distances shown from El Salvador(pink line) and Guatemala(orange line). This makes this region peculiar as the land is located above the shallower portion of the subduction zone.
Subduction zones, being a plate boundary, allow for massive amounts of stress buildup to occur at the plate interface. This interface forms a large fault zone, known as the megathrust, which is where the Cocos and Caribbean plates are in contact with one another. At a typical subduction zone, the buildup of stress is eventually released as large, often destructive earthquakes. The section of the subduction zone that encompasses the Nicoya Peninsula experiences such earthquakes, the most recent being a magnitude M7.6 event in 2012. However, what makes this region particularly interesting is that it also experiences “silent” earthquakes in the form of slow slip events (SSEs), where the stress release occurs over longer periods of time lasting days to weeks. These SSEs are undetectable to those living in the area since they do not cause ground shaking like normal earthquakes, but scientists can detect them using GPS stations that track the movement of the plates. Why do some portions of the Nicoya Peninsula experience SSEs while others have destructive earthquakes? Scientists have proposed several hypotheses, but we don’t yet know the exact reason why. This is one of the questions we aim to answer through the data we collect on the Nicoya peninsula.
How exactly do we plan to study something located far beneath the ground? For this, we will be using an electromagnetic geophysics method called Magnetotellurics (magneto-tell-er-ics), or MT for short. The MT method records very small changes in the Earth’s magnetic field and the (very) small electric currents flowing in the ground that are generated by the changing magnetic field. By recording how well electric current travels through the subsurface, we can get measurements of how conductive or resistive the underground structures located far beneath our feet are. Being able to see how the conductivity changes can help us create a picture of what is happening at the subduction zone and what might be present.
The oceanic plate (that has been underwater for millions of years and is wet or ‘hydrated’) is being pulled down into the hot mantle, meaning that this water-rich crust is now entering the subsurface. We know water is very conductive (it carries electric current easily) and the MT method we are using to “see” into the earth is sensitive to changes in conductivity, this means by using MT we can image the movement of fluids in and around the subduction zone. We want to know how these fluids affect the mechanics of stress build-up at the subduction zone, which lead to the creation of earthquakes and may have an influence on the slow slip events that I mentioned earlier.
In order to investigate our hypothesis, we have to go out and collect MT data in our study area. This will be done over the next month as we travel around the Nicoya peninsula to dig a bunch of holes and trenches and deploy as many MT receivers as time allows. I’ll be updating y’all as our journey progresses!