seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

how do asperities fracture an experimental study of unbroken asperities abstract the fracture of a shear fault containing several unbroken asperities in a granitic porphyry is examined in detail via acoustic emission ae data collected by a high speed multi channel waveform recording system the experimental results reveal that a quasi static nucleation of the shear faulting corresponds to the fracture of coupled asperities on the fault plan aes caused by the fracture of individual asperities exhibit similar characteristics to the sequence for natural earthquakes including foreshock mainshock and aftershock events foreshocks initiated at the edge of the asperity occur with an event rate that increases according to a power law of the temporal distance to the mainshock and with a decreasing b value from 1 1 to 0 5 one or a few mainshocks then initiate at the edge of the asperity or the front of the foreshocks the aftershock period is characterized by a remarkable increase and subsequent gradual decrease in b value and a decreasing event rate obeying the modified omori law which has been well established for earthquakes the fracture of neighboring asperities is then initiated after the mainshock of a particular asperity presumably due to redistribution of the strain energy accumulated within an asperity which is released by the mainshock resulting in enhancement of the stress concentration around the nearest neighboring intact asperities the progressive fracturing of multiple coupled asperities during the nucleation of shear faulting results in short term precursory fluctuations in both b value and event rate which may prove useful information in the prediction of failure of the main fault plane of seismic earthquakes

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

sole marks in storm event beds in the permo carboniferous talchir formation raniganj basin india abstract sole marks in storm event beds are described for the first time from talchir formation permo carboniferous of raniganj basin india gutters and grooves trending subparallel to wave ripple crests were formed by shore parallel geostrophic flow at the initial phase of storms during intensification of storms oscillation dominated shore normal current produced gutters grooves and bipolar bimodal prod marks and bounce marks at nearly perpendicular or highly oblique to the axis of major wave ripple crests superposition of shore normal oscillatory flow on shore parallel geostrophic current during peak storms is evident from curvilinear crosscutting and turnabout groove marks hydrodynamic interpretation of these sole marks interrelationship between palaeogeographic constraints governing the overall sediment dispersal pattern and the stratification architecture in storm event beds signify complex interplay of mutually perpendicular shore parallel and shore normal components of storm current in shallow marine setting

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

slab pileup in the mantle transition zone and the 30 may 2015 chichi jima earthquake the 30 may 2015 chichi jima m8 earthquake is one of the largest deep focus earthquakes ever recorded and its depth of 682 km puts it near the base of the mantle transition zone before source mechanisms and slip models of this earthquake can be reliably assessed a better understanding of the tectonic setting and structures of the region near the origin is required here we present evidence from receiver functions a method of isolating subsurface material contrast with converted seismic waves that the earthquake initiated within the upper mantle transition zone above a significantly depressed 660 km phase boundary additionally we observe multiple conversions within and below the transition zone which we associate with seismic waves passing into and out of segments of the subducting pacific plate from this we infer slab material is piling up at the base of the transition zone and segments are penetrating into the lower mantle

fine details of the wadati benioff zone under indonesia and its geodynamic implications using about 5200 handpicked p s pp sp pcp and scp phases from digitally recorded seismograms together with international seismological centre reported phases we obtain improved hypocentral locations for 1790 earthquakes 983 of them having 90 confidence limits 50 km in the period 1962 to september 1996 along the indonesian subduction zone we use an improved joint hypocenter determination method in which the solution is more stable than in the original one we show that for deep earthquakes use of the core reflected phases is as good as use of depth phases and since such core phases often have larger amplitudes and sharper onsets than the depth phases their arrival times can be read very accurately the positions of the relocated hypocenters show that 1 a portion of the indonesian arc between 110 e and 123 e longitude and deeper than 500 km is dipping southward at an angle of 75 that is in a direction opposite to the upper part of the north dipping slab and 2 east of about 108 e the seismic zone is wider near 670 km than near the 500 km depth the first suggests southward lateral flow in the mantle relative to the plate motion vector here from the contortion of the seismic zone along the eastern portion of the indonesian arc we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10 16s 1 over the last 10 20 myr

shelf storm flow dynamics insight from the mesoproterozoic rampur shale central india abstract the intracratonic rift related mesoproterozoic rampur shale in kudri india formed in the marine mid and outer shelf domain it bears a dominant imprint of shore parallel storm driven flow as predicted by oceanographers for modern shelves and thus differs from other ancient storm deposits in the geological record in the rampur shale gutters are shore parallel while tool marks of various kinds are both shore parallel and shore normal and show bipolarity in both directions hydrodynamic analysis and inter relationships between the flow features suggest that the storm driven flow was shore parallel dominantly unidirectional and steady initially and that it became increasingly unsteady thereafter while also becoming dominantly shore normal during peak oscillation as storm conditions waned the gutters were filled by sediments under dominant wave influence these rampur data are consistent with observations made in modern seas and suggest that storm driven flows in general have evolved in the same way over time although their geological records may vary in details because of local constraints

triggering of the 2014 mw7 3 papanoa earthquake by a slow slip event in guerrero mexico since their discovery two decades ago slow slip events have been shown to play an important role in accommodating strain in subduction zones however the physical mechanisms that generate slow slip and the relationships with earthquakes are unclear slow slip events have been recorded in the guerrero segment of the cocos north america subduction zone here we use inversion of position time series recorded by a continuous gps network to reconstruct the evolution of aseismic slip on the subduction interface of the guerrero segment we find that a slow slip event began in february 2014 two months before the magnitude m w 7 3 papanoa earthquake on 18 april the slow slip event initiated in a region adjacent to the earthquake hypocentre and extended into the vicinity of the seismogenic zone this spatio temporal proximity strongly suggests that the papanoa earthquake was triggered by the ongoing slow slip event we demonstrate that the triggering mechanism could be either static stress increases in the hypocentral region as revealed by coulomb stress modelling or enhanced weakening of the earthquake hypocentral area by the slow slip we also show that the plate interface in the guerrero area is highly coupled between slow slip events and that most of the accumulated strain is released aseismically during the slow slip episodes

unusually deep bonin earthquake of 30 may 2015 a precursory signal to slab penetration abstract an m7 9 earthquake occurred on 30 may 2015 at an unusual depth of 680 km downward and away from the well defined wadati benioff wb zone of the southern bonin arc to the north northern bonin the subducted slab is stagnant above the upper lower mantle boundary at 660 km depth where the wb zone bends forward to sub horizontal to the south northern mariana it penetrates the boundary where the wb zone extends near vertically down to the boundary thus the southern bonin slab can be regarded as being in a transitional state from slab stagnation to penetration the transition is shown to happen rapidly within the northern half of the southern bonin slab where the heel part of the shoe like configured stagnant slab hits the significantly depressed 660 km discontinuity the mainshock and aftershocks took place in this heel part where they are sub vertically aligned in approximate parallel to their maximum compressional axes here the dips of the compressional axes of wb zone earthquakes change rapidly across the thickness of the slab from the eastern to western side and along the strike of the slab from the northern to southern side suggesting rapid switching of the downdip compression axis in the shoe shaped slab elastic deformation associated with the wb zone seismicity is calculated by viewing it as an integral part of the slab deformation process with this deformation the heel part is deepened relative to the arch part and is compressed sub vertically and stretched sub horizontally a tendency consistent with the idea of progressive decent of the heel part in which near vertical compressional stress is progressively accumulated to generate isolated shocks like the 2015 event and eventually to initiate slab penetration

statistical methods for comparing directions to the orientations of focal mechanisms and wadati benioff zones we propose simple graphical methods for evaluating whether a set of direction vectors are clustered with respect to particular directions axes or planes as examples of these techniques we present data concerning the relationship between 1 aftershock initial event geometry and nodal planes of focal mechanisms 2 aftershock initial event geometry and the plane of the wadati benioff zone and 3 b axes of focal mechanisms and the plane of the wadati benioff zone

a multiple sulfur and organic carbon isotope record from non conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup south africa abstract multiple sulfur isotope ratios 36 s 34 s 33 s 32 s and organic carbon isotope ratios 13 c 12 c were measured from 198 non conglomeratic sedimentary samples that were collected from five deep diamond drill cores that cover the majority of the ca 2 96 2 82 ga witwatersrand supergroup 13 c org 34 s py and 33 s py values of the sample set range from 44 3 to 21 9 3 7 to 16 5 and 4 0 to 1 9 respectively these geochemical data vary relative to depositional facies proximal marine distal marine and fluvial dominated and tectonic setting trailing margin and foreland basin in the trailing margin setting of the 2 96 ga hospital hill subgroup the proximal marine depofacies is characterized by relatively high organic carbon contents up to 0 9 wt and 13 c org values around 28 in contrast to the distal marine depofacies that yields low organic carbon contents 0 01 wt and high 13 c org values up to 22 both depofacies yield low sulfur contents 0 02 wt a narrow range of 34 s values 3 2 and positive 33 s values up to 1 9 this data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production preservation in deeper water settings due to longer transport distances and effective biological degradation e g fe reducing microbes positive 33 s values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir in the foreland basin tectonic setting of the 2 94 ga government and 2 92 ga jeppestown subgroups shelf deposits associated with fluvial braidplain depofacies are characterized by 13 c depleted organic carbon 44 to 38 relatively high sulfur contents 0 2 1 3 wt variable 34 s values 3 7 to 16 5 and small negative 33 s values 0 4 these data suggest that the microbial community in the fluvial dominated depofacies may have consisted of photoautotrophs methanogens anaerobic methanotrophs and sulfate reducers sulfate was derived from a variety of sources that include photochemical crustal and marine sulfur reservoirs the occurrence of three regionally persistent diamictite deposits in the government subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly 13 c depleted organic carbon suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the mesoarchean glaciations drawdown of h 2 and ch 4 however the link between large 33 s anomalies 4 0 to 1 2 and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures

gutenberg richter law for deep earthquakes revisited a dual mechanism hypothesis deep earthquake b values appear to vary with slab thermal state and earthquake magnitude the physical reason for the variations and the relation with deep rupture mechanisms are still unclear here i confirm the spatial variations of b value and the dependence on slab temperature using about 40 yr of data from the global centroid moment tensor catalog a new bimodal pattern is observed for the 500 700 km depth range b is close to 1 in the cold tonga slab while in warmer slabs e g south america japan kuril izu bonin mariana b is close to 0 5 for intermediate magnitudes m w 5 3 6 5 and increases to 1 for large magnitudes m w 6 5 to explain these observations i propose a dual mechanism hypothesis in which deep earthquakes nucleate only within the metastable olivine wedge mow but can rupture outside mow by a different mechanism the fractal dimension of earthquake size distribution changes from 2 to 1 as the thermally controlled mow thickness decreases and back to 2 as the mechanism outside mow dominates

wrinkle structures a critical review abstract in this paper a variety of so called wrinkle structures is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces and true microbially induced wrinkle structures two types of small scale microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence 1 elephant skin textures characterized by reticulate patterns of sharp crested ridges forming mm to cm scale polygons occurring on argillite or argillaceous veneers above fine grained sandstone and likely reflecting growth structures of microbial mats 2 kinneyia structures characterized by mm scale flat topped winding ridges and intervening troughs and pits sometimes resembling small scale interference ripples kinneyia structures usually occur on upper surfaces of siltstone sandstone beds themselves frequently event deposits and are thought to have formed beneath microbial mats additionally more linear variations of mat growth structures partly resembling small scale petees may be developed finally some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved these may appear as arcuate belts of non penetrative small scale folds or as wrinkled bulges on otherwise flat surfaces wrinkle structures as indicators for the former presence of mats gain in importance if other mat related structures are additionally observed in the same clastic succession e g sand chips sandy intraclasts or spindle shaped or sinuously curved to circular sand cracks frequently combined in networks furthermore appropriate lithologies and facies are required for instance if compared with the distribution of modern cohesive microbial mats laminated siltstone argillite with intercalated siltstone sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development within a variety of physically induced small scale wrinkly structures miniature load structures may above all be misinterpreted as microbially induced wrinkle structures due to their similar size and appearance and their comparatively frequent occurrence

kaapvaal craton changing first and second order controls on sea level from c 3 0 ga to 2 0 ga abstract well preserved supracrustal basin fills including the oldest large depository witwatersrand known makes the kaapvaal craton an ideal example for studying controls on first and second order stratigraphic cyclicity the duration of the three basins studied witwatersrand ventersdorp and transvaal from c 3 0 to c 2 0 ga is almost twice that of the entire phanerozoic period from which the paradigms of classical sequence stratigraphy have been derived amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through earth history this does not appear to be borne out by the precambrian record studied here instead a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first and second orders is preferred first order cycles within the c 3 0 2 0 ga period on kaapvaal are synonymous with the entire evolution of a supracrustal basin fill defined by a specific basin type second order cycles define basin fill packages of unique character which reflect large scale shifts in the accommodation sedimentation balance early to middle archaean evolution of the kaapvaal craton encompassed amalgamation of a mosaic of subdomains to produce an early south southeastern nucleus by c 3 1 ga accretion of composite terranes from both north and west followed from c 3 0 2 7 ga this was contemporaneous with evolution of the c 3 0 2 8 ga witwatersrand basin the high grade limpopo terrane was amalgamated with the kaapvaal craton between 2 95 and 2 69 ga although a younger age for this orogensis is also possible the witwatersrand first order depositional cycle was controlled largely by subduction related tectonic loading and had a duration of c 190 my the c 5 my first order cycle of the c 2 7 ga ventersdorp basin was essentially controlled by thermal uplift induced extensional subsidence and that of the c 650 my transvaal basin by extensional and subsequent thermal subsidence it is noteworthy in the three basins examined that first order cycles controlled by plate tectonics witwatersrand and transvaal basins were about two orders of magnitude longer relative to the ventersdorp plume tectonic cycle second order cycles also varied greatly from c 100 my to c 1 my within the witwatersrand and transvaal basins subsidence allowed extensive seaways to develop within which classical sequence stratigraphic systems tracts can be recognized reflecting different stages of shoreline shifts within the short lived ventersdorp continental basin system sedimentological criteria can be used to discriminate high and low accommodation systems tracts the transvaal first order cycle was terminated at c 2 0 ga by the bushveld complex plume event

deep earthquake rupture histories determined by global stacking of broadband p waveforms 1 we conduct a systematic investigation of time histories of moment release in deep earthquakes whose physical mechanism remains unknown we have constructed source time functions stfs of 111 large deep earthquakes depth 100 km and mw 6 4 by stacking p waveforms from broadband seismograms recorded by the global seismographic network aligning and stacking waveforms cancels noise and allows us to identify the rupture initiation and termination times accurately each stf is scaled to a reference moment of 1019 n m using the duration moment relation m01 3 in order to investigate depth dependence of the stfs we bin them into three depth ranges 100 350 km 350 550 km and deeper than 550 km based on physical properties of the mantle and the distribution of the data we infer a change in character near 550 km earthquakes deeper than about 550 km are shorter have simpler time functions and are more uniform in their characteristics further we find longer than expected scaled durations and greater complexity in the 350 550 km depth range the scaled durations of the 350 550 km earthquakes do not decrease with depth as 1 vs instead possessing a slightly longer average scaled duration than those 100 350 km deep we further categorize the earthquakes by subduction zone and find a similar depth dependence within individual zones finally the events that have achieved highest moment rate in the first 1 2 s after initiation tend to be deeper and or larger events the latter suggesting that dynamic rupture propagation affects the final size of the earthquake the depth dependence of duration and complexity suggests more complicated rupture histories between 350 and 550 km and a possible change in the physical mechanism of rupture at about 550 km

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

storm event beds in a paleoproterozoic rift basin aravalli supergroup rajasthan india abstract storm event beds in the paleoproterozoic riftogenic sedimentary succession of aravalli supergroup are described from a 12 8 m thick sandstone mudstone interbedded unit in zawar area rajasthan india the storm event beds include different primary structural assemblages indicating deposition from waning storm current sequential arrangement of beds with characteristic primary structural assemblages suggests deposition under a transgressive phase and overall retrogradational evolution of the storm succession provides evidence in favour of faster downsagging of the basin floor the pb zn sulphide ore bearing sedimentary succession of zawar records repeated downsagging and exhumation of the basin floor in the frame of continental rift tectonics

alluvial fan to storm dominated shelf transition in the mesoproterozoic singhora group chattisgarh supergroup central india abstract mesoproterozoic siliciclastic strata at the basal part of the singhora group central india provide an example of a pre vegetation alluvial fan and braid delta depositional system detailed process based facies and paleoenvironmental analysis have led to the identification of 15 facies types grouped under four different facies associations which record paleoenvironmental settings ranging between continental alluvial fan and distal marine shelf two formations investigated are the rehtikhol formation formed in a conglomeratic or coarse arenaceous alluvial fan braid plain and delta setting and the overlying saraipalli formation which preserved the sedimentation record of a storm infested inner to distal shelf setting the preponderance of granular and sandy sediment gravity flow products with subordinate rock fall deposits within the alluvial fan system together with a pervasive west northwesterly paleocurrent is suggestive of a gently sloping beveled source area in the east southeast distal to the inferred fan a braid plain system developed with the decrease in the depositional slope streams within the braid plain had widths up to several hundred meters the braid plain system at the margin of the marine basin developed a delta with the deposition of wave reworked fluvial dominated sediments the crest trends of wave generated bedforms within the inferred delta sediments depict the northeast southwest shoreline interpreted for the singhora sea the prodeltaic basinal part of the singhora braid delta was storm infested the inner shelf acted as a zone of throughput with high velocity storm flows eroding deep narrow scours gutter casts and is interpreted as a zone of bypass operation of geostrophic currents on the singhora shelf has been inferred from the directional relationship of gutters with the inferred orientation of the shoreline a proximal distal relationship has been established within the shelf products based on the relative dominance of erosional and depositional features the singhora alluvial fan and braid delta facies are arranged in a fining upward sequence denudation of the source area increasing bathymetry with relative rise in sea level or a combination of both is inferred both extrabasinal and intrabasinal forcings possibly contributed to the deepening upward trend

preslip and cascade processes initiating laboratory stick slip recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a cascade up process to explore such a case in the laboratory we study the initiation of dynamic rupture stick slip of a smooth saw cut fault in a 76 mm diameter cylindrical granite laboratory sample at 40 120 mpa confining pressure we use a high dynamic range recording system to directly compare the seismic waves radiated during the stick slip event to those radiated from tiny m 6 discrete seismic events commonly known as acoustic emissions aes that occur in the seconds prior to each large stick slip the seismic moments focal mechanisms locations and timing of the aes all contribute to our understanding of their mechanics and provide us with information about the stick slip nucleation process in a sequence of 10 stick slips the first few microseconds of the signals recorded from stick slip instabilities are nearly indistinguishable from those of premonitory aes in this sense it appears that each stick slip begins as an ae event that rapidly 20 s grows about 2 orders of magnitude in linear dimension and ruptures the entire 150 mm length of the simulated fault we also measure accelerating fault slip in the final seconds before stick slip we estimate that this slip is at least 98 aseismic and that it both weakens the fault and produces aes that will eventually cascade up to initiate the larger dynamic rupture

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

constraining the source location of the 30 may 2015 mw 7 9 bonin deep focus earthquake using seismogram envelopes of high frequency p waveforms occurrence of deep focus earthquake at the bottom of a subducting slab in this study the source location of the 30 may 2015 mw 7 9 deep focus bonin earthquake was constrained using p wave seismograms recorded across japan we focus on propagation characteristics of high frequency p wave deep focus intraslab earthquakes typically show spindle shaped seismogram envelopes with peak delays of several seconds and subsequent long duration coda waves however both the main shock and aftershock of the 2015 bonin event exhibited pulse like p wave propagations with high apparent velocities 12 2 km s such p wave propagation features were reproduced by finite difference method simulations of seismic wave propagation in the case of slab bottom source the pulse like p wave seismogram envelopes observed from the 2015 bonin earthquake show that its source was located at the bottom of the pacific slab at a depth of 680 km rather than within its middle or upper regions

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

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

the zambezi delta mozambique channel east africa high resolution dating combining bio orbital and seismic stratigraphies to determine climate palaeoprecipitation and tectonic controls on a passive margin abstract the zambezi delta draining the southern african plateau and the southern part of the east african rift is one of a the largest delta of africa with a long lasting history starting during early cretaceous with more than 12 km of sediments deposited the zambezi delta is therefore a unique archive of the past topographic evolution of southern and eastern africa and their related deformations but also of the climate changes global and regional consequences of local topographic growths understanding this archive supposes to get a high resolution dating of the sediments our two objectives are here 1 to construct an age model of the zambezi cenozoic delta using a combination of biostratigraphy orbital stratigraphy and sequence stratigraphy and 2 to determine the palaeoprecipitation variations of the zambezi catchment from the oligocene to present day in a known tectonic framework the neogene sequences were dated at high resolution assuming that the third order sequences are of eustatic origin and record long term eccentricity cycles the sequences were correlated in ages on the calculated earth orbital solutions of laskar for the time intervals provided by the biostratigraphy nannofossils planktonic foraminifers the palaeoprecipitation record was based on the definition of a humidity index based on pollen analysis and associated botanical associations the late oligocene was a quite wet period getting dryer in the uppermost chattian the base tortonian 11 ma was a humid period the messinian was a dry period with a slight increase of the humidity during the zanclean and a sharp increase around the zanclean piacenzian boundary the zambezi delta has recorded the uplifts of the southern african plateau around 85 ma and around 25 ma and those of the southward migration of the east african rift since 5 5 ma

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

precambrian continental freeboard and geological evolution a time perspective abstract continental crustal growth rates crustal volumes thicknesses and continental freeboard are considered to be intimately related however none are easy to quantify least of all in the precambrian additionally crustal volumes thicknesses and isostasy are related to changing mantle heat and the concomitant variation in thickness of ocean crust another set of variables would be the interaction of plate tectonics and the onset of plate tectonics a contentious issue in itself and mantle plumes superplumes further complication is provided by the ranges of freeboard elevations and eustatic sea level changes overlapping in scale and sharing common genetic causes the constant freeboard model suggests that the net results of the interaction of this complex set of variables has been that continental elevation relative to sea level has remained essentially similar on a global average basis over much of geological time assuming an essentially constant ocean volume since c 3 8 ga application of this model to the precambrian geological record suggests that sigmoidal crustal growth rate models are probably the most reliable a conundrum of the constant freeboard model would be that freeboard would tend to become lowered towards mean sea level for any specific craton over time this freeboard equilibrium would be interrupted for a particular craton by active geodynamic processes resulting from the interplay of plate tectonics and mantle thermal instabilities in this paper we test the validity of this thesis of alternating approximate freeboard equilibrium and active geodynamics by examining the volcano sedimentary record of a number of cratons kaapvaal singhbhum and pilbara for the singhbhum craton and bearing in mind its poor chronological data base freeboard equilibrium appears to have prevailed until c 2 1 ga but for kaapvaal and pilbara until c 1 8 ga the geodynamic processes largely outweighed the tendency towards equilibrium except for c 200 my from c 2 6 to 2 4 ga when global sea level appears to have remained high and transgression was the norm we thus tentatively add the plate tectonic cycle and mantle thermal processes to the already complex association of concepts controlling continental freeboard over time

repeating deep earthquakes evidence for fault reactivation at great depth we have identified three groups of deep earthquakes showing nearly identical waveforms in the tonga slab relocation with a cross correlation method shows that each cluster is composed of 10 to 30 earthquakes along a plane 10 to 30 kilometers in length some of the earthquakes are colocated demonstrating repeated rupture of the same fault and one pair of events shows identical rupture complexity suggesting that the temporal and spatial rupture pattern was repeated recurrence intervals show an inverse time distribution indicating a strong temporal control over fault reactivation runaway thermal shear instabilities may explain temporally clustered earthquakes with similar waveforms located along slip zones weakened by shear heating earthquake doublets that occur within a few hours are consistent with events recurring before the thermal energy of the initial rupture can diffuse away

storm wave development of shore normal grooves gutters on a steep sandstone beach face abstract shore normal grooves gutters cut into the seabed have been reported widely from the marine geological record grooves commonly are spaced regularly across plane consolidated surfaces in the littoral and sub littoral zones and may be deeply incised despite their common occurrence in the rock record there are few detailed descriptions of examples from modern environments previously reported examples have been ascribed to erosion by wave induced currents especially storm driven near shore flows in particular examples from beach faces have been related to either wave swash or backwash however no conceptual model exists to explain the presence of grooves their morphology or their spacing alongshore herein quasi regularly spaced grooves on a soft sandstone beach face are described and interpreted to have formed due to wave breaking and swash zone processes consequent upon exceptional storms at sea the groove morphologies are quantified using terrestrial laser scanning numerical modelling of the translation from offshore waves to nearshore breaking waves provides estimates of the swash zone parameters a consideration of swash zone processes provides an explanation for formation of the grooves in particular the swash zone shear stress distribution and consequent bed erosion is a dome shaped function of distance across the beach face and this controls the cross shore variability in groove depths high speed sheet flows such as swash and backwash develop periodic shore normal high and low speed streaks alongshore consequent streaky erosion produces the quasi regular alongshore groove spacings however on any given beach face the specific spacing of grooves is likely a property not only of the local sheet flow attributes but also of larger scale morphological forcing this outcome suggests that spacing is an emergent property of the coupled sheet flow and larger scale forcing and thus specific spacings on any beach face remain unpredictable

decay and expansion of the early aftershock activity following the 2011 mw9 0 tohoku earthquake the 2011 m w9 0 tohoku earthquake was followed by an abundant amount of seismicity providing a unique opportunity to analyze the triggering mechanism of great earthquakes although the tohoku earthquake occurred close to a dense seismic network many aftershocks that occurred in the first few hours after the mainshock are not recorded in the earthquake catalogs here we use a template waveform approach to recover as many as possible missing events in the first 12 hours following the tohoku mainshock our analysis is able to detect about 1 4 times more events than those listed in the high sensitivity seismograph hi net earthquake catalog combining our new dataset with earthquakes that occurred at latter times we are able to observe a continuous decay of the aftershock rate and along strike expansion of aftershock area we relate the latter observation to the occurrence of post seismic slip over the deep interface

topography of the 660 km seismic discontinuity beneath izu bonin implications for tectonic history and slab deformation we analyze the p wave codas of 65 paths from deep northwestern pacific earthquakes recorded by arrays of stations in germany the western united states india and turkmenistan we identify a phase resulting from a near source s to p conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition we migrate these data along with 39 more from wicks and richards 1993 accounting for three dimensional ray bending by the sloping discontinuity to produce a high resolution topography map of the 660 km discontinuity in the izu bonin region assuming an equilibrium phase transition we interpret the discontinuity depth in terms of local temperatures the slab if defined by a thermal anomaly greater than 400 k is only about 100 km thick near 28 n suggesting the slab is penetrating into the lower mantle with little or no advective thickening farther to the north however cold material appears spread out over a wide region consistent with the slab having been laid down flat on the 660 km discontinuity as the trench retreated 2000 km eastward both the narrow slab to the south and the flat lying slab to the north are consistent with recent high resolution tomographic images the depression to 745 km along the arc is consistent with a maximum thermal anomaly of about 1100 k along the entire arc the depression occurs directly beneath the deepest earthquakes even where seismicity is dipping at 45 and stops at 450 km depth suggesting that the slab steepens to a vertical dip at the deepest seismicity this change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease

extended nucleation of the 1999 mw 7 6 izmit earthquake laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture we report here that one of the best recorded large earthquakes to date the 1999 moment magnitude mw 7 6 izmit turkey earthquake was preceded by a seismic signal of long duration that originated from the hypocenter the signal consisted of a succession of repetitive seismic bursts accelerating with time and increased low frequency seismic noise these observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust this slip accelerated slowly initially and then rapidly accelerated in the 2 minutes preceding the earthquake

unusual strong ground motion across japan from the 680 km deep 30 may 2015 ogasawara islands earthquake the mw 7 9 earthquake beneath the ogasawara islands on 2015 may 30 at 680 km depth about 100 km deeper than other seismicity in the vicinity produced significant shaking over a broad area of japan in the epicentral distance range 1000 2000 km usually deep earthquakes in the subducting pacific slab develop a band of large ground motion along the east coast of northern japan due to efficient guiding of high frequency 1 hz waves in the subducting slab however for this very deep earthquake the large ground acceleration arises from relatively low frequency 1 hz s wave pulses with a long tail of long period 0 1 hz signals the arrival of the slab guided high frequency signal was very late and weak compared with normal pacific slab events numerical simulation of seismic wave propagation reveals that the observed low frequency pulses are the direct s wave from the very deep source followed by a large sv to p conversion at the free surface with near critical s wave incidence these phases produce a long period shear coupled pl s pl wave by constructive interference between wide angle p and s reflections in the crust and incoming s wave to the crust the s pl wave travels several hundred kilometers at regional distances with reinforcement from continuing s wave input from the very deep source such an unusual observation of strong low frequency strong ground motion was due to its very great source depth outside the main slab so the high frequency pacific slab guided wave was weak

trench motion controlled slab morphology and stress variations implications for the isolated 2015 bonin islands deep earthquake the subducted old and cold pacific plate beneath the young philippine sea plate at the izu bonin trench over the cenozoic hosts regional deep earthquakes we investigate slab morphology and stress regimes under different trench motion histories with mantle convection models viscosity temperature and deviatoric stress are inherently heterogeneous within the slab which we link to the occurrence of isolated earthquakes models expand on previous suggestions that observed slab morphology variations along the izu bonin subduction zone exhibited as shallow slab dip angles in the north and steeper dip angles in the south are mainly due to variations in the rate of trench retreat from the north where it is fast to the south where it is slow geodynamic models consistent with the regional plate tectonics including oceanic plate age plate convergence rate and trench motion history reproduce the seismologically observed principal stress direction and slab morphology we suggest that the isolated 680 km deep 30 may 2015 m w 7 9 bonin islands earthquake which lies east of the well defined benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined benioff zone can be explained by pacific slab buckling in response to the slow trench retreat

sole marks and combined flow storm event beds in the brixton formation of the siliciclastic archean witwatersrand supergroup south africa abstract sole marks and associated storm event beds are described for the first time from the middle archean witwatersrand supergroup the sole marks are excellently preserved and include a variety of marks formed by combined flow turnabout curvilinear and low angle intersecting groove marks bipolar prod marks recurved flute marks and shallow scour hollows the sole marks are intimately associated with other sedimentary structures formed by combined flow or oscillatory flow such as wave ripple marks undulatory parallel lamination hummocky cross stratification and hummock and swale structures the storm event beds with sole marks were deposited in a transitional shelf setting and grade upwards into swaly and trough cross bedded orthoquartzite deposited on the inner shelf interbedded storm event beds display a variety of sole mark assemblages reflecting widely varying flow conditions in the storm generated bottom boundary layer oscillatory currents were mainly oriented perpendicular to paleoshore and were combined in some cases with offshore directed storm surge relaxation currents some sole mark assemblages specifically those with turnabout and low angle intersecting groove marks contain evidence for the presence of shore oblique or shore parallel geostrophic currents the storm event deposits in the witwatersrand supergroup are thus compared with models derived from odern storm deposition

the role of microbial mats in the movement of stones on playa lake surfaces abstract this paper sheds light into the debated mechanisms that move rocks across low gradient surfaces during storm episodes microbial mats are recognised to play a crucial role in sediment destabilisation and the subsequent motion of rocks in a playa lake from central spain widespread countless scars are present on the playa lake sediment surface each terminating at a stone and or a mound of overfolded fragments of microbial mats all available objects including cobble sized stones on the surface were transported for several metres leaving behind furrow like tracks the starting point of the tracks was found underwater on a surface veneered by microbial mats poorly attached to the substrate the flotation and transportation of the microbial upper layer by wind generated water currents are suggested to be a critical element promoting the destabilisation and subsequent transportation of the attached sediment including the rocks the sudden exposure of bubble separated sediment is also envisaged as a source of buoyancy required to lift out the rocks at the starting point of the tracks and to reduce the friction on the base of the rock at the end of the tracks the rocks and the sediment mounds often overlap which reveals that rocks were embedded and transported by sediment rafts across the playa lake surface the combination of the specific sedimentary and weather conditions involved in the transportation of rocks occurs periodically in mudflats the implications are that this is not a local phenomenon but rather is a geographically widespread sedimentary process that may have occurred repeatedly throughout geological time the effects of storm episodes can be inferred in ancient mudflat deposits by the assemblage of variably sized stones and microbial mat related structures

transient triggering of near and distant earthquakes we demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes we assume that earthquake triggering is a stick slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring slider model and a rate and state dependent friction constitutive law a critical triggering threshold is implicit in such a model formulation our first hypothesis is that transient loads lead to clock advances i e transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased i e triggering may be delayed these simple clock advance models predict complex relationships between the triggering delay the clock advance and the transient characteristics the triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied this implies that the stress required to bring about failure does not depend linearly on loading time even when the fault is loaded at a constant rate the timing of instability also depends nonlinearly on the transient loading rate faster rates more rapidly hastening instability this implies that higher frequency and or longer duration seismic waves should increase the amount of clock advance these modeling results and simple calculations suggest that near tens of kilometers small moderate earthquakes and remote thousands of kilometers earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load i e accumulated strain energy would have been relieved via other mechanisms we test this using two new seismicity models that 1 are inherently unstable but slide at steady state conditions under the background load and 2 are conditionally stable such that instability occurs only for sufficiently large perturbations for the new seismicity models very small amplitude transients trigger instability relative to the clock advance models the unstable steady state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude as in the clock advance models we were unable to generate delayed triggering with conditionally stable models for both new seismicity models the potential for triggering is independent of when the transient load is applied or equivalently of the prestress unlike in the clock advance models in these models a critical triggering threshold appears to be inversely proportional to frequency further advancement of our understanding will require more sophisticated quantitative models and observations that distinguish between our qualitative yet distinctly different model predictions

the isolated 680 km deep 30 may 2015 m w 7 9 ogasawara bonin islands earthquake deep focus earthquakes located in very high pressure conditions 300 to 700 km below the earth s surface within sinking slabs of relatively cold oceanic lithosphere are mysterious phenomena the largest recorded deep focus earthquake m w 7 9 in the izu bonin slab struck on 30 may 2015 beneath the ogasawara bonin islands isolated from prior seismicity by over 100 km in depth and followed by only a few small aftershocks globally this is the deepest 680 km centroid depth event with m w 7 8 in the seismological record seismicity indicates along strike contortion of the izu bonin slab with horizontal flattening near a depth of 550 km in the izu region and rapid steepening to near vertical toward the south above the location of the 2015 event this event was exceptionally well recorded by seismic stations around the world allowing detailed constraints to be placed on the source process analyses of a large global data set of p sh and pp seismic phases using short period back projection subevent directivity and broadband finite fault inversion indicate that the mainshock ruptured a shallowly dipping fault plane with patchy slip that spread over a distance of 40 km with a multi stage expansion rate 5 km s down dip initially 3 km s up dip later during the 17 s total rupture duration the radiated energy was 3 3 10 16 j and the stress drop was 38 mpa the radiation efficiency is moderate 0 34 intermediate to that of the 1994 bolivia and 2013 sea of okhotsk m w 8 3 deep earthquakes indicating that source processes of very large deep earthquakes sample a wide range of behavior from dissipative more viscous failure to very brittle failure the isolated occurrence of the event much deeper than the apparently thermally bounded distribution of bonin slab seismicity above 600 km depth suggests that localized stress concentration associated with the pronounced deformation of the izu bonin slab and proximity to the 660 km phase transition likely played a dominant role in generating this major earthquake

seismic observations indicating that the 2015 ogasawara bonin earthquake ruptured beneath the 660 km discontinuity the termination of deep earthquakes at a depth of 700 km is a key feature for understanding the physical mechanism of deep earthquakes the 680 km deep may 30 2015 ogasawara bonin earthquake mw7 9 and its aftershocks were recorded by seismic stations at distances from 7 to 19 synthetic seismograms indicate that the p waveforms depend on whether the earthquake is located above or below the 660 km discontinuity in this study i show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660 km velocity discontinuity recordings of the p wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter theoretical waveforms computed with a 660 km discontinuity above the aftershock and mainshock match the observed waveforms more accurately these observations may indicate earthquake ruptures due to mantle minerals other than olivine or strong deformation of the 660 km phase transition

the role of a hidden fault in stress triggering stress interactions within the 1935 mw 7 1 hsinchu taichung earthquake sequence in central taiwan abstract hidden faults often lead to misestimation of earthquake hazard in western taiwan where many blind faults are located how the stress interactions correlate with potentially significant earthquakes and what is the role of blind faults in earthquake triggering are the important questions for earthquake hazard assessment given sequential rupture of active faults with an unexposed fault segment involved the 1935 mw 7 1 hsinchu taichung earthquake sequence in central taiwan provides an exceptional case to study this destructive earthquake sequence took 3000 lives and was composed of four m 6 earthquakes occurred within three months the mw 7 1 mainshock and a subsequent mw 6 8 event that occurred 12 s later caused surface ruptures on the tuntzuchiao fault ttcf and shihtan fault stf which are separated by 25 km these two ruptures were characterized by different types of faulting thrust faulting on the stf and right lateral faulting on the ttcf fault about 24 min later an mw 6 0 event occurred 45 km north of the mainshock three months later on july 17 the last event mw 6 2 occurred 30 km northwest of the mainshock in this study we revisit the data available for the 1935 sequence in an effort to place constraints on the fault models using five different fault models a series of coulomb stress calculations are conducted to understand whether static stress transfer advances slip during the subsequent events we propose that key features of the 1935 hsinchu taichung earthquake triggering can be explained by the existence of an unexposed fault segment in between the segments of ttcf and stf we also show that the 1935 earthquake sequence may play an important role in activating post 1935 earthquake activity along the sanyi puli seismic zone where the sequential ruptures of the 1935 events encourage ruptures of a nw se trending seismic zone at later times

earth s earliest microbial mats in a siliciclastic marine environment 2 9 ga mozaan group south africa this study provides evidence for the existence of filamentous cyano bacteria forming sediment stabilizing mats in shelf environments at 2 9 ga the oldest known occurrence of microbial mats in siliciclastic rocks the mesoarchean mozaan group south africa features fine grained quartzites of an ancient shallow shelf environment these sandstones contain wrinkle structures which in thin section reveal filamentous textures forming carpet like microbial mat fabrics the textures resemble the trichomes of modern cyanobacteria chloroflexi or sulfur oxidizing proteobacteria mineralogical geochemical and isotopic analyses acre consistent with a biological origin of the filament like textures carbon filaments with biogenic isotopic signatures 1 3 c 24 2 0 5 are closely associated with hematite goethite and chert minerals which may derive from the former presence of oxygen within the microbial mats detrital quartz zircon and rutile in the mats could indicate baffling trapping and binding of the bacterial communities

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

multiple slow slip events during a foreshock sequence of the 2014 iquique chile mw 8 1 earthquake to obtain a precise record of the foreshock sequence before the 2014 iquique chile mw 8 1 earthquake we applied a matched filter technique to continuous seismograms recorded near the source region we newly detected about 10 times the number of seismic events listed in the routinely constructed earthquake catalog and identified multiple sequences of earthquake migrations at speeds of 2 10 km d both along strike and downdip on the fault plane updip of the main shock area in addition we found out repeating earthquakes from the newly detected events likely indicating aseismic slip along the plate boundary fault during the foreshock sequence these observations suggest the occurrence of multiple slow slip events updip of the main shock area the final slow slip event migrated toward the main shock nucleation point we interpret that several parts of the plate boundary fault perhaps experienced slow slip causing stress loading on the prospective largest slip patch of the main shock rupture

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

subduction zone guided waves and the heterogeneity structure of the subducted plate intensity anomalies in northern japan 1 the subducting pacific plate acts an efficient waveguide for high frequency signals and often produces anomalously large intensity on the eastern seaboard of northern japan during deep earthquakes the waveform records in the region of high intensity show a low frequency f 2 hz later arrivals with a long coda this behavior is not explained by a simple subduction zone model comprising a high velocity plate with low attenuation from the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the pacific subduction zone we demonstrate that the high frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0 5 km in thickness the standard deviation of wave speed fluctuations from the averaged background model is about 2 this new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness but low frequency waves with long wavelengths can easy tunnel through such lamina structure the result is frequency selective propagation characteristics with a faster low frequency phase followed by large and high frequency signals with very long coda a low wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high frequency coda three dimensional simulations using the earth simulator supercomputer for modeling of high frequency seismic wave propagation in the pacific subduction zone including plate heterogeneity clearly demonstrate the scattering waveguide effects for high frequency seismic waves traveling in the plate the region of large intensity for the heterogeneous model migrates away from the hypocenter into northern japan with an elongated zone along the pacific coast almost comparable to the observations from deep events in the pacific plate

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

rupture complexity of the 1994 bolivia and 2013 sea of okhotsk deep earthquakes the physical mechanism of deep earthquakes depth 300 km remains enigmatic partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations the two largest deep earthquakes the recent great 2013 sea of okhotsk earthquake m 8 3 depth 607 km and the great 1994 bolivia earthquake m 8 3 depth 637 km together provide a unique opportunity to compare their rupture patterns in detail here we extend a travel time sub event location method to perform full teleseismic p waveform inversion this new method allows us to explain the observed broadband records with a set of sub events whose model parameters are robustly constrained without smoothing we find that while the okhotsk event is mostly unilateral rupturing 90 km along strike with a velocity over 4 km s the bolivia earthquake ruptured about half this distance at a slow velocity about 1 5 km s and displayed a major change in rupture direction we explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered

neoproterozoic tsunamiite upper bhander sandstone central india abstract this paper addresses a distinctive event bedset encased by coastal erg margin deposits at a preferred stratigraphic level near the base of the neoproterozoic upper bhander sandstone in central india the bedset is composed of couplets of sandstone beds that exhibit incisive amalgamation although they differ in geometry structures at soles within and at tops of beds vertical grain size variation as well as palaeocurrent pattern and direction the wide extent of the bedset is evident from several exposures spread over a distance of more than 50 km roughly in strike parallel direction flow and depositional dynamics interpreted from the coupled event beds are more consistent with a tsunami origin than alternative palaeogeography compatible models of climate induced storm flash flood or accentuated tide deposits a palaeotsunamiite model is thus discussed with separate incoming and outgoing components considering the overall depositional setting to be an epeiric sea coast in an intracratonic sag basin the relevant bedset is inferred to reflect the record of a teletsunamiite it would also be one of the very few precambrian tsunamiites known so far exceptional preservation of this possible tsunamiite was facilitated by sheltered deposition behind the backshore zone and the berm as well as by rapid burial by wind deflated sands and advancing aeolian dunes

spatial and temporal distribution of microbially induced sedimentary structures a case study from siliciclastic storm deposits of the 2 9 ga witwatersrand supergroup south africa abstract microbially induced sedimentary structures miss arise from the interaction of microbial mats with the physical sedimentary dynamics of shallow marine settings the structures occur in siliciclastic deposits of paleo archean age to modern where they record biostabilization and baffling trapping and binding of photoautotrophic microbenthos in the brixton formation witwatersrand supergroup south africa erosional remnants and pockets wrinkle structures and oscillation cracks include carbon and pyrite rich filament like textures that resemble modern trichomes of cyanobacteria c isotope ratios of 22 0 1 indicate potential photoautotrophy the miss occur at the turning points of regression transgression and are restricted to a specific sedimentary facies characterized by 2 20 cm thick fine sandstone beds predominantly composed of quartz the sandstone beds display ripple marks which record moderate hydraulic reworking this spatial and temporal distribution of miss in the brixton formation is consistent with occurrences of those structures in comparable environments throughout earth history