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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. Plain Language Summary: The 22 July 2020 Mw7.8 Simeonof earthquake ruptured east of the Shumagin Islands along the interface of the Alaska‐Aluetian subduction zone, where the Pacific plate subducts beneath the North American plate. This earthquake occurred ∼250 km from a seafloor geodetic GNSS‐Acoustic site IVB1, which acts as a seafloor GPS station and provides the only direct offshore measurement of seafloor deformation following the earthquake. In late September 2020, we observed 0.6 ± 0.7 of eastward offset and −15.5 ± 0.8 cm of northward offset at the station IVB1 and used this measurement to refine a model of the earthquake rupture area that previously only used the GPS and seismic stations installed on the nearby Aleutian Islands. The seafloor offset was not large enough to require additional rupture to the south of where previous studies have imaged the earthquake, confirming that the Simeonof earthquake was a deep megathrust earthquake. Prior to the earthquake, station IVB1 was moving with the down‐going Pacific plate with a velocity of 3.78 ± 1.15 cm/yr despite resting on the North American plate, implying that the two plates are partially stuck together. We expect that there could still be another future earthquake beneath IVB1 since the offset at IVB1 following the Simeonof earthquake was smaller than expected. Key Points: We observed seafloor offset at a GNSS‐Acoustic site along the Alaska subduction zone following the 22 July 2020 Mw7.8 Simeonof earthquakeAdditional slip updip of the main earthquake rupture area is not required to generate the observed seafloor offsetThe shallow plate interface was partially locked prior to the Simeonof earthquake and may still host unrelieved strain [ABSTRACT FROM AUTHOR]

Many types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S‐wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a subducted seamount. Our findings corroborate a previous interpretation, based on burst‐type repeating earthquakes and intermittent tremor, that localized slow slip and tremor around the seamount was triggered by fluid migration following the large plate boundary slow slip event and indicate tremor occurred continuously rather than as isolated and sporadic individual events. Plain Language Summary: Slow earthquakes and tremor are characterized by slow fault rupture. Their generation mechanism has not been well understood. The slow earthquake itself does not cause damage but may inform forecasts of large earthquakes that could cause strong ground shaking or tsunamis. A large slow slip event occurred on the Hikurangi subduction plate boundary, offshore New Zealand, in 2014 directly beneath a temporary ocean bottom seismometer network. We applied new methods to this data to detect and characterize an earthquake tremor signal. The methods determine the polarization of the wave emanating from the tremor and the fast direction of the anisotropic subsurface structure through which the tremor wave propagates. These parameters enable us to detect small amplitude tremors and to determine their spatial and temporal distribution. We observed continuous tremor activity for about 2 weeks duration while the slow slip event was waning. This tremor activity occurred over a mapped subducted seamount and on the plate boundary, which likely experienced large stress changes due to the slow slip event. Our results are consistent with previous studies that used different methods on the same data, which indicate that the tremor activity in the vicinity of the seamount was triggered by fluid migration. Key Points: S‐wave splitting and polarization analyses of continuous offshore data detect small amplitude tremor activityTremor activity with stable polarization direction started near the end of the 2014 Gisborne slow slip event (SSE) and was continuous for about 2 weeksTremor activity occurred around the landward edge of the subducted seamount and was triggered by fluid migration following the SSE [ABSTRACT FROM AUTHOR]

Isolating the source of nontidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the nontidal components, as these are largely common‐mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, installed in the Hikurangi subduction zone offshore New Zealand. We find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms the result of previous studies from Cascadia that nontidal components are more similar along isobaths even if the distance between sites is large. Furthermore, in order to reduce noises, the required depth difference between site pairs also varies with site depths. Plain Language Summary: Coherent signals of ocean bottom pressure are observed along common water depths within an ocean bottom pressure array offshore New Zealand. We statistically evaluated the similarity of the seafloor pressure collected in 2014 offshore the North Island's east coast, where the Pacific Plate dives or "subducts" along the Hikurangi subduction zone beneath the North Island. This is important for removal of noise caused by oceanographic processes, which must be done to detect centimeter‐level vertical movement of the seafloor crust during slow slip events using seafloor pressure records. We measured the similarity of pairs of seafloor pressure records at a range of water depths. Similar to previous studies offshore the Cascadia subduction zone, our results confirm that seafloor pressure records from similar depths (but at large horizontal distances from each other) can be used effectively to reduce oceanographic noise in sea floor pressure data to reveal the seafloor crustal deformation. Key Points: The similarity of the nontidal components of bottom pressure varies with relative water depth, rather than distance between sitesBottom pressure data show that reference sites at similar depths will optimize oceanographic noise removal for seafloor geodetic studiesOceanographic models with baroclinicity represent the observed depth dependence of the nontidal variations better than barotropic models [ABSTRACT FROM AUTHOR]

Variations in pore-fluid pressure along subduction megathrusts are often invoked to explain differences in fault slip behavior between margins. However, many other parameters vary between subduction zones, making it difficult to isolate the causes of elevated pore fluid pressures and their effects on megathrust behavior. Here we show evidence from pre-stack depth migrations of multichannel seismic reflection data along the subduction zone off the Alaska Peninsula for significant, systematic along-strike variations in the thickness, continuity, P-wave velocity, and estimated pore-fluid pressure of the subducting sediment layer that lines the shallow plate interface within 25 km of the trench. These variations appear to correlate with changes in seismicity, locking, and earthquake rupture history. The currently locked and seismically quiet Semidi segment has a continuous, thick (600-900 m) subducted sediment layer that is characterized by low seismic velocities and elevated pore pressure (λ* ≈ 0.4-0.8). The subducted sediment in the neighboring Shumagin Gap, a region with low geodetic coupling and abundant small earthquakes, is thinner, irregular and has lower pore pressure (λ* < 0.2-0.3). We suggest that the thicker and weakly faulted sediment layer entering the trench at Semidi is associated with a continuous and overpressured sediment layer lining the shallow plate interface, but forms a large coherent asperity as it dewaters and consolidates at greater depths, thus favoring large earthquakes. In contrast, thinner incoming sediment disrupted by outer rise faulting at Shumagin results in a plate interface without elevated pore-fluid pressures, but which is heterogeneous and complex at all depths, contributing to creep and frequent small earthquakes. [ABSTRACT FROM AUTHOR]