gulf of mexico shelf framework interpretation using a bed form attribute from spectral imaging this article introduces a new attribute called spice spectral imaging of correlative events that calculates a bed form boundary framework from the seismic data and highlights detail from subtle changes in the seismic wavelet the concept of spectral imaging is presented in the context of a well log model and its relationship to impedance layering further validation of the method is provided by a detailed sequence stratigraphy analysis using well logs and seismic data from the northern gulf of mexico the technique based on wavelet transform decomposition and singularity analysis of migrated seismic data uses the localization properties of the wavelet transform in time and frequency to produce a unique display that provides a boundary framework of the subsurface that is rich in structural and stratigraphic detail this advances seismic interpretation closer to the goal of producing accurate geologic mapping of the subsurface additionally this attribute offers a straightforward way to interpret a seismic section similar to a geologist in the field who maps beds and faults directly from the outcrop most seismic interpretation today is performed on variable area displays of the seismic amplitude of a wavelet that changes with depth this has historically posed a number of significant problems for the interpreter reflections in seismic data are blurred representations of the actual stratigraphy the interference of a changing pulse shape with a wide variety of impedance contrasts in the subsurface adds a significant element of uncertainty to the final mapping of reservoir seal and trap the richness and nonstationary character of the seismic trace require a process that localizes rapidly changing features in the spectrum the unique calculation reduces the uncertainty in picking subtle bed form boundaries and brings out the full extent of the resolution of the seismic amplitude data spice is computed in the continuous wavelet transform cwt domain

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

the mechanism of great banda sea earthquake of 1 february 1938 applying the method of preliminary determination of focal mechanism to a historical event abstract a moment tensor solution is derived for the great 1938 banda sea earthquake one of the largest events for which no definitive focal geometry has been published we apply the method of preliminary determination of focal mechanism which consists of inverting for a moment tensor using exclusively the spectral amplitudes of mantle rayleigh and love waves while discarding the phase information which can be affected by uncertainties in relative timing between historical records from a dataset of 17 records at seven stations we obtain a robust solution featuring a very large moment of 8 37 10 28 dyn cm the centroid depth being constrained around 60 km by minimizing the rms residual of the inversion the method involves an inherent two fold 180 indeterminacy on the orientations of strike and slip which is lifted by the examination of the polarization of long period body waves read individually on original records resulting in a mostly thrust faulting mechanism strike 276 dip 63 slip 70 this approach coupled with more traditional relocation efforts allows a rare glimpse of this exceptional event confirmed here to rank among the 10 largest moments ever published within the extremely complex regional plate tectonics pattern the 1938 event took place in a region of sparse seismicity and away from the presumed locations of block boundaries the deeper than normal focus of the event agrees well with the generally minor level of reported damage and with the generation of a relatively benign tsunami available modern focal mechanisms in its vicinity are too few and too scattered to allow any meaningful comparison but the 1938 event shares its compressional axis with that of the smaller and deeper 1963 shock to the southwest which probably expresses coherence in the regional contortion of the subducting australian plate lithosphere

3d seismic image processing for unconformities abstractin seismic images an unconformity can be first identified by reflector terminations i e truncation toplap onlap or downlap and then it can be traced downdip to its corresponding correlative conformity or updip to a parallel unconformity for example in topsets unconformity detection is a significant aspect of seismic stratigraphic interpretation but most automatic methods work only in 2d and can only detect angular unconformities with reflector terminations moreover unconformities pose challenges for automatic techniques used in seismic interpretation first it is difficult to accurately estimate normal vectors or slopes of seismic reflectors at an unconformity with multioriented structures due to reflector terminations second seismic flattening methods cannot correctly flatten reflectors at unconformities that represent hiatuses or geologic age gaps we have developed a 3d unconformity attribute computed from a seismic amplitude image to detect unconformities by highlighting the

a re evaluation of the great aleutian and chilean earthquakes of 1906 august 17 summary we investigate two great earthquakes that occurred in the aleutian islands and chile within 30 min of each other on 1906 august 17 based on a collection of seismograms compiled shortly after the events by scientists at strasbourg the method of preliminary determination of focal mechanisms pdfm is applied to 14 mantle waves from seven stations in order to resolve the moment tensors of the two shocks it is complemented by examination of body wave polarities at japanese stations to lift the remaining indeterminacy in focal mechanism the chilean earthquake occurring second is a regular subduction event whose moment 2 8 1028 dyn cm is revised downwards from previous estimates except kanamori s suggesting that its rupture did not involve more than 200 km of fault the aleutian earthquake occurring first has a larger moment 3 8 1028 dyn cm but features mantle wave radiation patterns and body wave polarities incompatible with both underthrusting at the aleutian subduction zone and tensional buckling at the outer rise rather we suggest that it is an intraplate somewhat deeper 50 km earthquake this is supported by tentative relocation of available arrival times north of it the origin of the earthquake may be related to the presence of the bowers ridge north of the amchitka pass in the epicentral area finally hydrodynamic simulations using our source mechanisms support the observation that the chilean event was the source of the reported transpacific tsunami the report of a 3 5 m wave at maui constitutes a misassociation as its timing is shown to be non causal for both events

revision of magnitudes of large shallow earthquakes 1897 1912 abstract in previous research trace amplitudes of surface wave maxima recorded by undamped milne seismographs were used to determine the surface wave magnitudes ms of large shallow earthquakes which occurred prior to 1912 for this purpose the effective gain of these instruments was calibrated by using the surface wave magnitudes ms gr which were calculated from the unpublished worksheets for seismicity of the earth of gutenberg and richter in this paper the real quality of ms gr is critically re evaluated by using independent sets of data it is found that ms gr for the period 1904 1909 is considerably overestimated the average excess from the real ms is 0 5 units for 1904 1906 0 4 for 1907 0 3 for 1908 1909 and 0 0 for 1910 1912 this overestimation is so systematic and large that the previous results are all redetermined the average effective gain of milne instruments is revised to be 21 9 previously the gain depended on ms this revision results in systematic reduction in the previously assigned magnitudes the revised values of ms for 264 shallow earthquakes with ms 6 8 and over in the period 1897 1912 inclusive are listed the present revision is large enough to preclude the possibility of the high activity of large shallow earthquakes around the turn of the century the present results have a direct effect on all the magnitude catalogues of shallow earthquakes which occurred prior to 1909

the shikotan earthquake of october 4 1994 lithospheric earthquake a large m w 8 2 earthquake occurred off shikotan is one of the kurile is on october 4 1994 we inverted 32 body wave records to determine the rupture pattern using an iterative deconvolution method the mechanisms of the subevents were allowed to vary during rupture the source parameters obtained are the location of the initial break 43 48 n 147 40 e the centroid depth 56 km strike dip rake 49 75 125 for the total source the seismic moment m o 2 6 10 21 nm m w 8 2 source time duration t 42 s the average rupture velocity 2 5 km s we also determined the mechanism using long period love and rayleigh waves from 14 stations the solution for a finite source distributed over a depth range from 0 to 90 km is strike dip rake 54 76 129 with mo 2 3 10 21 nm in good agreement with that from body waves referring to the extent of the aftershock area and the subevent distribution we estimated the fault area s 120 60 km the average slip d 5 6m and the stress drop 11 mpa we computed synthetic waveforms as well as static displacements using either the steep or the low angle plane as the fault plane and found that the steep dip fault model fits the data better our result the mechanism large centroid depth high stress drop strongly suggests that the 1994 shikotan earthquake is a lithospheric earthquake an intra plate event that ruptures through a substantial part of the subducting oceanic lithosphere this type of lithospheric earthquake is relatively common

intraplate seismicity of the pacific basin 1913 1988 we establish here a comprehensive database of intraplate seismicity in the pacific basin relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913 1988 these numbers do not include earthquake swarms which account for another 838 events most of the remainder 304 events are actually plate boundary earthquakes that have been erroneously located in intraplate regions a significant number occur in recent years when location capabilities should have guarded against this situation relocations involve a careful linear inversion of p and s arrivals accompanied by a monte carlo statistical analysis we have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins

the 1985 central chile earthquake a repeat of previous great earthquakes in the region a great earthquake surface wave magnitude 7 8 occurred along the coast of central chile on 3 march 1985 causing heavy damage to coastal towns intense foreshock activity near the epicenter of the main shock occurred for 11 days before the earthquake the aftershocks of the 1985 earthquake define a rupture area of 170 by 110 square kilometers the earthquake was forecast on the basis of the nearly constant repeat time 83 9 years of great earthquakes in this region an analysis of previous earthquakes suggests that the rupture lengths of great shocks in the region vary by a factor of about 3 the nearly constant repeat time and variable rupture lengths cannot be reconciled with time or slip predictable models of earthquake recurrence the great earthquakes in the region seem to involve a variable rupture mode and yet for unknown reasons remain periodic historical data suggest that the region south of the 1985 rupture zone should now be considered a gap of high seismic potential that may rupture in a great earthquake in the next few tens of years

preliminary determination of focal mechanisms from the inversion of spectral amplitudes of mantle waves abstract we present a method for the preliminary determination of focal mechanisms pdfm based on the inversion of deviatoric moment tensor components from depleted datasets of mantle wave spectral amplitudes the latter are found to be largely insensitive to errors in epicentral estimates which can hamper the inversion of the full complex spectrum under real time operational conditions robust solutions can be obtained from rayleigh wave spectra at as few as three stations and from a combination of rayleigh and love waves at only two stations provided they sample 60 in azimuth the systematic application of this method to a dataset of 43 large earthquakes during 1996 1999 yielded mechanisms rotated an average of only 17 from the published harvard cmt a figure comparing favorably with the case of other catalogues such as the usgs moment tensor solutions

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

spatiotemporal patterns in the energy release of great earthquakes for the past 80 years the energy released in great strike slip and thrust earthquakes has occurred in alternating cycles of 20 to 30 years this pattern suggests that a global transfer mechanism from poloidal to toroidal components of tectonic plate motions is operating on time scales of several decades the increase in seismic activity in california in recent years may be related to an acceleration of global strike slip moment release as regions of shear deformation mature after being reached by stresses that have propagated away from regions of great subduction decoupling earthquakes in the 1960s

nonequilibrium air clathrate hydrates in antarctic ice a paleopiezomdter for polar ice caps abstract craigite the mixed air clathrate hydrate found in polar ice caps below the depth of air bubble stability is a clathrate mixed crystal of approximate composition n2o2 6h2o recent observations on the byrd station antarctic core show that the air hydrate is present at a depth of 727 m well above the predicted depth for the onset of hydrate stability we propose that the air hydrate occurs some 100 m above the equilibrium phase boundary at byrd station because of piezometry i e that the anomalous depth of hydrate occurrence is a relic of a previous greater equilibrium depth along the flow trajectory followed by vertical advection of ice through the local phase boundary depth flowline trajectories in the ice based on numerical models show that the required vertical displacement does indeed occur just upstream of byrd station air hydrate piezometry can thus be used as a general parameter to study the details of ice flow in polar ice caps and the metastable persistence of the clathrate phase in regions of upwelling blue ice

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

on the age vs depth and optical clarity of deep ice at south pole the first four strings of phototubes for the amanda high energy neutrino observatory are now frozen in place at a depth of 800 to 1000 m in ice at the south pole during the 1995 96 season an addit

mm a variable period mantle magnitude this paper lays the theoretical groundwork for a variable period mantle magnitude mm based on the measurement of the spectral amplitude x of very long period rayleigh waves we retain the concept of magnitude by restricting ourselves to single station measurements ignoring the focal mechanism and the exact depth of the shallow earthquakes considered our measurements are made at a series of periods in all cases greater than 40 s and the largest value is retained this procedure effectively avoids the well known interference effects leading to saturation of magnitude scales defined at a fixed period two corrections are used a period dependent distance correction cd and a source correction cs also period dependent compensating for the variation of the excitation of rayleigh waves with period both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory the result is a formula of the type mm log10 x cd cs c0 in which all coefficients including the constant c0 are justifiable on sound theoretical grounds the analysis of a data set of 256 records from the broadband seismograph at papeete tahiti the ultra long period system at pasadena and stations of the geoscope network shows that the mean error in the estimation of the seismic moment is on the order of 0 1 0 2 units of magnitude with the standard deviation at each station also on the order of 0 2 units of magnitude no significant trend with either distance period or station can be identified the method can also be transposed to the time domain under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the pacific basin both versions of the method lend themselves well to automation thus by providing a real time estimate of the seismic moment of distant earthquakes mm has considerable potential for tsunami warning purposes its concept can easily be extended to love waves and also to intermediate and deep earthquakes

multichannel seismic evidence bearing on the origin of bowers ridge bering sea bowers ridge is a large arcuate sub marine ridge that extends north and west from the aleutian ridge and separates the abyssal aleutian and bowers basins in the bering sea two multichannel seismic reflection lines recorded in 1976 over bowers ridge and the adjacent basins confirm the existence of 8 to 10 km thick sediment wedges on the north side of bowers ridge and at the base of the bering continental margin deformed sediment within the bowers wedge indicates that subduction of the adjacent ocean crust beneath the ridge probably occurred prior to middle cenozoic time flat lying reflectors near the bottom of the trench suggest that a bathymetric trough and large ridge existed in mesozoic time the major period of underthrusting subsidence and in filling of the trench probably occurred from mesozoic to early tertiary time small amounts of underthrusting may have continued after the early tertiary development of the aleutian ridge however by middle miocene time the formerly subaerial bowers ridge had subsided below sea level the multichannel seismic data do not show evidence for a buried spreading center within the eastern aleutian basin consequently the sediment wedges trenches at both bowers ridge and the bering continental margin are believed to be the consequence of subduction that occurred during the convergence of the ridge and the margin the large size of bowers ridge suggests that a large amount of convergence has occurred since mesozoic time if bowers ridge was a large feature in mesozoic time as suggested by the apparent bathymetric trough then the ridge may be as old or older than the aleutian ridge to which it connects

volumetric estimates of seismic reflector rotation and convergence tools for mapping rotation about faults and seismic stratigraphy geometric attributes such as coherence and curvature are commonly used for mapping structural deformation and depositional environment coherence proves useful for identification of faults channel edges reef edges and collapse features while curvature images folds flexures subseismic conjugate faults that appear as drag or as folds adjacent to faults rollover anticlines diagenetically altered fractures karst and differential compaction over channels unfortunately these two attributes have limited value in imaging classic seismic stratigraphy features such as progradation and erosional truncation seismic stratigraphy refers to the analysis of the configuration and termination of seismic reflection events packages of which are then interpreted as stratigraphic patterns these packages are then correlated to well known patterns such as toplap onlap downlap hummocky clinoforms and so forth which in turn represent architectural elements of a depositional environment mitchum et al 1977 by

rupture process of the february 4 1965 rat islands earthquake the great rat islands underthrusting earthquake mw 8 7 of february 4 1965 represents subduction of the pacific plate beneath the north american plate along a 600 km segment of the western end of the aleutian islands body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the rat islands earthquake we have deconvolved world wide standard seismograph network long period teleseismic p wave seismograms to obtain source time functions directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s unilateral rupture in the direction 300 fault length of 420 km and average rupture velocity of 2 5 km s the three pulses of moment release are located along the fault and these regions of high moment release are interpreted as asperities the first asperity extends from the epicenter to 100 km to the wnw this is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s the second pulse of moment release is very jagged is less coherent between stations and is centered 200 km wnw of the epicenter the third pulse of moment release extends from 360 to 420 km wnw of the epicenter although the aftershock area is 600 km in length we can not resolve any moment release from the p waves beyond 420 km wnw of the epicenter the rat islands event was closely followed by a large tensional outer rise event on march 30 1965 ms 7 5 which is located oceanward of the largest moment release associated with the rat islands mainshock rupture detailed analysis of the p waves for this large outer rise event confine the depth extent to the upper 30 5 km of the crust the spatial and temporal association between the february 4 mainshock and the march 30 tensional outer rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter the overriding plate along the western aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins geist et al 1988 we suggest that the central undeformed parts of the blocks have the strongest coupling with the down going plate and hence are the sites of the largest moment release during an underthrusting earthquake the three asperities determined from the p waves correspond to the rat buldir and near tectonic blocks respectively hence the p wave seismic moment release of the rat islands earthquake is controlled by the lateral segmentation of the overriding plate

determination of magnitude for large shallow earthquakes 1898 1917 abstract in order to arrive at a uniform basis for a study of the global seismicity the surface wave magnitude ms of shallow earthquakes for 1898 1917 is newly determined by using maximum amplitudes of surface waves recorded by undamped milne and omori seismographs the effective gain of these instruments needs to be calibrated with great care because of the lack of a damping device the calibration of milne instruments is based on a comparison of amplitudes measured by milne instruments with those by more modern damped instruments the effective gain varies from 7 to 20 in the range of ms from 6 to 8 5 the reported amplitudes for the years 1910 1912 are twice as large as the standard and need to be corrected by applying these results to a wealth of the milne data ms is determined for a large number of earthquakes between 1898 and 1912 to supplement the data for 1913 1917 records of undamped omori seismographs are used for the entire period from 1898 to 1917 308 shallow earthquakes with m s 7 0 are listed here both this list and the previous catalogue of abe 1981 for 1918 1980 provide a catalogue of large shallow earthquakes much more uniform than any previously established between 1899 and 1910 the annual frequency of shocks with m s 7 0 is 20 which is twice as large as the annual average of 10 during 1911 and 1980

the 1957 great aleutian earthquake the 9 march 1957 aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded 1200 km however due to a lack of high quality seismic data the actual source parameters for this earthquake have been poorly determined we have examined all the available waveform data to determine the seismic moment rupture area and slip distribution these data include body surface and tsunami waves using body waves we have estimated the duration of significant moment release as 4 min from surface wave analysis we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50 100 1020 nm using the tsunami waveforms we estimated the source area of the 1957 tsunami by backward propagation the tsunami source area is smaller than the aftershock zone and is about 850 km long this does not include the unalaska island area in the eastern end of the aftershock zone making this area a possible seismic gap and a possible site of a future large or great earthquake we also inverted the tsunami waveforms for the slip distribution slip on the 1957 rupture zone was highest in the western half near the epicenter little slip occurred in the eastern half the moment is estimated as 88 1020 nm or m w 8 6 making it the seventh largest earthquake during the period 1900 to 1993 we also compare the 1957 earthquake to the 1986 andreanof islands earthquake which occurred within a segment of the 1957 rupture area the 1986 earthquake represents a rerupturing of the major 1957 asperity

use of the mantle magnitude m m for the reassessment of the moment of historical earthquakes the mantle magnitudemm is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments which in many cases had been previously estimated only on the basis of the earthquake s rupture area we provide 27 new or revised values ofmo based on the spectral amplitudes of surface waves recorded at a number of stations principally uppsala and pasadena among them and most significantly we document a large low frequency component to the source of the 1923 kanto earthquake the low frequency seismic moment is 2 9 1028 dyn cm in accord with geodetic observations on the other hand we revise downwards the seismic moment of the 1906 ecuador event which did not exceed 6 1028 dyn cm

the great kurile earthquake of october 4 1994 tore the slab the fault geometry depth and slip distribution of the kurile earthquake of oct 4 1994 are estimated using seismic waveforms aftershock distribution geodetic measurements and tsunami waveforms previous large earthquakes in the kurile arc had typical underthrusting focal mechanisms due to the local subduction seismic wave inversions of the 1994 event indicate a thrust type mechanism with a large strike slip component this does not represent an underthrust event at the subduction interface the focal depth is estimated at 50 kin just beneath the previously estimated subduction interface we calculate crustal deformation and tsunami waveforms from the two possible fault planes of the focal mechanism solution and find that both fault planes explain the observations we choose the north south striking plane because of the occurrence of a large m w 7 8 intermediate depth earthquake on dec 6 1978 northwest of the 1994 rupture area the lineation of aftershock distributions of these two events suggests that both earthquakes were due to a tearing of the slab perpendicular to the trench the joint inversion of geodetic and tsunami data shows that the largest slip about 17 m occurred at the upper end of the slab

seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america a quantitative reappraisal the seismic potential of the chilean and southern peruvian margins of south america is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years 1984 2004 long term estimates of seismic potential or the conditional probability of recurrence within a specified period of time are based on 1 statistical analysis of historic repeat time data using weibull distributions and 2 deterministic estimates of recurrence times based on the time predictable model of earthquake recurrence both methods emphasize the periodic nature of large and great earthquake recurrence and are compared with estimates of probability based on the assumption of poisson type behavior the estimates of seismic potential presented in this study are long term forecasts only as the temporal resolution or standard deviation of both methods is taken to range from 15 to 25 of the average or estimated repeat time at present the valparaiso region of central chile 32 35 s has a high potential or probability of recurrence in the next 20 years coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971 1973 average repeat times also suggest this area is due for a great shock within the next 20 years flanking segments of the chilean margin coquimbo illapel 30 32 s and talca concepcion 35 38 s presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years in contrast the rupture zone of the great 1960 earthquake 37 46 s has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years in the north the seismic potentials of the mollendo arica 17 18 s and arica antofagasta 18 24 s segments which last ruptured during great earthquakes in 1868 and 1877 are also high but poorly constrained