Earthquake nucleation in the lower crust by local stress amplification

Abstract

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep.

High transient stress in the lower crust: Evidence from dry pseudotachylytes in granulites, Lofoten Archipelago, northern Norway

“…Another possibility to generate transiently high stresses has been presented by Hawemann et al (2019), who suggested that high stresses in the lower crust arise from the jostling of strong blocks within a shear zone network (Musgrave Ranges, Australia). This hypothesis was supported by observations from Nusfjord East in Lofoten (Campbell et al, 2020), where pseudotachylytes occur in blocks between granulite-facies shear zones. In this part of Lofoten, ∼15 km from the Nusfjord ridge, no shear zones have been observed so far.…”

“…Some of these pseudotachylytes were then mylonitized and developed into shear zones as described by Menegon et al (2017) for Nusfjord. Stress amplifications within blocks bounded by shear zones may subsequently have triggered the formation of a second generation of pseudotachylytes (Campbell et al, 2020).…”

Mechanisms of melt extraction during lower crustal partial melting

“…There has been some recent interest in the likelihood of transient pulses of high stress in the middle to lower crust (Campbell & Menegon, 2019; Campbell, Menegon, Fagereng & Pennacchioni, 2020; Hawemann, Mancktelow, Pennacchioni, Wex & Camacho, 2019; Jamvelt, Ben‐Zion, Renard & Austrheim, ; Trepmann & Stockhert, 2003). While some of these may be associated with localized melting, for example in fault‐related pseudotachylites, we do not consider that they have wide application to large‐scale melting events, and so have not considered them here.…”

A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: The South Malawi Active Fault Database

“…and White, 1989). Finally, we note that though the lower crust in southern Malawi may be laterally heterogenous with localised zones of viscously deforming material (Fagereng, 2013;Hellebrekers et al, 2019;Wedmore et al, 2020a), there is geological evidence from exhumed metamorphic terranes that in dry lower crust, earthquakes may both nucleate and propagate within a predominantly viscous regime (Campbell et al, 2020;Menegon et al, 2017).…”