destructive earthquake and disastrous tsunami in the indian ocean what next abstract a major earthquake event and a disastrous tsunami struck on december 26 2004 in the indian ocean which led to the death of nearly 250 000 people within a short period of time this has given a major jolt not only to the scientists but also to the authorities planners and various national and international organizations discussions regarding what to do and how to cope with such situations are taking place and also on various plans and programmes for the future we have analyzed the available scientific data related to the sumatra earthquake and tsunami and also the earlier disastrous events of the tsunami this includes seismicity of the region numerical modeling studies and crustal deformation studies based on earlier reports of disastrous events warning systems related to earthquakes and tsunamis we suggest scientific studies that need to be taken up in the near future in the region our study will provide some direction for the ongoing discussions to prepare better plans and programmes in the indian ocean by various south east asian countries

coda q distribution in the canary islands abstract the canary archipelago near the african plate between 27 and 30 n and 13 and 19 w is included in the oldest region 135 m y of the eastern atlantic ocean the tectonic seismicity is moderate with low magnitudes and it is almost totally related to an active fault located between the islands of tenerife and gran canaria the purpose of this paper is to provide a clear idea of the anelasticity of coda waves in the region in order to compute master dispersion curves as a function of the quality factor q we have applied inversion methods to obtain analytical magnification curves for all the seismographic stations used fp t data corresponding to the coda wave dispersion of 52 local and regional seismic events recorded at 6 seismographic stations have been obtained the fit of the data to the master curves determined yields estimates of coda q values and their frequency dependence most of the q0 values determined at 1 hz vary between 100 and 300 showing a strong frequency dependence in the range 0 2 2 hz the characteristic v value degree of frequency dependence of q in the region given by the law q q0 fv is v 0 9 by applying regionalization techniques an iso q0 map for a reference frequency of 1 hz has been obtained for the canary archipelago this map shows important lateral variations in q0 reflecting the tectonics and the volcanism of the region the lowest q0 values are found along the trend of the fault and beneath the islands

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

temporal variations of qc 1 and b values associated to the december 2000 january 2001 volcanic activity at the popocatepetl volcano mexico abstract we correlated temporal variations of coda attenuation q c 1 at 6 hz and b values with the 11 december 2000 23 january 2001 major eruptive activity at the popocatepetl volcano mexico we interpreted a total of 225 volcanic earthquakes with magnitudes between 1 9 and 3 3 located between 2 and 8 km from the summit of the volcano and registered from january 1999 to june 2001 q c 1 was estimated using a single scattering model and values of b were obtained applying the method of maximum likelihood our results indicate that from january 1999 to march 2000 period 1 q c 1 varied within the same range without significant changes then values of q c 1 steadily decreased during april mid december 2000 period 2 from about mid december 2000 onset of the major volcanic activity up to february 2001 period 3 q c 1 sharply increased reaching its peak value from march to june 2001 period 4 q c 1 decreased and increased in response to the post eruptive phase of dome formation and destruction process that took place during periods 2 and 3 average b values behaved opposite to the q c 1 trend within period 2 when q c 1 decreased b slowly increased at the start of period 3 b reached its peak value and returned to low values by the end of this time interval we associate the temporal changes of the two analyzed parameters to variations of pressure and or the variation in the degree of saturation of pore fluids that took place before during and after the major eruption during periods 2 and 3 q c 1 correlated negatively with b with a correlation coefficient of 0 61 and 0 83 respectively the significance of correlation cannot be rejected at the confidence level of 0 975 our observations may have important implications for future prediction studies of large eruptions at the popocatepetl volcano

aftershock propagation characteristics during the first three hours following the 26 december 2004 sumatra andaman earthquake abstract within three hours of the mainshock rupture of the 26 december 2004 sumatra andaman earthquake 45 aftershocks occurred that are distributed all along the mega thrust fault plane and also along the west andaman fault seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50 76s indicating an overall rate of aftershock propagation to the tune of 167 meters sec seismic moment calculated from fault parameters gives a value of 1 2 1030 dyne cm three separate fault segments are identified from distribution of aftershocks with propagation rates 330 250 and 85 meters sec in the southern central and northern segments these 7 unilaterally propagating shocks along the mega thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously

seismic coda q and scaling law of the source spectra at the aeolian islands southern italy the model developed by aki and chouet for the coda wave generation and propagation has been used to calculate the quality factor q for the zone of the aeolian islands southern italy in the frequency range of 1 to 12 hz and the scaling properties of the seismic spectrum in the magnitude range of 0 4 to 4 7 the q found for the aeolian area has a frequency dependence of the form q qfv the absolute values of q seem to be dependent on the station and location of the seismic events confirming the strong lateral heterogeneities in the geological structure beneath the aeolian arc a temporal variation has been noted in the q calculated at vulcano station vpl in a period of 3 weeks soon after the occurrence of a main shock of ml 5 5 located near the station the scaling behavior of this sequence is similar to that obtained in two areas of california and one portion of japan with a corner frequency that remains constant with an increasing seismic moment between magnitudes 1 and 4 it differs substantially from the scaling properties of the hawaian earthquakes that show a linear pattern without an increase of the stress drop with magnitude the fact that vulcano is an active volcano seems not to influence the scaling properties of the seismic sequence localized very near it it probably indicates that the aftershocks used for calculating the scaling law are generated out of the volcanic complex lipari vulcano in a zone with a good capability of accumulating the stress

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

numerical modeling of thermal structure circulation of h2o and magmatism metamorphism in subduction zones implications for evolution of arcs abstract numerical models on thermal structure convective flow of solid generation and transportation of h2o rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes heat balance circulation of h2o magmatism metamorphism growth of arcs and continental margins a large scale convection model with steady subduction of a cold old slab 130 myr old predicts rapid 100 myr cooling of subduction zones resulting in cessation of magmatism the model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab i e the age of the subducting slab if subduction of a young hot slab including ridge subduction occurs every 60 to 120 myr as is suggested for eastern asia the average temperature beneath arcs is increased by about 300 c which may explain the long lasting magmatism in eastern asia associated with subduction of young slabs and ridges thermal structure and circulation of h2o are greatly modified to cause a transition from 1 normal arc magmatism 2 forearc mantle melting to 3 slab melting to produce a significant amount 100 km3 of granitic melts associated with both high p t and low p t type metamorphism the last stage of 3 can result in formation of a granitic batholith belt and a paired metamorphic belts synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths synchronous formation of batholith and regional metamorphic belts and p t conditions of the paired metamorphism even the high p t metamorphism requires an elevated geothermal structure in the forearc region associated with ridge subduction although the emplacement of the batholiths and the regional metamorphic belts and the mass balance in subduction zones are not well constrained at present the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins as well as for the long term heat balance in subduction zones

induction models of southern india and the effect of off shore geology abstract simple numerical tests based on thin sheet modelling are used to define the conductors most probably responsible for current channelling in the region including southern india a combination of crustal conductors located in the indo ceylon graben and underneath the comorin ridge best reproduces the observations at periods of 50 100 min in so far as channelling is concerned daytime variations do not help to define the location of conductors any better than night time data and may thus be ignored

qp structure of mount etna constraints for the physics of the plumbing system 1 in this study we present the three dimensional qp structure of the mount etna volcano italy obtained by using local earthquake data the attenuation along ray paths t computed from the high frequency decay of velocity spectra is used to reconstruct the deep structure of the volcano down to 15 km depth the tomographic images reveal two broad low qp anomalies at 0 and 3 km depth located to the south and southwest of the summit area and extending at depth to the west of a central high qp anomaly located between 6 and 15 km depth the joint analysis of p wave attenuation and velocity allows us to better constrain the physics of the plumbing system the shallow low qp anomalies are associated with high vp anomalies beneath the top of the volcano and normal low vp toward its western borders we interpret the regions of low qp and normal low vp as shallow volumes where magmatic fluids are stored conversely the low qp high vp region may indicate intensely fractured rock volumes filled by fluids surrounding the magma branches at depth the low qp anomaly defines the path of the magma ascent to the west of an extremely high qp high vp volume located beneath the southeastern side of the summit area the high qp high vp body is interpreted to be the compact solidified high density intrusive body formed by nonerupted material during past volcano activity

q c of three component seismograms of volcanic microearthquakes at campi flegrei volcanic area southern italy digital recordings of three component microearthquake codas from shallow seismic events in the volcanic region of campi flegrei southern italy were used with an automatic technique to calculate the attenuation factorq c codaq in the hypothesis of singles tos backscattering results show the same value ofq for each of the three components this result is interpreted as due to isotropics wave radiation pattern a check of the coda method was performed using a single station method based on simple assumptions on the direct sh wave spectrum single stationq was averaged over the stations and over the earthquakes results show that the two methods lead to comparable results a frequency dependence quite different from that evaluated in active tectonic regions was found for coda attenuation comparable to other volcanic areas throughout the world this is interpreted as due to the presence of magma that affects anelasticity and scattering

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

spatial variations of seismic attenuation in the north west of iranian plateau from analysis of coda waves abstract in this study spatial variation of coda wave attenuation in the north western nw iranian plateau is explored using local earthquakes single backscattering method is used to map the spatial variations of coda q qc using 14 969 earthquakes which are recorded digitally by sixteen permanent stations observed spatial attenuation maps are well correlated with geological setting of study area north west of iranian plateau is dominated with high attenuation which is well correlated with quaternary deposits as well as the sahnad sabalan volcanic zone and tabriz fault these observation is in accordance with numerous 2d velocity tomograms which have shown very low velocity in the nw iran the region characterized by volcanic structures exhibits mainly higher seismic attenuation than the surrounding regions whereas paleo tethys basin in the eastern part of covered area are categorized by smaller attenuation than the other zone our observation shows good correlation with reported lower crustal low velocity region beneath the sahand and sabalan volcanos the sahand and sabalan volcanoes are very large structures and they dominate the pliocene quaternary magmatic landscape of nw iran in addition our observation has clearly shown existence of a probable fault zone trending in ne sw direction between tabriz city and urmia lake which is consistent with the trend of meizoseismal zone of the deh khareqan modern azarshahr earthquake m 6 8 in 1641

focal parameters of seismic sources during the 1981 and 1983 eruption at mt etna volcano sicily italy in this study 50 seismic events preceding and accompanying the eruptions occurring in 1981 and 1983 have been considered seismic moments fault radii stress drops and seismic energies have been calculated using brune s model j geophys res 75 4997 5009 1970 j geophys res 76 5002 1971 site anelastic attenuation along the propagation path geometrical spreading and interaction with the free surface effects are taken into account for each event we have also estimated the equivalent wood anderson magnitude mwaeq scherbaum and stoll in bull seism soc am 73 1321 1343 1983 relations among all these source parameters have been determined furthermore the hypothesis of self similarity aki in j geophys res 72 1217 1231 1967 is not verified for events with seismic moments 1012 n m in fact the relationship between log stress drop and log moment is linear up to a moment of 1012 n m events of 1981 eruption while for higher moments events of 1983 eruption the slope of the regression line is not significantly different from zero we suppose that such a behaviour is related to a heterogeneous medium with barriers on the faults finally the main conclusion is that eruptions of 1981 and 1983 differ from one another both in eruptive and seismic aspects analysis of seismic energies indicates an increase in mt etna s activity confirmed by studies performed on the following lateral eruption of 1991 1993 patane et al in bull volcanol 47 941 952 1995 occurring on the same structural trend

temporal change in coda q at nevado del ruiz volcano colombia abstract an analysis of coda q q c intrinsic q q i scattering q q s and total q q t was made at nevado del ruiz volcano nrv for local earthquakes recorded during 1992 1994 coda q has been calculated for the first time at nevado del ruiz volcano nrv using the single scattering model proposed by aki and chouet 1975 estimations of q c were made for five frequency bands centred at 1 5 3 0 6 0 12 0 and 24 0 hz the estimated q c varies from around 60 at 1 5 hz to around 980 at 24 hz depending on the frequency with q c 30 1 2 f 1 03 0 001 in general q c values at nrv are rather similar to those obtained in other volcanic areas in the world a steady trend in q c 1 was found from january 1992 to january 1994 this variation is coincident with the stability in the volcanic activity since the last phreato magmatic eruption on 1 september 1989 in the middle of april 1994 a new activity started at nrv and a small volcanic crisis occurred continuous increasing was observed in q c 1 around two months before the crisis this suggests a possibility that the temporal variation of q c can be useful for monitoring the volcanic activity at nrv q i and q s were separated from q t by using a multiple scattering model called multiple lapse time window analysis mltwa fehler et al 1992 estimations for q i q s and q t were made for four frequency bands 3 0 6 0 12 0 and 24 0 hz q i values were found to be near to those of q c for frequencies higher than 6 0 hz for low frequencies q s values were near to q c values this can suggest that the intrinsic absorption could be the predominant attenuation phenomenon in the nrv area at frequencies higher than 6 hz and the scattering phenomenon is predominant for low frequencies therefore the change in q c at high frequencies is considered to reflect variations in the inner conditions of the volcano such as changes in temperature movement of fluids and so on

seismicity and plate deformation below the andaman arc northeastern indian ocean abstract the seismic activity originating below the andaman arc sea region is generally discernible into fore and back arc seismic zones which are traceable for nearly 1500 km in a n s direction at the junctures between the indian burma and se asia plates the fore arc seismicity displays an east dipping 40 55 benioff zone upto about 200 km focal depths details of the benioff zone in correspondence to the observed gravity field are discussed in four n s sectors which suggest some significant variations in the configuration of the benioff zone the back arc seismicity affects only the top 40 45 km of the lithosphere below the andaman sea where the back arc spreading ridge splits the volcanic arc stress distribution and faulting due to earthquakes below the andaman west sunda arc are studied here using 68 focal mechanism solutions their most significant results are low angle thrust events occur along the upper edge of the descending indian plate downdip tensional events have steeply dipping 60 nodal planes and normal faulting takes place in most parts of the benioff zone along moderately dipping 30 45 planes downdip compressional events high angle reverse fault nodal plane dip 60 or reverse faulting along moderately dipping 30 45 nodal planes also occur below the andaman arc the compressive earthquakes dominate the shallower level of the subducting slab and the tensional stress observed locally in north part of the andaman sea may be an outcome of the weak coupling between the descending and overriding plates generally a more or less complete sequence of faulting i e thrusting below the trench normal faulting below the fore arc and strike slip motion along the inner edges of the fore arc characterize the andaman west sunda arc in the southern andaman region a rather oblique convergence between the indian ocean and the se asia plates is needed to explain the existence of a somewhat contorted benioff zone in which compressional stress dominates in deeper lithosphere oceanward the ninetyeast ridge also impinges on the subduction zone in this region left lateral shear motion along the east margin of the ninetyeast ridge is further inferred by the results of focal mechanism solutions

anatomy of a forearc submarine fan upper eocene oligocene andaman flysch group andaman islands india abstract detailed facies analysis in the upper eocene oligocene andaman flysch group reveals fourteen different facies grouped into three different associations these facies associations represent two anatomical divisions of a submarine fan viz inner fan and depositional lobe in middle fan paleocurrent pattern high percentage of quartz in the sandstones and outer arc derived clasts in the inner fan channel conglomerates suggest dual sediment supply in this fan a longitudinal geometry of the basin has been inferred where juxtaposition of sediments of different proximality deterred development of any definite sequence pattern a tectonically induced active sediment supply in a rising sea level stand is thought to be responsible for development of the fan

rise and fall of the eastern great indonesian arc recorded by the assembly dispersion and accretion of the banda terrane timor abstract new age petrochemical and structural data indicate that the banda terrane is a remnant of a jurassic to eocene arc trench system that formed the eastern part of the great indonesian arc the arc system rifted apart during eocene to miocene supra subduction zone sea floor spreading which dispersed ridges of banda terrane embedded in young oceanic crust as far south as sumba and timor in timor the banda terrane is well exposed as high level thrust sheets that were detached from the edge of the banda sea upper plate and uplifted by collision with the passive margin of nw australia the thrust sheets contain a distinctive assemblage of medium grade metamorphic rocks overlain by cretaceous to miocene forearc basin deposits new u pb age data presented here indicate igneous zircons are less than 162 ma with a cluster of ages at 83 ma and 35 ma 40ar 39ar plateau ages of various mineral phases from metamorphic units all cluster at between 32 38 ma these data yield a cooling curve that shows exhumation from around 550 c to the surface between 36 28 ma after this time there is no evidence of metamorphism of the banda terrane including its accretion to the edge of the australian continental margin during the pliocene these data link the banda terrane to similar rocks and events documented throughout the eastern edge of the sunda shelf and the banda sea floor

variation of seismic coda wave attenuation in the garhwal region northwestern himalaya seismic coda wave attenuation q text c 1 characteristics in the garhwal region northwestern himalaya is studied using 113 short period vertical component seismic observations from local events with hypocentral distance less than 250 km and magnitude range between 1 0 to 4 0 they are located mainly in the vicinity of the main boundary thrust mbt and the main central thrust mct which are well defined tectonic discontinuities in the himalayas coda wave attenuation q text c 1 is estimated using the single isotropic scattering method at central frequencies 1 5 3 5 7 9 12 16 20 24 and 28 hz using several starting lapse times and coda window lengths for the analysis results show that the q text c 1 values are frequency dependent in the considered frequency range and they fit the frequency power law q text c 1 left f right q 0 1 f n the q0 qc at 1 hz estimates vary from about 50 for a 10 s lapse time and 10 s window length to about 350 for a 60 s lapse time and 60 s window length combination the exponent of the frequency dependence law n ranges from 1 2 to 0 7 however it is greater than 0 8 in general which correlates well with the values obtained in other seismically and tectonically active and highly heterogeneous regions the attenuation in the garhwal region is found to be lower than the qc 1 values obtained for other seismically active regions of the world however it is comparable to other regions of india the spatial variation of coda attenuation indicates that the level of heterogeneity decreases with increasing depth the variation of coda attenuation has been estimated for different lapse time and window length combinations to observe the effect with depth and it indicates that the upper lithosphere is more active seismically as compared to the lower lithosphere and the heterogeneity decreases with increasing depth

attenuation and source parameters of shallow microearthquakes at mt etna volcano italy we estimated the attenuation laws of high frequency seismic waves in the shallow crust depth 5 km and earthquake source parameters by using a se lected data set of 320 shallow events 2 6 md 4 2 recorded at mt etna volcano during the last two flank eruptions occurring in 2001 and 2002 2003 the quality factor q was estimated from spectra of p and s waves for 24 stations of the local permanent network by applying a spectral ratio technique the results show varia tions in both qp and qs as a function of frequency according to the power law q q0f n with n ranging between 0 3 and 1 3 for p waves and between 0 2 and 0 9 for s waves as typical of volcanic environments strong azimuthal variations of qp were also found suggesting the presence of local strong lateral heterogeneities and or of fluid filled cracked volumes after correction for attenuation we estimated the source parameters seismic moment source radius and stress drop of a subset of 66 shallow events under the assumption of a circular dislocation the estimated seismic moments m0 range from 10 13 to 10 15 n m the source radii r are confined between 100 and 1000 m and stress drop dr ranges between 0 2 mpa and about 4 mpa combining the source parameters obtained in this study with those calculated by pataneet al 1997 for an old data set of smaller microearthquakes 10 9 m0 10 14 n m recorded in the same area we re evaluated the scaling relationship between seismic moment m 0 and corner frequency f c for the earthquakes with m 0 ranging between 10 13 and 10 15 n m we confirm that microearthquakes at mt etna seem not to obey a

attenuation of short period seismic waves at etna as compared to other volcanic areas codaq for etna volcano is frequency dependent and theq frequency pattern and the numerical values ranging from about 100 at 1 hz to about 300 at 18 hz are similar to the values obtained for other volcanoes campi flegrei aeolian islands and hawaii moreover the frequency pattern and the numerical values of coda quality factor for most of the seismically active zones of italy are very different from those of the volcanic zones

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

stress associated intrinsic and scattering attenuation from laboratory ultrasonic measurements on shales seismic attenuation is sensitive to stress induced subtle changes in the physical state of rocks in this study the stress and frequency associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses as an improvement to the single scattering model the elastic monte carlo method is employed to investigate multiple scattering attenuations by incorporating the boundary reflections and wave conversions our results show that as the axial stress increases the intrinsic attenuation decreases in all directions while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions these discrepancies result from different attenuation mechanisms both the intrinsic and scattering attenuation are found to be largest in the direction 45 to the bedding but least in the perpendicular direction the s wave attenuation is larger and more sensitive to stress changes than p wave attenuation due to its shorter wavelength as expected from sandstone examples the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation the frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses but varies in different directions of an individual rock with different heterogeneity and anisotropy scales the peak frequency of s coda is smaller and its peak scattering attenuation is larger than p coda in conclusion the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions indicating significant anisotropy and heterogeneity

stratigraphic paleoecology bathymetric signatures and sequence overprint of mollusk associations from upper quaternary sequences of the po plain italy upper quaternary sequences of the po plain italy were used to assess the informative strength and sequence stratigraphic overprint of quantitative paleoecological patterns three densely sampled cores 89 samples 98 genera 23 280 specimens dominated by extant mollusk species with known environmental distributions were analyzed with detrended correspondence analysis dca the dca scores calibrated using extant genera provided outstanding estimates of bathymetry 3 m and related environmental parameters depth related successions of mollusk associations delineated by using dca were consistent with independent sequence stratigraphic interpretations and yielded insights inaccessible via routine techniques e g depth estimates for maximum flooding surfaces the dca ordination demonstrates the severity of the sequence stratigraphic overprint samples are highly uniform taxonomically during late transgressive systems tracts and highly variable during the following highstand systems tracts when analyzed across comparable systems tracts similar species associations repeat during the last and current interglacial cycles suggesting that po plain mollusk associations have remained remarkably stable over the past 125 k y the results are consistent with the bathymetric interpretation of the dc axis 1 postulated previously for the paleozoic fossil record demonstrate the sequence stratigraphic overprint of paleoecological patterns predicted by computer modeling and illustrate the utility of quantitative paleoecological patterns in augmenting sequence stratigraphic interpretations

attenuation of coda waves at the tres virgenes volcanic area baja california sur mexico we estimated the coda wave attenuation quality factor q c at the tres virgenes volcanic area from 26 local seismic events recorded by a portable network of seven digital stations the quality factor q c was estimated using the single scattering attenuation model of sato 1977 in the frequency range from 6 to 24 hz the magnitudes of the analyzed events range from 1 to 3 these events occurred at hypocentral depths ranging from 3 to 6 km and were recorded at epicentral distances of up to 30 km within the variation of q c six out of seven stations analyzed showed quite similar values at all frequencies data from these six stations led to an average q c given by q c f 50 3 f 0 65 0 20 in the indicated frequency range for the seventh station which is located near a steam production well anomalous low q c values were obtained for this particular site we found q c f 4 3 0 6 f 1 33 0 05 in this zone we observed an abnormally strong reduction of q c leading to attenuation values that are three to four times larger than for the whole area of study results from other geophysical studies suggest the presence of a highly conductive body below this particular area a correlation of the observed coda attenuation with a geothermal anomaly is suggested manuscript received 27 february 2000

facies clustering in turbidite successions case study from andaman flysch group andaman islands india abstract bed thickness data of two turbidite sections viz corbyn s cove section south andaman and kalipur section north andaman those belong to oligocene andaman flysch group a forearc submarine fan system were assessed for facies clustering employing hurst statistics both the sections show hurst phenomenon and reveal clustering in terms of thick and thin beds forcing behind event bed depositions in either of the studied sections was assessed statistically and inferred to be non random and with cyclicities of irregular physical length the inferred paleogeography through hurst criteria though worked well for distal fan setting i e basin floor sheet sandstones of corbyn s cove section its unequivocal application in proximal fan deposits remains to be tested the mismatch in paleogeographic interpretation between hurst test result lobe interlobe and field observation channel levee for the inner fan deposit is explained through differential facies stacking between fans grow in sea level lowstand and highstand lower bed amalgamation poor sand to mud ratio and subordinately present thick event deposits may be the result of active growth of andaman flysch fan in sea level highstand and expressed in lower hurst k value for inner fan channel levee association kalipur section compared to many of the channel levee deposits of lowstand fan systems observed world over

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

aftershock investigation in the andaman nicobar islands of india and its seismotectonic implications six three component short period digital seismograph four reftek and two kinemetrics stations were established in different parts of the andaman nicobar islands following the 26 december 2004 devastating sumatra andaman mainshock m w 9 3 here we analyze about 18 000 aftershocks m 3 0 recorded from 6 january to 16 march 2005 to better understand the seismotectonics of the region a sudden burst of aftershock activity with irregular trend in the month of january 2005 was observed at almost all seismograph stations the estimate of p value 0 9532 from aftershocks m 4 5 is near to normal value of 1 0 which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system creep effects and history dependent stress changes the frequency magnitude relation of the aftershocks followed the power law with average b value of 0 7723 and it varies from 0 49 to 1 03 indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault the distribution of the located aftershocks by a multistation method shows a north south trending aftershock cluster in an area of about 800 300 km 2 which reflects an approximate rupture dimension of the mainshock beneath the andaman nicobar islands most of aftershocks occurred in a depth range of 5 65 km the determination of composite fault plane solutions of the best located aftershock clusters at three different depth ranges 0 15 16 30 and 31 km in the ten blocks of the region suggests that the mainshock rupture propagated through normal reverse and strike slip earthquakes the areas in the vicinity of barren and narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces suggesting the facilitation of brittle failure in the weakened crust by the process of underheating the zone near baratang mud volcano is associated predominantly with thrust fault at depth of 0 30 km suggesting a high compressive force beneath the baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 december 2004 sumatra andaman earthquake m w 9 3 here we propose that the relocation of our large aftershock dataset using converted sp depth phase technique for precise depth control is needed and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region

regional conductance map of andaman and nicobar region abstract to image the electrical conductivity distribution fluxgate magnetometers are operated at five sites in andaman and nicobar region transfer functions are estimated for the period range 8 128 min from nighttime transient geomagnetic variations using robust regression analysis the observed induction arrows in andaman islands are found to point towards east despite deep sea located towards its west this indicates that fore arc basin andaman nicobar deep is more conducting than the region of outer non volcanic island arc thin sheet model requires the conductance of 10 000 35 000 s with increase conductivity towards the south for explaining the observed induction pattern the observed induction pattern at andaman nicobar stations can be explained in terms of high conducting cretaceous tertiary sediments filling the andaman nicobar deep high conductivity over invisible bank has been attributed to the partial melts volatile fluids derived from the subducting indian plate that are intruding into the eastern margin of fore arc basin through the west andaman fault waf the induction pattern at great nicobar station campbell bay may be related to the highly conducting sediments filling the mergui basin along with mafic intrusions also crustal transition occurs below the mergui terrace at the malayan coast contributing to the enhanced conductivity anomaly

regional electromagnetic induction around the indian peninsula and sri lanka a three dimensional numerical model study using the thin sheet approximation abstract further to other recent numerical studies of geomagnetic induction in southern india the thin sheet algorithm developed at the university of victoria has been used to investigate the three dimensional problem of induction by a uniform source over the indian peninsula and sri lanka with this algorithm it is not necessary to assume that the region is surrounded by a normal structure but the limitations imposed by grid size and the given conductivities for the crust and mantle have restricted our study to an inducing field of 20 min period solutions have been computed for two different models 1 with only the conductivity contrast arising from land and seawater of variable depth present and 2 with additional sub surface conductors included in the palk strait gulf of manowar region underneath or near the comorin ridge and along the west coast fault results are presented for uniform magnetic sources oriented in both the northward and westward directions to distinguish the effect of different polarizations the correspondence between the observed and computed parkinson arrows at the southern array stations suggests the existence of the three sub surface conductors

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions