From fault creep to slow and fast earthquakes in carbonates

Abstract

A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L'Aquila M w 6.1 2009 and Norcia M w 6.5 2016 in Central Italy. Surprisingly, within this region, fast (≈3km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite of its relevance for seismic hazard studies, the transitions from fault creep to slow and fast seismic rupture propagation are still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures, are obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100 o C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of regular seismicity in Central Italy. The transition from slow to fast rupture is explained by the increase of the ambient temperature, which enhances the elastic loading stiffness of the fault and consequently the slip velocity during the nucleation stage, allowing flash weakening. The activation of such weakening induces the propagation of fast ruptures radiating intense high frequency seismic waves.

Initial effective stress controls the nature of earthquakes

“…Note that the strain gages data allow to record slip front velocity up to a maximum of 180 m/s. For faster events, we used acoustic records to track the rupture front velocity, as used in previous studies 39 – 41 .…”

Structural Complexity and Mechanics of a Shallow Crustal Seismogenic Source (Vado di Corno Fault Zone, Italy)

“…These evidences are mainly concentrated within the fault core cataclastic units (CU1 and CU2) and the high‐strain damage zone (HSDZ) which are characterized by in situ shattered fault rocks and discrete fault slip zones with extreme shear strain localization (Figure 6; Demurtas et al, 2016). These slip zones display potential evidence of crystal‐plastic deformation mechanisms and fluid pressurization during fast coseismic slip propagation (e.g., mirror‐like slip surfaces sharply truncating dolostone clasts, intensely sheared calcite veins with calcite grains foam‐like texture, and cataclastites and utracataclasites with “fluidization textures”; Demurtas et al, 2019; De Paola et al, 2015; Fondriest et al, 2013; Siman‐Tov et al, 2013, 2015; Smeraglia et al, 2017; Smith et al, 2013, 2017; Passelegue et al, 2019).…”

Fault Stability Across the Seismogenic Zone

“…Because calcite is prone to deform plastically at room temperature under relatively moderate confining pressures (De Bresser & Spiers, 1997;Fredrich et al, 1989), carbonates can be used to investigate earthquake nucleation within the semibrittle regime. An extensive number of laboratory studies have experimented the transition from brittle to semibrittle rock deformation in intact carbonates (Fredrich et al, 1989;Nicolas et al, 2017;Schubnel et al, 2006;Vajdova et al, 2004Vajdova et al, , 2010Wong & Baud, 2012), carbonate fault gouges (Smith et al, 2015;Verberne et al, 2013Verberne et al, , 2014Verberne et al, , 2015, or carbonate saw cut samples (Harbord, 2018;Passelègue et al, 2019). Studies on intact samples highlighted a transition from localized to ductile (i.e., distributed) deformation with increasing confining pressure.…”

“…The recent discovery of a broad range of fault slip behaviors from slow to fast velocities (Obara & Kato, 2016; Passelègue et al, 2019; Peng & Gomberg, 2010) has shed a new light on the heterogeneous character of natural faults. Although observed, this spectrum of fault slips is not completely understood in terms of physical mechanisms.…”