Shallow Slow Slip Events Identified Offshore the Osa Peninsula in Southern Costa Rica From GNSS Time Series.

Using new continuous geodetic time series, we identify five shallow slow slip events (SSEs) offshore and beneath the Osa peninsula in southern Costa Rica. An early event was detected by one station in 2013, and two events occurring in close succession in both 2018 and 2022 were detected by multiple stations, indicating a preliminary recurrence interval of ∼4–5 years. While SSEs have been observed to the northwest at Nicoya, this is their first documentation in southern Costa Rica. Modeled slip distributions of the 2018 and 2022 events indicate they likely ruptured the same or overlapping patches of the plate interface, near the trench, updip of the 1983 Mw 7.4 Osa event. Immediately offshore, estimated cumulative slip from the 2018 and 2022 events is sufficient to close the slip deficit from tectonic loading over the recurrence interval, potentially limiting the magnitude and spatial slip distribution of future large ruptures. Plain Language Summary: Slow slip events (SSEs), effectively slow earthquake ruptures, are a phenomena that have been observed in many subduction zones around the world. While SSEs in subduction zones are often observed at depth, this is likely due to our observational capability being better when the events occur under land. By contrast, shallow events are more difficult to observe because they often occur under the ocean, despite their potential relationship to large, damaging, and/or tsunamigenic earthquakes. In this paper, we present research that identifies shallow slow slip events in a location where they have not been previously observed, the Osa Peninsula in southern Costa Rica. In total we observe five shallow SSEs, sometimes occurring in pairs, that appear to rupture every ∼4–5 years. Additionally we suggest that the amount of slip in these events may be sufficient to account for all the tectonic convergence in some locations, potentially limiting the size of future large earthquakes. Key Points: We identify five slow slip events in southern Costa Rica, one in 2013, and two events occurring in close succession in both 2018 and 2022The recent four events likely ruptured the same shallow patch of the megathrust, extremely close to the Middle American TrenchEvents in 2018 and 2022 seem to release nearly all slip accumulated by plate motion over a ∼4 year recurrence interval in some locations [ABSTRACT FROM AUTHOR]