a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

in situ gas hydrate hydrate saturation estimated from various well logs at the mount elbert gas hydrate stratigraphic test well alaska north slope abstract in 2006 the u s geological survey usgs completed detailed analysis and interpretation of available 2 d and 3 d seismic data and proposed a viable method for identifying sub permafrost gas hydrate prospects within the gas hydrate stability zone in the milne point area of northern alaska to validate the predictions of the usgs and to acquire critical reservoir data needed to develop a long term production testing program a well was drilled at the mount elbert prospect in february 2007 numerous well log data and cores were acquired to estimate in situ gas hydrate saturations and reservoir properties gas hydrate saturations were estimated from various well logs such as nuclear magnetic resonance nmr p and s wave velocity and electrical resistivity logs along with pore water salinity gas hydrate saturations from the nmr log agree well with those estimated from p and s wave velocity data because of the low salinity of the connate water and the low formation temperature the resistivity of connate water is comparable to that of shale therefore the effect of clay should be accounted for to accurately estimate gas hydrate saturations from the resistivity data two highly gas hydrate saturated intervals are identified an upper 43 ft zone with an average gas hydrate saturation of 54 and a lower 53 ft zone with an average gas hydrate saturation of 50 both zones reach a maximum of about 75 saturation

rate and state model casts new insight into episodic tremor and slow slip variability in cascadia advances in geodetic and seismic observations have led to the discovery of episodic tremor and slow slip ets ets in cascadia subduction zone occurs semiregularly and shows intriguing spatio temporal variability reportedly associated with frictional properties and stress conditions yet the origin of complex ets behaviors remains largely unknown here we develop a laboratory based rate and state asperity in matrix subduction fault model supported by geological observations of exhumed fault with heterogeneous frictional properties and pore pressure variation to reproduce all ets variability in good agreement with observations our results show that differential pore pressure plays a crucial role in affecting fault behaviors regions of asperities with decreased pore pressure tend to have increased tremor our study suggests that ets variability can be used to probe otherwise enigmatic fault zone properties

very low frequency earthquakes related to small asperities on the plate boundary interface at the locked to aseismic transition 1 very low frequency vlf seismic signals observed in southwestern japan are evidently radiated from shear slips on the upper surface of the subducting philippine sea plate we used grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes between 3 1 and 3 8 from continuous seismograms recorded over a 5 year period by using a very dense broadband seismic network at least 5 10 sequences of repetitive activity were observed during the 5 years the vlf events formed clustered distributions along the 35 km isodepth contour of the subducting plate surface the nodal planes which dip landward of moment tensor solutions of the vlf events reflected the configuration of the subducting plate interface the slip vectors were consistent with the direction of movement of the subducting plate the dip and strike of the slip vectors clearly reflected the configuration of the upper surface of the subducting plate we found that the rates of seismic moment release per unit area associated with five major vlf clusters were very similar although both the seismic magnitudes and sizes of the clusters varied considerably the rate of seismic moment released from detectable vlf sources was 0 1 of the rates of short term slow slip events suggesting that the source areas occupied only 0 1 of the fault segment on which the short term slow slip events occurred

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

dynamic modeling of slow earthquakes based on thermoporoelastic effects and inelastic generation of pores 1 we numerically simulate features of slow earthquakes to understand their generation mechanism in the framework of dynamic modeling their typical features will be that the fault rupture velocity and stress drop are markedly lower than those of ordinary earthquakes we assume a fault in a thermoporoelastic medium taking account of fluid flow and inelastic creation of pores on the fault this paper is an extension of our studies published in the work of suzuki and yamashita 2006 2007 2008 in which a nondimensional parameter su was shown to play a critical role in dynamic fault rupture the parameter su represents the dominance of the effect of inelastic pore creation over that of frictional heating under the condition of no fluid flow however it can be shown that su plays an important role even if the fluid flow is considered in the present study we successfully simulate slow fault rupture growth and low stress drop which characterize the slow earthquakes critical ingredients of our modeling are assumptions of 1 su considerably larger than assumed for the simulation of ordinary earthquakes 2 fluid flow into the inelastically created pores and 3 initial shear stress significantly smaller than assumed for the simulation of ordinary earthquakes the assumption of a large value of su corresponds to that of a slip resistant and low stress drop zone the fluid inflow can promote the fault rupture in the slip resistant zone

influence of a subducted oceanic ridge on the distribution of shallow vlfes in the nankai trough as revealed by moment tensor inversion and cluster analysis

very low frequency earthquakes within accretionary prisms are very low stress drop earthquakes 1 we report characteristic p wave spectra and stress drops of very low frequency vlf earthquakes that occurred within the accretionary prism along the nankai trough southwestern japan although many vlf earthquakes showed no distinct phases on a raw seismogram a few showed slightly distinct phases the arrival times of some phases were consistent with the calculated p wave arrival times the corner frequencies of the vlf earthquakes were calculated using the stacked p wave spectra assuming an omega square spectrum stress drops were very low i e in the range of 0 1 10 kpa which corresponds to 0 1 1 of those of ordinary earthquakes such extremely low stress drops of the vlf earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

short term and long term tremor migration patterns of the cascadia 2004 tremor and slow slip episode using small aperture seismic arrays 1 tectonic tremor has been recorded at many subduction zones including the nankai cascadia mexican and alaskan subduction zones this study the first to use small aperture seismic arrays to track tremor deployed three small aperture seismic arrays along the cascadia subduction zone during a tremor and slow slip episode in july 2004 the tremor was active during virtually all up to 99 minutes of the analyzed tremor episode using 5 min sample windows individual wave phases were tracked across the arrays and used to derive slowness vectors these were compared with slowness vectors computed from a standard layered earth model to derive tremor locations locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume the migration between volumes was not smooth and the movement of the sources within the volume followed no specific pattern overall migration speeds along the strike of the subduction zone were between 5 and 15 km d smaller scale migration speeds between volumes reached speeds up to 2 km min uncertainties in the best locations were 5 km in epicenter and 10 km in depth for this data set and processing methodology tremor does not locate predominately on the primary subduction interface our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed at least in part of low frequency earthquakes broadly distributed in location

fluid overpressure along an oligocene out of sequence thrust in the shimanto belt sw japan abstract out of sequence thrusts osts exposed in ancient accretionary prisms are considered as fossil analogs of present day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity in modern as well as in ancient accretionary prisms first order megasplay faults or osts truncate or merge with faults of lesser importance called second order osts structural analysis of the makinokuchi fault a branch of an oligocene to lower miocene second order ost in the tertiary shimanto belt of central kyushu sw japan brings information about the conditions of deformation at the time of thrusting the studied exposure shows that the fault footwall and to a much lesser extent the fault hanging wall consist of quartz cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component the footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias the continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions 245 285 c and 5 8 km depth these similarities suggest that the fluids responsible for syn tectonic hydraulic brecciation were collected from the footwall through the conduits the fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone our analysis shows that fluid overpressures can play a key role in the growth and activity of second order osts in accretionary prisms and suggests that fluids collected along second order osts or splay faults may flow upward along first order osts or megasplay faults

scaling relationships of source parameters for slow slip events to better understand the physical mechanisms of slow slip events sses detected worldwide we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes these scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault healing characteristics the static stress drop remains constant for different sized events as is observed for earthquakes however the static stress drop of slow slip events is within a range of 0 01 1 0 mpa 1 2 orders of magnitude lower than that found for earthquakes which could be related to the low stress state on the fault the average rupture velocity ranging from kilometers per second to kilometers per day decreases linearly with increasing seismic moment in log log space unlike earthquakes that are nearly constant this inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties slow slip events typically have ratios of event duration over dislocation rise time less than 3 while earthquakes have ratios greater than 3 this indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed the recurrence statistics of slow slip events on the northern cascadia subduction zone are weakly time predictable and moderately antislip predictable that is the event size and preevent recurrence interval are anticorrelated which may indicate that healing between events strengthens the fault with time online material table of source parameters and data sources

friction experiments under in situ stress reveal unexpected velocity weakening in nankai accretionary prism samples abstract the nankai trough hosts diverse fault slip modes ranging from slow slip events to megathrust earthquake ruptures we performed laboratory friction experiments on samples collected by the integrated ocean drilling program offshore kii peninsula japan this study systematically investigates the effect of effective normal stress on frictional strength and the velocity dependence of friction for natural fault zone and wall rock samples collected from depths of 270 to 450 meters below seafloor mbsf and over a range of shearing velocity spanning from 0 01 30 m s in addition cohesive strength was determined before and after each velocity step experiment while the sample was unloaded we tested both powdered and intact specimens at estimated in situ effective stresses as well as at higher stresses representing deeper portions of the megasplay fault sites c0004 c0010 and the frontal thrust zone sites c0007 c0006 the apparent coefficient of sliding friction s varies between 0 22 0 53 and correlates inversely with clay content direct measurements of cohesive strength show that on average 11 of the residual shear strength and up to 30 for some specimens can be attributed to cohesion friction coefficient slightly decreases as a function of increasing effective normal stress attributed to a decreasing proportion of cohesive strength the lowest s values are observed for samples from the frontal thrust zone site c0007 and the megasplay fault site c0010 all samples show a combination of velocity strengthening and velocity weakening behavior but intact samples tested under in situ effective normal stress and low shearing velocities a b we propose that fault zones in the nankai subduction zone are likely to be velocity weakening and thus potentially able to host the nucleation of unstable slip from seismogenic depths to the seafloor we also find that testing disaggregated fault zone samples and employing effective stresses exceeding those in situ lead to overestimation of a b emphasizing the importance of testing intact samples under in situ conditions in laboratory friction studies

crustal structure across the coseismic rupture zone of the 1944 tonankai earthquake the central nankai trough seismogenic zone 1 differences in the coseismic rupture process between the 1944 tonankai and the 1946 nankaido earthquakes have been studied by many fault models to understand what factors control coseismic rupture zones it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes the previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the neogene quaternary accretionary prism however little is known about the structure of the rupture zone of the 1944 earthquake to obtain a complete image of the seismogenic zone of the 1944 earthquake a wide angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land our model for the crustal structure is based on ocean bottom seismographic data the crustal structure appears characteristic for subducting oceanic crust and a neogene quaternary accretionary prism bounded by an island arc crust the neogene quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front the subducting oceanic crust can be traced down to 35 km the subduction angle becomes steeper landward reaching up to 11 beneath the island arc crust the depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km while the updip limit is located beneath the island arc upper crust similar structures of the updip and downdip limits are also published for several other subduction zones

geological and frictional aspects of very low frequency earthquakes in an accretionary prism 1 very low frequency earthquakes vlfe occur within accretionary prisms and near subduction plate boundaries at slip rates of 0 05 2 mm s however the geological and frictional aspects of vlfe remain poorly understood the thrusts in the shimanto accretionary complex exhumed from source depths of vlfe are composed of quartz rich fault rocks with or without clay foliations we examined the frictional velocity dependence of thrust materials at slip rates of 0 0028 0 28 mm s the powder sample from non foliated fault rock shows velocity weakening behavior while that from foliated fault rock exhibits velocity strengthening behavior both samples show velocity strengthening behavior at slip rates of 0 28 2 8 mm s microstructural analysis reveals that the velocity weakening sample shows a shear localization while the velocity strengthening sample is marked by clay foliations oblique and parallel to shear zone boundaries our results imply that frictional instability generates along quartz rich fault rock but slip becomes stable at higher slip rates regardless of mineral composition this is favorable for the occurrence of vlfe in subduction zones

a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone accretionary prism and incoming philippine sea plate new insights into episodic deformation at the nankai trough subduction zone are provided by data from two ocean drilling program borehole hydrologic observatories drilled into the philippine sea plate site 1173 and the seaward part of the nankai accretionary prism off southwestern japan site 808 and from an array of high sensitivity borehole accelerometer and velocity seismometers on shikoku island hi net fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site and relaxation of the accretionary prism toe the rate of strain ca 10 6 yr 1 inferred from the rates of pressure rise at site 1173 up to 5 kpa yr 1 is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide opposing these trends are transient pressure anomalies observed in late june early july 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism concurrent with these transients a swarm of very low frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust reverse fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism we suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about june 26 2003 at or near the up dip limit of the locked portion of the subduction thrust and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas in addition to demonstrating that the prism is far from inactive during interseismic intervals the observations may also provide a small amplitude analog for strain and hydrologic response at the time of great subduction earthquakes

episodic deformation and inferred slow slip at the nankai subduction zone during the first decade of cork borehole pressure and vlfe monitoring abstract with new data recovered in december 2011 and november 2012 from two ocean drilling program odp cork circulation obviation retrofit kit borehole observatories in the toe of the nankai subduction zone accretionary prism hole 808i and in the subducting philippine sea plate hole 1173b off southwestern japan records of formation fluid pressure now span over 10 yr over nearly this same period of time the japanese terrestrial hinet array has enabled detection of small earthquakes across the breadth of the accretionary prism the records include several formation fluid pressure anomalies and concurrent local very low frequency earthquake vlfe swarms in the subducting plate pressure anomalies are most commonly slow negative steps that are inferred to reflect dilatational strain associated with slow slip on the subduction thrust fault in the areas of vlfe activity in three instances concurrent positive impulsive anomalies are observed in the prism these are inferred to reflect contraction when slip reaches the location of hole 808i the spatial distribution of vlfes suggests that slow slip may occur in patches that cumulatively span the seaward half of the subduction prism two anomalies occurred at times of the largest earthquakes in the region the september 2004 mw 7 7 off kii earthquake 220 km to the northeast and the march 2011 mw 9 0 tohoku earthquake roughly 900 km to the northeast in the subducting plate the observed changes in pressure are abrupt and consistent in sign with the expected coseismic strain contractional and dilatational respectively in the case of the tohoku earthquake the co seismic pressure decrease was followed 12 days later by an additional but slow decrease and by an impulsive but complex and long lived increase in the accretionary prism hole 808i in this instance co seismic cross strike stress may have triggered post seismic slip and the deformation seen in the plate and prism given the great distance from the tohoku epicenter and small change in stress estimated at the nankai observatory sites ca 7 kpa shear stress triggering of local slip would require the outer subduction thrust and overlying prism in this region to be very fragile to what degree this state might vary with time is not known but tracking episodic slip in the seaward part of this and other subduction zones using formation fluid pressure and vlfe monitoring may provide valuable clues about the evolution of subduction faults through their thrust earthquake cycles

very low frequency earthquakes in cascadia migrate with tremor we find very low frequency earthquakes vlfes in cascadia under northern washington during 2011 episodic tremor and slip event vlfes are rich in low frequency energy 20 50 s and depleted in higher frequencies higher than 1 hz compared to local earthquakes based on a grid search centroid moment tensor inversion we find that vlfes are located near the plate interface in the zone where tremor and slow slip are observed in addition they migrate along strike with tremor activity their moment tensor solutions show double couple sources with shallow thrust mechanisms consistent with shear slip at the plate interface their magnitude ranges between mw 3 3 and 3 7 seismic moment released by a single vlfe is comparable to the total cumulative moment released by tremor activity during an entire episodic tremor and slip event the vlfes contribute more seismic moment to this episodic tremor and slip event than cumulative tremor activity and indicate a higher seismic efficiency of slow earthquakes in cascadia than previously thought spatiotemporal correlation of vlfe and tremor activity suggests that they are the results of the same physical processes governing slow earthquakes

fluid rich damage zone of an ancient out of sequence thrust kodiak islands alaska 1 the uganik thrust is a fossil out of sequence thrust fault which was active over a period of 3 ma during the early tertiary until activity ceased with the subduction of the kula farallon spreading ridge at 57 ma during this period the fault experienced at least 1 km of throw and developed a strongly asymmetric damage zone the brittle damage zone in the footwall of the fault acted as a conduit for fluid advection during the active faulting a similar asymmetrical footwall damage zone has been interpreted as a fluid conduit at the nobeoka thrust shimanto belt sw japan thermal indicators in the uppermost footwall give similar maximum paleotemperatures to those in the hanging wall 280 c while previous work elsewhere in the footwall formation suggests maximum burial temperatures of 240 c in this case similar to the irish canyon thrust in the franciscan accretionary complex the location of the thermal anomaly is spatially offset from the structural fault which caused it owing to thermal overprinting in the vicinity of the fault

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

dynamic deformation of the accretionary prism excites very low frequency earthquakes 1 we have detected anomalous very low frequency earthquakes within the accretionary prism along the nankai trough southwestern japan centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at 10 km depth above the upper surface of the subducting philippine sea plate and within 50 70 km landward of the trough axis the focal mechanisms indicate reverse faulting their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea floor topography these observations suggest that the occurrence of very low frequency earthquakes is related to numerous reverse fault systems within the accretionary prism and that the earthquakes reflect the dynamics of deformation within this accretionary prism

evidence of localized failure along altered basaltic blocks in tectonic melange at the updip limit of the seismogenic zone implications for the shallow slow earthquake source

modeling of long period ground motions in the nankai subduction zone model simulation using the accretionary prism derived from oceanfloor local s wave velocity structures the accretionary prism in the subduction zone which consists of thick low velocity oceanic sediments significantly affects the propagation of seismic waves for shallow offshore earthquakes including large interplate earthquakes in order to simulate long period 5 s ground motions in the nankai subduction zone we constructed a three dimensional 3d seismic velocity structure model of the accretionary prism by interpolation extrapolation of local s wave velocity structures beneath 46 oceanfloor seismic stations donet which are deployed just above the accretionary prism off the southern kii and eastern shikoku regions we modeled local s wave velocity structures using a simple two parameter depth varying velocity function to investigate the effects of the accretionary prism on ground and seafloor motions we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern japan the simulations reasonably reproduced the observed seismograms not only for the period ranges of the moment tensor inversion 50 s but also for the strong long period ground motions in the sedimentary basins 5 s especially in the region where donet stations are densely deployed since depth varying local s wave structures significantly improve the reproducibility of long period ground motions our modeling procedure is useful for modeling long period ground motions of local and regional offshore subduction zone earthquakes

spatial distribution and energy release of nonvolcanic tremor at parkfield california nonvolcanic tremors nvts are observed in transition zones between freely slipping and locked sections of faults and normally occur below the seismogenic zone based on nvt recordings in the parkfield region of the san andreas fault we provide a novel approach to assess the energy release of these events and assign magnitudes me that are compatible with size estimates of small earthquakes in the same region to assess the energy magnitude of a detected tremor we refine the estimate of its duration and perform a spectral analysis that accounts for local attenuation for the 218 nvts that we were able to process we resolve me values in the range of 0 67 to 0 84 for events which we could not process using the spectral analysis technique we propose a statistical model to estimate me values using observable characteristics such as peak amplitude spectral velocity at the source corner frequency and duration we furthermore provide seismic moment and moment magnitude estimates and calculate stress drops in a range of 3 10 kpa as a result of our spectral analyses we find strong indications regarding the ongoing debate about potential nvt location hypotheses the parkfield nvts have a higher probability to be located in the proposed three dimensional cloud like cluster than in any other suggested location distribution

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

the 2015 gorkha earthquake a large event illuminating the main himalayan thrust fault the 2015 gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the main himalayan thrust mht to overcome limitations due to unaccounted lateral heterogeneities we perform centroid moment tensor inversions in a 3 d earth model for the main shock and largest aftershocks in parallel we recompute s top and p to s receiver functions from the hi climb data set inverted centroid locations fall within a low velocity zone at 10 15 km depth and corresponding to the subhorizontal portion of the mht that ruptured during the gorkha earthquake north of the main shock hypocenter receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the mht our analysis of the main shock indicates that long period energy emanated updip of high frequency radiation sources previously inferred this frequency dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids

hanging wall deformation of a seismogenic megasplay fault in an accretionary prism the nobeoka thrust in southwestern japan the structure and occurrence of deformation within the hanging wall of the nobeoka thrust in kyushu japan was investigated to understand the dynamic aspects of splay faulting in relation to seismic events from field observations hanging wall is suggested to have undergone four phases of deformation the first phase involved horizontal shortening as documented by folding and thrusting followed by a phase of vertical loading shown by the development of horizontal slaty cleavages pressure solution and cleavage parallel mineral vein precipitation a third phase involved shearing and deformation along cleavage restricted to near the nobeoka thrust while the fourth phase produced widespread brittle fracturing associated with the development of pseudotachylyte bearing faults and tension crack filling veins high angle to cleavage these four phases can be explained as follows during the inter seismic period an extensionally stable taper was maintained in the inner wedge of the accretionary prism by dominant vertical loading 1 in combination with a lesser amount of horizontal compression 2 related to the locking of the mega thrust elastic strain energy in the hanging wall of the inner wedge was co seismically released by slip on the mega thrust and horizontal shortening in the outer wedge associated with dynamic ductile weakening of the fault plane this sudden release of elastic strain caused brittle fracturing with 1 at a high angle to the shear surface of the nobeoka thrust most of the displacement resulting from deformation of the footwall

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

near field observations of an offshore mw 6 0 earthquake from an integrated seafloor and subseafloor monitoring network at the nankai trough southwest japan an mw 6 0 earthquake struck 50 km offshore the kii peninsula of southwest honshu japan on 1 april 2016 this earthquake occurred directly beneath a cabled offshore monitoring network at the nankai trough subduction zone and within 25 35 km of two borehole observatories installed as part of the international ocean discovery program s nantroseize project the earthquake s location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate sized offshore earthquake we use the offshore seismic network to locate the main shock and aftershocks seafloor pressure sensors and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation and seafloor pressure observations to model the resulting tsunami contractional strain estimated from formation pore pressure records in the borehole observatories equivalent to 0 37 to 0 15 strain provides a key to narrowing the possible range of fault plane solutions together these data show that the rupture occurred on a landward dipping thrust fault at 9 10 km below the seafloor most likely on the plate interface pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock the earthquake and its aftershocks are located within the coseismic slip region of the 1944 tonankai earthquake mw 8 0 and immediately downdip of swarms of very low frequency earthquakes in this region illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone

tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip at subduction zones slip along the shallowest parts of the plate boundary is generally thought to be aseismic observations of very low frequency earthquakes occurring at shallow levels on the plate boundary of the nankai trough subduction zone imply that slow but seismic slip can occur there and could potentially generate tsunamigenic earthquakes

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone an example from the nobeoka thrust drilling project 1 we examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the nobeoka thrust drilling project which provide a high resolution transect of properties across the main fault zone the footwall cataclasite exhibits higher averages of neutron porosity 7 6 and lower values of electric resistivity 232 m compared to the hanging wall phyllite 4 8 453 m this clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation despite the contrast observed between the hanging wall and footwall in macroscopic scale the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using archie s law suggesting the similarities in microstructures and mineralogy in this low porosity range above the main fault core of the nobeoka thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow archie s law damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the nankai trough but with much thicker width whereas the footwall damage zone is more extensive in the nobeoka thrust splay faults may exhibit strong deformation in the hanging wall in the early stage and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault the damage effect may eventually be enlarged in the footwall

effects of shear heating slip induced dilatancy and fluid flow on diversity of 1 d dynamic earthquake slip we theoretically study effects of thermal pressurization slip induced dilatancy and fluid flow on slip evolution assuming 1 d fault model we generalize the analysis made in our former papers by introducing an upper limit for the inelastic porosity evolution the expression for nondimensional parameter ta which is related to the upper limit is derived in the present paper we find that the parameter ta together with two nondimensional parameters su and su derived in our former papers completely determine the qualitative nature of system behavior once the initial condition is given for example changes in su and ta generate two qualitatively different slip behaviors both of which can be models for ordinary earthquake the slip is accelerated with time after experiencing an initial deceleration in one of them while the slip ceases after monotonic deceleration in the other the initial slip deceleration observed in the former case may be interpreted as a dynamic event preceding the main shock in seismological observations we mathematically derive the expression for the ranges of the nondimensional parameters in which the above two slip behaviors appear slow earthquakes are also modeled in the same framework and nonzero values of the nondimensional parameter su together with relatively large values of su and ta are found to contribute to the generation of such slow earthquakes the mathematical framework constructed here can be applied to understanding of some other natural phenomena such as a kind of reaction diffusion system

along strike structural changes controlled by dehydration related fluids within the philippine sea plate around the segment boundary of a megathrust earthquake beneath the kii peninsula southwest japan 1 active and passive seismic experiments in the kii peninsula southwest japan revealed prominent structural features around the segment boundary of a megathrust earthquake associated with the subduction of the philippine sea plate phs a distinct reflection band in the uppermost part of the phs shows significant lateral variation along its strike the thicker reflection band which corresponds to the deeper extension of the fault area of the 1944 tonankai earthquake is interpreted to be a zone of high pore fluid pressure causing a state of conditionally stable slip on the plate boundary by the reduction in effective normal stress a thinner reflective band corresponds to the deepest part of the rupture area of the 1946 nankai earthquake where plate coupling is stronger due to less effect of fluids this along strike structural variation controls the differences in frictional properties and the lateral limit of rupturing along the plate boundary

spatiotemporal distribution of low frequency earthquakes in southwest japan evidence for fluid migration and magmatic activity abstract low frequency earthquakes lfes in the lower crust and uppermost mantle are widely observed in southwest japan and they occur not only along the subducting philippine sea phs slab interface but also beneath active arc volcanoes the volcanic lfes are still not well understood because of their limited quantities and less reliable hypocenter locations in this work seismic tomography is used to determine detailed three dimensional 3 d p and s wave velocity vp and vs models of the crust and upper mantle beneath southwest japan and then the obtained 3 d vp and vs models are used to relocate the volcanic lfes precisely the results show that the volcanic lfes can be classified into two types pipe like and swarm like lfes and both of them are located in or around zones of low velocity and high poisson s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes the pipe like lfes may be related to the fluid migration from the lower crust or the uppermost mantle whereas the swarm like lfes may be related to local magmatic activities or small magma chambers the number of lfes sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration the spatiotemporal distribution of the lfes may indicate the track of migrating fluids as compared with the tectonic lfes along the phs slab interface the volcanic lfes are more sensitive to fluid migration and local magmatic activities high pore pressures play an important role in triggering both types of lfes in southwest japan

coseismic versus interseismic ground deformations fault rupture inversion and segmentation revealed by 2003 mw 6 8 chengkung earthquake in eastern taiwan 1 the 2003 chengkung earthquake mw 6 8 provided diagnostic evidence for a source model showing the deformation process of the seismogenic chihshang fault in eastern taiwan the aftershocks show a fault bend at a depth of 18 km coseismic ground displacements recorded by strong motion records allow us to deduce instant rupturing of this event our resulting model shows a fault length of 33 km and dip slip dominant rupture on fault plane deeper than 18 km estimated coseismic displacements constrain two fault planes one at 5 18 km depth dipping 60 e and 18 36 km depth dipping 45 e the uppermost fault plane of the chihshang fault 0 5 km did not break immediately after the main shock however it may have a major role in after slip and even interseismic ground deformation the taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements

implications of basement rock alteration in the nankai trough japan for subduction megathrust slip behavior abstract the nankai trough is an exceptionally well studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip outcrop studies of exhumed analogues for the modern subduction thrust as well as seismic reflection images of the active subduction zone suggest that the basaltic basement participates in plate boundary shearing at depths of a few km and greater we obtained altered basaltic upper basement from the modern nankai trough seaward of the trench recovered by drilling on iodp expedition 333 we performed laboratory friction experiments on bare surfaces and gouge powders of this material in addition to and in combination with other materials for comparison the altered basalt exhibits predominantly velocity strengthening frictional behavior indicating a tendency for stable slip for bare surface experiments few instances of velocity weakening friction occur and are restricted to slip rates of

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms

low frequency tremors associated with reverse faults in a shallow accretionary prism abstract the shallow part of a seismogenic subduction zone is thought to play an important role in tsunami genesis during large interplate thrust earthquakes near the updip portion of the seismogenic zone along the nankai trough splay faults which are major active reverse faults in the accretionary prism likely rupture during large interplate earthquakes such as the 1944 m w 8 2 tonankai earthquake off kii peninsula we observed low frequency tremors associated with reverse faults in a shallow accretionary prism off kii peninsula at close range by using ocean bottom seismographs the tremors were characterized by a dominant frequency range of 2 8 hz and a lack of energy in the frequency range above 10 hz their duration ranged from tens of seconds to a few minutes their source locations lay in three clusters on the landward slope of the nankai trough activity in each cluster continued for from one day to two weeks many of the low frequency tremors were located near the shallowest part of a major splay fault the episodic activity of low frequency tremors and of previously reported very low frequency earthquakes which indicate reverse faulting in the shallow accretionary prism suggests that the reverse faults in the accretionary prism are conditionally stable faults that can become unstable under sufficiently strong dynamic loading such as that caused by a large earthquake such reverse faults as typified by a splay fault can rupture during large interplate earthquakes and generate large tsunamis

probing the slip weakening mechanism of earthquakes with electrical conductivity rapid transition from asperity contact to gouge comminution 1 despite the great importance of slip weakening during earthquakes the fault weakening mechanism inside the fault remains to be clarified here we propose a micromechanism of slip weakening at seismic and subseismic slip rates as demonstrated by electrical conductivity observation across the fault at the seismic slip rate the formation of melt patches and the subsequent growth to molten layer during frictional melting were confirmed by the rapid increases in conductivity in two stages at the subseismic slip rate the conductivity data successfully captured rapid asperity breakage phases and the subsequent gouge evolution phases during the slip weakening process the present results suggest that the gouge comminution process is one of the key mechanisms controlling the slip stability at the subseismic slip rate which may cause a variety of natural earthquakes