a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

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

scale dependence in the dynamics of earthquake propagation evidence from seismological and geological observations abstract we attempt to reconcile current understanding of the earthquake energy balance with recent estimates of fracture energy from seismological investigations and surface energy from geological observations the complex structure of real fault zones suggests that earthquakes in such fault structures are dominated by scale dependent processes we present a model for an inelastic fault zone of finite thickness embedded in an elastic crust represented at a macroscopic scale by a mathematical plane of zero thickness the constitutive properties of the fault zone are governed by physical processes controlling gouge and damage evolution at meso and micro scale however in order to model and interpret seismological observations we represent dynamic fault weakening at the macroscopic scale in terms of traction evolution as a function of slip and other internal variables defining a phenomenological friction or contact law on the virtual mathematical plane this contact law is designed to capture the main features of dynamic fault weakening during earthquake rupture in this study we assume that total shear traction is friction and corresponds to shear resistance of the whole fault zone we show that seismological observations depending on finite and limited wavelength and frequency bandwidth can only provide an estimate of breakdown stress drop and breakdown work a more general definition of seismological fracture energy representing a lower bound of the total intrinsic power of dissipation on the fault zone we emphasize that geological estimates of surface energy can be compared with seismological estimates of breakdown work only if they are representative of the same macroscopic scale in this case it emerges that contrary to surface energy seismological breakdown work represents a non negligible contribution to the earthquake energy budget

numerical simulations on megathrust rupture stabilized under strong dilatancy strengthening in slow slip region 1 episodic slow slip events sses typically involve a few millimeters to centimeters of slip over several days to months at depths near or further downdip of megathrust seismogenic zones despite its widespread presence in subduction margins it remains unknown how sses interact with the seismogenic zone and affect megathrust ruptures here i construct a 2 d thrust fault model governed by rate state friction to investigate how fault dilatancy influences the amplitude and spatial distribution of coseismic slip afterslip and sses model results illustrate that under strong dilatancy and high pore pressure around the friction stability transition coseismic rupture stops at the onset of sses modeled sses have lower velocities longer recurrence intervals and durations and larger slip amounts as dilatancy becomes stronger demonstrating a transition from short term to long term type of sse behavior these results qualitatively explain the range of spatial distributions of sses and megathrust ruptures observed or inferred in natural subduction zones furthermore the relative depths of sses and megathrust afterslip may serve as an indicator of dilatancy effectiveness

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

intense foreshocks and a slow slip event preceded the 2014 iquique mw 8 1 earthquake the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 the moment magnitude mw 8 1 iquique earthquake of 1 april 2014 broke a highly coupled portion of this gap to understand the seismicity preceding this event we studied the location and mechanisms of the foreshocks and computed global positioning system gps time series at stations located on shore seismicity off the coast of iquique started to increase in january 2014 after 16 march several mw 6 events occurred near the low coupled zone these events migrated northward for 50 kilometers until the 1 april earthquake occurred on 16 march on shore continuous gps stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred

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

fault zone architecture and deformation processes within evaporitic rocks in the upper crust 1 recently in the northern apennines geophysical data have identified the triassic evaporites te anhydrites and dolomites as the source region of the major extensional earthquakes of the area m 6 in order to characterize fault zone architecture and deformation processes within the te we have studied exhumed evaporite bearing normal faults within the upper crust the structure of large displacement 100 m normal faults is given by 1 a zoned fault core with a wider portion of fault parallel foliated ca sulphates ductile deformation overprinted by an inner fault core ifc of localized brittle deformation and 2 wide dolostones to absent ca sulphates damage zones of fault fracture patterns fault rock assemblage within the ifc is characterized by fault breccia gouge and cataclasites of different grain size most of the deformation within the ifc is localized along thin and fault parallel principal slip surfaces pss made of dolomite rich fine grained cataclasite sem analyses show an evolution from ca to st to gypsum rich mineralization due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation the development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones progressive deformation within the fault core leads to the development of fault parallel dolomite rich cataclastic layers their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the ifc

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

evolution of the elastic moduli of seismogenic triassic evaporites subjected to cyclic stressing seismic cycles lead to variations in rock physical properties quantifying these changes is of key importance in building reliable crustal deformation models here we report laboratory measurements of the uniaxial compressive strength ucs young s modulus and poisson s ratio both static es s and dynamic ed d of the seismogenic triassic evaporites of the northern apennines triassic evaporites are composed of dolostones anhydrites and gypsum gypsum was the weakest lithology with ucs values ranging from 10 to 26 mpa anhydrite exhibited intermediate values from 52 to 144 mpa and dolostones were the strongest with a maximum ucs of 228 mpa during uniaxial cyclic stressing experiments we observed complex variations in es and s with large increases are observed in the early cycles stage 1 followed by essentially constant values stage 2 before es decreases and s increases approaching failure stage 3 complementary microseismicity acoustic emission ae data show no significant ae during stage 1 then the stress needed to induce ae remained essentially constant stages 2 and 3 integration of mechanical data with microstructural observations suggests a first stage dominated by compaction and strengthening a second stage characterised by quasi elastic behaviour associated with the development of randomly oriented microfractures and a third stage of weakening due to the growth of macrofractures parallel to the direction of the load laboratory dynamic elastic moduli are on average in agreement with dynamic elastic moduli used in crustal modelling however static values of young s modulus are about 50 lower than dynamic ones and static values of poisson s ratio are about 40 higher with respect to dynamic values these observations suggest that the frequency effect on the difference between laboratory and crustal scale dynamic moduli values is rather small and that static values of modulus are more appropriate for crustal deformation modelling than seismically derived values

constraints on slow earthquake dynamics from a swarm in central italy several clustered slow earthquakes have been recorded by a geodetic interferometer in central italy the strain rise times of the events range from tens to thousands of seconds and the seismic moment scales with the square root of the rise time this scaling law contrasts with the conservative assumption of constant rupture velocity in fault modeling but is consistent with the occurrence of a slow rupture propagation analogous to heat diffusion in a slab

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

magnetotelluric soundings structure and fluids in the southern canadian cordillera the cordillera of western canada lies in a region of oceanic and island arc lithosphere accreted to north america during subductions of the last 200 ma magnetometer arrays have shown the crust of the region to be highly conductive magnetotelluric mt soundings across the intermontane and omineca tectonic belts between 50 n and 54 n reveal structure in terms of electrical resistivity pseudosections of phase and apparent resistivity and preliminary resistivity depth sections are shown for three transects the resistivity range is from less than one ohm metre to several thousands of ohm metres in old continental shields crustal resistivities cover a similar four decade range transposed up two decades i e 102 106 m we show that the observed resistivities can be produced by water with nacl and or co2 in solution at the high temperatures of the cordilleran crust in fractured rock of effective porosity 4 5 the resistivity variations may represent varying fracture densities by following structu

earthquakes as a triggering mechanism for debris flows and turbidites on the calabrian ridge abstract debris flows and turbidites have been described in piston cores bottom photographs and side looking sonar records from basin floors amid the rugged irregular topography of the calabrian ridge a sediment transport process is inferred which began with a rotational slump on the steep basin wall subsequently transformed into a debris flow and turbid cloud such events occurred approximately every 1500 years because the debris flow turbidite deposits are correlative between basins some relatively widespread mechanism such as an earthquake is required to initiate the slope failure the estimated intensity of ground shaking as a result of historical earthquakes on the nearby calabrian and hellenic arcs has been used to develop a relationship between frequency of occurrence and intensity of ground shaking extrapolation of this relation to infrequent large magnitude events shows that the 1500 yr event is small enough to be geologically plausible but large enough to cause slope failure on basin walls it is possible that the historical record fortuitously contains one of these large magnitude long repeat time events which may have caused the most recent of the observed debris flows in 1903

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

a slip pulse model with fault heterogeneity for low frequency earthquakes and tremor along plate interfaces 1 deep low frequency earthquakes lfes and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes including strong anisotropy in their migration velocity and source spectra displaying 1 f decay we show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow slip events lfe tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a gaussian distribution the difference in their migration speeds along dip and along strike was reproduced without anisotropic rheological properties by introducing alignments of their sources similar to observed streaks of lfes tremor the key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate this model provides new insights into the physical source process of lfes and tremor and should find practical use in assessing properties of deep plate interfaces

an earthquake like magnitude frequency distribution of slow slip in northern cascadia 1 major episodic tremor and slip ets events with mw 6 4 to 6 7 repeat every 15 2 months within the cascadia subduction zone under the olympic peninsula although these major ets events are observed to release strain smaller tremor swarms without detectable geodetic deformation are more frequent an automatic search from 2006 2009 reveals 20 000 five minute windows containing tremor which cluster in space and time into 96 tremor swarms the 93 inter ets tremor swarms account for 45 of the total duration of tremor detection during the last three ets cycles the number of tremor swarms n exceeding duration follow a power law distribution n 0 66 if duration is proportional to moment release the slip inferred from these swarms follows a standard gutenberg richter logarithmic frequency magnitude relation with the major ets events and smaller inter ets swarms lying on the same trend this relationship implies that 1 inter ets slip is fundamentally similar to the major events just smaller and more frequent and 2 despite fundamental differences in moment duration scaling the slow slip magnitude frequency distribution is the same as normal earthquakes with a b value of 1

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

geologic heterogeneity can produce aseismic slip transients 1 slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones 10 103 m composed of rocks with marked differences in mechanical properties here we model quasi dynamic rupture along faults composed of material mixtures characterized by different rate and state dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting keeping other parameters fixed the relative proportions of velocity weakening vw and velocity strengthening vs materials control the sliding character stable slow or dynamic along the fault the stability boundary between slow and dynamic is accurately described by linear analysis of a double spring slider system with vw and vs blocks our results place bounds on the volume fractions of vw material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones

insights on the seismogenic layer thickness from the upper crust structure of the umbria marche apennines central italy 1 we reconstruct the subsurface geology in a region of the northern apennines central italy where a protracted extensional sequence occurred in 1997 1998 with maximum magnitude m 6 0 our study is mainly based on the interpretation of three reprocessed seismic reflection profiles crossing the epicentral area which constrain the subsurface geometry to a depth of about 12 km where most of the shallow seismicity occurs comparing the subsurface setting with accurately determined earthquake locations we find that the seismicity is located entirely within the sedimentary cover and does not penetrate the underlying basement this is explained by considering that the sedimentary cover is rather thick and composed of relatively strong lithologies platform carbonates and evaporites while the upper part of the basement consists of weak phyllites and siliciclastic rocks this weak horizon is also evidenced by the low vp values measured in deep wells of the region its effect is to decouple the sedimentary cover from the crystalline basement where only microseismicity occurs our study indicates that local structure and stratigraphy can significantly influence the distribution of seismicity within the upper crust particularly in complex geological environments such as thrust and fold belts

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event