The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up-dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self-calibrating pressure recorder (ASCPR). A piston-gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey-style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long-term, sensor-independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co-located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation.

We report on a feasibility study for an offshore instrument network in the Cascadia subduction zone to improve earthquake and tsunami early warning. The global DART buoy network provides effective warning for far-field tsunamis but near-field tsunami warning is challenging because the lead time is short and near-source observations are rarely available to directly measure the sea surface disturbance and evolution. Near-field tsunami warnings presently rely on rapid point source seismic inversions that do not estimate tsunami wave height. Efforts are underway to incorporate GNSS data into rapid source inversions that would support an initial near-field tsunami prediction. Offshore observations would contribute further to near-field tsunami warnings by providing: first, direct observations of seafloor and sea surface displacements during earthquake rupture and second, ongoing measurements for continued forecast refinement. Offshore instruments could also detect tsunamis triggered by submarine landslides and by so-called “tsunami” or “slow” or “silent” earthquakes that can generate unexpectedly large tsunamis but are characterized by shaking intensity so low as to be undetected or ignored. Pressure observations in the source zone will be challenging to interpret because they are dominated by seafloor accelerations and hydroacoustic waves rather than changes in hydrostatic pressure. In an effective system, pressure observations may need to be complemented by other observations such as inertial measurements of seafloor displacement, GNSS buoys and high-frequency coastal radar. It may also be important to place pressure sensors just seaward of the source zone to measure the developing tsunami in a region with an undisturbed seafloor. We will discuss alternative design options for an offshore instrument network in Cascadia, the research and development that must to be completed to determine the best approach, and the role of offshore observations in a holistic plan for tsunami mitigation.

Slow slip events were discovered in many subduction zones during the last two decades thanks to recordings of the displacement of Earth’s surface by dense GNSS networks. Slow slip can last from a few days to several years and have a relatively short recurrence time (months to years), compared to the recurrence time of regular earthquakes (up to several hundreds of years), allowing scientists to observe and study many complete event cycles. In many places, tectonic tremor is also observed in relation to slow slip and can be used as a proxy to study slow slip events of moderate magnitude where surface deformation is hidden in GNSS noise. However, in subduction zones where no clear relationship between tremor and slow slip occurrence is observed, these methods cannot be applied, and we need other methods to be able to better detect and quantify slow slip. Wavelets methods such as the Discrete Wavelet Transform (DWT) and the Maximal Overlap Discrete Wavelet Transform (MODWT) are mathematical tools for analyzing time series simultaneously in the time and the frequency domain by observing how weighted averages of a time series vary from one averaging period to the next. In this study, we use wavelet methods to analyze GPS time series and seismic recordings of slow slip events in Cascadia. We use detrended GPS data, apply the MODWT transform and stack the wavelet details over several nearby GPS stations. As an independent check on the timing of slow slip events, we also compute the cumulative number of tremors in the vicinity of the GPS stations, detrend this signal, and apply the MODWT transform. In both time series, we can then see simultaneous waveforms whose timing corresponds to the timing of slow slip events. We assume that there is a slow slip event whenever there is a peak in the wavelet signal. We verify that there is a good correlation between slow slip events detected with only GPS data, and slow slip events detected with only seismic data. The wavelet-based detection method detects all events of magnitude higher than 6 as determined by independent event catalogs (e.g. Michel et al., 2019, Pure Appli. Geophys.).