rupture directivity of the 2011 mw 5 2 lorca earthquake spain 1 on may 11th 2011 a rather small earthquake caused nine fatalities in the city of lorca se spain we analyze seismograms from a dense network to characterize the source of this earthquake we estimate an oblique reverse faulting mechanism moment magnitude of 5 2 and a shallow hypocenter 4 6 km at only 5 5 km epicentral distance from the city center double difference relocations yield a 5 km long ne sw trending distribution of aftershocks sw of the mainshock suggesting a sw propagating rupture along the alhama de murcia fault we use the mw 4 6 foreshock and an mw 3 9 aftershock as empirical greens functions to estimate apparent source time functions observing a clear directivity effect we model apparent durations with a unilateral and asymmetric bilateral rupture in both cases obtaining rupture directivity of n220 e towards lorca in addition to the near epicenter and shallow depth directivity may have contributed to the significant impact

stuck in the mud earthquake nucleation and propagation through accretionary forearcs 1 subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis this requires the earthquake to rupture through clay rich sediments of the accretionary wedge which are largely aseismic as found previously the frictional properties of a range of wet clays at low slip velocity are velocity strengthening thus inhibiting earthquake nucleation however at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy we interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge the lack of fracture energy can explain how a large rupture propagating from depth might not be arrested by clay rich velocity strengthening sediments as is commonly seen the results suggest that generally earthquakes may be difficult to nucleate on mature faults dominated by clay but the propagation of earthquakes through these zones is energetically very favourable

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

3d modeling of the cycle of a great tohoku oki earthquake considering frictional behavior at low to high slip velocities 1 we perform 3d quasi dynamic modeling of the cycle of a megathrust earthquake in the offshore tohoku region japan using a rate and state dependent friction law with two state variables that exhibits strong velocity weakening at high slip velocities we set several asperities where velocity weakening occurs at low to intermediate slip velocities outside of the asperities velocity strengthening occurs at low to intermediate slip velocities at high slip velocities strong velocity weakening with large displacements occurs both within and outside of the asperities the rupture of asperities occurs at intervals of several tens of years whereas megathrust events occur at much longer intervals several hundred years megathrust slips occur even in regions where velocity strengthening occurs at low to intermediate slip velocities but where velocity weakening is dominant at high slip velocities the proposed model explains that megathrust earthquakes occur in the same subduction zone as large thrust earthquakes

frictional sliding of dolomite a variation in constitutive behavior fault slip occurs via earthquakes or stable creep depending on the manner in which frictional resistance varies with sliding velocity and displacement if the form of this dependence is a function of rock type observed spatial variations in slip behavior along faults could be explained by lithologic variation we have used a rotary shear apparatus to investigate the frictional response of dolomite marble to changes in sliding velocity we have modeled the results using a state variable constitutive law for rock friction and a numerical method to solve for the interaction between the friction law and the compliance of our machine dolomite marble displays steady state velocity strengthening as contrasted with granite and quartzite which show velocity weakening at the same conditions to model the dolomite results it is necessary to use two state variables and surprisingly the parameters specifying the decay magnitudes associated with these state variables differ in sign from each other the magnitude of the parameter associated with the longer decay is negative leading to a downward decay of frictional strength associated with a decrease in velocity if the shorter decay is of sufficient magnitude as we observe at low velocity it is possible for the machine rock system to exhibit instability and oscillatory behavior even though the steady state response is velocity strengthening another way in which dolomite contrasts with granite and quartzite is that the friction parameters vary strongly with velocity a transition from unstable to stable sliding with increasing velocity has been observed previously and our observed variation of the constitutive parameters with velocity can explain this behavior there is some indication that the evolution of the state variables for dolomite occurs as a function of time rather than displacement in contrast with granite and quartzite the frictional behavior of dolomite shows that the constitutive laws that have been developed based on results from granite and quartzite are inadequate to describe all rock types and that more general laws are required

crustal motion and strain accumulation in western bulgaria abstract global positioning system gps data acquired between 1996 and 2004 and fault plane solutions for four seismic zones are analyzed to obtain the velocity and strain rate fields for western bulgaria the gps derived velocities suggest that southwestern bulgaria moves to the s to sse at a rate of 1 mm year with respect to northern bulgaria and southern romania defining an approximately ese trending extensional boundary that marks the northernmost extent of the aegean extensional domain the boundary includes the e w trending sub balkan graben system of central bulgaria and its westward continuation into the sofia graben active faults within the boundary region trend ene to wnw and they have normal or oblique normal and strike slip displacements consistent with the velocity field within the western part of the boundary region extension is transferred to the north of the sofia graben across the westernmost part of the stara planina mountains and the ridges of the western sredna gora the geodetically derived n s extension is in agreement with the seismic data that show a variable pattern but with projections of the t axes mostly lying within the n s quadrant the predominant type of faulting is caused by sub horizontal extensional stresses in an approximate n s direction this is consistent with extension and oblique strike slip interpreted from geological studies that show numerous wnw ene to e w trending active normal faults in western bulgaria

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

frictional properties of exhumed fault gouges in dfdp 1 cores alpine fault new zealand principal slip zone gouges recovered during the deep fault drilling project dfdp 1 alpine fault new zealand were deformed in triaxial friction experiments at temperatures t of up to 350 c effective normal stresses n of up to 156 mpa and velocities between 0 01 and 3 m s chlorite white mica bearing dfdp 1a blue gouge 90 62 m sample depth is frictionally strong friction coefficient 0 61 0 76 across all experimental conditions tested t 70 350 c n 31 2 156 mpa it undergoes a transition from positive to negative rate dependence as t increases past 210 c the friction coefficient of smectite bearing dfdp 1b brown gouge 128 42 m sample depth increases from 0 49 to 0 74 with increasing temperature and pressure t 70 210 c n 31 2 93 6 mpa the positive to negative rate dependence transition occurs as t increases past 140 c these measurements indicate that in the absence of elevated pore fluid pressures dfdp 1 gouges are frictionally strong under conditions representative of the seismogenic crust

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

are the frictional properties of creeping faults persistent evidence from rapid afterslip following the 2011 tohoku oki earthquake 1 geophysical observations and numerical studies have shown that creeping portions of faults have persistent rate strengthening frictional properties and can act as barriers to earthquake rupture propagation on the basis of gps data following the 2011 mw 9 0 tohoku oki earthquake in japan we find that the evolution of afterslip and postseismic shear stress on the plate interface is inconsistent with persistent rate strengthening frictional properties but is consistent with slip rate dependent frictional properties that exhibit less rate strengthening with increasing slip rate such slip rate dependent frictional properties tend to prevent creeping regions from acting as barriers to rupture propagation and therefore could be an important factor in determining the spatial extent of individual earthquakes

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

frictional constraints on crustal faulting we consider how variations in fault frictional properties affect the phenomenology of earthquake faulting in particular we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes we model these variations using a rate and state dependent friction law where we differentiate velocity weakening behavior into two fields the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening similarly we differentiate velocity strengthening behavior into two fields the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening the strong seismic field comprises the seismic slip concentrations or asperities the two intermediate fields weak seismic and compliant have frictional velocity dependences that are close to velocity neutral these fields modulate both the tectonic loading and the dynamic rupture process during the interseismic period the weak seismic and compliant regions slip aseismically while the strong seismic regions remain locked evolving into stress concentrations that fail only in main shocks the weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically this mixed frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance the model also provides a mechanism for rupture arrest dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas producing broad areas of accelerated afterslip aftershocks occur on both the weak seismic and compliant areas around a fault but most of the stress is diffused through aseismic slip rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed we test this frictional model by comparing the seismicity and the coseismic slip for the 1966 parkfield 1979 coyote lake and 1984 morgan hill earthquakes the interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip these regions are interpreted as either weak seismic or compliant depending on whether or not they manifest interevent seismicity

towards inferring earthquake patterns from geodetic observations of interseismic coupling ultimately seismotectonic studies seek to provide ways of assessing the timing magnitude and spatial extent of future earthquakes ample observations document the spatial variability in interseismic coupling defined as a degree of locking of a fault during the period of stress build up between seismic events fully or nearly locked fault patches are often surrounded by aseismically creeping areas however it is unclear how these observations could help assess future earthquakes here we simulate spontaneous seismic and aseismic fault slip with a fully dynamic numerical model our simulations establish the dependence of earthquake rupture patterns and interseismic coupling on spatial variations of fault friction we consider the long term evolution of slip on a model fault where two seismogenic locked segments are separated by an aseismically slipping patch where rupture is impeded we find that the probability for a large earthquake to break through the rupture impeding patch is correlated with the interseismic coupling averaged over this patch in addition the probability that an earthquake breaks through the rupture impeding patch and interseismic coupling are both related to fault friction properties through a single non dimensional parameter our study opens the possibility of predicting seismic rupture patterns that a fault system can produce on the basis of observations of its interseismic coupling and suggests that regions of low interseismic coupling may reveal permanent barriers to large earthquakes

the alhama de murcia fault se spain a seismogenic fault in a diffuse plate boundary seismotectonic implications for the ibero magrebian region received 20 december 2002 revised 15 september 2003 accepted 26 september 2003 published 2 january 2004 1 the shortening between the african and the iberian plates is absorbed by a number of faults distributed over a very wide zone with very low slip rates and long periods of seismic loading thus a seismotectonic map based only on faults associated with seismicity or with expressive geomorphic features is incomplete it is possible to characterize seismogenic faults using paleoseismology first paleoseismological results based on trenching analysis in the eastern betics lorca totana segment of the alhama de murcia fault are presented the main paleoseismic parameters of this fault segment are 1 a minimum of two to three mw 6 5 7 earthquakes in the last 27 kyr shortly before 1650 a d between 830 and 2130 b c and shortly before 16 7 ka respectively with a mean recurrence period of 14 kyr and a very short elapsed time and 2 a net slip rate of 0 07 0 6 mm yr during the last 30 kyr these results were extrapolated to the rest of the known active faults in the eastern betics and were added to the slip rates of the active faults at the african margin the total slip rate of the transect which crosses de alhama de murcia fault in spain and reaches the cheliff basin algeria would represent 21 82 of the total shortening between africa and eurasia estimated from plate motion models and seismic moment summation a number of factors could account for this discrepancy 1 hidden seismogenic faults in the emerged areas 2 absence of correlation between current and late pleistocene slip rates 3 extensive small faults that are undetected and that absorb a significant amount of the deformation and 4 possible overestimation of the convergence rates index terms 7221 seismology paleoseismology 7230 seismology seismicity and seismotectonics 7215 seismology earthquake parameters 8150 tectonophysics plate boundary general 3040 keywords eastern betics alhama de murcia fault paleoseismology trenching seismotectonics plate boundary

effect of clay content and mineralogy on frictional sliding behavior of simulated gouges binary and ternary mixtures of quartz illite and montmorillonite 1 we investigated the frictional sliding behavior of simulated quartz clay gouges under stress conditions relevant to seismogenic depths conventional triaxial compression tests were conducted at 40 mpa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz montmorillonite and illite in all cases frictional strengths of mixtures fall between the end members of pure quartz strongest and clay weakest the overall trend was a decrease in strength with increasing clay content in the illite quartz mixture the trend was nearly linear while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers two micromechanical models were used to analyze the critical clay fractions for the two regime transitions on the basis of clay porosity and packing of the quartz grains the transition from regime 1 high strength to 2 intermediate strength is associated with the shift from a stress supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains manifested by the development of p foliation and reduction in riedel shear angle the transition from regime 2 intermediate strength to 3 low strength is attributed to the development of shear localization in the clay matrix occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects

earthquake rupture propagation inferred from the spatial distribution of fault rock frictional properties abstract the frictional properties of fault rocks that rupture during earthquakes are expected to affect the nucleation and propagation of seismic slip but these properties and their variation under in situ earthquake conditions are typically unknown here we present experimental results on the variation of frictional properties of fault rocks from the alhama de murcia fault amf in se spain which ruptured in the 2011 lorca earthquake in the epicentral area the fault zone is characterized by the presence of abundant phyllosilicate rich gouges that surround more competent fractured lenses of the phyllitic basement measurements of lineaments and orientations of r shears in the gouges are consistent with cmt solutions of the 2011 lorca earthquake suggesting that the bulk of displacement on the amf was accommodated within similar gouges at depth in order to evaluate the frictional properties of the fault rocks of the amf we performed rotary shear experiments under hydrothermal conditions of samples obtained from surface outcrops of the amf zone progressively simulating deeper levels in the crust by stepping temperature effective normal stress and fluid pressure a negative velocity dependence of friction expressed as a negative value of the rate and state friction rsf parameter a b and which is a prerequisite for the nucleation of an instability was observed only for samples derived from competent lenses under hydrothermal conditions gouge derived samples exhibited only velocity strengthening properties i e positive values of a b which increase with deeper conditions in particular with increasing effective normal stress combined with our outcrop observation of the anastomosing nature of gouges surrounding more competent lenses of fractured protolith our results suggest that the 2011 lorca earthquake nucleated in the competent lenses followed by propagation of slip into the frictionally weaker gouges the inferred upward propagation direction of the 2011 lorca earthquake is consistent with propagation of the rupture into the velocity strengthening gouges which at shallower levels provide a smaller barrier to seismic slip due to lower values of effective normal stress and a b our results suggest that the spatial variation of the frictional properties along the amf was an important factor controlling the nucleation and propagation of seismic slip which together with the shallow hypocenter close to the city of lorca led to serious damage we infer that understanding of such variations in frictional properties may significantly improve seismic hazard evaluations in tectonically active regions

quantitative mapping of the oxidative effects of mantle metasomatism the oxidation state of fe in garnets in a garnet peridotite xenolith from the wesselton kimberlite south africa was quantitatively mapped using x ray absorption near edge structure xanes spectroscopy maps of fe3 fe were produced by recording the fluorescence intensity at discrete energies rather than recording the full spectrum at each point the intensity at each point in the map was quantitatively converted to fe3 fe with reference to a linear calibration derived from garnet standards for which fe3 fe had been determined previously by mossbauer spectroscopy the resolution of these maps approaches that of elemental maps obtained using an electron microprobe the maps reveal zoning in fe3 fe between the core 0 075 and rim 0 125 that correlates with zoning of other elements the rims record an oxidizing metasomatic event in the lithospheric mantle the oxygen fugacity fo2 of this metasomatism is considerably higher than expected from studies of homogeneous garnets that exhibit metasomatic signatures such garnets may represent a re equilibrated average of the original core and metasomatic rim fo2 values metasomatism of the lithospheric mantle may thus have a greater impact on diamond stability than previously thought

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

repeating earthquake finite source models strong asperities revealed on the san andreas fault 1 we investigate the rupture process of a sequence of repeating mw 2 1 earthquakes on the san andreas fault in parkfield spanning the occurrence of the september 28 2004 mainshock by inverting seismic moment rate functions obtained from empirical green s function deconvolution the results show that these events have extremely concentrated slip patches with radii on the order of 10 20 m with peak slip between 8 4 and 11 4 cm the rupture speed and rise time are consistent with values of larger earthquakes the spatial distribution of stress drop for the events shows low average values 2 5 5 6 mpa and very large peak values of 66 7 93 9 mpa the results show that strong asperities can exist at small scales on an otherwise weak fault and helps reconcile differences between traditional spectra based and tectonic loading methods for determining the stress drop of small repeating earthquakes

nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation crystal plasticity at sub seismic slip rates at 18 150 c a central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic seismic transitions in fault gouges and to identify microstructural indicators for such transitions we present new data on the slip stability of calcite fault gouges and on microstructural development down to the nanometer scale our experiments consisted of direct shear tests performed dry at slip rates of 0 1 10 m s at a constant normal stress of 50 mpa at 18 150 c the results show a transition from stable to potentially unstable slip above 80 c all samples recovered showed an optical microstructure characterized by narrow 30 40 m wide riedel and boundary shear bands marked by extreme grain comminution and a crystallographic preferred orientation cpo boundary shear bands sectioned using fib sem focused ion beam scanning electron microscopy revealed angular grain fragments decreasing from 10 to 20 m at the outer margins to 0 3 m in the shear band core where dense aggregates of nanograins also occurred transmission electron microscopy applied to foils extracted from boundary shears using fib sem combined with the optical cpo showed that these aggregates consist of calcite nanocrystals 5 20 nm in size with the 104 201 dislocation glide system oriented parallel to the shear plane and direction our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity because crystal plasticity is strongly thermally activated we infer that the transition to velocity weakening slip is likely due to enhanced crystal plasticity at 80 c this implies that tectonically active limestone terrains will tend to be particularly prone to shallow focus seismicity

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data

seismic signature of intracrustal magmatic intrusions in the eastern betics internal zone se iberia 1 receiver functions at three broad band stations located in the most easterly inner betics have been analyzed to investigate the structure of its underlying crust and uppermost mantle these stations are located within a geologically distinctive block bounded by the palomares alhama de murcia faults which is characterized by high heat flow values widespread strike slip faulting and neogene volcanism our analysis shows that a low velocity zone pervades the uppermost mantle beneath the stations and that the overlying crust has a high vp vs ratio and a prominent intracrustal low velocity zone i between the palomares and alhama de murcia faults and ii east of murcia perhaps thermally perturbing the upper crust north and east of cartagena independent studies show that the seismic velocities of the intervening mantle lid are normal beneath the stations and we suggest that our observations result from rapidly ascending magma diapirs ponding at intracrustal levels within the distinctive block

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

episodic fault creep events in california controlled by shallow frictional heterogeneity stable fault slip or creep is thought to occur in unconsolidated sediments that form shallow parts of continental strike slip faults numerical simulations show that creep events observed on faults in california also require the presence of a shallow unstable layer of rock the thickness of which influences the duration of the creep event

geologic evolution of the betic cordilleras in the western mediterranean miocene to the present abstract the betic cordilleras were formed during the eo meso and neo alpine evolution of the western mediterranean area during the neo alpine stage the northward subduction of the african plate took place and the algerian provencal basin opened with the creation of oceanic floor associated space problems produced the expulsion and extensive stretching of the internal zones to the west which in turn disrupted and deformed the external zones of the betic and rif cordilleras these occurrences essentially took place in the burdigalian and continued with diminished intensity into the middle miocene following the suturing of the transcurrent contact between the internal and external zones important n60 90 e and nw se faults occurred the betic cordilleras are characterized as having been deformed by regional horizontal shortening compression oriented wnw ese or nw se which gradually rotated to the nnw sse in the late miocene when its most important intramontane neogene basins were formed the alboran basin with a thinned continental crust was formed as a western prolongation of the opening of the algerian provencal basin with its basic features already apparent in the burdigalian the volcanism present in the southeast of the betic cordilleras and in northeast morocco crossing the alboran basin is mainly late miocene in age it is linked to fracturing and exhibits chemistry related to the different thickness of the crust

frictional behaviour of simulated quartz fault gouges under hydrothermal conditions results from ultra high strain rotary shear experiments abstract numerous experimental studies have been conducted to explore the mechanical behaviour and strength properties of rock materials most of these studies however do not take the effects of fluids into account though it has long been recognized that fluids might play an important role in the deformation of crustal rocks under brittle ductile conditions moreover all these studies are limited in the total shear strain that could be reached in this study we report on ultra high strain experiments performed on simulated quartz fault gouges in a rotary shear apparatus under hydrothermal conditions effective normal stresses of 20 100 mpa a fluid pressure of 200 mpa temperatures of 400 600 c sliding velocities of 0 01 1 m s and total shear strains up to 50 the experiments show strain hardening up to a shear strain of 0 6 1 8 followed by strain weakening of up to 30 towards a steady state value at a strain of 8 12 this strain weakening effect is much higher than previously reported for quartz gouge the steady state shear strength increases with decreasing grain size increasing sliding velocity and decreasing temperature the microstructure of the deformed quartz gouge is characterized by the presence of a through going boundary parallel y shear some samples also show riedel shears oriented oblique to the shear zone boundary deformation in these tests was largely by cataclastic processes with most displacement being accommodated along the boundary parallel y shear causing the marked weakening observed intergranular pressure solution did not accommodate significant shear strain but appears to play a role in smoothing and weakening the localized slip surface and in controlling gouge compaction

an alternative suggestion for the pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of north atlantic ocean ice rafted debris abstract the onset of abundant ice rafted debris ird deposition in the nordic seas and subpolar north atlantic ocean 2 72 millions of years ago ma is thought to record the pliocene onset of major northern hemisphere glaciation nhg due to a synchronous advance of north american laurentide scandinavian and greenland ice sheets to their marine calving margins during marine isotope stage mis g6 numerous marine and terrestrial records from the nordic seas region indicate that extensive ice sheets on greenland and scandinavia increased ird inputs to these seas from 2 72 ma the timing of ice sheet expansion on north america as tracked by ird deposition in the subpolar north atlantic ocean however is less clear because both europe and north america are potential sources for icebergs in this region moreover cosmogenic dating of terrestrial tills on north america indicate that the laurentide ice sheet did not extend to 39 n until 2 4 0 14 ma at least 180 ka after the onset of major ird deposition at 2 72 ma to address this problem we present the first detailed analysis of the geochemical provenance of individual sand sized ird deposited in the subpolar north atlantic ocean between mis g6 and 100 2 72 2 52 ma ird provenance is assessed using laser ablation lead pb isotope analyses of single ice rafted 150 m feldspar grains to track when an ice rafting setting consistent with major nhg first occurred in the north atlantic ocean during the pliocene intensification of nhg inhg we investigate when the pb isotope composition 206pb 204pb 207pb 204pb 208pb 204pb of feldspars deposited at dsdp site 611 first resembles that determined for ird deposited at this site during mis 100 2 52 ma the oldest glacial for which there exists convincing evidence for widespread glaciation of north america whilst quaternary magnitude ird fluxes exist at site 611 during glacials from 2 72 ma we find that the provenance of this ird is not constant instead we find that the pb isotope composition of ird at our study site is not consistent with major nhg until mis g2 2 64 ma we hypothesise that ird deposition in the north atlantic ocean prior to mis g2 was dominated by iceberg calving from greenland and scandinavia we further suggest that the grounding line of continental ice on northeast america may not have extended onto the continental shelf and calved significant numbers of icebergs to the north atlantic ocean during glacials until 2 64 ma

tectonic and seismic implications of an intersegment rupture the damaging may 11th 2011 mw 5 2 lorca spain earthquake on may 11th 2011 a mw 5 2 earthquake stroke the city of lorca in the se spain this event caused 9 fatalities 300 injuries and serious damage on the city and the surrounding areas the lorca earthquake occurred in the vicinity of a region bounding two well known segments of a large active fault the alhama de murcia fault amf the lorca earthquake offers a unique opportunity to study how strain is accommodated in an intersegment region of a large strike slip fault we map recent tectonic structures in the epicentral region and we use radar interferometry to analyze the coseismic deformation combining these data with seismological observations of lorca seismic sequence we first model the source of the earthquake then we analyze the influence of our preferred model in the adjacent segments by coulomb failure stress modeling the proposed earthquake source model suggests that this event ruptured an area of 4 3 km within the complex structure that limits the gonar lorca and lorca totana segments of the amf the induced static stress change on the adjacent segments of the fault represents a seismic cycle advance equivalent to 200 to 1000 years of tectonic loading

new constraints on megathrust slip stability under subduction zone p t conditions abstract to understand and model subduction zone seismogenesis data are needed on the frictional properties of meta pelitic subduction zone materials under in situ megathrust conditions here we report the results of rotary shear friction experiments on simulated illite quartz fault gouge at an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa at 150 500 c and sliding velocities of 1 100 m s the results show three temperature dependent regimes characterized by velocity strengthening at 150 250 c velocity weakening at 250 400 c and velocity strengthening at 400 500 c the regimes are defined by a decrease in the rate and state friction rsf parameter a b at 150 300 c followed by an increase at 300 500 c these trends correlate with systematic changes in other rsf parameters as well as an increase in friction coefficient above 300 c we explain the effects of increasing temperature in terms of a transition from frictional granular flow 150 250 c through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking scc of quartz clasts 250 400 c to non dilatant slip on the phyllosilicates with accommodation at quartz clasts by scc 400 500 c taking into account the effects of sliding velocity the observed velocity weakening regime broadly explains the extent of the seismogenic zone within subduction megathrusts