Rapid shallow megathrust afterslip from the 2021 M8.2 Chignik, Alaska earthquake revealed by seafloor geodesy

Brooks, Benjamin A.; Goldberg, D.; DeSanto, J. B.; Ericksen, T. L.; Webb, Spahr C.; Nooner, S. L.; Chadwell, C. D.; Foster, James H.; Minson, S. E.; Witter, Robert C.; Haeussler, Peter J.; Freymueller, J. T.; Barnhart, William D.; Nevitt, J. M.

doi:10.1126/sciadv.adf9299

Cited by 23 publications

(13 citation statements)

References 89 publications

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“…In this study the earthquake we observed had a primarily deep rupture that was verified by the small offset at the seafloor GNSS‐A site. In comparison, other studies have identified cases where coseismic slip greater than 1 m did extend to shallow depths, such as has been observed for the Chignik (Brooks et al., 2023) and Tohoku (Kido et al., 2011; Sato et al., 2011a) earthquakes. Thus, even though this study confirms previously published models of the Simeonof earthquake (Crowell & Melgar, 2020; Liu et al., 2020; Ye et al., 2021), the seafloor geodetic measurements are necessary to understand the shallow slip behavior in subduction zone earthquakes.…”

Section: Discussionmentioning

confidence: 71%

“…Furthermore, there was not a large tsunami that such a displacement would likely have produced. Another hypothesis is that the offset at IVB1 is primarily postseismic rather than coseismic, as was inferred at the SEM1 array following the neighboring Chignik earthquake in 2021 (Brooks et al., 2023). However, this hypothesis would also yield a rupture model with little shallow slip similar to the one we present in this study.…”

Section: Discussionmentioning

confidence: 95%

“…This technique has previously been used to capture the offshore surface deformation from past earthquakes, including two M7 earthquakes off the Kii Peninsula in 2004 (Kido et al., 2006; Tadokoro et al., 2006), the 2005 Mw 7.2 earthquake off Miyagi prefecture (Matsumoto et al., 2006; Sato et al., 2011b), and the 2011 Mw 9.0 Tohoku‐Oki earthquake (Kido et al., 2011a; Sato et al., 2011a). The data we present are part of an array (Figure 1) that directly captured the horizontal co‐seismic and early postseismic surface deformation of the Simeonof, Sand Point, and Chignik (Brooks et al., 2023) earthquakes along the trench of the Alaska‐Aleutian Subduction Zone, and are the first seafloor displacement observations of an earthquake outside of Japan coastal waters. With these new measurements we are able to further resolve the updip extent of the seismic rupture and its implications for strain in the shallow subduction zone.…”

Section: Introductionmentioning

confidence: 99%

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Limited Shallow Slip for the 2020 Simeonof Earthquake, Alaska, Constrained by GNSS‐Acoustic

DeSanto,

Webb,

Nooner

et al. 2023

Geophysical Research Letters

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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.

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Section: Discussionmentioning

confidence: 71%

Section: Discussionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

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Limited Shallow Slip for the 2020 Simeonof Earthquake, Alaska, Constrained by GNSS‐Acoustic

DeSanto,

Webb,

Nooner

et al. 2023

Geophysical Research Letters

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“…To mitigate the error caused by poorly constrained sound speed, Spiess (1985) outlined a method in which only the assumptions of thermal stratification and rigid block motion are required: acoustic ranging to a symmetric array of seafloor transponders will determine the array's horizontal center with high precision, forcing uncertainty in sound speed to impact only the vertical component of the array if it moves as a rigid block. Based on this method, GNSS‐A has been successfully applied to a number of seafloor geodesy projects, yielding centimeter or better precision in horizontal seafloor positioning (e.g., Gagnon et al., 2005; Chadwell & Spiess, 2008; Tomita et al., 2017; Yokota & Ishikawa, 2020; Brooks et al., 2023). The method, though proven effective, requires at least three transponders to form an array, limiting the array's reliability and number of equivalent sites (e.g., three transponders to form an equilateral triangle array, four transponders to form a square array, etc.…”

Section: Introductionmentioning

confidence: 99%

Shallow Water Seafloor Geodesy With Wave Glider‐Based GNSS‐Acoustic Surveying of a Single Transponder

Xie,

Zumberge,

Sasagawa

et al. 2023

Earth and Space Science

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Due to the blockage of seawater, seafloor displacement cannot be directly measured by space geodesy. The combination of Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) has been used to overcome the electromagnetic barrier, so that a GNSS‐determined sea surface vessel's coordinates can be transformed to seafloor benchmarks in a global reference frame. Due to the high cost and science priorities, previous GNSS‐A studies mainly targeted relatively deep water and a minimum of three transponders were used to form an array, equivalent to a precision geodetic station. With recent developments in unmanned autonomous surface vessels, low cost GNSS‐A surveys are poised to become practical. Here we demonstrate that with a carefully designed surveying trajectory, Wave Glider‐based GNSS‐A surveying of a single transponder in shallow water can provide centimeter‐level accuracy on horizontal seafloor positioning, even if the sound speed model deviates from the actual value by a few meters per second. Results from a nine‐month experiment conducted at ∼54 m water depth show that the repeatability of the seafloor horizontal positioning is better than 2 cm. When conditions allow, the acoustic observations should be collected symmetrically about the transponder and data redundancies are recommended to reduce the error associated with time‐dependent variations in sound speed.

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“…Seafloor geodetic data have been shown to increase resolution of geodetic studies in subduction zone regions (Iinuma et al., 2012; Tomita et al., 2020; Yokota et al., 2016), and it is anticipated that the addition of GNSS‐A sites will enable better recovery of slip distributions at subduction zones (Bürgmann & Chadwell, 2014; Sathiakumar et al., 2017). These instruments are placed on the seafloor, very close to the trench (Brooks et al., 2023)—that is, in a region with very significant bathymetry that is likely to bias slip estimates unless it is accounted for.…”

Section: Introductionmentioning

confidence: 99%

Accuracy of Finite Fault Slip Estimates in Subduction Zone Regions With Topographic Green's Functions and Seafloor Geodesy

Langer

Ragon

2023

JGR Solid Earth

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Until recently, the lack of seafloor geodetic instrumentation and the use of unrealistically simple, half‐space based forward models have resulted in poor resolution of near‐trench slip in subduction zone settings. Here, we use a synthetic framework to investigate the impact of topography and geodetic data distribution on coseismic slip estimates in various subduction zone settings. We calculate surface displacements in two synthetic topographic domains that have topography similar to that of Chile and Japan, respectively. We then attempt to image target slip distributions by using a Bayesian approach to solve for slip with two sets of Green's functions—one that accounts for topography and one that does not—and five sets of 50 or more observation points selected from the synthetic surface displacements. Three of these sets of observation points are entirely onland, and two include 5–10 seafloor geodetic sites. We find that the use of topographic Green's functions always improves inferred slip models, and with seafloor geodetic data, it enables an almost perfect recovery of a target slip model, even in the near‐trench region. Critically, our results demonstrate that it would be impossible for non‐topographic Green's functions to properly recover the true slip distribution, particularly in the near‐trench region. We also perform a parameter study with approximately 4,000 slip models estimated using a least‐square approach, and find that topographic Green's functions yield significantly more accurate slip models in cases where good data (well distributed and reasonably dense) are available, even in the absence of seafloor geodetic sites.

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Rapid shallow megathrust afterslip from the 2021 M8.2 Chignik, Alaska earthquake revealed by seafloor geodesy

Cited by 23 publications

References 89 publications

Limited Shallow Slip for the 2020 Simeonof Earthquake, Alaska, Constrained by GNSS‐Acoustic

Limited Shallow Slip for the 2020 Simeonof Earthquake, Alaska, Constrained by GNSS‐Acoustic

Shallow Water Seafloor Geodesy With Wave Glider‐Based GNSS‐Acoustic Surveying of a Single Transponder

Accuracy of Finite Fault Slip Estimates in Subduction Zone Regions With Topographic Green's Functions and Seafloor Geodesy

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