Simeonof Island Research Articles

‘Double Puzzle’ at the Shumagin seismic gap, Alaska Peninsula: intraslab strike-slip faulting loaded by lateral variations in megathrust fault friction

SUMMARY Along the Aleutian megathrust, the Shumagin seismic gap (162° W–158.5° W) had not hosted a large megathrust earthquake during the observational period 1946–2020. Geodetic evidence suggests a prominent trench-parallel transition from strong to weak kinematic coupling in this segment, indicating varying frictional properties of the megathrust. In 2020 July and October, the occurrence of two large, but dissimilar earthquakes in this seismic gap (the Mw 7.8 Simeonof Island thrust event on 2020 July 22 followed by the Mw 7.6 Sand Point intraslab strike-slip event on 2020 October 19) presented a unique opportunity to examine the interaction between stresses on the megathrust and within the downgoing slab. We use geodetic and geophysical evidence to derive a more accurate kinematic coupling model of the megathrust in this area and show that the Mw 7.6 Sand Point earthquake within the downgoing Pacific slab, likely occurred as a result of both trench-perpendicular interseismic shear stress caused by variable kinematic coupling of the megathrust and coseismic stress changes resulting from the Simeonof Island event. Furthermore, we show that the location of the strike-slip event coincides with along-strike change in the megathrust gravitational anomaly and flexural bending of the downgoing plate, suggesting a long-term interaction between megathrust frictional properties and the structure of the plate interface and downgoing slab.

Aseismic slip and recent ruptures of persistent asperities along the Alaska-Aleutian subduction zone

The frictional properties and slip behaviors of subduction thrusts play a key role in seismic and tsunami hazard assessment, especially in weakly coupled “seismic gaps”. Here, we rely on GPS observations in the Shumagin Gap of the Aleutian subduction zone to derive the slip distribution of the 2020 Mw 7.8 Simeonof Island, Alaska earthquake and of the subsequent afterslip during the first 87-day period. Our modeling results show that the mainshock ruptured at depths of ∼30–40 km beneath Simeonof Island. Kinematic and stress-driven models indicate that the afterslip occurred both updip and downdip of the mainshock rupture. Physically plausible locking models derived from interseismic GPS velocities suggest that the 2020 Simeonof and 2021 Mw 8.2 Chignik earthquakes ruptured persistent asperities on the subduction thrust. We infer that there are several additional persistent asperities at depths of 20–50 km west ∼157°W. However, it is still uncertain whether there are additional locked asperities at shallow depths because of the current lack of geodetic observations close to the trench.

AutoCoulomb: An Automated Configurable Program to Calculate Coulomb Stress Changes on Receiver Faults with Any Orientation and its Application to the 2020 Mw 7.8 Simeonof Island, Alaska, Earthquake

AbstractCoulomb stress change is the change in resultant force of shear stress and friction imposed on a receiver fault plane. The resulting stress change is often computed using the Coulomb 3.4 and the postseismic Green’s functions and postseismic components (PSGRN-PSCMP) programs. Notwithstanding both preferences, both have incomplete optimally oriented failure planes (OOPs) and are inconvenient to resolve Coulomb stress changes on various fault planes placed in varying depths. Here, we present an alternative program termed AutoCoulomb. It leverages the shell command-line tool to automatically batch-process Coulomb stress changes on all sorts of receiver fault planes. We first validate the program. We then apply it to the 2020 Mw 7.8 Simeonof Island, Alaska, earthquake, as a case study. Our results show that Coulomb stress changes resolved on fixed receiver faults, using the three programs, are in line with each other. So are those resolved on 3D OOPs using the PSGRN–PSCMP and the AutoCoulomb programs. Nevertheless, Coulomb stress changes on 2D OOPs, generated by the AutoCoulomb program, always outweigh those done by the Coulomb 3.4 program, indicating that 2D OOPs constrained by the latter are not the most optimal. Some nonoptimal 2D OOPs result in the reversal of the signs of Coulomb stress changes, posing a risk of misleading stress shadows with negative Coulomb stress changes. For the case study, the 28 July 2020 Mw 6.1 aftershock received a positive coseismic Coulomb stress change of ∼3.5 bars. In contrast, the compounded coseismic Coulomb stress changes at the hypocenters of the 1946 Mw 8.2, the 1948 Mw 7.2, and the 2020 Mw 7.8 earthquakes are within a range from −1.1 to 0.1 bar, suggesting that coseismic Coulomb stress changes promoted by preceding mainshocks alone are not responsible for these mainshocks. Other factors, such as postseismic viscoelastic relaxation, afterslip, and slow slip, may contribute to promoting their occurrence.

Slipping the Shumagin Gap: A Kinematic Coseismic and Early Afterslip Model of the Mw 7.8 Simeonof Island, Alaska, Earthquake

AbstractIn July 2020, a Mw 7.8 earthquake initiated directly to the east of Simeonof Island offshore of the Alaska Peninsula. The earthquake ruptured the eastern part of the Shumagin Gap, a region devoid of large earthquakes over the last century and characterized by low geodetic coupling. Here, we investigate the rupture kinematics of the earthquake using a joint inversion of high‐rate GNSS and strong‐motion data. We find that the rupture was focused between depths of 30–45 km, starting east of the Shumagin Islands and rupturing downdip towards the northwest, with little slip west of 160°W. Early postseismic observations indicate that the entirety of the Shumagin Gap at depths between 40–60 km ruptured with aseismic afterslip and aftershocks. Historically, this earthquake resembles the Shumagin Islands earthquake of 1917, indicating that a possible rupture asperity exists to explain low interseismic coupling and repeating ~M7.5–8 earthquakes.

Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska

Can a predominantly creeping segment of a subduction zone generate a great (M > 8) earthquake? Despite Russian accounts of strong shaking and high tsunamis in 1788, geodetic observations above the Aleutian megathrust indicate creeping subduction across the Shumagin Islands segment, a well-known seismic gap. Seeking evidence for prehistoric great earthquakes, we investigated Simeonof Island, the archipelago's easternmost island, and found no evidence for uplifted marine terraces or subsided shorelines. Instead, we found freshwater peat blanketing lowlands, and organic-rich silt and tephra draping higher glacially smoothed bedrock. Basal peat ages place glacier retreat prior to 10.4 ka and imply slowly rising (<0.2 m/ka) relative sea level since ~3.4 ka. Storms rather than tsunamis probably deposited thin, discontinuous deposits in coastal sites. If rupture of the megathrust beneath Simeonof Island produced great earthquakes in the late Holocene, then coseismic uplift or subsidence was too small (≤0.3 m) to perturb the onshore geologic record.

Modern Salt-Marsh and Tidal-Flat Foraminifera From Sitkinak and Simeonof Islands, Southwestern Alaska

We describe the modern distribution of salt-marsh and tidal-flat foraminifera from Sitkinak Island (Trinity Islands) and Simeonof Island (Shumagin Islands), Alaska, to begin development of a dataset for later use in reconstructing relative sea-level changes caused by great earthquakes along the Alaska-Aleutian subduction zone. Dead foraminifera were enumerated from a total of 58 surface-sediment samples collected along three intertidal transects around a coastal lagoon on Sitkinak Island and two intertidal transects on Simeonof Island. Two distinctive assemblages of salt-marsh foraminifera were recognized on Sitkinak Island. Miliammina fusca dominated low-marsh settings and Balticammina pseudomacrescens dominated the high marsh. These two species make up >98% of individuals. On Simeonof Island, 93% of individuals in high-marsh settings above mean high water were B. pseudomacrescens . The tidal flat on Simeonof Island was dominated by Cibicides lobatulus (60% of individuals), but the lower limit of this species is subtidal and was not sampled. These results indicate that uplift or subsidence caused by repeated earthquakes along the Alaska-Aleutian subduction zone could be reconstructed in coastal sediments using alternating assemblages of near monospecific B. pseudomacrescens and low-marsh or tidal-flat foraminifera.

Bryophytes from Simeonof Island in the Shumagin Islands, southwestern Alaska

Bryophytes from Simeonof Island in the Shumagin Islands, southwestern Alaska

Lichens from Simeonof Wilderness, Shumagin Islands, Southwestern Alaska

One hundred eighty-eight taxa of lichens are reported from Simeonof Island in the Shumagin Islands of southwestern Alaska. Wide-ranging arctic-alpine and boreal species dominate the lichens; a coastal element is moderately represented, while amphi-Beringian species form a minor element. The lichen component of Empetrum nigrum dwarf shrub heath, the dominant vegetation type, was analyzed to identify the most frequently occurring lichens within this community.

Wildlife and Historical Notes on Simeonof Island, Alaska

Wildlife and Historical Notes on Simeonof Island, Alaska

Alaskan Species of Acanthocephalan Genus Corynosoma Luehe, 1904

Recently the author has had the opportunity to collect many specimens of various species of Corynosoma from marine birds and mammals in Alaskan waters extending from Southeastern Alaska to Bristol Bay. Specimens from two host species, Enhydra lutris (sea otter) and Delphinapterus leucas (Beluga or white whale), are of special interest. Van Cleave (1953) recognized the presence of an undescribed species of Corynosoma in a sea otter taken on Simeonof Island off the southern coast of the Alaskan Peninsula. He