Shumagin Seismic Gap 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.

Coseismic Coulomb stress changes induced by a 2020–2021 MW > 7.0 Alaska earthquake sequence in and around the Shumagin gap and its influence on the Alaska-Aleutian subduction interface

Three MW > 7.0 earthquakes in 2020–2021 occurred in the Shumagin seismic gap and its adjacent area of the Alaska-Aleutian subduction zone, including the MW7.8 Simeonof thrust earthquake on July 22, 2020, the MW7.6 Sand Point strike-slip earthquake on October 19, 2020, and the MW8.2 Chignik thrust earthquake on July 29, 2021. The spatial and temporal proximity of these three earthquakes prompts us to probe stress-triggering effects among them. Here we examine the coseismic Coulomb stress change imparted by the three earthquakes and their influence on the subduction interface. Our results show that: (1) The Simeonof earthquake has strong loading effects on the subsequent Sand Point and Chignik earthquakes, with the Coulomb stress changes of 3.95 bars and 2.89 bars, respectively. The Coulomb stress change caused by the Sand Point earthquake at the hypocenter of the Chignik earthquake is merely around 0.01 bars, suggesting the negligible triggering effect on the latter earthquake; (2) The triggering effects of the Simeonof, Sand Point, and Chignik earthquakes on aftershocks within three months are not well pronounced because of the triggering rates of 38%, 14%, and 43% respectively. Other factors may have played an important role in promoting the occurrence of these aftershocks, such as the roughness of the subduction interface, the complicated velocity structure of the lithosphere, and the heterogeneous prestress therein; (3) The three earthquakes caused remarkable coseismic Coulomb stress changes at the subduction interface nearby these mainshocks, with an average Coulomb stress change of 3.2 bars in the shallow region directly inwards the trench.

Optimizing a Model of Coseismic Rupture for the 22 July 2020MW7.8 Simeonof Earthquake by Exploiting Acute Sensitivity of Tsunami Excitation Across the Shelf Break

AbstractThe Shumagin seismic gap along the Alaska Peninsula experienced a major,MW7.8, interplate thrust earthquake on 22 July 2020. Several available finite‐fault inversions indicate patchy slip of up to 4 m at 8–48 km depth. There are differences among the models in peak slip and absolute placement of slip on the plate boundary, resulting from differences in data distributions, model parameterizations, and inversion algorithms. Two representative slip models obtained from inversions of large seismic and geodetic data sets produce very different tsunami predictions at tide gauges and deep‐water pressure sensors (DART stations), despite having only secondary differences in slip distribution. This is found to be the result of the acute sensitivity of the tsunami excitation for rupture below the continental shelf in proximity to an abrupt shelf break. Iteratively perturbing seismic and geodetic inversions by constraining fault model extent along dip and strike, we obtain an optimal rupture model compatible with teleseismicPandSHwaves, regional three‐component broadband and strong‐motion seismic recordings, hr‐GNSS time series and static offsets, as well as tsunami recordings at DART stations and regional and remote tide gauges. Slip is tightly bounded between 25 and 40 km depth, the up‐dip limit of slip in the earthquake is resolved to be well‐inland of the shelf break, and the rupture extent along strike is well‐constrained. The coseismic slip increased Coulomb stress on the shallow plate boundary extending to the trench, but the frictional behavior of the megathrust below the continental slope remains uncertain.

Subduction earthquakes controlled by incoming plate geometry: The 2020 M > 7.5 Shumagin, Alaska, earthquake doublet

In 2020, an earthquake doublet, a M7.8 on July 22nd and a M7.6 on October 19th, struck the Alaska-Aleutian subduction zone beneath the Shumagin Islands. This is the first documented earthquake doublet involving a megathrust event and a strike-slip event. The first event partially ruptured a seismic gap, which has not hosted large earthquakes since 1917, and the second event was unusual as it broke a trench-perpendicular fault within the incoming oceanic slab. We used an improved Bayesian geodetic inversion method to estimate the fault slip distributions of the major earthquakes using Interferometric Synthetic Aperture Radar (InSAR) wrapped phase and Global Navigation Satellite Systems (GNSS) offsets data. The geodetic inversions reveal that the Shumagin seismic gap is multi-segmented, and the M7.8 earthquake ruptured the eastern segment from 14 km down to 44 km depth. The coseismic slip occurred along a more steeply, 26∘ dipping segment, and was bounded up-dip by a bend of the megathrust interface to a shallower 8∘ dip angle connecting to the trench. The model for the M7.6 event tightly constrained the rupture depth extent to 19-39 km, within the depth range of the M7.8 coseismic rupture area. We find that the M7.6 event ruptured the incoming slab across its full seismogenic thickness, potentially reactivating subducted Kula-Resurrection seafloor-spreading ridge structures. Coulomb stress transfer models suggest that coseismic and/or postseismic slip of the M7.8 event could have triggered the M7.6 event. We conclude that the segmented megathrust structure and the location of intraslab fault structures limited the rupture dimensions of the M7.8 event and are responsible for the segmentation of the Shumagin seismic gap. Our study suggests that the western and shallower up-dip segments of the seismic gap did not fail and remain potential seismic and tsunami hazard sources. The unusual earthquake doublet provides a unique opportunity to improve our understanding of the role of the subducting lithosphere structure in the segmentation of subduction zones.

The deep Shumagin gap filled: Kinematic rupture model and slip budget analysis of the 2020 Mw 7.8 Simeonof earthquake constrained by GNSS, global seismic waveforms, and floating InSAR

On 22 July 2020, the Mw 7.8 Simeonof megathrust earthquake struck offshore of the Alaska Peninsula. This was the largest event since 1917 in the enigmatic area known as the Shumagin seismic gap, a region of transitional plate interface coupling from highly coupled to the east to creeping to the west. Hence, this event provides a rare chance to understand rupture mechanics on such heterogeneously coupled faults. In this study, we examine the rupture process of the 2020 Simeonof earthquake with a combination of static GNSS displacements, InSAR displacements, high-rate GNSS waveforms and tele-seismic waveforms. Due to the discontinuous nature of the deformation field, we use InSAR data for individual islands and tie the displacement field either to GNSS observations or keep these “floating”, i.e. we estimate an ambiguity parameter during the inversion. Our results demonstrated that the rupture process of this event is unidirectional, initiating at the hypocenter and propagating westward for about 130 s with an average rupture velocity of ∼1.9 km/s. The highest slip was centered below the Koniuji Islands and occurred between 20 s to 50 s after the rupture initiation. We find that InSAR observations, especially “floating” data on near-field islands, provide essential constraints for the slip inversion, building confidence in the slipping of the less coupled region. Comparison with an existing and an alternative plate coupling model demonstrated that the remaining slip budget is unlikely to be able to generate a large event at depths from 30 to 40 km, as the Shumagin seismic gap has been mostly filled by the 2020 Simeonof earthquake at those depths. However, both coupling models suggest a substantial slip deficit in shallow near trench regions, suggesting that a significant earthquake can still occur. Seafloor geodetic observations are required to further constrain the near-trench plate coupling.

Ring-shaped seismicity structures, being formed in the Alaska region: Justi-fied prediction of the place and magnitude of the Simeonof earthquake of July 22, 2020 (Mw 7.8)

We have been studying some seismicity characteristics connected with Simeonof earthquake of July 22, 2020 (Mw 7.8) in the area of Shumagin seismic gap, located in the region of South Alaska. 8 years before this event, the authors picked up here ring-shaped seismicity structure. Using characteristics of this structure we made a conclusion on preparation for large earth-quake (Mw~8), similarly to many events in subduction zones. We used data on threshold magnitude and size of the ring structure, and earlier obtained dependences of these parame-ters on magnitudes of main events for the subduction zones in the Eastern Pacific. Accumula-tion of new seismicity data prior to 2020/07/22 allowed us to specify the source location and magnitude of the Simeonof earthquake: Mw=7.9±0.3. It was shown that after this event new ring structures continued to form, which can correspond to preparation for larger earthquake: Mw=8.2±0.2. Geodynamical processes, which lead to formation of the ring structures, are discussing.

Anisotropy-revealed change in hydration along the Alaska subduction zone

AbstractMegathrust earthquake behavior in subduction zones is controlled by a variety of factors including the hydration state of the subducting slab. Increased hydration reduces the occur-rence of great, damaging earthquakes by diminishing the strength of the material along the interface between tectonic plates. Understanding variations in hydration in subductions zones is necessary for properly assessing the overall hazard posed by each region. Fortunately, seismic anisotropy is strongly dependent upon hydration of the subducting crust and litho-sphere. I present shear-wave splitting measurements that illuminate changes in anisotropy, and therefore hydration, of the subducting Pacific plate beneath the Alaska subduction zone (northern Pacific Ocean). Variations in shear-wave splitting directly correlate to changes in the behavior of great, megathrust earthquakes. My measurements show that the Shumagin seismic gap is characterized by a hydrated subducting slab, explaining the long-term lack of great earthquakes. Observations in the immediately adjacent Semidi segment, which experiences great events regularly, indicate a far less hydrated slab. These results are driven by the preferential alignment of paleo-spreading fabrics of the Pacific plate. Where fabrics are more closely aligned with the orientation of the trench, outer-rise faulting and plate hydration is enhanced. These results highlight the importance of changes in preexisting slab structures and subsequent hydration in the production of great, damaging earthquakes.

The 22 July 2020 MW 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust

The earthquake potential of the Shumagin seismic gap along the Alaska Peninsula (∼162°W to ∼158.5°W) has been debated for more than 40 years. On 22 July 2020, the eastern half of the gap hosted an MW 7.8 earthquake involving a patchy rupture of the megathrust in the depth range of 20 to 45 km. The space-time slip distribution is determined by joint inversion of teleseismic P and SH waves and static displacements from regional GPS stations. The event initiated near the epicenter of the 10 November 1938 (MW 8.2) event, and ruptured westward, with little/no overlap with the 1938 rupture zone. The main slip patch has peak slip of ∼3.8 m below the Shumagin Islands, and produced ∼30 cm uplift and ∼25 cm SSE horizontal displacement on Chernabura Island. The slip model predicts well the small (<1 cm) tsunami signals persisting for more than ten hours observed at deep-water DART seafloor pressure recordings along the Alaska-Aleutian arc. Aftershocks with depths from 20 to 40 km fringe the large-slip patches, and show westward concentration during the first month after the mainshock. Aftershocks up-dip of the 1948 MW 7.1 event contribute to the high level of modest-size background seismicity extending to the trench in the region of very low seismic coupling (0.0–0.1) in the western Shumagin gap east of the 1 April 1946 (MW 8.6) rupture zone. The 31 May 1917 event is the last major earthquake to rupture the eastern half of the Shumagin gap, and has a lower surface wave magnitude (MSG-R 7.4, horizontal components) compared to the 2020 event (MSG-R 7.7, vertical components). Comparison of instrument-equalized waveforms for the 1917 and 2020 events indicates similar size contrast and differences in overall rupture duration and slip complexity. The 2020 rupture has average slip of ∼1.9 m over the 3600 km2 region with co-seismic slip ≥1 m. This is much less than the ∼6.7 m of potentially accumulated slip deficit since 1917, consistent with geodetic estimates of low average seismic coupling coefficient of 0.1–0.4. The megathrust seaward of the 2020 event has low seismicity and may either be aseismic or capable of comparable size ruptures. Comparisons are made with other subduction zones that have experienced relatively deep megathrust slip in regions with moderate seismic coupling.

The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin

The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin

Strain accumulation in the Shumagin Islands: Results of initial GPS measurements

Deformation in the Shumagin seismic gap has been monitored with repeated trilateration (EDM) in the 1980–1987 interval and with the Global Positioning System (GPS) in the 1987–1991 interval. The geodetic network extends for 100‐km across the Shumagin Islands to the Alaska Peninsula. Results from the GPS surveys are consistent with those previously reported for the EDM surveys: we failed to detect significant strain accumulation in the N30°W direction of plate convergence. Using the method of simultaneous reduction for position and strain rates, we found the average rate of extension in the direction of plate convergence to be −25±25 nanostrain/yr (nstrain/yr) during the 1987–1991 interval of GPS surveys compared with −20±15 nstrain/yr during the 1981–1987 interval of complete EDM surveys. We found a marginally significant −26±12 nstrain/yr extension rate in the 1981–1991 interval covered by the combined EDM and GPS surveys. Strain rates are higher, but not significantly so, in the part of the network closest to the trench. Spatial variation in the deformation is observed in the 1980–1991 average station velocities, where three of the four stations closest to the trench have an arcward velocity of a few mm/yr. The observed strain rates are an order of magnitude lower than the −200 nstrain/yr rate predicted by dislocation models.

Aftershocks of the 13 May 1993 Shumagin Alaska earthquake

The 13 May 1993 Ms 6.9 Shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1.5 by 1 June 1993. Of these aftershocks, 79 were located by using S‐P travel times at the only two stations within 570 km of the mainshock epicenter. The rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1.0 m and a 28 bar stress drop. The magnitude‐frequency plots give a b‐value for the aftershock sequence of about 0.4, which is low compared to the background value of approximately 0.8. The decay of the aftershock sequence followed the modified Omori law with a p‐value of 0.79, which is also lower than the typical values of about 1.1 observed in Alaska. Both of these facts can be interpreted as indicating relatively high ambient stress in the Shumagin seismic gap and the possibility that the 13 May earthquake was a foreshock to a larger gap‐filling event to occur within the next few years.

On some future tsunamis in the Pacific Ocean

Possible tsunamis in the Pacific Ocean, especially in its northeastern part, are discussed in relation to a predicted major earthquake in the Shumagin Seismic Gap (located in the eastern part of the Aleutian Island Chain) and to a major eruption of the St. Augustine volcano in Cook Inlet, Alaska. The deep-water propagation of the tsunami generated in the Shumagin Gap is simulated through the use of a spherical polar coordinate grid of the approximate size of 14km. The tsunami generated by the St. Augustine volcano is studied through the fine mesh grid confined to the Cook Inlet only. The numerical models were calibrated against historical tsunami data. The properties of the tsunami signal are described by the maximum amplitude which occurs in the tsunami record. This allows us to single out the direction along which a maximum tsunami is to be expected.

Absence of strain accumulation in the Shumagin Seismic Gap, Alaska, 1980–1987

Measurements of the deformation of a trilateration network in the Shumagin seismic gap in the interval 1980–1987 failed to detect any significant strain accumulation (observed extension rate in the direction of plate convergence 0.00±0.03 μstrain/yr). Dislocation models of the subduction process and measurements at a comparable network at a known seismic subduction zone (Nankai Trough, Japan) suggest that a rate of the order of −0.2 μstrain/yr should have been observed if the main thrust zone beneath the Shumagin Islands were locked. The simplest explanation of the observed absence of strain accumulation in the Shumagin seismic gap is that the main thrust zone beneath the Shumagin Islands is not presently locked. Other possible explanations depend upon very particular circumstances.

Structural development of an accretionary prism by thrust and strike-slip faulting: Shumagin region, Aleutian Trench

Research Article| May 01, 1988 Structural development of an accretionary prism by thrust and strike-slip faulting: Shumagin region, Aleutian Trench STEPHEN D. LEWIS; STEPHEN D. LEWIS 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar JOHN W. LADD; JOHN W. LADD 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar TERRY R. BRUNS TERRY R. BRUNS 2U.S Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1988) 100 (5): 767–782. https://doi.org/10.1130/0016-7606(1988)100<0767:SDOAAP>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation STEPHEN D. LEWIS, JOHN W. LADD, TERRY R. BRUNS; Structural development of an accretionary prism by thrust and strike-slip faulting: Shumagin region, Aleutian Trench. GSA Bulletin 1988;; 100 (5): 767–782. doi: https://doi.org/10.1130/0016-7606(1988)100<0767:SDOAAP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A combination of SEABEAM swath bathymetric data, closely spaced single-channel seismic reflection profiles, and multichannel seismic reflection profiles from the Shumagin Islands region indicates that the large-scale deformation of the accretionary prism in this segment of the Aleutian Trench occurs progressively through several stages: (1) folding along axes perpendicular to the plate-convergence direction in the region, (2) thrust faulting in the direction of plate convergence and (3) oblique strike-slip faulting along conjugate west-northwest-trending right-lateral and north-northeast-trending left-lateral faults. Strike-slip faults occur in distinct domains, with right-lateral oblique faults present west of a major transverse fault at long 161°W. and left-lateral oblique faults prevalent east of 161°W.The structural development of the modern Aleutian Trench fore arc in this region is probably affected by pre-Eocene inherited structures that are still active in the present-day stress regime. A major domain boundary fault located at long 161°W. is probably part of a strike-slip fault system that also bounds Sanak Basin farther landward on the Shumagin shelf. This fault zone is nearly coincident with the mapped position of the western edge of the Shumagin seismic gap. These transverse structures in the Shumagin fore arc probably influence the spatial distribution of stress buildup related to subduction, and hence the locations of rupture zones of great interplate earthquakes, along this segment of the Aleutian Trench. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Improved sea level monitors for measuring vertical crustal deformation in the Shumagin Seismic Gap, Alaska

We have improved the relative vertical deformation detection capability of a network of sea level gauges in the Shumagin seismic gap, Alaska. An examination of the present noise levels suggests that the network is now capable of providing relative vertical deformation data that is quieter than data from leveling, the Global Positioning System, Very Long Baseline Interferometry, or Satellite Laser Ranging.

Influence of the Size, Shape and Orientation of the Earthquake Source Area in the Shumagin Seismic Gap on the Resulting Tsunami

Abstract The directional properties of tsunamis generated by circular and elongated sources are being studied by means of maximum amplitude contours. For the tsunami source located in the Gulf of Alaska along the Aleutian Islands, the main lobe of energy is directed towards the south and southeast. The time dependent tsunami signal over the shelf and at the open ocean depicts a considerable difference, probably a result of the shelf resonance. Coriolis effects are only noticeable over the shelf regions where the longer period tsunami waves are generated.

Strain accumulation in the Shumagin seismic gap, Alaska : Savage, J C; Lisowski, M J Geophys Res V91, NB7, 10 June 1986, P7447–7454

Strain accumulation in the Shumagin seismic gap, Alaska : Savage, J C; Lisowski, M J Geophys Res V91, NB7, 10 June 1986, P7447–7454

A densely spaced array of sea level monitors for the detection of vertical crustal deformation in the Shumagin Seismic Gap, Alaska

We have installed a network of sea level gauges with ∼40 km spacing in the Shumagin Islands in order to detect relative vertical motion, in particular, possible crustal motion precursory to an expected major earthquake. This required the development and deployment of a pressure sensor sea level gauge suitable for installation on remote and harsh coastlines. Data are collected in near‐real‐time via satellite, both in order to exploit fully any precursors that may be observed and to provide continuous information on the status of the instruments. Using Wiener filtering techniques, we have determined conservatively that no relative vertical crustal motion &gt;0.1 m between stations has occurred during 1981–1985. This is consistent with independent geodetic leveling and trilateration data, though evidence exists for more rapid deformation during 1978–1980. The sea level data collected so far have an rms noise level of ∼0.04 m after processing; this is limited in part by the pressure sensor and associated electronics. Improved pressure gauges with better long‐term stability have recently been installed. A short‐baseline tiltmeter operating in a tunnel has demonstrated that suitably designed and located land‐based tiltmeters have a lower noise level, and hence better precursor detection characteristics than the sea level gauges, at monthly and shorter periods.

Strain accumulation in the Shumagin Seismic Gap, Alaska

Strain accumulation in the Shumagin seismic gap was estimated from repeated trilateration surveys made between 1980 and 1985. The measured rate of shear‐strain accumulation (0.01 ± 0.03 μrad/a) was an order of magnitude less than the rate (0.18 μrad/a) predicted for seismic subduction. The absence of measurable shear‐strain accumulation in the Shumagin Islands suggests that the main thrust zone there may not be locked but rather may slip stably. If this is indeed the case, the probability of a great, shallow thrust earthquake in the Shumagin gap is greatly diminished. A moderate earthquake (mb = 6.0) occurred at the seaward end of the trilateration network 4 months after the most recent survey. However, no significant anomalies that could be interpreted as precursors were detected in that survey.

Seismicity patterns (1963-1983) as stress indicators in the Shumagin seismic gap, Alaska

Earthquakes (1963 to 1983) of magnitude 5.0 to 6.5 form a rim of high seismic activity around a central region of relative seismic quiescence that coincides with the Shumagin seismic gap. This pattern, which can be inferred both from local network and teleseismic earthquake locations, shows high activity near the eastern and western ends of the seismic gap and along the down-dip end of the main thrust zone. The rim of seismicity surrounding an area of seismic quiescence may reflect strong couplign along the elastic-brittle part of the plate boundary where the great earthquakes occur. The temporal behavior of the microseismicity recorded by the Shumagin network is characterized by a burst of activity in 1978 and 1979, and a low or average level of activity from 1980 to January 1983. The increased microearthquake activity during 1978-1979 is located mainly near the down-dip end of the main thrust zone and along the Benioff zone down to 100 to 200 km depth. Composite fault plane solutions of earthquakes occurring in 1978-1979 show down-dip tension between 50 to 120 km depth in the upper plane of the Benioff zone. Composite fault plane solutions of 1981 earthquakes, however, indicate in-plate compression in the same region. Hence, the rate of occurrence and focal mechanisms of microearthquakes located in the Benioff zone below the main thrust both show coincident temporal and spatial variations that may reflect fluctuations in local stresses.