Tremor activity inhibited by well-drained conditions above a megathrust

Abstract

Tremor occurs on megathrusts under conditions of near-lithostatic pore-fluid pressures and extremely weakened shear strengths. Although metamorphic reactions in the slab liberate large amounts of fluids, the mechanism for enhancing pore-fluid pressures along the megathrust to near-lithostatic values remains poorly understood. Here we show anti-correlation between low-frequency earthquake (LFE) activity and properties that are markers of the degree of metamorphism above the megathrust, whereby LFEs occur beneath the unmetamorphosed overlying plate but are rare or limited below portions that are metamorphosed. The extent of metamorphism in the overlying plate is likely controlled by along-strike contrasts in permeability. Undrained conditions are required for pore-fluid pressures to be enhanced to near-lithostatic values and for shear strength to reduce sufficiently for LFE generation, whereas well-drained conditions reduce pore-fluid pressures at the megathrust and LFEs no longer occur at the somewhat strengthened megathrust. Our observations suggest that undrained conditions are a key factor for the genesis of LFEs.

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

“…Here we have focused on unresolved issues concerning (i) the long-recognized spatial anticorrelation between epicenters of tremor/low-frequency earthquakes and crustal earthquakes in northern Cascadia (e.g., Kao et al, 2009) and (ii) the mass-balance discrepancy that arises in explaining low fore-arc Poisson's ratios through silica precipitation of slab-derived fluids (Audet & B€ urgmann, 2014;Hyndman et al, 2015;Ramachandran & Hyndman, 2012). The anticorrelation of microseismicity and tremor observed in other locations (southwest Japan (Nakajima & Hasegawa, 2016); northern California Plourde et al, 2015); and eastern Alaska (Wech, 2016;Chuang et al, 2017)) and genetic associations between seismicity, fluids, and quartz vein networks suggest that the model detailed here may have broad relevance to warm convergent margin processes and the development of orogenic gold deposits. These slab-derived fluids may deposit minor silica but their primary effect is to promote metasomatism at greenschist and lower amphibolite facies within deforming continental crust.…”

Subduction zone megathrust earthquakes

“…Recent locations of LFEs in Nankai correlate variations in LFE location with Vp and Vs anomalies. Nakajima and Hasegawa (2016) found gaps in LFE occurrence in areas where they observed small Vp and Vs anomalies, indicative of a well-drained and low-porepressure megathrust; the LFEs are instead concentrated in areas of higher Vp and Vs anomalies, serving as proxies for higher-pore-fluid pressure.…”

Age of the Subducting Philippine Sea Slab and Mechanism of Low‐Frequency Earthquakes

“…A high fluid content has been suggested by studies of seismic tomography, which show that the LFEs occur in areas with a high Poisson's ratio, low Q , and low seismic velocity (Liu & Zhao, , ; Matsubara et al, ; Wang et al, ; Z. W. Wang et al, ). A persistent spatial gap of ~50 km wide in the LFE distribution is observed in the Kii channel (Obara, ), which is generally considered to be associated with a low fluid content (Aso et al, ; Liu & Zhao, ; Nakajima & Hasegawa, ; Z. W. Wang et al, ) and the lack of hydrous minerals (Seno & Yamasaki, ). These weak seismic events ( M < 2.5) have also been observed in other young (<50 Myr) and warm subduction zones (Figure S1 in the supporting information) (Beroza & Ide, ; Peng & Gomberg, ), such as the Cascadia (Rogers & Dragert, ), Central California (Nadeau & Dolenc, ), Mexico (Payero et al, ), and Costa Rica (Brown et al, ).…”