The 2021 M8.2 Chignik Earthquake as Seen by MERMAID Seismometers in the South Pacific Ocean

We present observations of the 29 July 2021 M 8.2 earthquake from MERMAID seismometers which form part of the SPPIM (South Pacific Plume Imaging and Modeling) project. MERMAID (Mobile Earthquake Recording in Marine Areas by Independent Divers) instruments are seismometers which freely float ~1500 m below the ocean surface, recording seismoacoustic waves from a range of local, regional and teleseismic earthquakes (Simon et al., SRL & GJI, 2021), as well as hydroacoustic signals of non-earthquake origin (Pipatprathanporn & Simons, GJI, 2021). The SPPIM array comprises 50 instruments deployed between 2018-2019; on the day of the 2021 Chignik earthquake 46 were operational, recording at positions between 175E and 106W, and from roughly the equator to 32S. We focus here on the records gathered by 16 SPPIM MERMAIDs operated by Princeton University. The Chignik earthquake was initially reported by eight Princeton SPPIM MERMAIDs; records were requested from the remaining eight. The instruments for which the automatic detection system was triggered immediately rose to the surface and provided seismograms ~240 seconds long, containing P waves which traveled through the lower mantle, reaching depths of ~1800-2400 km. Longer seismic records were requested from the other seismometers, in order to assess both whether the P wave was detectable, and to examine the data for other seismic arrivals, including surface waves and T phases. We compare the records from the M8.2 Chignik earthquake to those from two earthquakes from the previous year which may be foreshocks -- the M7.8 event on 22 July 2020 (seven reporting) and the M7.6 event on 19th October (nine reporting) -- and the aftershock on 14th August 2021. We also investigate why some instruments did, and others did not, detect the events, considering both the earthquake properties and the near-MERMAID environment. All the triggered seismograms from Princetons SPPIM instruments are deposited at IRIS; those from three MERMAIDs are available without temporal restriction.