Relations between earthquake distributions, geological history, tectonics and rheology on the continents

Abstract

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400–600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record'.

Variations in the Seismogenic Thickness of East Africa

“…The transition in seismogenic thickness in northern Tanzania, as with those observed at larger scale across the rest of East Africa, and globally (Craig et al, 2011;Jackson et al, 2021;Sloan et al, 2011), is coincident with an increase in lithospheric thickness (see Figure 4) imaged using both geophysical and petrological data, and therefore coincides with a change in the thermal structure on the same scale. We therefore assess whether the change in temperature structure is itself sufficient to account for the observed change in seismogenic thickness.…”

Understanding earthquakes using the geological record: an introduction

“…For example, the recognition of transient and mostly aseismic slip in some parts of subduction zones being coupled to field-based analysis of mixed-lithology fault zones has resulted in a new understanding of the mechanisms for generating transient fault slip at velocities intermediate between earthquakes and plate motions [1][2][3][4][5]. Similarly, observations of earthquake faulting in the lower continental crust made both petrologically (preserved as pseudotachylytes) and seismologically has resulted in a new appreciation of the diversity of rheology in the continental crust, and the importance of trace amounts of water in controlling the deformation [6][7][8][9][10]. In February 2020, the Royal Society hosted a Hooke discussion meeting to further explore links between geological and geophysical methods of studying faulting, titled 'Understanding earthquakes using the geological record'.…”

“…Dunkel et al [8], Jackson et al [9] and Menegon et al [10] investigate the links between hydration state, temperature, deformation mechanism and rheology in the continental crust. At the present day, most regions where continental earthquakes occur in the mid to lower crust (e.g.…”

“…deeper than 25 km) are spatially associated with regions of thick lithosphere. However, the effect of the lithosphere thickness on the temperature structure is insufficient on its own to explain the depth of the earthquakes, and implies (by comparison to the oceanic lithosphere) that the continental lower crust in these regions is anhydrous [9]. This inference is consistent with observations from the geological record, where evidence of earthquake slip in the mid to lower crust recorded by the presence of pseudotachylytes is commonly associated with anhydrous metamorphic rocks [8,10], which have had melt (and therefore volatiles) extracted from them.…”

“…This inference is consistent with observations from the geological record, where evidence of earthquake slip in the mid to lower crust recorded by the presence of pseudotachylytes is commonly associated with anhydrous metamorphic rocks [8,10], which have had melt (and therefore volatiles) extracted from them. The resulting lateral variations in lithosphere hydration state, and therefore strength, can be seen to have had an important influence on the geological history of the continents [9].…”