The April 2017 M_w6.5 Botswana Earthquake: An Intraplate Event Triggered by Deep Fluids

Abstract

Large earthquakes in stable continental regions remain puzzling as; unlike at plate boundaries, they do not result from the local buildup of strain driven by plate tectonics. The 2017 Mw6.5, Bostwana normal faulting earthquake occurred in a region devoid from recent tectonic activity and where present‐day deformation is negligible. The depth of the event (29 ± 4 km), in a felsic lower crust where ductile deformation is expected, likely requires a transient pulse of fluids from a deep source to activate brittle faulting. The mainshock was preceded by two foreshock swarm‐like sequences that may be further evidence for fluid movement in a critically loaded fault network. Contrary to plate boundary events, the Mw6.5 Botswana earthquake did not require prior localized stress or strain accumulation. We propose that the crust in stable continental regions, even long after the last tectonic episode, constitutes a reservoir of elastic stress that can be released episodically, for instance, as a result of deep fluid migration.

The Preseismic and Postseismic Phases of the 700‐km Deep M7.9 Bonin Islands Earthquake, Japan

“…The Waveform Matched‐Filter Technique is based on the computation of the similarity between a window that contains the template, A, and the continuous signals, B. To do so, we compute the coherency value (C) in the frequency domain (equation , Gardonio et al, ). The template window starts 2 s before the P wave arrival and lasts 10 s. Coherency is calculated by sliding the 10‐s template over the continuous signal in 2‐s increments.…”

Earthquake nucleation in the lower crust by local stress amplification

“…However, earthquakes also nucleate in the continental lower crust in mechanically strong lithologies [2][3][4] . Deep continental earthquakes tend to nucleate along intraplate faults, or faults cutting thick, cold cratons [5][6][7][8] . Earthquakes in continental interiors have resulted in significantly higher casualties than earthquakes at plate boundaries 9 .…”

“…% H 2 O 17 ) lower crustal rocks at depths ≥25-30 km require either transiently high differential stresses or local weakening mechanisms (e.g. high pore fluid pressure 5 ). One explanation for transient high stresses is the downward propagation of an earthquake rupture from the shallower seismogenic zone 4,18 .…”

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

“…are also close to the Zoetfontein fault whereas in the ODR, event F5 is near the Lecha fault and event F9 is relatively close to the Chobe fault. Foreshock activity in Figure 4 does not reveal any clustering around the mainshock, although according to [22], the mainshock was preceded by two foreshock swarm-like sequences.…”