quantifying noble gas contamination during terrestrial alteration in martian meteorites from antarctica we investigated exterior and interior subsamples from the martian shergottite meteorites allan hills alh a77005 and roberts massif rbt 04261 for secondary minerals oxygen isotopes ar ar and noble gas signatures electron microprobe investigations revealed that rbt 04261 does not contain any visible alteration even in its most exterior fractures whereas fracture fillings in alha77005 penetrate into the meteorite up to 300 m beyond which the fractures are devoid of secondary minerals light noble gases seem to be almost unaffected by terrestrially induced alteration in both meteorites thus a shock metamorphic overprint of 30 35 gpa can be deduced from the helium measurements in rbt 04261 oxygen isotopes also seem unaffected by terrestrially weathering and variations can easily be reconciled with the differences in modal mineralogy of the exterior and interior subsamples the measurements on irradiated samples ar ar showed a clear martian atmospheric contribution in alha77005 but this is less apparent in our sample of rbt 04261 exterior and interior subsamples show slight differences in apparent ages but the overall results are very similar between the two in contrast krypton and xenon are severely affected by terrestrial contamination demonstrating the ubiquitous presence of elementally fractionated air in rbt 04261 although seemingly contradictory our results indicate that rbt 04261 was more affected by contamination than alha77005 we conclude that irrespective of on which planet the alteration occurred exposure of martian rocks to atmosphere or brine introduces noble gases with signatures elementally fractionated relative to the respective atmospheric composition into the rock and relationships of that process with oxygen isotopes or mineralogical observations are not straightforward

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

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

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

site dependent coda q at mono craters california abstract the coda q of the early coda of shear waves was determined from a network of seismic stations deployed in the mono craters area during the summer of 1982 coda amplitudes determined from a moving window analysis were fit to single scattering theory with an l1 norm for center frequencies from 1 5 to 12 0 hz we find that station to station variations in coda q observed for frequencies between 6 and 12 hz are well correlated with near site geology coda q for stations on crystalline rocks is larger and more frequency dependent than for stations sited on bishop tuff adjacent to mono craters coda q from sites on bishop tuff away from mono craters is still lower and less frequency dependent these observations suggest that site effects dominate the early coda in the mono craters region and that the single scattering model is inappropriate the mechanisms generating these site effects are inferred to be 1 multiple scattering from resonant structures such as sedimentary basins volcanic edifices variable basement topography and surface topography and 2 near site attenuation

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

evaluation of methods for estimating linear site response amplifications in the los angeles region we have evaluated two methods of estimating linear site response amplifications and the standard index for classifying sites by comparing the results of each technique to observed ground motions at 33 sites in the los angeles region using velocity and density profiles from 33 boreholes we evaluated the use of the average 30 m shear wave velocity and associated site classifications the quarter wavelength method and the haskell propagator matrix method we correlated the average 30 m shear wave velocity and nehrp site classification at the borehole sites colocated within 290 m of a site with observed ground motion the observed data follow the expected trend of higher ground motion amplifications for lower average shear wave velocities but there is a significant degree of variability also there is a great deal of scatter in the observed amplifications within each national earthquake hazards reduction program nehrp class we used the velocity and density information in the database to calculate the average frequency dependent site response in the frequency ranges 1 3 hz 3 5 hz and 5 7 hz for a one dimensional flat layered structure using the haskell propagator matrix method and the quarter wavelength method there is a general correlation with the ground motion data however once again there is a great degree of scatter although slightly less for the quarter wavelength method we note that both the observed and predicted amplifications can change dramatically over a distance as small as 1 km or less even though all techniques considered give results that follow the expected trend of higher amplifications for softer sediments the predicted site response at any particular site with the current level of information may not be representative of the shaking that will actually occur during an earthquake the disparity between observed and predicted amplifications appears to be a result of oversimplification inherent in the amplificationestimation methods such as the use of average or assumed values for the site conditions in the absence of measured values the smoothing effect of using an average velocity limiting the properties considered to the uppermost 30 m of material and complexities in the wave propagation that are not addressed by these methods

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

site amplification of coda waves from local earthquakes in central california the goals of this study are two fold 1 to examine the site effect of coda waves believed to be the average site effect of shear waves in order to understand its spatial and frequency dependent behavior and 2 to learn what we can about the processes that generate the coda itself we use digital data from over 90 earthquakes each recorded by a subset of some 150 stations in the coast ranges of central california between san francisco and san luis obispo results from the band 1 5 to 24 hz indicate that site amplification depends strongly on surface geology and frequency at low frequencies 1 5 to 3 0 hz changes in amplification up to a factor of 20 are observed amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and franciscan basement sites at high frequencies 6 to 24 hz the pattern changes many granite sites the majority in the gabilan range exhibit increasing amplification with frequency relative to the average station this behavior differs strikingly from that observed at sediment and franciscan sites which roll off at an intermediate rate and many sediment sites adjacent to the san andreas fault zone which decay away even more rapidly with frequency these results can be explained qualitatively by appealing to variable near site impedance and attenuation however complicating phenomena such as mode excitation in valley sediments must exist at low frequencies peaked behavior at a few hard rock sites at high frequency 12 hz currently lacks explanation at a few sediment sites the low frequency excitation is so strong that the coda envelopes begin to take on different shapes this contradicts the assumption based on previous observations that the coda shape is independent of source receiver location thus some site measurements may be artificially high the working model of coda waves as body wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near site resonance excited by incident direct and coda wave energy

seismic tracking of lahars using tremor signals debris flows or lahars frequently occur in volcanic regions and cause serious disasters to nearby communities lahars are commonly observed as long lasting high frequency tremor in seismic records we propose a seismic technique to locate the source of tremor originating from lahars by using waveform data from a seismic network in this method we fit the surface wave amplitudes from point sources positioned over the surface of a volcano to the observed tremor amplitudes and estimate the spatial distributions of the normalized residuals in successive time windows the observed tremor amplitudes at individual stations are corrected by site amplification factors independently estimated from coda waves of tectonic earthquakes the positions of time windows applied to the observed tremor signals are corrected for the travel times of individual source and receiver pairs we track the source of tremor or lahars as temporal changes of the best fit locations in the residual distributions this method is an extension of the existing methods used to locate seismic events and tremor associated with volcanic eruptions we applied this technique to tremor signals of lahars observed by a seismic network at cotopaxi volcano ecuador our analysis showed regions with small residuals on the ne flank of the volcano this suggests that lahars descended on this flank which is consistent with the result of a field survey that found fresh lahar deposits on the ne flank our method which can be easily automated may be useful for the rapid detection and monitoring of lahars

site amplification and attenuation of strong ground motion abstract at the mcgee creek california site three component strong motion accelerometers are located at depths of 166 m 35 m and 0 m the surface material is glacial moraine to a depth of 30 5 m overlying hornfels accelerations were recorded from two california earthquakes round valley m l 5 8 23 november 1984 18 08 utc and chalfant valley m l 6 4 21 july 1986 14 42 utc anti plane shear strains for the round valley and chalfant valley events are less than 1 8 10 5 and 1 2 10 5 respectively by separating out the sh components of acceleration we were able to determine the orientations of the downhole instruments peak sh accelerations at the surface were 5 7 times greater than those at 166 m in both earthquakes a constant phase velocity haskell thomson model was applied to generate synthetic sh seismograms at the surface using the accelerations recorded at 166 m in the frequency band 0 10 to 10 0 hz we compared the filtered synthetic records to the filtered surface data the onset of the sh pulse and the reflections from the interface at 30 5 m are clearly seen in both the synthetics and the recorded data the synthetic records closely match the data in both amplitude and phase the fit between the synthetic accelerograms and the data indicates that the seismic amplification at the surface is a result of the resonance of the surface layers and the contrast of the impedances shear stiffness of the near surface materials the impedance contrast of the materials does not account for all of the amplification of the peak accelerations at the surface the resonances of the layer system below 10 hz contribute the most to this amplification we find that a linear model predicts the soil response for frequencies up to 10 hz material damping is added to the soil layers of the model in the form of a quality factor q the damping moderates the strong resonance effects by allowing energy to radiate into the half space the quality factor q 10 is constant in this frequency range but does not accurately model the attenuation at frequencies above 10 hz

experimental study of site effects in the fort de france area martinique island abstract we used weak motion recordings of regional events to quantify the site effects in the fort de france martinique area in the frequency band 1 to 25 hz the studied area extends over three distinct geological units the morne pitault volcanic massif 15 to 11 m a the carbet volcanic massif 4 to 2 m a and the lamentin alluvial plain quaternary age the topography being complex 31 sites were selected in order to get a representative sampling of the various topographical and geological configurations the large number of studied sites also allowed us to define a reference site whose selection was not obvious large broadband amplifications are observed in the lamentin plain instead of the expected peak resonances more surprising such resonance is observed around 2 to 3 hz on several sites of the volcanic vauclain pitault chain though no sedimentary layer is present on the other hand in the inner fort de france where significant effects were expected because of the very complex topography no large amplication is observed except for at one site clear three dimensional 3d effects are also observed at some stations though not very well documented the low frequency behavior deduced from teleseismic observations seems rather different illustrating the difficulty in finding a convenient reference site in such a volcanic area

attenuation of coda waves in northern greece the single scattering model has been applied for the estimation of codaq values for local earthquakes that occurred in northern greece during the period 1983 1989 and recorded by the telemetered network of the geophysical laboratory of the university of thessaloniki codaq estimations were made for four frequency bands centered at 1 5 hz 3 0 hz 6 0 hz and 12 0 hz and for the lapse time windows 10 20 sec 15 30 sec 20 45 sec 30 60 sec and 50 100 sec the codaq values obtained show a clear frequency dependence of the formq c q 0 f n whileq 0 andn depend on the lapse time window q 0 was found equal to 33 andn equal to 1 01 for the time window of 10 to 20 sec while for the other windowsq 0 increased from 60 to 129 withn being stable close to 0 75 this lapse time dependence is interpreted as due to a depth dependent attenuation the high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity in agreement with studies in other seismic regions

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

seismic site amplification factors in campi flegrei southern italy the method of spectral ratios of s waves from local small earthquakes recorded at various sites in the campi flegrei volcanic area southern italy has been used to evaluate the seismic site response in the frequency range 1 15 hz the spectra of the s wave trains recorded at the short period seismic stations are corrected for attenuation using attenuation parameters quality factor q previously determined for the area using the coda method a reference station has been used selected on the basis of the highest number of recorded earthquakes and not on geological considerations the relative uniformity of the geological conditions and the absence of hard rock sites conditioned such choice the method has been checked using the coda wave method to estimate the spectral site response and similar results were obtained the results show a relative peak of the spectral ratio pattern centred at low frequency 2 3 hz and a minimum of 0 2 at 8 hz for two stations located on yellow tuff such results are interpreted as being caused by a filtering effect produced by selective attenuation such a pattern could be of interest for seismic engineering purposes in view of the abundance of tuff over a wide area in this densely inhabited region

effect of bed roughness on tsunami bore propagation and overtopping abstract the accurate modelling of overtopping of coastal defences by tsunami waves is of vital importance for the formulation of disaster management strategies to improve knowledge of this phenomena the authors conducted experiments on coastal structure overtopping using bores that were generated by a dam break mechanism three types of structures were tested namely a coastal dyke a wall and a wall of infinite height the results highlight the necessity to consider the energy present in a bore to determine if a structure will be overtopped or not as a result of these experiments an empirical formula to determine the height of overtopping given the incident bore height and velocity was validated the study highlights the importance of clearly modelling the velocity and froude number of a tsunami such experiments should be conducted on rough beds for which a suitable manning s n seems to be around 0 06 sm 1 3 the study also contrasted the results obtained to those of the asce7 method and concludes that the manning s n values recommended in asce7 are probably too low

a precursorlike change in coda excitation before the western nagano earthquake ms 6 8 of 1984 in central japan temporal variations of coda duration were studied in relation to a crustal earthquake september 14 1984 which took place near the nw corner of the kanto tokai microearthquake observation network of the national research center for disaster prevention in japan vertical seismograms recorded at station ger close to the main shock epicenter 24 km between february 1982 and december 1984 were analyzed a precursorlike change was found in the relation between coda duration and average magnitude for 185 earthquakes which occurred in a small volume in the middle of the aftershock region the average magnitude is the arithmetic mean of station magnitudes which are calculated from the maximum amplitudes of direct waves coda durations for earthquakes of the same average magnitude were significantly longer during the 16 month period preceding the main shock than those before and after this period with a confidence level of 99 9 the increase in coda durations was about 3 s on average in this period the coda duration anomaly was significant for lapse time shorter than 30 s which corresponds to a travel radius less than 50 km although no clear temporal change was found in the residuals of station magnitude measured at ger from average magnitude an attenuation change in the preevent period can not be ruled out due to the large scatter of data crack formation however is the most plausible mechanism for an increase in scattering intensity around the focal region in the stress accumulation stage and is consistent with other precursory observations clusters of cracks in the heterogeneous earth medium might be big enough to scatter high frequency seismic waves even though each individual crack may be very small

source process of long period seismic events at taal volcano philippines vapor transportation and condensation in a shallow hydrothermal fissure 1 we analyzed observations of a swarm of more than 40 000 long period lp seismic events at taal volcano philippines in 2010 2011 the event waveforms are strongly correlated to each other consistent with a fixed source location and begin with a dilatational first motion they have a peak frequency around 0 8 hz and a quality factor q of 6 waveform inversion of the events pointed to a tensile crack source dipping 30 60 at a shallow 100 200 m depth a simulation using a fluid filled crack model indicated that the complex frequencies of the waveforms are explained by the fundamental longitudinal mode resonance of a vapor filled crack 188 m long a satellite thermal infrared image acquired during the swarm period suggests that the lp events were not accompanied by surface gas releases we considered a vapor transportation model in which vapor exsolved from magma and rose in a fissure extending to the lp source this model yielded estimates that 105 107 m3 of magma was involved in the lp swarm and that the temperature of vapor in the lp source crack was around 600 k we modeled a triggering mechanism of the crack resonance based on sudden condensation of vapor at the crack tip in a cold aquifer this model explained observed characteristics of the events including the dilatational first motion the total volumetric change and the fixed source location

site amplification in the san fernando valley california variability of site effect estimation using the s wave coda and h v methods during the months that followed the 17 january 1994 m 6 7 northridge california earthquake portable digital seismic stations were deployed in the san fernando basin to record aftershock data and estimate site amplification factors this study analyzes data recorded on 31 three component stations from 38 aftershocks ranging from m 3 0 to m 5 1 and depths from 0 2 to 19 km site responses from the 31 stations are estimated from coda waves s waves and ratios of horizontal to vertical h v recordings for the coda and the s waves site response is estimated using both direct spectral ratios and a generalized inversion scheme results from the inversions indicate that the effect of qs can be significant especially at high frequencies site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other depending on the choice of the reference site the difference is reduced when an average of six rock sites is used as a reference site in addition when using this multi reference site the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the s waves however for nonrock sites the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the s waves the h v method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods the amplifications determined from the hiv method are however different from the amplifications determined from the other methods finally the stations were grouped into classes based on two different classifications general geology and a more detailed classification using a quaternary geology map for the los angeles and san fernando areas average site response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification

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

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

the contribution of topographic scattering to teleseismic coda in southern california given that seismic coda is largely the result of scattering near the receiver a tie to geological complexity i e to crustal heterogeneity is expected but a systematic and predictive understanding of the important aspects of geology is lacking we have developed a technique for mapping the origins of coda energy as recorded by regional networks that provides insight into the proximal generation of seismic coda here the role of topography in the production of surface propagating coda energy both as a scatterer and as a proxy of shallow geologic structure is examined for the portion of the san andreas fault system within the southern california seismic network we find significant correlation of p to rg scattering potential with topographic roughness that extends to relatively subtle topography the correlation peaks at a migration velocity of 2 9 km s confirming the identification of scattered rg energy and offering an alternative means of estimating regional shallow velocity structure