Abstract
Decadal forerunning seismic activity is examined for very large, shallow earthquakes along strike-slip and intraplate faults of the world. It includes forerunning shocks of magnitude Mw ≥ 5.0 for 21 mainshocks of Mw 7.5 to 8.6 from 1989 to 2020. Much forerunning activity occurred at what are interpreted to be smaller asperities along the peripheries of the rupture zones of great mainshocks at transform faults and subduction zones. Several great asperities as ascertained from forerunning activity agree with the areas of high seismic slip as determined by others using geodetic, mapping of surface faulting, and finite-source seismic modeling. The zones of high slip in many great earthquakes were nearly quiescent beforehand and are identified as the sites of great asperities. Asperities are strong, well-coupled portions of plate interfaces. Different patterns of forerunning activity on time scales of up to 45 years are attributed to the sizes and spacing of asperities (or lack of). This permits at least some great asperities along transform faults to be mapped decades before they rupture in great shocks. Rupture zones of many great mainshocks along transform faults are bordered either along strike, at depth or regionally by zones of lower plate coupling including either fault creep forerunning activity, aftershocks and/or slow-slip events. Forerunning activity to transforms in continental areas is more widespread spatially than that adjacent to oceanic transforms. The parts of the San Andreas fault themselves that ruptured in great California earthquakes during 1812, 1857 and 1906 have been very quiet since 1920; moderate to large shocks have been concentrated on their peripheries. The intraplate shocks studied, however, exhibited few if any forerunning events, which is attributed to the short period of time studied compared to their repeat times. The detection of forerunning and precursory activities for various time scales should be sought on the peripheries of great asperities and not just along the major faults themselves. This paper compliments that on decadal forerunning activity to great and giant earthquakes along subduction zones.
Highlights
Great earthquakes have occurred at shallow depths along active transform faults and subduction zones and within the interiors of lithospheric plates
This permits at least some great asperities along transform faults to be mapped decades before they rupture in great shocks
Sykes [1] found that 69% of shallow earthquakes worldwide of Mw ≥ 7.7 were located at subduction zones, 7% involved normal-faulting and most of the rest occurred with strike-slip mechanisms either along transform plate boundaries or within lithospheric plates
Summary
Great earthquakes have occurred at shallow depths along active transform faults and subduction zones and within the interiors of lithospheric plates. They are important to understand since many have caused extensive destruction and loss of life. Emphasis is given to the spatial and in some cases the temporal patterns of activity of moderate to large forerunning events that preceded them in the previous decades. The rupture zones and forerunners of three older earthquakes in California and Guatemala are described It is surprising how little has been written about decadal preceding events, which I call forerunning earthquakes to distinguish them from foreshocks of shorter-time duration. The first aim of this paper is to describe the spatial distribution of forerunning events to a large number of mainshocks
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