Krabbenhoeft Anne, von Huene Roland, Miller John J, Lange Dietrich, Vera Felipe
GEOMAR Helmholtz Center for Ocean Research Kiel, Wischhofstr. 1-3, 24148, Kiel, Germany.
U.S. Geological Survey, Scientist Emeritus, 800 Blossom Hill Road, Los Gatos, CA, 95032, USA.
Sci Rep. 2018 Sep 12;8(1):13706. doi: 10.1038/s41598-018-32071-4.
Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 M 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and post-processes of the nearby great 1964 M 9.2 megathrust earthquake.
大洋岩石圈中的大型板内地震很少见，通常与大洋板块内的弥散变形区域有关。2018年1月23日发生在阿拉斯加湾的7.9级走滑地震，使科迪亚克岛近海的一个大洋断裂带系统发生破裂。测深图显示，由于覆盖大洋基底的沉积物较厚，断裂带的地形表现不明显，但断裂带系统可通过南北向磁异常的偏移和垂直重力梯度中的东西向线性带识别出来。全球地震台站的反演结果表明，初始破裂首先从震中向北传播，可能是沿着与洋壳构造平行的薄弱带破裂。随后破裂方向转向东，大部分能量在阿加断裂带释放，导致了一次不同寻常的多断层地震。同样，余震表现出复杂的活动特征，与两种不同的构造结构有关：（1）沿南北走向的洋壳构造发生的地震，主要为走滑破裂，此外还有正滑和斜滑机制；（2）沿东西向断裂带发生的走滑地震。为了解释这种复杂的断层活动行为，我们采用经典的应力和应变分配概念，提出了一个俯冲带附近沟斜向断裂带/大洋板块构造的大洋内大型走滑地震的广义模型。考虑到1964年最大余滑的科迪亚克凹凸体位置和外隆库仑应力分布，我们认为2018年阿拉斯加湾异常的矩释放是附近1964年9.2级巨大逆冲型地震的同震和震后过程中应力向进入的大洋板块转移的结果。
