Characterizing the Foreshock, Main Shock, and Aftershock Sequences of the Recent Major Earthquakes in Southern Alaska, 2016–2018

B G Bukchin,V G Kossobokov,A S Fomochkina,A K Nekrasova

doi:10.3389/feart.2020.584659

Abstract

For each of three major M ≥ 7.0 earthquakes (i.e., the January 24, 2016, M7.1 earthquake 86 km E of Old Iliamna; the January 23, 2018, M7.9 earthquake 280 km SE of Kodiak; and the November 30, 2018, M7.1 earthquake 14 km NNW of Anchorage, Alaska), the study considers characterization of the foreshock and aftershock sequences in terms of their variations and scaling properties, including the behavior of the control parameterηof the unified scaling law for earthquakes (USLE), along with a detailed analysis of the surface wave records for reconstruction of the source in the approximation of the second moments of the stress glut tensor to obtain integral estimation of its length, orientation, and development over time. The three major earthquakes at 600 km around Anchorage are, in fact, very different due to apparent complexity of earthquake flow dynamics in the orogenic corner of the Pacific and North America plate boundary. The USLE generalizes the classic Gutenberg-Richter relationship taking into account the self-similar scaling of the empirical distribution of earthquake epicenters. The study confirms the existence of the long-term periods of regional stability of the USLE control parameter that are interrupted by mid- or even short-term bursts of activity associated with major catastrophic events.

Highlights

The territories around Anchorage have recently experienced four major earthquakes (Figure 1) and associated seismic sequences which illustrate the complexity of the tectonic environment at the orogenic corner of the boundary between the Pacific and North America plates where an Eulerian plate model is not expected to be accurate (Bird, 2003)
It is notable that the corresponding distances are much shorter for M ≥ 4.0 foreshocks
The sizes and shapes of aftershocks’ time series are very different in Figure 5: for the 2016, M7.1 event, we observe a nearly straight line extending for about 50 km; for the January 20518, M7.9 event, the pattern of epicenters looks like a butterfly, FIGURE 6 | Values of the unified scaling law for earthquakes (USLE) control parameter η vs. earthquake origin time within angular distance of 1° from the epicenters of MW7.1 January 24, 2016 and November 30, 2018 and within 2.5° from the epicenter of MW7.9 January 23, 2018 main shocks and their averages per 50 events

Summary

Introduction

The territories around Anchorage have recently experienced four major earthquakes (Figure 1) and associated seismic sequences which illustrate the complexity of the tectonic environment at the orogenic corner of the boundary between the Pacific and North America plates where an Eulerian plate model is not expected to be accurate (Bird, 2003). The complexity of the megathrust is characterized with a multiple rupture of several segments of subducting Pacific plate, including the lateral transition faulting along the Yakutat block at the corner of the Pacific–North America plate boundary. That is why simple methods should be used to produce robust constrains to improve resolution of authoritative comprehensive determinations

Methods

Results

Conclusion