John B. DeSanto

The 22 July 2020 Mw7.8 Simeonof earthquake was a deep megathrust event that ruptured along the Shumagin segment of the Alaska‐Aleutian subduction zone. This earthquake occurred ∼250 km from a seafloor geodetic GNSS‐Acoustic site IVB1, where we observed a velocity of 3.78 ± 1.15 cm/yr with the down‐going slab prior to the earthquake followed by 0.6 ± 0.7 eastward and −15.5 ± 0.8 cm northward coseismic offset. We computed a slip model of the coseismic rupture using the static offset at IVB1 alongside regional continuous GNSS and strong motion stations. The small static horizontal offset at the site precludes significantly shallower rupture than previously inferred from terrestrial observations, confirming that the Simeonof earthquake was a deep megathrust earthquake. The observed site velocity implies partial locking prior to the earthquake, implying significant shallow strain accumulation such that the small coseismic offset is unlikely to have relieved all of the accumulated strain since the last coseismic rupture.

The shallower portions of subduction zone megathrust faults host Earth’s most hazardous tsunamigenic earthquakes, yet understanding how and when they slip remains elusive because of challenges making seafloor observations. We performed Global Navigation Satellite System Acoustic seafloor geodetic surveys before and ~2.5 months after the 29 July 2021 M _w (moment magnitude) 8.2 Chignik, Alaska, earthquake and determine ~1.4 meters cumulative co- and post-seismic horizontal displacement ~60 kilometers from the megathrust front. Only for the 2011 M _w 9 Tohoku event have closer subduction zone earthquake displacements been observed. We estimate ~2 to 3 meters of megathrust afterslip shallower than 20 kilometers, a portion of the megathrust on which both inter- and co-seismic slip likely had occurred previously. Our analysis demonstrates that by 2.5 months, shallower and deeper moment had effectively equilibrated on the megathrust, suggesting that its tsunamigenic potential remains no more elevated than before the earthquake.

One commonly used method to measure and detect centimeter‐scale changes on land is structure‐from‐motion (SfM) photogrammetry, wherein sets of digital images are used to generate three‐dimensional spatial models. Subcentimeter accuracy useful for geodetic studies is achievable when these surveys are conducted on land or from the air. This method has made its way into marine environments by way of scuba, remotely operated vehicle, and autonomous underwater vehicle‐based surveys largely in the context of mapping. Repeated SfM photogrammetry surveys could be used to monitor changes in the stability of seafloor benchmarks and key sites as the interest in and need for marine geodetic measurements continues to grow. These studies require centimeter‐level accuracy or better, and some rely on accurate placement, positioning, or relocation of geodetic monuments, benchmarks, and instruments. We assessed the accuracy of the photogrammetric method for this purpose by simultaneously conducting a remotely operated vehicle‐based SfM photo survey to produce three‐dimensional spatial data, and a precise pressure survey to accurately measure height information. We found that the SfM survey agreed with the pressure survey height within its uncertainty of ±6.0 cm while using a standard, off‐the‐shelf camera adapted for underwater use. This level of relative accuracy would allow us to detect changes in geodetic benchmarks or instrumented sites between repeated surveys where large changes or the accumulation of small changes is expected within the scene over several years. We believe that centimeter‐level accuracy is obtainable with adjustments to the photogrammetry survey parameters, a higher‐resolution camera, and inclusion of additional coded targets.

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3 km by 20 km) on the port and starboard side of the ship are used to estimate vertical and across-track displacement. We present displacement measurements from a survey of the Ayu Trough southwest of the Marianas Trench using a 12 kHz multibeam. Vertical and across- track displacement errors for the 12 kHz multibeam sonar are typically 0-2 m with RMS uncertainties of 0.25-0.67 m in the across-track and 0.37-0.75 m in the vertical as determined by 3-way closure tests. The uncertainty of the range-averaged sound velocity is a major error source. We estimate that variations in the sound velocity profile, as quantified using expendable bathythermographs (XBTs) during data collection, contribute up to 0.3 m RMS uncertainty in the across-track direction and 1.6 m RMS uncertainty in the vertical direction.

Seafloor geodetic studies such as Global Positioning System (GPS)-Acoustic experiments often require the measurement platform on the sea surface to be positioned accurately to within a few centimetres. In this paper, we test the utility of Precise Point Positioning (PPP) for this application with two experiments. The first fixed platform experiment is a comparison between three independent processing software packages: Positioning and Navigation Data Analyst (PANDA), Global Navigation Satellite System-Inferred Positioning System and Orbit Analysis Simulation Software (GIPSY-OASIS), and the Canadian Spatial Reference System (CSRS)) and a more accurate solution based on conventional differential processing of a remote GPS station in the Aleutian Islands. The second moving platform experiment is a comparison among the three PPP software packages using 40 hours of ship navigation data collected during theRoger RevelleRR1605 cruise 170 nautical miles southwest of Palau in May 2016. We found the PPP solutions were repeatable to 5·49 cm in the horizontal components and 12·4 cm in the vertical component. This demonstrates not only that PPP is a useful tool for positioning marine platforms in remote locations, but also that modern ship navigation instruments such as the Kongsberg Seapath 330 + are suitable for seafloor geodetic application.

Accurate seafloor geodetic methods are critical to the study of marine natural hazards such as megathrust earthquakes, landslides, and volcanoes. We propose digital image correlation of repeated shipboard sidescan sonar surveys as a measurement of seafloor deformation. We test this method using multibeam surveys collected in two locales: 2500 m deep lightly sedimented seafloor on the flank of a spreading ridge and 4300 m deep heavily sedimented seafloor far from any plate boundary. Correlation of these surveys are able to recover synthetic displacements in the across‐track (range) direction accurate to within 1 m and in the along‐track (azimuth) direction accurate to within 1–10 m. We attribute these accuracies to the inherent resolution of sidescan data being better in the range dimension than the azimuth dimension. These measurements are primarily limited by the accuracy of the ship navigation. Dual‐frequency GPS units are accurate to ∼10 cm, but single‐frequency GPS units drift on the order of 1 m/h and are insufficient for geodetic application.