estimation of coda wave attenuation for nw himalayan region using local earthquakes abstract the attenuation of seismic wave energy in nw himalayas has been estimated using local earthquakes most of the analyzed events are from the vicinity of the main boundary thrust mbt and the main central thrust mct which are well defined tectonic discontinuities in the himalayas the time domain coda decay method of a single back scattering model is employed to calculate frequency dependent values of coda q qc a total of 36 local earthquakes of magnitude range 2 1 4 8 have been used for q c estimation at central frequencies 1 5 3 0 6 0 9 0 12 0 and 18 0 hz through eight lapse time windows from 25 to 60 s starting at double the time of the primary s wave from the origin time the estimated average frequency dependence quality factor gives the relation q c 158 f 1 05 while the average q c values vary from 210 at 1 5 hz to 2861 at 18 hz central frequencies the observed coda quality factor is strongly dependent on frequency which indicates that the region is seismic and tectonically active with high heterogeneities the variation of the quality factor q c has been estimated at different lapse times to observe its effect with depth the estimated average frequency dependent relations of q c vary from 85 f 1 16 to 216 f 0 91 at 25 to 60 s lapse window length respectively for 25 s lapse time window the average q c value of the region varies from 131 36 at 1 5 hz to 2298 397 at 18 hz while for 60 s lapse time window its variation is from 285 95 at 1 5 hz to 2868 336 at 18 hz of central frequency the variation of q c with frequency and lapse time shows that the upper crustal layers are seismically more active compared to the lower lithosphere the decreasing value of the frequency parameter with increasing lapse time shows that the lithosphere acquires homogeneity with depth

evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

time variation of b and qc at mt etna 1981 1987 abstract time variations of seismic attenuation qc and of the coefficient b in the frequency magnitude formula are presented for mt etna during 1981 1987 significant variations of both parameters were found suggesting long period variations of the physical state of the volcano

evidence for stability in coda q associated with the egion central greece earthquake of 15 june 1995 one hundred seventy two seismograms recorded at one seismological station located close to the source region of the egion ms 6 2 15 june 1995 earthquake were used to measure values of q from the decay of the earthquake coda the collected data were compared between events that occurred before and after the main event the analysis showed no significant temporal variation in the value of coda q for the region

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

temporal change in coda q before the tangshan earthquake of 1976 and the haicheng earthquake of 1975 a new earthquake precursor called coda q is demonstrated for the tangshan earthquake of 1976 and for the haicheng earthquake of 1975 the precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake during the 3 year period preceding the tangshan earthquake the coda q was about 3 times lower than it was before or after this period a comparable change also occured at the time of the haicheng earthquake the observed change cannot be due to the change in the source parameters such as the focal depth or the source spectrum the coda q precursor appears to detect the same physical process detected by other precursors such as b value quiescence radon water quality vp vs and animal behavior but has advantages over them in its clearer physical mechanism easier sampling and better defined anomalous region

piecewise linear model for large strain radial consolidation a piecewise linear model for radial consolidation called rcs1 is presented rcs1 accounts for vertical strain soil self weight hydraulic conductivity anisotropy radial and vertical flows soil

coda q before and after the 1986 andreanof islands earthquake coda q values before and after the may 7 1986 andreanof islands earthquake in the central aleutian subduction zone were calculated from microearthquake seismograms on the basis of aki s single backscattering model digital waveforms from 329 microeathquakes in 5 4 years before the mainshock and 40 aftershocks recorded by 11 stations of the central aleutians seismic network were utilized before the mainshock coda q in the rupture zone west of the mainshock epicenter was 15 higher than theq outside the rupture zone to the west a lowq in the region east of the mainshock epicenter is inferred but lacks solid evidence the highq region accords with the part of the rupture zone where most seismic moment was released during approximately two months after the mainshock coda q in the rupture zone decreased about 10 precursoryq changes were not found in the 5 4 years before the mainshock it is deduced that a coda q precursor if it existed was of small magnitude 10 or its duration was either shorter than one year or longer than four years

temporal variation of coda q during hualien earthquake of 1986 in eastern taiwan the may 20 1986 hualien earthquake sequence occurred on the northeastern coast of taiwan them 6 1 gs mb mainshock was followed by a large number of closely clustered aftershocks with the largest being anm 5 5 event one seismic station twd of the taiwan telemetered seismographic network is located in the surface projection of the source region and provides excellent recordings of the entire earthquake sequence these recordings plus events occurring in the same source area preceding the mainshock offer a unique opportunity to study the spatial and temporal variations of codaq in a region of active subduction a simple technique is devised that uses the envelope of the coda waveform to enable a quick determination of the codaq from drum records for recordings with a peak power at about 8 hz the following findings have been obtained 1 the ambient codaq near an active subduction region was as low as 145 2 there was no significant decrease in codaq within the period beginning one year and four months prior to the mainshock 3 there was a significant drop of codaq immediately after the mainshock this drop lasted approximately two days before returning to the ambient level 4 codaq values varied with focal depth

temporal change of qp 1 in focal area of 1984 western nagano japan earthquake as derived from pulse width analysis on the basis of the pulse width broadening of initial p waves temporal change of qp 1 was investigated for the focal area of the 1984 western nagano central japan earthquake of m 6 8 qp 1 in the focal area ranging between 3 9 10 3 and 10 0 10 3 was significantly higher than that of the surrounding area 2 8 10 3 throughout the 1 3 year postseismic stage that we examined the highest value however appeared not just after the main shock but for the period from 1 day to 2 months following the main shock after the maximum attenuation was attained qp 1 in the focal area monotonically decreased approaching the value of the surrounding area this characteristic feature of qp 1 change is successfully accounted for by the model in which newly opened cracks are gradually saturated by interstitial fluid qp 1 of the focal area was much higher than the surrounding area even for the period prior to the main shock the high attenuation of the preseismic stage is probably a consequence of the earthquake swarm which preceded the m 6 8 earthquake however there remains a possibility that attenuation intensity in the focal area increased as a precursor to the western nagano earthquake

estimates of q in central asia as a function of frequency and depth using the coda of locally recorded earthquakes digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the hindu kush region of afghanistan were used to determine the attenuation factors of the s wave coda qc and primary s waves q an anomalously rapid decay of the coda shortly after the s wave arrival observed also in a study of coda in central asia by rautian and khalturin 1978 seems to be due primarily to depth dependent variations in qc in particular we deduce the average qc in the crust and uppermost mantle 100 km depth is approximately four times lower than the deeper mantle 400 km depth over a wide frequency range 0 4 to 24 hz further while qc generally increases with frequency at any depth the degree of frequency dependence of qc depends on depth except at the highest frequency studied here 48 hz the magnitude of qc at a particular frequency increases with depth while its frequency dependence decreases for similar depths determinations of q and qc agree suggesting a common wave composition and attenuation mechanism for s waves and codas comparison of these determinations of qc in afghanistan with those in other parts of the world shows that the degree of frequency dependence of qc correlates with the expected regional heterogeneity such a correlation supports the prejudice that qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments

coda waves a review the analysis and interpretation of coda waves have received increasing attention since the early seventies in the past few years interest in this subject has spread worldwide and the study of high frequency seismic coda waves has become a very important seismological topic as a conclusion of the studies accomplished in this time coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves p ands waves play a particularly important role in this interaction the process introduces an attenuation which added to the intrinsic absorption gives the observed apparent attenuation therefore coda waves constitute a thumbprint left by the heterogeneities on the seismograms coda waves offer decisive information about the mechanism of how scattering and attenuation take place this review describes coda waves in detail and summarizes the work done in this subject to 1986 the relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed particular attention has been given to the application of coda waves to estimate source and medium parameters the state of the art of the temporal variations of coda wave shape and the possible use of these variations as an earthquake precursor also are presented care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering

site amplification from s wave coda in the long valley caldera region california following the work of phillips 1985 we have computed site amplification factors for coda waves at many sites in the long valley region in the eastern sierra nevada we computed ratios of coda amplitudes measured at 15 stations in and around long valley caldera relative to a granitic site mmpm for six frequency bands centered at 1 5 3 0 6 0 9 0 12 0 and 15 0 hz all station sites located within the caldera experienced large ground motion amplification at 1 5 and 3 0 hz ranging between five and 17 times that of the reference site however at higher frequencies these same sites exhibited significantly less amplification than the reference granite site this is attributed to the competing effects of an impedance contrast between the basement rock and caldera fill and very high absorption in the caldera fill at high frequencies station mmlm located on top of a volcanic plug displayed the largest amplitudes of all the sites studied for frequencies between 9 0 and 15 0 hz a dike structure attached to the plug couples the basement rock to the surface at high frequencies the resulting large amplitudes at mmlm are not due to amplification resulting from a strong impedance contrast rather the absorption under this site is very low perhaps lower than at the reference site mmpm outside the caldera another hard rock site located at devil s postpile mdpm generally behaved like the reference site for all frequencies the lowest amplifications observed came from a site outside the caldera mdcm located on thin pyroclastic ash deposits overlying granitic basement this can be attributed to a dominance of absorption over the amplification caused by lower impedance of this layer variations among sites on similar surface geology may be due to small local variations in impedance and absorption under and adjacent to the site the range in the spectral decay parameter between caldera and rock sites are comparable to results of anderson and hough 1984 for sites on alluvium and rock in the san fernando region these surprisingly different amplifications support the need for additional site specific studies amplifications determined in this study for the frequency range 1 5 and 3 0 hz correlate remarkably well with eaton s 1990 residuals for duration magnitude fmag and amplitude magnitude xmag for the usgs northern california seismic array further supporting the use of coda waves in determining site specific amplification

differential analysis of coda q using similar microearthquakes in seismic gaps part 1 techniques and application to seismograms recorded in the anza seismic gap abstract a large but conflicting literature exists that suggests that the scattering properties of the earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes to search for temporal variations in coda q in the vicinity of an expected moderate earthquake nucleation region along the san jacinto fault zone pairs and clusters of earthquakes recorded by the anza seismic network between 1982 and 1992 are examined to minimize differences arising from path and source variability only the most similar examples of microearthquake wave fields are intercompared to assess coda q differences between similar microearthquake signals we search for systematic temporal changes in moving window spectral amplitude ratios for a single scattering coda model with an envelope decay function of the form t m e ft qc the relationship between the natural log amplitude spectral ratio and q 1 q 1 c 2 q 1 c 1 is linear i e ln r f t ft q 1 f w f where w f is a time independent relative source term to obtain robust uncertainty estimates of q 1 f and w f we utilize multi taper spectral analysis coupled with a nonparametric monte carlo confidence interval estimation procedure for the most similar events in the anza region coda cross correlation values of approximately 0 7 for 16 sec of coda we find that coda q as a function of time is stable at a typical 1 resolution of only between 25 and 50 of the reference coda q value for the area even in the best constrained frequency bands comparison of similar microearthquake coda signals with both short and long temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda q to obtain estimates of relative coda q that are more precise than this level the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the anza region such earthquakes have been observed to occur in other regions of the san andreas fault system and are analyzed in antolik et al 1996

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

a temporal change in coda wave attenuation observed during an eruption of mount st helens during the past few years there have been numerous reports of changes in coda wave attenuation occurring before major earthquakes these observations are important because they may provide insight into stress related structural changes taking place in the focal region prior to the occurrence of large earthquakes the results of these studies led us to suspect that temporal changes in coda wave attenuation might also accompany volcanic eruptions by measuring power decay envelopes for earthquakes at mount st helens recorded before during and after an eruption that took place during september 3 6 1981 we found that coda q sup 1 for frequencies between 6 and 30 hz was 20 30 higher before the eruption than after the change is attributed to an increase in the density of open microcracks in the rock associated with inflation of the volcano prior to the eruption q sup 1 was found to be only weakly dependent on frequency and displayed a slight peak near 10 hz the weak frequency dependence is attributed to the dominance of intrinsic attenuation over scattering attenuation since it is generally accepted that intrinsic attenuation is constant with frequency whereas scattering attenuation decreases strongly at higher frequencies the weak frequency dependence of q sup 1 more at mount st helens contrasts with results reported for studies in nonvolcanic regions the peak in q sup 1 near 10 hz at mount st helens is attributed to the scale length of heterogeneity responsible for generating backscattered waves results for nonvolcanic regions have shown this peak to occur near 0 5 hz thus a smaller scale length of heterogeneity is required to explain the 10 hz peak at mount st helens copyright american geophysical union 1988 less

hypocenter migration as an explanation for temporal changes in coda q in the 1993 earthquake swarm near the norris lake community georgia qc coda q and b value exhibit systematic temporal variations on a scale of months and hours the long term monthly variations correlate with the swarm s growth and decay examination of the hypocenters demonstrates that q is a strong function of hypocenter and the temporal variations of q reflect a systematic migration of hypocenters the b value appears to vary in this swarm because the method of magnitude determination is dependent on qc throughout the duration of the swarm the active areas migrated from a high qc area eastward to a low qc area variations of qc over periods of hours were explained by a shifting ofhypocenters between high and low qc areas significant changes in qc occur for hypocenters that are separated by distances 10 km the nonuniform fractal distribution of relative scattering coefficients plays a major role in determining the spatial dependence of q by using forward coda modeling on the distribution of relative scattering coefficients derived for this earthquake swarm we can reproduce seismic coda with the same spatial dependence on q as found in the earthquakes throughout the swarm the relative locations of earthquakes associated with different q values and our ability to model different q values from a known distribution of relative scattering coefficients confirm that the migration of hypocenters and not a temporal change in attenuation properties determines the temporal variation of qc observed during the norris lake community earthquake swarm

temporal and spatial variation on coda q 1 associated with the north palm springs earthquake of july 8 1986 a temporal and spatial change of codaq 1 associated with the occurrence of the north palm springs earthquake of july 8 1986 was studied by using 242 small local earthquakes in the vicinity of the mainshock we found that the codaq 1 of earthquakes which occurred before the mainshock was significantly higher than that of the aftershocks in the mainshock area while the codaq 1 for the surrounding area remained almost constant throughout 1986 codaq 1 was determined separately for the lapse time windows of 10 to 20 sec and 15 to 40 sec for the period from 1981 to 1987 the result for the time window 10 to 20 sec showed a peak in codaq 1 before the time of mainshock at all frequencies the peak appeared earlier at lower frequencies there was no significant change in codaq 1 for the time window 15 to 40 sec probably because the change was restricted to a small area

changes in source and site effects compared to coda q 1 temporal variations using microearthquakes doublets in california we analyse spectral ratio of the coda of doublets of microearthquakes our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake a moving window cross spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram for several frequency bands sr t f plot from a certain pattern in thesr t f plot we should infer a temporal change in coda attenuation several doublets recorded in central california by the usgs network are analysed

s wave coda q from 3 to 20 hz and p wave q for foreshocks and aftershocks of the petatlan earthquake in order to understand better the variations of mechanical properties close to the source and the failure process in the upper portion of the crust in a geologically very young subduction zone the seismic s wave coda qc from 3 to 20 hz and p wave half pulse width qp from 6 to 12 hz were determined for selected foreshocks and aftershocks of the 1979 petatlan mexico ms 7 6 earthquake to limit the region being sampled the quality factor of s coda qc was obtained from the first 10 s following the s onset qc exhibited a strong spatial temporal and frequency dependence and in the small foreshock portion of the aftershock area qc and to a lesser extent qp were significantly lower before the mainshock than afterward the frequency dependence n in qc qo n was practically constant for aftershocks in four different regions n 1 32 0 03 and for foreshocks n 1 43 0 07 only in one peripheral region station pair did we find a significantly different n n 0 97 0 09 our observations indicate that qc and perhaps the frequency dependence n are sensitive to the physical property changes in the source region and might be useful as predictors of large earthquakes in a shallow subduction zone environment

a preliminary study of coda q in california and nevada the decay rate of local earthquake coda is characteristic of the area in which the epicenter and stations are located aki 1969 aki and chouet 1975 rautian and khalturin 1978 tsujiura 1978 aki 1980 interpreted the coda decay rate in terms of the attenuation property of s waves in the lithosphere under the area the q of s waves inferred from coda waves is called coda q recently several papers reported temporal changes in coda q prior to the occurrence of large earthquakes they are 1 the tangshan earthquake m 7 8 of 1976 and the haicheng earthquake m 7 4 of 1975 jin and aki 1986 2 three earthquakes m 8 in the kuril kamchatka region gusev and lemzikov 1985 3 the kalapana hawaii earthquake m 7 2 of 1975 wilson et al 1983 and 4 the eastern yamanashi japan earthquake m 6 0 of 8 august 1983 sato 1986 in all these cases the coda q was reported to be anomalously low before the occurrence of the earthquake however in a detailed study by chouet 1979 a strong fluctuation of coda q with time was observed in bear valley california which was not followed by a significant earthquake a recent coda q study in the adak seismic zone by scherbaum and kisslinger 1985 also did not find significant coda q changes that could be related to a mb 5 8 earthquake which occurred in the area of investigation in order to evaluate the feasibility of using coda q for earthquake prediction research in california and nevada we began a systematic study of coda q using digital waveform data from the regional microearthquake networks operated by the u s geological survey and the university of nevada

temporal variations of coda q an attenuation coefficient view abstract spatial and temporal variations of coda wave attenuation were identified in many studies and particularly in relation to major earthquakes and volcanic eruptions both the coda quality factor qc and its frequency dependence often change following such events which is often attributed to variations in the properties of large volumes of the subsurface however qc is also strongly sensitive to the assumed theoretical models which are usually insufficiently accurate for constraining the actual relationships between the geometrical spreading anelastic dissipation and scattering this inaccuracy often leads to significant exaggeration of attenuation effects and complicates the interpretation of temporal variations to resolve this problem this study uses a phenomenological approach based on the temporal attenuation coefficient instead of qc the attenuation coefficient often linearly depends on frequency f with intercept 0 related to the geometrical spreading and slope giving the effective quality factor qe as d df q e 1 two published examples of temporal variations of local earthquake coda are revisited a non volcanic stone canyon in central california and volcanic area mount st helens washington in both cases linear f patterns are found with the effects of on coda decay rates being significantly stronger than those of q e 1 at stone canyon ranged from 0 035 to 0 06 s 1 and qe varied from 3000 to 10 000 with increasing and qe decreasing during the winter season at mount st helens remained constant at 0 18 s 1 and qe changed from 400 before the eruption to 750 after it the observed temporal variations are explained by the near surface changes caused by seasonal variations in the non volcanic case and gas magma and geothermal system related in the volcanic case scattering attenuation does not appear to be a significant factor in these areas or otherwise it may be indistinguishable due to its fundamental trade off with the background structure and anelastic attenuation in the data

regional coda q variations in the western alps northern italy abstract to study the microseismicity of the central segment of the western alps a local temporal seismic network of six stations was installed in addition to the regional centralized network operating in the area in 3 months about 220 shocks with magnitude ranging between 1 0 and 3 8 have been analyzed and located the epicenter distribution shows a possible subdivision into seismogenetic bands this subdivision is also recognizable from the spectral characteristics of the analyzed seismic signals a study of the wave attenuation in a restricted area well covered by the local network was carried out through a coda analysis the results show strong spatial qc variations in agreement with the strong lateral heterogeneities that characterize this area in particular the brianconnais domain the crystalline massifs and the inner part of the alpine arc appear to behave as decreasing attenuation bands

origins of amplitude variations in seismic doublets source or attenuation process summary accurate estimates of spectral ratios of two doublets i e similar seismic events occurring along the san andreas fault in the hollister region have been computed using a cross spectral method both events of each doublet occurred within a 24 hr interval although both waveforms of a doublet are remarkably similar computed spectral ratios exhibit strong but steady variations with frequency in the first arrivals the attenuation process can explain neither the sign nor the order of magnitude of these variations on the contrary they are closely related to the source mechanism we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events one doublet exhibits an azimuthal distribution of the spectral ratio variations that is interpreted as the result of a change in the position of the hypocentres relative to the rupture area for this doublet location of hypocentres varies only from 10 to 30 per cent of the source length this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets i e in the amplitude of each event for the other doublet the variations are related to a change in source length we also have computed spectral ratios all along the seismograms using a moving window technique when an azimuthal distribution of the spectral ratio is observed in the first arrivals we notice a decrease of the spectral ratio all along the seismograms the doublet with strong but non azimuthal variations of the spectral ratio in the first arrivals does not indicate such a decrease of the spectral ratio along the seismogram this implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters these variations have the same order of magnitude as those provoked by a temporal change in coda q of 10 per cent they can occult time changes of spectral ratio for instance in case the attenuation in the crust would vary before a strong earthquake

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

q determined from local earthquakes in the south carolina coastal plain abstract coda q or qc is determined from the rate of coda decay in the coastal plain near charleston south carolina the mean of qc calculated from individual estmates is near 190 at 1 hz and increases to over 1650 at 10 hz a possible temporal change in qc is identified high values being associated with increased seismic activity during late 1977 temporal changes have been observed elsewhere and laboratory measurements of q under different stress states correlate high q with high stress a spatial variation in qc is identified increasing from west qc at 10 hz equal to 850 to east qc at 10 hz greater than 2100 across the coastal plain the demarcation of high to highest zones of qc is coincident with the western boundary of the innermost isoseismal of the charleston 1886 earthquake it is hypothesized that the 1886 earthquake was mislocated originally because of this spatial variation in energy attenuation that was unrecognized at that time

cs3 large strain consolidation model for layered soils abstracta numerical model called cs3 is presented for one dimensional large strain consolidation of layered soils the algorithm accounts for vertical strain soil self weight conventional constitutive relationships changing material properties during consolidation unload reload time dependent loading and boundary conditions an externally applied hydraulic gradient and multiple soil layers with different material properties cs3 can accommodate equilibrium and nonequilibrium profiles for the initial void ratio as well as variable profiles for preconsolidation stress and applied stress increment verification checks show excellent agreement with available analytical and numerical solutions several numeric examples are used to illustrate the capabilities of cs3 and highlight errors that may occur when multilayer systems are modeled as a single layer with average properties finally settlement estimates obtained using cs3 are in good agreement with field measurements for the gloucester test fill

temporal change in attenuation intensity before and after the eastern yamanashi earthquake of 1983 in central japan a crutal earthquake of magnitude 6 0 took place on august 8 1983 in the middle of the kanto tokai microearthquake observation network of the national research center for disaster prevention in central japan by analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock attenuation change was estimated by using two different kinds of analyses in the first analysis station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before in the second analysis s coda duration from the origin time was measured and was compared with the average magnitude the increase of s coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter the simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 hz qs 1 1 4 10 2f 1 since other precursors appeared in the preparation stage it is possible to interpret the change in qs 1 as a precursor the attenuation intensity would have been anomalously large before the main shock

time space variation of seismic s wave coda attenuation qc 1 and magnitude distribution b values for the petatlan earthquake average s wave coda q qc at 6 hz and to about 10 sec after the s wave arrival shows 1 temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 petatlan mexico earthquake 2 spatial variation through the aftershock area positively correlated with b high qc b values at the two activity centers intermediate values in between and landward and low values toward the middle america trench which remain low for two weeks after the main shock and 3 five foreshock average qc 135 1 6 and eleven aftershock average qc 175 1 3 for the eastern asperity the b values and possibly qc values seem to correlate with the level of aftershock activity

seismicity and coda q values in the eastern pyrenees first results from the la cerdanya seismic network seismicity in the la cerdanya region of the eastern pyrenees has been accurately mapped for the first time using data from a local seismic network the majority of earthquakes lies on or near the la cerdanya fault or secondary faults to the south coda magnitudes determined for these earthquakes using magnitude relations from other regions range between 0 5 and 2 2 these are however presumed to be underdetermined values sinceq values appear to be very low in the la cerdanya region codaq values at a frequency of 1 5 hz range between 17 and 120 the lowest values being obtained for the most seismically active regions codaq values also increase with increasing distance a result which indicates decreasing seismic attenuation with increasing depth in the crust

coda q before and after the 1983 misasa earthquake of m 6 2 tottori prefecture japan to detect any temporal change in coda q associated with an earthquake of m 6 2 digitized data from analog magnetic records of microearthquakes for a small confined region just around its hypocenter have been processed with special attention to measuring errors the m 6 2 earthquake took place at the misasa town tottori prefecture in southwest japan with preceding anomalous seismicity changes theq values were elevated 20 around three years before the m 6 2 event for frequencies of 5 to 20 hz and tended to decrease around two years before data with high quality show undulated temporal variation with a period of 5 6 years before the earthquake which is correlated with the regional seismic activity there is a possibility that the observedq change stands for precursory effect of the medium scale earthquake the aftershock sequence also shows an undulated temporal variation ofq with a period of 150 days for around 10hz correlating also with the seismic activity the fracturing processes by numerous microearthquakes may be responsible for the attenuation property of randomly scattered seismic waves

spatial variation of crustal coda q in california coda wave data from california microearthquakes were studied in order to delineate regional fluctuations of apparent crustal attenuation in the band 1 5 to 24 hz apparent attenuation was estimated using a single back scattering model of coda waves the coda wave data were restricted to 30 s following the origin time this insures that crustal effects dominate the results as the backscattered shear waves thought to form the coda would not have had time to penetrate much deeper results indicate a strong variation in apparent crustal attenuation at high frequencies between the franciscan and salinian regions of central california and the long valley area of the sierra nevada although the coda q measurements coincide at 1 5 hz q c 100 at 24 hz there is a factor of four difference between the measurements made in franciscan q c 525 and long valley q c 2100 with the salinian midway between q c 900 these are extremely large variations compared to measures of seismic velocities of comparable resolution demonstrating the exceptional sensitivity of the high frequency coda q measurement to regional geology in addition the frequency trend of the results is opposite to that seen in a compilation of coda q measurements made worldwide by other authors which tend to converge at high and diverge at low frequencies however the worldwide results generally were obtained without limiting the coda lengths and probably reflect upper mantle rather than crustal properties our results match those expected due to scattering in random media represented by von karman autocorrelation functions of orders 1 2 to 1 3 the von karman medium of order 1 3 corresponding to the franciscan coda q measurement contains greater amounts of high wavenumber fluctuations this indicates relatively large medium fluctuations with wavelengths on the order of 100 m in the highly deformed crust associated with the franciscan however the influence of scattering on the coda q measurement is currently a matter of controversy

vertical and radial consolidation analysis of multilayered soil using the spectral method a new easy to implement solution to the consolidation of multilayered soil based on the spectral method is presented combined vertical and radial drainage under instantaneous or single ramp loading is considered ignoring well resistance flow in the vertical direction is based on the average hydraulic gradient at a particular depth which allows smear effects to be included the excess pore water pressure profile across all soil layers is described by a single expression calculated with common matrix operations average excess pore pressures within or across any number of layers are easily calculated from the single expression the new model is verified against other solutions from the current literature indicating that the more general spectral method model can replace the separate solutions developed for specific problems

an integrated hydrological model for rain induced landslide prediction this paper describes an extension to the combined hydrology and stability model chasm to fully include the effects of vegetation and slope plan topography on slope stability the resultant physically based numerical model is designed to be applied to site specific slopes in which a detailed assessment of unsaturated and saturated hydrology is required in relation to vegetation topography and slope stability applications are made to the hawke s bay region in new zealand where shallow seated instability is strongly associated with spatial and temporal trends in vegetation cover types and the mid levels region in hong kong an area subject to a variety of landslide mechanisms some of which may be subject to strong topographic control an improved understanding of process mechanism afforded by the model is critical for reliable and appropriate design of slope stabilization and remedial measures copyright 2002 john wiley sons ltd

temporal stability of coda q 1 in southern california some studies of coda q 1 have found temporal changes that may be associated with earthquake activity but these analyses are subject to biases due to differences in source locations and other nonstationary behavior in earthquake catalogs these biases can be greatly reduced by using clusters of repeating earthquakes studies using this approach have generally found no resolvable changes in coda q 1 we examine coda q 1 variations across southern california using 22 similar event clusters identified from a recent large scale waveform cross correlation project to improve earthquake locations these clusters are found across the region and span the time period between 1981 and 2005 we apply the method of beroza et al 1995 to compute differential coda q 1 using waveforms from similar earthquake pairs and analyze the results to constrain any possible temporal variations results from individual event pairs show a great deal of scatter in differential coda q 1 but exhibit no clear temporal variations or changes associated with large earthquakes application of a median filter to smooth the results shows that any persistent large scale changes in coda q 1 during this time period are less than about 30

anisotropic scattering and travel time delay analysis in kilauea volcano hawaii earthquake coda waves we study the s wave coda of microearthquake multiplets earthquakes showing similar seismograms recorded on the kilauea volcano and we analyze the direction of emission for wavelets composing the coda our data set consists of 71 small magnitude well relocated clustered events that occurred in an area of about 1 km2 in the southern flank of kilauea recorded at hypocentral distances of 10 to 20 km for each station we apply a cross spectral moving window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets these delays are computed for consecutive windows starting at the p wave arrival time and ending at 4 times the s wave travel time they are next used to estimate the slowness vector for each window along the seismograms our analysis shows 1 that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the s wave travel time and 2 that for some stations the azimuthal plane rotates during the seismogram with a clear travel time delay signature our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects this conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust our results do not support a coda diffracted isotropically in the crust in the magnitude range and at the hypocentral distances we investigated

seismic albedo from the total seismic energy dependence on hypocentral distance in southern california abstract scattering and absorption of seismic waves have been studied in southern california to determine their respective contributions to the measured apparent attenuation we measured the total seismic energy from the onset of the s waves for local earthquakes recorded at stations on granitic rock reduced to a common source by eliminating the source and site effect by the coda method then applied wu s method based on the radiative transfer theory which has the facility to separate the intrinsic absorption from the scattering effect the observed total energy as a function of hypocentral distance exhibits a striking behavior increasing to a maximum at about 50 km then decreasing exponentially as a function of hypocentral distance decay of the total energy as a function of distance is remarkably similar for all frequency bands centered at 1 5 3 0 6 0 and 12 0 hz in good agreement with earlier results on q of coda and s waves in southern california comparison with wu s theoretical plots which shows curves that differ in shape because of differences in seismic albedo if b 0 s s a where s and a are the scattering and absorption coefficients respectively leads to the conclusion that the s wave energy is primarily redistributed in space by scattering rather than intrinsically absorbed by the earth our results from southern california agree with those of toksoz et al who found seismic albedo b0 to be 0 8 0 9 in eastern north america coda q in southern california however closely agrees with scattering q using wu s model in disagreement with the conclusions of toksoz et al for eastern north america where coda q is close to the absorptive q obtained from the total energy study the agreement between wu s theoretical curves and our observations is not perfect departure of the observed curves from the theory may be due in part to inhomogeneous distribution of scatterers and absorbers as well as non isotropic scattering we need more realistic models which can also account for inhomogeneous bounded media

properties of scattered elastic waves in the lithosphere of kamchatka parameters and temporal variations abstract records of near earthquakes from the shipunsky station were used to estimate s wave absorption for seven frequency bands ranging from 0 4 to 24 hz three techniques were employed and three rows of consistent estimates were obtained two of the three techniques give estimates that are independent of q anisotropic scattering was studied by observing the broadening of the s wavelet the estimates obtained show that anisotropic scatter may be significant the time variation of the coda envelope shape was studied using the records of 1 s regional seismographs three epicentral regions were chosen in the kurile kamchatka zone where m 8 earthquakes have occurred time series of the parameter which is the anomaly of the logarithmic steepness of coda envelope were constructed for vertical and horizontal channels of several stations pronounced forerunner type variations of were revealed we show that the probable cause of these variations is a qs decrease in the vicinity of impending m 8 earthquake this decrease is about 20 with a lead time of about a year the forerunner anomaly is not of the bay type and recovers 1 2 years after a large earthquake

stability of coda q in the region of parkfield california view from the u s geological survey parkfield dense seismograph array many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate to large earthquakes parkfield california on the san andreas fault is a promising location for studying premonitory changes in coda q qc because a large earthquake is likely to occur there we have investigated qc using recordings from the u s geological survey parkfield dense seismograph array which is a digital array with 14 triaxial sensors and an aperture of about 1 km for each earthquake we can measure qc from up to 42 recordings their average is more stable than the measurement from a single station using clustered seismicity we have developed criteria for selecting events and reducing scatter in the measurement the qc value determined from a seismogram depends on the position and length of the analysis window thus qc should always be measured from the same length window starting at the same lapse time regardless of the source location in addition the band limited signal to noise ratio at the end of the analysis window is important qc determined in two frequency bands 4 8 hz and 8 16 hz from a tight cluster of 26 events which occurred between december 1989 and january 1994 has not changed despite m 4 7 and m 4 6 events in october 1992 and november 1993 qc measured from local events 60 km in three frequency bands shows larger scatter but has also not changed during this period for monitoring qc observations should include array averaged measurements from a single lapse time because qc measurements made using an analysis window that starts at a constant multiple of the s wave lapse time depend on epicentral distance a procedure combining the evaluation of the time and distance dependences of qc also gives stable observations

the permeability of natural soft clays part ii permeability characteristics reply the permeability characteristics of a number of intact natural soft clays from quebec the usa and sweden have been investigated in the laboratory the variation of permeability with void ratio is best represented in terms of a linear e vs lg k relation which is generally valid in the range of volumetric strains encountered in engineering practice the permeability at the in situ void ratio is shown to be a complex function of such parameters as the void ratio the clay fraction the plasticity index and the fabric of the clay permeability anisotropy is negligible in marine clays the e vs lg k relationships of the various clays tested are well ordered in terms of an empirical parameter ip clay fraction irrespective of the geological origin of the clay the slope of the e vs lg k relationship referred to as permeability change index ck is simply related to the initial void ratio by ck 0 5eo keywords permeability clays laboratory tests permeability anisotropy permeability void ratio re

coda q before the 1983 hawaii ms 6 6 earthquake abstract coda q values were derived for more than 300 microearthquakes that occurred in a 6 yr period before the 16 november 1983 kaoiki hawaii earthquake m s 6 6 the sources were located within a 14 16 km rectangular region surrounding the main shock epicenter and most of them occurred at depths between 5 and 10 km digital recordings from three stations at epicentral distances ranging from 0 to 18 km were used coda q was calculated from the amplitude decay rate of the s wave coda in nine frequency bands from 4 5 to 27 hz the average q of the nw part of the studied area is about 15 per cent higher than that of the se part these two subregions also showed differences in seismicity b value and microearthquake source mechanisms in the high q volume the b value was 1 0 and the rate of earthquakes per unit volume was about 50 per cent of the rate in the low q volume where b 1 3 one interpretation of these observations is that more extensive faulting in the se kaoiki fault zone leads to lower q higher b value and a higher seismicity rate during the 5 to 6 yr before the mainshock the 1 yr average q values were stable no significant q change could be identified as a precursor to the main shock

some geomorphological considerations for the future development of landslide models this paper reviews the commonly available models used to model landslides the main purpose is then to examine some of the wider issues associated with the developments that are required if we are to develop 3 d applications and to extrapolate their use to long time scales the paper does not discuss landslide hazard mapping models but concentrates on the 3 d and temporal problems associated with the modelling of single slides

temporal change in coda associated with the round valley california earthquake of november 23 1984 seven thousand seismograms of small earthquakes in the mammoth lakes bishop area were used to measure values of q from the decay of the earthquake coda these measurements were compared between events that occurred before and after the round valley earthquake m 5 7 we found that in regions near the main shock epicenter measurements of coda q 1 for earthquakes that occurred after the main shock were higher than for those earthquakes that occurred prior to the main shock the opposite behavior was found for regions farther away from the main shock namely lower coda q 1 after the main shock than before measurements of coda q 1 in the long valley caldera outside of the immediate source region of the earthquake were higher than in surrounding areas before the main shock but the difference disappeared after the occurrence of the main shock this indicates that the temporal variation in coda q 1 is comparable to its spatial variation the doughnut model seismicity quiescence surrounded by a zone of activity which was invoked for explaining the precursory seismicity pattern appears to be similar with the observed coda q 1 variation associated with the round valley earthquake the observed spatial variations in coda q 1 also help to reconcile conflicting results published in previous studies of the coda q 1 precursor

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

spatial and temporal correlation between coda q and seismicity in china a map of coda q at 1 hz q was constructed for china by analyzing more than 1000 seismograms from 80 seismic stations distributed throughout china the resultant contour map of q shows a strong correlation with the epicenters of historic great earthquakes in china the high q regions are devoid of the epicenters and the low q regions are full of them a remarkably linear relation was found between the logarithm of coda q and the maximum earthquake within the area sampled by coda waves radius of about 150 km from each station when we consider earthquakes which occurred from 1700 to present roughly speaking if q is 1000 m max is about 4 and if q is 100 m max is about 8 this relation will be very useful for siting critical facilities such as nuclear power plants and waste disposal sites we also found evidence in support of the migration of low q region coincident with the well known migration of high seismicity region over the distance of several hundred kilometers during the period of several hundred years if the migration is due to a diffusion process the diffusion constant is of the order of 100 km2 yr which agrees very well with those associated with the aftershock migration fluid injection and vp vs anomaly comparison of coda q with q1 estimated from the intensity data shows that they agree very well for q1 150 supporting the validity of the s to s single backscattering model for coda waves for q1 150 however coda q becomes constant independent of q1 this behavior is qualitatively consistent with numerical experiments of frankel and clayton and suggests that coda q may be measuring the intrinsic q of about 150 for these regions where q1 150 this limiting coda q value may depend on regions because coda q lower than 100 has been observed in some active regions in this study and also coda q as low as 30 to 40 has been reported in california mexico and italy

some measurements of the permeability of kaolin this paper reports an experimental investigation of the vertical and horizontal permeabilities of speswhite kaolin clay the permeabilities were measured using falling head permeability tests a mo

suction controlled triaxial testing laboratory procedures in relation to resistance envelope methods in certain tropical slopes it appears possible that soil suction may play a significant role in maintaining stability this communication outlines a laboratory suction controlled triaxial test that can be undertaken to validate the threshold suction predicted by resistance envelope methods it is shown that for two sites examined in st lucia west indies the results from such a test confirm results obtained by use of resistance envelopes

higher order interrelations between seismogenic structures and earthquake processes abstract aki k 1992 higher order interrelations between seismogenic structures and earthquake processes in t mikumo k aki m ohnaka l j ruff and p k p spudich guest editors earthquake source physics and earthquake precursors tectonophysics 211 1 12 to advance our understanding of higher order smaller scale interrelations between the fault zone structure and the earthquake process we start with the observation that the source parameters of major california earthquakes deviate systematically from the self similarity this departure from the self similarity is interpreted in terms of both the specific barrier model and the asperity model the asperity model however is found to be inconsistent with the following observations 1 sharp impulsive displacements waveform in the direction perpendicular to the fault trace 2 the complementary relationship between fault slip and aftershock and 3 existence of barriers in the creeping segment of the san andreas fault the application of the specific barrier model on the other hand leads to a predominant subevent crack size of a few hundred meters in the creeping segment this result is consistent with the negative correlation between coda q 1 and b value observed in the stone canyon area for 1 5 3 temporal increase in coda q 1 is sometimes accompanied by an increase in the rate of surface deformation but not always so this is attributed to the erratic coupling between the creeping brittle region and the overlying brittle region the applicability of the conclusions obtained from california earthquakes to other regions is investigated using the observed scaling laws of seismic source spectra and it is suggested that the crack size of the creeping brittle zone may vary regionally from a few hundred meters to a few kilometers as an alternative to the widely held view that the aseismic creep in the plate boundary may occur as a giant slow earthquake our result suggests that it may be occurring as small fractures with scale lengths from a few hundred meters to a few kilometers

lateral variations of lg coda q in southern mexico broad band digital three component data recorded at unm a geoscope station were used to estimate lg coda q for 34 medium size 3 9 m b 6 3 earthquakes with travel paths laying in different geological provinces of southern mexico in an effort to establish the possible existence of geological structures acting as wave guides and or travel paths of low attenuation between the pacific coast and the valley of mexico the stacked spectral ratio method proposed by xie and nuttli 1988 was chosen for computing the coda q the variation range of q 0 q at 1 hz and the frequency dependence parameter estimates averaged on the frequency interval of 0 5 to 2 hz for the regions and the three components considered are i guerrero region 173 q 0 182 and 0 6 q 0 0 7 ii oaxaca region 183 q 0 198 and 0 6 q 0 0 8 iii michoacan jalisco region 187 q 0 204 and 0 7 q 0 0 8 and iv eastern portion of the transmexican volcanic belt tmvb 313 q 0 335 and 0 9 the results show a very high coda q for the tmvb as compared to other regions of southern mexico this unexpected result is difficult to reconcile with the geophysical characteristics of the tmvb e g low seismicity high volcanic activity and high heat flow typical of a highly attenuating low q region visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the tmvb the amplitude decay of lg coda is anomalously slow as compared to other earthquakes in southern mexico thus it seems that the high q value found does not entirely reflect the attenuation characteristics of the tmvb but it is probably contaminated by a wave guide effect this phenomenon produces an enhancement in the time duration of the lg wave trains travelling along this geological structure this result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the september 19 1985 michoacan earthquake the overall spatial distribution of coda q values indicates that events with focus in the michoacan jalisco and oaxaca regions yield slightly higher values than those from guerrero this feature is more pronounced for the horizontal component of coda q a slight dependence of average coda q 1 on earthquake focal depth is observed in the frequency range of 0 2 to 1 0 hz approximately on the horizontal component deeper h 50 km events yield lower values of q 1 than shallower events for frequencies higher than 1 0 hz no clear dependence of q 1 on focal depth is observed however due to the estimates uncertainties this result is not clearly established