Subduction zones are convergent plate boundaries that act as one of the main driving forces of plate tectonics. They host a spectrum of slip events from earthquakes to slow-slip events. Subduction zones also show strong along-strike variations in plate locking and earthquake rupture. But what material properties of the plate- boundary differentiate the slip modes and cause the along-strike variations is unclear and it remains challenging to image subduction plate boundary faults down to seismogenic depth.

Mendocino Triple Junction is located at the southern end of the Cascadia subduction zone and is one of the most seismic active area in North America, making it an ideal site to image subduction plate boundary structures using high frequency seismic waves from local earthquakes.

We develop workflows to identify P-to-S and S-to-P converted phases and guided waves scattered from subducted slab interface from local earthquakes and use these phases to image the subduction plate boundary structure of southernmost Cascadia. We observe strong velocity heterogeneity of the plate boundary fault in both along-dip and along-strike directions and tentative correlations between plate locking factors with converted phases amplitudes and existance of guided waves. We plan further develop these methods to improve the imaging resolution of subduction plate-boundary fault at depth.

Oceanic transform faults (OTFs) play a key role in plate tectonics. They accommodate relative motion between adjacent oceanic plates through both seismic and aseismic slips. More than 50% of fault slip is inferred to be aseismic along global OTFs from teleseismic studies.  However, how different slip modes are distributed and interact at OTFs remain elusive due to limited near-field observations.

Quebrada-Discovery-Gofar (QDG) transform faults are three oceanic transform fault systems located along East Pacific Rise at ~4ºS. Despite their adjacent locations and similar slipping rates, they exhibite distinct seismic behaviors. Magnitude (M) 5-6 earthquakes occur quasi-periodically on several segments of Discovery and Gofar transform faults with a period of 5-6 years, while Quebrada rarely host M5 earthquakes.  

We develop a workflow using multiple machine-learning and open-source software to detect and locate microearthquakes recorded by ocean bottom seismometers along QDG to identify different fault slip modes.  For instance, earthquake distribution indicates that the G3 segment of Gofar is distinctly segmented into five zones with different responses to the M6 mainshock and the December seismic swarm in 2008, indicating various slip behaviors and strong interactions between different zones. Collaborating with colleagues applying other geophysical analysis methods for QDG, we will gain a more comprehensive view of the seismogenic properties of OTFs.