Seismic imaging of the Cook Inlet portion of the Aleutian-Alaska Subduction Zone
Background
This project takes us far from the fields and gentle topography of Wisconsin, to one of the most seismogenic locations on Earth - Alaska! Our setting is the Cook Inlet region of the Alaska-Aleutian subduction zone. This area experiences hazardous megathrust earthquakes, but also a subtler form of seismicity known as slow slip. In this region, slow slip happens recurrently for years at a time.
Location of study area, modified from Iwamoto et al. (2022). Red triangles are volcanoes of Aleutian Arc, black lines are approximate slab depth contours, and grey dots are tremor locations. Purple ellipses are approximate locations of recurring Slow Slip Events according to Rousset et al. (2019).
Slow slip is often thought of as paired with a phenomenon called tremor — repeated, low-frequency seismic signals that are distinct from earthquakes. But we also can find examples where slow slip happens without accompanying tremor. So what’s happening here? What controls these two processes? Both have been associated with increased fluid pressure in the subduction zone — fluid brought down by subduction, or released by metamorphic reactions in the subducting oceanic crust. Previous seismic imaging work, in Alaska and other subduction zones, indicates that seismic velocity is lower in this oceanic crust (sometimes called a Low-Velocity Layer, or LVL), which is consistent with increased fluid content. However, it’s not clear if slow slip and tremor originate due to the fluid pressure itself, or due to the metamorphic reactions. It’s also not clear if this LVL looks different between regions that do and do not undergo slip and/or tremor.
The Cook Inlet region of the Aleutian-Alaska subduction zone is a great natural laboratory for investigating the processes and properties that control slow slip and tremor — there are two recurring, long-lived Slow Slip Events (SSEs) that have been located in this region: one, north of Anchorage, has extensive tremor during slip (Wech 2016). The other, further south, has slip but no tremor has been observed thus far (my student Eryck is verifying this relationship using more recent data). Unfortunately, this interesting southern SSE region has not been heavily instrumented compared to other parts of the subduction zone. The goal of our project is to fill this data gap, and to use seismic imaging to evaluate how properties like fluid content and rock type in the LVL vary along the Aleutian-Alaska subduction zone in the Cook Inlet region.
Broadband locations for Alaska BADGER experiment (yellow triangles).
A “typical” station setup for the Alaska BADGER experiment. A broadband instrument deployed at station BA05, Anchor Point. The sensor is wrapped in insulation to reduce noise; the datalogger is in the yellow box next to it. Placement on concrete floors has coupling quality comparable to burial vaults in this region.
Our scientific goals are to use a variety of seismic imaging methods, including receiver functions and P-wave autocorrelations, to map the subjecting plate interface and oceanic Moho. Azimuthal variations in receiver functions can give us additional information about the geometry and rock type within the oceanic crust, which is crucial to evaluating whether the slow-slip regions are hydrated or eclogitized.
Broadband Data Collection
To this end, we have deployed the Alaska Broadband Accessory Deployment for GEophysical Research (Alaska BADGER). Other experiments in the area take their name from animals (SALMON, BEAAR, WVLF, to name a few), so it is fitting that the UW mascot, the badger, joins the zoo of data in south-central Alaska!
We have placed 11 broadband seismometers along the western edge of the Kenai Peninsula, giving us good coverage in the historical slow-slip region and the locked portion. We worked with local radio stations and schools to locate hosts for our instruments, targeting indoor site locations. Typically, experiments of this scale will bury sensors outside and power them using solar panels. However, we are trying to keep accessible sites that are indoors (on concrete or dirt floors) and can be plugged in. This saves on cost and minimizes risk of damage to instruments; it also means that we have more accessible sites in locations that are car-accessible and less risky for people who don’t feel safe in very rugged/remote locations. Inclusivity and accessibility in fieldwork is important to me, and I hope that this experiment can showcase ways to keep fieldwork open to all types of scientists!
In the field. Eryck Ochoa (Master’s student) and Eva performing a servicing check on station BA03, in Clam Gulch, AK.
A Novel Dataset
One of the most novel aspects of this project is that we are expanding the data we use beyond the traditional broadband data collection described above. We are also leveraging, with help from collaborators Brad Lipovsky and Marine Denolle at University of Washington, Distributed Acoustic Sensing (DAS) data from fiber optic cables from within the Cook Inlet! This offshore data will give us a chance to 1) supplement our existing data and expand sensitivity further down dip, 2) compare the sensitivity of DAS and broadband instruments to local and tele seismic events, and 3) develop further methods for combining traditional and cutting-edge instrumentation.
We’re excited to get started on this portion of the project. Check back here for future updates!