the influence of fluid sensitive dispersion and attenuation on avo analysis summary analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation but this is generally ignored during amplitude versus offset avo analysis the effect can be modelled with equivalent medium theory based on the squirt flow concept but the excess attenuation is associated with strong velocity dispersion consequently when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency the impact of this variation depends on the avo behaviour at the interface class i reflections tend to be shifted to higher frequency while class iii reflections have their lower frequencies amplified we calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations and find that these effects are predicted to be detectable on stacked data two field data sets show frequency anomalies similar to those predicted by the analysis and we suggest that our modelling provides a plausible explanation of the observations

three dimensional splay fault geometry and implications for tsunami generation megasplay faults very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism are thought to play a role in tsunami genesis we imaged a megasplay thrust system along the nankai trough in three dimensions which allowed us to map the splay fault geometry and its lateral continuity the megasplay is continuous from the main plate interface fault upwards to the sea floor where it cuts older thrust slices of the frontal accretionary prism the thrust geometry and evidence of large scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time contrary to the usual seaward progression of accretionary thrusts the megasplay fault has progressively steepened substantially increasing the potential for vertical uplift of the sea floor with slip we conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis such as the 1944 moment magnitude 8 1 tonankai event and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis

microseismicity around rupture area of the 1944 tonankai earthquake from ocean bottom seismograph observations abstract along the nankai trough southwestern japan the philippine sea plate psp is subducting beneath the eurasian plate and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100 200 years the most recent large thrust event in the eastern nankai trough off kii peninsula was the 1944 tonankai earthquake in this region current seismicity is very low and hypocenters are not determined accurately by the land seismic network we conducted microseismicity observations around the rupture area of the 1944 tonankai earthquake using ocean bottom seismographs obss hypocenters were determined using a 2 d seismic velocity structure model based on an airgun obs seismic survey results obtained show that the seismicity was relatively active near the trough axis these earthquakes may relate to deformation of the subducting philippine sea plate on the other hand microseismicity in the rupture area of the 1944 tonankai earthquake was very low this low level seismic activity in the co seismic rupture area of the 1944 tonankai earthquake likely relates to a single large asperity off kii peninsula

deformation structures from splay and decollement faults in the nankai accretionary prism sw japan iodp nantroseize expedition 316 evidence for slow and rapid slip in fault rocks

seismicity in the incoming subducting philippine sea plate off the kii peninsula central nankai trough 1 along the nankai trough southwestern japan the philippine sea plate phs is subducting beneath the eurasian plate and large interplate earthquakes have occurred repeatedly the latest large thrust earthquake at the central nankai trough off the kii peninsula was the 1944 tonankai earthquake mw 8 2 although a previous ocean bottom seismograph obs experiment from november 2001 to february 2002 showed seismicity around the rupture area of the 1944 tonankai earthquake there were large uncertainties regarding hypocenter locations especially with respect to depth we conducted another obs experiment to obtain accurate locations of earthquakes and their focal mechanisms the new results show earthquake clusters at the trough axis within the oceanic crust the clusters were located near faults cutting the oceanic crust and thus the earthquakes may have occurred along preexisting faults in the incoming phs seismicity in the subducting phs is characterized by seismic quiescence in the oceanic crust of the coseismic rupture area of the 1944 tonankai earthquake the change in seismicity in the oceanic crust can be explained by a change in the stress in the incoming subducting phs the stress in the oceanic crust of the phs changes from trough normal tension at the trough axis caused by the bending of the phs to downdip compression beneath the kii peninsula the seismicity in the subduction zone likely relates to the balance of stress in the incoming and the subducting plate as well as to coupling between the subducting and overriding plates

characteristics of slow earthquakes in the very low frequency band application to the cascadia subduction zone deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency vlf signals detectable by broadband seismometers but usually hidden in large ambient noise by stacking broadband seismograms relative to tremor hypocentral times we can recover the vlf signals the stacked signals are then inverted to determine a moment tensor solution using a procedure previously applied to vlf signals in japan taiwan and mexico here we apply this method to the cascadia subduction zone where tremors and slow slip events are clearly observed as expected we successfully recover vlf signals for almost the entire tremor region beneath southern vancouver island and northern washington state the moment tensors are mostly well determined as low angle thrust type but source depths are poorly constrained the slip direction is slightly rotated counterclockwise with respect to the local plate motion direction probably due to bending of the subducting plate the seismic moment measured in vlf band is proportional to the seismic energy of tremors with a scaled energy of about 3 10 9 the widespread observability of vlf signals suggests that the deformation associated with tremors and sses is actually a very broadband phenomenon as suggested by stochastic models

the frictional hydrologic metamorphic and thermal habitat of shallow slow earthquakes slow earthquakes have recently been discovered on the shallow parts of some subduction zones a review of the conditions under which these quakes form reveals that they could be common in most subduction zones globally

recurring and triggered slow slip events near the trench at the nankai trough subduction megathrust the discovery of slow earthquakes has revolutionized the field of earthquake seismology defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults we report on a family of recurring slow slip events sses on the plate interface immediately seaward of repeated historical moment magnitude mw 8 earthquake rupture areas offshore of japan the sses continue for days to several weeks include both spontaneous and triggered slip recur every 8 to 15 months and are accompanied by swarms of low frequency tremors we can explain the sses with 1 to 4 centimeters of slip along the megathrust centered 25 to 35 kilometers km from the trench 4 to 10 km depth the sses accommodate 30 to 55 of the plate motion indicating frequent release of accumulated strain near the trench

the nature of the landers mojave earthquake line the landers california earthquake of 28 june 1992 magnitude 7 3 is the latest of six significant earthquakes in the past 60 years whose epicenters and slip directions define a 100 kilometer alignment running approximately n15 w across the central mojave region this pattern may indicate a geologically young throughgoing fault that replaces numerous older strike slip faults by obliquely cutting across them these older faults and perhaps also the bend in the san andreas fault may be losing their ability to accommodate upper crustal deformation because they have become unfavorably oriented with respect to the regional stress field

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

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

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

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

thermal effects of hydrothermal circulation and seamount subduction temperatures in the nankai trough seismogenic zone experiment transect japan 1 we examine the thermal effects of seamount subduction seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position prior to subduction seamounts provide high permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust as the seamount is subducted the high permeability pathway is closed shutting down the advective transfer of heat if significant fluid flow occurs it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system additionally subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge we apply these features of seamount subduction to a thermal model for the nankai trough seismogenic zone experiment transect on the margin of japan models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench they yield temperatures of 100 c 295 c for the rupture area of the 1944 tonankai earthquake temperatures in the region of episodic tremor and slip are estimated at 290 c 325 c 70 c cooler than a model with no fluid circulation

active faulting near the cajon pass well southern california implications for the stress orientation near the san andreas fault the dosecc cajon pass well is located 4 km ne of the san andreas fault in a region that includes a diversity of secondary structures active faults close to the well are subparallel to the right lateral san andreas fault but exhibit left lateral and normal slip the stress orientation measured in the well is not consistent with right lateral slip on the san andreas fault or the stresses inferred from focal mechanisms of earthquakes within 10 km of the fault but may drive the e striking slightly normal left lateral cleghorn fault the closest active fault to the well if the in situ stress is everywhere consistent with the highly variable styles of active secondary faulting like the cleghorn the orientation of stresses along the san andreas fault may be quite variable

waveform tomography imaging of a megasplay fault system in the seismogenic nankai subduction zone abstract we apply frequency domain waveform tomography to form quantitative high resolution p wave velocity images of a megasplay fault system within the central nankai subduction zone offshore of southwest japan using controlled source ocean bottom seismometer obs data originally acquired in 2004 the waveform tomography method exploits recorded seismic waveforms beyond their first arrivals and thus achieves a much higher resolution of the order of a wavelength than that of conventional traveltime tomography methods frequency domain waveform tomography facilitates a multi scale approach to stabilize the inversion in which initial traveltime tomography results are sequentially improved on by first fitting low frequency components of the seismic records starting at 2 25 hz higher frequency components up to 8 5 hz are then introduced progressively our final waveform tomography image allows velocity anomalies as small as 700 m horizontally and 350 m vertically to be discerned and interpreted as confirmed by checkerboard modeling tests the improved explanatory power of the final images is verified by observing that synthetic waveforms calculated from the final results yield much better fit to the observed waveforms than those estimated from the original traveltime tomography image apparent lithological boundaries from waveform tomography agree well with corresponding reflections on seismic migration images providing further confidence in the validity of the results the megasplay fault is evident on the final waveform tomography image as a sharp velocity discontinuity delineating the upper surface of a velocity reduction of nearly 1 km s with respect to the regional 1d velocity trend the megasplay fault can be traced continuously throughout the entire image from a nearly horizontal section at the landward extent moving seaward through to a steeper angle penetrating the old accretionary prisms with several additional splays appearing to branch in the shallow subsurface the location of the low velocity zones imaged by our waveform tomography method coincides with two previously identified low velocity zones the image reveals a low velocity zone that is continuous from deeper to shallower portions of the subsurface suggesting that pore fluids may be transported from the inner wedge to the transition zone and to the surface through fluid conduits associated with the megasplay fault system

three dimensional tomography of the 1992 southern california earthquake sequence constraints on dynamic earthquake rupture tomographic inversion of p wave arrival times from aftershocks of 1992 southern california earthquakes is used to produce three dimensional images of subsurface velocity the preliminary 1992 data set augmented by the 1986 m 5 9 north palm springs sequence consists of 6458 high quality events recorded by the permanent regional network providing 76306 raypaths for inversion the target area consisted of a 104 x 104 x 32 km 3 volume divided into 52 x 52 x 10 rectilinear blocks significant velocity perturbations appear to correlate with rupture properties of recent major earthquakes preliminary results indicate that a low velocity anomaly separates the dynamic rupture of the m 6 5 big bear event from the m 7 4 landers main shock a similar low velocity region separates the m 6 1 joshua tree sequence from the landers rupture high velocity anomalies occur at or near nucleation sites of all four recent main shocks north palm springs joshua tree landersbig bear a high velocity anomaly is present along the san andreas fault between 5 and 12 km depth through san gorgonio pass this high velocity area may define an asperity where stress is concentrated and where future large earthquakes may begin

an apparent shear zone trending north northwest across the mojave desert into owens valley eastern california strain rates measured at four geodetic networks in eastern california situated between northern owens valley and transverse ranges along a small circle drawn about the pacific north america pole of rotation are remarkably consistent each exhibits 0 14 mu rad yr simple right lateral engineering shear strain accumulation across the local vertical plane tangent to the small circle local faults e g owens valley garlock helendale traversing these networks are not as closely aligned with the vertical planes of maximum shear strain accumulation as is the local tangent to the small circle a fifth network slightly east of the small circle shows no significant strain accumulation thus a shear zone trending n35 degree w from near the eastern end of the big bend of the san andreas fault to northern owens valley is indicated by these data this corresponds to the eastern california shear zone proposed on geological evidence by dokka and travis the shear zone carries approximately 8 mm yr of the pacific north america relative plate motion from the san andreas fault north northwest across the mojave desert into owens valley and the northern basin and range province the shear zone observed at the surface may be a manifestation of a through going subcrustal fault

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

the san andreas fault system through the transverse ranges as illuminated by earthquakes the pattern of seismogenic faulting in the san gorgonio pass san bernardino basin area of the san andreas fault saf zone is mapped from about 7000 quality selected focal mechanisms 1981 nov 92 they are derived from phase data of the southern california network by a relocation procedure based on location dependent station corrections and by a grid search procedure about 3 4 of the mechanisms have been interpreted as planes of rupture or slip planes that delineate many distinct faults one of these faults is right lateral dips steeply northeast and is continuous trough the san gorgonio pass area we interpret it as the main branch of the saf a very large earthquake seems possible on this portion of the saf because of its continuity and because of the unusually deep reach of the seismicity 23 km southwest of this fault a volume of diffuse and persistent seismicity has a sharp downward cut off which may reflect a basal detachment dipping 20 northeast and intersecting the saf along the deepest seismicity northeast of the saf the floor of the seismicity is shallower by as much as 10 km stress characteristics are derived from slip planes in 10 selected subregions the regime is transpressional near the constricting bend of the saf at san gorgonio pass it is dominated by horizontal extension in the san bernardino basin area and it is again transpressional in the eastern san gorgonio mountains north of the cucamonga thrust the broad features of this zonation may be related to the intersection of the saf with the pinto mountain fault and with the san jacinto fault a change of fault kinematics in the yucaipa cluster coincides with the 1992 landers and big bear main shocks and may be a manifestation of static stress change

triggered tectonic tremor in various types of fault systems of japan following the 2012 mw8 6 sumatra earthquake tectonic tremor an extremely stress sensitive seismic phenomenon occurring in the brittle ductile transition section of a fault is associated with the shearing mechanism of slow slip observations of triggered tremor can facilitate the evaluation of the existence of background ambient tremor and slow slip events this paper presents widespread triggered tremor sources in japan initiated by the surface waves of the 2012 mw8 6 sumatra earthquake on strike slip and thrust faults in the deep volcanic low frequency earthquake active area in the shallow tectonic tremor and very low frequency earthquake active regions and in the subduction zone in most regions the amplitudes of triggered tremor are generally logarithmically proportional to the dynamic stress caused by various triggering earthquakes our observations suggest that triggered tremor in the newly discovered sources is the result of a more rapid rate of background ambient tremor and evidence has suggested the existence of ambient tremor in some regions

seismicity at the eastern end of the 1944 tonankai earthquake rupture area the rupture areas of the large interplate thrust earthquakes along the nankai trough offshore southwestern japan are divided into several segments the 1944 tonankai earthquake ruptured one of the segments off of the kii peninsula in 2005 we conducted an ocean bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake little seismic activity was observed on the plate interface most earthquake activity was within the subducting and overriding plates aftershocks of the 2004 off kii peninsula earthquake which was an intraplate earthquake in the subducting philippine sea plate were mainly located in the subducting oceanic crust and uppermost mantle however several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge with a focal mechanism indicating deformation by a subducting seamount the earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 tonankai earthquake in contrast few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling which in turn caused the observed differences in the deformation of the sedimentary wedge the spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the nankai trough

seismicity related to heterogeneous structure along the western nankai trough off shikoku island 1 along the western nankai trough southwest japan heterogeneous crustal structures such as scattered deep strong reflectors and a subducted topographic high showed a spatial relationship to along trough variation in the coseismic slip during the 1946 nankai earthquake which is the most recent of repeated interplate earthquakes of magnitude 8 range in this area we observed micro earthquakes using a temporary ocean bottom seismograph network off shikoku island western nankai trough near the trough axis off cape muroto earthquakes in the subducting oceanic plate were associated with structures in the oceanic crust and the uppermost mantle of the subducting plate beneath the trough landward slope the observed earthquakes suggest heterogeneous interplate coupling related to the crustal structure variations the crustal structure variations could affect the interplate coupling during the interseismic period as well as coseismic rupture propagation during large interplate earthquakes along the western nankai trough

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

decomposition of structural amplitude effect and avo attributes application to a gas water contact the structural amplitude effect associated with focusing and defocusing due to the reflector curvature importantly contributes to reflection seismic amplitudes this paper develops a conciliatory approach for estimating the structural amplitude effect and the attributes of amplitude variation versus offset avo the avo attributes are extracted from raw amplitudes in which the structural effect is taken into account explicitly based on a structural model reconstructed from travel time inversion one of the goals is to conduct the avo analysis not just locally per cdp but also horizontally to see the global variation along the reflection the lateral variations of avo attributes are decomposed by the chebyshev expansion the method is demonstrated with an example of weak shallow gas water contact appearing on a 2 d seismic profile of a site survey in the north sea

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

the impact of reservoir scale on amplitude variation with offset to investigate the vertical and horizontal impact of reservoir scale on the amplitudeversus offset characteristics we conduct seismic numerical simulations on models containing spatially confined lithologic units with different scales we find that the reservoir scale has a nonlinear effect on the amplitude versus offset intercepts and gradients as the reservoir width increases amplitude versus offset intercept and gradient both first increase then decrease and finally remain stable the amplitudeversus offset intercept is maximum when the reservoir width is 80 larger than the fresnel zone radius whereas the amplitude versus offset gradient peaks at 1 5 times the fresnel zone radius both amplitude versus offset intercept and gradient are approximately proportional to the reservoir width prior to reaching their maxima when the lateral extent of the reservoir is more than three times the fresnel zone radius the amplitude versus offset attributes are constant modelling the reservoir thickness shows that intercept and gradient behave in a manner similar to that of tuning of thin beds both the amplitude versus offset intercept and gradient first increase and then decrease with the thickness peaking at the tuning thickness the thickness contribution to amplitude versus offset variations is negligible when the thickness is larger than 1 6 times of the tuning thickness considering the magnitude of the changes in amplitude versus offset intercept and gradient caused by reservoir scale the width causes a maximum 433 intercept increase and a 344 gradient increase whereas the thickness causes a maximum 100 intercept increase and a 73 gradient increase cross plotting the amplitude versus offset intercept and gradient shows the reservoir scale change gives rise to an anti clockwise spiraling effect in conclusion the lateral and vertical extents of the reservoir both play an important role in amplitude variation with offset our analysis shows that the lateral reservoir extent has a larger impact on the amplitude variation with offset than the vertical tuning effect

mixed deformation styles observed on a shallow subduction thrust hikurangi margin new zealand geophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults but geological signatures and underlying deformation processes remain poorly understood international ocean discovery program iodp expeditions 372 and 375 investigated new zealand s hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow slip events we report direct observations from cores that sampled the active p paku splay fault at 304 m below the seafloor this fault roots into the plate interface and comprises an 18 m thick main fault underlain by 30 m of less intensely deformed footwall and an 10 m thick subsidiary fault above undeformed footwall fault zone structures include breccias folds and asymmetric clasts within transposed and or dismembered relatively homogeneous silty hemipelagic sediments the data demonstrate that the fault has experienced both ductile and brittle deformation this structural variation indicates that a range of local slip speeds can occur along shallow faults and they are controlled by temporal potentially far field changes in strain rate or effective stress

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

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

hydrocarbon seepage and its sources at mud volcanoes of the kumano forearc basin nankai trough subduction zone twelve submarine mud volcanoes mv in the kumano forearc basin within the nankai trough subduction zone were investigated for hydrocarbon origins and fluid dynamics gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from mvs 2 4 5 and 10 molecular ratios c1 c2 40 v pdb indicate that hydrate bound hydrocarbons hcs at mvs 2 4 and 10 are derived from thermal cracking of organic matter considering thermal gradients at the nearby iodp sites c0009 and c0002 the likely formation depth of such hcs ranges between 2300 and 4300 m below seafloor mbsf with respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of hcs fueling the mvs is derived from sediments of the cretaceous to tertiary shimanto belt below pliocene pleistocene to recent basin sediments considering their sizes and appearances hydrates are suggested to be relicts of higher mv activity in the past although the sporadic presence of vesicomyid clams at mv 2 showed that fluid migration is sufficient to nourish chemosynthesis based organisms in places distributions of dissolved methane at mvs 3 4 5 and 8 pointed at fluid supply through one or few mv conduits and effective methane oxidation in the immediate subsurface the aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth

dynamic role of tectonic melange during interseismic process of plate boundary mega earthquakes abstract tectonic melange has a key role in subduction zones because its thick pile forms the plate boundary itself therefore the plate boundary process is nearly identical to the melange forming process we examined three tectonic melanges in the shimanto belt in southwest japan to decipher their progressive deformation process with subduction especially within the seismogenic zone here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment dominated tectonic melange the necessity of tectonic melange accompanying fossil seismogenic decollement is unveiled several deformation processes continue by turn until the depth of the down dip limit of the seismogenic zone is reached the results support the space time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes

spatial relationship between shallow very low frequency earthquakes and the subducted kyushu palau ridge in the hyuga nada region of the nankai subduction zone

influence of subduction zone conditions and gouge composition on frictional slip stability of megathrust faults to understand the temperature depth distribution of destructive earthquakes in subduction megathrusts and the mechanisms of nucleation of these events data on the frictional behaviour of phyllosilicate quartz rich megathrust fault gouges under in situ conditions are needed we performed rotary shear friction experiments at effective normal stresses of 25 200 mpa pore fluid pressures of 50 200 mpa at 140 600 c and sliding velocities of 1 100 m s using gouge mixtures with an illite quartz ratio between 65 35 and zero experiments on 65 35 mixtures deformed at an effective normal stress neff of 170 mpa a pore fluid pressure pf of 100 mpa and 150 500 c provided a reference dataset this showed three temperature dependent slip stability regimes with potentially unstable velocity weakening behaviour at 250 400 c and velocity strengthening at lower and higher temperatures the velocity weakening regime was found to shift towards higher temperatures with decreasing neff being located at 350 600 c at 50 mpa increasing quartz content and decreasing sliding velocity also displaced the velocity weakening regime towards lower temperatures increasing pf increased a b at all temperatures narrowing the temperature extent of the velocity weakening regime we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and a b with velocity and temperature are brought about by changes in the relative importance of deformation of the clast phase by thermally activated stress corrosion cracking and pressure solution versus athermal granular flow of the mixture accompanied by dilatation our results imply that the depth extent of the seismogenic zone on subduction megathrusts depends not only on temperature and that to predict its location it is essential to have well constrained depth profiles for pore fluid pressure and effective normal stress

what do the cajon pass stress measurements say about stress on the san andreas fault comment on in situ stress measurements to 3 5 km depth in the cajon pass scientific research borehole implications for the mechanics of crustal faulting by mark d zoback and john h healy

episodic slow slip events in the japan subduction zone before the 2011 tohoku oki earthquake abstract we describe two transient slow slip events that occurred before the 2011 tohoku oki earthquake the first transient crustal deformation which occurred over a period of a week in november 2008 was recorded simultaneously using ocean bottom pressure gauges and an on shore volumetric strainmeter this deformation has been interpreted as being an m6 8 episodic slow slip event the second had a duration exceeding 1 month and was observed in february 2011 just before the 2011 tohoku oki earthquake the moment magnitude of this event reached 7 0 the two events preceded interplate earthquakes of magnitudes m6 1 december 2008 and m7 3 march 9 2011 respectively the latter is the largest foreshock of the 2011 tohoku oki earthquake our findings indicate that these slow slip events induced increases in shear stress which in turn triggered the interplate earthquakes the slow slip event source area on the fault is also located within the downdip portion of the huge coseismic slip area of the 2011 earthquake this demonstrates episodic slow slip and seismic behavior occurring on the same portions of the megathrust fault suggesting that the faults undergo slip in slow slip events can also rupture seismically

effect of pore pressure buildup on slowness of rupture propagation pore fluid pressure is known to play an important role in brittle fracture initiation and propagation yet the underlying mechanisms remain unclear we conducted triaxial experiments on saturated porous sandstones to investigate effects of pore pressure buildup on the slowness of shear rupture propagation at different confining pressures at low to intermediate confinements rocks fail by brittle faulting and pore pressure buildup causes a reduction in rock s shear strength but does not induce measurable differences in slip behavior when the confinement is high enough to prohibit dynamic faulting rocks fail in the brittle ductile transitional regime in the transitional regime pore pressure buildup promotes slip instability on an otherwise stably sliding fracture compared to those observed in the brittle regime the slip rate stress drop and energy dissipated during rupture propagation with concurrent pore pressure buildup in the transitional regime are distinctively different when decreasing confining pressure instead the slip behavior resembles the ones of the brittle regime emphasizing how the observed slowness is related to excess pore pressure beyond the effective pressure phenomenon analysis of the mechanical data using existing theoretical models confirms these observations quantitative microstructural analyses reveal that increasing pore pressure lessens the dilatancy hardening during failure thus enhances slip along the localized zone in the transitional regime our experimental results suggest that pore pressure buildup induces slow slip in the transitional regime and slip rates along a shear fracture may vary considerably depending on effective stress states

variations in strength and slip rate along the san andreas fault system convergence across the san andreas fault saf system is partitioned between strike slip motion on the vertical saf and oblique slip motion on parallel dip slip faults as illustrated by the recent magnitude m s 6 0 palm springs m s 6 7 coalinga and m s 7 1 loma prieta earthquakes if the partitioning of slip minimizes the work done against friction the direction of slip during these recent earthquakes depends primarily on fault dip and indicates that the normal stress coefficient and frictional coefficient vary among the faults additionally accounting for the active dip slip faults reduces estimates of fault slip rates along the vertical trace of the saf by about 50 percent in the loma prieta and 100 percent in the north palm springs segments

coseismic frictional heating and fluid rock interaction in a slip zone within a shallow accretionary prism and implications for earthquake slip behavior 1 to understand the relation between seismic events and the geological structure in a shallow accretionary prism we investigated a major reverse fault in a fossil accretionary prism the emi group burial depth 1 4 km boso peninsula japan we examined the slip zone rocks and the surrounding host rocks microscopically and analyzed their trace elements and isotopes using the fluid mobile trace element spectrum which is sensitive to fluid rock interaction at high temperatures we estimated that the slip zone experienced frictional heating of 350 c caused by high velocity sliding by numerical analysis of the frictional heating and thermal diffusion using various values for slip displacement the frictional coefficient depth and fault dip we determined the conditions sufficient for producing the estimated high temperature frictional heating at shallow depth 1 2 km the results suggest that such faulting in a shallow accretionary prism could potentially generate huge tsunamis

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

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

episodic aseismic slip in a two degree of freedom block spring model 1 we consider a two degree of freedom block spring model in which two blocks block 1 and block 2 are connected by a spring and driven by a slowly moving driver assuming a rate and state dependent friction law we set the friction parameters such that dynamic instability occurs at block 1 episodic aseismic slip occurs at block 2 when its frictional parameters are near the stability transition when the stress has accumulated to approximately a steady state level during an interseismic period block 2 starts a slow slip quasi static oscillation in the stress and slip velocity occurs with decaying fluctuation amplitudes leading to the steady state values the decaying oscillation could be a plausible generation mechanism of the episodic aseismic slip observed in the tokai district japan since 2001 we also consider various slip modes on a plate boundary based on this two block model associated with the interaction between different frictional properties

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

shallow episodic tremor near the nankai trough axis off southeast mie prefecture japan we analyzed long term continuous seismic records from september 2015 to april 2016 of dense ocean floor network system for earthquake and tsunamis an ocean floor observation system deployed around the fore arc slope of the nankai subduction zone to investigate shallow tremor near the trough axis we found that the activity of shallow tremor was concentrated in two time periods 6 days in october 2015 and 2 weeks in april 2016 during the episode in april 2016 migration and triggering of tremor were observed these characteristics are similar to those of tremor in the deeper part of the subduction zone the triggering of tremor indicates that the tremor activity is very sensitive to nearby stress perturbation in the area of this study which is near the initiation points of past large earthquakes along the nankai trough therefore it is very important to monitor tremor activity in this region for understanding the stress accumulation process of megathrust earthquakes

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

rupture process of the 1944 tonankai earthquake ms 8 1 from the inversion of teleseismic and regional seismograms 1 we digitized teleseismic and regional records of the 1944 tonankai earthquake we used the multiple time window method to invert these records for the spatial and temporal distribution of slip and rake we assume a 220 140 km fault with a spacing of 20 20 km and a maximum rupture velocity of 2 5 km s the inversion resolved an asperity along the accretionary wedge and under the shima and atsumi peninsulas the rupture propagated from southwest to northeast and there was little slip near the hypocenter consistent with the absence of uplift in the overlying submarine forearc basin the peak slip is 2 3 m and the total seismic moment is 2 4 1028 dyn cm mw 8 2 we compared the predicted and observed vertical geodetic displacements and inferred that the limit of rupture is consistent with the change from subsidence to uplift west of the atsumi peninsula this suggested that the earthquake did not rupture the plate interface in the tokai gap we modeled the tsunami using sea bottom displacements generated from this slip model the tide gauge amplitudes and frequency content are consistent with those predicted resolution tests from inverting synthetic seismograms computed using a simplified slip model indicated that the combination of teleseismic and regional data sets best recovers the asperity locations and peak slips given similar station geometry and degrees of freedom an inversion of the synthetic regional data set indicates that slip artifacts can occur early in the rupture process due to many clipped records while the inversion of synthetic teleseismic data set had less resolution and recovered only 50 of the peak slip

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

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

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

detailed structural image around splay fault branching in the nankai subduction seismogenic zone results from a high density ocean bottom seismic survey 1 to investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone it is important to know the detailed structure around the plate boundary and active splay fault system the nankai trough southwestern japan is among the best studied subduction zones with splay faults in the world this paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 tonankai earthquake in the central nankai trough based on results from a wide angle high density ocean bottom seismograph obs survey our seismic image clearly shows for the first time the subduction structure along with the splay fault reflections imaged on a previously obtained seismic reflection profile the correspondence between the splay fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time migrated section to a depth section by using a velocity model estimated from the wide angle obs data in this study the high resolution seismic image obtained by the high density obs survey demonstrates a zone of low velocity gradient just above and along the splay fault we focused specifically on the detailed structure around the splay fault a simple amplitude analysis of the wide angle reflections showed that it is likely that a thin low velocity layer with velocities 0 5 1 5 km s slower than those in the surrounding rock exists above the splay fault the presence of this layer suggests elevated fluid pressure in the fault zone which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes

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

co seismic frictional melting along an out of sequence thrust in the shimanto accretionary complex implications on the tsunamigenic potential of splay faults in modern subduction zones abstract out of sequence thrusts ost play an important role in the thickening of accretionary prisms formation of forearc basins and tsunami generation in subduction zones by detailed investigations of geological and paleogeothermal structures an ancient seismogenic ost is identified in the shimanto accretionary complex southwest japan this ost consists of a mappable en echelon fault system developed at a late stage of the accretionary process the estimated accumulated displacement of 2 5 8 5 km and formation depth of 2 5 5 5 km based on a thermal analysis indicates much higher fault activities sense of slip with a sinistral slip and the large displacement was distributed among multiple small faults each with a small offset most of the faults in the en echelon fault system recorded repeated brittle failure and one fault preserves pseudotachylyte a fault rock indicative of seismic faulting this is the first report of pseudotachylyte along an ost in an accretionary prism repeated fracturing in the same narrow shear zones each only a few millimeters thick suggests the osts perform as major shear localized zone in the accretionary prism shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone these findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region this ost showed high activity and repeated faulting in the shallow portion of the accretionary prism comparable to that of the submarine tsunamigenic ost in the present nankai trough

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

high precision slip distribution of the 1944 tonankai earthquake inferred from tsunami waveforms possible slip on a splay fault abstract inversion of tsunami waveforms is a well established technique for estimating the slip distributions of subduction zone earthquakes with some of the most detailed results having been obtained for earthquakes in the nankai trough sw japan the present study although it uses a method and tsunami waveform data set almost identical to previous study aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami green s functions specifically we incorporated four improvements in the present study 1 we used a realistic plate model based only on seismic survey results and assumed it to be the fault plane of the 1944 tonankai earthquake 2 the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis 3 we included the effect of horizontal displacement of the ocean bottom on tsunami generation and 4 we performed a checkerboard resolution test as obtained in previous studies a zone of high slip 2 0 m was resolved off the shima peninsula however the more precise calculation of tsunami green s functions has revealed additional detail that was not evident in previous studies which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism while there was little or no slip near the trench axis in the eastern part of the rupture zone there was up to 1 5 m of slip resolved within 30 km of the trough axis in the western part along the coast of the kii peninsula this troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys furthermore it is also clear that the upper edge of the enshu fault off shima and atsumi peninsulas is consistent with the up dip limit of slip in the eastern part of our model we tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 tonankai earthquake and find that slip on this splay fault is also consistent with the data although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface we further suggest that the position of the enshu fault may be determined by the subduction of topographic highs and that such faults may have an important influence on the up dip rupture limit of the 1944 tonankai and potentially other subduction zone earthquakes

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

textural record of the seismic cycle strain rate variation in an ancient subduction thrust active faults slip at different rates over the course of the seismic cycle earthquake slip c 1 m s 1 interseismic creep c 10 100 mm year 1 and intermediate rate transients e g afterslip and slow slip events studies of exhumed faults are sometimes able to identify seismic slip surfaces by the presence of frictional melts and slow creep by textures diagnostic of rate limited plastic processes the pasagshak point thrust preserves three distinct fault rock textures which are mutually cross cutting and can be correlated to different strain rates ultrafine grained black fault rocks including pseudotachylyte were formed during seismic slip on layers up to 30 cm thick well organized s c cataclasites 7 31 m thick were formed by slow creep with pressure solution as a dominant rate limiting mechanism these must have formed at strain rates consistent with long term plate boundary motion but solution creep healing acted to reduce porosity of the cataclasites and eventually restricted fluid connectivity such that creep by this mechanism could not continue disorganized non foliated rounded clast cataclasites were formed at shear rates faster than solution creep and are interpreted as representing shear at intermediate strain rates these could have formed during afterslip or delocalization of slip associated with an earthquake rupture

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

migrating tremor off southern kyushu as evidence for slow slip of a shallow subduction interface detection of shallow slow earthquakes offers insight into the near trench part of the subduction interface an important region in the development of great earthquake ruptures and tsunami generation ocean bottom monitoring of offshore seismicity off southern kyushu japan recorded a complete episode of low frequency tremor lasting for 1 month that was associated with very low frequency earthquake vlfe activity in the shallow plate interface the shallow tremor episode exhibited two migration modes reminiscent of deep tremor down dip of the seismogenic zone in some other subduction zones a large scale slower propagation mode and a rapid reversal mode these similarities in migration properties and the association with vlfes strongly suggest that both the shallow and deep tremor and vlfe may be triggered by the migration of episodic slow slip events

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

the 1988 and 1990 upland earthquakes left lateral faulting adjacent to the central transverse ranges two earthquakes m l 4 6 and m l 5 2 occurred at almost the same location in upland southern california in june 1988 and february 1990 and had similar strike slip focal mechanisms with left lateral motion on a northeast striking plane the focal mechanisms and aftershock locations showed that the causative fault was the san jose fault an 18 km long concealed fault that splays west southwest from the frontal fault of the central transverse ranges left lateral strike slip faults adjacent to the frontal faults may play an important role in the deformation of the transverse ranges and the los angeles basin as suggested by these upland earthquakes the left lateral strike slip 1988 m l 4 9 pasadena earthquake on the raymond fault 30 km to the west of upland and scattered background seismicity along other active left lateral faults these faults may transfer slip away from part of the frontal fault toward the south alternatively these faults could represent secondary faulting related to the termination of the northwest striking right lateral strike slip faults to the south of the range front the 1988 and 1990 upland earthquakes ruptured abutting or possibly overlapping segments of the san jose fault the edges of the overlapping aftershock zones which are sharply defined together with background seismicity outline a 14 km long aseismic segment of the san jose fault the 1988 mainshock originated at 9 5 km depth and caused aftershocks between 5 and 12 km in contrast the 1990 mainshock focus occurred at the top of its aftershock zone at 5 km and caused aftershocks down to 13 km depth these deep aftershocks tapered off within 2 weeks the rate of occurrence of aftershocks in magnitude time space was the same for both sequences the state of stress reflected in the focal mechanisms of the aftershocks is identical to that determined from background activity and did not change with time during the aftershock sequence the constant stress state suggests that the 1988 and 1990 events did not completely release all the stored slip on that segment of the fault the presence of 14 km of unbroken fault the abrupt temporal termination of deep aftershocks and the constant stress state all suggest that a future moderate sized earthquake m l 6 0 6 5 on the san jose fault is possible with a rupture length of at least 14 km and possibly 18 km

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

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults