reassessment of the 1907 sumatra tsunami earthquake based on macroseismic seismological and tsunami observations and modeling on 4 january 1907 an earthquake occurred off the west coast of sumatra indonesia with an instrumental surface wave magnitude ms in the range of 7 5 8 0 at periods of 40 s the tsunami it generated was destructive on the islands of nias and simeulue where it killed hundreds and gave rise to the legend of the s mong this tsunami was also observed in other parts of the indian ocean basin relative to its instrumented magnitude the size of the tsunami was anomalous qualifying the event as a tsunami earthquake however unusually for a tsunami earthquake the shaking on nias was severe 7 ems we revisit the 1907 earthquake with a multidisciplinary approach by extracting evidence describing shaking effects or the tsunami from written documents and by acquiring new seismograms combining these we discriminate two large earthquakes within an hour of each other with clear differences in seismological character the first we interpret to be a tsunami earthquake with characteristic low levels of shaking an estimated average seismic moment m0 of 2 5 1028 dyn cm mw 8 2 in the frequency band 6 8 mhz and an epicentral location close to the front of the sunda megathrust the seismograms we analyzed also document a regular growth of moment with period approaching mw 8 4 at the longest resolvable period 170 s for the second earthquake that caused damage on nias we estimate ms 7 based on seismograms and phase data we also identify two ms 6 aftershocks within 24 h of the mainshock additionally we present a dataset of 88 locations within the indian ocean basin where the tsunami was observed using a subset of these we forward modeled the tsunami to propose a seismic rupture model extending along the sunda megathrust for about 220 km 94 7 e to 97 e with a maximum modeled slip of 21 m our new rupture model provides an acceptable fit to our new dataset of tsunami runup and inundation values from 88 local and far field locations in the indian ocean basin we also urge caution against an over reliance on the s mong legend for tsunami evacuation as its premise that a tsunami will only follow an earthquake with very severe ground motions is rendered ineffective for tsunami earthquakes

crustal anisotropy in southern california from local earthquake data 1 shear wave splitting measurements were made for local earthquakes in southern california at incidence angles larger than the critical incidence angle for the region of san bernardino mountains the fast polarization directions vary strongly but are consistently fast for directions roughly perpendicular to the ray paths this observation is most readily explained by transverse isotropy with a vertical symmetry axis and is probably associated with horizontally foliated gneisses or schists in the upper crust other measurements show a predominance of north south fast directions while some data have fast directions that are parallel to the san andreas fault these observations are related to azimuthal anisotropy as found in other local shear wave splitting and sks splitting studies the data show that apart from the free surface effect shear wave splitting measurements for shear waves at shallow incidence angles can have an imprint of transverse isotropy as well as azimuthal anisotropy

the age and origin of the central scotia sea summary opening of the drake passage gateway between the pacific and atlantic oceans has been linked in various ways to cenozoic climate changes from the oceanic floor of drake passage the largest of the remaining uncertainties in understanding this opening is in the timing and process of the opening of the central scotia sea all but one of the available constraints on the age of the central scotia sea is diagnostic of or consistent with a mesozoic age comparison of tectonic and magnetic features on the seafloor with plate kinematic models shows that it is likely to have accreted to a mid ocean ridge between the south american and antarctic plates following their separation in jurassic times subsequent regional shallowing may be related to subduction related processes that preceded backarc extension in the east scotia sea the presence of a fragment of jurassic cretaceous ocean floor in the gateway implies that deep water connections through the scotia sea basin complex may have been possible since eocene times when the continental tips of south america and the antarctic peninsula first passed each other

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

new thermochronometric constraints on the rapid palaeogene exhumation of the cordillera darwin complex and related thrust sheets in the fuegian andes thermal modelling of new fission track and u th sm he data from the fuegian andes reveals rapid cooling 12 48 c ma 1 during the middle and late eocene followed by slow cooling 1 5 c ma 1 to the recent we interpret the rapid cooling as a result of exhumation from contractional uplift within the crystalline interior of the orogen this interpretation is consistent with independent evidence of eocene shortening flexural subsidence and provenance changes in the study area and is approximately coeval with marine geochemical evidence of the onset of drake passage opening in light of the palaeogene history of nazca south american plate convergence and the differences in shortening magnitudes and exhumation histories between the fuegian and patagonian andes our data support eocene development of the patagonian orocline which also provides a plausible explanation for early opening of drake passage

the tectonic crustal stress field and style of faulting along the pacific north america plate boundary in southern california we invert for the state of stress in the southern california crust using a catalogue of high quality earthquake focal mechanisms 1981 2010 the stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region from the stress field we determine the maximum horizontal compressive stress s hmax orientations and the style of faulting across southern california the trend of s hmax exhibits significant regional and local spatial heterogeneities the regional trend of s hmax varies from north along the san andreas system to nne to the east in the eastern california shear zone as well as to the west within the continental borderland and the western transverse ranges the transition zones from one state of stress to the other occur over a distance of only a few kilometres following a trend from yucca valley to imperial valley to the east and the western edge of the peninsular ranges to the west the local scale heterogeneities in the s hmax trend include nnw trends along the san andreas fault near cajon pass tejon pass and the cucapah range as well as nne trends near the northern san jacinto fault and the wheeler ridge area the style of faulting exhibits similar complexity ranging from predominantly normal faulting in the high sierra nevada to strike slip faulting along the san andreas system to three consecutive bands of thrust faulting in the wheeler ridge area and the western transverse ranges the local variations in the style of faulting include normal faulting at the north end of the san jacinto fault and scattered areas of thrust faulting the regional variations in the shmax trends are very similar to the pattern of the gps measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper mantle deformation field the mean trend of s hmax departs about approximately 14 to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle

continental stretching preceding the opening of the drake passage evidence from tierra del fuego age estimates for the onset of a seaway through the drake passage range from middle eocene to early miocene complicating interpretations of the relation between ocean circulation and cenozoic global cooling here we present evidence for the presence of a latest paleocene early eocene extensional basin i e lateral rift in tierra del fuego an accurately dated postrift unconformity indicates that extensional faulting ended in the studied area ca 49 ma in concurrence with a previously reported eightfold increase in south america antarctica separation rate and the proposed onset of oceanic basins in the incipient drake passage the coincidence of these facts indicates progressive strain concentration on the zone of future crustal separation i e drake passage after abandonment of lateral rifts ca 49 ma although the described extensional depocenters did not allow the exchange of water between the pacifi c and atlantic oceans they represent a direct indication of continental lithospheric stretching preceding the recently proposed eocene opening of embryonic basins in the west scotia sea

the tectonic history of drake passage and its possible impacts on global climate this study provides an integrated review of plate tectonic models of the evolution of the antarctica patagonia connection compared to geological records collected on land in patagonia and tierra del fuego and offshore along the northern edge of the scotia sea a temporal framework for the sedimentary and tectonic events of the north scotia ridge and tierra del fuego is constructed with additional data compiled from entire patagonia and the austral basin this review provides robust correlations of seaways and tectonic events along the scotia and south america plates and indicates that the opening of the drake passage was not steady state since ca 30 ma rather the regions forming the present day northern limit of this gateway experienced important paleogeographic changes from deep marine basins to shallow ridges and emerged regions during the late oligocene and early middle miocene time our compilation of geological data shows that emergence along the north scotia ridge and tierra del fuego was achieved at 23 22 ma and has been followed by elimination of the patagoniano sea in patagonia starting at 22 23 ma and achieved at 20 ma this transition towards more continental sedimentation in southern south america is correlated with more shallow marine conditions in the austral basin this succession of events had a strong influence on the general geometry of the drake passage corresponding to a constriction of its northern limit starting in the window 29 22 ma and achieved at 21 ma this period of active deformation in southern south america also corresponds to a period of the global climate having two anomalies well known from the isotopic records the late oligocene warming around 26 ma and the mid miocene climatic optimum which ended between 15 and 14 ma the possible effects of the post oligocene tectonic evolution of the drake passage region on general oceanic circulation are discussed causes for the synchronicity between tectonic events and these global warming events are examined

3 d rotation of double couple earthquake sources summary we discuss 3 d rotations by which one double couple earthquake source can be rotated into another arbitrary double couple due to the symmetry of double couple sources there are four such rotations an algorithm is obtained in analytical form which is also available as a computer program solving the inverse problem of 3 d rotation of double couple earthquake sources i e for each pair of focal mechanisms or seismic moment tensor solutions the program finds all four rotations which rotate one mechanism into another this algorithm may be used in a wide variety of studies of stress field causing earthquakes investigations of the relationship between the focal mechanisms and the tectonic features of a seismogenic region etc the same inversion algorithm can be used to study the 3 d rotation of any symmetric second rank tensor such as the stress or strain tensor

have the southernmost andes been curved since late cretaceous time an analog test for the patagonian orocline the kinematic evolution of the enigmatic arc shaped southernmost andes of patagonia and tierra del fuego has been a subject of debate for most of the past century we compared the results from analog sandbox experiments with the tectonic evolution and actual configuration of the mountain chain in order to elucidate whether oroclinal bending took place during the tertiary or if the southernmost andes have been a curved orogen since at least late cretaceous time experiments simulating oroclinal rotation produced strong along strike variations in shortening and failed to account for structural data compiled from the fuegian andes results from experiments simulating an l shaped concave to foreland indenter were in agreement with the known tertiary structural evolution of the southernmost andes the diachronicity of principal shortening events previously recognized in patagonia and tierra del fuego could only be reproduced by moving the indenter in two successive orthogonal directions first approximately northward to form the fuegian fold and thrust belt and then approximately eastward to propagate thrusting in the patagonian andes this two phase evolution is consistent with a recorded change in the convergence direction of the farallon nazca plate that occurred at ca 27 ma

automatic measurement of shear wave splitting and applications to time varying anisotropy at mount ruapehu volcano new zealand 1 we present an automatic shear wave splitting measurement tool for local earthquakes with the sole manual step of choosing an s arrival time we apply the technique to three data sets recorded on mount ruapehu volcano in new zealand that have previously been determined to have fast polarizations that vary in time and with earthquake depth the technique uses an eigenvalue minimization technique applied over multiple measurement windows the dominant period of each waveform sets minimum and maximum window lengths cluster analysis determines the best solution among all the windows and quality grading criteria assess the results automatically when the same filters are used for events determined to be high quality from manual studies the automatic technique returns virtually identical results scatter increases when the automatic technique chooses the best filter but the average automatic results remain consistent with the manual results when the automatic technique is used on sets that include data previously judged as poor quality some stations yield distributions of fast polarizations that include peaks that were not present in previously published results the difference may stem from two factors automatic grading lets through some measurements that independent analysts consider poor quality but also unconscious bias in the manual selection process may downgrade measurements that do not fit expectations nonetheless the new objective analysis confirms changes in the average fast polarizations between 1994 and 2002 and between shallow and deep events therefore this new technique is valuable for objective assessment of anisotropy and its variation in time

sks splitting beneath southern california measurements of sks phase splitting were obtained from nineteen seismic stations in southern california the fast polarization directions are 53 at the southern end of the great valley 82 8 in the western transverse ranges and northern peninsular ranges 95 4 in mojave desert and 70 on san clemente island the splitting time ranges from 0 8 to 1 8 seconds which is consistent with an anisotropic layer of 100 to 200 km thick for 4 anisotropy

mapping stress and structurally controlled crustal shear velocity anisotropy in california we present shear velocity anisotropy data from crustal earthquakes in california and demonstrate that it is often possible to discriminate structural anisotropy polarization of the shear waves along the fabric of major active faults from stress induced anisotropy polarization parallel to the maximum horizontal compressive stress stress directions from seismic stations located near but not on the san andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault in contrast seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust inducing shear wave polarizations parallel to the fault plane

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

modeling shear wave splitting due to stress induced anisotropy with an application to mount asama volcano japan we use numerical modeling to investigate the proposed stress based origin for changing anisotropy at mount asama volcano japan stress induced anisotropy occurs when deviatoric stress conditions are applied to rocks which are permeated by microcracks and compliant pore space leading to an anisotropic distribution of open crack features changes to the local stress field around volcanoes can thus affect the anisotropy of the region the 2004 eruption of mount asama volcano coincided with time varying shear wave splitting measurements revealing changes in anisotropy that were attributed to stress changes associated with the eruption to test this assertion we create a model that incorporates knowledge of the volcanic stress ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model anisotropy is calculated in two ways by considering a basic case of having uniform crack density and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress our results show that this approach is sensitive to crack density crack compliance and the regional stress field all of which are poorly constrained parameters in the case of dry crack closure results show that modeled stress conditions produce a much smaller degree of anisotropy than indicated by measurements we propose that the source of anisotropy changes at asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes such as the movement of pore fluid

deep embrittlement and complete rupture of the lithosphere during the m w 8 2 tehuantepec earthquake subduction zones where two tectonic plates converge are generally dominated by large thrust earthquakes nonetheless normal faulting from extensional stresses can occur as well rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending such earthquakes are limited to regions of the subducting slab cooler than 650 c and can be highly tsunamigenic producing tsunamis similar in amplitude to those observed during large megathrust events here we show from analyses of regional geophysical observations that normal faulting during the moment magnitude mw 8 2 tehuantepec earthquake ruptured the entire cocos slab beneath the megathrust region we find that the faulting reactivated a bend fault fabric and ruptured to a depth well below the predicted brittle ductile transition for the cocos slab including regions where temperature is expected to exceed 1 000 c our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension in the subducted slab possibly due to fluid infiltration we conclude that lithosphere can sustain brittle behaviour and fail in an earthquake at greater temperatures and ages than previously considered geophysical observations of the 2017 tehuantepec earthquake suggest that oceanic lithosphere can sustain brittle behaviour and rupture in an earthquake at greater depths than previously assumed

near fault anisotropy following the hector mine earthquake 1 we present anisotropy measurements from shear wave splitting along the hector mine rupture zone six major arrays were deployed in four locations in the year following the m7 1 hector mine earthquake the dense station coverage wide distribution of the arrays and repeated deployments show a clear predominant fast direction and spatial variation of splitting along the fault but no resolvable temporal variations we determined splitting parameters using an automated cross correlation method discarding fast directions with initial source polarizations near crack parallel or perpendicular directions only two of the four array locations give reliable measurements of anisotropy at depth fast directions and delay times are constant across the 1 km wide array length however some spatial variation of splitting is observed along fault strike delay times decrease from north to south with greater splitting in areas of higher slip a change in splitting parameters along fault strike likely reveals the orientation of cracking in the near fault region during a major quake average fast directions are between fault parallel and the regional maximum compressive stress direction we do not see temporal evolution in anisotropy however measured splitting suggests a heterogeneous stress field partially created during rupture that persists over at least a 1 year timescale

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

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

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

distinguishing between stress induced and structural anisotropy at mount ruapehu volcano new zealand 1 we have created a benchmark of spatial variations in shear wave anisotropy around mount ruapehu new zealand against which to measure future temporal changes anisotropy in the crust is often assumed to be caused by stress aligned microcracks and the polarization of the fast quasi shear wave is thus interpreted to indicate the direction of maximum horizontal stress but can also be due to aligned minerals or macroscopic fractures changes in seismic anisotropy have been observed following a major eruption in 1995 96 and were attributed to changes in stress from the depressurization of the magmatic system three component broadband seismometers have been deployed to complement the permanent stations that surround ruapehu creating a combined network of 34 three component seismometers this denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined using an automated shear wave splitting analysis we examine local earthquakes in 2008 we observe a strong azimuthal dependence of and so introduce a spatial averaging technique and two dimensional tomography of recorded delay times the anisotropy can be divided into regions in which agrees with stress estimations from focal mechanism inversions suggesting stress induced anisotropy and those in which is aligned with structural features such as faults suggesting structural anisotropy the pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using coulomb modeling with a dike like inflation source we suggest that the stress induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity

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

on an improved method to obtain the moment tensor and depth of earthquakes from the amplitude spectrum of rayleigh waves abstract a new method is presented to invert for the moment tensor and depth using the amplitude spectra of vertical component rayleigh waves in the period range 20 to 100 sec the technique follows a similar approach to that suggested by romanowicz 1982a to invert for the moment tensor from the complex spectra of rayleigh waves and presents some distinct advantages to the method proposed originally by mendiguren 1977 it eliminates some biases and errors in the data arising for example from inaccurate propagation corrections furthermore it is substantially faster computationally and permits us to study independently the variance reduction as a function of depth of each of the momenttensor elements resulting in better focal depth resolution the method is applied to three earthquakes in the tien shan mountains of central asia the eastern cordillera of peru and the gibbs transform fault in the north atlantic in all three cases the results of the moment tensor inversion agree with those determined using long period body wave modeling

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

the deficient t waves of tsunami earthquakes summary we develop an algorithm quantifying the energy flux of t phases recorded at island stations following major teleseismic events which we further scale by the seismic moment m0 of the earthquake to define a t phase efficiency we apply this concept to a set of six recognized tsunami earthquakes which generated tsunamis larger than expected from their conventional seismic waves through comparison with nearby reference events the t waves of which were recorded at the same sites we find that the tsunami earthquakes exhibit a deficiency in ranging from 1 5 to 2 5 orders of magnitude this result settles a 50 yr old controversy on the possible correlation between t wave generation and tsunami genesis the deficient character of the t waves from tsunami earthquakes readily supports the proposed model of an exceedingly slow rupture velocity for this class of events and the close examination of t wave trains supports the concept of a jerky rupture in at least two cases the computation of is straightforward in real time and could become a valuable contribution to real time tsunami warning in the far field

combining shallow and deep geophysical information the yehuin chepelmut fault zone in the magallanes fold and thrust belt tierra del fuego argentina the outer fuegian fold and thrust belt is the deformed sector of the austral magallanes basin which underwent a contractional phase since the late cretaceous and a strike slip phase during the neogene the area hosts two neogene basins yehuin and chepelmut which have been mainly developed by the influence of wrench deformation and were subsequently modified by glacial activity of the ewan and fuego glacier lobes of the fagnano palaeoglacier geophysical surveys using electrical resistivity tomography onland shallow seismic surveys and magnetometry have been combined with outcrop analysis and deep seismic lines to recognize both shallow and intermediate deep structures related to the lacustrine basins four sets of faults with different deformation history were involved in the basin genesis comparison with the deseado fault zone indicates similarities of the structural features between the two zones the stress field interpreted as responsible for the origin of the studied basins is analogous to the one in the deseado valley located in the western tierra del fuego therefore a transcurrent fault zone here named yehuin chepelmut fault zone is proposed as part of the diffuse boundary between south america and scotia plates

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

source parameters of the 1957 aleutian earthquake from tsunami waveforms we studied the source parameters of the 1957 aleutian earthquake using tsunami waveform data recorded on tide gauges around the pacific ocean using a finite difference computation this tsunami can be numerically simulated the tsunami records were inverted for the slip distribution on the rupture area results show that slip was concentrated in the western half of the aftershock zone with a maximum displacement of 7m the moment computed from the slip distribution is 88 1020 nm giving the 1957 earthquake a moment magnitude of mw 8 6 the waveform inversion confirms that no slip occurred in the unalaska island area making this area a possible seismic gap with a potential to rupture in a great earthquake and generate a pacific wide tsunami

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

the 2009 samoa tonga great earthquake triggered doublet great earthquakes having seismic magnitudes of at least 8 usually involve abrupt sliding of rock masses at a boundary between tectonic plates such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks as is commonly observed in the trench slope region seaward of a great subduction zone thrust event1 the earthquake sequence addressed here involves a rare instance in which a great trench slope intraplate earthquake triggered extensive interplate faulting reversing the typical pattern and broadly expanding the seismic and tsunami hazard on 29 september 2009 within two minutes of the initiation of a normal faulting event with moment magnitude 8 1 in the outer trench slope at the northern end of the tonga subduction zone two major interplate underthrusting subevents both with moment magnitude 7 8 with total moment equal to a second great earthquake of moment magnitude 8 0 ruptured the nearby subduction zone megathrust the collective faulting produced tsunami waves with localized regions of about 12 metres run up that claimed 192 lives in samoa american samoa and tonga overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries with the triggered thrust faulting only being revealed by detailed seismic wave analyses extensive interplate and intraplate aftershock activity was activated over a large region of the northern tonga subduction zone

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

the opening of the magellan strait and its geodynamic implications terra nova 25 13 20 2013 abstract the existence of a neogene quaternary rift system responsible for the opening of the eastern magellan strait has been one of the more peculiar tectonic features of the southernmost andes since its discovery more than a decade ago however until now there has been no explanation pinpointing the specific tectonic factors that led to the development of the rift this work focuses on the interaction between the fault systems and the tectonic regimes that have affected tierra del fuego north of the south america scotia left lateral strike slip plate boundary due to a good match between a jurassic extensional fault array orientation and the principal stress directions related to the strike slip plate boundary we conclude that jurassic transfer faults reactivated extensionally generating the magellan rift system this process was enhanced by northward continental drift and has implications for the geodynamic behind the separation of south america from antarctica

stress and structure induced anisotropy in southern california from two decades of shear wave splitting measurements we measure shear wave splitting sws parameters i e fast direction and delay time using 330 000 local earthquakes recorded by more than 400 stations of the southern california seismic network 1995 2014 the resulting 232 000 sws measurements 90 000 high quality ones provide a uniform and comprehensive database of local sws measurements in southern california the fast directions at many stations are consistent with regional maximum compressional stress hmax however several regions show clear deviations from the hmax directions these include linear sections along the san andreas fault and the santa ynez fault geological blocks nw to the los angeles basin regions around the san jacinto fault the peninsular ranges near san diego and the coso volcanic field these complex patterns show that regional stresses and active faults cannot adequately explain the upper crustal anisotropy in southern california other types of local structures such as local rock types or tectonic features also play significant roles

teleseismic estimates of radiated seismic energy the e m 0 discriminant for tsunami earthquakes we adapt the formalism of boatwright and choy for the computation of radiated seismic energy from broadband records at teleseismic distances to the real time situation when neither the depth nor the focal geometry of the source is known accurately the analysis of a large data set of more than 500 records from 52 large recent earthquakes shows that this procedure yields values of the estimated energy ee in good agreement with values computed from available source parameters for example as published by the national earthquake information center neic the average logarithmic residual being only 0 26 units we analyze the energy to moment ratio by defining log10 ee m0 for regular earthquakes this parameter agrees well with values expected from theoretical models and from the worldwide neic catalogue there is a one to one correspondence between values of that are deficient by one full unit or more and the so called tsunami earthquakes previously identified in the literature as having exceedingly slow sources and believed due to the presence of sedimentary structures in the fault zone our formalism can be applied to single station measurements and its coupling to automated real time measurements of the seismic moment using the mantle magnitude mm should significantly improve real time tsunami warning

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

historical seismograms for unravelling a mysterious earthquake the 1907 sumatra earthquake history of instrumental seismology is short seismograms are available only for a little more than 100 years high quality seismograms are available only for the last 50 years and the seismological database is very limited in time to extend the database seismograms of old events are of vital importance many unusual earthquakes are known to have occurred but their seismological characteristics are poorly known the 1907 sumatra earthquake is one of them 1907 january 4 m 7 6 gutenberg and richter located this event in the outer rise area of the sunda arc this earthquake is known to be anomalous because of its extensive tsunami which is disproportionate of its magnitude the tsunami affected the coastal areas over 950 km along the sumatran coast we investigated this earthquake using the historical seismograms we could collect from several seismological observatories we examined the p wave arrival times listed in the strassburg bulletin 1912 and other station bulletins the scatter of the observed computed traveltime residuals ranges from 30 to 30 s too large to locate the event accurately the uncertainty of the epicentre estimated from an s p grid search relocation study is at least 1 110 km we interpreted the omori seismograms from osaka mizusawa and tokyo and the wiechert seismograms from gottingen and uppsala by comparing them with the seismograms simulated from modern broad band seismograms of the 2002 2008 and two 2010 sumatra earthquakes which occurred near the 1907 earthquake from the amplitude of rayleigh waves recorded on the omori seismograms we conclude that the magnitude of the 1907 earthquake at about 30 to 40 s is about 7 8 i e 7 5 to 8 0 the sh waveforms recorded on the gottingen and uppsala seismograms suggest that the 1907 earthquake is a thrust earthquake at a shallow depth around 30 km the most likely scenario is that the 1907 earthquake initiated on the subduction interface and slowly ruptured up dip into the shallow sediments and caused the extensive tsunami although their quantity and quality are limited historical seismograms provide key quantitative information about old events that cannot be obtained otherwise this underscores the importance of preserving historical seismograms

split mode evidence for no ultraslow component to the source of the 2010 maule chile earthquake we present a comprehensive study of the earth s gravest spheroidal modes excited by the maule chile earthquake of 27 february 2010 using uninterrupted time series extending over a minimum of 19 and a maximum of 93 days for each of 101 station mode combinations we use the formalism of stein and geller 1977 to compute the relative excitation of the 2 l 1 singlets in the relevant geometry and obtain an estimate of the seismic moment by best fitting the observed spectrum to that of the resulting synthetic computed for the same recording window the average results for seven spheroidal and two radial modes deviate no more than 12 from the global centroid moment tensor moment of graphic 1 with no evident trend with frequency in other words we fail to document an ultralow frequency component expressed as an increase of moment with period in the source of the 2010 event this result indicates that such components are not universal features of megaquakes even though they had been documented for the 1960 chilean 2004 sumatran and 1964 alaskan events the only three events with larger moments in the past 50 years in this respect the 2010 earthquake is most comparable to the slightly smaller 2005 nias event which incidentally also featured a bilateral rupture 1 embed inline graphic 1 gif

s wave splitting in the aftershock region of the 1995 hyogo ken nanbu earthquake the 1995 hyogo ken nanbu earthquake kobe earthquake m7 2 occurred on january 17 1995 a temporary aftershock observation was carried out by the geophysical research group organized by universities for prediction seismology in 1995 from october 1995 to january 1996 a dense seismic network was deployed in and around the aftershock region and some seismic stations were set directly on the active fault traces we analyze the s wave splitting in and around the aftershock region of the hyogo ken nanbu earthquake we detect a spatial variation of s wave splitting which can be related to crack orientations in an area where a large earthquake has occurred at stations more than 1 km away from the hyogo ken nanbu earthquake fault zone the leading shear wave polarization directions lspd are found to be parallel to the axis of the regional maximum horizontal compressional stress e w suggesting that cracks caused by regional tectonic stress exist in the area however at stations within 500 m of the earthquake fault zones the lspds are parallel to the fault strikes n45 50 e this suggests that new fractures were produced parallel to the faults by shear faulting of the hyogo ken nanbu earthquake and resulted in s wave splitting along the earthquake fault zone nevertheless shear fault origin anisotropy was not evident at stations on the active faults which ruptured in 1596 this implies that healing processes have already closed the fractures produced by shear faulting in 1596 therefore we can expect that s wave splitting can be a useful tool for monitoring healing processes of active faults

apparent stress fault maturity and seismic hazard for normal fault earthquakes at subduction zones summary the behavior of apparent stress for normal fault earthquakes at subduction zones is derived by examining the apparent stress a es m 0 w here es is radiated energy and m 0 is seismic moment of all globally distributed shallow depth h 1m pa are also generally intraslab but occur where the lithosphere has just begun subduction beneath the overriding plate they usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis or where there are local contortions or geometrical complexities of the plate boundary lower a 1m pa i sassociated with events occurring at the outer rise or complex between the or and the trench axis as well as with intracrustal events occurring just landward of the trench the average apparent stress of intraslab normal fault earthquakes is considerably higher than the average apparent stress of interplate thrust fault earthquakes in turn the average a of strike slip earthquakes in intraoceanic environments is considerably higher than that of intraslab normal fault earthquakes the variation of average a with focal mechanism and tectonic regime suggests that the level of a is related to fault maturity lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements from hundreds to thousands of kilometres in contrast immature faults such as those on which intraslab normal fault earthquakes generally occur are found in cold and intact lithosphere in which total fault displacement has been much less from hundreds of metres to a few kilometres also faults on which high a oceanic strike slip earthquakes occur are predominantly intraplate or at evolving ends of transforms at subduction zones earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults we have identified earthquake pairs in which an interplate thrust and an intraslab normal earthquake occurred remarkably close in space and time the intraslab normal member of each pair radiated anomalously high amounts of energy compared to its thrust fault counterpart these intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which me an energy magnitude is shown to be a far better estimate of the potential for earthquake damage than mw this discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in latin america than the fatalities associated with interplate thrust events that represented much higher total moment release

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

progressive deformation of a coulomb thrust wedge the eastern fuegian andes thrust fold belt fil torres carbonell pablo juan consejo nacional de investigaciones cientificas y tecnicas centro austral de investigaciones cientificas argentina

seismic anisotropy in the shallow crust of the loma prieta segment of the san andreas fault system we study seismic anisotropy beneath the santa cruz mountains using digital recordings of s waves from 60 aftershocks of the 1989 loma prieta earthquake m 7 0 recorded at 18 local stations three component particle motion covariance matrix decomposition combined with a cross correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves results from this procedure are compared with those obtained through visual inspection of horizontal particle motions inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of s to p converted phases from our study uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20 and 10 ms respectively about half of the s wave records examined show clear evidence of shear wave splitting two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events 52 of the polarizations are parallel to the strike of the san andreas fault nw se and 24 are parallel to the probable direction of local maximum horizontal compressive stress ne sw the fault parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary the fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the san andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy eda where parallel alignment of fluid filled fractures produces the anisotropy the predominance of fault parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results the splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth this suggests that the seismic anisotropy is no deeper than 2 km the depth of the shallowest earthquakes analyzed for an anisotropic zone 2 km thick the average velocity anisotropy is about 5 in our study region

the large andaman islands earthquake of 26 june 1941 why no significant tsunami we present a modern seismological study of the earthquake of 26 june 1941 in the andaman islands the largest pre 2004 event along that section of the india burma plate boundary despite a large conventional magnitude m text pas 8 1 it generated at best a mediocre tsunami for which no definitive quantitative reports are available we show that the 1941 earthquake took place under the andaman accretionary prism and consisted of a composite event whose nucleating phase had a strike slip mechanism incompatible with a data set of spectral amplitudes of mantle rayleigh and love waves combining this initial phase with a larger normal faulting mechanism can reconcile them with p wave first motions reports of subsidence on the eastern coast of the andaman islands and the small amplitudes of any putative tsunami the small tsunami results from a combination of that mechanism and of a source located under the islands themselves and in shallow water implying a reduction in amplitude under green s law when transitioning to a deeper basin

stress induced upper crustal anisotropy in southern california 1 we use an automated method to analyze shear wave splitting from local earthquakes recorded by the southern california seismic network between 2000 and 2005 the observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression hmax suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern california is regional stress however at other stations fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock splitting delay times range widely within 0 2 s these upper crustal anisotropy observations together with previous studies of sks shear wave splitting surface waves and receiver functions suggest different mechanisms of anisotropy at different depths under southern california anisotropy in the upper crust appears to be in response to the current horizontal maximum compression hmax which differs from the cause of anisotropy in the lower crust and mantle we also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays

twenty five years of progress in the science of geological tsunamis following the 1992 nicaragua and flores events we review a set of 47 tsunamis of geological origin triggered by earthquakes landslides or volcanoes which have occurred over the past 25 years and provided significant new insight into theoretical experimental field or societal aspects of tsunami science among the principal developments in our command of various aspects of tsunamis we earmark the development of the w phase inversion for the low frequency moment tensor of the parent earthquake the abandonment of the concept of a maximum earthquake magnitude for a given subduction zone controlled by simple plate properties the development and implementation of computer codes simulating the interaction of tsunamis with initially dry land at beaches thus introducing a quantitative component to realistic tsunami warning procedures and the recent in situ investigation of current velocities in addition to the field of surface displacements during the interaction of tsunamis with harbors continued research remains warranted notably in the field of the real time identification of tsunami earthquakes whose tsunamis are larger than expected from their seismic magnitudes especially conventional ones the recent tragedy during the 2018 krakatau flank collapse along a scenario which had been quantitatively forecast also emphasizes the need for a continued effort in the education of the populations at risk

structure controlled seismic anisotropy along the karadere duzce branch of the north anatolian fault revealed by shear wave splitting tomography abstract we use a three dimensional 3d shear wave splitting sws tomography method developed by zhang et al 2007 to map the spatial distribution of crustal anisotropy in the karadere duzce branch of the north anatolian fault naf in western turkey the input data consists of 20 751 measurements of the sws delay times from 7856 aftershocks of the 1999 mw 7 4 izmit and mw 7 1 duzce earthquakes the results show a continuous belt like highly anisotropic zone along the karadere duzce branch of naf generally 3 km wide and down to 5 km deep the observed asymmetric pattern of anisotropy along the karadere segment is qualitatively consistent with asymmetric damages from the unilateral eastward propagation of the izmit rupture another strong anisotropy region was found near the end of the izmit rupture zone close to the dipping direction of the fault segment that ruptured during the subsequent duzce earthquake these results are generally consistent with the shallow fault zone anisotropy inferred from previous studies while the anisotropy generally becomes weaker at depth and or outside the fault zone we also identify several highly anisotropic regions as deep as 10 km primarily within the almacik block these isolated anisotropy regions could be explained by individual intrusive igneous rock bodies with different mineral alignment overall seismic anisotropy in upper crust of the karadere duzce branch of the naf is structurally controlled

a great earthquake doublet and seismic stress transfer cycle in the central kuril islands temporal variations of the frictional resistance on subduction zone plate boundary faults associated with the stick slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate here we report on two great earthquakes that occurred near the kuril islands which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting on 15 november 2006 an event of moment magnitude 8 3 ruptured the shallow dipping plate boundary along which the pacific plate descends beneath the central kuril arc the thrust ruptured a seismic gap that previously had uncertain seismogenic potential although the earlier occurrence of outer rise compressional events had suggested the presence of frictional resistance within minutes of this large underthrusting event intraplate extensional earthquakes commenced in the outer rise region seaward of the kuril trench and on 13 january 2007 an event of moment magnitude 8 1 ruptured a normal fault extending through the upper portion of the pacific plate producing one of the largest recorded shallow extensional earthquakes this energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

new paleomagnetic and 40ar 39ar geochronological results for the south shetland islands west antarctica and their tectonic implications

the depth of the deepest historical earthquakes we usep ands times listed in the international seismological summary to relocate 23 historical earthquakes 1927 1963 reported as occurring at or below 670 km in all cases our relocated hypocenters are shallower than the starting depths furthermore all events converge to 691 km or less with a precision estimated at 10 km this study upholds the results of stark and frohlich who had usedpp p times of post wwssn earthquakes to constrain reliable hypocentral depths to no greater than 684 km in particular we reject rothe s claim that a 1963 event in the vicinity of new guinea occurred at a depth of more than 780 km

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

the great 1933 sanriku oki earthquake reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

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

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

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

new insights into the tectono stratigraphic evolution of the malvinas basin offshore of the southernmost argentinean continental margin abstract a detailed tectono stratigraphic analysis of the malvinas basin was achieved by the interpretation of around 65 000 km of 2d seismic reflection profiles five main seismo stratigraphic units and their sub units informally named units u1 to u5 a b bound by major unconformities were identified and correlated with the mesozoic to cenozoic main tectonic phases of the basin unit u1 pre 168 ma represents the seismic basement unit u2 168 150 5 ma syn rift phase thickens and deepens southwards units u1 and u2 are affected by several syn rift normal faults that exhibit a main ne sw strike direction in the south of the basin and a nw se strike direction in the centre of the basin this suggests that the malvinas basin may have developed initially as a rift basin with two different extensional directions 1 a nw se directed extension probably linked with the opening of the weddell sea early mid jurassic and 2 a ne sw directed extension most likely linked with the jurassic back arc extension of gondwana and probably later with the onset of the opening of the south atlantic during late jurassic to early cretaceous time unit u3 150 5 68 ma sag phase is mainly an aggradational wedge shaped unit unit u4 68 42 5 ma transtensional foredeep phase overlies unconformable unit u3 it deepens in the south because of an n s nw se directed extensional regime accumulation rates decrease during units u2 u3 and u4 from 4 84 to 1 23 to 0 8 km3 ma 103 units u5a and u5b 42 5 5 5 and 5 5 0 ma transpressional foredeep phase finally represent a change from aggradation to progradation sedimentary pattern and to a left lateral transpressional regime in the south the sediment supply was considerably higher than before and a thick sedimentary wedge has been deposited until today accumulations rates increased in units u5a and u5b from 2 28 to 8 91 km3 ma 103

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999 m7 4 izmit and m7 1 duzce turkey earthquake sequences summary we analyse spatiotemporal variations of crustal anisotropy along the karadere duzce branch of the north anatolian fault from similar earthquakes in the aftershock regions of the 1999 mw7 4izmit and mw7 1 duzce earthquakes the similar earthquake clusters are identified by performing cross correlation on waveforms generated by 18 000 earthquakes depending on the applied similarity criterion about 4 60 per cent of the events belong to similar event clusters splitting parameters averaged within each cluster show significant variations for slightly different ray paths indicating strong spatial variations of crustal anisotropy in this area we also find clear changes in the spatiotemporal seismicity patterns following the duzce main shock large apparent co seismic changes up to 30 per cent of shear wave splitting delay times are observed across the time of the duzce main shock at stations near the epicentral region however the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity rather than changes in the properties of the anisotropic medium splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the duzce main shock the results do not show systematic precursory changes before the duzce main shock

timing and climatic consequences of the opening of drake passage age estimates for the opening of drake passage range from 49 to 17 million years ago ma complicating interpretations of the relationship between ocean circulation and global cooling secular variations of neodymium isotope ratios at agulhas ridge southern ocean atlantic sector suggest an influx of shallow pacific seawater approximately 41 ma the timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals circulation productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide these results also indicate that drake passage opened before the tasmanian gateway implying the late eocene establishment of a complete circum antarctic pathway

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

provenance of eastern magallanes foreland basin sediments heavy mineral analysis reveals paleogene tectonic unroofing of the fuegian andes hinterland abstract documenting variations in the composition of foreland basin detrital sediments is a useful tool for reconstructing the kinematic history of associated orogenic systems herein we report the results of our sem eds heavy mineral analysis of the late cretaceous to early miocene eastern magallanes basin of southernmost argentina our data indicate that campanian to middle eocene sediments had a mafic ophiolitic provenance which we interpret as being derived from the patagonian fuegian magmatic arc and the mafic floor of the preceding rocas verdes marginal basin upper middle eocene to lower miocene heavy minerals on the other hand indicate a metamorphic metasedimentary provenance which we interpret as being derived from the cordillera darwin metamorphic complex together these data indicate an abrupt shift in sediment provenance in middle to late eocene time thereby corroborating recent interpretations of the basin s detrital zircon geochrononology and providing further support for temporal and possibly genetic relationships between development of the patagonian orocline the opening of drake passage and the oi 1 glaciation of antarctica

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

early foreland deformation of the fuegian andes argentina constraints from the strain analysis of upper cretaceous danian sedimentary rocks abstract this contribution addresses new structural data from upper cretaceous danian sedimentary rocks of the fuegian andes orogenic front in argentina the structures studied called d were formed during the early foreland deformation of the orogen in the late cretaceous danian in times when south america and antarctica were still connected the strain analysis of these structures indicates that deformation occurred at upper crustal depths and was characterized by flexural folding accompanied by formation of pressure solution tectonic foliations in zones of higher strain deformation intensities increase toward the hinterland and with depth the history of deformation involved progression from layer parallel shortening to folding above a detachment and further formation of a forward propagating thrust wedge layer parallel shortening and incipient folding recorded in maastrichtian danian rocks indicate the leading edge of d deformation non coaxial finite strain orientations were involved during formation of d structures along the fuegian andes front these finite strain orientations cannot be explained with the sw ne regional contraction usually assumed to have driven andean deformation in this region alternatively we consider that n s contraction combined with buttressing against the cratonic foreland comprise a more suitable interpretation for d and younger deformation

crustal and upper mantle anisotropy related to fossilized transpression fabric along the denali fault northern canadian cordillera we analyze crustal and upper mantle structure near the denali fault northern canadian cordillera using 11 broadband seismic stations receiver functions at five stations within 5 30 km of the fault trace display a strong p to s conversion within the midcrust 10 25 km depth that systematically varies with back azimuth stacking and velocity inversion along complementary northwest southeast and northeast southwest back azimuth ranges yield a strong anisotropy 10 in the midcrust with complex crustal anisotropic behavior at least two layers close to the denali and duke river faults junction anisotropy occurs in a low velocity zone with the fast axis parallel to the denali fault trend three additional stations close to the denali fault 15 30 km as well as two stations further away 100 km show no significant crustal anisotropy shear wave sks splitting analysis indicates similar upper mantle anisotropy with a fast axis parallel to the trend of the denali fault for all stations except for station why located 125 km away we relate this crustal and upper mantle anisotropy to fossilized structural and mineral fabrics due to eocene transpression on the denali fault these results suggest the main transpression phase 400 km displacement was accommodated in a shear zone at least 50 60 km less than 125 km wide in the midcrust and upper mantle lack of clear anisotropy in the lowermost crust may relate to complex deformation within a detachment layer orogenic float model online material table of sks splitting information and figures of receiver function analysis

paleomagnetic evidence for a pre early eocene 50 ma bending of the patagonian orocline tierra del fuego argentina paleogeographic and tectonic implications abstract the southernmost segment of the andes of southern patagonia and tierra del fuego forms a 700 km long orogenic re entrant with an interlimb angle of 90 known as patagonian orocline no reliable paleomagnetic evidence has been gathered so far to assess whether this great orogenic bend is a primary arc formed over an articulated paleomargin or is due to bending of a previously less curved or rectilinear chain here we report on an extensive paleomagnetic and anisotropy of magnetic susceptibility ams study carried out on 22 sites 298 oriented cores predominantly sampled in eocene marine clays from the external magallanes belt of tierra del fuego five sites out of six giving reliable paleomagnetic results containing magnetite and subordinate iron sulphides yield a positive fold test at the 99 significance level and document no significant rotation since 50 ma thus the patagonian orocline is either a primary bend or an orocline formed after cretaceous earliest tertiary rotations our data imply that the opening of the drake passage between south america and antarctica probably causing the onset of antarctica glaciation and global climate cooling was definitely not related to the formation of the patagonian orocline but was likely the sole consequence of the 32 2 ma scotia plate spreading well defined magnetic lineations gathered at 18 sites from the magallanes belt are sub parallel to mostly e w local fold axes while they trend randomly at two sites from the magallanes foreland our and previous ams data consistently show that the fuegian andes were characterized by a n s compression and northward displacing fold thrust sheets during eocene early miocene times 50 20 ma an unexpected kinematics considering coeval south america antarctica relative motion both paleomagnetic and ams data suggest no significant influence from the e w left lateral magallanes fagnano strike slip fault system mffs running a few kilometres south of our sampling sites we thus speculate that strike slip fault offset in the fuegian andes may range in the lower bound values 20 km among those proposed so far in any case our data exclude any influence of strike slip tectonics on the genesis of the great orogenic bend called patagonian orocline