| Abstrakt: |
We present seafloor pressure records from the Cascadia Subduction Zone, alongside oceanographic and geophysical models, to evaluate the spatial uniformity of bottom pressure and optimize the geometry of sensor networks for resolving offshore slow‐slip transients. Seafloor pressure records from 2011 to 2015 show that signal amplitudes are depth‐dependent, with tidally filtered and detrended root‐mean‐squares of <2 cm on the abyssal plain and >6 cm on the continental shelf. This is consistent with bottom pressure predictions from circulation models and comparable to deformation amplitudes from offshore slow slip observed in other subduction zones. We show that the oceanographic component of seafloor pressure can be reduced to ≤1‐cm root‐mean‐square by differencing against a reference record from a similar depth, under restrictions that vary with depth. Instruments at 100–250 m require depths matched within 10 m at separations of <100 km, while locations deeper than 1,400 m are broadly comparable over separations of at least 300 km. Despite the significant noise reduction from this method, no slow slip was identified in the dataset, possibly due to poor spatiotemporal instrument coverage, nonideal deployment geometry, and limited depth‐matched instruments. We use forward predictions of deformation from elastic half‐space models and hindcast pressure from circulation models to generate synthetic slow‐slip observational records and show that a range of slip scenarios produce resolvable signals under depth‐matched differencing. For future detection of offshore slow slip in Cascadia, we recommend a geometry in which instruments are deployed along isobaths to optimize corrections for oceanographic signals. Plain Language Summary: Slow‐slip earthquakes, a special class of earthquake in which slip along a fault occurs over periods of days to years without producing shaking, have increasingly been found to occur in the subsea segments of subduction zones worldwide. There is evidence that these offshore slow‐slip earthquakes can occur within the same region where large, damaging earthquakes are generated and that they may precede and potentially trigger these events. Offshore slow‐slip earthquakes cause the seafloor to move vertically by up to a few centimeters, which can be detected by measuring the pressure on the seafloor if the effects of ocean tides and circulation can be corrected for. In this study, we search for evidence of slow‐slip earthquakes in seafloor pressure data from the Cascadia Subduction Zone, off the U.S. and Canadian west coast, and assess how to best reduce oceanographic signals in these data in order to reliably observe slow‐slip deformation. No evidence for these events is seen in Cascadia pressure data from 2011 to 2015. Using a combination of observational data and model simulations, we show that oceanographic signals can be largely eliminated from seafloor pressure measurements if instruments are placed in rows of constant depth. A network of 22 instruments would be sufficient to monitor for large offshore slow‐slip earthquakes offshore central Oregon, but smaller events would be difficult to detect with this method. By increasing our ability to detect these slow‐slip earthquakes, we can better understand subduction zone processes and their relation to large, damaging megathrust earthquakes. Key Points: In Cascadia, differencing tidally filtered, depth‐matched seafloor pressure records can reduce oceanographic noise to less than 1‐cm RMSModels of offshore SSEs in Cascadia suggest that a network of depth‐matched sensors at 30‐km spacing can detect Mw 6.4 and larger eventsNo offshore slow‐slip earthquakes are detected in the available Cascadia seafloor pressure data from 2011 to 2015 [ABSTRACT FROM AUTHOR] |
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