Slow rupture of an aseismic fault in a seismogenic region of Central Italy

Abstract

Slow earthquakes and afterslips prove that the Earth does not have just two response time scales, i.e. that of tectonic loading and that of regular earthquakes. A swarm of slow earthquakes, with time constants of the order of hundreds of seconds, has been detected by a laser interferometer below the Gran Sasso massif (Italy). We analyse and model these observations to identify a very plausible source in a local fault, with no historic seismic behavior. While slow earthquakes occurring in subduction zones, and at the transition between locked and stably sliding segments of the San Andreas fault, are often associated with seismic events, in the case of the Apennines there is no correlation between local seismicity and slow earthquakes. Slow earthquakes, therefore, may also represent a specific failure behavior for a seismically locked fault, adding further complexity to the interpretation of geologic data for seismic hazard estimates.

Fault slip controlled by gouge rheology: a model for slow earthquakes

“…Because no far‐field wavelet is observed in the recorded strain signals, their source must be close to the interferometer (see fig. 2 in Amoruso et al 2002).…”

“…A detailed analysis of recorded strain signals, independent of the dynamic modelling described here, allows to retrieve the main features of the sources of the slow events (Amoruso et al 2002). In particular, left‐end fault slip U is approximately 4 cm and fault length L for t s ≃ 100 s is approximately 220 m. Diffusion time along the fault for L ≈ 200 m is thus approximately 1000 s, giving, as previously mentioned, ν∼ 50 m 2 s −1 .…”

“…in which ν = η/ρ is the kinematic viscosity. Because d ≤ 1 m from geology and L ≥ 100 m from observations (Section 3 and Amoruso et al 2002), it follows τ τ . Velocity diffusion along the y-axis is thus much faster than diffusion along the x-axis.…”

The geodetic laser interferometers at Gran Sasso, Italy: Recent modifications and correction for local effects

“…In their previous configuration, the laser interferometers at Gran Sasso already produced interesting results as regards slow earthquakes (Crescentini et al 1999;Amoruso et al, 2002) and free oscillations of the Earth (Park et al 2008). However, their electro-optical set-up has been recently modified with respect to the original version to reduce costs and improve reliability.…”

“…Among local phenomena, we can mention single-site measurement of seismic phase velocities (comparison of straingrams and seismograms recorded during the transit of seismic waves provide information on wave characteristics which cannot be derived from observations with either instrument alone; in particular, it is possible to determine the phase velocity of the seismic wave by taking ratio of both records, e. g. Sacks et al, 1976), slow earthquakes (discontinuous, earthquakelike events that release energy over a period much longer than usual earthquakes, e. g. Crescentini et al, 1999;Amoruso et al, 2002), deformation induced by seasonal charging and discharging of the local or regional aquifer. Global phenomena include the study of the free oscillations of the Earth (the capability of strainmeters of observing fundamental toroidal modes is much higher than that of horizontal seismometers, e. g. Park et al, 2008) and strain tides (as regards the diurnal resonance due to the free core nutation, see e. g. Mukai et al, 2004).…”