Cabled Axial Seamount
The Axial Seamount is formed by the intersection of the Juan de Fuca mid-ocean ridge and the Cobb-Eickelberg hotspot. As part of the Ocean Observatories Initiative, this active submarine volcano is monitored in real-time with a range of instruments that includes seismometers, hydrophones, bottom pressure sensors, and tilt meters. The volcano erupted in April-May 2015. Using high-precision earthquake locations, we imaged an outward-dipping caldera ring fault that accommodated both inflation before the eruption and deflation during the eruption (Wilcock et al., 2016).
We are currently working on producing an openly-available near-real-time double-difference earthquake catalog to monitor seismicity at this volcano. We are also using ambient noise-based methods to measure temporal velocity changes associated with the eruption in 2015. Preliminary observations of decrease in seismic velocity and increase in both seismicity rate and tilt magnitude suggest there might be a shallow, localized magma recharge event starting ~6 weeks before the eruption.
Seismic Monitoring at 9º50’N East Pacific Rise
From October 2003 to January 2007, up to 12 ocean bottom seismometers (OBSs) were deployed and recovered annually in a 4-by-4 km area. This time period encompasses an eruption in January 2006. This is the first eruption at a mid-ocean ridge that was recorded by in situ OBSs, and the lava flow buried and damaged several instruments.
We detected and located various types of seismic events e.g. earthquakes, long-period events, and lava-related events, and used their spatiotemporal evolution to characterize the eruption. We showed that the eruption happened over the course of a week, mostly in the first 48 hours. This is contrary to previous geochemical theories that the eruption happened in multiple pulses over ~6 months. We further inferred that a significant portion of the seafloor-spreading was controlled by the buildup of tectonic stress to a critical level rather than magma overpressure in the multiple underlying magma lenses (Tan et al., 2016).
In addition, we found that the timings of the ~100,000 microearthquakes located correlate with tides. This suggests that some of the microearthquakes were triggered by tidal stresses. We examined how the microearthquakes’ response to semidiurnal tides changes temporally to probe how the stress state at the mid-ocean ridge evolved over an eruption cycle. We find that tidal triggering was strong pre-eruption but disappeared immediately post-eruption. Since the tidal triggering signal did not vary systematically in the 2+ years leading up to the eruption, we conclude that tidal triggering signal may not be a useful short-term tool to forecast mid-ocean ridge eruptions (Tan et al., 2018).
Tidal Triggering of Earthquakes at the Alaska-Aleutian Subduction Zone
Significant tidal triggering of earthquakes has been observed in the years to decades precursory to large subduction zone earthquakes (Tanaka 2002, 2010, 2012). We worked on using tidal triggering of earthquakes to probe the stress state of the Alaska-Aleutian plate boundary. This could improve forecasting of future subduction zone earthquakes in this region.