UC Berkeley

Understanding of Earth’s evolution has been hindered, in part, by the technical challenges associated with placing seismic instruments on the seafloor. As technology improves, more arrays of ocean bottom seismometers are being deployed around the ocean basins. Perhaps the most ambitious such array, the Cascadia Initiative, covered the entirety of a small oceanic plate, the Juan de Fuca plate, with ocean bottom seismometers in a four-year experiment. That array is the primary motivation for this study.

The two models presented in this work, CASC16-P and CASC19-S, are teleseismic tomographic studies. They are each the first of their kind to use offshore and onshore data to simultaneously image the P- and S-wave seismic velocity structure of the mantle beneath the Juan de Fuca and North American plates. These models have provided insight into the tectonics of oceanic plates: how they are created, how they might interact with the mantle over the course of their life, how they might be influenced by an overriding continental plate, how they behave in the mantle after subduction, and how subduction could cease.

This work focuses on the on- and offshore regions of the Pacific Northwest of the United States—in particular, on the interaction between the Juan de Fuca plate, the North American plate, and the upper mantle. But it also demonstrates the value in oceanic arrays. The offshore instruments used in this study have allowed the identification of new features offshore, and better resolution of features near the coastlines. Ocean bottom seismology represents a rapidly growing new frontier that has the potential to investigate earth evolution in detail that was not previously possible. The work we present here has both helped illuminate the tectonics of the Pacific Northwest, and helped generate detailed hypotheses about the nature of global tectonics that will be tested elsewhere.