THE SEISMIC CYCLE IN CARBONATE-BEARING FAULTS

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1 THE SEISMIC CYCLE IN CARBONATE-BEARING FAULTS PhD candidate: MICHELE FONDRIEST, II course Tutor: Prof. GIULIO DI TORO Co-Tutors: PhD STEVEN SMITH, Prof. DARIO ZAMPIERI Cycle: XXVI Abstract The study of exhumed fault zones allows to characterize the structure of potential seismic sources and earthquake-related processes. For instance, it is not clear if mirror-like fault surfaces are produced during seismic ruptures. The Foiana Line (FL) is a major sinistral transpressive fault cutting dolostones in the Italian Southern Alps. The fault zone is m wide and consists of pulverized dolostones cut dense networks of faults with mirror-like surfaces. Slow- to high-velocity rotary shear experiments performed on dolomite gouges produced mirror-like slip surfaces only at seismic slip rates (v 0.1 m/s) at applied normal stresses and displacements consistent with those estimated for the FL. Both distribution of the pulverized dolostones and kinematics of the mirror-like slip surfaces along the FL compare favourably with results of 3D rupture simulations. Therefore the association of pulverized dolostones and mirror-like slip surfaces might represent a potential indicator of seismic rupture propagation. Introduction Fault zones cutting carbonate rocks represent significant seismogenic sources in the Mediterranean area, including Italy, where most earthquakes nucleate within (aftershocks surely do) and propagate through upper crustal sedimentary sequences dominated by limestones and dolostones (e.g. Friuli (Italian Fore-Alps) earthquake, 1976, M w = 6.4; L Aquila (Central Apennines), 2009, M w = 6.1). The seismogenic behavior of a fault strongly depends on fault zone internal structure and fault rock constitutive properties (Scholz, 2002). In the last few years increasingly high-resolution seismological techniques (e.g. high-precision hypocenters relocation, seismic tomography) yielded new constraints on the broad geometry of active seismogenic fault zones (Chiaraluce et al., 2011b; Di Stefano et al., 2011). Further information on the coseismic off-fault damage distribution are retrieved from 3D numerical simulations of seismic ruptures (e.g. Ma and Andrews, 2010). Nevertheless the study of exhumed fault zones remains the best tool to characterize the geometry of the faults (e.g. fault roughness) at scales relevant for seismic rupture dynamics. At the same time the study of natural and experimentally-produced fault rocks is the only method to gain direct information about the physical and chemical processes occurring on faults during the seismic cycle, which are of paramount importance in understanding earthquake source physics (i.e. coseismic shear stress, earthquake energy budgets, dynamic fault weakening mechanisms and slip rate variability; Kanamori and Brodsky, 2004). This PhD project deals with the investigation of earthquake mechanics in carbonate host rocks, more specifically dolostones, by means of a multidisciplinary approach including: field description of dolostone-cutting fault zones exhumed from seismogenic depths rock mechanics experiments: (i) slow- to high-velocity rotary shear friction experiments; (ii) split Hopkinson bar tests (i.e. pulverization experiments ) mineralogical and microstructural characterization of natural and experimental fault materials. The principal aims of the project are (i) to characterize the internal structure of ancient seismic faults in dolostones, (ii) to recognize mineralogical and microstructural indicators of seismicity (e.g. coseismic slip and seismic rupture propagation), and (iii) to estimate several earthquake source parameters. The Foiana Line (FL) is a dolostone-cutting fault which has been investigated during the second year of my PhD. Detailed aerial and field structural survey highlighted the presence of significant volumes of pulverized dolostones along the FL. Such dolostones are cut by dense networks of faults with mirror-like (light reflective) slip surfaces. Slow- to high-velocity rotary shear friction experiments performed on dolomite gouge demonstrate that the mirror-like slip surfaces develop only at seismic slip rates and for applied normal stresses and displacements consistent with those estimated on the natural faults. At these 1

2 experimental conditions the frictional power density dissipated on the samples is comparable to that of natural earthquakes. Both spatial distribution of the pulverized dolostones and kinematics of the mirrorlike slip surfaces along the FL compare favorably with off-fault damage distribution predicted by 3D rupture simulations along strike-slip faults. These field and experimental observations suggest that the association of pulverized dolostones and mirror-like slip surfaces might record the propagation of seismic ruptures along the FL. Methods Field work along the FL included systematic rock samples collection along structural profiles oriented parallel and perpendicular to the strike of the main faults. In the southern portion of the FL an aerial survey using an unmanned aerial vehicle (UAV; collaboration with Chartagena Srl) was also performed. The produced geo-referenced photomosaic was used for detailed structural mapping of the fault zone over an area of ~ 6*10 4 m 2 with the aim to quantify the damage distribution within dolostones. Rock friction experiments were performed with SHIVA (Slow- to High Velocity Apparatus, INGV, Rome), a rotary shear apparatus which allows to deform gouges at conditions typical of seismic slip. Tests were performed on 3 mm thick layers of dolomite gouge (grain size < 250 µm) from the FL outcrops using a purpose-built gouge sample holder with metal confinement. The latter allows to impose normal stresses up to 30 MPa: the apparatus installed in Padua can apply normal stress up to 5 MPa on gouges. Oriented thin sections were cut for both natural and experimental fault rocks. Microstructures were studied by optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM). Mineral composition was determined by semi-quantitative X-ray powder diffraction (XRPD) whereas elemental composition was measured by wavelength dispersion spectroscopy (WDS) at the electron microprobe (EMP). Pulverized dolostones along the FL The Foiana Line (FL) is a major NNE-SSW-trending sinistral transpressive fault cutting sedimentary Triassic dolostones in the Italian Southern Alps. The FL has a cumulative vertical throw that reduces toward its southern termination from ~ 1.5 km to ~ 0.3 km. The fault zone is m wide and is exposed for ~ 10 km along strike within several outcrops exhumed from increasing depths from the south (1 km) to the north (2.5 km). The outcrops are characterized by shattered dolostones affected by pervasive extensional fracturing with rock particles up to few millimeters in size. Shattered dolostones are locally cut by dense networks of 1-20 m in length faults with mirror-like slip surfaces (wavelength of surface roughness < 1 µm). Microstructural investigations of the shattered dolostones beneath the mirror-like slip surfaces show: 1) lack of significant shear strain (even at a few micrometers from the slip surface), 2) extensional fracturing down to the micrometer scale, 3) exploded clasts with radial fractures, and 4) chains of split clasts (resulting from force chains) oriented at degrees to the slip surfaces. Similar features have been reported in natural and experimental pulverized rocks, which are considered to be produced by the propagation of seismic ruptures at shallow depth (Dor et al. 2006a; Yuan et al., 2011). The southern portion of the FL is characterized by a NE-SW-trending fault bend bounding large exposures (tens to hundreds of meters) of pulverized dolostones resulting in badlands topography over an area of ~ 6*10 4 m 2. Mirror-like slip surfaces have a dominant ENE-WSW strike, dip-slip reverse kinematics and displacements ranging between 0.04 m and 0.5 m. Resolved normal stress on the slip surfaces is estimated in the range MPa at hydrostatic pore pressures. The northern portion of the FL consists of NNE-SSW-trending sub-parallel fault strands spaced 2-5 m apart, surrounded by 2-3 m thick bands of pulverized dolostones. The fault strands are characterized by smooth to mirror-like sub-vertical slip surfaces with dominant strike-slip kinematics. Overall, deformation is more localized moving from South to North along the FL. 3-Dimensional rupture simulations along strike-slip faults predict (1) off-fault damage distributions with flower-like shapes (broad damage zone near the surface that rapidly narrows with increasing confining pressure, e.g., Ma and Andrews, 2010) and (2) formation of secondary faults/fractures with disperse attitudes and kinematics (reverse to normal) near the surface, with strike-slip at depth. 2

3 Qualitatively, these theoretical results compare favorably with increasing strain localization and a switch in fault kinematics recognized along the FL moving from the southern, shallower portion of the fault zone to the northern, deeper portion. Mirror-like slip surfaces in dolostone: a record of seismic slip To understand how the mirror-like slip surfaces formed, slow- to high-velocity rotary shear friction experiments were performed on dolomite gouge using SHIVA (INGV, Rome) at conditions approaching those estimated for the FL. Tests were conducted at slip velocities of m/s, normal stresses up to 26 MPa and displacements in the range m. At seismic slip rates of 1 m/s the dolomite gouge shows a dramatic reduction of the friction coefficient (µ) from a peak value of ~ 0.7 to a steady-state value of ~ The gouge starts to weaken above a threshold velocity in the range m/s after a transient phase of strengthening. During the tests the instantaneous power density (shear stress*slip rate) dissipated on the sample reaches values of 6-10 MW/m 2 over distances of m. At 26 MPa of normal stress weakening is linked to development of a mirror-like slip surface after only 0.07 m of slip. At intermediate slip rates (0.1 m/s) only moderate reductions in µ are observed. Instantaneous power density is ~ 1 MW/m 2 and the mirror-like slip surface starts to develop after 0.1 m of slip. At sub-seismic slip rates ( m/s) µ remains ~ 0.7, instantaneous power density is ~ 0.02 MW/m 2, and no mirror-like slip surface are formed. Experimental results demonstrate that mirror-like slip surfaces are developed when the frictional power density instantaneously dissipated on the samples exceeds a threshold value of ~1 MW/m 2 (i.e. slip rate 0.1 m/s for the applied normal stresses). However, the well-developed mirror-like slip surfaces observed along the FL have measured displacements as small as 0.04 m. At similar experimental displacements mirror-like slip surfaces were formed only when the slip velocity was ~ 1 m/s and the instantaneous power density approached values typical of natural earthquakes ( MW/m 2 ). Microstructural observations suggest that experimental slip zones contain a compacted layer up to 20 µm thick immediately beneath the mirror-like slip surface in which deformation is strongly localized. The layer consists of partially-welded dolomite clasts µm in size and larger clasts that are sharply truncated by the mirror-like slip surfaces. Chemical analyses recognized small (< 100 µm of length), discontinuous patches of periclase and Mg-calcite nanoparticles (i.e. dolomite decomposition products) lining some of the mirror-like slip surfaces. Natural slip zones from the FL can be well compared to the experimental ones since they are characterized by lack of significant shear strain, grain compaction and truncation of larger clast approaching the slip surfaces. The combination of such observations indicate that small-displacement mirror-like slip surfaces in dolostones represent a geological marker of extreme frictional power dissipation during seismic slip. Ongoing and future work on the FL Ongoing and future work on the FL will focus: (i) detailed structural mapping of pulverized dolostones along the FL; (ii) comparison between natural and experimentally deformed (with SHIVA) dolomite gouges; (iii) microstructural evolution with displacement of dolomite gouges deformed at seismic slip rates; (iv) roughness measurements of natural and experimental mirror-like slip surfaces using White Ligth Interferometry (WLI, collaboration with Thibault Candela); (v) split Hopkinson bar tests ( pulverization experiments ) on dolostones. The last two points should allow to respectively constrain: (iv) the relationship between mirror-like slip surfaces and frictional power dissipation during seismic slip, and (v) the deforming conditions (pressure, strain, strain rate etc.) of pulverized dolostones. References CHIARALUCE, L., VALOROSO, L., PICCININI, D., DI STEFANO, R. and DE GORI, P. 2011b. The anatomy of the 2009 L Aquila normal fault system (central Italy) imaged by high resolution foreshock and aftershock locations. Journal Geophys. Res., 116, doi: /2011jb DI STEFANO, R., CHIARABBA, C., CHIARALUCE, L., COCCO, M., DE GORI, P., PICCININI, D. and VALOROSO, L Fault properties heterogeneity affecting the rupture evolution of the 2009 (M w 3

4 6.1) L Aquila earthquake (Central Italy): insights from seismic tomography. Geophys. Res. Let., 38, doi: /2011gl DOR, O., BEN-ZION, Y., ROCKWELL, T.K., BRUNE, J., 2006a. Pulverized rocks in the Mojave section of the San Andreas Fault Zone. Earth and Planetary Science Letters, 245, KANAMORI, H. and BRODSKY, E The physics of earthquakes. Rep. Prog. Phys., 67, MA, S. and ANDREWS, D. J Inelastic off fault response and three dimensional dynamics of earthquake rupture on a strike slip fault. Journal Geophys. Res., 115, doi: /2009JB SCHOLZ, C. H The Mechanics of Earthquakes and Faulting. Cambridge Univ. Press, New York, ed. 2. YUAN, F., PRAKASH, V. and TULLIS, T., Origin of pulverized rocks during earthquake fault rupture. Journal of Geophysical Research, doi: /2010jb

5 SUMMARY OF ACTIVITY IN THIS YEAR Courses: C. PASSCHIER: Microtectonics course, March 2012, Johannes Gutemberg Universität, Institut fϋr Geowissenschaften, Mainz, Germany. G. PENNACCHIONI, N. MANCKTELOW, S. F. COX, L. MENEGON, D. SCHMID, M. STIPP and C. TREPMANN: Structural analysis of crystalline rocks, Second EGU Summer School, August 2012, Nevessee area, South Tyrol, Italy. R. J. ANGEL: Scientific Communication in English, October-November 2012, Università degli Studi di Padova, Padua, Italy. Communications: DI TORO, G., NIELSEN, S. B., SPAGNUOLO, E., SMITH, S. A. F., VIOLAY, M., FONDRIEST, M., Keynote Talk. Friction during earthquakes from rock deformation experiments, ECGS Workshop 2012, Earthquake source physics at various scales, October 2012, Luxembourg. FONDRIEST, M*., SMITH, S. A. F., DI TORO, G., NIELSEN, S. B., Mirror-like slip surfaces in dolostone: natural and experimental constraints on a potential seismic marker, GIGS Annual Meeting 2012, October 2012, Modena, Italy. FONDRIEST, M., SMITH, S. A. F., DI TORO, G., NIELSEN, S. B., Mirror-like slip surfaces in dolostone: natural and experimental constraints on a potential seismic marker, to be presented to AGU Fall Meeting 2012, December 2012, San Francisco, USA. DI TORO, G., SMITH, S. A. F., FONDRIEST, M., BISTACCHI, A., NIELSEN, S. B., MITCHELL, T., MITTEMPERGHER, S., GRIFFITH, A., Invited Talk. Is the fault core-damage zone model representative of seismogenic faults? Pre-existing anisotropies and fault zone complexity, to be presented to AGU Fall Meeting 2012, December 2012, San Francisco, USA. SMITH, S. A. F., DI TORO, G., NIELSEN, S. A. F., FONDRIEST, M., Invited Talk. Field and experimental constraints on seismic localization in granular fault gouge, to be presented to AGU Fall Meeting 2012, December 2012, San Francisco, USA. Posters: DI TORO, G., FONDRIEST, M., SMITH, S. A. F., ARETUSINI, S., Structure of a seismogenic fault zone in dolostones: the Foiana Line (Italian Southern Alps), to be presented to AGU Fall Meeting 2012, December 2012, San Francisco, USA. Publications: FONDRIEST, M., SMITH, S. A. F., DI TORO, G., ZAMPIERI, D. and MITTEMPERGHER S., Fault Zone Structure and Seismic Slip Localization in Dolostones, an example from the Southern Alps, Italy. Journal of Structural Geology, in press. FONDRIEST, M., SMITH, S. A. F., DI TORO,G., NIELSEN, S. B., CANDELA, T. and MAIR, K., Mirror-like slip surfaces and frictional power dissipation during earthquakes, in preparation. Teaching activities: Teaching assistant: 25 hours, Complementi di Geologia, Laurea Triennale in Scienze Geologiche (2011/2012). Other: This PhD project is funded by the European Research Council Starting Grant Project (USEMS) No Full title of the project: Uncovering the Secrets of an Earthquake: Multidisciplinary Study of Physico-Chemical Processes During the Seismic Cycle. Principal investigator: Giulio Di Toro. *Grants and Awards: Premio miglior presentazione orale giovani non strutturati, GIGS Annual Meeting 2012, October 2012, Modena, Italy. 5