local site effects in atakoy istanbul turkey due to a future large earthquake in the marmara sea summary since the 1999 izmit and duzce earthquakes in northwest turkey many seismic hazard studies have focused on the city of istanbul an important issue in this respect is local site effects strong amplifications are expected at a number of locations due to the local geological conditions in this study we estimate the local site effects in the atakoy area southwestern istanbul by applying several techniques using synthetic data hybrid 3 d modelling and 1 d modelling and comparing to empirical data we apply a hybrid 3 d finite difference method that combines a complex source and wave propagation for a regional 1 d velocity model with site effects calculated for a local 3 d velocity structure the local velocity model is built from geological geotechnical and geomorphological data the results indicate that strongest spectral amplifi cations sa in the atakoy area occur around 1 hz and that amplification levels are largest for alluvial sites where sa reaching a factor of 1 5 2 can be expected in the case of a large earth quake we also compare our results to h v horizontal to vertical component of the recorded signal spectral ratios calculated for microtremor data recorded at 30 sites as well as to ambient noise synthetics simulated using a 1 d approach because the applied methods complement each other they provide comprehensive and reliable information about the local site effects in atakoy added to that our results have significant implications for the southwestern parts of istanbul built on similar geological formations for which therefore similar sa levels are expected

an approach to improve rayleigh wave ellipticity estimates from seismic noise application to the los angeles basin s u m m a r y we present an approach for seismic noise which improves rayleigh wave ellipticity estimates by reducing the influences from body waves and love waves the method requires three component seismographs and uses the phase shift information between the vertical and horizontal components we select data that show the phase shift of about 90 in order to separate rayleigh waves from body waves and love waves in comparison to the usual h v approach the estimates for rayleigh wave ellipticity are systematically different by about 20 per cent because the existence of s waves and love waves systematically increases horizontal amplitudes the differences in the inverted s wave velocity structure reach up to about 20 per cent in the upper 20 km the method is limited to a relatively low frequency range below 0 3 hz as the 90 phase shift peaks become difficult to identify above 0 3 hz despite this limitation in the frequency range this approach can improve the s wave velocity structure in the upper 5 10 km which should lead to a better prediction of long period ground motions for periods of about 5 10 s an improved prediction of such long period ground motion is important for hazard mitigation in the world s metropolitan areas with high rise buildings this approach is applied to the structure in the broader los angeles basin in southern california and improvement of the reference velocity structure in the upper 5 10 km is demonstrated

can broad band earthquake site responses be predicted by the ambient noise spectral ratio insight from observations at two sedimentary basins site effect assessments performed through earthquake based approaches such as the standard spectral ratio ssr require good quality records of numerous earthquakes in contrast the use of ambient noise appears to be an attractive solution for ease and rapid computation of site responses with sufficient spatial resolution microzonation especially in low seismicity areas two main approaches are tested here the horizontal to vertical spectral ratio hvsr and the noise based ssr ssrn the hvsr uses the relative amplitude of the horizontal and vertical components of the ambient noise instead the ssrn defines the spectral ratio between the seismic noise recorded simultaneously at a site and at a rock reference station similar to earthquake based ssr while the hvsr is currently used in hundreds of site specific studies the ssrn approach has been gradually abandoned since the 1990s in this study we compare the results obtain from these two approaches with those of earthquake based ssr this comparison is carried out for two sedimentary basins in provence southeastern france and in argostoli western greece in agreement with the literature the hvsr does not provide more than the fundamental resonance frequency of the site f0 the ssrn leads to overestimation of the ssr amplification factors for frequencies higher than the minimal f0 of the basin f0min this discrepancy between ssrn and ssr is discussed and appears to be mainly dependent on the local geological configuration we thus introduce the hybrid standard spectral ratio ssrh approach which aims to improve upon the ssrn by adding an intermediate station inside the basin for which the ssr is known this station is used in turn as a local reference inside the basin for the ssrn computation the ssrh provides site transfer functions very similar to those of the ssr in a broad frequency range based on these results the ssrn or ssrh should be further tested and should receive renewed attention for microzonation inside sedimentary basins

microtremor measurement to evaluate site characteristics by horizontal to vertical spectral ratio technique in sikkim northeast himalayas india abstract the measurement of site amplification of microtremors is an important tool for evaluating seismic hazard at local and regional level the h v spectral ratio hvsr method is applied to estimate the amplification of ground motion at resonance frequency the fundamental resonance frequency is estimated at 8 different sites in study region and shows the maximum vulnerability of the site to the earthquakes this frequency ranges between a minimum of 2 8 hz to a maximum of 9 3 hz the thickness of the sediments below the surface of recording sites is calculated using the empirical relationship that comes out to be in a particular range of 9 5 m 33 1 m above the bed rock the sedimentary thickness and the resonance frequency clearly matched with geological setting of the study area the observation shows a slow reduction in the thickness of the sediments and increase in the resonance frequency from south western part to north eastern part of the study region the results show that most of the sites are highly vulnerable to seismic risks with high amplification of seismic waves in the area thus current study provides a deep insight in to the seismic hazard of the sikkim region with a clear emphasis on the site amplification and the sedimentary cover for the region

stability assessment and quantitative evaluation of h v spectral ratios for site response studies in kumaon himalaya india using ambient noise recorded by a broadband seismograph network the kumaon himalaya region in india has accumulated considerable seismic risk potential from moderate to high seismicity due to ongoing tectonic evolutionary processes to assess conditions of high seismic risk arising from local site effects at populated locations we applied the empirical horizontal to vertical h v spectral amplitude ratio method nakamura in quarterly reports of the railway technical research institute tokyo 30 25 33 1989 using ambient seismic noise recorded by a network of 32 digital broadband seismographs from june 2005 to june 2008 the data and the estimated parameters were subjected to stability tests to assess the effect of seasonal variations seasonal variations in the ambient noise data seemed to show a lesser effect on fundamental frequency estimates and a slightly greater effect on the peak h v amplitudes validation of the ambient noise results was done by complementary tests using h v ratios of local and regional earthquakes the peak corresponding to the fundamental resonance frequency is prominently present in both the ambient noise and the earthquake datasets the study locations showed distinct h v curve topologies corresponding to the effects of both velocity contrast at well defined frequencies and characteristic topographic effect around a certain frequency range the clearly indicated peaks in the h v curves possibly correspond to velocity contrasts created by weathered sediments overlying hard rock basements in rocky hills our study indicates high site responses at many populated locations near the surface trace of the seismically active main central thrust mct and other fault boundaries the fundamental resonance frequencies estimated from the site response studies at the 32 locations could be useful in preliminary site characterization ground motion prediction and seismic hazard analysis

new relationships between vs thickness of sediments and resonance frequency calculated by the h v ratio of seismic noise for the cologne area germany noise measurements were carried out in the cologne area germany and the resonance frequency of each site was estimated from the main peak in the spectral ratio between the horizontal and vertical component for 32 of these sites the thickness of the sedimentary cover was known from boreholes and a clear correlation between resonance frequency and sediment thickness was observed a formula that correlates cover thickness with frequency of the main peak in the horizontal to vertical spectral ratio was derived in addition a best fitting shear wave velocity distribution with depth vs z as well as a relationship between average shear wave velocity vs and thickness of the sedimentary cover was calculated by using all of the noise measurements and applying the derived relationships we obtained a subsoil classification for the cologne area

a comparison and test of various site response estimation techniques including three that are not reference site dependent abstract we compare various site response estimation techniques using after shock data of the 1989 loma prieta earthquake collected in oakland california because of recent interest in comparing results from weak and strong motion to infer any nonlinearity and between direct s and coda waves we pay particular attention to the uncertainties first sediment to bedrock spectral ratio estimates between pairs of sites are compared with those obtained from various generalized inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously we find that the site amplification factors are very similar among these approaches but that the uncertainties can be significantly different depending on how the data are weighted we also examine and test three site response estimation techniques that do not rely on a reference site to estimate source and path effects the first involves a parameterized source and path effects inversion even when the bedrock data are excluded from consideration this approach is found to reveal the frequency dependence of site response at each of the sediment sites the second technique involves taking horizontal to vertical component spectral ratios receiver function type estimates of shear wave aftershock data these are also found to reveal the frequency dependence of site response at the sediment sites and the results for the bedrock site are relatively flat and near unity the third estimate is formed by taking horizontal to vertical component ratios of ambient seismic noise and these are shown to reveal the fundamental resonant frequency of the sediment sites unfortunately discrepancies exist among all of the site response estimates and with one dimensional predictions with respect to a frequency independent scaling factor nevertheless the highly frequency dependent character of site response is well constrained and the fact that non reference site dependent methods are capable of revealing this is promising for site specific hazard assessments in regions that lack adequate reference sites

investigation of site properties in adapazari turkey using microtremors and surface waves determination of suitable areas for new residential buildings is crucial to increase the resilience of buildings against earthquake hazards after the 1999 izmit earthquake mw 7 4 the city of sakarya has been expanding rapidly in terms of both the population and number of superstructures despite the fact that sakarya suffered several large earthquakes in the last two decades the geophysical properties of the region have not been adequately investigated in this study the site properties of sakarya university esentepe campus and its surrounding serdivan district which is one of the important parts of the city are determined using microtremor measurements and surface wave analysis nakamura s spectral ratio method spectral ratio between the horizontal and vertical components h v or hvsr was used to determine the fundamental frequency and site amplification values the shear wave velocity profiles of the studied sites were determined using the multichannel analysis of surface waves method seismic measurements were performed at 34 locations to record the surface waves in the area the fundamental frequency and site amplification values are determined as 1 02 11 68 and 1 33 5 96 hz respectively from the microtremor measurements the fundamental frequency is between 4 0 and 11 5 hz in the campus area and between 1 3 and 2 0 hz near the d 100 intercity highway the site amplification was determined to be 1 2 5 in the campus area the greatest site amplification 6 is obtained at thick alluvium deposits in the valley between hills the average shear wave velocity values are within the range of 300 and 1120 m s some parts located on the hill area have better soil condition b categories according to the national earthquake hazard reduction program and have comparatively high shear wave velocities in the range of 740 1080 m s whereas low velocity values are found at the thick alluvium deposits d we found that the geological properties and topographies play very important roles on the shear wave velocity and the amplification factors in the investigated 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

ambient noise and weak motion excitation of sediment resonances results from the tiber valley italy abstract we recorded ambient noise and weak earthquake ground motion on holocene sediments in the tiber valey north of rome in order to help predict the response of these sediments to strong ground motion ambient noise within the valley is high and sharply peaked near 2 5 hz similar peaks are observed along the tiber valley towards the south in and south of rome a small local earthquake simultaneously recorded within the valley and at a reference hard rock site is dominated by energy at 4 to 20 hz at the reference site but appears to excite low frequency resonances at the sediment site we also recorded noise at a small array of four stations within the valley and apply the spatial auto correlation method of aki 1957 1965 to obtain rayleigh wave dispersion between 1 and 5 hz the observed resonance and dispersion can be used to determine velocity structure in the near surface sediments between 1 and 5 hz we obtain phase velocities of approximately 400 to 500 m sec implying a shear wave velocity of 440 to 530 m sec the holocene sediment layer is constrained from geotechnical data to have a thickness of 40 m our velocity results thus predict a quarter wavelength resonance of 2 8 to 3 3 hz close to that observed in the ambient noise and weak motion data

hvsr technique in near surface thermal basin characterization the example of the caldiero district north east italy in the present work non invasive microtremor survey is applied for the preliminary characterization of the thermal district of caldiero in the eastern po plain italy a thermal resource known since the roman times for the presence of warm springs the work s aim was to test the suitability of the microtremor passive technique for the preliminary characterization of a thermal basin in terms of resonance properties between the alluvial deposit covers and the rocky bedrock in particular the horizontal to vertical spectral ratio hvsr single station technique is adopted to estimate the resonance properties of the geothermal basin more than 100 hvsr single station measurements are collected and analyzed in the area studied hvsr information is integrated with the findings of the analysis of the data relative to more than 20 deep wells such an integration permits the calibration for validation of the geophysical data the correlation between the results from microtremor survey lidar data analysis and wells information highlighted the main geological structures of the site which are potentially responsible for the thermal waters arising hvsr results are then interpreted in terms of regional geological setting textural and hydrogeological properties of the superficial deposits and the underlying rocky bedrock the results of this method confirm the existence of significant correlations between the resonance behavior of the basin as determined by the hvsr technique and the geological assessment this suggests that the single station technique could be a very promising tool for a rapid and preliminary non invasive method for the near surface general characterization of geothermal basins

integrating ambient noise with gis for a new perspective on volcano imaging and monitoring the case study of mt etna abstract the timely estimation of short and long term volcanic hazard relies on the availability of detailed 3d geophysical images of volcanic structures high resolution seismic models of the absorbing uppermost conduit systems and highly heterogeneous shallowest volcanic layers while particularly challenging to obtain provide important data to locate feasible eruptive centres and forecast flank collapses and lava ascending paths here we model the volcanic structures of mt etna sicily italy and its outskirts using the horizontal to vertical spectral ratio method generally applied to industrial and engineering settings the integration of this technique with web based geographic information system improves precision during the acquisition phase it also integrates geological and geophysical visualization of 3d surface and subsurface structures in a queryable environment representing their exact three dimensional geographic position enhancing interpretation the results show high resolution 3d images of the shallowest volcanic and feeding systems which complement 1 deeper seismic tomography imaging and 2 the results of recent remote sensing imaging the study recovers a vertical structure that divides the pre existing volcanic complexes of ellittico and cuvigghiuni this could be interpreted as a transitional phase between the two systems a comparison with recent remote sensing and geological results however shows that anomalies are generally related to volcano tectonic structures active during the last 17 years we infer that seismic noise measurements from miniaturized instruments when combined with remote sensing techniques represent an important resource to monitor volcanoes in unrest reducing the risk of loss of human lives and instrumentation

measuring the sediment thickness in urban areas using revised h v spectral ratio method abstract for the sake of safety it is crucial for civil engineers to determine the thickness of the sediment layers in the urban areas however the noisy detection environment in a city presents a significant challenge to the traditional artificial source geophysical methods which are used to estimate and monitor of sediment thickness given this otherwise challenging detection environment the h v spectral ratio method provides an effective and cheap means of detecting sediment thickness it exploits microtremor signals to obtain the resonant frequency of the surface layer which is then used to determine site sediment thickness with an empirical equation using the data from a single observation station in this study we collected microtremor data at 109 locations in the aoshan bay area of the shandong peninsula and determined the resonant frequency and analysed the morphological features of the h v spectral ratio curves in order to calibrate the frequency bedrock depth equation accurately to the local conditions vs profile estimation is used to fine tune the frequency bedrock depth equation by comparing the s wave velocity structure obtained using the microtremor spac method with that found from borehole samples we were able to verify the reliability and accuracy of application of the microtremor spac method by using the h v spectral ratio method and the modified equation that transforms resonance frequencies to sediment thickness we generated a thickness map of the sedimentary layers in the aoshan bay area of the shandong peninsula the map provides useful information for ensuring the safe construction of the city s infrastructure we conclude that the use of microtremor signals can provide important geological information as a cheaper and quick way for the construction and development of urban infrastructure

using microtremors to assess potential earthquake site response a case study in flushing meadows new york city abstract we have used portable 5 sec period seismometers to record ambient noise across a glacially cut valley filled with unconsolidated holocene sediments the valley located in flushing meadows new york city is surrounded by a densely populated urban environment microtremors were recorded at several sites on and adjacent to the valley while some raw spectra of valley site microtremors exhibit no predominant peaks spectral ratios taken with respect to recordings made at a reference station outside the valley show a sharp peak near 0 8 hz a secondary peak near 2 4 hz is also visible in some of the spectral ratios the frequency of the predominant peak remains constant across the valley width while the relative amplitude decreases systematically from a maximum at the valley center to background levels near the valley edge resonant frequencies predicted by one dimensional modeling of the valley center site response show excellent agreement with observations the 0 8 hz spectral ratio peak is interpreted as the fundamental resonance mode and the 2 4 hz peak is interpreted as the first harmonic of the region above the base of an organic clay layer observed in borehole data the amplification factors implied by the observed spectral ratios are unrealistically high up to 100 which can be attributed to the close proximity of microtremor sources an experiment involving the use of a pile driver source located outside the valley was also performed signals obtained at valley sites during times when the pile driver was in operation show a two fold increase in the amplitude of a spectral peak when compared to signals obtained when the pile driver was turned off this observation gives additional evidence that observed spectral peaks represent soil layer resonances in the valley based on the results of this and other previous studies we conclude that easily obtained microtremor measurements can contribute valuable microzonation and site response information to a study of regional earthquake hazard

frequency dependent acoustic seismic coupling of meteor shock waves simultaneous infrasonic and seismic observations of meteor shock waves have been recorded at a collocated array in southwestern ontario canada analysis of signals from seven events that exhibited acoustic seismic coupling representing 0 6 of the meteor flux detected using other systems suggests signifi cant filtering and amplitude enhancement indicative of a frequency dependent acous tic transfer function the acoustic response at the site possesses the properties of an 10 hz high pass filter on incident shock waves with ground motions up to an order of magnitude larger than theoretical predictions in terms of energy the air to ground energy coupling efficiency is measured at 1 3 0 8 above this cutoff overall cou pling efficiencies however are much lower between 0 0007 and 0 071 as the dominant frequencies of the incident shock waves typically 0 1 10 hz are weak or nonexistent in the seismic signals such strong frequency dependent effects suggest that without detailed knowledge of the site and its influences on the subsequent ground motion transformation of seismic data to produce pseudopressure data for the purposes of quantitative source energy estimation may be unreliable for seismom eters located within thick overburden

observation of 1 to 5 second microtremors and their application to earthquake engineering part iii a two dimensional study of site effects in the san fernando valley abstract a two dimensional study of the influence of deep sediment on seismic ground motions was conducted by recording and analyzing long period microtremors in the san fernando valley california the recordings were made at 50 regularly distributed sites in the valley three reference sites were employed at the baserock outcrop around the valley so as to observe the time dependent characteristics of the microtremors 1 amplitudes in a period range of several seconds correlate with the thicknesses of the sedimentary layers 2 a site amplification effect which was evaluated in terms of sediment to rock spectral ratios is qualitatively consistent with available geological and strong motion data 3 the practical field observation procedure designed for this study the two dimensional approach can be easily used in other surveys for estimating the spatial characteristics of earthquake ground motions in the period range of several seconds

seismic microzonation in australia abstract since the 1980s seismic microzonation studies have been undertaken in australia to assess the likely effects of earthquakes on urban centres built on unconsolidated sediments presently the nakamura method is used for processing data so far parts of perth adelaide cairns gladstone rockhampton newcastle sydney and launceston have been zoned the launceston tasmania study was the pilot study for many of these as it refined the methodology used and the data obtained were incorporated into a gis database building heights and site factor zoning maps were produced for the launceston city council one of the major activities of the new initiative by the australian geological survey organisation agso popularly known as the cities project is coordinating seismic microzonation throughout australia microzonation data have been included in agso s geohazards gis database this is helping local councils zone land for seismic hazards state emergency services use the information to plan for emergencies resulting from the effects of earthquakes these practical applications of seismic microzonation data will help mitigate the destructive effects of any future large earthquakes occurring near major urban centres in the launceston case it was found that there is a variable risk dependant on epicentral distance and the nature of relatively unconsolidated sediments in various parts of the city disastrous amplification could occur at some sites

microtremor observations of deep sediment resonance in metropolitan memphis tennessee we measured site resonance periods throughout metropolitan memphis tennessee the site resonance was derived from recordings of ambient microtremor and is a direct measurement of seismic ground shaking in the memphis metropolitan area the period t0 in s is predicted by the empirical relation t0 0 39899 7 8922 10 3h 3 0634 10 6h2 350 h 1100 m where h is the thickness in m of the unconsolidated sediments that overly hard paleozoic rocks assuming that a quarter wavelength relationship governs the relationship between resonant period sediment thickness and average shear wave speed we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses vs 521 15 0 37459h 350 h 1100 m the scatter in the observations and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work

assessment of the stability of h v spectral ratios from ambient noise and comparison with earthquake data in the cologne area germany abstract situated in an active tectonic region the highly industrialised cologne area germany suffers from moderate sized earthquakes our contribution to the mitigation of earthquake risk includes a microzonation study using ambient seismic noise and earthquake recordings from two field campaigns an analysis of these data using the horizontal to vertical h v as well as the classical spectral ratio csr technique led to the following observations and conclusions 1 the spatial variation in the thickness of the sedimentary cover is reliably retrieved using the fundamental resonance frequency estimated from the peak in the h v ratio of ambient noise 2 this fundamental resonance frequency is stable in time but the amplitude of the peak the amplification factor is not 3 the relative amplification variation of the h v ratio in the area should therefore be checked systematically with repeated measurements before drawing conclusions about its significance 4 the analysis of the h v ratio of ambient noise provides the lower frequency bound above which amplification may occur 5 the shapes of the spectral ratios obtained by the different methods are generally in good agreement however the analysis of earthquake data shows that significant amplification of ground motion may also occur at frequencies higher than the fundamental one even when thick sediments are present

a study of the seismic response of the city of benevento southern italy through a combined analysis of seismological and geological data abstract a previous analysis improta l g di giulio and a rovelli 2005 variations of local seismic response in benevento southern italy using earthquakes and ambient noise recordings j seism 9 191 210 of small magnitude earthquakes recorded at 12 sites within the city of benevento has stressed the significant role played by near surface geology in causing variability of the ground motion in this paper we extend the study of the seismic response from 12 sites to the entire urban area based on inferences from the comparison at the 12 sites between earthquake and ambient vibration results we have collected ambient noise at about 100 sites within the city intensifying measurements across the main shallow geological variations we use borehole data to interpret ambient noise h v spectral ratios in terms of near surface geology comparing h v curves to theoretical transfer functions of 1d models along five well constrained profiles on the basis of geological geotechnical and seismic data we identify three main typologies of seismic response in the city each type of response is associated to zones sharing common soil conditions and similar soil classes according to building codes for seismic design moreover we find that the spatial variation of the seismic response in the ancient town area is consistent with the damage pattern produced by a very destructive well documented historical earthquake that struck the city in 1688 causing mcs intensity of ix x in benevento finally we use ground motions recorded during the experiment by improta et al improta l g di giulio and a rovelli 2005 variations of local seismic response in benevento southern italy using earthquakes and ambient noise recordings j seism 9 191 210 to generate synthetic seismograms of moderate to strong mw 5 7 molise 2002 and ms 6 9 1980 irpinia earthquakes we calibrate the random summation technique by ordaz et al ordaz m j arboleda and s k singh 1995 a scheme of random summation of an empirical green s function to estimate ground motions for future large earthquakes bull seism soc am 85 1635 1647 using recordings of these earthquakes available in benevento after a satisfactory fit between observed and synthetic seismograms we compute response spectra at different sites and speculate on effects of the geology class at large level of shaking including soil nonlinearity we find that large discrepancies from design spectra prescribed by seismic codes can occur for a wide sector of benevento especially for periods

a method to estimate phase velocities of rayleigh waves using microseisms simultaneously observed at two sites the frequency wave number f k method and the spatial autocorrelation spac method can both be applied to estimate the subsurface structure by using array observations of long period 1 10 sec microtremors since these methods require more than four to seven sets of simultaneous recordings the allocation of many resources not only of instruments but also of personnel must be provided for the observations to avoid this difficulty we propose an alternative method based on the conventional spac method it is called the two site spac 2sspac method and requires only two sets of seismographs under the assumption that the wave fields are spatially and temporally stationary after formulating the analytical background of the 2sspac method we use data from actual microtremors to discuss the limitations of the assumption and the applicability of this method through a comparison with conventional methods this discussion leads to the conclusion that the 2sspac method provides reasonable values for phase velocities in the frequency range lower than about 1 0 hz at the sites studied

site effect evaluation in the basin of santiago de chile using ambient noise measurements summary we performed extensive ambient vibration measurements in the basin of santiago de chile chile and we look for testing the reliability of the horizontal to vertical amplitude spectra ratio method h v as a tool to provide qualitative and quantitative information of site effects in complex geological media the interpretation of the h v data was carried out conformably to the sesame project consensus criteria and outlines three major patterns 1 clear peaked h v curves related to sharp underground velocity contrast 2 h v peak of low amplitude and flat curve related to weak contrast and 3 broad h v peak indicating the presence of strong lateral variations of underground structure h v measurements however reveal a discrepancy between the computed soil resonance frequencies and the expected building resonance therefore not leading to a straight interpretation of the intensity distribution derived from observed damage to one storey houses in santiago after the 1985 valparaiso earthquake indeed the h v technique mostly maps the first fundamental frequency however it fails to show higher resonance modes in the case of the city of santiago this method works well for assessing the seismic hazard for high rise buildings but is questionable for smaller structures as is the case of a great percentage of constructions in the city

on bias and noise in passive seismic data from finite circular array data processed using spac methods the finite nature of typical small seismic arrays used in conjunction with spatial autocorrelation spac processing for observing the microtremor wavefield causes predictable perturbationsofthespacspectrumwhensourcesofseismic noiseareconfinedtoarestrictedrangeofazimuths suchperturbations are especially evident at higher frequencies where wavelengths are on the order of the array radius the effects arereadilymodeledandshowthatthetriangulararraygeometries commonly used for microtremor studies require azimuthal distributions of wave energy on the order of 60 or greatertohaveahighprobabilityofbeingfreeofsuchperturbations the imaginary component of the spac spectrum which is ideally zero for a sufficiently dense circular array and or a sufficiently isotropic wavefield is in practice often nonzero and provides three quality control indicators 1 an indication of insufficient spatial averaging 2 an empirical measure of the level of statistical uncertainty in spac spectralestimates and3anindicationofdeparturesfromplanewavestationarityoftheseismicnoisewavefield

modeling of strong motion data in northeastern india q stress drop and site amplification abstract in this article point source seismological model is used to model the available strong motion accelerograms in northeastern india key seismic parameters such as quality factor q value kappa factor 0 site amplification and stress drop are derived from the strong motion data a total of 111 three component records coming from seven earthquakes with magnitudes ranging from m w 5 2 to 7 2 are used in obtaining the source and path parameters the q value of the indo burma tectonic domain which is a subduction zone is obtained as 431 f 0 7 for the bengal basin shillong plateau region the quality factor is estimated as 224 f 0 93 the kappa factors for vertical and horizontal components are obtained as 0 013 and 0 033 for soft rock type sites for firm ground sites 0 is estimated as 0 025 for vertical and 0 041 for horizontal components respectively the site amplification functions for soft rock and firm ground conditions are found from the horizontal to vertical ratio the obtained stress drops of the seven events lie between 128 and 282 bars the source parameters obtained in this study provide a useful framework for working out ground motion relations in northeastern india

seismic zonation of barcelona based on numerical simulation of site effects seismic responses of different sites of barcelona have been investigated through numerical modelling geological maps and geotechnical data available from drillings for buildings and infrastructures have been used to determine the dynamical properties of the soils through different correlations between standard geotechnical data and dynamical parameters obtained in other regions an estimation of the depth of the palaeozoic basement has been obtained through an inversion of a detailed gravity survey a 1 d equivalent linear method has been used to compute complete transfer functions and other spectral responses such as psa and psv for various damping values with the purpose of classifying zones with similar behaviour given the uncertainties associated with the input data a montecarlo s simulation process has been carried out four zones characterized by their corresponding transfer function and by pga amplifications are proposed the numerical results are compared with those previously obtained through microtremor measurements showing that predominant periods derived from nakamura s technique should be taken carefully

site specific relationships between bedrock depth and hvsr fundamental resonance frequency using kik net data from japan it is commonly accepted that the horizontal to vertical spectral ratio hvsr technique enables the detection of the fundamental resonance frequency f text hvsr of a given site the utility of this f text hvsr is analyzed using the nonlinear regression relationships between f text hvsr and bedrock depth h the derived relationships are mostly site specific so that the present paper consists of two main parts the first is a literature review for the available empirical relationships between f text hvsr and h the aim of this part is to highlight the practical limitations of these established relationships and to make fair comparisons the second is to generate new relationships taking advantage of the very wide range of available lithological geophysical and geotechnical borehole drilling data of the 697 kik net seismic stations in japan for this purpose hvsr are calculated using 10 000 weak earthquakes or linear events recorded at kik net stations to determine the f text hvsr and correlate it with the corresponding h the overlying layers bedrock interface falling within sedimentary igneous or metamorphic layers significantly affect the derived frequency depth relationships in addition these relationships are strongly reproduced by the v text p v text s ratio of the bedrock in the range of 1 6 2 2 interestingly it is found that f text hvsr less than 1 hz corresponding to h more than 100 m leads the trend of the overall frequency depth relationship

quantifying seismic vulnerability dynamical shear strain and liquefaction of the quaternary deposits in the doon valley near the main boundary thrust in the northwest himalaya india abstract erosion and deposition are two conjugate surface dynamical processes that affect differently to the soft soil site characteristics of quaternary deposits in the doon valley and at its northern topographic transition zone near the main boundary thrust mbt during an impending large magnitude earthquake what would be the seismic vulnerability and liquefaction susceptibility of the soft soil at both the regions we quantified seismic vulnerability index effective dynamical shear strain and the expected ground deformation in the valley soft soil through h v site characteristics using seismic ambient noise data at the in accessible topographic transition zone near the mbt site effects were studied using moderate and light magnitude earthquakes recorded by three broad band seismographs bbs results show unique dual peak mean resonant frequencies 1 11 and 1 38 hz and amplification factors 1 69 and 1 51 at the topographic transition zone which highlights a bimodal distribution of site effects where the top soil formation is influenced by erosion and deposition due to topographic slope instability apart from fluvial re working here relatively compact oldest doon gravel is present in the foothill terraces with the re worked fluvial deposition of the youngest doon gravel however in the valley and its terraces the soft soil formed under fluvial environ is relatively thick 10 to 2 and mean resonant frequency of 1 1 hz the spot resonant frequencies increases from 1 hz to more than 6 hz from the hft region to the mbt while the amplification factor decreases from 14 to 1 in general the valley soft soil is susceptible for liquefaction because the estimated dynamical shear strain is 15 25 millistrains under medium 2 2 m s 2 225 years and long 4 6 m s 2 2500 years term strong motion conditions which is much higher than the soil plasticity limit of ten millistrains that lead to liquefaction dynamical shear strain distribution with depth at eight boreholes in the valley indicate 20 m is the liquefaction potential depth where the pore pressure induced peak shear strain exceeds the threshold shear strain of 0 01 during the passage of seismic waves isolated locations in the valley like that at dharmawala h1 where all site parameters are very high and would be hazardously vulnerable to liquefaction however near the mbt the liquefaction possibility of soft soil is less but would experience ground deformation of more than 50 cm

detection of spurious maxima in the site amplification characteristics estimated by the hvsr technique spurious peaks in the site amplification characteristics estimated by horizontal to vertical spectral ratios hvsr of microtremors can be generated when ground motion is characterized by negligible spectral amplitudes and instrumental numerical noise is dominant in order to individuate hvsr maxima that cannot be safely considered as an effect of actual ground motion characteristics a new statistical procedure is proposed in particular statistics can be computed with the assumption that random noise is gaussian in this case it results that the proposed statistics are independent from the level of noise and only depends on the procedures adopted for the computations of spectral amplitudes from the monitored signal in order to use such statistics to discriminate suspect hvsr peaks its major sampling properties in the case of signal dominated by random noise have been deduced by numerical simulations an application of this methodology to hvsr measurements carried out at a test site in northern italy is described and discussed manuscript received 21 december 1999

the use of ambient seismic noise measurements for the estimation of surface soil effects the motril city case southern spain this paper presents a study of the ambient seismic noise features in motril city the predominant resonant period at each site was determined using the horizontal to vertical noise ratio hvnr of the microtremors records it was applied to 91 points which cover the city and we present the distribution of the periods over a map which shows important characteristics requiring consideration in a study focussing on the mitigation of the seismic risk using these results two zones can be distinguished north of the gg profile with predominant periods smaller than 0 25 sec which corresponds with a hard soil and south of gg with periods ranging from 0 25 to 0 6 indicating a medium soft soil microtremors are a preliminary useful tool that compensates for the lack of seismic recorded data and that can complement other valuable information borehole logs geology etc to evaluate the seismic risk on places as motril city where some of this information is unavailable through the predominant period the hvnr indicates the kind of soil we are dealing with

lateral propagation effects observed at parkway new zealand a case history to compare 1d versus 2d site effects abstract the importance of 2d or 3d site effects has been shown by theoretical studies however there is a lack of observational evidence of site effects due to lateral heterogeneities we have evaluated 1d and 2d site effects in the data obtained by a dense seismograph array that operated for more than 2 months in the alluvial basin of parkway new zealand 1d site effects were evaluated using three independent techniques spectral ratios relative to a reference station spectral ratios of horizontal relative to vertical ground motion and a generalized inversion scheme site effects due to lateral heterogeneity were investigated in detail for two events using frequency wavenumber analysis our results show that it is not possible to separate 1d from 2d site effects using frequency domain techniques this explains the scatter obtained from standard 1d data analysis the 1d resonance peaks will be more or less contaminated with laterally propagating waves for different events

distribution of ground motion intensity inferred from questionnaire survey earthquake recordings and microtremor measurements a case study in christchurch new zealand during the 1994 arthurs pass earthquake the areal distribution of seismic ground motion intensity in the city of christchurch new zealand during the 1994 arthurs pass earthquake ml 6 6 was evaluated using an intensity questionnaire together with local site amplifications inferred from seismic recordings and microtremors in order to estimate the intensity in parts of the city where no intensity data were available intensity data were compared to relative levels of shaking determined from both weak motion and microtremor recordings weak ground motion amplification factors were determined using ratios of ground accelerations at five sediment sites with respect to a rock site microtremor amplification factors were determined from horizontal to vertical spectral ratios at a 1 km spacing throughout the city a positive correlation between weak motion and microtremor amplification factors allowed extrapolation of microtremor amplification to estimated mm intensity emmi emmi ranged from 3 to 6 and was consistent with the questionnaire intensity and geological conditions and showed detailed information on the areal distribution of ground motion intensity in the city

effect of local geology on ground motion in the city of palermo italy as inferred from aftershocks of the 6 september 2002 mw 5 9 earthquake on 6 september 2002 a m w 5 9 earthquake occurred in the southern tyrrhenian sea 40 km off the coast of palermo italy in the days after the mainshock eight temporary stations were installed in the city to record aftershocks on different geological formations seismograms of about 30 earthquakes with magnitude 2 8 are analyzed the data analysis confirms the role of near surface geology in causing locally significant variations of the ground shaking level as already inferred by guidoboni et al 2003 from historical damage scenario studies the largest spectral variations estimated through aftershock recordings result in a factor of 10 difference between stiff and soft sites in frequency bands varying from 1 to 3 5 hz the geological structure of the study area is reconstructed by using data from more than 2000 boreholes organized in a geographic information system specifically dedicated to the assessment of natural hazards in urban areas vertically varying velocity models are used for a comparison with the observed data in general 1d transfer functions fit the largest amplification frequency but underestimate amplitudes of observations probably because of 2d and 3d complexity because the seismic stations were not installed in free field but at ground or underground level inside buildings a possible influence of the structure was also investigated simultaneous ambient noise measurements were performed on the top at the base and outside the buildings where stations were installed for all but one site this analysis shows that the estimated ground motion amplifications do not reflect the building vibration modes and therefore in these cases soil structure interaction does not bias the free field response of the study sites finally speculations on the effect of the local geology in terms of response spectra of the m w 5 9 mainshock are discussed in the framework of the eurocode 8 prescriptions

hvsr deep mapping tested down to 1 8 km in po plane valley italy abstract the horizontal to vertical spectral ratio hvsr of seismic noise is extensively used in seismic microzonation for its capability to provide a good approximation to the subsoil main resonance frequencies of geotechnical and seismic engineering interest this implies in turn that it has also an approximate passive subsoil mapping capability independent of the level of noise illumination albeit limited to relatively shallow depths since tilt sensitivity makes hvsr unreliable below 0 1 hz however we have experimentally verified that hvsr subsoil mapping capability extends to depths in the kilometer range by applying it to the largest sedimentary basin of italy the po plain valley there we were able to resolve the major stratigraphic discontinuities down to the sediment bedrock interface for which we estimated a depth of 1 6 km with a 25 uncertainty while the surface mapped from oil exploration indicates a depth of 1 8 km quarter of an hour recordings gave always stable signals that fitted to synthetic curves using as a constraint the parameters of the shallow subsoil provided a stratigraphic map consistent with the independent survey this candidates hvsr as a fast and inexpensive first order subsoil mapping tool down to depths of geological and exploration interest

study of sediment nonlinearity using explosion data in the mississippi embayment strong and weak motion data from the mississippi embayment seismic excitation experiment esee were analyzed for signatures of nonlinear site responses this experiment was performed jointly by the university of memphis and u s geological survey in october 2002 by detonating two explosions of 2500 and 5000 lbs intrinsic and scattering q estimates qi and qs from the coda of the strong motion data were found to be very low compared to previously determined q values of p and rayleigh waves of weak motion data from the same explosions the qi estimates from p wave late coda of the strong motion data are less by more than 100 at 3 hz and by more than 200 at 10 hz compared to the p wave q values determined from the weak motion data by langston et al 2005 also qi determined from the late coda of strong motion rayleigh wave data is less by more than 200 at 0 5 hz and by more than 50 at 3 0 hz compared to q values determined from rayleigh wave weak motion data a resonance peak spectral amplitude of the early part of a strong motion seismogram is shifted to lower frequencies compared to that from a later part of the same seismogram spectral amplitude ratios between transverse and vertical components of the strong motion data are degraded between frequencies 2 and 10 hz for p waves and less than 4 hz for rayleigh waves compared to the weak motion transverse to vertical spectral ratio all these are signatures of nonlinear site responses during strong ground motion this study proves the non transportability of weak motion attenuation results to estimate ground motion from a future large earthquake that may take place in areas like the new madrid seismic zone

microtremor analyses at teide volcano canary islands spain assessment of natural frequencies of vibration using time dependent horizontal to vertical spectral ratios we use time dependent horizontal to vertical spectral ratios hvsr of microtremors to determine the dominant frequencies of vibration of the geological structures beneath several recording sites in the vicinity of teide volcano canary islands spain in the microtremors the time dependent hvsrs ratiograms are a useful tool to discriminate between the presence of real dominant frequencies linked to resonances of the subsurface structure and the spurious appearance of peaks due to local transients we verified that the results are repeatable in the sense that microtremors recorded at the same site but at different times yield a very similar hvsr function two types of results are found 1 sites where there is no resonance of the propagating microtremors and therefore no value of a dominant frequency can be assessed and 2 sites where a stationary peak in the hvsr is found and a dominant frequency related to resonance of the shallow structure can be estimated these resonant frequencies show substantial spatial variations even for nearby sites which reflects the complexity of the shallow velocity structure in the las canadas area large dominant frequencies occur near the caldera walls and also at a few locations that coincide with the intersections of the inferred rims of the three calderas forming las canadas small dominant frequencies also occur near the caldera rim and may be due to discontinuities in the caldera wall and or to local velocity anomalies intermediate frequencies are mostly found in the eastern part of the caldera where a tentative profile of the basement depth has been obtained intermediate frequencies have also been measured south of ucanca and south of montana blanca in view of the present results we conclude that the use of ratiograms constitutes an improvement of the hvsr method and provides an appropriate tool to investigate the shallow velocity structure of a volcanic region

local site effects during the orkney earthquake of 5 august 2014 site response analysis is conducted at 37 seismic stations located in the gauteng north west and free state provinces in south africa using the nakamura h v spectral ratio technique on records of the orkney 5 august 2014 earthquake the earthquake of magnitude ml 5 5 led to the unfortunate death of one person and damage of more than 600 houses intensity data collected soon after the event showed that the effects of the earthquake appeared to vary significantly across the region this motivated the authors to conduct a more detailed investigation of the effects of site conditions on seismic station records in the region resonance frequency values obtained from the h v ratios were observed to vary strongly across the region and also within seismic station clusters similar behaviour was observed with the peak amplitude of the ratios at the resonance frequency except for the johannesburg area whose results showed a relatively simple shape of the ratios implying less complex velocity structure all the h v ratios exhibit dominant peaks at resonant frequencies that varied between 0 5 and 35 hz the average observed resonant frequency was f 7 9 hz the amplitude of the dominant peaks also varied strongly from 1 66 to 11 69 with only two sites exhibiting maximum peaks with amplitude smaller than 2 these results serve as a strong motivation or justification for the on going microzonation studies in south africa where a detailed study of the velocity structure will be used to obtain reliable information on site amplification and resonance

subsoil structure using spac measurements along a line the spac spatial autocorrelation method was proposed almost 50 years ago by aki 1957 this method allows a phase velocity dispersion curve to be obtained from microtremor measurements using an array of stations arranged in a circle the subsoil structure is subsequently derived from the inversion of that dispersion curve in this article we show that it is possible to get similar results using microtremors recorded along a line we use microtremor records obtained by using four broadband seismographs disposed along a line with different interstation spacings 5 10 20 and 40 m our data are precessed by using the standard spac procedure with the exception of the azimuthal average the final subsoil structure as determined from the inversion of the phase velocity dispersion curve shows excellent agreement with previous results at the site of our measurements our results suggest that the use of the spac method is not restricted to a particular geometry of the array provided that the basic requirement of stationarity is fulfilled

evaluation of site effect within the tectonic basin in the northern side of ankara abstract this study mainly concentrates on the determination of site effects for the plio quaternary and especially alluvial soils of the cubuk district and its close vicinity in ankara turkey in the study area particularly in the vicinity of the asymmetric graben that has formed due to the tectonic activities along the ne sw trending normal faults a microtremor survey was implemented to identify the local site response under a possible seismic event the results were compared and correlated by using passive and active surface wave measurements engineering geological and geotechnical deep borehole logs along with information on local geology and geological tectonic setting regarding the microtremor results larger fundamental periods were acquired than expected over the area and the effect of the tectonic deformation on the stiffness of the soils was also observed at either the h v curves or the shear wave velocity profiles the results have demonstrated that geotechnical information down to a depth of 30 m may not be compared with the h v parameters the h v amplitudes were not always accompanied with the higher periods in the quaternary sediments this could be due to the basin and basin edge effects observed particularly at the center and boundary of the basin respectively additionally the microtremor survey showed that the spectral ratio amplitudes derived by the h v ratio are debatable and are not a direct indicator of local soil behavior under a seismic excitation

a site effect study in acapulco guerrero mexico comparison of results from strong motion and microtremor data abstract the city of acapulco is located near or above the mature seismic gap of guerrero along the mexican subduction zone with the purpose of studying the character of strong ground motion on soft sites four digital accelerographs have been installed in the city on such sites these instruments have been in operation since 1988 two additional instruments part of the guerrero accelerograph array are located on hard sites in the area one of these vnta has been in operation since 1985 and the other acan since 1989 these stations have recorded several earthquakes we use data from eight events 4 2 m 6 9 to study spectral amplification of seismic waves at the soft sites with respect to vnta the s waves are amplified by a factor of 6 to 25 at the soft sites in a fairly broad range of frequencies both the amplification and the frequency band over which it occurs depend upon the site although the largest earthquake in our data set m 6 9 gave rise to a peak horizontal acceleration exceeding 0 3 g at one of the soft sites no clear evidence of nonlinear behavior of the subsoil is found spectral amplifications of s wave coda are very similar to those of s waves we also measured microtremors at the strong motion sites the microtremor spectra were interpreted using reasonable assumptions to test the feasibility of this technique in reproducing the spectral amplifications observed during earthquakes our results show that only a rough estimate of site response can be obtained from this technique at least in acapulco caution is warranted in its use elsewhere

lateral heterogeneities and microtremors limitations of hvsr and spac based studies for site response abstract we report a site effect study carried out in a small alluvial valley nari basin in ulleung island republic of korea in that small valley a permanent broad band array of five stations records ground motion continuously in addition we performed microtremor measurements using two arrays of stations a small aperture array 60 m and a medium aperture one 500 m ambient vibration was also recorded at 38 points inside the basin the records were analyzed using horizontal to vertical spectral ratios hvsr to estimate dominant frequency and maximum amplification and the spatial autocorrelation spac method to estimate the subsoil structure via phase velocity dispersion curves nari basin is clearly a 3d structure but it was expected that a laterally irregular model might be built from the interpolation of several 1d models dominant frequency maps based on results from hvsr suggest a simple sedimentary structure with mostly smooth lateral variations in contrast maximum amplification distribution is very irregular and shows no correlation with dominant frequency in the case of nari it seems that the complexity of the basin structure cancels the usefulness of hvsr we could estimate a phase velocity dispersion curve for 81 of the station pairs recording simultaneously ambient vibration in our arrays in all the cases the resulting dispersion curve had a simple shape suggesting that the subsoil structure consists of a single layer over a half space however the dispersion curves are incompatible among them even for different station pairs covering the same path something impossible if the subsoil structure were 1d it seems that the passive methods we have used in nari basin are not adapted to its structure this makes a tomography unfeasible and we are unable to propose a subsoil structure for nari basin our results show that surface wave dispersion may be estimated using the spac method even when the medium is far from 1d we believe that this problem is enhanced by the small size of this basin which amplifies the effect of lateral heterogeneities

a seismic passive imaging step beyond spac and remi abstractthe basic property of passive imaging is that given any two points one of them can be taken as the source of the waves and the other as the recording station this property can be derived from the statistical self alignment of the rays along the vector joining the two points and applies also to nondiffuse wavefields like seismic tremor it provides a statistical basis for the use of the stationary phase integral allowing passive interferometry under the mild constraint of mechanical homogeneity at a local scale combined with the tremor s large spectral bandwidth it allows one to recover the local green s function from spatial correlation furthermore combining this property also with the azimuthal isotropy of either the wavefield or the array and using the statistical mode as the estimator provides a new technique to measure the local velocity dispersion in the subsoil this technique exploits the potential of spatial autocorrelation spac and refraction microtremor remi allowing one

coda q distribution in the iberian peninsula summary several attenuation studies have established a frequency dependence law of the anelastic attenuation factor q in the form q qo f fo v for the approximate 1 10 hz frequency range we propose a method that leads to the determination of q which is a function of the reference frequency fo and the real exponent v with a single station to carry out the problem we determine a set of master curves as a function of y we discuss the method and the different features of the master curves when it is applied to the complicated regions of the iberian peninsula and to several instruments with different responses using this new method and the seismographic stations available in the iberian peninsula we have mapped iso qo lines at a reference frequency of 1 hz applying inversion methods the q values determined for iberia vary between about 100 and about 600 values close to 100 correspond to the southern part of iberia in general q values increase from south to north with values about 600 near the nw part of iberia the pyrenees mountains and adjacent areas present q values between about 200 and about 350 these results suggest a strong qo lateral variation in iberia a considerable frequency dependence of coda q has also been determined the v values vary between 0 3 and 0 8 the q values obtained in the iberian peninsula show very good agreement with several q values obtained in other regions of the world comparison between the iso q lines and other geophysical parameters like regional variations of p velocities heat flow isoseismal intensity distribution and crustal thickness indicates that lower q values are associated with higher isoseismal intensity attenuation higher heat flow lower p velocities and thinner crust

the impact of a buried high velocity layer in the seismic site amplification of the city of llolleo chile programa de riesgo sismico prs at the university of chile comision nacional de investigacion cientifica y tecnologica conicyt 2017 21171480 fondo nacional de desarrollo cientifico y tecnologico fondecyt 1170430 advanced mining technology center amtc fb0809 pia conicyt

local site effects in the town of benevento italy from noise measurements the study of ground motion amplification produced by surface geology is extremely interesting in the benevento area southern italy as it is characterized by high seismic hazard the present moderate to low seismicity makes the noise method appropriate to estimate the seismic site response in the area the three components of seismic noise have been recorded in five sites in the benevento metropolitan area characterized by different surface geology in order to estimate the seismic site response in evaluating site amplification effects we used the direct interpretation of amplitude spectra and standard spectral ratio techniques evaluating sediment to bedrock sediment to average and h v spectral ratios the temporal evolution of the noise spectra is analysed within one day in order to assess the stationarity of the noise signal the noise wavefield properties have been studied through polarization analyses in selected bands of frequency where spectral peaks are observed to dominate to better understand the real nature of those peaks results give evidence of low amplification levels missing any correlation between spectral amplitudes and sediment thickness over the basement we interpret this result as due to the poor impedance contrast between sediments and basement which is characterized by low values of shear waves velocity moreover sharp amplitude peaks are observed in the raw spectra of the sediment sites in the 2 4 hz frequency band a numerical simulation interprets this effect as possibly associated with a wide scale structure invoking the presence of a sharper impedance contrast at greater depth at high frequencies the action of ambient noise sources mainly active on horizontal components of motion is retained dominant to generate the prominent peaks observed in the h v spectral ratios in some cases the presence of a near surface low velocity layer can contribute to amplify the seismic motion generated at these frequencies

microtremor survey in talchir india to ascertain its basin characteristics in terms of predominant frequency by nakamura s ratio technique abstract the talchir basin one of india s oldest basins has been a subject of interest because of its rich coal deposits the maximum thickness of the basin is about 1500 m beyond the basin is the hard metamorphic rocks of precambrian age the ambient noise survey data have been analyzed for the talchir basin using nakamura s technique of horizontal vertical spectral ratio hvsr to ascertain the basin structure in terms of the predominant frequency the predominant frequency varies from 0 25 hz to 7 8 hz but a major portion of the basin comes under the range of 0 3 hz 2 4 hz while on the metamorphic rocks it is as high as 7 8 hz the variation in predominant frequency shows a good correlation with the sediment thickness of the basin the results have been compared with the previous studies by other researchers and it shows consistency with the northerly dip of the basin the present study has also been compared with the results of the synthetic seismogram that was performed for the talchir basin the predominant frequency obtained from hvsr technique complements well with the frequency at which the peak response spectra ratio is observed the present study of the predominant frequency identifies quite well the characteristics of talchir basin and is in good agreement with the synthetic ground motion modeling of the region

a physically sound way of using noise measurements in seismic microzonation applied to the urban area of napoli noise measurements have been done in the urban area of napoli italy a densely and crowded populated city of southern italy with historical architectonic heritage often damaged by tectonic earthquakes noise has been recorded by kinemetrics q330 station with a 3 component episensor broadband accelerometer the h v ratio between the mean of the fourier spectra of the horizontal components and the spectrum of the vertical component has been computed for 20 s windows and averaged on about 150 windows of noise data noise has been measured at sites with detailed shear wave velocity profiles obtained from the non linear inversion hedgehog method of the group velocity dispersion curve of rayleigh fundamental mode extracted with ftan method from active seismic surveys ellipticities of all modes rayleigh waves and spectral amplifications both with sophisticated algorithms like mode summation and finite difference techniques and simplified 1 d techniques have been computed it turns out that the fundamental and at times also higher modes rayleigh waves are present at the resonance frequency of the soil cover where the maximum spectral value of noise h v shows resemblance to the main peak of computed spectral amplifications although the match at the resonance frequency realistic computed amplifications must be assumed for seismic microzonation hence a novel use of noise measurements is proposed which consists of a rapid detailed mapping of the computed spectral amplifications and average physical properties starting from the fit of the main peak frequency 2007 elsevier b v all rights reserved

advances and problems in understanding the seismic response of potentially unstable slopes abstract the influence of site effects on landslide triggering during earthquakes has been inferred in several studies but its evaluation is made difficult by the complexity of factors controlling the dynamic response of potentially unstable slopes and also by the lack of local ground motion instrumental observations this work explores this problem and reports new findings based on an ongoing long term accelerometric monitoring conducted on a landslide prone test area in the apennine mountains italy where the presence of site effects enhancing seismic susceptibility of local slopes has been invoked on the basis of historic accounts of landsliding triggered at large epicentral distance the recordings relative to low to moderate magnitude earthquakes showed significant amplifications affecting hillslope portions covered by thick 5 m colluvia and pronounced amplification maxima oriented along the local maximum slope direction on a recent deep seated landslide while the amplifications seem most likely linked to high impedance contrast between surface materials and underlying substratum the causes of directivity are less clear the case of the monitored test site together with evidence of site response directivity identified on other hillslopes suggest that the directivity phenomena can result from a combination of topographic lithological and structural factors that act together to re distribute shaking energy focusing it on site specific directions thus it is difficult to single out the critical factors controlling such phenomena and no general criterion for the identification of sites affected by directivity is proposed here nevertheless the presence and orientation of site response directivity can be revealed through reconnaissance techniques by using recordings of seismic weak motion and or ambient microtremors and calculating azimuthal variation of shaking energy and horizontal to vertical ground motion spectral ratios a comparison with the recordings obtained during the recent m w 6 3 earthquake that hit the abruzzo region in april 2009 demonstrated that analysis relying on data from low energy events can furnish valid indications for slope behaviour also under stronger shaking provided the data are well differentiated in terms of distance azimuth and source characteristics furthermore the comparative analysis of the abruzzo earthquake recordings at a landslide and reference sites showed that directivity properties of strong shaking had been correctly anticipated using earlier weak motion observations however under the complex slope conditions the identification of resonance frequencies from horizontal to vertical spectral ratios estimated from weak motion accelerometric recordings does not seem reliable and better results have been obtained by velocimetric microtremor recordings

microtremor measurements for the microzonation of dinar the geotechnical site conditions in dinar town located in western turkey were investigated after the 1995 dinar earthquake based on borings in situ penetration tests seismic wave velocity measurements and microtremor records the variation of damage distribution within the town was evaluated with respect to 23 district damage ratios calculated based on the detailed damage survey conducted by the general directorate of disaster affairs site amplifications were estimated from microtremor spectral ratios and microzonation was performed using a gis methodology the results of in situ penetration tests and seismic wave velocity measurements as well as the damage distribution were compared with the amplification zonation obtained from microtremor records the results indicate the applicability of microtremor spectral ratios for assessing the local site conditions and site amplifications

a mobile multi depth borehole sensor set up to study the surface to base seismic transfer functions abstract the best method to evaluate the seismic site response is by means of borehole vertical arrays that use earthquake records from different depths in this paper we introduce the implementation of a single borehole sensor system synchronized to a sensor on the surface that is fixed at variable depths within a single well this system is used for recording small amplitude earthquake signals at variable stiffness conditions in depth to compute empirical borehole transfer functions the computed average empirical borehole transfer functions allow the estimation of an s wave velocity model that is constrained using the frequency peak observed in the h v ratio curve pairs of surface and borehole earthquake records were obtained with the borehole sensor placed at 10 20 50 and 100 m depth in a test site in managua nicaragua the average velocity of the final model down to 100 m appeared to be in good agreement with the average velocity computed via cross correlation using the surface and borehole signals likewise an inverted masw profile and h v ratio at the same site agree with the s wave velocity model obtained

site response and basin waves in the sacramento san joaquin delta california the sacramento san joaquin delta is an inland delta at the western extent of the central valley levees were built around swampy islands starting after the civil war to reclaim these lands for farming various studies show that these levees could fail in concert from shaking from a major local or regional earthquake resulting in salty water from the san francisco bay contaminating the water in the delta we installed seismographs around the delta and on levees to assess the contribution of site response to the seismic hazard of the levees cone penetrometer testing shows that the upper 10 s of meters of soil in the delta have shear wave velocities of about 200 m s which would give a strong site response seismographs were sited following two strategies pairs of stations to compare the response of the levees to nearby sites and a more regional deployment in the delta site response was determined in two different ways a traditional spectral ratio tsr approach of s waves using station bdm of the berkeley digital seismic net as a reference site and using sh sv ratios of noise or nakamura s method both estimates usually agree in spectral character for stations whose response is dominated by a resonant peak but the most obvious peaks in the sh sv ratios usually are about two thirds as large as the main peaks in the tsrs levee sites typically have large narrow resonances in the site response function compared to sites in the farmland of the delta these resonances at a frequency of about 1 3 hz have amplitudes of about 15 with tsr and 10 12 with nakamura s method sites on farmland in the delta also have amplifications but these are typically broader and not as resonant in appearance late slow rayleigh waves were recorded at stations in the delta have a dominant period of about one second and are highly monochromatic results from a three station array at the holland marina suggest that they have a phase velocity of about 600 m s and arrive at about the same azimuth as the straight line back azimuth to the source a dispersion curve determined for the basin or valley waves yields a shallow velocity profile that increases from about 350 m s in the upper 0 2 km to about 1 1 km s at a depth of about 2 km

hydrothermal circulation and the dike gabbro transition in the detachment mode of slow seafloor spreading one of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast spreading ocean crust this boundary marks the brittle ductile transition at the ridge crest and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi steady state melt lens in contrast at slow spreading ridges the crustal structure appears chaotic with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor recent work suggests that as much as 50 of the atlantic ocean crust formed by a detachment mode of seafloor spreading including the formation of oceanic core complexes capped by long lived convex upward detachment faults these detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall thus explaining the link between faulting and black smoker systems we suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike gabbro transition in the detachment mode of spreading inevitably leading to exposure of gabbros on the seafloor through continued faulting this concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates

hvsr passive seismic stratigraphy for the investigation of planetary volcanic analogues abstract this paper presents new results of the application of passive seismic surveys for the stratigraphic investigation of planetary volcanic analogues we tested hvsr horizontal to vertical spectral ratio surveys that seem particularly suitable for planetary subsurface exploration in order to exploit the potential of these surveys in stratigraphic applications hvsr surveys were undertaken at the tinguaton volcanic region lanzarote canary islands seismic noise measurements were collected during the pangaea x 2018 testing campaign which formed part of the european space agency s esa astronaut training program single station free field seismic noise data were collected along two orthogonal profiles one crossing the tinguaton volcano the other passing alongside the tinguaton caldera seismic stratigraphic sections obtained from the shear wave velocity constrained inversion of the main h v peaks provided the main impedance contrasts between layers and a bulk estimate of the shear wave velocity in the layers contour maps of the hvsr patterns were used to recognize and identify the main volcanic units such as scoria deposits and different basaltic lava flows geological structures such as a regional fault related to the volcanic vent source and morphological characteristics such as the tinguaton caldera this study also identified several pitfalls that may limit the accuracy of such investigations these include the multilayer setting of scoria deposits and partly of underlying basalts as well as the presence of very thin horizons or a very gradual increase in shear wave velocity our study also shows the topographic effect due to the volcano s sharp topography along crater rim is significant and results in severe artifacts in the data from our work it is clear that hvsr method is an effective method to support the investigation of planetary volcanic terrains on the moon and mars with similarly volcanic complex geological settings even where horizontal layering and the isotropic site response are not fully verified

an experimental study of site effects near thessaloniki northern greece ground motion amplification by local geology conditions is investigated near thessaloniki greece using spectral ratios of s waves from local and regional earthquakes we present results for two different experiments using an array of five digital three component seismographs in the first instruments were installed across vasilika sedimentary valley southeast of thessaloniki three on sediments and two on rock sites outside the valley empirical transfer functions relative to one of the rock sites show significant amplification for low frequencies between 1 and 6 hz at the stations on the sediments and at high frequencies between 8 and 15 hz for a station on a weathered rock site our spectral ratios for local earthquakes have shapes similar to those for regional earthquakes but the amplification levels are different in the second experiment the seismographs were installed at five sites within the city of thessaloniki on four different geological formations according to our results site response does not vary significantly between gneiss quartzites and recrystallized limestone two stations on the same neogene formation a series of red silty clays with mica and calcareous concretionary bodies show relative amplification which attains a factor of 5 relative to the rock site but the shapes of the transfer functions are different between the two stations finally we show that our results correlate well with macroseismic information from the 1978 seismic sequence in thessaloniki consisting of both regional isoseismal maps and damage distribution within the city this substantiates the use of weak motion records to estimate local ground motion amplification in earthquake engineering

using diffuse field theory to interpret the h v spectral ratio from earthquake records in cibeles seismic station mexico city abstract it has been recently demonstrated that averaging the autocorrelations of fields produced by various almost vertical incoming elastic body plane waves upon a layered system approximately leads to the imaginary part of the corresponding 1d green s functions for deep sources located underneath the receiver kawase et al 2011 thus the ensemble of these waves from deep earthquakes recorded in a station located in the epicentral zone is interpreted as a diffuse field in this short note we extend the study to consider earthquakes recorded in a station located at epicentral distances of up to hundreds of kilometers we consider the horizontal to vertical spectral ratio hvsr of the averaged p s and coda waves and full earthquake records at the cibeles station mexico city accelerometric network and compare these with the results obtained with the corresponding hvsr for the 1d kawase et al 2011 and the 3d sanchez sesma rodriguez et al 2011 diffuse fields models using the signals of 90 earthquakes recorded at cibeles we find that the experimental results have distinctive features compatible with the 3d signature of a diffuse field we interpret this result as a consequence of the multiple paths that seismic waves undergo from the subducting slab to the mexico city valley and to the multiple scattering in a complex tectonic environment our study strongly suggests that we can use strong motion records from earthquakes and apply similar techniques to the ones used to analyze the ambient seismic field online material earthquake catalog

observation of 1 to 5 second microtremors and their application to earthquake engineering part ii evaluation of site effect upon seismic wave amplification due to extremely deep soil deposits abstract the usefulness of long period microtremor observation for earthquake engineering problems at extremely deep soil deposit site was examined in this study microtremor observations were made in the niigata plain japan and in los angeles california where depths to the basement rock are several kilometers these two locations were selected because strong motion records obtained during the niigata earthquake of 1964 and the san fernando earthquake of 1971 contain large long period amplitudes understanding why these predominate long period motions were recorded is important for the evaluation of seismic input motions for large scale structures long period microtremor observations were carried out in both areas in order to evaluate the characteristics of deep soil observations were performed simultaneously at a number of stations to distinguish the nature of deep soil deposit the result of fourier analysis showed that amplitude of microtremors in long period range increases systematically from the point of basement rock outcrop to deep deposit site with the increase corresponding to the depth to basement rock this relationship coincides with that observed in the strong motion earthquake records these analyses indicated that simultaneous observation of long period microtremors at a number of stations can provide insight to the characteristic of deep soil amplification and therefore permit estimation of seismic input motions for large scale structures

array observations of microseismic noise and the nature of h v in the mississippi embayment abstract ambient ground motion data were collected using phased seismic arrays in fall 2002 and spring 2007 within the mississippi embayment and at a single station external to the embayment these data allowed us to determine the wave field composition of ambient noise for understanding wave propagation mechanisms giving rise to spectral peaks using nakamura s h v technique ambient ground motions in the frequency band of 0 1 0 33 hz 10 3 sec period were dominated by spatially localized rayleigh and love wave microseisms generated by high ocean waves along the north american seaboard in the time periods of analysis seismic waves important in forming the h v peak near 4 sec period are composed of relatively high phase velocity rayleigh and love waves that convert to propagating homogeneous shear waves in the thick unconsolidated sediments of the embayment the h v resonant period is controlled by both constructive and destructive interference of these shear waves a simple relationship for the h v peak is given using a propagator matrix formulation that predicts the resonance frequency of a layered medium for surface wave motion at the base of the system the amplitude of the observed h v peak however does not give an accurate estimate of shear wave amplification because it depends on the slowness of the incident wave the inconsistency in estimated average shear wave velocities using the h v method and differential travel times of local earthquake sp phases in the mississippi embayment may be explained by misidentification of sp wave conversion points from deeper interfaces

characterizing surface geology liquefaction potential and maximum intensity in the kachchh seismic zone western india through microtremor analysis abstract this study presents results of microtremor analysis carried out over diverse geological formations and liquefaction sites in the kachchh seismic zone of western india which is host to the deadliest intraplate earthquake m w 7 7 microtremors were recorded at 36 sites with single stations and 7 sites with an array the microtremor array measurements were made using triangular arrays with sensors kept at 30 90 m radii the rayleigh wave velocities are estimated from the array data using high resolution frequency wavenumber f k analysis and the 1d shear wave velocity structure is determined by inversion we found a good correlation between surface geology resonant frequency and amplitude of the horizontal to vertical spectral ratio hvsr curve and shear wave velocity the ground vulnerability index k g values in the liquefied areas are found to be higher than those in the adjacent areas devoid of liquefaction broad or multiple peaks in the hvsr curves and large variations in the shear velocity models could be attributable to the structural and tectonic complexity in the maximum intensity area further our results show that the average fundamental frequencies are around 0 32 hz for the quaternary 0 80 hz for tertiary 1 8 hz for cretaceous 1 8 hz for jurassic and 2 3 hz for the deccan trap formations the inverted s wave velocity varies from 190 to 1350 m s down to a depth of 200 m for the quaternary 220 to 680 m s down to 100 m for tertiary 236 to 1160 m s for cretaceous 670 to 1780 m s for jurassic and 260 to 2440 m s for the deccan trap sites the results of microtremor analysis corroborate the observations from the available geophysical geological and borehole data

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

geophysical reconstruction of buried geological features and site effects estimation of the middle valle umbra basin central italy abstract the middle valle umbra central italy is a nw se 20 km long and 10 km wide quaternary extensional basin located in the internal sector of the apennine chain this area historically experienced strong earthquakes that caused significant damages to the outstanding historical heritage the same area has been recently hit by the 2016 seismic sequence of amatrice visso norcia with the aim to reconstruct the buried geological structures of the basin a multi technique geophysical approach was performed an extended campaign of ambient noise measurements was carried out to investigate the subsurface setting and to identify the main geological units we performed three 2d passive arrays to analyze two different sites within the basin their aperture was between 150 and 752 m for one site and of 48 m for the other site to characterize the geological units in terms of sediment thickness and shear wave velocity profile data collected were processed with f k and mspac analysis to extract dispersion curves with good resolution in a frequency range of 0 5 10 hz and 4 5 18 hz for the two sites respectively spectral ratios were computed for every single station ambient noise measurement performed and for all the stations of the bigger array our final target is to extend these results to the whole valley in order to retrieve the attitude of the main geological units and propose a reliable reconstruction of the subsurface geometry of the basin another point of this work is to evaluate the site response in the middle of the valley through the analysis of the earthquakes recorded by the accelerometric station it csa belonging to the italian civil protection and the corresponding recordings of the nearby rock station it ass

microtremor measurements used to map thickness of soft sediments the observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers the most suitable method for this determination is nakamura s technique which is the ratio of the horizontal component noise spectrum and that of the vertical component i i v spectrum the frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site using an extensive database of microtremor measurements carried out in the western lower rhine embayment germany it was possible to show that this correlation is clearly valid for a wide range of thickness namely from tens of meters to more than 1000 m a simple formula was derived that for the sediments to be found in the area investigated directly calculates the cover thickness from the frequency of the main peak in the h v spectrum a comparison with calculated resonant frequencies sug gests the relation derived from the noise measurements depending on the velocity depth function of the shear wave classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the res onant frequency of the subsoil the practical relevance of the investigation is illus trated by means of cross sections constructed from results of the microtremor anal yses which provide a convincing image of the surficial structure of the areas investigated

ambient noise level and site characterization in northern egypt the characterization of the background seismic noise in any seismological network is an essential task for checking the performance of the recording stations hence a funded project from science and technology funding authority stdf was awarded by the national research institute of astronomy and geophysics nriag in 2017 to detect the recording capabilities of the egyptian national seismological network ensn established after the cairo earthquake which occurred in egypt on october 12 1992 this article covers the first output of the project which shows the results of noise assessment for nine broadband stations located in northern egypt for a proper assessment of seismic data quality for a part of the ensn network the passcal quick look extended pqlx software was used which is based on the calculation of power spectral density psd and probability density function pdf for the background noise the analysis covered almost one year of continuously recorded seismic data the results show that some stations have very low detectability and higher noise level in some frequency bands which affects the recording of seismic events while others show good performance complete site analysis for all stations was done to illustrate the present site response to be able to recommend whether to keep a station or to relocate it to achieve this the seismic noise data extracted from all stations of the considered zone was studied to determine the horizontal to vertical h v spectral ratio method to study the amplification parameters of the recording site the output of this study is useful for detecting operational problems in the northern part of the egyptian seismological network and could be relevant for the reconfiguration and optimization of ensn stations

comparative study of microtremor analysis methods during a multidisciplinary microzonation pilot project in the city of heraklion crete greece microtremor data were collected at the top of exploratory boreholes specifically designed for the purposes of the project over a period of 5 days for 4 h day at 125 hz continuous recordings the data were analysed with the ssr and h v ratio techniques using the standard fft applied to long data series and a multi variate maximum entropy mv maxent spectral analysis method both techniques implemented with both spectral analysis methods identify the same major resonance frequency band albeit with different amplification levels the mv maxent however is effective in handling short data lengths while yielding high resolution spectra and addressing several shortcomings of the conventional fft windowing zero padding etc thus it yields competitively similar results with only a fraction a few minutes of the data required by the lower resolution fft method and appears to be a powerful tool for site effect investigations moreover the results of both microtremor based techniques are consistent and remarkably similar to the results of microzonation methods that require expensive borehole data

microseisms observations in the santa clara valley california we have investigated ground motion amplification in the santa clara valley scv using microseisms observed during the 1998 deployment of 41 short period seismometers the santa clara valley seismic experiment scvse lindh et al 1999 fletcher et al 2003 recorded many local regional and teleseismic events in our previous work we investigated the 3d velocity structure of the scv by modeling the teleseismic p waves recorded during the scvse dolenc et al 2005 to complement these results we now focus on the microseisms that were recorded during the same period and relate these observations to local earthquake wave amplification it is found that the seismic noise is related to the ocean wave heights measured on the weather buoy west of half moon bay california the spectral ratio of the horizontal to vertical h v microseisms at each scvse site is stable with time and the period of the dominant peak in the h v ratio is related to the basin depth the results of this study show that seismic noise can be used in the assessment of the effects of deep sediments on long period earthquake ground motions

some aspects of source characteristics of the 19 september 1985 michoacan earthquake and ground motion amplification in and near mexico city from strong motion data abstract the fourier amplitude ratio of the 19 september m s 8 1 to the 21 september 1985 m s 7 6 michoacan earthquakes obtained from data in and near mexico city increases from about 3 5 at 0 5 hz to about 10 at 0 2 hz this increase is neither seen in the coastal data nor in the teleseismic broadband gdsn p wave spectra it is also not predicted by 2 or gusev9s scaling laws we tentatively attribute the increase at frequencies of less than 0 5 hz to a special path and or a depth of energy release effect for the 19 september earthquake the damage to mexico city is mostly related to 0 5 hz energy since at f 0 5 hz the fourier amplitude ratios in and near mexico city along the coast and at teleseismic distances are nearly equal we attribute this energy to the source characteristics of the 19 september earthquake the evidence suggests that the 19 september was about twice more energetic at f 0 5 hz than expected from scaling laws and from spectral regression of coastal earthquakes recorded in mexico city the analysis of strong motion data recorded in mexico city shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in ciudad universitaria the frequency at which the maximum relative amplification occurs varies from site to site and lies between 0 2 and 0 7 hz the data suggest that f o 4 h where f o frequency at which maximum relative amplitude occurs and and h are the shear wave velocity and the thickness of the clay layer respectively relatively small if any nonlinear behavior of the clay is found at strains of about 0 2 per cent during the great 19 september earthquake for a given lake bed site maximum relative amplitude and f o are roughly constant independent of magnitude depth and azimuth of sources located more than about 200 km ground motion at lake bed sites in mexico city with f o 0 5 hz appear to be amplified 75 times with respect to hard rock coastal sites at equal distances from the source this is also confirmed by m l calculation of the 19 and 21 september earthquakes for the 19 and 21 september earthquakes the average values of m l at hard rock coastal sites 100 distance 250 km are 7 4 and 6 7 whereas they are 8 7 and 8 4 in the lake bed zone distance 300 km respectively

hvsr technique as tool for thermal basin characterization a field example in n e italy the euganean thermal basin located in the eastern po plain italy is one of the most important thermal districts of italy euganean berician thermal district the presence of hot water in the basin has brought about its exploitation over time the thermal area has been known since the roman empire and is still of crucial interest for the profitable use of its hydro geological resources for these reasons the water cycle of the thermal area has been thoroughly studied in particular with regard to its chemical and hydrogeological properties the effects of well pumping such as subsidence and water temperature steadiness have been monitored over time in the present work the horizontal to vertical spectral ratio hvsr single station microtremor technique was applied as a non invasive geophysical tool for the characterization of a part of the euganean berician thermal district involving the municipalities of abano and montegrotto towns this study represents the first correlation between the results from microtremor geophysical surveys and the thermal basin geological structure including remote sensing data and thermal well information more than 50 hvsr measurements from single stations were collected and analysed in close correlation with the vast amount of available borehole logs well exploitation data and sar interferometry measurements collected for the monitoring of land subsidence induced by thermal water pumping the aim of the work was to evaluate the suitability of the microtremor technique for the preliminary characterization of a thermal basin in terms of resonance properties of the alluvial deposits covering the rocky bedrock evaluated by means of hvsr methods hvsr results were interpreted in terms of regional geological setting and hydrogeological properties of the subsoil our field results confirmed the existence of significant correlations between the resonance behaviour of the basin determined by the hvsr technique and the geological structures suggesting that single station measurements are a promising tool for creating a rapid and preliminary non invasive contribution to the characterization of the geothermal basin

site effects in a volcanic environment a comparison between hvsr and array techniques at colima mexico colima city is the capital of the mexican federal state of the same name it is located close to the pacific coast and is subjected to a large seismic risk we present a microzonation study in this city based on microtremors using single station and array measurements we applied horizontal to vertical spectral ratios hvsr analysis to single station measurements at 310 sites within the city concentrating measurements in zones that were damaged by the january 2003 m 7 4 earthquake the results show that a seismic zonation based exclusively on single station microtremor measurements is not a reliable alternative when the local geology is complex and site effects are not the result of a single impedance contrast for this reason we applied two independent analysis techniques to array measurements of microtremors the spatial autocorrelation spac method and the refraction microtremor remi method we used linear arrays to record 25 sec microtremor windows at eight sites within the city which were analyzed with those two techniques the result of both techniques of analysis is a phase velocity dispersion curve which can be inverted to obtain a shallow s wave velocity profile two of the sites were the location of shallow 50 m boreholes where p and s wave velocity profiles were measured using a p s suspension log the phase velocity dispersion curves obtained from the remi and spac analyses of the microtremor records showed very good agreement the velocity profiles inverted from the phase velocity dispersion curves showed good agreement with the suspension logging measurements at one of the two sites where they were available and poor agreement at the other site the transfer functions computed from the inverted soil profiles are in good agreement with previous estimates of local amplification from spectral ratios analysis of earthquake records our results are compatible with previous indications of site effects and explain the failure of single station microtremor measurements when the concept of dominant frequency loses its meaning finally we propose an estimate of local site amplification at the city of colima which will be useful for future predictions of ground motion at this city