Recognizing Signatures of Slow Slip in Plate Boundary Observatory Borehole Pore Pressure Data

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Within the past 15-20 years, large unusual earthquakes that slip over the course of days to weeks, rather than seconds, have been discovered to repeatedly occur along subduction zone faults. Although these slow slip earthquakes do not create seismic shaking, recognizing their occurrence is important because they can influence the potential of regular earthquakes and tsunamis. These slow slip earthquakes are sometimes associated with extremely quiet seismic rumblings, but are otherwise identified through continuous Global-Positioning-System monitoring that reveals small motions occurring at dedicated stations. The aim of this study is to evaluate a potential complementary method for identifying these events through the use of groundwater pressure data. As a fault slips, it squishes and relaxes the water saturated rocks below the ground like a sponge. With sensitive instruments within wells / boreholes, the resulting water pressure changes can sometimes be observed. This project seeks to improve the ability and speed at which slow slip earthquakes are detected by incorporating the analysis of groundwater pressure data from existing monitoring facilities along the Cascadia subduction zone within the United States Pacific Northwest. The project will study and robustly identify water pressure changes associated with fault slip and will support graduate students to conduct research as part of the project. The results will be disseminated through published papers and public presentations.This project will detect and analyze suspected signatures of subduction zone slow slip hidden within continuous pore pressure records from Earthscope's Plate Boundary Observatory borehole array. The science is motivated by an aim to improve our understanding of the temporal and spatial distribution of slow slip within subduction zones. Hydrogeologic signatures of fault slip behavior can provide an additional constraint on large-scale slow slip beyond seismic tremor detections or transients in continuous positioning measurements. Suspected pore pressure signatures of fault slip, however, must be differentiated from unrelated local hydrogeologic disturbances and meteorological effects. This study addresses these issues by analyzing existing borehole pore pressure data during known large-scale slow slip events and uses signal processing and statistical techniques with a variety of data types to improve the robust identification and discrimination of suspected signals. The results are expected to improve our understanding of the hydrogeologic response to fault slip, help further assess the timing and extent of large-scale slow slip events, and potentially provide a real-time statistically-based slow slip detection algorithm independent of continuous positioning data or seismologic observations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.