Investigation of coupling between surface processes and induced flow in the lower continental crust as a cause of intraplate seismicity

Abstract

Many studies have highlighted the role of coupling between surface processes and flow in the lower continental crust in deforming the crust and creating topographic relief over Quaternary timescales. On the basis of the rheological knowledge gained, it is suggested that intraplate seismicity can also be caused by coupling between surface processes and flow in the lower continental crust. This view is shown to be a natural consequence of the modern idea that isostatic equilibrium is maintained by flow in the weak lower crust in response to erosion and sedimentation. It is supported by a general correlation between the vigour of surface processes and rates of intraplate seismicity, and by instances of seasonal seismicity that correlates with seasonal climate. Human interference in the environment can affect surface loading: for instance, deforestation for agriculture or urban development can cause increased erosion rates; global warming is expected to cause increased storminess (and thus increased erosion rates) and/or global sea-level rise. The possibility of increased rates of seismicity resulting from these processes should thus be considered in future hazard assessment.Surface processes as a cause of intraplate seismicity Figure 1. (a) Graph of estimated temperature T against depth z for the Palaeozoic continental crust beneath the Atlantic Coastal Plain (ACP) of South Carolina. Calculations use T , e.g., Lachenbruch, 1970), where T 0 is the mean surface temperature, K is the thermal conductivity of the crust, A 0 and D are the surface radiogenic heat production and its scale depth, and q 0 is the heat flow at the base of the lithosphere. T 0 is set to 20°C (cf. Bollinger et al., 1985). Values of K (3·29 W m −1°C−1 ) and A 0 (2·70 µW m −3 ) are from the borehole at Springfield, SC, ~150 km NW of Charleston. D = 8 km (typical for the ACP); q 0 is set to 48·5 mW m −2 (close to the 48·2 mW m −2 typical for the ACP) to give the observed surface heat flow at Springfield of 70·1 mW m −2 . All these geothermal data are from Costain et al. (1986). This calculated geotherm roughly matches the local ~15 km depth limit of seismicity (with 90 per cent of events shallower than 13 km), which is thought to represent thẽ 300°C isotherm (see, e.g., Bollinger et al., 1985), and indicates a Moho temperature (for 35 km depth) of 588°C. Extrapolated, it predicts the ~1400°C asthenosphere temperature at ~90 km depth, a conservative value given the ~80 km lithosphere thickness from seismic tomography (van der Lee, 2002). (b) Predicted variation of viscosity η with depth z across this lower crust, which from (a) is taken as spanning 15-35 km depth. η is calculated as η = η L exp(G/RT) (Westaway, 1998), where R is the molar gas constant (8·3143 J K −1 mol −1 ) and T(z) is the temperature predicted at each depth in (a). G (150 kJ mol −1 ) and η L (6·67 × 10 7 Pa s) parametrize the lower-crustal rheology (see Table I). Figure 2.Diagram illustrating an eroding region that provides the sediment source for an adjacent depocentre, with isost...

Climatically controlled river terrace staircases: A worldwide Quaternary phenomenon

“…Prediction uses W i =8 km, u=20 °C km -1 , κ=1.2 mm 2 s -1 , t o1 =18 Ma, ∆T e1 =-10.0 °C, t o2 =3.1 Ma, ∆T e2 =-11.2 °C, t o3 =0.9 Ma and ∆T e3 =-5.5 °C. Based on data from Westaway (2006b). Age constraint is poor but the low terrace probably dates from MIS 2.…”

“…The minimal role of fault movement in accommodating lateral variations in uplift generated by lower crustal flow has been discussed elsewhere (Westaway, 2006b). Examples are known where slip on intraplate active faults is superimposed onto regional uplift, but this results in minor warping of terraces rather than the formation of additional features (e.g.…”

The importance of characterizing uncertainty in controversial geoscience applications: induced seismicity associated with hydraulic fracturing for shale gas in northwest England

“…2 of Hornbach et al, 2015), the water load arising from this instance of wastewater injection having been concentrated over the hanging-wall of the basement normal fault, which has slipped in the induced seismicity in a normal sense. Since this basement normal fault is very steep (dip ~70°), from previous experience (e.g., Westaway, 2002Westaway, , 2006) the main effect of this load will be to increase the shear stress (in the sense conducive to normal faulting) across this fault, thus unclamping it. If this is the correct explanation for this instance of induced seismicity, a very simple mitigation measure is feasible, namely to switch the wastewater injection to the footwall and/or to balance it in future on both sides of the fault so no differential loading effect, which would act to unclamp the fault, can develop.…”

“…The mechanisms whereby human activities can affect seismicity have been widely discussed in recent years (e.g., Seeber, 2002;Westaway, 2002Westaway, , 2006Klose 2007aKlose ,b, 2013Ellsworth 2013;Rubinstein and Mahani, 2015; see below). Following Klose (2013), an 'anthropogenic earthquake' can be defined as any seismic event for which a human activity can reasonably be shown to be the cause, or at least a major influence on timing.…”

“…In this case, given the geometry envisaged in Fig. 1, with the fault and fracture network oblique and with the former approaching the end of the latter, based on previous experience (e.g., Westaway, 2002Westaway, , 2006 it can be envisaged that the opening of the fracture network will increase the component of right-lateral shear stress across the fault (itself conducive to 'unclamping' the fault, but favouring the opposite sense of slip to that observed; Fig. 5) but will also increase the normal stress across the fault (itself conducive to 'clamping' the fault).…”