No abstract

An important question of earthquake science is to what extent fluids such as water play an important role in modulating seismicity. While many models of earthquake rupture assume that fluids are critical for dynamic weakening during slip and evidence for fluids is well documented for shallow injection-induced or hydrothermally induced earthquakes, there has been limited conclusive evidence for the role of fluids for tectonic earthquakes in the middle-to-lower crust. Here, we provide evidence for fluid migration during the 2016 Meinong, Taiwan, aftershock sequence, occurring at 10-20 km depth within a classic fold-and-thrust belt. We find high V p /V s ratios, characteristic of highly fluid saturated regions, in the Meinong aftershock region and that the V p /V s ratios in the central aftershock region change with time during the aftershock sequence. The central aftershock sequence distinguishes itself from other aftershocks by having a swarm-like magnitude distribution and aftershock decay rate, as well as a slower migration rate that may be related to fluid diffusion. The estimated permeability (~3.8 × 10 −15 m 2) and temporal changes in V p /V s suggest that moderate earthquakes may be able to strongly affect permeabilities in the midcrust. The results also suggest that fluid processes play a critical role in regulating seismicity in a classic continental collision tectonic setting and may also have a role in modifying earthquake hazards more generally. Plain Language Summary A long-standing question is how fluids like water influence earthquakes. While the effect of fluids on earthquake rupture is widely observed for shallow injection-induced or hydrothermally induced earthquakes, we know relatively less about their effect on deep earthquakes. Here, we provide evidence for fluid movement during the 2016 Meinong aftershock sequence in southwestern Taiwan. The analysis shows that seismic waves traveling through the aftershock region change their speed during the aftershock sequence, which is strongly suggestive of fluid movement. An estimate of the speed of fluid migration also suggests that the midcrust of southwestern Taiwan can easily transport fluids. These results suggest that fluids may generally play an important role for deep crustal earthquakes and affect earthquake hazards.