Research ArticleGEOPHYSICS

Triggering an unexpected earthquake in an uncoupled subduction zone

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Science Advances  24 Mar 2021:
Vol. 7, no. 13, eabf7590
DOI: 10.1126/sciadv.abf7590
  • Fig. 1 Tectonic setting of the 2020 Alaska-Aleutian subduction zone earthquake sequence.

    Here, the Pacific (PA) plate subducts under the North American (NA) plate at 65 mm/year (45). The 10-km depth contours of the subducting slab are indicated by dashed lines (43). Circles and focal mechanisms represent earthquakes from the USGS Comprehensive Catalog, scaled by magnitude and colored by date. Red shaded areas are rupture zones of previous great megathrust earthquakes, inferred from aftershock zones. Geodetic interseismic coupling is indicated along the subduction trench (8,9). The Shumagin Gap area outlined by the dashed white line has little to no mechanical coupling but is pseudocoupled (hence its low partial geodetic coupling). The shaded blue area down-dip of the Shumagin Gap is the aftershock/afterslip zone of the 21 July earthquake. The dark orange dashed line indicates the expected extent of high pseudocoupling [~80% according to (21)] surrounding the coupled parts of the interface. The area of the 1946 earthquake appears geodetically uncoupled, so we indicate the uncertain plate interface behavior in this region with hatched fill. Cross section A-B shows earthquakes and their focal mechanisms projected in side view. These indicate the dominance of deeper thrust faulting plate interface events from July to October, followed by mostly strike-slip events along the top of the slab since 19 October.

  • Fig. 2 Modeled stresses in the slab near the Shumagin Gap before the 2020 earthquake sequence.

    (A) Coulomb stresses along the surface of the subducting slab, resolved onto trench-perpendicular, right-lateral, strike-slip faults. This model has two locked patches (indicated by black rectangles with cross-hatching) and a 100-km-long freely sliding zone between them. Red areas are where right-lateral slip is promoted, and blue areas are where that slip is inhibited. The stress magnitudes correspond to 82 years of interseismic loading. Arrows represent the displacements at the top of the slab relative to the displacement in the center of the locked zone and show the shearing that develops across the edges of the uncoupled gap. (B) Coulomb stresses along the surface of the subducting slab for a model with only one locked patch on the east side of the uncoupled zone. Without a locked zone in the west, the stresses in the slab under the uncoupled regions favor exclusively the right-lateral faulting.

  • Fig. 3 Modeled stress changes in the slab during the July to October 2020 earthquake sequence.

    (A) Coulomb stress changes produced by the July Mw 7.8 earthquake (slip area indicated by the rectangle) in the slab, resolved onto right-lateral strike-slip faults with the geometry of the October earthquake. If the target fault is steeply dipping (left), then the stress changes at the location of the October Mw 7.6 event (orange circle) are near zero to slightly negative. In contrast, if the target fault is more shallowly (50°) dipping (right), then the July event positively loads the Mw 7.6 hypocentral region. (B) For either target fault geometry, the subsequent afterslip and aftershocks down-dip of the Shumagin Gap load the Mw 7.6 hypocenter.

  • Fig. 4 Synoptic view of the plate tectonic setting and plate interface conditions in the Shumagin Gap region.

    (A) Coupling conditions since the mid-20th century earthquakes bounding the gap region. The asperity that ruptured in 1938 is assumed to have relocked, producing both fully coupled and pseudocoupled regions. The presence of a similar asperity on the west side of the Shumagin Gap associated with the 1946 rupture is less certain because geodetic observations indicate low coupling there. The region down-dip of the coupled zones (and extending into the Shumagin Gap) is a transition zone where coseismic slip, aftershocks, and postearthquake afterslip may occur. Within the Shumagin Gap, the transition from coupled to uncoupled next to the 1938 asperity favors right-lateral slip perpendicular to the trench. (B) Conditions associated with the 2020 earthquake sequence. The July Mw 7.8 event (blue) ruptured across the edge of the coupled zone. It produced aftershocks and afterslip in the vicinity of the earthquake and also within the afterslip region down-dip of both the coupled and uncoupled sections of the plate boundary. The coseismic slip, afterslip, and aftershock activity in this down-dip region increased the Coulomb stress within the slab favoring triggering of the October Mw 7.6 strike-slip event (orange).

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