Plate coupling and strain in the far western Aleutian arc modeled from GPS data

Abstract

The Aleutian and Kuril‐Kamchatka arcs meet at a triple junction of the Pacific (PAC), Bering (BER), and North American (NAM) plates. We invert GPS observations from the westernmost Aleutian (Komandorsky) Islands and Kamchatka for the fault locking depth and block motion in the far western Aleutian transform boundary. Three boundary models were considered: (1) only the Aleutian thrust fault without a trench‐normal component, (2) only a strike‐slip fault in the back arc north of the Komandorsky Islands, and (3) a rigid Komandorsky sliver bounded by the Aleutian and back‐arc faults. Observed velocities prefer Model 3, with a secular westward sliver velocity of 51 mm/a relative to NAM (two thirds of the total PAC‐NAM motion). The observed velocities are ~10% slower because of elastic strain from boundary faults. The best fitting locking depth of faults bounding the sliver is 12 km, which is similar to depths observed in diverse tectonic environments.

Rupture Along 400 km of the Bering Fracture Zone in the Komandorsky Islands Earthquake (MW 7.8) of 17 July 2017

“…Uncertainty in the mechanism, size, and slip location of these historic events limits our ability to evaluate whether the 2017 event is a recurrence event. However, the large size of the 2017 event, comparable to the 1906 San Francisco earthquake in both moment and length of rupture, establishes that the Bering Fracture Zone is serving as the likely primary plate boundary in the westernmost Aleutians, as proposed by Kogan et al ().…”

“…Recent GPS measurements indicate motion of the KS almost parallel to the Pacific Plate motion (Figure b), at 5.1 cm/yr, about two thirds of the Pacific‐North America rate (Kogan et al, ). Paleomagnetic observations (Minyuk & Stone, ) of the Komandorsky Islands (Bering and Medny) indicate localized internal clockwise rotation within the KS.…”

“…The upper plate in this region is the Bering Plate (or Bering Block) (e.g., Cross & Freymueller, 2008;Lander et al, 1994;Mackey et al, 1997), which has minor clockwise rotation relative to North America, so the Pacific-Bering relative motion is almost the same as the Pacific-North America motion. The BFZ is thus proposed to be the primary tectonic plate boundary in the westernmost Aleutians (Figure 1), with right-lateral strike-slip faulting (Kogan et al, 2017). About 2.7 cm/yr of relative motion between the Pacific Plate and the KS is accommodated either by strike-slip faulting on the shallow-dipping megathrust or by deformation on fore-arc faults (Geist & Scholl, 1994 foreshocks with m b ≥ 4 occurred within 12 h before the main shock near the hypocenter (Figure 2b), commencing with an M W 6.3 strike-slip event (strike 130°, dip 68°, rake À179°).…”

“…The possibility that the deeper slip actually involved horizontal shearing on the underlying megathrust is considered, but two-fault models assuming such a geometry do not provide significant trench-parallel slip on a shallow-dipping plane. The depth of the megathrust is~40 km below the seafloor at the BFZ, based on northwestward extrapolation from the Near Islands region of the slab contours for the Slab 1.0 model (Hayes, Wald, & Johnson, 2012), and using the 18°dip of the megathrust inferred by Kogan et al (2017). We vary the depth from 25 to 40 km, but the megathrust radiation pattern is not favorable for accounting for the deep slip in our single-fault models.…”

Aleutian island arc magma production rates and mechanisms

“…West of 173°E, the structure is more complex. There is a long strike-slip structure named the Bering Fracture Zone (BFZ), which is considered to be the main tectonic plate boundary of the westernmost end of the Aleutian islands (Lay et al, 2017;Kogan et al, 2017). The BFZ separates the Aleutian island arc from the Kamchatka Peninsula (Jonathan M. Lees et al, 2000;Lay et al, 2017;Cross and Freymueller, 2008;Chekhovich et al, 2014) and the Komandorsky Basin (Jonathan M. Lees et al, 2000).…”