My Master of Science degree is in geological sciences from UC Santa Barbara. I mostly focused on geophysics, applying my undergraduate physics background and programming experience to do several seismology projects. In addition, I was fortunate enough to go on an interdisciplinary research cruise to the Galapagos midocean ridge and hotspot.
Seismic structure under the Los Angeles Basin using ocean-generated seismic noise
My master’s thesis centered around using data from the California Integrated Seismic Network to determine seismic structure under Los Angeles. CISN is a network of seismic stations all around California, and I was able to use information about the travel time of seismic energy between stations to reconstruct the wave speed in different areas – a technique known as seismic tomography (the same computational technique used in CAT scans and the like). Seismic tomography for very deep regions of the earth is the only way we know about interior structure – for example, that the core is made up of liquid and solid parts. In the shallow crust, understanding the speed at which seismic energy moves through the earth helps to predict the severity of shaking at the surface. The Los Angeles basin is full of sediment, which has slow wave speeds and tends to amplify shaking. The kind of energy I was using to determine this structure was ocean-generated seismic noise – known as “microseisms” – in the frequency range 0.05 to 0.3 Hz. Microseisms originate offshore where ocean waves interact with the continental shelf, setting up vibrations that can be measured well inland. Understanding wave propagation in LA is key due to the high amount of seismic activity from thrust faults (where the two sides are pushed together, one over the other – this is caused by the motion of the Pacific and North American plates and the local shape of the plate boundary) and the dense human population which is affected by it.
Lunar crustal structure from thermal expansion-generated seismic noise
Using the same methods as with ocean-generated seismic noise (the travel time between stations are measured using a signal processing technique called “cross-correlation”), my advisor, Toshiro Tanimoto, my fellow graduate student Tomoko Yano, and I used lunar seismic data from the Apollo missions to determine seismic structure in the lunar crust. In this case, though the moon has no oceans to cause seismic noise, the temperature extremes between day and night on the moon set up enough expansion due to heating that the rock cracks. These cracking episodes create enough seismic noise for our technique to work. This work was published in the Journal of Geophysical Research-Planets, and referenced by planners for the InSight mission to Mars, which landed successfully on November 26, 2018.
Probably the most exciting experience from my time at UCSB was joining a large group of principal investigators on a 6-week research cruise at the Galapagos midocean ridge and hotspot on the R/V (research vessel) Thomas G Thompson. Though the Galapagos islands are famous for their wildlife, the islands themselves are a product of a geological hotspot, thought to be supplied from deep in the earth’s mantle by a plume of magma rising and impinging on the crust just below the islands. Hawaii and Iceland are two other examples of hotspots. What is particularly interesting about the Galapagos hotspot is that it is not exactly in the middle of a midocean spreading ridge (like Iceland), nor is it in the middle of a tectonic plate (like Hawaii). Instead, it is near the Galapagos midocean ridge. Midocean ridges are the places where new oceanic crust is made, with magma extruding out onto the seafloor and supplying heat and heavy minerals such as sulfur to the deep ocean. In some places, seawater percolates down into cracks and becomes super-heated and saturated with these minerals, then shoots back up into the cold ocean above. Many forms of life have adapted to use this energy, including tube worms and crabs. Our goal on this cruise was to determine if the hotspot provided more heat and caused more “black smoker” hydrothermal vents, or if the additional magma supply would thicken the crust and prevent seawater from getting down to the heat source, effectively shutting off the vents.
We did bathymetric surveys of the seafloor and chemical samples of the ocean water along the midocean ridge to look for likely smokers, and eventually put a camera system down near the seafloor to search for vents. We did find several vent sites, with their associated unusual ecosystems. I was one of several graduate and undergraduate students who helped with record keeping in shifts while we conducted surveys, and I also worked with the bathymetric data. After having crossed the equator, I am also now a Noble Shellback.