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

Abstract

The 17 July 2017 Komandorsky Islands M W 7.8 earthquake involved arc-parallel right-lateral patchy strike-slip faulting along~400 km of the Bering Fracture Zone (BFZ) in the westernmost Aleutian Islands back arc. The large size of the earthquake indicates that the BFZ serves regionally as the primary plate boundary extending from the Near Islands to Kamchatka, with the fore-arc Komandorsky Sliver translating rapidly parallel to the Aleutian Trench. The slip distribution is determined by analysis of seismic, tsunami, and geodetic observations. Fault displacements of 4 to 8.5 m, mostly in the upper 15 km, but with localized extension to 20 to 30 km depth along a~100 km long segment of the BFZ, are comparable to the possible slip deficit since the last major earthquakes in this region in 1849 and 1858, given an estimated 5.1 cm/yr rate between the Komandorsky Sliver and the Bering Plate. Plain Language Summary A large earthquake struck the westernmost portion of the Aleutian Island arc on 17 July 2017. In this region the Pacific plate is moving relative to the North American plate parallel to the plate boundary, with no convergence. As a result the plate motion is accommodated on a strike-slip fault in the upper plate, located along the Bering Fracture Zone. The earthquake produced slip and aftershocks along a 400 km long stretch of the Bering Fracture Zone in a magnitude 7.8 earthquake. The slip distribution and time history are determined by modeling seismic, geodetic, and tsunami data. The event is comparable in length and seismic moment to the 1906 San Francisco earthquake on the San Andreas Fault. Recent GPS measurements indicate motion of the KS almost parallel to the Pacific Plate motion (Figure 1b), at 5.1 cm/yr, about two thirds of the Pacific-North America rate (Kogan et al., 2017). Paleomagnetic observations (Minyuk & Stone, 2009) 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).

Aleutian island arc magma production rates and mechanisms

“…1), and it extends more than 3000 km from the Kamchatka Peninsula on the west to the Alaska on the east (LEWIS et al, 1988;Jicha et al, 2006;Liu et al, 2018;Dimalanta et al, 2002). The convergence rates and incoming plate ages decrease from west to east (Müller et al, 2008;DeMets et al, 2010;Lay et al, 2017). The formation of the Aleutian island arc is related to a series of plate tectonic events in the northern Pacific region (Jicha et al, 2006;Steven Holbrook et al, 1999 subducted northward, and a convergent boundary was formed between the Kula Plate and the North American Plate (Jicha et al, 2006;Liu et al, 2018).…”

“…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).…”

“…The subducted Pacific Plate has diverse thermal structures (Levin et al, 2005) and slab dips (Hayes et al, 2018). In addition, there are fracture zones and ridges near the Aleutian Trench (Singer et al, 2007;Scholl, 2007), as well as slab window due to the tearing of the Pacific Plate west of 173°E (Levin et al, 2005;Yogodzinski et al, 2001;Lay et al, 2017). Therefore, the Aleutian island arc is an ideal region for studying the island arc magma production rate and its potential influencing factors, including slab dips, fracture zones and ridges.…”

Scaling of two corner frequencies of source spectra for earthquakes of the Bering fault

“…Using the level of source displacement spectra, one can estimate the seismic moment, and based on seismic moment the moment magnitude [Kanamori, 1977;Hanks and Kanamori, 1979]. This traditional approach uses the theoretical model [Brune, 1970]: the amplitudes are determined based on the ray seismic, and spectral levels on the different rays are averaged, with focal-sphere-averaged radiation Figure 3.…”

“…Since one can expect high level of anisotropy around the Aleutian and Bering faults, we checked the existence of two separate corner frequencies by the spectral ratio method, which theory was proposed in [Yokoi and Irikura, 1991] and successfully implemented using observational data in [Imanishi and Ellsworth, 2006]. The key advantage of the method is that all uncertainties caused by media inhomogeneous and respective wave propagation can be neglected because we divide the spectra obtained for hypocenters closely located.…”