a geomagnetic excursion in the brunhes epoch recorded in new zealand basalts abstract anomalous geomagnetic directions are recorded in the thermoremanent magnetisation trm of alkali basalt lavas from the auckland volcanic field new zealand the concordance of paleomagnetic directions both in different sites from individual eruptive centres and in sites from several different centres as well as their relatively short age span compared with that of the field as a whole indicates that these anomalous directions have a geomagnetic origin the directions which are dated between approximately 25 ka and 50 ka do not represent full reversals of the geomagnetic field and are interpreted as excursions very few excursions have been reported from the southern hemisphere this is the first such record known from igneous rocks and further supports the notion that excursions are global phenomena the excursions here are tentatively correlated with those reported from lake mungo mono lake and laschamp

seismicity variations associated with aseismic transients in guerrero mexico 1995 2006 primarily aseismic deformation transients in subduction zones sometimes associated with tremors and low frequency earthquakes are a newly recognized mode of deformation stressing in the up dip seismogenic zone is increased episodically due to down dip transientslips andeacheventmaymakeitmorepronetofailureinalargethrustearthquake itisimportantforseismichazardassessment to search and identify patterns of seismicity variation associated with transients the guerrero mexico region is chosen for this study because of long term continuous geodetic observations and abundant seismicity in the shallow subduction zone we search the gcmt and neic catalogs for earthquakeswithdepths lessthan100kmbetween1995and2006withintheareacoveringthe regionaffected by major transients since 1996 a completeness magnitude of mc 4 5 is determined for the neic catalog used in this study based on the maximum likelihood method three large transients in 1998 2001 2002 and 2006 are all temporally correlated with high seismic rates in the studied area in particular transients are either preceded by a cluster of extensional earthquakes relatively far inland from the trench or followed by shallow thrustearthquakes closetothe trench in somecases such asthe 2001 2002 transient both typesofactivityare foundbordering the transient the assembled evidence suggests that transients may serve as a mechanism of stress communication between distant seismicity clusters in shallow subduction zones 2007 elsevier b v all rights reserved

source parameters and time dependent slip distributions of slow slip events on the cascadia subduction zone from 1998 to 2008 1 we invert for the time dependent slip history of slow slip events on the cascadia subduction zone using gps data from 1998 to 2008 the 16 slip transients have sufficient station coverage to solve for the slip distribution on the plate interface gps time series are inverted for fault slip using the extended network inversion filter limited station coverage south of portland 45 5 n latitude restricts our analysis to events on the northern half of the subduction zone slip is resolved at the base of the seismogenic zone and the slip distributions suggest a potential segment boundary near seattle 47 6 n that correlates roughly with geologic and tectonic boundaries events that initiate to the north and south tend to overlap at about this latitude we compile statistics on source parameters such as propagation rate recurrence interval and stress drop which can be used to constrain proposed models of the source mechanics over a 10 year period total strain release from slow slip events is nonuniform along strike with the greatest cumulative slip 27 cm centered beneath port angeles 48 1 n this slip patch also exhibits the most regular recurrence of mw 6 events relative to other locations along strike the spatial extent of the slip patch beneath port angeles correlates with the along strike bend of the cascadia subduction zone in northwestern washington suggesting that plate geometry plays an important role in controlling the along strike characteristics of slow slip

resonant slow fault slip in subduction zones forced by climatic load stress global positioning system gps measurements at subduction plate boundaries often record fault movements similar to earthquakes but much slower occurring over timescales of 1 week to 1 year these slow slip events have been observed in japan1 2 cascadia3 4 5 6 7 mexico8 9 alaska10 and new zealand11 the phenomenon is poorly understood but several observations hint at the processes underlying slow slip although slip itself is silent seismic instruments often record coincident low amplitude tremor in a narrow 1 5 cycles per second frequency range12 also modelling of gps data3 7 9 and estimates of tremor location13 indicate that slip focuses near the transition from unstable stick slip to stable friction at the deep limit of the earthquake producing seismogenic zone perhaps most intriguingly slow slip is periodic at several locations with recurrence varying from 6 to 18 months depending on which subduction zone or even segment is examined4 5 6 9 here i show that such periodic slow fault slip may be a resonant response to climate driven stress perturbations fault slip resonance helps to explain why slip events are periodic why periods differ from place to place and why slip focuses near the base of the seismogenic zone resonant slip should initiate within the rupture zone of future great earthquakes suggesting that slow slip may illuminate fault properties that control earthquake slip

a palaeomagnetic secular variation record for new zealand during the past 2500 years abstract a record of the secular variation of the geomagnetic field over new zealand for the past 2500 years has been compiled by measuring and averaging logs of the primary magnetization of five cores of lacustrine sediment from lake pounui a time scale is provided by four radiocarbon age determinations the record is constrained by historical observations for the last 350 years and is in good agreement with instantaneous palaeomagnetic directions from lava flows this is the first continuous record of secular variation from new zealand and one of only a few from the southern hemisphere together with existing australian records the results show significant evidence of westward drift of the geomagnetic field over the past 2500 years it contains new information for modelling the geomagnetic secular variation in the southern hemisphere and will also be valuable for dating applications within new zealand

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

repeated pulses of vertical methane flux recorded in glacial sediments from the southeast bering sea 1 there is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period in this study of two sediment cores from the southeast bering sea 700 m and 1467 m water depth we identify multiple episodes during the last glacial period of intense methane flux reaching the seafloor within the uncertainty of the radiocarbon age model the episodes are contemporaneous in the two cores and have similar timing and duration as dansgaard oeschger events the episodes are marked by horizons of sediment containing 13c depleted authigenic carbonate minerals 13c depleted archaeal and bacterial lipids which resemble those found in anme 1 type anaerobic methane oxidizing microbial consortia and changes in the abundance and species distribution of benthic foraminifera the similar timing and isotopic composition of the authigenic carbonates in the two cores is consistent with a region wide increase in the upward flux of methane bearing fluids this study is the first observation outside santa barbara basin of pervasive repeated methane flux in glacial sediments however contrary to the clathrate gun hypothesis kennett et al 2003 these coring sites are too deep for methane hydrate destabilization to be the cause implying that a much larger part of the ocean s sedimentary methane may participate in climate or carbon cycle feedback at millennial timescales we speculate that pulses of methane in these opal rich sediments could be caused by the sudden release of overpressure in pore fluids that builds up gradually with silica diagenesis the release could be triggered by seismic shaking on the aleutian subduction zone caused by hydrostatic pressure increase associated with sea level rise at the start of interstadials

quake clamps down on slow slip using continuous gps cgps data from the hikurangi subduction zone in new zealand we show for the first time that stress changes induced by a local earthquake can arrest an ongoing slow slip event sse the cgps data show that the slip rate in the northern portion of the 2013 2014 kapiti sse decreased abruptly following a nearby intraslab earthquake we suggest that deceleration of the kapiti sse in early 2014 occurred due to a tenfold increase in the normal stress relative to shear stress in the sse source induced by the nearby mw 6 3 earthquake consistent with expectations of rate and state friction our observation of an abrupt halting slowing of the sse in response to stress changes imposed by a local earthquake has implications for the strength of fault zones hosting sses and supports the premise that static stress changes are an important ingredient in triggering or delaying fault slip

volcanic records of the laschamp geomagnetic excursion from mt ruapehu new zealand abstract we present palaeodirectional records of the laschamp geomagnetic excursion from lavas on mt ruapehu new zealand fourteen lava flows on the northwestern and southern flanks of mt ruapehu with 40ar 39ar weighted mean plateau ages that range from 46 3 2 0 to 39 9 1 4 ka were studied the youngest and older flows carry a normal polarity magnetization however six flows dated between 46 3 2 0 and 42 7 1 8 ka record excursional directions three of these flows record southerly palaeomagnetic declinations and negative inclinations that agree well with a published laschamp record from the auckland volcanic field avf together the avf and mt ruapehu lavas currently represent the only volcanic records of the laschamp excursion outside the chaine des puys region france thus they make an important contribution to the global set of laschamp excursion records virtual geomagnetic pole vgp groups for the new zealand and french records early in the excursion are compatible with a dipole dominated field that rotated to an equatorial orientation while simultaneously decaying in strength in contrast younger excursional flows from france and new zealand yield separate vgp groups which suggest either that the field had a nondipolar morphology in this later phase or that the vgp groups were not synchronous 40ar 39ar ages for the mt ruapehu record are on average slightly older than published northern hemisphere ages and from the relative palaeointensity minimum in the glopis sedimentary stack although few individual ages differ significantly at the 2 level the spread suggests an overall excursion duration that is longer than the currently accepted 1500 years this age spread may result from excess ar in magmas at the time of the eruption biasing the results to slightly older ages or from non synchronous excursional field behaviour at near antipodal locations or possibly a precursory phase prior to the main excursion

marine magnetic anomalies evidence that tiny wiggles represent short period geomagnetic polarity intervals since the 1960 s the geomagnetic polarity time scale which is based on marine magnetic anomalies has become fundamentally important in geochronology despite the importance of marine magnetic anomaly records there has been longstanding uncertainty about the meaning of the smallest anomalies observed in these records small amplitude short wavelength anomalies are frequently observed in marine magnetic anomaly records across fast spreading oceanic crust s 50 mm yr the origin of these small scale anomalies referred to as tiny wiggles has remained controversial over the last 30 years tiny wiggles have been interpreted to represent either short period polarity intervals or largescale fluctuations in the ancient field intensity we present palaeomagnetic evidence from a sedimentary record from the north pacific ocean which demonstrates that two short but clearly resolvable polarity zones in addition to a probable geomagnetic excursion occur within chron c5n 2n 9 92 10 95 ma where three tiny wiggles have been reported on marine magnetic anomaly profiles relative palaeointensity data indicate that the field collapsed prior to and during the reversals and during the excursion but that it recovered to higher field intensities within the polarity intervals before collapsing to low values at the succeeding polarity transition this indicates that some tiny wiggles represent real short period geomagnetic polarity intervals while others may represent geomagnetic excursions the existence of such short polarity intervals confirms the predictions of statistical analyses of geomagnetic reversal frequency and indicates that tiny wiggles represent the maximum resolution of geomagnetic polarity intervals in marine magnetic anomaly records fl 2000 elsevier science b v all rights reserved

slow slip transients along the oaxaca subduction segment from 1993 to 2007 summary we use data from 12 continuous gps stations in southern mexico including eight new stations to better characterize transient slip episodes along the mexican subduction zone continuous gps recording in oaxaca that began 14 years ago constituting the longest continuous gps record in southern mexico defines nine distinct episodes of transient slip from 1993 to 2007 including previously unreported transient slip episodes in early 1995 and 2006 all transient slip episodes recorded in oaxaca city were also recorded at a gps site 400 km away in the state of guerrero after measurements began there in early 1997 demonstrating that transient slip affects widespread areas of southern mexico well recorded transients in 2004 and 2006 appear to have originated along the oaxaca trench segment hundreds of kilometres from the potentially hazardous guerrero seismic gap during the 2004 slip transient displacements of more than 20 mm were recorded at stations in the oaxaca gps array a factor of 2 3 larger than at sites in guerrero elastic half space modelling of the 2004 displacements at sites throughout southern mexico indicates that most slip was focused beneath oaxaca in deeper areas of the subduction interface immediately downdip from previous megathrust rupture zones but that slip also may have extended significantly updip into a seismic gap along the oaxaca coast the best fitting model is also able to explain transient vertical offsets of up to 30 mm observed during this event shortly before the onset of transient slip in 2004 a pair of moderate m w 5 1 5 5 earthquakes ruptured the downdip end of the seismogenic zone immediately east of the oaxaca seismic gap suggesting a possible relationship between the two in 2006 transient slip began 1 3 months earlier in oaxaca than in other areas offering the clearest evidence to date for the existence of one or more source regions for transient slip outside the guerrero seismic gap where large amplitude transient slip originated in 2002 the still limited data indicate that transient slip that originates elsewhere in mexico may trigger aseismic slip in guerrero and further indicate that transient slip beneath oaxaca is limited to areas of the subduction interface that surround the rupture zones of previous large shallow thrust earthquakes

interplate coupling and transient slip along the subduction interface beneath oaxaca mexico summary we describe and model gps measurements of surface deformation from the oaxaca segment of the mexican subduction zone to characterize interseismic strain accumulation and episodic transient slip in this region and test seismologically based models of strain accumulation and release along subduction interfaces deformation measured from 2001 to 2007 within our dense 31 station gps array has consisted of 1 trench normal horizontal contraction at rates that decrease monotonically inland from the coast 2 rapid coastal subsidence that changes gradually to slow uplift at locations more than 100 km inland and 3 periods of transient slip that interrupt the otherwise steady deformation inverse modelling of transient station offsets in 2004 and 2006 indicates that transient slip along the subduction interface occurred downdip from the rupture limits of previous large earthquakes in this region in both 2004 and 2006 gps site velocities that are corrected for the effects of this transient slip vary significantly over distances of only tens of kilometres both along the coastline and inland implying that similar spatial variations occur in the degree of locking across the subduction interface deformation rates measured along the coast reach their maximum above the core of the rupture zone of the 1978 ms 7 8 shallow thrust earthquake and generally decrease outwards towards the edges of the rupture zone bounded value inverse modelling of the interseismic gps velocity field with a finite element mesh that simulates the study region indicates that much of the rupture zone of the 1978 shallow thrust earthquake is fully locked at the plate convergence rate but that this region is surrounded by weakly locked areas of the subduction interface which may slow or arrest the propagation of future earthquakes much of the deeper region of transient slip downdip from the seismogenic zone is also fully locked between the episodes of transient slip however the elastic energy that accumulated due to locking of this deeper transitional zone between 2002 and 2006 appears to have been mostly or completed released by the 2004 and early 2006 transient slip events the approximately balanced energy budget for the deeper zone of transient slip implies that this region is unlikely to contribute significant elastic energy to future earthquakes that originate along the seismogenic zone our results support a model in which seismic asperities coincide with regions of strong locking between earthquakes the potential elastic energy that has re accumulated since 1978 in the seismogenic zone is already sufficient to cause a repeat of the 1978 earthquake

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

tectonic tremor and slow slip along the northwestern section of the mexico subduction zone abstract the southwestern coast of mexico is marked by active subduction of the rivera and cocos plates producing megathrust earthquakes that tend to recur every 50 100 yr herein we use seismic and gps data from this region to investigate the potential relationship between earthquakes tectonic non volcanic tremor and transient slip along the westernmost 200 km of the mexico subduction zone visual examination of seismograms and spectrograms throughout the 18 month long mars seismic experiment reveals clear evidence for frequent small episodes of tremor along the rivera and cocos subduction zones beneath the states of jalisco colima and michoacan using a semi automated process that identifies prominent energy bursts in envelope waveforms of this new data analyst refined relative arrival times are inverted for source locations using a 1 d velocity model the resulting northwest southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 mw 8 0 colima jalisco and 2003 mw 7 2 tecoman subduction thrust earthquakes and just below the regions of afterslip triggered by these earthquakes despite the close proximity between tremor and megathrust events there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor a well defined gap in the tremor beneath the western colima graben appears to mark a separation along the subducted rivera cocos plate boundary from the position time series of 19 continuous gps sites in western mexico we present the first evidence that slow slip events occur on the rivera plate subduction interface unlike the widely recorded large amplitude slow slip events on the nearly horizontal cocos plate subduction interface below southern mexico slow slip events below western mexico have small amplitudes and are recorded at relatively few mostly coastal stations the smaller slow slip beneath western mexico may be due to the steeper dip causing a narrower zone where the conditions for slow slip are met the prominent afterslip following the m 8 1995 colima jalisco and m 7 2 2003 tacoma megathrust earthquakes may also shrink the area available for episodic slow slip

laschamp and mono lake geomagnetic excursions recorded in new zealand eight basaltic lavas from the auckland volcanic field new zealand record three distinct sets of excursional geomagnetic field directions and low paleointensities however the timing and therefore paleomagnetic significance of these records have been poorly understood radiocarbon k ar and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites forty 40ar 39ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods at 39 1 4 1 and 31 6 1 8 ka coincident with 40ar 39ar astrochronologic and 14c ages determined for the laschamp and mono lake excursions respectively samples from a lava flow associated with a third cluster of virtual geomagnetic poles vgps are complicated by low concentrations of radiogenic argon in the presence of excess argon and thus yield discordant age spectra and an imprecise age of 26 6 8 1 ka our findings indicate that the laschamp and mono lake excursions until recently identified unequivocally and isotopically dated only in the northern hemisphere were globally synchronized at 40 1 and 32 1 ka however the vgps of lavas that record the laschamp excursion in new zealand and france are inconsistent with a simple clockwise looping geometry inferred from vgp paths obtained in eight marine sediment cores spanning the laschamp excursion we suggest that the differences in vgps recorded at the various sites are significant and may point to non axial dipole components and lower mantle control on transitional fields during short lived excursions

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

time variability of the earth s gravity field hydrological and oceanic effects and their possible detection using grace the grace satellite mission scheduled for launch in 2001 is designed to map out the earth s gravity field to high accuracy every 2 4 weeks over a nominal lifetime of 5 years changes in the gravity field are caused by the redistribution of mass within the earth and on or above its surface grace will thus be able to constrain processes that involve mass redistribution in this paper we use output from hydrological oceanographic and atmospheric models to estimate the variability in the gravity field i e in the geoid due to those sources we develop a method for constructing surface mass estimates from the grace gravity coefficients we show the results of simulations where we use synthetic grace gravity data constructed by combining estimated geophysical signals and simulated grace measurement errors to attempt to recover hydrological and oceanographic signals we show that grace may be able to recover changes in continental water storage and in seafloor pressure at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer with accuracies approaching 2 mm in water thickness over land and 0 1 mbar or better in seafloor pressure

relationship between the deceleration of earth s rotation and earthquakes that occurred before the m s 8 0 wenchuan earthquake for earthquakes ml 2 5 that occurred from january 1990 to april 2008 around the ms 8 0 wenchuan earthquake we investigated the correlation between the phase of earth s rotation and the occurrence of earthquakes via schuster s test to determine the signals that triggered earthquakes before the ms 8 0 wenchuan earthquake we statistically investigated the correlation between the occurrence of earthquakes and earth s rotation the results were evaluated based on the p value where a smaller p value corresponded to a higher correlation between the occurrence of an earthquake and earth s rotation we analyzed the temporal evolution of p values for earthquakes and found that some lower p values i e significant correlation were calculated for earthquakes that occurred before the ms 8 0 wenchuan earthquake furthermore we investigated the spatial distribution of p values along the longmenshan fault and obtained a result exhibiting a considerably lower p value region in the northeastern part of the study area on the edge of which the epicenter of the ms 8 0 wenchuan earthquake was located among the earthquakes with lower p values a vast majority of them occurred during the deceleration of earth s rotation the lower p value obtained in this study reveals earth s rotation triggered earthquakes within the rupture region prior to the occurrence of the ms 8 0 wenchuan earthquake

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

rare palaeomagnetic evidence of long term mantle control of the geodynamo and possible role of the nad field in the reversal process the degree to which the lower most mantle influences behavior of the geodynamo has been debated over the past quarter century our analysis of a comprehensive set of 17 cenozoic paleomagnetic transitional field records obtained from lavas in the southern hemisphere provides robust evidence of stable mantle control since the pliocene the records come from a region where given a significantly weakened axial dipole the magnetic field today would be largely controlled by the non axial dipole nad flux patch currently emanating from earth s outer core beneath western australia the paleomagnetic recording sites from west to east include the south indian ocean eastern downloaded from https academic oup com gji advance article abstract doi 10 1093 gji ggz480 5602599 by guest on 17 november 2019 australia new zealand and french polynesia the analyzed records contain from 2 to 25 sequential transitional virtual geomagnetic poles vgps ten of the 17 records supply at least one vgp within a narrow longitudinal band 10 wide between 60 s and the equator centered along longitude 102 4 e that is transitional data from 59 of the cenozoic recordings are found to reside in a region that encompasses a mere 2 8 of the vgp transitional area on earth s surface a robust monte carlo approach applied to this dataset one that takes into account the number of transitional vgps contained in each record finds this result highly improbable p value 0 0006 the present day pattern of vertical flux at the core mantle boundary shows an anomalously strong thin southern hemisphere longitudinal band off the west coast of australia that strikingly coincides with this unusual paleomagnetic finding we conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 myr seemingly independent of the behavior of the axial dipole our findings indicate it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity

episodic tremor and slip on a frictional interface with critical zero weakening in elastic solid summary non volcanic tremor nvt and related relatively weak and slow slip events termed episodic tremor and slip ets are observed below the seismogenic sections of numerous subduction zones and several major strike slip faults these events have various characteristics that distinguish them from regular tectonic earthquakes here we show that a frictional fault in elastic solid with a region below the seismogenic zone having on average a critical near zero weakening during slip provides a simple unified explanation for the diverse observed phenomenology of nvt ets the results imply that nvt ets have little predictive power on the occurrence of large events in the overriding seismogenic zone additional model predictions that may be tested with future high resolution observations are fractal slip distributions and failure areas discrete power law frequency moment statistics with exponent 3 2 and exponential tapering overall scale invariant potency moment magnitude time histories triggered periodic nvt with evolving size and rate correlated with the stressing rate of the periodic triggering mechanisms and parabolic or exponential source time functions for event sizes measured by duration or moment potency

reflection and refraction by tilted layers an explanation for vhf auroral backscatter at large aspect angles auroral structures are often accompanied by tilts to the electron density contours on the underside of the e region by including the effects of these tilts on radio wave refraction and reflection it is possible to understand many of the previously puzzling observations of vhf auroral backscatter at large angles away from perpendicularity to the earth s magnetic field it is shown that many of the characteristics of echoes from the vhf canadian bistatic auroral radar system bars are consistent with this process for bars observations for which the tilt hypothesis is correct the resulting doppler velocities can be interpreted in the same way as those from stare and used to give electric fields and convection patterns over the bars field of view

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law

simultaneous chatanika radar and s3 2 satellite measurements of ionospheric electrodynamics in the diffuse aurora simultaneous chatanika radar and s3 2 satellite measurements have been used to investigate the spatial variation of the auroral zone ionosphere near the equatorward edge of the diffuse aurora during early evening satellite passes on 2 days in february 1976 the equatorward boundary of the diffuse aurora was nearly overhead at chatanika the configurations of e region ionization and conductivity were nearly identical in both passes but the latitudinal variations of f region ionization were quite dissimilar during both passes the ionospheric electric field in the diffuse aurora was directed northward as a result the closing current of the field aligned current system was primarily a pedersen current and the eastward electrojet was larger than the northward ionospheric current although the morphology of ionospheric conductivity was very similar on the 2 days different patterns were observed for the latitudinal distribution of electric field and field aligned current

slow slip on the northern hikurangi subduction interface new zealand 1 in october 2002 a surface displacement episode of 20 30 mm magnitude was observed over a 10 day period on two continuous global positioning system gps instruments near gisborne north island new zealand we interpret this to result from slow slip on the northern hikurangi subduction interface using ten years of regional campaign gps 1995 2004 and recent continuous gps data we estimate the recurrence interval for similar events to be 2 3 yrs in november 2004 a similar slow slip event occurred within this recurrence period the 2002 event can be modeled by 18 cm of slow slip near the down dip end of the seismogenic zone on the subduction interface offshore of gisborne the campaign gps data show that the 2002 slow slip event had little effect on regional strain patterns

transient deformation in southern mexico in 2006 and 2007 evidence for distinct deep slip patches beneath guerrero and oaxaca 1 we model three slow slip events in 2006 and 2007 recorded by continuous gps stations in central and southern mexico to test for overlap between their source regions along the mexican subduction interface and whether they intrude upward into the rupture zones of previous large earthquakes inverse modeling yields source regions beneath central oaxaca for two of the three slow slip events sse where a previously described sse occurred in 2004 and beneath guerrero for the third where slip events previously occurred in 2001 2002 and possibly 1998 along with previously published results our work suggests there are persistent differences between the depths and magnitudes of transient slip beneath oaxaca and guerrero transient slip beneath oaxaca in 2004 2006 and 2007 had a common source region downdip from the seismogenic zone and released elastic strain energy equivalent to mw 7 0 earthquakes equaling most or all energy that accumulated below the seismogenic zone transient slip beneath guerrero in 2006 had a larger moment magnitude mw 7 3 and extended somewhat farther updip possibly to seismogenic depths transient slip thus appears to relieve some elastic strain that accumulates at shallow levels in the guerrero seismic gap we find no evidence for spatial or temporal correlations of slow slip along these two widely separated source regions although better data are needed to test more definitively for any interaction between them

influence of a meteorological cycle in mid crustal seismicity of the nepal himalaya abstract the process of interseismic strain accumulation across the nepal himalaya is associated with micro seismicity also called mid crustal seismicity which occurs along the mid crustal ramp on the main himalayan thrust mht the seismicity shows strong annual periodicity in response to the annual stress variation of hydrological loads we report dominance of annual periodicity in the eastern nepal as compared to that of western nepal and overall annual periodicity dominance in the small magnitude earthquakes mw 0 3 rainfall seismicity cross correlation is significantly higher in the eastern nepal region as compared to that of western nepal with a prominent phase lag of 5 6 months with respect to the occurrence of monsoonal rainfall seasonal modulation can be explained by coulomb failure stress model and fault resonance hypothesis induced by meteorological cycle this new observation upholds the existing hypothesis that evaporation induced unloading in the himalayan foothills and adjacent indo gangetic plains during the post monsoon period i e during winter add significant component of horizontal compression to the interseismic contraction at the mht which is the main driving mechanism for the seasonal modulation

high precision 40 ar 39 ar dating of quaternary basalts from auckland volcanic field new zealand with implications for eruption rates and paleomagnetic correlations abstract the auckland volcanic field avf which last erupted ca 550 years ago is a late quaternary monogenetic basaltic volcanic field ca 500 km 2 in the northern north island of new zealand prior to this study only 12 out of the 53 identified eruptive centres of the avf had been reliably dated careful sample preparation and 40 ar 39 ar analysis has increased the number of well dated centres in the avf to 35 the high precision of the results is attributed to selection of fresh non vesicular non glassy samples from lava flow interiors sample selection was coupled with separation techniques that targeted only the groundmass of samples with 10 m wide coupled with ten increment furnace step heating of large quantities up to 200 mg of material the overall avf age data indicate an onset at 193 2 2 8 ka an apparent six eruption flare up from 30 to 34 ka and a 10 kyr hiatus between the latest and second to latest eruptions such non uniformity shows that averaging the number of eruptions over the life span of the avf to yield a mean eruption rate is overly simplistic together with large variations in eruption volumes and the large sizes and unusual chemistry within the latest eruptions rangitoto 1 and rangitoto 2 our results illuminate a complex episodic eruption history in particular the rate of volcanism in avf has increased since 60 ka suggesting that the field is still in its infancy multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions with five identified in the mono lake two within the laschamp one within the post blake or blake and two possibly within the hilina pali

implications of transient deformation in the northern basin and range western united states transient deformation events observed in global positioning system gps data from the basin and range extensional province may illuminate qualitatively similar transient events observed in subduction zones and other tectonic environments we model gps time series at 22 sites using a combination of hyperbolic tangent function analysis and elastic load deformation estimated from climatological data we identify two transient events 2000 4 and 2004 4 with roughly similar timing and displacement to those described previously by other researchers the first few years of gps observations adopted as a reference state in earlier studies are found to be anomalous our results differ from previous studies in two respects first a significant component of northward transient motion occurs during both events despite a reversal of sign in east component motion second sites move coherently in the eastern as well as the western basin and range surface mass loading the largest source of transient stress forcing in the region exhibits no evidence of a simple relationship to the deformation transients prior studies inferred slip on a single megadetachment at the moho but that hypothesis assumes negligible ductile deformation of the lower crust and a dry olivine rheology for the uppermost mantle recent measurements of crustal quartz abundance and effective elastic thickness suggest both assumptions are unlikely basin and range transients can be reconciled with the frictional slip mechanism widely accepted for subduction zone transients provided that slip is occurring on discontiguous detachment surfaces at midcrustal depths

postseismic deformation of the andaman islands following the 26 december 2004 great sumatra andaman earthquake 1 two years after the great sumatra andaman earthquake the 3 1 m wsw coseismic displacement at port blair andaman islands had increased by 32 cm postseismic uplift initially exceeded 1 cm per week and decreased to 1 mm week by 2007 points near port blair had risen more than 20 cm a 24 reversal of coseismic subsidence uplift at eight gps sites suggests a gradual eastward shift of the coseismic neutral axis separating subsidence from uplift simulations of the gps postseismic displacements as viscoelastic relaxation of coseismic stress change and as slip on the plate interface indicate that slip down dip of the seismic rupture dominates near field deformation during the first two years postseismic slip beneath the andaman islands released moment equivalent to a magnitude mw 7 5 earthquake and the distribution suggests deep slip in the stable frictional regime accelerated to catch up to the coseismic rupture

palaeointensities of the auckland geomagnetic excursions by the ltd dht shaw method we report new palaeointensity results concerning the auckland geomagnetic excursions using the double heating technique of the shaw method with low temperature demagnetisation ltd dht shaw method the excursional palaeodirections recorded in six volcanoes of the auckland volcanic field new zealand have been classified into three groups north down nd west up wu and south up su directions in the present study five to six consistent palaeointensities have been obtained from each of five volcanoes recording the auckland geomagnetic excursions the wiri 27 ka crater hill and puketutu volcanoes nd group yielded mean palaeointensities of 10 6 1 2 1 11 8 2 8 and 11 1 0 4t respectively the hampton park volcano 55 ka wu group gave 9 5 1 2t while the mclennan hills volcano su group gave 2 5 0 5t it is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections nd group possibly supporting the reliability of the palaeointensity data these five palaeointensities for the auckland geomagnetic excursions correspond to virtual dipole moments vdms of 0 6 2 1 10 22 am 2 whereas three mean palaeointensities obtained from the auckland volcanoes having non excursional palaeodirections are 13 1 40 0 t giving stronger vdms of 2 1 6 9 10 22 am 2 these results suggest that the dipole component of the geomagnetic field reduced to about 2 10 22 am 2 or less during the auckland geomagnetic excursions

stress rates in the central cascadia subduction zone inferred from an elastic plate model gps vectors and surface tilt and uplift rates from northwestern oregon and southwestern washington are inverted to estimate rates of stress changes along the cascadia thrust fault and base of the overriding plate using a finite thickness elastic plate model the data are fit by elevated shear stress and coulomb failure function cff rates within 80 km of the trench by contrast an elastic half space dislocation model does not fit as well and predicts significant amounts of locking and elevated cff rates near the coast

the auckland volcanic field new zealand geophysical evidence for structural and spatio temporal relationships abstract geophysical data from the monogenetic auckland volcanic field reveal complex structural and spatio temporal relationships at different scales the volcanic field is coincident with regional magnetic and gravity anomalies that mark a major crustal suture and with a discontinuity marking a significant structural asperity here the linear regional magnetic anomaly splays into a wide band of nnw trending lineaments arising from serpentinised shear zones in the upper crust that matches the extent of the volcanic field and that may reflect a region of crustal weakness creating preferential permeability however there appears to be no simple correlation between the locations of individual vents and these lineaments that might delineate more shallow structural controls with this orientation probably as a consequence of other structural influences high resolution aeromagnetic data over the volcanic field show that the volcanoes have a wide range of magnetic signatures indicating a variability of subsurface structure scoria cone volcanoes typically have strong anomalies up to several 100 nt whilst tuff ring volcanoes typically have weak anomalies less than 50 nt though the surface geology is not always an indicator of the nature and extent of the subsurface deposits both cone and tuff ring volcanoes in the auckland field appear to be underlain by subsurface bowl shaped bodies of basalt implying that their eruption histories commonly involve lava ponding into early excavated craters the present geophysical data give no evidence for subsurface dyke like structures or for substantial near surface volumes of basaltic rocks where there are no known eruption centres or buried flows aeromagnetic and palaeomagnetic data suggest that a number of adjacent vents with an implied structural linkage may be contemporaneous though other examples occur where vents of clearly different ages exploit the same apparent structure a unique feature of the auckland field is that at least 5 widely separated and structurally unrelated volcanoes are contemporaneous these observations highlight the spatial and temporal heterogeneities that can occur in monogenetic fields and have an important bearing on recurrence rates and temporally linked eruptions which are probably more common than is generally supposed and are key factors in statistically based hazard assessments

distal record of multi sourced tephra in onepoto basin auckland new zealand implications for volcanic chronology frequency and hazards we have documented 80 tephra beds dating from ca 9 5 to 50 ka contained within continuously deposited palaeolake sediments from onepoto basin a volcanic explosion crater in auckland new zealand the known sources for distal 190 km from vent tephra include the rhyolitic taupo volcanic centre 4 and okataina volcanic centre 14 and the andesitic taranaki volcano 40 and tongariro volcanic centre 3 the record provides evidence for four new events between ca 50 and 28 ka mangaone subgroup suggesting okataina was more active than previously known the tephra record also greatly extends the known northern dispersal of other mangaone subgroup tephra ten rhyolitic tephra pre date the rotoehu eruption ca 50 ka and some are chemically dissimilar to post 50 ka rhyolites some of these older tephra were produced by large magnitude events however their source remains uncertain eight tephra from the local basaltic auckland volcanic field avf are also identified interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from taranaki volcano in the 27 5 9 5 ka period in addition 28 older events are recognised the tephra are trachytic to rhyolitic in composition all have high k2o contents 3 wt and there are no temporal trends this contrasts with the proximal lava record that shows a trend of increasing k2o with time by combining the onepoto tephra record with that of the previously documented pukaki crater 15 avf basaltic fall events are constrained at 34 6 30 9 29 6 29 6 25 7 25 2 24 2 23 8 19 4 19 4 15 8 and 14 5 ka and three pre 50 ka events this provides some of the best age constraints for the avf and the only reliable data for hazard recurrence calculations the minimum event frequency of both distal and local fall events can be estimated and demonstrates the auckland city region is frequently impacted by ash fall from many volcanoes

a detailed palaeointensity and inclination record from drill core soh1 on hawaii abstract a new record of absolute palaeointensity was obtained from drill core scientific observation hole 1 soh1 on kilauea volcano hawaii kilauea s high eruption rate resulted in a relatively continuous record and stratigraphic constraints preserved the chronological order three hundred and sixty samples were studied with the thellier thellier technique which gave 195 successful palaeointensity and 271 successful inclination determinations three geomagnetic excursions were observed which exhibited intensity reductions of about 50 initial age control from k ar and ar ar dating only constrained the total age between 20 and 120 ka the final age model was obtained by stretching the soh1 record relative to other hawaiian palaeomagnetic data this gave an age range of 0 45 ka for the flows and identified the excursions as the hilina pali mono lake and laschamp events the soh1 record of the hilina pali event is the most detailed ever incorporating data from around 40 flows this age model suggests that kilauea had a burst of activity at the soh1 site around 20 ka all available data was combined to form a composite record of palaeointensity and inclination on hawaii for 0 45 ka

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs a review abstract more than a decade after the discovery of deep episodic slow slip and tremor or slow earthquakes at subduction zones much research has been carried out to investigate the structural and seismic properties of the environment in which they occur slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner where three major structural elements are in contact the subducting oceanic crust the overriding forearc crust and the continental mantle in this region thermo petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions and their consumption by hydrating the mantle wedge these fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore fluid pressure and or reducing friction in fault gouges resolving the fine scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging and most advances have been made using teleseismic scattering techniques e g receiver functions in this paper we review the teleseismic structure of six well studied subduction zones three hot i e cascadia southwest japan central mexico and three cool i e costa rica alaska and hikurangi that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior we conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior and that near lithostatic pore fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence

a model for calculating eruptive volumes for monogenetic volcanoes implication for the quaternary auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios especially transitions between explosive and effusive behaviour and the length of eruptions erosion superposition and lack of exposure limit the accuracy of volume determination even for very young volcanoes in this study a systematic volume estimation model is developed and applied to the auckland volcanic field in new zealand in this model a basaltic monogenetic volcano is categorised in six parts subsurface portions of volcanoes such as diatremes beneath phreatomagmatic volcanoes or crater infills are approximated by geometrical considerations based on exposed analogue volcanoes positive volcanic landforms such as scoria spatter cones tephras rings and lava flow were defined by using a light detection and ranging lidar survey based digital surface model dsm finally the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes considering only those parts with high reliability the overall magma output converted to dense rock equivalent for the post 250 ka active auckland volcanic field in new zealand is a minimum of 1 704 km3 this is made up of 1 329 km3 in lava flows 0 067 km3 in phreatomagmatic crater lava infills 0 090 km3 within tephra tuff rings 0 112 km3 inside crater lava infills and 0 104 km3 within scoria cones using the minimum eruptive volumes the spatial and temporal magma fluxes are estimated at 0 005 km3 km2 and 0 007 km3 ka the temporal volumetric evolution of auckland is characterised by an increasing magma flux in the last 40 ky which is inferred to be triggered by plate tectonics processes e g increased asthenospheric shearing and backarc spreading of underneath the auckland region

spatial relationship between large aspect angle vhf radio aurora and 557 7 nm emissions evidence for refraction a recently proposed explanation for large aspect angle vhf backscatter observations based on refraction through auroral ionization structures in the e region moorcroft 1989 has been confirmed by an experimental comparison of observations of auroral e region coherent backscatter at 48 5 mhz made with the bistatic auroral radar system or bars with simultaneous 557 7 nm emissions detected with a ccd based all sky imager the apparently large aspect angle radio auroral echoes are found to occur predominantly on the side of the visual aurora away from the radar for both the premidnight and the postmidnight sectors for the red lake bars radar this means that the strongest echoes occur to the north of the visual form in sharp contrast with previous comparisons particularly for the premidnight sector which found that radar echoes occur predominantly southward of the visual aurora these results are consistent with the proposed explanation based on refraction in the auroral e region

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

age of the mono lake excursion and associated tephra abstract the mono lake excursion mle is an important time marker that has been found in lake and marine sediments across much of the northern hemisphere dating of this event at its type locality the mono basin of california has yielded controversial results with the most recent effort concluding that the mle may actually be the laschamp excursion earth planet sci lett 197 2002 151 we show that a volcanic tephra ash 15 that occurs near the midpoint of the mle has a date not corrected for reservoir effect of 28 620 300 14c yr bp 32 400 gisp2 yr bp in the pyramid lake basin of nevada given the location of ash 15 and the duration of the mle in the mono basin the event occurred between 31 500 and 33 300 gisp2 yr bp an age range consistent with the position and age of the uppermost of two paleointensity minima in the napis 75 stack that has been associated with the mle philos trans r soc london ser a 358 2000 1009 the lower paleointensity minimum in the napis 75 stack is considered to be the laschamp excursion philos trans r soc london ser a 358 2000 1009

paleomagnetic constraints on the ages of the holocene malpais de zacapu lava flow eruptions michoacan mexico implications for archeology and volcanic hazards four monogenetic holocene lava flows located within the michoacan guanajuato volcanic field mexico were sampled for paleomagnetic dating these flows namely el infiernillo malpais las viboras

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

slow slip events in mexico revised from the processing of 11 year gps observations 1 the world s largest observed slow slip events sse occurred in 2001 2002 and 2006 in the guerrero subduction zone mexico using an improved gps processing that accounts for time varying atmospheric phenomena as well as oceanic atmospheric and hydrologic loading corrections the 11 year gps position time series in guerrero show a noise reduction of 50 with respect to previous studies thanks to the improved position time series and in particular the simultaneous analysis of the three dimensional gps observations we can provide new information about sses in the studied area first we detect seven nonperiodic anomalous displacements with subcentimeter amplitude but no quasi annual anomalies as proposed previously the displacements seem to occur simultaneously with the observed peaks of non volcanic tremor activity in the area second we refine the characteristics of the two major sses in terms of timing duration and cumulative displacements and highlight the complex surface spatiotemporal evolution of the displacements during these sses in particular we observe a clear initiation phase for the 2006 sse as well as ending phases for both large sses the ending phase shows a strong deceleration of the anomalous displacements with respect to the main displacement phase already observed for the 2001 2002 and 2006 sses the duration of the sses increases by 30 40 including the initiation and ending phases for the 2006 sse the main displacement phase also shows spatiotemporal complexity our results demonstrate the need for improved three dimensional gps processing technique in order to undertake detailed studies of sses

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

microearthquake streaks and seismicity triggered by slow earthquakes on the mobile south flank of kilauea volcano hawai i 1 we perform waveform cross correlation and high precision relocation of both background seismicity and seismicity triggered by periodic slow earthquakes at kilauea volcano s mobile south flank we demonstrate that the triggered seismicity dominantly occurs on several preexisting fault zones at the hilina region regardless of the velocity model employed the relocated earthquake epicenters and triggered seismicity localize onto distinct fault zones that form streaks aligned with the slow earthquake surface displacements determined from gps due to the unknown effects of velocity heterogeneity and nonideal station coverage our relocation analyses cannot distinguish whether some of these fault zones occur within the volcanic crust at shallow depths or whether all occur on the decollement between the volcano and preexisting oceanic crust at depths of 8 km nonetheless these hilina fault zones consistently respond to stress perturbations from nearby slow earthquakes

absolute palaeointensity study of the mono lake excursion recorded by new zealand basalts abstract one of the geomagnetic excursions recorded in basalts of the auckland volcanic field new zealand has recently been correlated unequivocally with the mono lake excursion making it the only confirmed record from the southern hemisphere this record is also exceptional in occurring in five separate monogenetic basaltic volcanoes absolute palaeointensity determinations using the microwave technique based on a comprehensive suite of samples of the auckland basalts recording the excursion show that the geomagnetic field in new zealand at the time of the mono lake excursion was reduced to about 14 t i e to 30 of its normal value this result confirms previous estimates based on more limited sampling in addition it provides a comparison of the microwave and ldt dht shaw methods of measuring palaeointensity which give results that are statistically indistinguishable the palaeointensities determined from the five different volcanoes are also indistinguishable though the palaeodirection data suggest that a very small segment of the vgp path may have been recorded this study confirms the reliability of palaeointensity and palaeodirection determinations from these particular new zealand basalts which together with their definitive 40ar 39ar ages establishes this record of the mono lake excursion as one of the best documented consequently there is significant potential in searching for records of this excursion elsewhere in the pacific region for use as a stratigraphic marker in studies of recent volcanism and palaeoclimate reconstructions

odp site 1063 bermuda rise revisited oxygen isotopes excursions and paleointensity in the brunhes chron an age model for the brunhes chron of ocean drilling program odp site 1063 bermuda rise is constructed by tandem correlation of oxygen isotope and relative paleointensity data to calibrated reference templates four intervals in the brunhes chron where paleomagnetic inclinations are negative for both u channel samples and discrete samples are correlated to the following magnetic excursions with site 1063 ages in brackets laschamp 41 ka blake 116 ka iceland basin 190 ka pringle falls 239 ka these ages are consistent with current age estimates for three of these excursions but not for pringle falls which has an apparent age older than a recently published estimate by 28 kyr for each of these excursions termed category 1 excursions virtual geomagnetic poles vgps reach high southerly latitudes implying paired polarity reversals of the earth s main dipole field that apparently occurred in a brief time span 2 kyr in each case several times shorter than the apparent duration of regular polarity transitions in addition several intervals of low paleomagnetic inclination low and negative in one case are observed both in u channel and discrete samples at 318 ka mis 9 412 ka mis 11 and in the 500 600 ka interval mis 14 15 these category 2 excursions may constitute inadequately recorded category 1 excursions or high amplitude secular variation

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

afterslip and aftershocks in the rate and state friction law istituto nazionale di geofisica e vulcanologia italy ingv centre national de la recherche scientifique cnrs exxonmobil upstream research company

combined paleointensity and 40ar 39ar age spectrum data from volcanic rocks of the west eifel field germany evidence for an early brunhes geomagnetic excursion results of virtual geomagnetic pole position paleointensity and isotopic age determinations have been derived from lavas of the quaternary west eifel volcanic field germany the characteristic remanent magnetization carm directions and corresponding positions of virtual geomagnetic poles vgp have mainly been determined by bohnel et al they are exclusively of normal polarity the vgps form a strongly non fisherian distribution characterized by about 30 pole positions with latitudes below 60 n which are mainly confined to the longitude sector from 20 w to 120 e eight independent poles are clustered at 40 n and 45 e geomagnetic paleointensity values from 37 volcanic units of the west eifel have been determined paleointensities range from 7 to 62 t with an unexpectedly large number of paleointensity values below 30 t about half of the low paleointensity values are associated with intermediate vgp latitudes isotopic age determinations from west eifel volcanic rocks have been published fuhrmann and lippolt additional isotopic ages are presented here the west eifel volcanic field is usually devided into an older part in the nw and a younger one in the se based on petrological models and geological evidence all available isotopic ages of the west eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr for seven of the clustered low latitude vgps it was possible to determine paleointensities and 40ar 39ar ages within their error limits six poles have identical virtual dipole moments vdm at 3 5 1022a m2 furthermore the individual isotopic age intervals of the 40ar 39ar data overlap the assigned age of 510 30 kyr is considered to be the age of a geomagnetic excursion for higher latitude vgp positions 45 n vdm values range from 2 to 10 1022a m2 the mean vdm value about 7 5 1022a m2 of the younger volcanoes ages estimated to be below 100 kyr is not remarkably different from the present dipole moment of the earth in contrast the older volcanoes with higher latitude vgp positions yielded a mean value of only 5 3 1022a m2 and a bimodal frequency distribution of the vdm values isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion

comparison of frictional strength and velocity dependence between fault zones in the nankai accretionary complex accretionary complexes host a variety of fault zones that accommodate plate convergence and internal prism deformation including the decollement imbricate thrusts and out of sequence thrusts or splays these faults especially the decollement and major splay faults are considered to be candidates for hosting slow slip events and large magnitude earthquakes but it is not clear what modes of slip should be expected at shallow levels or how they are related to fault rock frictional properties we conducted laboratory experiments to measure the frictional properties of fault and wall rock from three distinct fault zone systems sampled during integrated ocean drilling program expedition 316 and ocean drilling program leg 190 to the nankai trough offshore japan these are 1 a major out of sequence thrust fault termed the megasplay site c0004 2 the frontal thrust zone a region of diffuse thrust faulting near the trench site c0007 and 3 the decollement zone sampled 2 km from the trench site 1174 at 25 mpa effective normal stress at slip rates of 0 03 100 m s and in the presence of brine as a pore fluid we observe low friction 0 46 for all of the materials we tested however the weakest samples 0 30 are from the decollement zone material from the megasplay fault is significantly weaker than the surrounding wall rocks a pattern not observed in the frontal thrust and decollement all samples exhibit primarily velocity strengthening frictional behavior suggesting that earthquakes should not nucleate at these depths a consistent minimum in the friction rate parameter a b at sliding velocities of 1 3 m s 0 1 0 3 m d is observed at all three sites suggesting that these shallow fault zones may be likely to host slow slip events

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

eruption recurrence rates in a basaltic volcanic field based on tephra layers in maar sediments implications for hazards in the auckland volcanic field long term eruption recurrence rates in monogenetic basaltic volcanic fields are difficult to assess because of low eruption frequencies but they are important because of the spread of human infrastructure into such fields auckland city new zealand is built on the auckland volcanic field a young 250 ka basaltic volcanic field in the absence of abundant material for radiocarbon and isotopic dating an eruptive chronology based on basalt tephra layers deposited in maar lakes was developed interbedded well dated tephra layers from silicic volcanoes some 220 270 km to the south were used as age constraints the basalt tephra layers reveal a pattern of activity not evident from the temporal spatial distribution of volcanic landforms twenty four basalt tephra layers over the last 80 k y represent an average frequency of one per 3 5 k y recurrence times vary from 0 5 k y to 20 k y and show no temporal trend the tephra record shows a major flare up in explosive activity at 32 2 ka during heightened activity at 34 24 ka this was related to a period of simultaneous eruptions from several volcanoes across the field revealed by paleomagnetic and isotopic ages in contrast the field has been relative quiet during the last 20 k y punctuated by the construction of a shield volcano at 0 7 ka thus the surface manifestation of magmatism varies greatly with time complicating long term volcanic hazard forecasting despite the high risk from future basalt activity because of proximity ash fall from distant andesite volcanoes is the most frequently occurring volcanic hazard in auckland city

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

the determination of time stationary two dimensional convection patterns with single station radars at the present time most ground based radar estimations of ionospheric convection use observations from single station facilities this approach requires certain assumptions as to the spatial and or temporal uniformity of the convection in this paper the authors present a critical examination of the accuracy of these vector velocity determinations using realistic modeled flow patterns that are time stationary but not spatially uniform they find that under certain circumstances the actual and inferred flow fields show considerable discrepancy sometimes not even agreeing in the sense of flow direction specifically they show that the natural curvature present in ionospheric convection on varying spatial scales can introduce significant error in the velocity estimate particularly when the radius of curvature of the flow structure is less than or equal to the radar range to the scattering volume the presence of flow curvature cannot be detected by radars which determine velocities from measurements in two viewing directions and it might not be detected by radars using azimuth scanning techniques thus they argue that every effort should be made to measure the ionospheric convection by bidirectional or multidirectional observations of a common ionospheric volume and that a synthesis of coherent and incoherent radar observations from different sites ismore preferable to multidirectional single station observations using either radar alone these conclusions are applicable to any doppler measurement technique and are equally valid for high latitude wind patterns using fabry perot interferometer techniques less

fault frictional parameters and material properties revealed by slow slip events at kilauea volcano hawai i 1 we categorize slow slip events at kilauea volcano into two distinct families based on gps measurements of the surface displacement patterns an event correlation filter confirms that eastern and western families are statistically distinguishable with the western family notably self similar the western family exhibits quasi periodicity with regular repeat times while eastern family events are aperiodic or have complicated periodicity if the decollement is the source fault for both families of events it must have varying frictional properties at the 10 km scale of separation the temporal slip and spatial scaling behavior are consistent with a simplistic rate and state dependent frictional formalism provided that the characteristic slip distance for state evolution dc is of the order of millimeters rather than the 10 100 m typically found in lab studies and the shear rigidity is around 2 gpa consistent with fault gouge material

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

transient fault slip in guerrero southern mexico the guerrero region of southern mexico has ac cumulated more than 5 m of relative plate motion since the last major earthquake in early 1998 a continuous gps site in guerrero recorded a transient displacement modeling indicates that anomalous fault slip propagated from east to west along strike of the subduction megathrust campaign gps and leveling data corroborate the model the moment release was equivalent to an mw 6 5 earthquake no m 5 earthquakes accompanied the event indicating the frictional regime is velocity strengthening at the location of slip

diurnal radio aurora variations at 50 mhz measured by the bistatic auroral radar system radars mean doppler velocity and intensity data obtained from the normal operating mode alternating single pulse double pulse mode as also used by stare of the bars radars from selected intervals between march 1986 and july 1987 have been used to describe typical morphological characteristics and diurnal variations in the backscatter comparisons with the maria magnetometers at gillam and eskimo point verify that the bars drift velocities obtained by standard merging for which one assumes that the radar doppler shifts depend on the cosine of the angle between the electron hall drift and the radar look direction are in qualitative agreement with the equivalent currents three basic time intervals could be identified 1 an evening interval between 2100 and 0200 ut 1500 to 2000 mlt with westward e b drift which occurs only under disturbed conditions and for which the nipawin backscatter intensity is very low most of the time 2 a breakup interval dominated by eastward drift but including westward turnings which under disturbed conditions can extend from 0200 to 1000 ut 2000 to 0400 mlt but which occurs even under relatively quiet conditions when no backscatter precedes or follows it in which case it lasts about 2 hours around 0600 ut 3 an interval with oscillations around a mainly eastward drift backscatter in this interval does not seem to depend on the occurrence of the previous two types and lasts from 1000 up to 1500 ut 0400 to 0900 mlt no backscatter has yet been measured during the rest of the day 1500 to 2100 ut or 0900 to 1500 mlt the period from 2130 ut may 28 to 1430 ut may 29 1987 is used as a sample day during which all three backscatter types occurred

longevity of a small shield volcano revealed by crypto tephra studies rangitoto volcano new zealand change in eruptive behavior of a basaltic field abstract the life span of small volcanoes in terrestrial basaltic fields commonly considered monogenetic can be difficult to assess due to a paucity of datable materials capable of providing a 102 103 year age resolution we have used microscopic tephra layers crypto tephra in lake sediments to determine the longevity of rangitoto volcano a small shield that represents the most recent volcanism in the auckland volcanic field avf new zealand previous studies suggested construction in a relatively short interval at 550 500 cal yrs bp in contrast the tephra record shows evidence of intermittent activity from 1498 140 to at least 504 6 cal yrs bp a longevity of 1000 years rangitoto volcano is thought to represent about half the magma erupted in the 250 ka history of avf thus the avf has experienced a dramatic shift to prolonged and voluminous central vent volcanism in its most recent history this demonstrates the difficulty in determining time erupted volume relationships in such fields previous avf hazard risk modeling based on isolated short lived

mid holocene change in types of degassing volcanoes using indium in antarctic ice as a tracer of volcanic source type 1 proportions of trace metals in antarctic ice samples indicate that the type of volcanoes that dominated atmospheric emissions changed at about the middle of the holocene from relatively mafic deep source volcanoes to more silicic shallower source volcanoes we base this inference on the strong contrast in the abundances of the trace metal indium in relative to other trace metals present in ice deposited at different times in the past and on contrasting in abundances in modern emissions of volcanoes of different types indium is more abundant in the emissions of deep source mafic volcanoes than in more felsic shallower source volcanoes earlier workers have shown on the basis of petrologic and some meteoritic evidence that in may be partitioned to the interiors stony mantles of differentiated planets or enriched in the liquids of partly crystallized mafic melts

slow but not quite silent regular earthquakes release large amounts of strain accumulated in earth9s crust within a very short time recently a different type of earthquake has been reported from subduction zones around the world deep in earth9s crust slow earthquakes may last weeks or months in their perspective melbourne and webb explain that while slow these earthquakes are not silent as originally assumed the authors highlight the report by rogers and dragert who show that slow earthquakes in the cascadia subduction zone off the western coast of north america are accompanied by a tremor that is absent when slow faulting is not occurring the earthquakes are likely caused by fluid flow a mechanism that may also apply to regular earthquakes

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

seismic reflection character of the hikurangi subduction interface new zealand in the region of repeated gisborne slow slip events summary we use seismic reflection data to map the geometry and character of the subduction interface in the gisborne area of the hikurangi subduction margin new zealand which experiences repeated shallow 15 km slow slip events the reflection character and geometry in this area is highly variable which we interpret to be related to the subduction of seamounts and underthrust sediments three zones of high amplitude interface reflectivity hrz 1 2 and 3 are interpreted to be the result of fluid rich sediments that have been entrained with subducting seamounts the interface above the hrz zones is shallower than the surrounding areas by 2 4 km due to the warping of the interface to accommodate seamount subduction these zones of high amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008 we hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high amplitude reflectivity due to 1 the enhanced underthrusting of fluid rich sediment 2 reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and 3 potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations from our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events

on the biannually repeating slow slip events at the ryukyu trench southwestern japan 1 global positioning system data show that about 20 slow slip events occurred during 1997 2007 in the southwestern part of the ryukyu arc japan where large interplate thrust earthquakes are not known to have occurred in spite of relatively fast plate convergence they recur fairly regularly on one patch of the subduction fault which is as deep as 20 40 km and mechanically isolated in an uncoupled subduction zone they have time constants of a month or so and release seismic moment equivalent of mw 6 6 on average this is one of the best recorded sequences of repeating fault slips with enough events for statistical analyses they have fairly constant recurrence interval of 0 5 year but do not show preferred months of occurrence in a year suggesting that they are not controlled by seasonal forcing the correlation between intervals and amounts of fault slip show statistically significant time predictable recurrences there is one significant disruption in the rhythm but the stress perturbations due to nearby earthquakes do not appear to play a significant role this part of the fault might be an altered asperity that used to rupture fast but now slips only slowly because of the increased ambient temperature

pole tide modulation of slow slip events at circum pacific subduction zones episodic slow slip ess events have been detected at several circum pacific subduction zones such as cascadia japan and mexico notably at least eight ess events along the northern cascadia subduction zone recurred with a period of 13 16 months we study the relationship between pole tide associated with chan dler wobble with a period of 14 months induced stress and the occurrence of the ess events our quantitative analysis shows that 14 of the 20 documented ess events occurred during the ascension phase prior to the maximum of a pole tide induced coulomb failure stress change and three events occurred at the stress change peak the pole tides modulate the stress field at the downdip edge of the transition zone along the plate interface and may trigger ess events when conditions are favorable the phase advance of the triggered events with respect to the induced coulomb failure stress change may reflect that the fault slip is dictated by a rate and state dependent friction law inferred from laboratory experiments

volcanic risk ranking for auckland new zealand ii hazard consequences and risk calculation in a companion paper a methodology for ranking volcanic hazards and events in terms of risk was presented and the likelihood and extent of potential hazards in the auckland region new zealand investigated in this paper the effects of each hazard are considered and the risk ranking completed values for effect are proportions of total loss and as with likelihood and extent are based on order of magnitude

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

geomagnetic excursion captured by multiple volcanoes in a monogenetic field 1 five monogenetic volcanoes within the quaternary auckland volcanic field are shown to have recorded a virtually identical but anomalous paleomagnetic direction mean inclination and declination of 61 7 and 351 0 respectively consistent with the capture of a geomagnetic excursion based on documented rates of change of paleomagnetic field direction during excursions this implies that the volcanoes may have all formed within a period of only 50 100 years or less these temporally linked volcanoes are widespread throughout the field and appear not to be structurally related however the general paradigm for the reawakening of monogenetic fields is that only a single new volcano or group of closely spaced vents is created typically at intervals of several hundred years or more therefore the results presented show that for any monogenetic field the impact of renewed eruptive activity may be significantly under estimated especially for potentially affected population centres and the siting of sensitive facilities