orthogonal relation between wavefield polarization and fast s wave direction in the val d agri region an integrating method to investigate rock anisotropy 1 wavefield polarization is investigated using 200 seismograms recorded by a network of 20 stations installed on rock outcrops in the val d agri region that hosts the largest oil fields in the southern apennines italy polarization is assessed both in the frequency and time domains through the individual station horizontal to vertical spectral ratio and covariance matrix analysis respectively we find that most of the stations show a persistent horizontal polarization of waveforms with a ne sw predominant trend this direction is orthogonal to the general trend of quaternary normal faults in the region and to the maximum horizontal stress related to the present extensional regime according to previous studies in other areas such a directional effect is interpreted as due to the presence of fault related fracture fields polarization being orthogonal to their predominant direction a comparison with s wave anisotropy inferred from shear wave splitting indicates an orthogonal relation between horizontal polarization and fast s wave direction this suggests that wavefield polarization and fast velocity direction are effects of the same cause the existence of an anisotropic medium represented by fractured rocks where shear wave velocity is larger in the crack parallel component and compliance is larger perpendicularly to the crack strike the latter is responsible for the observed anisotropic pattern of amplitudes of horizontal ground motion in the study area

structural control on the directional amplification of seismic noise campo imperatore central italy abstract seismic signals propagating across a fault may yield information on the internal structure of the fault zone here we have assessed the amplification of seismic noise i e ambient vibrations generated by natural or anthropogenic disturbances across the vado di corno fault campo imperatore central italy the fault zone is considered as an exhumed analogue of the normal faults activated during the l aquila 2009 earthquake sequence detailed structural geological survey of the footwall block revealed that the fault zone is highly anisotropic and is affected by a complex network of faults and fractures with dominant wnw ese strike we measured seismic noise with portable seismometers along a 500 m long transect perpendicular to the average fault strike seismic signals were processed calculating the horizontal to vertical spectral ratios and performing wavefield polarization analyses we found a predominant ne sw to nne ssw i e ca perpendicular to the average strike of the fault fracture network amplification of the horizontal component of the seismic waves numerical simulations of earthquake induced ground motions ruled out the role of topography in controlling the polarization and the amplitude of the waves therefore the higher seismic noise amplitude observed in the fault perpendicular direction was related to the measured fracture network and the resulting stiffness anisotropy of the rock mass these observations open new perspectives in using measures of ambient seismic noise which are fast and inexpensive to estimate the dominant orientation of fracture networks within fault zones

fault zone amplified waves as a possible seismic hazard along the calaveras fault in central california the calaveras fault lies within a low velocity zone lvz 1 2 km wide near gilroy california accelerographs g06 located in the lvz 1 2 km from the calaveras fault and g07 4 km from g06 recorded both the m 6 2 1984 morgan hill and the m 6 9 1989 loma prieta earthquakes comparison of the ground motions shows that a large 0 6 1 0 hz velocity pulse observed at g06 during the morgan hill event may be amplified by focussing caused by the lvz such amplified waves might be a mappable seismic hazard and the zone of increased hazard can extend as much as 1 2 km from the surface trace of the fault finite difference simulations of ground motions in a simplified lvz model show a zone of amplified motion similar to the observations

wavefield polarization in fault zones of the western flank of mt etna observations and fracture orientation modelling ambient noise measurements performed on the western flank of mt etna are analyzed to infer the occurrence of directional amplification effects in fault zones the data were recorded along short 500 m profiles crossing the ragalna fault system ambient noise records were processed to compute the horizontal to vertical noise spectral ratio as a function of frequency and direction of motion wavefield polarization was investigated in the time frequency domain as well peaks of the spectral ratios generally fall in the frequency band 1 0 6 0 hz pointing out directional amplifications that are also confirmed by the results of the time frequency analysis the largest amplification occurring with high angle to the fault strike a variation of the frequency of the spectral peak is observed between the two sides of the fault possibly related to a damage fault asymmetry measurements performed several kilometers away from the fault zone do not show behavior that is as systematic as in the fault zone and this suggests that the observed directional effects can be ascribed to the fault fabric we relate the polarization effect to compliance anisotropy in the fault zone where the presence of predominantly oriented fractures makes the normal component of ground motion larger than the transversal one in order to test the direction and the type of fractures that are expected in the fault zone we modeled the brittle deformation pattern of the investigated fault theoretical results are in good agreement with field observations of the fracture strike

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

seismic fault zone trapped noise systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array the frequency dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field a critical frequency fc 0 5 hz defines a threshold above which the in fault scattered wavefield has increased isotropy and coherency compared to the ambient noise the increased randomization of in fault propagation directions produces a wavefield that is trapped in a waveguide cavity like structure associated with the low velocity damage zone dense spatial sampling allows the resolution of a near field focal spot which emerges from the superposition of a collapsing time reversed wavefront the shape of the focal spot depends on local medium properties and a focal spot based fault normal distribution of wave speeds indicates a 50 velocity reduction consistent with estimates from a far field travel time inversion the arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern providing a noise based imaging tool that can complement analyses of trapped ballistic waves the results can have wide applicability for investigating the internal properties of fault damage zones because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves

difference information criterion for the analysis of a seismotectonic influence on a radon time series at the ksm site japan summary this research investigates how a difference information criterion combined with a function of observed seismotectonic activity can be used to analyse the influence of seismotectonic activity on the variation in a radon time series observed at the ksm site izu peninsula japan the difference information criterion is used to identify significant variance in the radon time series the seismotectonic activity is represented by an aggregated univariate function of seismic observations a flexible definition of the function is proposed enabling an analysis of various geometrical and energetic relations between the seismotectonic activity and the monitoring site the results are compared with previous results obtained at the same site with a different approach identifying 12 post seismic signals and one pre seismic signal to 23 earthquakes with mw 6 0 and within 1000 km hypocentral distance during the period considered the combination of a difference signal criterion and the introduced aggregation function of seismic observations is more sensitive for identifying seismotectonic related variance in the radon time series than the use of confidence intervals as proposed in former works

evidences for strong directional resonances in intensely deformed zones of the pernicana fault mount etna italy 1 in this paper we investigate ground motion properties in the western part of the pernicana fault this is the major fault of mount etna and drives the dynamic evolution of the area in a previous work rigano et al 2008 showed that a significant horizontal polarization characterizes ground motion in fault zones of mount etna both during earthquakes and ambient vibrations we have performed denser microtremor measurements in the ne rift segment and in intensely deformed zones of the pernicana fault at piano pernicana this study includes mapping of azimuth dependent horizontal to vertical spectral ratios along and across the fault frequency wave number techniques applied to array data to investigate the nature of ambient vibrations and polarization analysis through the conventional covariance matrix method our results indicate that microtremors are likely composed of volcanic tremor spectral ratios show strong directional resonances of horizontal components around 1 hz when measurements enter the most damaged part of the fault zone their polarization directions show an abrupt change by 20 to 40 at close measurements between the northern and southern part of the fault zone recordings of local earthquakes at one site in the fault zone confirm the occurrence of polarization with the same angle found using volcanic tremor we have also found that the directional effect is not time dependent at least at a seasonal scale this observation and the similar behavior of volcanic tremors and earthquake induced ground motions suggest that horizontal polarization is the effect of local fault properties however the 1 hz resonant frequency cannot be reproduced using the 1 d vertically varying model inferred from the array data analysis suggesting a role of lateral variations of the fault zone although the actual cause of polarization is unknown a role of stress induced anisotropy and microfracture orientation in the near surface lavas of the pernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone

focal spot imaging based on zero lag cross correlation amplitude fields application to dense array data at the san jacinto fault zone we image the subsurface below a dense seismic array straddling the clark branch of the san jacinto fault zone in southern california the analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross correlations computed between all stations of the dense array local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions the method provides simultaneous estimates of wave speed apparent attenuation and anisotropy without solving a tomographic inverse problem the obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far field rayleigh wave tomography we observe an anticorrelation between our apparent attenuation coefficient and seismic velocity and a fault parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault the results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low velocity structure to the southwest associated with a shallow sedimentary basin

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

large amplitude variations recorded by an on fault seismological station during the l aquila earthquakes evidence for a complex fault induced site effect 1 a station fagn installed on a segment of the fault system that generated the april 2009 l aquila earthquakes shows larger ground motions compared to nearby stations spectral ratios using 304 earthquakes result in a station amplification significantly varying event by event in the frequency band 1 8 hz the resulting pattern of amplitude dependence on causative earthquake location reveals that the strongest up to a factor of 10 amplifications occur for tightly clustered aftershocks aligned with the fault dip beneath fagn thus indicating a fault guided effect fault models are investigated in a grid search approach by varying velocity q width and depth of the fault zone although the problem solution is not unique and there are strong trade offs among the model parameters constraints from observations yield a deep trapping structure model where the most likely values of velocity reduction q and damage zone width are 25 20 and 280 m respectively

seismic radiation from an sh line source in a laterally heterogeneous planar fault zone a solution to the class of problems in which an sh line source is operating in a medium consisting of two quarter spaces with an arbitrary number of semi infinite vertical layers in between is given in a frequency wavenumber domain the general result is applied to a model with a single vertical layer and synthetic seismograms are calculated for a source specified as an sh line dislocation at the interface between the vertical layer and the faster quarter space the numerical examples demonstrate that moderate fault normal material heterogeneity can have significant effects on the seismic response of a medium the overall distribution of travel times amplitudes and motion polarities in the presence of fault zone material heterogeneity is different from a corresponding distribution in a homogeneous half space the effects are most prominent near the fault zone where head waves surface waves and trapped modes result in waveform complexities large amplifications over an order of magnitude in the fault zone itself and first motion polarities which are reversed from homogeneous medium predictions the results imply that material heterogeneity should be included in fault zone models which are used for the interpretation of observed seismic data and theoretical calculations of near fault responses such models can be utilized to obtain important fault zone and earthquake source parameters

shallow seismic trapping structure in the san jacinto fault zone near anza california summary we analyse fault zone trapped waves generated by 500 small earthquakes for high resolution imaging of the subsurface structure of the coyote creek clark valley and buck ridge branches of the san jacinto fault zone near anza california based on a small number of selected trapped waves within this data set a previous study concluded on the existence of a lowvelocity waveguide that is continuous to a depth of 15 20 km in contrast our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3 5 km this is based on the following lines of evidence 1 earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones 2 a traveltime analysis of the difference between the direct s arrivals and trapped wave groups shows no systematic increase moveout with increasing hypocentral distance or event depth estimates based on the observed average moveout values indicate that the propagation distances within the low velocity fault zone layers are 3 5 km 3 quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low velocity fault zone layers the results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones the waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch this may result from a preferred propagation direction of large earthquake ruptures on the san jacinto fault

empirical evidence of orthogonal relationship between directional site effects and fracture azimuths in an active fault zone the case of the mt etna lower eastern flank abstract mt etna in eastern sicily is a composite active volcano whose eastern flank is affected by several normal and strike slip active faults in this study geological structural field survey and ambient vibration measurements are performed in the lower eastern flank of mt etna in order to define the role of the tectono stratigraphic setting in the seismic site response the sites of measurements are mainly distributed inside the few hundreds of meters long s caterina graben which is located between two main seismogenic active faults of this volcano sector the acireale and the fiandaca faults our study aims at defining the contribution of the fault related active deformation in determining the local ground motion amplification also providing a subsoil geological model based on the integration of geological and geophysical investigation therefore structural measurements were made in seven sites revealing coseismic and aseismic extensional fractures mainly striking nnw the seismic site response was studied processing the ambient vibration measurements through horizontal to vertical spectral ratio the measured fundamental frequency falls in the range 1 0 4 0 hz that we linked to the presence of a velocity contrast at depth of 80 200 m by calibrating a frequency depth function moreover the seismic site effect is due to the stratigraphic setting and is markedly directional showing maximum amplification along the direction orthogonal to the strike of the observed isooriented extensional fracture field produced by the activity of the s caterina graben it is noteworthy that far from the graben on sites of measurements located near the fiandaca fault we observed directional effects parallel to the direction of maximum extension inferred for this tectonic lineament our results suggest a linkage between directional site effects and local stress tensors and fractures field orientation

the 2009 l aquila central italy mw6 3 earthquake main shock and aftershocks 1 a m w 6 3 earthquake struck on april 6 2009 the abruzzi region central italy producing vast damage in the l aquila town and surroundings in this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks the distribution of aftershocks 712 selected events with m l 2 3 and 20 with m l 4 0 defines a complex 40 km long nw trending extensional structure the main shock fault segment extends for 15 18 km and dips at 45 to the sw between 10 and 2 km depth the extent of aftershocks coincides with the surface trace of the paganica fault a poorly known normal fault that after the event has been quoted to accommodate the extension of the area we observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes

horizontal polarization of ground motion in the hayward fault zone at fremont california dominant fault high angle polarization and fault induced cracks summary we investigate shear wave polarization in the hayward fault zone near niles canyon fremont ca waveforms of 12 earthquakes recorded by a seven accelerometer seismic array around the fault are analysed to clarify directional site effects in the fault damage zone the analysis is performed in the frequency domain through h v spectral ratios with horizontal components rotated from 0 to 180 and in the time domain using the eigenvectors and eigenvalues of the covariance matrix method employing three component records the near fault ground motion tends to be polarized in the horizontal plane at two on fault stations where the local strike is n160 ground motion polarization is oriented n88 19 and n83 32 respectively at a third on fault station the motion is more complex with horizontal polarization varying in different frequency bands however a polarization of n86 7 similar to the results at the other two on fault stations is found in the frequency band 6 8 hz the predominantly high angle polarization from the fault strike at the hayward fault is consistent with similar results at the parkfield section of the san andreas fault and the val d agri area a quaternary extensional basin in italy in all these cases comparisons of the observed polarization directions with models of fracture orientation based on the fault movement indicate that the dominant horizontal polarization is near orthogonal to the orientation of the expected predominant cracking direction the results help to develop improved connections between fault mechanics and near fault ground motion

probing the sources of ambient seismic noise near the coasts of southern italy 1 in this study we probe the source of ambient noise in the southern apennines and the calabrian arc by crosscorrelating two months of ambient seismic noise records collected during the calabria apennine tyrrhenian subduction collision acretion network cat scan project significant rayleigh wave energy is observed on the vertical component of the noise correlation stacks and reveals multiple sources of ambient noise in southern italy the most dominant noise sources are found along 1 the tyrrhenian coast of northern calabria southern campania and 2 the adriatic sea near the gargano promontory enhanced ocean currents evident from buoy records during the study period could be responsible for the observed microseisms we validate the source locations using earthquake records and the consistency between noise and earthquake correlation functions supports the observed dominant directions of ambient seismic noise citation gu y j c dublanko a lerner lam k brzak and m steckler 2007 probing the sources of ambient seismic noise near the coasts of southern italy geophys res lett 34 l22315 doi 10 1029 2007gl031967

quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 landers california earthquake evidence for a shallow trapping structure summary we analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 landers earthquake a grid search method with station delay corrections is used to locate events that do not have catalogue locations the quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct s waves at stations close to and off the fault zone approximately 70 per cent of the events with s p times of less than 2 s including many clearly off the fault produce considerable trapped wave energy this distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the landers rupture zone the time difference between the s arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth the dispersion measured from the trapped waves is weak these results imply that the seismic trapping structure at the landers rupture zone is shallow and does not extend continuously along strike by more than a few kilometres synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2 4 km a width of approximately 200 m an s wave velocity reduction relative to the host rock of approximately 30 40 per cent and an s wave attenuation coefficient of approximately 20 30 the fault zone waveguide north of the array appears to be shallower and weaker than that south of the array the waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

quaternary geology and assessment of aggregate resources of korea for the national industrial resources exploration and development abstract demand for aggregates in korea fueled by construction has increased rapidly in recent years quaternary aggregate resources are associated with holocene alluvial deposits and shallow marine sands and with fluvial sequences developed during the late quaternary soil wedge structures developed in palaeosols on river terraces indicate that periglacial conditions prevailed during oxygen isotope stages 2 and 4 along the margins of the yellow sea sea level fluctuations involved a drop to 120 m a s l during the lgm ca 15 000 yr bp resulting in the formation of three seismosequences the uppermost sequence of which contains aggregate sediments repetitive episodes of sedimentation along the major river valleys have resulted in the production of fluvial aggregates preferred by construction engineers intensive exploitation of these resources however has forced korea to turn to other quaternary deposits for aggregate

seismic trapped modes in the oroville and san andreas fault zones three component borehole seismic profiling of the recently active oroville california normal fault and microearthquake event recording with a near fault three component borehole seismometer on the san andreas fault at parkfield california have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals these wave trains are interpreted as fault zone trapped seismic modes parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers as shallow as 4 kilometers and extend 12 kilometers along the fault on either side of the recording station selected oroville and parkfield wave forms are modeled as the fundamental and first higher trapped sh modes of a narrow low velocity layer at the fault modeling results suggest that the oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second whereas at parkfield the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1 1 to 1 8 kilometers per second these low velocity layers are probably the rupture planes on which earthquakes occur

directional site resonances observed from aftershocks of the 18 october 1989 loma prieta earthquake abstract two weeks after the 18 october 1989 loma prieta california earthquake 18 three component digital seismometers were deployed in the epicentral area to form three six station subarrays the subarray configuration allowed us to investigate the presence of direction and frequency dependent site resonances we measured the shear wave polarization from the recordings of 10 aftershocks from the loma prieta earthquake our observations show that the site response has a strong azimuthal dependence and that both the shear wave polarization and the spectral amplitude of the ground motions are affected by site characteristics in the frequency range from 1 to 18 hz the majority of stations examined showed preferred azimuths of ground motion for the scattered waves that did not depend either on the earthquake location or on the polarization of the shear waves expected from the known focal mechanism the measurements were made from 5 sec windows that included direct and scattered shear waves which contain the largest amplitude motions in the near source region and are therefore of most interest to earthquake engineers however in the 0 to 2 hz frequency range the first pulse of shear waves shows a polarization that is well predicted by the mechanism and location of the earthquake the rapid spatial variation of the preferred directions and their corresponding frequencies indicate that geologic structures within a distance of the order of 50 m probably control these site effects we suggest that the site amplified the motion of scattered waves in one preferred direction altering the resulting polarization and modulating the spectral amplitude

evaluation of complex site effects through experimental methods and numerical modelling the case history of arquata del tronto central italy abstract this work deals with the experimental and numerical evaluation of the local seismic response of arquata del tronto area marche region central italy severely struck by the mw 6 0 august 24th 2016 earthquake arquata main village rises on elongated wnw ese trending ridge of the central apennines thrust belt central italy at elevations about 170 m higher than the underlying alluvial valleys where borgo and san francesco hamlets are built on despite their proximity less than 500 m arquata del tronto borgo and san francesco reported a different damage distribution after the august 2016 mainshock suggesting that the seismic response of the area may be controlled by site effects in order to explore this hypothesis we evaluated the 2d numerical local seismic response along four representative geological cross sections passing through arquata del tronto san francesco and borgo additional 1d analyses were carried out at strategic points along the cross sections in order to explore the 2d physical phenomena governing the local response the satisfactory agreement between numerical amplification functions in linear range and experimental amplification functions obtained by the generalized inversion technique git applied to a large number of aftershocks confirms the substantial reliability of the subsoil models numerical analyses representative of the 2016 mainshock were carried out and processed in terms of peak and key ground motion parameters a comparison with the damage induced by the 2016 mainshock was finally undertaken substantially justifying the observed pattern this study provides general implications about site response and seismic microzonation in sites characterized by similar complex geological and morphological settings

localized damage caused by topographic amplification during the 2010 m 7 0 haiti earthquake microzonation maps use local geological conditions to characterize seismic hazard but do not generally consider topography ground motions during the haiti earthquake are found to have been significantly amplified along a high topographic ridge which caused substantial structural damage indicating that topography can play an important role in seismic hazard local geological conditions including both near surface sedimentary layers1 2 3 4 and topographic features5 6 7 8 9 are known to significantly influence ground motions caused by earthquakes microzonation maps use local geological conditions to characterize seismic hazard but commonly incorporate the effect of only sedimentary layers10 11 12 microzonation does not take into account local topography because significant topographic amplification is assumed to be rare here we show that although the extent of structural damage in the 2010 haiti earthquake was primarily due to poor construction topographic amplification contributed significantly to damage in the district of petionville south of central port au prince a large number of substantial relatively well built structures situated along a foothill ridge in this district sustained serious damage or collapse using recordings of aftershocks we calculate the ground motion response at two seismic stations along the topographic ridge and at two stations in the adjacent valley ground motions on the ridge are amplified relative to both sites in the valley and a hard rock reference site and thus cannot be explained by sediment induced amplification instead the amplitude and predominant frequencies of ground motion indicate the amplification of seismic waves by a narrow steep ridge we suggest that microzonation maps can potentially be significantly improved by incorporation of topographic effects

groundwater radon anomalies associated with earthquakes abstract earthquake related changes in groundwater radon have been detected at a sensitive observation site located right on a major active fault in northeast japan a time series analysis based on bayesian statistics was successfully applied to remove background variations from the observed radon data enabling us to examine the earthquake related changes in detail we set a simple criterion of amplitude and duration for an anomaly observed in our radon data we define an anomaly as a radon change that kept its level beyond 2 a standard deviation over the whole observation period during a period longer than one day we have observed 20 radon anomalies that satisfied this criterion from january 1984 to december 1988 most of these anomalies have turned out to be related to large earthquakes that occurred in east japan and its surrounding area we have identified 12 post seismic and 2 pre seismic radon anomalies out of a total of 30 earthquakes with magnitude m 6 0 and hypocentral distance d 1000 km the typical pattern of the post seismic anomalies is a radon decrease which started just after an earthquake lasting for periods ranging from a few days to more than one week the amplitude of the post seismic anomalies depends on both magnitude and hypocentral distance and can in general be expressed by a simple magnitude distance relationships a possible pre seismic anomaly was observed about one week before the largest earthquake that occurred in this region during the observation period march 6 1984 m 7 9 d 1000 km another possible pre seismic anomaly was observed about three days before two nearby large earthquakes that occurred at almost the same place in a time interval of 53 min february 6 1987 m 6 4 and m 6 7 d 130 km

time variable response characteristics of groundwater radon to earthquakes a clear coseismic anomaly of groundwater radon was observed for a magnitude 5 6 earthquake that occurred on may 11 1992 the coseismic radon anomaly was observed at a station which is located right on a major active fault in northeast japan and about 140 km away from the hypocenter this was the first time that an earthquake with m 6 had ever been accompanied by a clear radon anomaly at the station although we had observed 12 similar coseismic radon anomalies at the station during the observation period from 1984 to 1987 all of the earthquakes that were accompanied by radon anomalies in that period had been with magnitude 6 0 and over surprisingly the radon concentration has become more sensitive to show coseismic anomalies even for m 5 earthquakes since october 1992 this enhancement of sensitivity of the coseismic radon response may be attributed to the progress of micro crack formation in the fracture zone of the active fault which could be related to unusual stress accumulation in the region

low shear velocity in a normal fault system imaged by ambient noise cross correlation the case of the irpinia fault zone southern italy we extracted the green s functions from cross correlation of ambient noise recorded at broadband stations located across the apennine belt southern italy continuous records at 26 seismic stations acquired for 3 years were analyzed we found the emergence of surface waves in the whole range of the investigated distances 10 140 km with energy confined in the frequency band 0 04 0 09 hz this phase reproduces rayleigh waves generated by earthquakes in the same frequency range arrival time of rayleigh waves was picked at all the couples of stations to obtain the average group velocity along the path connecting the two stations the picks were inverted in separated frequency bands to get group velocity maps then used to obtain an s wave velocity model penetration depth of the model ranges between 12 and 25 km depending on the velocity values and on the depth of the interfaces here associated to strong velocity gradients we found a low velocity anomaly in the region bounded by the two main faults that generated the 1980 m 6 9 irpinia earthquake a second anomaly was retrieved in the southeast part of the region and can be ascribed to a reminiscence of the adria slab under the apennine chain

spectral element simulations of seismic waves generated by the 2009 l aquila earthquake abstract we adopt a spectral element method sem to perform numerical simulations of the complex wavefield generated by the 6 april 2009 m w 6 3 l aquila earthquake in central italy the mainshock is represented by a finite fault solution obtained by inverting strong motion and global positioning system data testing both 1d and 3d wavespeed models for central italy surface topography attenuation and the moho discontinuity are also accommodated including these complexities is essential to accurately simulate seismic wave propagation three component synthetic waveforms are compared to corresponding velocimeter and strong motion recordings the results show a favorable match between data and synthetics up to 0 5 hz in a 200 km 200 km 60 km model volume capturing features mainly related to topography or low wavespeed basins we construct synthetic peak ground velocity maps that for the 3d model are in good agreement with observations thus providing valuable information for seismic hazard assessment exploiting the sem in combination with an adjoint method we calculate finite frequency kernels for specific seismic arrivals these kernels capture the volumetric sensitivity associated with the selected waveform and highlight prominent effects of topography on seismic wave propagation in central italy online material movie of wave propagation waveform fits and stable of station parameters

instrumental evidence of normal mode rock slope vibration summary a unique field experiment was performed to constrain the seismic response of a large potentially unstable rock slope in the southern swiss alps small aperture seismic arrays were deployed to record ambient vibrations both inside and outside of the mapped instability boundary the recordings were analysed by means of the high resolution f k method site to reference spectral ratios and time frequency dependent polarization analysis all three methods indicated that the wavefield within the potentially unstable rock mass is dominated by normal mode motion standing waves rather than horizontal propagation of seismic waves both fundamental frequency and relative amplification could be recovered from ambient noise data the observed amplification is strongly directional and the maximum amplification is oriented perpendicular to open tension cracks mapped at the ground surface our results highlight site response characteristics relevant for analysis of earthquake triggered rock slope failures

fault zone guided waves from explosions in the san andreas fault at parkfield and cienega valley california fault zone guided waves were successfully excited by near surface explosions in the san andreas fault zone both at parkfield and cienega valley central california the guided waves were observed on linear three component seismic arrays deployed across the fault trace these waves were not excited by explosions located outside the fault zone the amplitude spectra of guided waves show a maximum peak at 2 hz at parkfield and 3 hz at cienega valley the guided wave amplitude decays sharply with observation distance from the fault trace the explosion excited fault zone guided waves are similar to those generated by earthquakes at parkfield but have lower frequencies and travel more slowly these observations suggest that the fault zone wave guide has lower seismic velocities as it approaches the surface at parkfield we have modeled the waveforms as s waves trapped in a low velocity wave guide sandwiched between high velocity wall rocks resulting in love type fault zone guided waves while the results are nonunique the parkfield data are adequately fit by a shallow wave guide 170 m wide with an s velocity 0 85 km sec and an apparent q 30 to 40 at cienega valley the fault zone wave guide appears to be about 120 m wide with an s velocity 0 7 km sec and a q 30

northridge earthquake damage caused by geologic focusing of seismic waves despite being located 21 kilometers from the epicenter of the 1994 northridge earthquake magnitude 6 7 the city of santa monica experienced anomalously concentrated damage with mercalli intensity ix an intensity as large as that experienced in the vicinity of the epicenter seismic records from aftershocks suggest that the damage resulted from the focusing of seismic waves by several underground acoustic lenses at depths of about 3 kilometers formed by the faults that bound the northwestern edge of the los angeles basin the amplification was greatest for high frequency waves and was less powerful at lower frequencies which is consistent with focusing theory and finite difference simulations

azimuth dependent amplification of weak and strong ground motions within a fault zone nocera umbra central italy 1 during three moderate magnitude earthquakes occurred in september october 1997 in the central apennines italy accelerations larger than 0 5 g were recorded in the town of nocera umbra 10 to 15 km n nw of the epicenters the accelerograph is sited in a fault zone close to a n30 e tectonic contact six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones the colfiorito sellano area s se of the array and the gualdo tadino area to the north the array data reveal large variations in terms of both peak ground motions and spectral amplitudes within the fault zone amplifications show a strong dependence on the source azimuth at the accelerograph site the effects are particularly large for events from s se peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 hz along the n30 e direction of motion parallel to the strike of the fault nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to m 0 1 2 x 10 25 dyn cm a particle motion analysis shows that the directional effect is also present in the strongest motions even though the amplification of peak ground motion decreases when m 0 increases results from stochastic simulations indicate that such a behavior is not due to nonlinearity applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily

an anomalous radon decrease in groundwater prior to an m6 0 earthquake a possible precursor a significant decrease in the radon concentration of artesian groundwater was observed at a well established observation site in northeast japan about two days before an m 6 0 earthquake occurred about 200 km from the site on the basis of a concurrent in situ minus 126 m temperature drop of the groundwater the authors infer that precursory changes in regional stress were locally amplified near this site and resulted in the temporary mixing of cooler radon poor groundwater into the normal artesian flow

diversity of fault zone damage and trapping structures in the parkfield section of the san andreas fault from comprehensive analysis of near fault seismograms summary we perform comprehensive analyses of trapped waves and signals of damaged fault zone rocks associated with time delays of body waves along the parkfield section of the san andreas fault saf waveforms generated by thousands of earthquakes and recorded by near fault stations in several permanent and temporary deployments are examined with attention to the possible influence of a lithology contrast across the fault on signals of the low velocity damage zone clear candidate trapped waves are identified at only three stations mmnb and two of the other near fault stations flip and pies further to the nw clear candidate trapped waves are not seen at any of the near fault stations to the se of mmnb the locations of the events generating good candidate trapped waves at mmnb and the other two stations are not distributed broadly in space but clustered in a small number of locations moreover events that generate clear candidate trapped waves at one station do not typically generate trapped waves at the other two stations these observations imply that the damage zone is highly variable along strike and that a coherent connected waveguide does not exist for distances along strike larger than at most 3 5 km the distance between stations synthetic waveform fits for observed trapped waves at stations mmnb and flip indicate that the most likely parameters of the trapping structures at these locations are widths of about 150 m depths of about 3 km velocity reductions of 30 40 per cent and q values of 10 40 synthetic calculations of trapped waves demonstrate that if there is a contrast of seismic velocity across the fault the trapped waves are delayed relative to the s wave arrival trapped waves at station mmnb and to a lesser extent also at stations flip and pies in the nw section show this characteristic this suggests a lithology contrast in the top few km at these locations in agreement with results from tomography and studies of head waves in the parkfield area at several mini across fault arrays where trapped waves are not observed a low velocity damage zone is detected from the delay in the arrival time of body wave phases relative to a nearby off fault station the observed delay of the s wave is greater than the p wave delay consistent with the existence of a damage zone with poisson ratio of about 0 33 the observations of time delays without trapped waves indicate that parts of the damage zone are insufficiently coherent to generate trapped waves a broader damage zone may exist in the region between the saf and the south west fracture zone the results highlight the diversity of damage structures along the 40 km of the saf examined in this study and imply that fault imaging based on data at single sites does not necessarily apply to a larger section

a methodological approach towards high resolution surface wave imaging of the san jacinto fault zone using ambient noise recordings at a spatially dense array we present a new technique for deriving detailed information on seismic velocities of the subsurface material from continuous ambient noise recorded by spatially dense seismic arrays this method uses iterative double beamforming between various subarrays to extract surface wave contributions from the ambient noise data in complex environments with unfavourable noise source distributions the iterative double beamforming extraction makes it possible to retrieve large amounts of rayleigh wave traveltime information in a wide frequency band the method is applied to data recorded by a highly dense nodal array with 1108 vertical geophones centred on the damage zone of the clark branch of the san jacinto fault zone south of anza california the array covers a region of 650 700 m 2 with instrument spacing of 10 30 m and continuous recording at 500 samples s 1 over 30 d in 2014 using this iterative double beamforming on subarrays of 25 sensors and cross correlations between all of the station pairs we separate surface waves from body waves that are abundant in the raw cross correlation data focusing solely on surface waves maps of traveltimes are obtained at different frequencies with unprecedented accuracy at each point of a 15 m spacing grid group velocity inversions at 2 4 hz reveal depth and lateral variations in the structural properties within and around the san jacinto fault zone in the study area this method can be used over wider frequency ranges and can be combined with other imaging techniques such as eikonal tomography to provide unprecedented detailed structural images of the subsurface material

fracture related wavefield polarization and seismic anisotropy across the greendale fault we investigate seismic signatures of fracturing in a newly ruptured strike slip fault by determining the wavefield polarization in the new zealand canterbury plains area and across the greendale fault which was responsible for the 3 september 2010 darfield mw 7 1 earthquake previous studies suggested that fractured rocks in fault damage zones cause directional amplification and ground motion polarization in the fracture perpendicular direction as an effect of stiffness anisotropy and cause velocity anisotropy with shear wave velocity larger in the fracture parallel component an array of 14 stations was installed following the darfield earthquake we assess polarization both in the frequency and time domains through the individual station horizontal to vertical spectral ratio and covariance matrix analysis respectively and compare the results to previously reported anisotropy measurements from shear wave splitting stations installed in the canterbury plains have an amplification peak between 0 1 and 0 3 hz for both earthquakes and ambient noise we relate the amplification to the resonance of a considerable thickness c 1 km of soft sediments lying over the metamorphic bedrock analysis of seismic events revealed the existence of another peak in amplification between 2 and 5 hz at two on fault stations which was not visible in the noise analysis in contrast to the lower frequency peak the ones between 2 and 5 hz are more strongly anisotropic attaining amplitudes up to a factor of 4 in the n52 direction to interpret this effect we model the fracture pattern in the fault damage zone produced by the fault kinematics we conclude that the horizontal polarization is orthogonal to extensional fractures which predominate in the shallow layers 2 km expected in n101 direction on the basis of the model thus different fracture distribution at different depths may explain different results for amplification compared to anisotropy we propose polarization amplification analysis as a complementary method to s wave splitting analysis polarization analysis is rapidly computed and robust and it can be applied to either earthquakes or ambient noise recordings giving useful information about the predominant fracture patterns at various depths

surface wave dispersion measurements from ambient seismic noise analysis in italy summary we present the surface wave dispersion results of the application of the ambient noise method to broad band data recorded at 114 stations from the istituto nazionale di geofisica e vulcanologia ingv national broad band network some stations of the mediterranean very broadband seismographic network mednet and of the austrian central institute for meteorology and geodynamics zamg vertical component ambient noise data from 2005 october to 2007 march have been cross correlated for station pairs to estimate fundamental mode rayleigh wave green s functions cross correlations are calculated in 1 hr segments stacked over periods varying between 3 months and 1 5 yr rayleigh wave group dispersion curves at periods from 8 to 44 s were determined using the multiple filter analysis technique the study region was divided into a 0 2 0 2 grid to invert for group velocity distributions checkerboard tests were first carried out and the lateral resolution was estimated to be about 0 6 the resulting group velocity maps from 8 to 36 s show the significant difference of the crustal structure and good correlations with known geological and tectonic features in the study region the po plain and the southern alps evidence lower group velocities due to soft alluvial deposits and thick terrigenous sediments our results also clearly showed that the tyrrhenian sea is characterized with much higher velocities below 8 km than the italian peninsula and the adriatic sea which indicates a thin oceanic crust beneath the tyrrhenian sea

amplification of ground motion and waveform complexity in fault zones examples from the san andreas and calaveras faults summary p wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of gaussian beams for point sources located in a low velocity wedge surrounding a fault the computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the san andreas and haywardcalaveras faults calculations in these models show that the 10 20hz vertical displacements of earthquakes located at 3 10 km depth are amplified by up to an order of magnitude in a 1 2 km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust this amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike slip earthquake depending on the source depth of the earthquake and the structure and velocity contrast of the wedge multiple triplications in the travel time curve of direct p and s waves will occur at stations in the fault zone a wedge model successfully predicts the triplications observed in the p waveforms of aftershocks of the coyote lake earthquake recorded in the fault zone showing that body waves from microearthquakes can be used to determine the three dimensional velocity structure of the fault zone the amplification waveform complexity and distortion of ray paths introduced by the lowvelocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone for realistic models of the wedge asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20hz numerical methods may be necessary to calculate accurately the wavefield at lower frequencies

shallow high resolution geophysical investigation along the western segment of the victoria lines fault island of malta abstract the victoria lines fault island of malta is a 15 km long and n260 striking segmented normal fault system which is probably inactive since the late pliocene in the westernmost part the fomm ir rih segment displays comparable geologic throw and escarpment height 150 170 m moreover its hangingwall hosts thin patches of middle pleistocene clastic continental deposits red beds which are poorly preserved elsewhere we acquired two seismic transects by collecting ambient vibration recordings processed by using horizontal to vertical spectral ratios complemented by one high resolution 2 d refraction tomography survey crossing this fault where it is locally covered by red beds and recent colluvial deposits we found a resonance peak at 1 0 hz in the hangingwall block whereas clear peaks in the range 5 0 10 0 hz appear when approaching the subsurface fault and we relate them to the fractured bedrock within the fault zone the best fit tomographic model shows a relatively high vp shallow body vp 2200 2400 m s that we relate to the weathered top of the miocene upper coralline limestone fm bounded on both sides by low vp regions 230 m s above the weathered top bedrock our results depict a clear seismic signature of the victoria lines fault characterized by low seismic velocity and high amplification of ground motion we hypothesize that during the middle pleistocene faulting may have affected the basal part of the red beds so that this part of the investigated complex fault system may be considered inactive since 0 6 myr ago

bouldery slope landforms on mt biseul korea and implications for paleoclimate and slope evolution identification of bouldery landforms in mountains and correctly understanding their formative processes play an important role in reconstructing the geomorphic history of a region we propose that blocks were liberated by frost cracking and wedging of cliff walls during the last glacial period however we further suggest and test four hypotheses comprising different scenarios for preconditioning by chemical weathering and subsequent block transport using terrain analysis characterization of boulders and 10 be exposure dating frost shattering from the backing cliff produced the boulders since the beginning of the last glacial period 80 ka and gelifluction transported them downslope throughout the last glacial period their activity then entered a dormant phase at the beginning of the holocene distribution patterns of exposure ages of tors and block streams are similar to those of previous studies implying that bouldery landscapes in the southern korean peninsula were likely to be formed by similar processes under periglacial conditions the timing of active periods in transport of block streams corresponds well with the cold periods identified in regional and global climate proxy records interestingly the activity of block streams in the study area reached a maximum during marine oxygen isotope stage 3 to 2 when the growth rate of nearby speleothems was lowest

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

last glacial and holocene fluvial wetland sedimentary stratigraphy comparison between soro ri and jangheung ri archeological sites korea abstract environmental changes in wetlands during the last glacial reflect the fluvial sedimentary sequences of south korea the stratigraphy of the latest pleistocene sequences in several fluvial drainage basins includes organic mud layers intercalated in fluvial deposits particularly including those on the soro ri and jangheung ri sites of the miho river and nam river respectively research methods included analyses of sedimentary facies 14 c dating and palynology the post last glacial maximum post lgm terrestrial sequences are interpreted as responses to fluctuations in fluvial environments in general fluvial sequences display cyclicity including fluvial channel sand and gravel deposits organic muds of backswamp origin and overbank flood deposited muds with intercalated paleosols fluvial sedimentary sequences are interpreted to be associated with backswamp organic muds of last glacial age the age of formation of the lower organic mud layer on the sorori site is greater than 36 ka bp and that of the upper organic mud layer ranges from 17 to 12 ka bp since 18 ka bp at the beginning of the post lgm period erosional processes became more pronounced from about 17 15 ka fluvial depositional processes prevailed to the end of the last glacial period the young fluvial sequences intercalated with organic mud layers are associated with the late glacial bolling and allerod intervals indicating mis 1 in korea

terrestrial heat flow around the aseismic front of the japanese island arc abstract very few data on heat flow are available in the area around the aseismic front of the japanese islands in order to remedy this situation measurements of the terrestrial heat flow were made at three locations in the eastern part of fukushima prefecture northeastern honshu japan the observed values of heat flow were 37 mw m 2 0 88 cal cm 2 s at soma 52 mw m 2 1 25 cal cm 2 s at kashima and 19 mw m 2 0 46 cal cm 2 s at naraha respectively these data partially fill the gaps in the terrestrial heat flow data on land in northeastern honshu japan these values are considerably lower than the average heat flow over the world but agree well with the previous estimation for the area

directional resonance variations across the pernicana fault mt etna in relation to brittle deformation fields summary the pernicana fault pf is the main structural element of mt etna and the northern boundary of a section sliding to the southeast observed ground motion records in the damage zone of the pf show strong variations of directional resonance in the horizontal plane the observed resonance directions exhibit an abrupt rotation of azimuth by about 30 across the fault varying from n166 on the north side to n139 on the south we interpret the directional resonance observations in terms of changes in the kinematics and deformation fields on the opposite sides of the fault the northern side is affected primarily by the left lateral strike slip movement whereas the southern side that is subjected also to sliding is under a dominant extensional stress regime brittle deformation models based on the observed kinematic field predict different sets of fractures on the opposite sides of the fault synthetic cleavages and extensional fractures are expected to dominate in the northern and southern sides respectively these two fracture fields have different orientations n74 and n42 respectively and both show a near orthogonal relation 88 in the northern sector and 83 to the south with the azimuth of the observed directional resonance we conclude that the direction of the largest resonance motions is sensitive to and has transversal relationship with the dominant fracture orientation the directional amplification is inferred to be produced by stiffness anisotropy of the fault damage zone with larger seismic motions normal to the fractures

are microtremors useful in site response evaluation we have reviewed the applicability of microtremor measurements to evaluate site response of soft soils to this end we evaluated three different techniques commonly used to estimate site effects from microtremor measurements interpretation of fourier amplitude spectra computation of spectral ratios relative to a firm reference station and finally computation of spectral ratios of horizontal components relative to the vertical component of ground motion nakamura s technique these techniques are applied to microtremor records obtained in three cities in mexico mexico city oaxaca and acapulco these cities differ in their local geological conditions and in their seismotectonic environment in order to evaluate the results obtained from microtremor measurements we compare them with standard spectral ratios of the intense s wave part of weak or strong motion records obtained at the same sites our results showed that microtremor measurements can be used to estimate the dominant period of a site with very acceptable reliability in the range 0 3 to 5 hz the best results were obtained with nakamura s technique which also gives a rough estimate of amplification of seismic waves when the local geology is relatively simple simple numerical simulations indicate that the underlying assumptions of nakamura s technique are consistent with the propagation of rayleigh waves these simple numerical simulations also explain why different researchers have been able to successfully characterize 1d site effects using microtremor records regardless of whether they consider microtremors to consist of surface or body waves our results strongly suggest that the technique by nakamura effectively compensates for source effects in microtremor measurements which eliminates a major limitation to their application in earthquake engineering

observed variations of earthquake motion across a sediment filled valley the response to earthquake motion of a site in one sediment filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal s azimuth and incidence angles the observed input motion was weak varying between 10 5 and 10 3 g measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites the valley was 400 m wide and 700 m long with a maximum sediment thickness of 60 m the average seismic impedance contrast between the sediments and underlying basement rock was about 6 1 ratios of fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten depending strongly on frequency and the distance of the site from the valley edge the scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes ground motion at valley edge and mid valley sites separated by less than 100 m differed by as much as a factor of five this difference could cause large differential motion in structures spanning these sites a theoretical flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge the lowest frequency responance apparently involves the sediments across the width of the valley not just the vertical column of soil beneath each site

normal faults and thrusts reactivated by deep fluids the 6 april 2009 mw 6 3 l aquila earthquake central italy 1 on 6 april 2009 a mw 6 3 earthquake occurred in the central apennines italy damaging the city of l aquila and the surrounding country we relocate the october 2008 to 6 april 2009 foreshocks and about 2000 aftershocks occurred between 6 and 30 april 2009 by applying a double difference technique and determine the stress field from focal mechanisms the events concentrate in the upper 15 km of the crust three main nw se to nnw sse striking 30 45 and 80 90 dipping faults were activated during the seismic sequence among these a normal fault and a thrust were reactivated with dip slip movements in response to ne sw extension the structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern apennines because of the lower strain rate of the central sector of the chain with respect to the southern one vp vs increases progressively from october 2008 to the 6 april 2009 main shock occurrence along a nw se strike because of an increment in pore fluid pressure along the fault planes pore pressure diffusion controls the space time evolution of aftershocks a hydraulic diffusivity of 80 m2 s 1 and a seismogenic permeability of about 10 12 m2 suggest the involvement of gas rich co2 fluids within a highly fractured medium suprahydrostatic high fluid pressure about 200 mpa at 10 km of depth within overpressurized traps bounded by preexisting structural and or lithological discontinuities at the lower upper crust boundary are required to activate the april 2009 sequence traps are the storage zone of co2 rich fluids uprising from the underlying about 20 km deep metasomatized mantle wedge these traps easily occur in extensional regimes like in the axial sector of apennines but are difficult to form in strike slip regimes where subvertical faults may cross the entire crust in the apennines fluids may activate faults responsible for earthquakes up to mw 5 6 deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges

the whisper of the hydrothermal seismic noise at ischia island abstract this work is devoted to the study of the background seismic noise at ischia island we highlight two persistent independent signals with dominant frequency peaks in the range 1 2 hz and 3 4 hz respectively the first signal is the most energetic and persistent and its polarization properties are well defined revealing a shallow propagation likely controlled by both tectonic volcanic structures and morphological features this signal is well correlated at all the stations in the casamicciola area and its group velocity is estimated as 500 600 m s the second signal shows similar polarization properties but the temporal pattern of its amplitude is strongly affected by anthropogenic noise revealing a modulation on a diurnal scale on the investigated spatial scales any clearly correlated signal is not evidenced we interpret the 1 2 hz correlated wave packets as the persistent whisper of the shallow circulation of fluids which are a mixing of sea and meteoric water and thermal fluids of the hydrothermal reservoir in the conceptual scheme of the self sustained musical instruments we hypothesize that the solid structure of the shallow hydrothermal system of ischia is formed by a network of channels continuously excited by the circulating hydrothermal fluids which produce a whispered sound i e the seismic noise this mechanism is able to produce persistent self sustained oscillations that we observe as the 1 2 hz seismic signal and eventually higher modes

site effects in unstable rock slopes dynamic behavior of the randa instability switzerland we evaluate the dynamic response of the large around five million cubic meters unstable rock slope at randa switzerland based on ground motion recordings during small earthquakes seismic measurements revealed strong polarization of the wavefield in the direction of maximum slope displacement with spectral amplification factors 5 within the unstable area results further highlighted resonant frequencies of both the large unstable rock mass and individual blocks within it block vibration was confirmed by phase analysis of in situ displacement measurements from two bounding tension fractures during a small earthquake numerical simulation using a discontinuous elastic model showed that the presence of steeply dipping compliant fractures is crucial for recreating the observed site response such fractures are common in rock slope instabilities and are here shown to contribute to polarization and amplification of seismic shaking site effects that are rarely considered for hard rock slopes and which may influence the potential for earthquake triggered failure

the structure of an exhumed intraplate seismogenic fault in crystalline basement abstract the 600 m thick gole larghe fault zone glfz is hosted in jointed crystalline basement and exposed across glacier polished outcrops in the italian alps ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes solidified frictional melts formed at 9 11 km depth and ambient temperatures of 250 300 c previous work focused on the southern part of the fault zone here we quantitatively document fault zone structure across the full width of the glfz and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike these new datasets indicate that the glfz has a broadly symmetric across strike damage structure and contains distinct southern central and northern zones distinguished by large variations in fracture density distribution of pseudotachylytes volume of fault rock materials and microfracture sealing characteristics the c 100 m wide central zone is bound by two thick 2 m and laterally continuous 1 km protocataclastic to ultracataclastic horizons within and immediately surrounding the central zone fracture density is relatively high due to cataclastic fault fracture networks that reworked earlier formed pseudotachylytes the fault fracture networks were associated with pervasive microcracking and fluid rock interaction resulting in the development of a c 200 m thick alteration zone delimited by lobate fluid infiltration fronts in the c 250 m thick southern and northern zones fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre existing magmatic cooling joints analysis of the structure of the glfz suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 au sable forks intraplate earthquake sequence in the northeastern usa including seismicity distributed across a fault zone 500 1000 m thick and large 100 mpa static stress drops associated with frictional melting

can seismic waves be trapped inside an inactive fault zone the case study of nocera umbra central italy in march 1998 a small aperture 350 m linear array of five broadband stations was installed across a pliocene strike slip fault crossing the town of nocera umbra central italy three component seismograms of local earthquakes recorded during the umbria marche seismic sequence have shown that the degradation of mechanical properties in damaged rock causes a significant amplification of ground motion within the fault zone marra et al 2000 during the operation of the array an m w 5 3 subcrustal earthquake occurred at a depth of 48 km and an epicentral distance of 10 km from nocera umbra the near vertical incidence of this event offers a unique opportunity for constraining the 2d geometry of the laterally heterogeneous low velocity structure responsible for the large amplification across the fault based on the observed symmetry of the amplification pattern a 2d wedge shaped model of the fault zone is used for the computation of antiplane sh synthetic waveforms to be compared with observations along the array two parameters of the wedge model can be constrained by independent geophysical and geological data 1 shear wave velocity in pristine rock outside the fault zone v s 2200 m sec and 2 the lateral extent of the damaged rock zone l 160 m the other two model parameters namely the wedge depth h and shear wave velocity of the fault zone v f s are varied computer simulations show that trapped waves which have been observed so far for earthquakes occurring inside the fault zone can develop inside a 2d wedge model when struck by external vertically incident seismic radiation results of numerical modeling indicate that in spite of its extreme simplicity such an idealized model is successful in reproducing many of the essential characters of ground motion within the fault zone of nocera umbra even though the best fit solution is not unique and the model parameters trade off each other a study of parameter sensitivity comparing synthetics with recorded data indicates that the variability of h and v f s can be restricted to a limited range of values the quality of fit is optimal when the shear velocity reduction lies in the range of 40 50 and the depth of the trapping zone is comprised between 1 and 2 km