In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10-20 MPa) of the pressure pulse overwhelms the typical (0.1-0.2 MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Faults can slip seismically or aseismically depending on their hydromechanical properties, which can be measured in the laboratory. Here, we demonstrate that fault slip induced by fluid injection in a natural fault at the decametric scale is quantitatively consistent with fault slip and frictional properties measured in the laboratory. The increase in fluid pressure first induces accelerating aseismic creep and fault opening. As the fluid pressure increases further, friction becomes mainly rate strengthening, favoring aseismic slip. Our study reveals how coupling between fault slip and fluid flow promotes stable fault creep during fluid injection. Seismicity is most probably triggered indirectly by the fluid injection due to loading of nonpressurized fault patches by aseismic creep.

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 16Recent friction experiments carried out under upper crustal P-T conditions have shown that 17 microstructures typical of high temperature creep develop in the slip zone of experimental 18 faults. These mechanisms are more commonly thought to control aseismic viscous flow and 19 shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled 20 experiments have been performed on carbonate gouges at seismic slip rates (1 ms -1 ), to 21 investigate whether they may also control the frictional strength of seismic faults at the higher 22 strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and 23 temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain 24 size reduction in a thin slip zone (150 µm). With increasing displacement (up to 15 cm) and 25 2 temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms 26 start to accommodate intragranular strain in the slip zone, and play a key role in producing 27 nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of 28 weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates 29 exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low 30 dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding 31 (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those 32 predicted by theoretical studies, and those observed during experiments on metals and fine-33 grained carbonates, where superplastic behaviour has been inferred. To a first approximation, 34 the measured drop in strength is in agreement with our flow stress calculations, suggesting 35 that strain could be accommodated more efficiently by these mechanisms within the weaker 36 bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. 37Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for 38 the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation 39 in nature. 40

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Temporal changes in seismic velocity during the earthquake cycle have the potential to illuminate physical processes associated with fault weakening and connections between the range of fault slip behaviors including slow earthquakes, tremor and low frequency earthquakes1. Laboratory and theoretical studies predict changes in seismic velocity prior to earthquake failure2, however tectonic faults fail in a spectrum of modes and little is known about precursors for those modes3. Here we show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults. We systematically altered the stiffness of the loading system to reproduce the transition from slow to fast stick-slip and monitored ultrasonic wave speed during frictional sliding. We find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation. Our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure, which suggests that real time monitoring of active faults may be a means to detect earthquake precursors.

1] We present seismological evidence for the existence of an actively slipping low-angle normal fault (15°dip) located in the northern Apennines of Italy. During a temporary seismic experiment, we recorded $2000 earthquakes with M L 3.1.* The microseismicity defines a 500 to 1000 m thick fault zone that crosscuts the upper crust from 4 km down to 16 km depth. The fault coincides with the geometry and location of the Alto Tiberina Fault (ATF) as derived from geological observations and interpretation of depth-converted seismic reflection profiles. In the ATF hanging wall the seismicity distribution highlights minor synthetic and antithetic normal faults (4-5 km long) that sole into the detachment. The ATF-related seismicity shows a nearly constant rate of earthquake production, $3 events per day (M L 2.3), and a higher b value (1.06) with respect to the fault hanging wall (0.85) which is characterized by a higher rate of seismicity. In the ATF zone we also observe the presence of clusters of earthquakes occurring with relatively short time delays and rupturing the same fault patch. To explain movements on the ATF, oriented at high angles ($75°) to the maximum vertical principal stress, we suggest an interpretative model in which crustal extension along the fault is mostly accommodated by aseismic slip in velocity strengthening areas while microearthquakes occur in velocity weakening patches. We propose that these short-lived frictional instabilities are triggered by fluid overpressures related to the buildup of CO 2 -rich fluids as documented by boreholes in the footwall of the ATF. Citation: Chiaraluce, L., C. Chiarabba, C. Collettini, D. Piccinini, and M. Cocco (2007), Architecture and mechanics of an active low-angle normal fault:

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