surface wave dispersion analysis in mexico city abstract in this article we present an observational investigation of ground motion at mexico city focused on surface waves our purpose is 2 fold first to understand incident ground motion during the great michoacan earthquake of 19 september 1985 and second to characterize surface waves propagating in the lake bed zone to this end we analyze the strong motion records obtained at mexico city for the large m s 8 1 earthquake of 19 september 1985 it is shown that in the low frequency range we observe the rayleigh fundamental mode in both the vertical and the radial components and the love fundamental mode in the transverse component at all the strong motion stations the vertical component also shows the first higher mode of rayleigh waves we use a very broadband record obtained at station cu for the smaller m s 6 7 earthquake of 14 may 1993 to verify that the dispersion computed from the model of campillo et al 1989 represents well the average surface wave propagation between the coast and mexico city in the 7 to 10 sec period range we use this result to assign absolute times to the strong motion records of the michoacan event this allowed us to identify additional wave trains that propagate laterally in directions other than great circle in the 3 to 5 sec period range these wave trains are identified as love waves in a second analysis we study a set of refraction data obtained during a small scale 250 m experiment on the virgin clay of the lake bed zone phase velocity dispersion curves for several modes of rayleigh waves are identified in the refraction data and inverted to obtain an s wave velocity profile this profile is used as the uppermost layering in a 2d model of mexico city valley the results of numerical simulation show that surface waves generated by lateral finiteness of the clay layer suffer large dispersion and attenuation we conclude that surface waves generated by the lateral heterogeneity of the upper most stratigraphy very significantly affect ground motion near the edge of the valley but their importance is negligible for distances larger than 1 5 km from the edge thus locally generated surface waves propagating through the clay layer cannot explain late arrivals observed for the 1985 event we suggest that the long duration of strong motion is due to the interaction between lateral propagation of waves guided by deep layers 1 to 4 km and the surficial clay layer this interaction is possible by the coincidence of the dominant frequency of the uppermost layers and the frequency of the deeply guided waves

tectonic setting and source parameters of the september 19 1985 michoacan mexico earthquake analysis of body waves and long period surface waves from the september 1985 earthquake in coastal michoacan mexico shows that the event was an interplate subduction event with a low dip angle fault plane 9 striking parallel to the mid america trench 288 and a small component of left lateral motion 72 with a point source depth of 17 km and a seismic moment in excess of 1 10 28 dyn cm the earthquake was a multiple event with a second source of identical moment fault geometry and depth occurring approximately 26 s after the first directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first this suggests that the earthquake first broke the northern portion of the michoacan gap propagated with low moment release through the rupture zone of the 1981 playa azul earthquake and then broke the remaining asperity in the southern section of the gap the seismic moment determined from rayleigh and love waves is between 1 0 1 7 10 28 dyn cm m w 7 9 8 1 the largest moment determined to date for a mexico subduction earthquake comparison of seismograms at pasadena with records of other large mexico events shows that the michoacan earthquake is basically the same size as the 1932 jalisco mexico earthquake and clearly larger than other significant events in mexico since 1932 the seismic moment and the time since the last large earth quake in michoacan in 1911 fit an empirical relation between moment and recurrence time found for the guerrero oaxaca region of the subduction zone the large aftershock on september 21 m s 7 5 has the same geometry as the mainshock a somewhat larger source depth 22 km a simple time function and a seismic moment between 2 9 4 7 10 27 dyn cm m w 7 6 7 7

acceleration spectra for subduction zone earthquakes we estimate the source spectra of shallow earthquakes from digital recordings of teleseismic p wave groups that is p pp sp by making frequency dependent corrections for the attenuation and for the interference of the free surface the correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths we apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from ms 6 2 to 8 1 obtaining corrected p wave acceleration spectra on the frequency band from 0 01 to 2 0 hz seismic moment estimates from surface waves and normal modes are used to extend these p wave spectra to the frequency band from 0 001 to 0 01 hz the acceleration spectra of moderate subduction zone earthquakes that is earthquakes whose seismic moments are less than 1027 dyn cm exhibit square or brune type spectra while the acceleration spectra of large subduction zone earthquakes that is earthquakes whose seismic moments are greater than 1027 dyn cm exhibit intermediate slopes where u 5 4 for frequencies from 0 005 to 0 05 hz for this set of earthquakes spectral shape appears to be a discontinuous function of seismic moment using reasonable assumptions for the phase characteristics we transform the spectral shape observed for large earthquakes into the time domain to fit ekstrom s 1987 moment rate functions for the ms 8 1 michoacan earthquake of september 19 1985 and the ms 7 6 michoacan aftershock of september 21 1985

effect of distance on local magnitudes found from strong motion records values of local magnitude m l are calculated from 56 strong motion accelerograms recorded in the imperial valley earthquake of 15 october 1979 according to procedures developed earlier kanamori and jennings 1978 these data plus similar data from the san fernando earthquake of 9 february 1971 and additional less numerous data from several other california earthquakes are used to investigate the use of different measures of distance in near field determinations of m l this investigation has relevance for similar uses of distances in determining seismic design criteria in addition the consistency of the values of m l found from the strong motion data is examined from the viewpoint of assessing the need for any correction in the standard attenuation curve log 10 a 0 it was found that the most consistent values of m l result when distance is measured to the closest point on the surface trace of the fault if a site lies within a circle with diameter equal to the extent of faulting and centered on the center of faulting center of energy release outside this circle the distance measured to the center of the circle is recommended a consistent trend in the values of m l found from strong motion records is seen in the data the values start at zero distance at essentially the far field value and then decrease to 1 4 unit at about 20 km then they rise to 1 4 unit at 50 to 60 km a smooth revision to the standard attenuation curve is presented which removes this systematic trend

propagation and resonance of sh waves in the salt lake valley utah abstract the salt lake valley is an alluvium valley where seismic amplification measured at some sites can exceed a factor of 10 relative to hard rock sites adjacent to the basin it is important to understand the causes for these amplifications in order to predict which sites in the salt lake valley are most prone to strong ground shaking towards this end finite difference synthetic seismograms were used to successfully characterize key parameters controlling low frequency sh wave amplification in the salt lake valley plane wave source synthetic seismograms reveal that sh wave amplification in the frequency band 0 2 to 2 0 hz is primarily controlled by the two shallowest lithologic units the 0 25 to 1 0 km thick unconsolidated alluvial quaternary sediments v s 0 98 km sec and the underlying 0 5 to 1 0 km thick semi consolidated tertiary sediments v s 1 42 km sec the quaternary sediments tend to increase the coda duration while the semi consolidated tertiary sediments act as a wave guide to trap the low frequency seismic energy it is also found that the basin can cast a seismic shadow over adjacent bedrock sites inhibiting seismic wave propagation to sites adjacent to the basin calibration of valley site amplification to these bedrock sites may result in overestimation of basin site amplification finite difference synthetic seismogram modeling can clearly demonstrate the significant effects of deep basin lithology and geometry on low frequency site amplification observed across the salt lake valley

strong motion in central mexico a model based on data analysis and simple modeling predictions of ground motion for mexico city during future large earthquakes are currently based on statistical analysis of the large quantity of data recorded by the mexico city accelerograph network mcan in spite of the robustness of these predictions large uncertanties come from our lack of a physical model that is able to account for the observations in this article we present such a model based on analysis of strong motion records obtained during the 25 april 1989 guerrero earthquake and on numerical modeling data analysis suggests that the large amplitudes and very long duration of ground motion observed in the lake bed zone in mexico city result from the interaction of diffracted wave trains of rayleigh waves and the very soft clay layer that covers the ancient lake of mexico city numerical modeling supports this hypothesis and indicates that this interaction is possible in spite of the largely different scales kilometers for the incident surface waves and tens of meters for the clay layer lateral heterogeneities split incoming wave trains into different modes each is strongly amplified and its duration is increased by the soft clay layer at its resonant frequency resulting in the very long monochromatic ground motion that has been the subject of a large number of articles since the 1985 michoacan earthquake manuscript received 23 august 2001

ice coring at mizuho station antarctica and core analyses a contribution from the glaciological research program in east dronning maud land antarctica between may 1983 and july 1984 glaciological parties of the 24th and 25th japanese antarctic research expeditions jare 24 and 25 carried out ice core drilling using a thermal drill down to 700 5 m depth at mizuho station 70 41 53 s 44 19 54 e as a part of the glaciological research program in east dronning maud land antarctica the thermal drill 3 9 m long and capable of taking a core 1 5 m long and 130 mm in diameter is an improved version of a drill used by jare 15 in 1975 the most important improvement was the monitoring system during drilling for which a micro computer was fitted in the drill by using this system such accidents as heater burn out tank overflow and failure of water suction would immediately be brought to our attention the drilling speed was about 1 6 m h when the optimum output was 3 6 kw the core recovery rate was above 99 the core quality was good down to a depth of 80 m between 80 and 120 m cracks were found at intervals of 0 15 0 5 m and horizontal cracks were found continuously at intervals of 0 01 m or less immediately after the core was pulled the stratigraphy was observed and bulk density was measured a dust band presumably volcanic particles was seen at only 500 2 m depth during stratigraphic observation the following analyses were carried out at mizuho station within a month of recovery 1 density determination by the hydrostatic method 2 measurement of total gas content 3 thin section analyses including observation of cracking around air bubbles and the crystalline texture and ice fabric studies the 700 5 m core has been brought to japan and the following analyses are now under way 1 oxygen isotope ratio 2 concentration of microparticles 3 electric conductivity 4 chemistry of soluble impurities

ground motion on stratified alluvial deposits for incident sh waves a boundary method is applied to study the response of a two dimensional horizontally stratified deposit of arbitrary shape under the incidence of sh waves the procedure combines the line source method with a discrete wavenumber representation for the stratified part in terms of propagator matrix comparison of results for a circular nonhomogeneous deposit with those obtained with the finite element method shows excellent agreement the response of a basin with two strata is analyzed variation of results as a function of incidence angle is presented for several frequencies transfer functions at several points on the surface of the deposit are obtained for vertical incidence and comparison with results from a one dimensional analysis show significant differences time response for an incident ricker wavelet clearly shows important effects due to lateral heterogeneity

the ml scale in southern california measurements 9 941 of peak amplitudes on wood anderson instruments or simulated wood anderson instruments in the southern california seismographic network for 972 earthquakes primarily located in southern california were studied with the aim of determining a new distance correction curve for use in determining the local magnitude m l events in the mammoth lakes area were found to give an unusual attenuation pattern and were excluded from the analysis as were readings from any one earthquake at distances beyond the first occurrence of amplitudes less than 0 3 mm the remaining 7 355 amplitudes from 814 earthquakes yielded the following equation for m l distance correction log a 0 log a 0 1 110log r 100 0 00189 r 100 3 0 where r is hypocentral distance in kilometers a new set of station corrections was also determined from the analysis the standard deviation of the m l residuals obtained by using this curve and the station corrections was 0 21 the data used to derive the equation came from earthquakes with hypocentral distances ranging from about 10 to 700 km and focal depths down to 20 km with most depths less than 10 km the log a 0 values from this equation are similar to the standard values listed in richter 1958 for 50 200 km the effect at close distances is consistent with that found in several other studies and is simply due to a difference in the observed 1 r geometrical spreading for body waves and the 1 r 2 spreading assumed by gutenberg and richter in the construction of the log a 0 table m l s computed from our curve and those reported in the caltech catalog show a systematic dependence on magnitude small earthquakes have larger magnitudes than in the catalog and large earthquakes have smaller magnitudes by as much as 0 6 units to a large extent these systematic differences are due to the nonuniform distribution of data in magnitude distance space small earthquakes are preferentially recorded at close distances relative to large earthquakes for large earthquakes however the difference in the two magnitudes is not solely due to the new correction for attenuation magnitudes computed using richter s log a 0 curve are also low relative to the catalog values the differences in that case may be due to subjective judgment on the part of those determining the catalog magnitudes the use of data other than the caltech wood anderson seismographs the use of different station corrections or the use of teleseismic magnitude determinations whatever their cause the departures at large magnitude may explain a 1 0 0 7 proportionality found by luco 1982 between m l s determined from real wood anderson records and those from records synthesized from strong motion instruments if it were not for the biases in reported magnitudes luco s finding would imply a magnitude dependent shape in the attenuation curves we studied residuals in three magnitude classes 2 0 m l 3 5 3 5 m l 5 5 and 5 5 m l 7 0 and found no support for such a magnitude dependence based on our results we propose that local magnitude scales be defined such that m l 3 correspond to 10 mm of motion on a wood anderson instrument at 17 km hypocentral distance rather than 1 mm of motion at 100 km this is consistent with the original definition of magnitude in southern california and will allow more meaningful comparison of earthquakes in regions having very different attenuation of waves within the first 100 km

soil basin and soil building soil interaction effects on motions of mexico city during seven earthquakes starting with the destructive 1985 michoacan mw 8 0 earthquake the lake zone of mexico city has been experiencing ground motions bearing the effects of strong soil amplification at specific site d

mexico 1985 the case for gravity waves summary the argument for the existence of gravity waves in sedimentary layers during strong earthquakes such as the 1985 mexico earthquake is based on a theory which predicts a discontinuity in surface wave propagation at a finite strain value b effects of stress strain non linearity in sediments and c observations suggesting the presence of ultra short wavelengths in soils during the 1985 mexico earthquake

evidence of the dominance of higher mode surface waves in the lake bed zone of the valley of mexico summary we compare ground motions recorded at the surface and in boreholes at five different locations of the lake bed zone of the valley of mexico with theoretical dispersion curves and eigenfunctions calculated for the first two modes of rayleigh and love waves we find that 1 the maximum in the horizontal to vertical displacement ratio which occurs at the dominant frequency of the site 0 4 hz corresponds to the higher mode rather than to the fundamental mode of the rayleigh waves 2 borehole records at depths from 0 to 100 m show that the normalized vertical displacement does not decrease rapidly below the superficial clay layer as should be the case for the fundamental mode but remains 0 8 and 3 the measured phase velocity at a period of about 2 5 s 2 0t0 5 km s x1 is too fast for the fundamental mode predicted for the known crustal velocity structure these observations lead us to conclude that the wavefield in the lakebed zone in mexico city is dominated by higher mode surface waves this provides a plausible explanation for the long duration of the coda in the lake bed zone although shear wave q is very small 10 20 in the clay layer the higher modes of surface waves do not propagate in the superficial clay layer but in the underlying structure where q values are likely to be relatively high thus while the clay layer plays the passive role of amplifying the ground motion its contribution in damping out the motion is insignificant the results have two important practical implications 1 the strain estimate from recorded ground velocity differs significantly for the fundamental mode as compared to the higher mode surface waves 2 if the ground motion is dominated by the fundamental mode then knowledge of the superficial layer and the velocity contrast with the underlying structure is sufficient for understanding and modelling of the ground motion if however the higher mode surface waves dominate then a detailed knowledge of the deeper structure is required

source characteristics of the 1985 michoacan mexico earthquake at periods of 1 to 30 seconds source characteristics of the sept 19 1985 michoacan mexico earthquake and its aftershock on sept 21 were inferred from broadband and short period teleseismic gdsn records we fourier transformed the p waves corrected for instrument response attenuation geometrical spreading and radiation pattern including the depth phases and then averaged to obtain the teleseismic source spectrum from 1 to 30 s the michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events source spectra for the sept 21 aftershock 1981 playa azul 1979 petatlan and 1978 oaxaca events follow a trend similar to that of the 1985 michoacan event this spectral trend may characterize the mexican subduction zone a station by station least squares inversion of the michoacan earthquake records for the source time function yielded three source pulses which we interpreted as events on the fault plane the first two are similar in moment and the third contains only 20 of the moment of the first directivity is evident in the timing at each station we measured the time differences between the pulses and performed a least squares nonlinear estimation of the strike distance and time separation between the events to locate them relative to one another the second event occurred 26 s after the first and 82 km southeast of it indicating southeastward rupture along the trench the third event occurred 21 s after the second and about 40 km seaward of it the two large events are also seen in the near field strong motions the mainshock records spectrum and time functions contain less high frequency radiation than those of the 1985 valparaiso chile earthquake apparently the michoacan earthquake ruptured two relatively smooth strong patches which generated large 30 s waves but small 1 to 10 s waves such behavior contrasts with the valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy this difference is consistent with the higher near field accelerations recorded for the valparaiso event

relationship between subduction and seismicity in the mexican part of the middle america trench two catalogs of earthquakes in the mexican part of the middle america trench are analyzed to elucidate principal relations between structure of the subducting cocos plate and seismicity a catalog of historical events that have occurred during the last two centuries with large magnitudes ms 6 0 is used to obtain cumulative seismic moment mo and seismic moment release rate m 0 distributions along the mexican subduction zone the catalog of instrumentally observed earthquakes from 1963 to 1990 international seismological center and u s geological survey with 4 5 mb 6 0 is applied to investigate background seismicity for the region the strength of coupling between the cocos and north american plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate v and age a of the cocos plate this correlates in general with a steady reduction in background seismicity and a slight average increase of m 0 and seismic energy release rate w at the sites where the main fracture zones of the cocos plate tehuantepec o gorman orozco and rivera undergo the subduction the general correlation breaks down the background seismicity increases at fracture zones while m0 and m 0 decrease significantly this feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted seismic slip rates along the trench obtained from m 0 are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the v model interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments variability of m 0 and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes an examination of space time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of 10 km yr

a virtual reference site for the valley of mexico we propose the use of the average spectra of northern strong motion stations located at the hill zone e g md tx es 64 as the reference for mexico city s ground motion this virtual site proposal is based on the analysis of recent data from the mexico city acelerometric network the northern stations show amplitudes both in time and frequency that are consistently smaller than those of hill zone stations located south and west of the city e g cu it is well known that cu the historical reference site in mexico city and other sites to the southwest present amplifications whereas the northern ones appear to be free of such effects the spectral ratio of the averages of the stations located in the south and west with respect to the northern stations shows a relatively constant amplification of up to 3 times in the 0 7 10 0 hz frequency band this amplification is a very unusual feature that should be explained the geologic conditions at the hill zone show that older miocene age deposits are located north of the city considering that northern sites represent the basement we assume that the configuration along the hill zone in the n s direction can be approximated by a simple dipping homogeneous layer we computed the antiplane seismic response for this model and averaged and compared it with the spectral ratio obtained from strong ground motion data the agreement is good and suggests how a smooth large scale feature could amplify seismic ground motion in a broad frequency band manuscript received 12 october 2001

local site effects on weak and strong ground motion abstract this is a review of the current state of the art in characterizing effects of local geology on ground motion a new horizon is clear in this aspect of strong motion studies non linear amplification at sediment sites appears to be more pervasive than seismologists used to think several recent observations about the weak motion and the strong motion suggest that the non linear amplification at sediment sites may be very common first on average the amplification is always greater at the younger sediment sites for all frequencies up to 12 hz in the case of weak motion while the relation is reversed for frequencies higher than 5 hz in the case of strong motion secondly the application of the amplification factor determined from weak motion overestimates significantly the strong motion at sediment sites observed during the loma prieta earthquake within the epicentral distance of about 50 km thirdly the variance of peak ground acceleration around the mean curve decreases with the increasing earthquake magnitude finally the above non linear effects are expected from geotechnical studies both in the magnitude of departure from the linear prediction and in the threshold acceleration level beyond which the non linearity begins

is there truly a hard rock site in the valley of mexico to understand the cause of observed amplification of seismic waves even at hill zone sites in the valley of mexico digital accelerographs have been installed at three especially chosen sites two of these sites madi and texc located on hard pleistocene lava a few meters in thickness overlying oligocene andesites were expected to be free of site effects analysis of the data recorded by these and other accelerographs during three moderate shallow subduction zone events however shows significant amplification at madi and texc between 0 2 and 0 6 hz with respect to established attenuation relations the cause of the amplification at hill zone sites in the valley of mexico including madi and texc may be pervasive low s wave velocity in and complex structure of the upper layers of the volcanic rocks if so then there may not be a truly hard rock site in the valley

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

nonlinear analysis on seismic site response of fuzhou basin china based on the parallel computing cluster platform of the abaqus software a large scale 2d finite element refined nonlinear modeling approach was used to study seismic site effects in the fuzhou basin such as peak ground acceleration pga spectral acceleration duration and acceleration transfer functions a 1d equivalent linear wave propagation analysis was conducted also with proshake for supplementation and calibration the simulation results demonstrated the following 1 pga amplification factors exhibited spatial variation characteristics that varied both laterally and with depth and exhibited a nonmonotonic decreasing characteristic with soil depth from the 2d results a greater motion amplification and focusing effect for some shallow soil layers was observed where there was significant fluctuation of the underlying bedrock interface the moderate and long period ground motion components were obviously amplified but were was less pronounced in the 1d result 2 for low moderate and high level earthquakes the surface pga amplification factors were 1 4 2 5 1 3 2 0 and 1 1 2 0 respectively and the predominant periods of the basin were 0 35 0 65 s 0 40 0 75 s and 0 50 1 05 s respectively 3 ground motion durations were prolonged to different degrees which were closely related to the characteristics of seismic bedrock motions 4 the sensitive frequency band of the seismic site response was from 0 5 to 2 hz 5 ground motion amplification in the downtown section of the city of fuzhou was generally larger to a certain extent the 2d results reflect the influences of the surface topography relief the fluctuation of the underlying bedrock interface and lateral heterogeneity of soils on seismic wave propagation

on the duration of seismic motion incident onto the valley of mexico for subduction zone earthquakes summary we have used finite difference simulations in 2 d models of the lithosphere to estimate the duration of long period 2 s ground motion incident onto the valley of mexico for subduction zone earthquakes our simulations suggest that two heterogeneous structures extend the duration of the ground motion between the subduction zone and mexico city by more than 1 min 1 the mexican volcanic belt and 2 two low velocity layers in the coastal region the accretionary prism and the water layer the duration generated by a crustal model including these structures is similar to that for earthquake records observed in between the coast and mexico city in the valley of mexico our models including only regional scale heterogeneity reproduce approximately one half of the observed duration the results suggest that both the regional and the local scale low velocity structures must be taken into account in order to explain the observed extended signal duration in the valley of mexico

the great jalisco mexico earthquakes of 1932 subduction of the rivera plate we have analyzed seismograms of the 3 june 1932 ms 8 2 and the 18 june 1932 ms 7 8 jalisco earthquakes and their aftershocks at manzanillo mnz guadalajara gum and tacubaya tac the aftershocks locations the first motions at mnz and the isoseismic maps of the two main shocks strongly suggest that a the 3 june 1932 earthquake initiated nw of but close to mnz and propagated nw for an estimated length of rupture of 220 km b the 18 june 1932 earthquake nucleated sw of mnz offshore and perhaps ruptured a length of about 60 km and c the width of rupture was approximately 80 km there is no evidence of events occurring se of mnz even up to 1 1 2 yr after the first main shock a gap of about 60 km remains between the aftershock areas of the 1932 jalisco and the 1973 colima earthquakes whose seismic potential is unknown although the boundary between the rivera and cocos plates is uncertain there is little doubt that the 1932 earthquakes broke the shallow part of the rivera subduction zone our results may help in quantifying seismic potential of tectonically similar areas such as the juan de fuca subduction zone in the nw united states

resonant response models for the valley of mexico i the elastic inclusion approach summary the model developed by bard and bouchon to study the earthquake response of open ended 2 d sediment filled valleys with simple geometries is generalized to describe completely enclosed basins with arbitrary geometries by solving the basin form that corresponds to the valley of mexico it is possible to understand via a two step process model several of the observed features of the 1985 september 19 earthquake that devastated downtown mexico city here a general approach that allows the treatment of the particular seismic behaviour of the valley of mexico which is a large sediment filled valley containing an extremely soft ground urban area with a very high water content using collective resonance methods that vary from elastic to acoustic models is established this will lead through a series of papers to a theory that takes into account the role played by any valley s collective modes for microzonation and seismic damage predictions

resonant response models for the valley of mexico ii the trapping of horizontal p waves summary the strong ground motion recorded during the 1989 april 25 earthquake ms 6 9 shows many of the previously observed features in the soft lacustrine clays over which downtown mexico city is built a dominant response frequency around 0 4 hz enormous time prolongation of the seismic signal with characteristic beating phenomena and conspicuous differential amplification following the two step process suggested in part i seligman et al 1989 we add two new theoretical aspects the generation of horizontally propagating sp waves in the layered structure beneath mexico city for incoming sv waves beyond critical incidence and the subsequent lateral trapping of these sp waves in the lacustrine clays the time evolution of the lateral resonant pattern produced by these waves is calculated for the realistic geometry of the lake beds synthetic accelerograms and fourier spectra at different sites within the clay region are thus obtained our results can explain the monochromaticity and the long duration of the strong motion observed systematically in the soft clays of the valley of mexico

experimental and numerical evidence of the clustering effect of structures on their response during an earthquake a case study of three identical towers in the city of grenoble france in this article interpretation of an equivalent to a macroseismic intensity survey performed in three identical stand alone buildings located in grenoble france after an m l 4 1 earthquake reveals a clustering effect resulting in different levels of perception of seismic loading by inhabitants the clustering effect is confirmed using numerical simulation the variation of the seismic response of the building in the middle of the cluster depends on the azimuth of the seismic source relative to the building cluster the major effect is the splitting of its resonance frequency accompanied by a decrease in vibration amplitude we conclude that clustering has an impact on urban effects calling into question the validity of seismic design which considers buildings in urban areas as stand alone constructions and the interpretation of macroseismic intensity surveys conducted in dense urban areas

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

spectral analysis and earthquake scaling of the petatlan earthquake m s 7 6 aftershocks recorded at stations along the pacific coast and between the coast and mexico city spectral parameters have been estimated for 214 petatlan aftershocks recorded at stations between petatlan and mexico city and between petatlan and acapulco the spectral parameters were used to obtain empirical relations for the estimation of seismic moment from coda length and fromm l stress drops using brune s model were calculated for these aftershocks six events with large stress drop are located within a previously suggested asperity and seven more suggest a boundary zone at the intersection of the petatlan and zihuatanejo aftershock rupture volumes stress drops increase with increasing seismic moment up to 1020 dyne cm but appear to be constant at greater moment values the peak horizontal velocity times distance of aftershocks recorded near the coast and between the coast and mexico city 30 to 270 km away scales linearly with seismic moment and predicts well the peak horizontal values of large m s 7 0 coastal thrust events recorded on rock sites at mexico city peak horizontal velocity is a straightforward measurement thus this relation allows us to evaluate expected ground motion between the pacific coast and mexico city from the seismic moment of subduction related earthquakes along the coast

site city seismic interaction in mexico city like environments an analytical study recent destructive earthquakes have confirmed the importance of hazard and vulnerability studies to predict and prevent the impact of large seisms most seismic risk analysis considers the buildings as a passive constituent integrated into the vulnerability analysis this approach neglects the possible contribution of the building vibration to the free field this article describes an analysis of site city effects that is the seismic interaction of the city with soft soil layers an analytic method derived from soil structure interaction studies is described and applied to the roma norte zone of mexico city the building parameters are derived from detailed studies of the ja and pc buildings the soil city system is subjected to the mexican m w 7 3 14 september 1995 earthquake the simulated ground surface motions which incorporate the wave field radiated from the buildings of roma norte district are compared to seismic records from this zone analytical procedures reproduce the long time duration and beating of the observed ground motion in mexico city parametric analysis are also performed to identify the predominant factors e g urbanization density soil to city stiffness ratio that favor the site city interaction effects a simple relation is proposed to estimate the expected efficiency of the site city interaction effects for any city

particle size and mineralogical composition of inorganic colloids in waters draining the adit of an abandoned mine goesdorf luxembourg particle size distributions and the mineralogy of inorganic colloids in waters draining the adit of an abandoned mine goesdorf luxembourg were quantified by single particle counting based on light scattering 100 nm 2 m combined with transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction this water system was chosen as a surrogate for groundwaters the dependence of the colloid number concentration on colloid diameters can be described by a power law distribution in all cases power law slopes ranged from 3 30 to 4 44 depending on water ionic strength and flow conditions the same main mineral types were found in the different samples 2 1 phyllosilicates illite and mica chlorite feldspars albite and orthoclase calcite and quartz with a variable number of fe oxide particles the colloid mineralogical composition closely resembles the composition of the parent rock spatial variations in the structure and composition of the rock in contact with the waters i e fissured rock versus shear joints are reflected in the colloid composition the properties of the study colloids as well as the processes influencing them can be considered as representative of the colloids present in groundwaters

1 d theoretical modeling for site effect estimations in thessaloniki comparison with observations we apply an algorithm based on the modal summation method to theoretically estimate the site effect at selected locations underlain by different geological formations within the city of thessaloniki greece complete strong motion synthetics are constructed for all components of motion at each site for a maximum frequency of 10 hz the anelastic local i d velocity models are based on cross hole data four point sources with different azimuths and distances from the city are used to compute the input signals the theoretical amplification is estimated through spectral ratios of accelerograms obtained by the local 1 d over those obtained by the regional 1 d velocity model the results from the numerical modeling are compared with those derived from experimental techniques such as of standard spectral ratio and horizontal to vertical spectral ratio which had been applied to acceleration data recorded at the same sites the comparison demonstrates that the theoretical amplifications based on known and simple subsurface geology can be used as a first order estimate while for cases of more complex geometries the use of at least 2 d modeling in site effects estimation is mandatory

high density macroseismic survey in urban areas part 2 results for the city of rome italy high density macroseismic surveys were carried out in rome following both the 14 october 1997 m w 5 6 i 0 viii mcs and the 26 march 1998 m w 5 3 i 0 vi vii mcs umbria marche central italy earthquakes the aim of the work was to investigate the relationship between intensity and local geology for the modern urban area of rome an issue yet to be examined in contemporary times intensity data for both earthquakes were correlated with near surface geology the results show a statistically significant amplification in holocene alluvial deposits in particular we found one degree of difference in mcs intensity between hard rock and recent alluvium formations furthermore the distribution of the earthquake effects in alluvium filled valleys was studied as a function of the shape ratio h l of the alluvial basins in the tiber river valley higher effects were found to be concentrated within a 150 m wide band along the bedrock alluvium surface contact suggesting the occurrence of an edge effect surprising results were observed in the minor alluvial valleys which displayed the strongest effects in terms of shaking the narrowest valleys width

local magnitude and sediment amplification observations from earthquakes in the northern baja california southern california region abstract the data set that is analyzed in this study consists of strong motion and digital velocity records obtained during the last 6 yr for earthquakes along faults in baja california north and southern california from this set local magnitudes for 60 earthquakes recorded at stations on hard rock and sediments are calculated following the technique proposed by kanamori and jennings 1978 the calculated near source motion magnitudes m lsm are then compared with reported local magnitude values m l which are in the range from 3 to 6 6 from these comparisons it is found that for earthquakes with magnitude m l between 3 and 5 5 almost all the m lsm values estimated from data registered on sediments exceed the m l values by amounts which at the shorter distances less than about 10 km may be as high as an order of magnitude or even higher for a few cases the strong motion records associated with these high m lsm values have highly coherent pulse like s wave signals of very short period and large accelerations similar estimations of local magnitude from more limited data obtained on hard rock were comparable or lower than the standard m l values we conclude that the high m lsm values are the result of a combination of high frequency source spectra and a strong shear wave energy amplification due to the presence of sediments a sediment amplification factor of 3 2 is inferred from the data and is in good agreement with a theoretical factor of 3 4 estimated on the basis of the mcmechan and mooney 1980 velocity model for the imperial valley it is also found from the analyzed data that near source local magnitude decreases between 0 and 10 km indicating that for smaller earthquakes the attenuation function log a 0 richter 1958 used in local magnitude determinations needs to be modified to take into account the characteristics of the seismic energy radiated at short distances near source strong motion data for the 1979 imperial valley m l 6 6 and 1980 victoria m l 6 1 events do not show the strong discrepancy between m lsm and m l observed for the smaller magnitude events factors that might be responsible for this include nonlinear sediment attenuation for larger amplitude waves as well as complex near source spreading out of the waveform resulting from larger source size near source magnitude saturation we give a numerical example using small earthquakes as green9s functions to show that for a complex source function represented as a superposition of smaller earthquakes m lsm and m l for the 1980 victoria earthquake can be explained by the magnitude saturation phenomenon

long period surface motion of the multi patch mw9 0 tohoku oki earthquake we show that it is possible to capture the oscillatory ground motion induced by the tohoku oki event for periods ranging from 3 to 100s using precise point positioning ppp we find that the ground motions of the sedimentary basins of japan were large respectively 0 15m s and 0 15m s2 for velocity and acceleration even for periods larger than 3s we compare geodetic observables with a ground motion prediction equation gmpe designed for japan seismicity and find that the spectral acceleration sa is well estimated for periods larger than 3s and distances ranging from 100 to 500km at last through the analysis of the displacement attenuation plots we show that the 2011 tohoku oki event is likely composed of multiple rupture patches as suggested before by time reversal inversions of seismic data

attenuation of short period p waves at mount st helens an increase of seismicity occurred at mount st helens between may and july 1998 with more than 900 events md 2 2 recorded by the pacific north west seismograph network this article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before january 1995 april 1998 during and after august december 1998 the period of higher seismicity and recorded on three different one component seismic stations epicentral distances of the studied events ranged between 0 5 and 15 km we performed the analysis distinguishing the deep depth 5 5 km from shallow depth 5 5 km events we used the frequency decay method to estimate the quality factor q and station site correction s for p waves in the frequency bands 2 7 and 18 30 hz the results show that the attenuation varies from site to site systematically and decreases with depth lower qp values are obtained for focal depths less than 5 5 km moreover the spatial variations of qp show minimum values 30 in the crater area that are interpreted as due to a low density mass distribution under the crater we find that qp was 30 higher before the period of high seismicity than after this change may be attributed to an increase of pressure in the magma chamber producing new cracks and or reopening of pre existing cracks which are the most viable mechanisms for increasing attenuation

modelling of near surface seismic structure beneath hawaii using reverberations summary the reverberation in near surface layers contains valuable information about the near surface velocity and density structure however previously the reverberation was often regarded as noise to be eliminated this paper presents an approach for modelling the shallow velocity structure with the reverberation in the near surface layers the method was applied to three component waveform data recorded by seismic arrays during a passive seismic experiment arrays for lithosphere observations in hawaii aloha the near surface multiple reflections were enhanced by stacking the seismograms of local upper mantle earthquakes with near vertical incidence angles by matching synthetics and observed p and s multiple reflections we have successfully modelled the impedance structure for the top 2 3km of the crust beneath seven stations of aloha arrays in southern hawaii reverberative site effects are similar for stations in an array but vary significantly from one array to another reflecting the lateral heterogeneity of the near surface structure two short profiles of near surface p wave velocity structure were constructed across the ainapo and south flank areas in southern hawaii based on interpretation of the multiple reflections recorded by two of the aloha arrays the profiles reveal the detailed and variable shallow structure near two active volcanoes kilauea and mauna loa and provide basic information about site effects for further investigations of seismic hazard assessment earthquake source properties and deeper structure

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

3d ground motion estimation in rome italy paleoseismic evidence and seismic hazard analysis suggest that the city of rome italy has experienced considerable earthquake ground motion since its establishment more than 2000 years ago seismic hazards in rome are mainly associated with two active seismogenic areas the alban hills and the central apennines regions located about 20 km southeast and 80 100 km east of central rome within the twentieth century m 6 8 and m 5 3 earthquakes in the apennines and the alban hills respectively have generated intensities up to mercalli cancani sieberg scale mcs vii in the city with a lack of strong motion records we have generated a 3d velocity model for rome embedded in a 1d regional model and estimated long period 1 hz ground motions for such scenarios from finite difference simulations of viscoelastic wave propagation we find 1 hz peak ground velocities pgvs and peak ground accelerations pgas of up to 14 cm sec and 44 cm sec2 respectively for a m 5 3 alban hills scenario largest near the northwestern edge of the tiber river our six simulations of a m 7 0 central apennine scenario generate 0 5 hz pgvs in rome of up to 9 cm sec as well as extended duration up to 60 sec the peak motions are similar to but the durations much longer than those from previous studies that omitted important wave guide effects between the source and the city the results from the two scenarios show that the strongest ground motion amplification in rome occurs in the holocene alluvial areas with strong basin edge effects in the tiber river valley our results are in agreement with earlier 2d sh wave results showing amplification of peak velocities by up to a factor of 2 in the alluvial sediments largest near the contact to the surrounding plio pleistocene formations our results suggest that both earthquakes from the alban hills and the central apennines regions contribute to the seismic hazards in rome although earthquakes from the former area may generate the larger peak motions seismic waves from the latter region may generate ground motions with extended durations capable of causing significant damage on the built environment

evidence for anomalous body wave radiation between 0 3 and 0 7 hz from the 1985 september 19 michoacan mexico earthquake summary in order to better understand the causes of unprecedented damage to mexico city during the 1985 september 19 michoacan earthquake mw 8 0 spectral ratios of teleseismic p waves of this earthquake are studied with respect to those from five recent large mexican subduction zone earthquakes 7 0 mw 7 7 the data are from vertical component digital seismographs it is found that the spectral ratios at stations in the ne quadrant are anomalously more energetic than those predicted by the 2 source model in the critical frequency range for mexico city 0 3 0 7 hz the evidence is especially convincing for the spectral ratios with respect to the earthquakes of 1985 september 21 mw 7 6 and 1986 mw 7 0 since the data are available from several stations in the ne quadrant the teleseismic p wave spectral ratio in this quadrant with respect to the 1985 september 21 earthquake in the critical frequency range is close to the acceleration spectral ratio found in and near mexico city also in the ne quadrant velocity traces in the epicentral region of the michoacan earthquake obtained by integrating the accelerograms also show oscillations with a frequency of about 0 4 hz furthermore a regression study of fourier acceleration spectra at a hill zone site in mexico city demonstrates that the michoacan earthquake was anomalously energetic in the city at the critical frequencies for an event of that magnitude and at that distance if the data from 7 0 mw 7 7 events can be extrapolated to estimate the ground motions from mw 8 0 earthquakes then the evidence supports an anomalously large body wave radiation towards mexico city between 0 3 and 0 7 hz during the michoacan earthquake this anomalous radiation and the dramatic local amplification of seismic waves in the lake bed zone of the city 10 50 times at frequencies between 0 3 and 0 7 hz appear to be the principal natural causes of the disaster the anomalous teleseismic p wave spectral ratios with respect to the earthquakes of 1985 september 21 and 1986 found in the ne quadrant are not observed in the data available from a small number of stations in the other quadrants if this observation is true then it suggests a directional property to the anomalous radiation

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

source mechanism of the september 19 1985 michoacan earthquake and its implications moment tensor inversion of low frequency teleseismic data for the september 19 1985 michoacan mexico earthquake gave an average strike 298 slip 268 and seismic moment 10 3 1027 dyne cm low frequency body waves clearly show the complex nature of this event cross correlation of the september 19 p wave with that of the smaller and colocated october 25 1981 playa azul earthquake show that the michoacan earthquake consisted of two subevents separated in time by 25 s and located approximately 30 km northwest and 40 km southeast respectively of the playa azul epicenter body wave inversion of the first subevent gave strike 298 slip 266 and seismic moment 3 0 1027 dyne cm the second subevent had a comparable seismic moment the cumulative moment for large earthquakes occurring along the michoacan northwest guerrero coast indicate that seismic displacement along this segment of the mexican subduction zone is similar to that occurring on other portions indicating that the former michoacan gap was not anomalously aseismic

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

edge diffracted 1 sec surface waves observed in a small size intramountain basin colfiorito central italy during the umbria marche central italy seismic sequence a small aperture 200 m four station array was operating in the colfiorito plain a few kilometers away from the epicenters of the ml 5 6 and 5 8 mainshocks of 26 september 1997 the array was deployed approximately 500 m from the eastern edge of the basin we analyze the three component seismograms of 12 aftershocks in a magnitude range of 2 5 to 4 1 amplitudes of the horizontal components are systematically higher than those of the vertical component with an average horizontal to vertical spectral ratio of about 3 at 1 hz in this frequency band earthquake induced ground shaking is highly coherent across the array a 1 sec running window zero lag cross correlation algorithm is used to compute apparent velocity and backazimuth of coherent wave trains in the frequency band 0 5 to 2 hz apparent velocity and backazimuth show a different behavior in the first part of the seismograms compared to the late coda the largest amplitude waves that is s waves and early coda are characterized by low apparent velocities mostly between 400 and 1200 m sec this suggests that near the rock edge the most significant part of seismic energy propagates horizontally in the basin backazimuth of these low frequency coherent wavetrains never coincides with the array to source direction the predominant backazimuth is peaked around n110 corresponding to the nearest steep outcrop of the basin edge the observed 1 sec coherent wave trains are interpreted as locally generated surface waves that are persistently diffracted from the nearby basin edge as long as a significant level of seismic radiation is incident to the bedrock when the bedrock excitation decreases a much larger variability of both apparent velocity and backazimuth is observed suggesting that in the coda randomly scattered waves within the basin and late arrivals of deeper origin become more important multipathing from the source to the site as well as multipathing within the basin are therefore interpreted as the main causes of the observed long duration coherent low frequency basin shaking

nonlinear soil response a reality geotechnical models consistently indicate that the stress strain relationship of soils is nonlinear and hysteretic especially at shear strains larger than 10 5 to 10 4 nonlinear effects such as an increase in damping and reduction in shear wave velocity as excitation strength increases are commonly recognized in the dynamic loading of soils on the other hand these effects are usually ignored in seismological models of ground motion prediction because of the lack of compelling corroborative evidence from strong motion observations the situation is being changed by recently obtained data explicit evidence of strong motion deamplification accompanied by changes in resonant frequencies are found in the data from the 1985 michoacan mexico and the 1989 loma prieta california earthquakes the events recorded by the vertical and surface accelerograph arrays in taiwan as well as a number of other events throughout the world evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of 100 to 200 gal nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils the findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong motion seismology

estimation of strong ground motions in mexico city expected for large earthquakes in the guerrero seismic gap we performed simulations of ground motions in mexico city expected for large earthquakes in the guerrero seismic gap in mexico the simulation method uses as empirical green s functions the accelerograms recorded in mexico city during four small to moderate earthquakes 8 feb 1988 m s 5 8 25 april 1989 m w 6 9 11 may 1990 m w 5 5 and 31 may 1990 mw 6 0 in the guerrero gap because these events occurred in the guerrero gap and have typical thrust mechanisms the propagation path and site effects can be accurately included in our simulation fault rupture patterns derived from the 1985 michoacan earthquake and source scaling relations appropriate for mexican subduction zone earthquakes are used if the guerrero event is similar to the 1985 michoacan event the resulting response spectrum in mexico city will be approximately twice as large as that of the 1985 michoacan earthquake at periods longer than 2 sec at periods shorter than 2 sec the amplitude will be 2 to 3 times larger than that for the michoacan earthquake if the events in the guerrero seismic gap occur as a sequence of magnitude 7 5 to 7 8 events as they did in the previous sequence around the turn of the century the strong motion in mexico city is estimated to be about half that experienced during the 1985 michoacan earthquake at periods longer than 2 sec however several factors affect this estimate the magnitude of the possible events has a significant range and if a rupture sequence is such that it enhances ground motion amplitude with constructive interference as occurred during the second half of the michoacan sequence some components of the ground motion could be amplified by a factor of 2 to 3 to aid in the interpretation of the simulated motion for purposes of design or hazard assessment design spectra for the cdao site in mexico city are derived from the response spectra of the simulated ground motions

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 study of site effects in ilan taiwan based on attenuation relationships of spectral acceleration abstract the purpose of this paper is to study the site effects for ilan in northeastern taiwan based on attenuation relationships of spectral acceleration sa over 2660 accelerograms recorded from 92 shallow earthquakes with m w magnitude ranging from 4 0 to 7 7 are used to develop these regional attenuation relationships site effects are investigated using the site residuals obtained from the sa attenuation relationships the results show the standard deviations of residuals decrease by 1 7 16 4 in horizontal sa after incorporating the site effect term v s 30 in the attenuation relationship of periods greater than 0 3 s clearly the attenuation relationships are improved by including the site effect term especially for the longer periods in addition from the contour maps of intraevent site residuals relative to an attenuation equation of our model 2 phase 1 for horizontal sa we find high amplification factors of short period 0 1 0 2 s reaching 2 7 in nanao with hard schist rocks high amplification factors of intermediate periods 0 3 1 0 s reach 1 9 and 2 0 respectively in the city of ilan and zhuangwei high amplification factors of long period 1 0 3 0 s reach 2 1 in zhuangwei where there is the greatest sediment thickness the results call for special attention to the seismic design of structures in these locations furthermore the intraevent residuals of the four site classes converging the 0 15 0 2 s period range are in agreement with the findings by aki 1988 accordingly site classes d and e have the largest ground motion amplification factor in the 0 15 0 5 s and 0 5 s period ranges respectively finally the predominant period contour maps deduced from v s 30 v s 75 and sediment depth in the ilan plain fall in the ranges of 0 4 1 0 1 0 1 5 and 1 5 8 0 s respectively this suggests that construction of buildings taller than nine stories in the zhuangwei area should pay special attention to avoid resonance of seismic excitation online material contour maps of relative corrected site amplification factors at 10 periods and tables of site parameters list of earthquakes intraevent site residuals and corrected site amplification factors

source spectra and rms acceleration of mexican subduction zone earthquakes strong motion sm data of six mexican subduction zone earthquakes 6 4 ms 8 1 recorded near the epicentral zone are analyzed to estimate their far field source acceleration spectra at higher frequencies f 0 3 hz apart from the usual corrections such as geometrical spreading 1 r average radiation pattern 0 6 free surface amplification a factor of 2 and equal partitioning of the energy into two orthogonal horizontal components a factor of 1 sqrt 2 the observed spectra are corrected for a frequency dependentq q 100f a site dependent filter e kf and amplification ofs waves near the surface a factor of about 2 atf 2hz we taker as the average distance from the rupture area to the site if we model the high frequency plateau f 1 hz of the source spectra by a point source 2 model and interpret them in terms of brune s model we obtain between 50 and 100 bars for all earthquakes the low frequency broadband teleseismicp wave spectra corrected witht 1 0 s agrees within a factor of two with sm source spectra near 1 hz the 2 model is inadequate to explain the observed source spectra in a broad frequency range these resemble spectra given bygusev 1983 with some differences

q of lg waves in the central mexican volcanic belt from seismograms of nine shallow coastal earthquakes recorded at a pair of broadband stations we estimate q of lg waves in the part of central mexican volcanic belt mvb that includes the valley of mexico the two stations straddle the central mvb and are located on cretaceous limestone a weighted least square fit to the q 1 f data in the frequency range 0 25 to 8 hz yields q f 98 f 0 72 this estimate of q is lower than the corresponding q in the forearc region that is given by q f 273 f 0 66 note that our estimate of q f corresponds to a 200 km wide zone of the mvb the result of this study sheds light on the characteristics of seismic waves as they traverse through the mvb where they undergo dramatic amplification in the valley of mexico it also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the mvb from future coastal earthquakes the lower q of lg waves in the mvb as compared with the forearc region seems correlated with lower resistivity reported in the mvb relative to the forearc region

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

velocity structure under the trans mexican volcanic belt preliminary results using correlation of noise summary prediction of ground motion in central mexico is of great consequence not only for mexico city but also for several other cities in the region however the geometry and velocity distribution of the geological structures in central mexico is unknown in this paper the velocity structure in central mexico is investigated using seismic noise records from three different arrays one permanent and two temporary distances between stations span 3 300 km seismic noise is analysed using time domain cross correlation to estimate the green s function between stations only the fundamental mode of rayleigh waves is recovered in the cross correlation of vertical component records between 5 and 10 s period at smaller periods we obtain no correlation even between closely spaced stations this suggests that the heterogeneity of the trans mexican volcanic belt tmvb hampers the propagation of rayleigh waves smaller than 9 km we observe a large velocity contrast between the guerrero terrain and the tmvb this velocity contrast measured here for the first time at this scale explains the appearance of regional amplification in central mexico and the origin of the diffracted waves behind the long duration of ground motion observed in mexico city in the large scale average rayleigh wave group velocity at 5 s period is about 2 5 km s 1 in the tmvb and larger than 3 6 km s 1 outside it significant lateral heterogeneity is evident in the stations of the dense meso american subduction experiment array we analysed we observe in addition a significant increase in velocity from north to south in the central portion of the tmvb our results are part of ongoing studies with the final objective of building a model of tmvb that may be used to simulate ground motion in central mexico from subduction zone earthquakes

evidence of low q below popocatepetl volcano and its implication to seismic hazard in mexico city seismograms recorded in mexico city reveal that the amplitudes of seismic waves whose wavepaths pass below popocatepetl presently an volcano before reaching the city are diminished by a factor of about one third at frequencies greater than 1 hz as compared to those which do not cross the volcano the high attenuation of seismic waves below the volcano may be attributed to the presence of magma and partial melting of rocks q of shear waves below the volcano is roughly estimated as 60 a consequence of the large attenuation of high frequency seismic waves is a decrease in the seismic hazard to low rise buildings in mexico city from intraplate earthquakes whose wavepaths cross the volcano

advancement in strong motion analysis by the use of evolutionary power spectra abstract illustrated by strong motion records from the michoacan earthquake 19 september 1985 and the loma prieta earthquake 17 october 1989 the application of the evolutionary power spectrum as a tool in strong motion analysis is demonstrated it is shown that a strong motion record is not a realisation of a simple stationary stochastic process it is a non stationary stochastic process which can be explained and modeled by some basic parameters and through the separation of the seismic process into stationary stochastic sub processes which are due to the dominant wave types

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

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

regional wave propagation from mexican subduction zone earthquakes the attenuation functions for interplate and inslab events the seismic waves from subduction zone earthquakes are significantly affected by the presence of 3d variation in crust and upper mantle structure around the source area these heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances this is illustrated by studying shallow interplate earthquakes along the mexican subduction zone and deeper inslab normal faulting earthquakes in the subducted cocos plate beneath mexican mainland the strong motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and mexico city we compare the wavefield from two large earthquakes of different source type during the shallow h 17 km interplate 1995 guerrero earthquake m w 7 3 the lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source the presence of a lateral velocity gradient in the crust caused by the subduction of the cocos plate enhances the lg wave amplitude which is then amplified further in the mexican volcanic belt by amplification in the low velocity volcanic rocks both effects lead to very large ground motions along the path from the coast to the mexican inland in the frequency band from 0 2 to 4 hz however for the deeper h 40 km inslab normal faulting 1999 oaxaca earthquake m w 7 5 the amplitude of the lg phase is too small to produce the abnormal wave propagation and the direct s wave and its multiple sms reflections between the free surface and moho show a simple attenuation with increasing distance we compare these observations with numerical simulations of seismic wave propagation using the fourier spectral method the results provide a key to the understanding of seismic wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3d heterogeneous structure

wave propagation and site response in the santa clara valley forty two portable digital instruments were deployed across the santa clara valley from june until early november 1998 this array recorded 14 small and moderate local events and 7 large teleseismic events we analyze the ground motion from these events to determine station delays and relative site amplification within the valley p waves from an event at the southern edge of the valley are early t 0 35 sec at stations over an axial ridge in the basement interface in the middle of the valley but late t 0 20 sec for stations over the cupertino and evergreen basins to either side the s wave delays are approximately twice as large teleseismic p waves from an m 7 0 event beneath the bonin islands show a similar pattern in travel time delays the p waves are amplified by factors of 1 5 3 for frequencies below 2 hz at stations within either basin compared with stations on the axial ridge the p wave coda appear enhanced at 2 3 sec but coda q estimates at frequencies from 0 2 to 1 1 hz are not markedly different at stations over the basin compared with stations on the ridge with the possible exceptions of consistently high values over the northern end of the evergreen basin we invert the s wave spectra for site specific attenuation and amplification from the 14 local events by assuming a common source spectra for each event 1 r geometrical spreading and constraining the inversion using the 30 m velocity profile at four stations in the array the largest amplifications occurred in the 1 to 6 hz band at stations near the northwest edge of the evergreen basin while the highest amplifications occur at stations with the lowest s wave velocities the scatter obscures the correlation between velocity and amplification the stations in the basins are characterized by higher attenuation than the stations on the basement ridge manuscript received 2 july 2001

elastic wave propagation and site amplification in the salt lake valley utah from simulated normal faulting earthquakes the two dimensional seismic response of the salt lake valley to near and far field earthquakes has been investigated from simulations of vertically incident plane waves and from normal faulting earthquakes generated on the basin bounding wasatch fault the response to normal faulting earthquakes was simulated using a two dimensional finite element method and the plane wave response was calculated from two dimensional finite difference simulations the plane wave simulations were then compared with observed site amplifications in the salt lake valley based on seismic recordings from nuclear explosions in southern nevada that show 10 times greater amplification within the basin than measured values on hard rock sites while previous studies attribute this increased site amplification to the near surface unconsolidated consolidated alluvial fill contact our synthetic seismograms suggest that in the frequency band 0 3 to 1 5 hz at least one half the site amplification can be attributed to the impedance contrast between the basin sediments and higher velocity basement rocks synthetic seismograms from vertically incident plane wave sources and buried double couple sources predict large amplitude rayleigh wave propagation from the edges of the basin and in general uniform site amplification in contrast near field simulations of basin bounding normal faulting earthquakes predict large amplitude rayleigh waves propagating westward from the fault across the basin spectra of synthetic accelerograms computed from the normal faulting earthquakes shows that spectral amplification within the basin is primarily due to source directivity with a maxima near the surface projection of the fault that decays rapidly away from the fault importantly the synthetic modeling of near field earthquake sources show that near field directivity effects are important and should be considered in an earthquake hazard assessment of the salt lake valley and similar geologic settings along the wasatch front

source parameters of microearthquakes at mount st helens usa summary we estimate the source parameters for a selection of microearthquakes that occurred at mount st helens in the period 1995 1998 excluding the activity of 2004 september this time period includes the most intense episode of earthquake activity since the last dome building eruption in 1986 october 200 seismograms were processed to obtain seismic moments source radii stress drops and average fault slip the source parameters were determined from the spectral analysis of p waves after correction for attenuation and site effects in particular p wave quality qp and site s factors have been previously calculated in the frequency ranges 2 7 hz and 18 30 hz because it was impossible to perform corrections for qp and s over the whole spectrum we applied a new approach based on the notion of holed spectrum to estimate spectral parameters the term holed spectrum indicates a spectrum lacking corrected spectral amplitude values at certain frequencies we carried out a statistical study to verify that dealing with the holed spectrum does not lead to significant differences in the estimates of spectral parameters we also investigated the dependence of spectral parameters low frequency level corner frequency and high frequency decay on the bandwidth of spectral hole and defined the threshold values for three different spectral models displacement holed spectra corrected by attenuation and site response are then used to determine spectral parameters in order to calculate seismic source parameters seismic moments range from 1017 to 1019 dyne cm source dimensions from 100 to 350 m and average fault slip from 0 003 to 0 1 cm self similarity seems to break down in that stress drops are very low 0 1 1 bars we postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime

seismic triggering by rectified diffusion in geothermal systems widespread seismicity was triggered by the june 28 1992 landers california earthquake at a rate which was maximum immediately after passage of the exciting seismic waves rectified diffusion of vapor from hydrothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant levels within the duration of the earthquake in a hydrothermal system modeled as a two component h 2o co 2 fluid in porous rock the pressure initially increases linearly with time the rate of pressure buildup depends sensitively on the mean bubble radius and is large for small bubbles the diffusion induced pressure is relaxed by percolation and resorption of vapor into the liquid solution the induced seismicity itself also relieves stress values of parameters used in the present calculations give results consistent with observations of triggered seismicity at long valley caldera after the landers earthquake for one representative condition at 250 c and 5 6 km depth oscillating strain acting on 10 m diameter bubbles increases pore pressure at the rate of 151 pa s resulting in a pressure increase of 12 kpa in the 80 s duration of the landers earthquake the elevated pressure induced by a single 26 m diameter cloud of bubbles in saturated rock relaxes by percolation through soil of 0 2 mdarcy permeability in 53 6 hours observations of earthquake swarms at other locations suggest that self induced buildup of pore pressure by rectified diffusion can provide a positive feedback mechanism for amplifying seismicity

investigation of surface geology and intensity variability in the palermo italy urban area after the 6 september 2002 earthquake this work was funded by the miur legge 488 92 delibera cipe 08 08 96 r catalano 60 2000 2001

a nodal discontinuous galerkin method for site effects assessment in viscoelastic media verification and validation in the nice basin we present a discontinuous galerkin method for site effects assessment the p sv seismic wave propagation is studied in 2 d space heterogeneous media the first order velocity stress system is obtained by assuming that the medium is linear isotropic and viscoelastic thus considering intrinsic attenuation the associated stress strain relation in the time domain being a convolution which is numerically intractable we consider the rheology of a generalized maxwell body replacing the convolution by a set of differential equations this results in a velocity stress system which contains additional equations for the anelastic functions expressing the strain history of the material our numerical method suitable for complex triangular unstructured meshes is based on centred numerical fluxes and a leap frog time discretization the method is validated through numerical simulations including comparisons with a finite difference scheme we study the influence of the geological structures of the nice basin on the surface ground motion through the comparison of 1 d and 2 d soil response in homogeneous and heterogeneous soil at last we compare numerical results with real recording data the computed multiple sediment basin response allows to reproduce the shape of the recorded amplification in the basin this highlights the importance of knowing the lithological structures of a basin layers properties and interface geometry

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

can horizontal p waves be trapped and resonate in a shallow sedimentary basin summary it has been proposed recently that horizontal p waves can be trapped and resonate laterally in a shallow sedimentary basin in this work it is shown both theoretically and numerically that in a sedimentary basin of finite lateral extent trapped horizontal p waves if they exist are not a stable way of propagation and therefore cannot resonate

factors contributing to the catastrophe in mexico city during the earthquake of september 19 1985 the extensive damage to high rise buildings in mexico city during the september 19 1985 earthquake is primarily due to the intensity of the ground shaking exceeding what was previously considered credible for the city by mexican engineers there were two major factors contributing to the catastrophe resonance in the sediments of an ancient lake that once existed in the valley of mexico and the long duration of shaking compared with other coastal earthquakes in the last 50 years both of these factors would be operative again if the guerrero seismic gap ruptured in a single earthquake

an overview on the seismic zonation and microzonation studies in india abstract the present study presents a review on the progressive development of the seismic zonation map of india both from official agencies and also from independent individual studies the zonation map have been modified and updated regularly with the occurrence of major destructive earthquakes over the years in the indian subcontinent with the addition of new data this study discusses the criteria chosen for the progressive zonation and the major earthquakes that were responsible for retrospection of the earlier published maps the seismic zonation maps of india have also been prepared by various independent workers by adopting different approaches to achieve the purpose of the zonation despite the endeavors from various sources to provide a solution for the problem of earthquake hazards in india there were many limitations on the zonation map as it gives the picture at a regional scale mostly on the bedrock level without addressing the local site conditions but nevertheless the seismic zonation map gives basic guidelines for any region to know the hazard scenario and if any city or urban population is under threat from seismic point of view further site specific seismic microzonation may be carried out in the international scenario the global seismic hazard assessment program gshap in 1999 prepared a hazard map for world in terms of peak ground acceleration pga with a 10 probability of exceedance in 50 years but it turned out to be an underestimation of the hazard parameter when compared with the observed pga to tackle the problem of seismic hazards there was a need to have a detail study on the local site conditions in terms of its geological geophysical and geotechnical properties with the advent of better instrumentation and knowledge on the mechanics of earthquakes it was possible to identify zones of hazards at a local level and this gives rise to the study of seismic microzonation seismic microzonation work has been carried out in india in some of the strategic important mega cities and industrial build up that has the potential of being damaged from future earthquakes as has been shown in the past though the microzonation map is not the final output map as it can still be updated at later stage with more input data it does provide a more realistic picture on the site specific seismic hazard