Nucleation process of the 2011 northern Nagano earthquake from nearby seismic observations

Abstract

The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely ‘modulated’ by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale.

The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

“…D ≈ 12-40 m 2 s -1 is well within the range suggested by Shapiro et al (2003) of 10 −2 to 10 −1 m 2 s -1 for crystalline rocks to 10 2 m 2 s -1 for a recently ruptured subduction megathrust fault. Observations of shaking induced swarm activity similarly indicates D in the order of 25-100 m 2 s -1 (Shimojo et al 2021).…”

“…But dynamic stresses act immediately (e.g. Shimojo et al 2021) or with a delay of no more than hours (Peña Castro et al 2019) and do not provide an explanation for the multimonth delays between the events.…”

“…Fluid triggering of the Muji earthquake may also account for the near-simultaneous rupture of both slip patches (Bie et al 2018). The swarm-like normal faulting sequence D, overlapping with the Sarez earthquake sequence, may have been initiated by dynamic perturbation of the hydraulic system through transient stresses from strong shaking, as has frequently been observed (Manga et al 2012;Shimojo et al 2021). The progression of a fluid front with time may be described by the square-root envelope-function of eq.…”

“…S8). Comparable earthquake migration velocities have been interpreted as a signature of propagating slow slip after a large earthquake (Kato et al 2016) or inside earthquake swarms (Roland & McGuire 2009;Shimojo et al 2021). Later aftershocks were concentrated at the northern end of the rupture (Fig.…”