Abstract
Silica precipitation is assumed to play a significant role in post-earthquake recovery of the mechanical and hydrological properties of seismogenic zones. However, the relationship between the widespread quartz veins around seismogenic zones and earthquake recurrence is poorly understood. Here we propose a novel model of quartz vein formation associated with fluid advection from host rocks and silica precipitation in a crack, in order to quantify the timescale of crack sealing. When applied to sets of extensional quartz veins around the Nobeoka Thrust of SW Japan, an ancient seismogenic splay fault, our model indicates that a fluid pressure drop of 10–25 MPa facilitates the formation of typical extensional quartz veins over a period of 6.6 × 100–5.6 × 101 years, and that 89%–100% of porosity is recovered within ~3 × 102 years. The former and latter sealing timescales correspond to the extensional stress period (~3 × 101 years) and the recurrence interval of megaearthquakes in the Nankai Trough (~3 × 102 years), respectively. We therefore suggest that silica precipitation in the accretionary wedge controls the recurrence interval of large earthquakes in subduction zones.
Highlights
Earthquake recurrence intervals are commonly used to predict the timescales of future earthquakes[1,2,3]
Silica is a dominant component of crustal rocks, and widespread quartz veins in subduction zones at seismogenic depths are taken as proof of significant fluid flow and silica precipitation[10,11,12]
No studies have quantitatively discussed the timescales of quartz vein formation and earthquake recurrence intervals within subduction zones
Summary
The Nobeoka Thrust is a major fault that bounds the northern and southern Shimanto belts of Kyushu, southwestern Japan[19] (Fig. 1a), traceable for >800 km in the Cretaceous–Neogene Shimanto Belt accretionary complex parallel to the modern Nankai Trough[19]. The stress inversion reveals that the extensional quartz veins formed under a normal faulting type stress regime, with the orientation of the minimum principal stress (σ3) axis being almost horizontal and trending roughly NNW−SSE in both the hanging wall and footwall[21] (Fig. 1b). Analyses of vitrinite reflectance and fluid inclusions reveal that vein formation occurs under both high P–T conditions (260–340 °C, 235–250 MPa) and low P–T conditions (140–250 °C and 150–190 MPa) in the hanging wall and footwall of the Nobeoka Thrust[19,28,29]. The normalized pore fluid pressure ratio, the lower bound of the maximum fluid pressure level[30], is ~0.95 for a tensile strength of 10 MPa21 This stress condition means that the tensile overpressure exceeds σ3 during the period of vein formation. The vein quartz grew on quartz grain surfaces in vein walls (Figs. 2c,d), and there is no evidence of repeated crack–seal events (i.e. inclusion bands[32])
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