fracture characterization of chalk in uniaxial and triaxial tests by rock mechanics fractographic and electromagnetic radiation methods abstract this study relates to fracture characterization of chalk in uniaxial and triaxial tests by combined application of rock mechanic fractographic and electromagnetic radiation emr methods we found high strength results 1 3 that ranged from 37 7 to 52 6 mpa we identified several key surface morphologies that were distinct from each other and expanded on their characterization according to tensile and shear categories tensile features include plumes or striae and stairs shear features include a sugary texture ridges and grooves and steps fracture surfaces induced by tension and shear show distinct morphologies longitudinal splitting domains can occur continuously on an axial tensile plane or adjacent parallel planes on the other hand shear failure into meso fractures up to about 10 cm in length is a process that combines a series of distinct domains on separate surfaces that slightly differ in curvature and orientation and are inclined to the sample axis four fracture sequences that initiated either in tensile or in shear surfaces were distinguished in the failed samples results obtained by rock mechanics and by emr methods are well correlated for microcracks and for meso fractures and may vary over five orders of magnitude in size and this correlation is partially supported by fractography in particular a very good linear relationship was obtained between the ratio of the time interval to reach emr pulse envelope maximum and the pulse frequency t and the largest fractures formed extrapolating this result enabled us to calculate fracture sizes when the value of t was known

stratigraphy and k ar ages across the west flank of the northeast iceland axial rift zone in relation to the 7 ma volcano tectonic reorganization of iceland lithostratigraphy magnetostratigraphy and new k ar ages from the region between eyjafjordur and skjalfandi north central iceland reflect tectonic reorganization of crustal accretion zone activity 7 ma two flood basalt piles are in unconformable contact along this western flank of the northeast iceland axial rift zone neiarz the older ranges in age from 9 5 to 13 ma and is largely composed of tholeiite flows in eastern dalsmynni the upper to middle portions of this older basaltic pile define a 15 35 se dipping monoclinal flexure developed 6 7 ma during early development of the present neiarz lavas of a flood basalt group younger than about 6 5 ma were deposited unconformably on the older flexured basalt pile these postflexure age flows mostly consist of compound doleritic basalts and olivine tholeiites possible tillites are found in the upper portion of this sequence the unconformity between the older and younger basalt piles represents a major structural temporal and lithological boundary in north central iceland zeolite zones appear subhorizontal through the dalsmynni flexure zone and have been largely reset subsequent to flexuring by westward onlap of the younger basalt sequence onto the older subsided and flexured flows radiometrie ages regional isochron patterns and timing constraints for tjornes fracture zone related shear deformation indicate the model of a singular 130 km eastward jump of the axial rift zone in north iceland 6 7 ma is too simple crustal accretion older than 12 ma along a proto neiarz is apparently required indicating that northern iceland has had multiple branched crustal accretion zones up until about 7 ma at which time a single spreading zone along the present neiarz developed an eastward shift of axial rift zone activity 6 7 ma in southwestern iceland is time correlative with reorganization of spreading activity in northern iceland our proposed 6 7 ma reorganization of axial rift zone activity on land is time correlative with an independently proposed increase in discharge from the iceland mantle plume

the miocene great basin of western north america as an extending back arc region abstract a narrow continental volcanic arc has lain near the western margin of the north american continent since mid miocene time its south end began retreating northward 18 m y ago the active remnant of this andesitic arc today is the cascade range of oregon washington and british columbia a large part of the great basin and region to its north was situated throughout this period first wholly then partially in a back arc position relative to the arc this region had and still has most of the characteristics of back arc regions elsewhere in the world a thinned faulted and extended crust and lithosphere limited development of a geophysical bilateral symmetry a mid to late miocene symmetry axis is marked by narrow swarms of basaltic dikes with associated positive linear magnetic anomalies that are parallel to the contemporary trench high heat flow low bouguer gravity and seismic wave velocity values notable seismic attenuation in the mantle and a fault plane solution record dominated by normal fault mechanisms this back arc region differs somewhat from most others those composed of oceanic crust because of its continental setting and recent geologic history despite its name the great basin stands high nearly 2 km above sea level this high elevation owes in part to the fact that extension and heating of the continental lithosphere are still underway though not as a back arc phenomenon the present lithospheric stretching results from oblique extension near the transform boundary of the north american plate the san andreas fault system the latest stretching has probably maintained the physical state of the lithosphere first developed during back arc spreading

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

correlation of plate motions with continental tectonics laramide to basin range some of the major tectonic and magmatic events of the last 150 ma in the western cordillera can be correlated with a new model for the displacement histories between western north america and adjacent oceanic plates sierra nevada plutonism ended and laramide compression began during increasingly rapid convergence of the farallon plate with north america and during a moderate increase in the westward motion of north america in the hotspot reference frame the end of the laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions the spectacular slowing of faralion north america convergence is attributed to the decreasing age of the farallon plate entering the trench and thus to a change from negative to positive buoyancy the transition from widespread arc related magmatism and rapid extension to basaltic volcanism and moderate extension in the basin and range province finds no ready explanation in the plate motions of the pacific basin a change to oblique spreading in the basin and range province accompanied growth in length of the san andreas fault as the mendocino triple junction progressed northward

seismic moment estimation based on fracture induced electromagnetic radiation abstract a new method to calculate the seismic moment based on electromagnetic radiation emitted during fracturing is presented results of this method for small fractures arising during the nucleation stages of an earthquake are shown to agree with former laboratory seismic measuring technique this method is quite easy to implement and can be used for measuring engineering failure intensity it can also be used to measure the seismic moment at the nucleation stages of an earthquake when seismic devices seismographs seismometers are not effective these measurements can then be used to calibrate the seismic devices for the estimation of the seismic moment during the catastrophic stages of earthquakes when these seismic devices are suitable and no electromagnetic radiation is present

characterization of nucleation during laboratory earthquakes 1 we observe the nucleation phase of in plane ruptures in the laboratory we show that the nucleation is composed of two distinct phases a quasi static and an acceleration stage followed by dynamic propagation we propose an empirical model which describes the rupture length evolution the quasi static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time the transition from quasi static to accelerating rupture is related to the critical nucleation length which scales inversely with normal stress in accordance with theoretical predictions and to a critical surfacic power which may be an intrinsic property of the interface finally we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions

a robust calibration technique for acoustic emission systems based on momentum transfer from a ball drop we describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events unlike previous calibration tech niques it does not require modeling of the wave propagation sensor response or signal conditioning rather this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical green s function to correctly apply this technique we develop mathematical expressions that link the seismic mo ment m0 of internal seismic sources i e earthquakes and acoustic emissions to the impulse or change in momentum p of externally applied seismic sources i e me teor impacts or in this case ball impact we find that at low frequencies moment and impulse are linked by a constant which we call the force moment rate scale factor c f mm0 p this constant is equal to twice the speed of sound in the material from which the seismic sources were generated next we demonstrate the calibration technique on two different experimental rock mechanics facilities the first example is a saw cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 mpa confining pressure the second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels using the empirical cali bration technique we are able to determine absolute source parameters including the seismic moment corner frequency stress drop and radiated energy of these magnitude 2 5 to 7 seismic events

electromagnetic emissions during dilating fracture of a rock abstract a lot of experiments on electromagnetic emissions emes have been reported under axial compressive fracture shear fracture indentation fracture and stick slip friction in lab and blasting in situ but there are rare reports on the in lab experimental work on emes during dilating fracture of a rock which is helpful in studying and understanding emes related to slow earthquakes and the earthquakes due to volcanic activities and water level changes of reservoirs therefore in the present paper in order to check whether there are detectable emes during dilating fracture of a rock in lab dilating fracture experiments were conducted the dry cuboid specimens of initially intact granodiorite and limestone were tested inside magnetic field free space mffs at room temperature we arranged evenly 20 eme antennas whose resonance frequencies range from 2 5 khz to 540 khz close to rock specimens our experimental results strongly indicate that detectable emes could generate during dilating fracture of a rock they were recorded only associated with some but not all phases of fracture their waveforms often took on the trend that a peak arrived at first and then attenuated sharply and followed by a series of low amplitude oscillations the electromagnetic em signals after eliminating the effects of eme antennae via deconvolution had the maximum peak to peak amplitudes of about 80 0 mv and 40 5 mv for granodiorite and limestone respectively their main spectral components often concentrated in the band of several khz to 60 khz and of several khz to 280 khz the emission of electrons and charged particles from fracture surfaces and or micro fracture electrification could be possible mechanisms for our experimental results

mechanisms of slip nucleation during earthquakes abstract slip nucleation during earthquakes is apparently analogous to rupture nucleation within an intact rock sample subjected to triaxial loading the observations indicate that both these nucleation processes initiate within a relatively small volume and in both the slip propagates unstably along a quasi planar surface in both processes a single pre existing shear fracture cannot nucleate the large scale slip and in both a process zone that includes several interacting fractures in a small volume are required to initiate the unstable slip both processes require rupture of intact rocks generate complex fracture geometry and are associated with intense energy release rate during slip recent observations and analyses are used to correlate rupture nucleation in laboratory tests with nucleation events of large earthquakes it is proposed that earthquake nucleation occurs by the interaction among multiple fractures within a small volume that develops into unstable yielding of the healed fault zone

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

neotectonics of hispaniola plate motion sedimentation and seismicity at a restraining bend abstract the pattern of neogene faulting volcanism and sedimentation in hispaniola helps to resolve a problem that arises in attempting to determine the direction of caribbean plate motion from earthquakes alone namely how well do earthquake mechanisms define plate motion hispaniola occupies a compressional north step or restraining bend in the generally east west trending motagua cayman trough puerto rico trench fault system which marks an active plate boundary zone pbz of left lateral strike slip motion between the north america and caribbean plates four areas are distinguishable in hispaniola from field mapping and from the interpretation of satellite imagery and conventional aerial photographs two of these areas consist of active left lateral strike slip fault systems that roughly define the northern and southern coasts of the island the structure of both fault systems suggest a roughly east west trending direction of relative plate motion consistent with previous findings the intervening area consists of en echelon mountain ranges thrust up at the restraining bend from the early miocene a pleistocene volcanic province within this area is interpreted as defining a diffuse extensional fault termination of the southern strike slip fault zone the fourth area in eastern hispaniola is interpreted as being underthrust by discontinuous fragments of lithosphere in response to convergence at the restraining bend it appears possible that thinner young island arc lithosphere may be more prone to subduction than to escape or flaking processes that are more important in older more buoyant lithosphere

paleomagnetism and crustal rotations along a shear zone las vegas range southern nevada in southern nevada paleozoic isopachs mesozoic structures and mountain ranges trend north south but as they approach the right lateral west northwest trending las vegas valley shear zone lvvsz they are bent clockwise up to 90 in oroflexes 30 km long in the las vegas range the declination of the characteristic magnetization of the cambrian bonanza king formation has likewise been rotated clockwise more than 50 for samples from eight sites 13 23 samples per site in the las vegas range and in its extension south of the lvvsz stepwise thermal demagnetization to 600 c removed recent and tertiary overprints revealing a nearly horizontal late paleozoic viscous partial thermoremanent magnetization vptrm which predates both the tertiary oroflexural bending and the sevier thrusting in the area the declination of the characteristic magnetization after correcting for the dip of bedding varies from 133 2 at a site 24 km north of the shear zone to 196 4 13 km north of it and 204 4 11 km south of the shear zone this parallels a similar change in the structural trend the paleomagnetic data indicate that the las vegas range is broken into rather small blocks 5 km which rotate in response to more continuous deformation at depth this deformation of the upper crust can be modelled using a thin non newtonian viscous plate model which when applied to the las vegas valley shear zone predicts rotations similar to those seen in the las vegas range

unloading induced rock fracture activation and maximum seismic moment prediction abstract hundreds of anthropogenic earthquakes have recently occurred worldwide due to underground space creation and energy extraction the mechanism behind the human induced geohazards is most likely associated with the reduction of normal stress on pre existing fractures and faults this study reports a series of laboratory experiments to investigate the mechanism of unloading induced fracture activation and proposes a simple approach to predict the maximum seismic moment for a critically stressed fracture the unloading driven shear test results exhibit that the unloading process induces the stress states of the sawcut and natural fractures to approach the mohr coulomb failure envelope and the normal stress unloading rate influences the peak slip rate the fracture instability is dependent on the relationship between the stiffness of the system and the slip weakening rate of the fracture and the shear dilation mainly occurs after the fracture activation the test results also show that the critical shear stress of the sawcut fracture during the unloading driven shear test is approximately equal to the residual shear strength after the displacement driven fracture slip this relationship inspires us to develop a new approach to estimate the maximum seismic moment our data demonstrate that the maximum seismic moments for both the fractures obtained from the unloading driven shear tests are all below the upper limit lines indicating that the proposed approach is reasonable the uncertainty analysis shows that the accurate estimation of fault size can improve the maximum seismic moment prediction

correlation between hydrocarbon reservoir properties and induced seismicity in the netherlands earthquakes induced by gas production are a social concern in the netherlands over the last two decades a total of about 350 such earthquakes have been recorded with magnitudes ranging up to 3 5 on richter s scale the new dutch mining law prescribes the operators to give a quantitative estimation of the likelihood of future seismic activity hazard and the associated damage risk this estimation has to be given for every onshore field producing or to be produced a traditional probabilistic seismic hazard analysis psha can not give an estimation of the hazard for a field before the occurrence of seismic activity we have therefore investigated the correlation between parameters related to reservoir and production properties and the occurrence of induced seismicity in a hydrocarbon field statistically using bayes theorem and the rule of succession three key parameters have been identified that show a good correlation with the occurrence of earthquakes pressure drop fault density of the reservoir and stiffness ratio between seal and reservoir rock based on the observed correlation a probability for the occurrence of earthquakes in fields that have no historical earthquake record has been calculated this has resulted in the definition of four groups of hydrocarbon fields having all a different probability 2006

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

shear wave structure of a transect of the los angeles basin from multimode surface waves and h v spectral ratio analysis we use broad band stations of the los angeles syncline seismic interferometry experiment lassie to perform a joint inversion of the horizontal to vertical spectral ratios h v and multimode dispersion curves phase and group velocity for both rayleigh and love waves at each station of a dense line of sensors the h v of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the green s function the presence of low frequency peaks 0 2 hz in h v allows us to constrain the structure of the basin with high confidence to a depth of 6 km the velocity models we obtain are broadly consistent with the scec cvm h community model and agree well with known geological features because our approach differs substantially from previous modelling of crustal velocities in southern california this research validates both the utility of the diffuse field h v measurements for deep structural characterization and the predictive value of the cvm h community velocity model in the los angeles region we also analyse a lower frequency peak 0 03 hz in h v and suggest it could be the signature of the moho finally we show that the independent comparison of the h and v components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

subsidence and thermal history of queen charlotte basin a tectonic model for the formation subsidence and thermal history of queen charlotte basin is developed based upon regional geological and geophysical data subsidence data from offshore wells in hecate strait and queen charlotte sound and thermal criteria derived from present heat flow and vitrinite reflectance information queen charlotte basin is seen to have resulted from two distinct mechanisms 1 during a period of broad regional uplift rifting and crustal extension occurred in queen charlotte sound up to about 17 ma ago and the queen charlotte islands were displaced northwards toward their present position by transcurrent motion along the louscoone inlet sandspit fault system the rifting generated a significant thermal anomaly and a restricted deep basin as a consequence of crustal thinning and subsequent thermal cooling 2 beginning about 6 ma ago oblique underthrusting commenced along the margin resulting in flexural uplift of the western part of the queen charlotte islands and comp

quantification of principal horizontal stresses inside a tunnel an application of fracture induced electromagnetic radiation femr technique in the darjeeling sikkim himalayas abstract characterization of recent stresses delineation of the locations of active faults landslide prone slip planes and forecasting earthquakes are some of the emerging applications of the fracture induced electromagnetic radiation femr technique it is believed that such radiation is polarized along the direction of maximum compression and proportional to the maximum applied shear stress in this paper we have demonstrated the use of the femr technique to calculate the magnitude and directions of principal horizontal stresses inside a tunnel of 240 m length over a hilly road between jorethang and mayong near the village of miyong in sikkim india by taking 11 cross sectional measurements using a portable femr recording device called angel m although a reverse stress regime is obvious and established over the himalayas the results show a normal stress regime prevailing over this shallow near surface region of the tunnel and the direction of maximum shear stress changes along the tunnel length the results were explained with the aid of a simple clay analogue model where normal reactivation of a thrust plane with changing dips was simulated with this communication we hope to encourage scientists and engineers to deploy the femr technique which is lesser known but quicker non destructive cheaper and more accessible than conventional methods in evaluating tunnel safety by calculating shallow stresses locally modified inside a tunnel

late cenozoic extension of northeastern baja california mexico the pattern and history of normal faulting in a portion of northeastern baja california strongly suggest that the area developed as a result of east west crustal extension during late cenozoic time geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto gulf of california and the distension of the basin and range province in addition these data coupled with regional tectonic timing relations preclude the application of an oblique rifting model to this area two intervals of extensional tectonics have affected the region since the early miocene at present it is not known if strain was continuous or discontinous with respect to time the first and most intense period initiated between 17 and 9 m y ago was east west directed as determined from tilt patterns orientation of intra terrane transcurrent faults and fault plane kinematic indicators this deformation was dominated by high angle normal faulting and resulted in the disruption of a region wide low relief erosion surface new patterns of sedimentation were established and controlled by north trending grabens that filled with alluvial fan and marine deposits sporadic silicic eruptions occurred in conjunction with the extension thus allowing resolution of the complex history of tilting that accompanied normal faulting tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault related monoclinal flexing and reverse drag effects extension during this interval was complete or substantially reduced by 6 m y b p the second interval of east west extension younger than 6 m y b p has been much less intense than the earlier event yet it seems to have utilized the same structures silicic volcanism also waned during this interval giving way to mafic eruptions related to rifting of the present gulf of california

latitudinal shift of pacific hotspots during the late cretaceous and early tertiary two frames of reference the geomagnetic field and hotspots have been widely used to measure the past motions and interactions of lithospheric plates several studies have found that these reference frames have undergone as much as 19 22 of relative motion often called true polar wander tpw in the past 200 myr1 4 moreover it also appears that this offset did not accumulate at a uniform rate some evidence suggests that spurts of tpw can occur at rates as high as 1 2 per myr3 5 rates even an order of magnitude less could pose serious problems for tectonic studies based on the hotspot reference frame because velocities of this magnitude cannot be considered insignificant compared to those of the plates unfortunately the imprecision of most palaeomagnetic data is such that large error bounds are common for many estimates of tpw rates in this report we present a record of tpw derived from pacific palaeomagnetic data that are relatively well constrained in age and palaeolatitude they show that offset between the two reference frames accumulated at 0 3 per myr 1 from late cretaceous to early tertiary

shallow crustal structure of the tanlu fault zone near chao lake in eastern china by direct surface wave tomography from local dense array ambient noise analysis ambient noise tomography ant has been used successfully to image shallow earth structure here we perform ant on a local dense seismic array around the tanlu fault zone tfz to the southeast of hefei city anhui province in eastern china the array consists of 53 stations with average spacing close to 5 km cross correlations of vertical component ambient noise data of different station pairs are computed in 1 h segments and stacked over 1 month from 17 march to 26 april 2017 clear fundamental mode rayleigh waves are observed between 0 2 and 5 s period we then use the direct surface wave tomographic method with period dependent ray tracing to invert group and phase dispersion travel time data simultaneously for three dimensional 3d shear wave velocity vs structure the vs model shows clear correlation with the known geologic features the tfz is associated with a high velocity anomaly zone in the shallow crust corresponding to metamorphic rocks due to magma intrusion low velocity anomaly zones are mainly located to the west of the tfz caused by thick sedimentary layers in the hefei basin our study shows that with ambient noise data recorded on a dense array and an advanced surface wave inversion method we can image detailed structure around the fault zone

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the quebradillas limestone miocene pliocene northern puerto rico and tectonics of the northeastern caribbean margin the uppermost formation of the limestone belt of northern puerto rico is herein redesignated the quebradillas limestone a type section is established within the original type locality north of the town of quebradillas as here redefined the quebradillas limestone consists of a limestone sequence overlying the uppermost amphistegina algal limestone of the aymamon limestone along highway 2 north of the town of quebradillas the formation consists of three types of limestone biomicrite which form the basis for subdividing the formation into three members the lower member consists of globigerine biomicrite and is 16 m thick in many areas the lower member contains thin beds of oyster shells and shell fragments inter bedded within marl both the matrix of the oyster bearing beds and the marl are rich in planktonic foraminifers the middle member is essentially operculinoides biomicrite and is slightly amphistegina algal biomicrite and its thickness is estimated to be 44 4 m the threefold subdivision has been recognized throughout the outcrop belt of the formation from the west coast of the island eastward to barceloneta it probably applies to the easternmost outcrops as well the quebradillas aymamon boundary is an erosional unconformity between the two formations the globigerine limestone of the lower member of the quebradillas fills solution cavities in the uppermost aymamon and in places the upper aymamon shows marked relief most of the quebradillas is pliocene in age although part of the lower member may be latest miocene the quebradillas limestone is a regressive sequence the lower member represents a lower neritic to upper bathyal environment the middle member an outer shelf environment and the upper member a forereef subtidal and intertidal beach environment the upper member is veneered by what appears to be a paleosol which in turn is overlain by quaternary eolianite marginal marine and marine deposits the quebradillas is not limited to only the onshore coastal area of the island but extends northward into the puerto rico trench above a water depth of 5 000 m the quebradillas covers an area of at least 4 000 km 2 on the southern slope of the trench and the total area could be as much as 9 000 km 2 seismic and dredge sample data suggest that the quebradillas consists of the same facies offshore as in both outcrop and one well onshore the presence of widespread quebradillas in the puerto rico trench has important tectonic implications for regional studies of both the puerto rico trench and the northern caribbean region

the lack of equipartitioning in global body wave coda correlation of the coda of large earthquakes can be used to retrieve body waves at teleseismic distances this retrieval depends on waves propagating with equal energy in all directions equipartitioning we carry out a beamforming analysis of body waves at periods of 10 and 40 s recorded at usarray and show that the late coda up to 10 h after the okhotsk earthquake is dominated by waves propagating in the great circle direction late coda waves are not equipartitioned this implies for seismic interferometry that teleseismic body waves can only be extracted from the coda when the earthquake and the receivers are located on a great circle this happens automatically when the employed stations are antipodal or when the autocorrelation of waves is used the use of long period ambient seismic noise relies on the distribution of sources because scattering only marginally randomizes directions this provides insight into recent observations of core phases extracted from seismic interferometry

use of electromagnetic radiation for potential forecast of earthquakes a forecast of an earthquake should identify the first stages of earthquake development nucleation and stick slip we show that a forecast cannot be achieved by seismic measurements due to their high attenuation but can be obtained by judicial filtering of electromagnetic radiation results show that electromagnetic radiation emitted from fractures femr during the early stages of an earthquake is less attenuated than seismic measurements due to the high frequencies involved but could be cluttered by external noise based on our previous studies an analysis of femr constructed on a profile of individual pulses can remove part of the clatter

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

on the origin and evolution of electrical signals during frictional stick slip in sheared granular material electromagnetic signals have been reported in association with geophysical phenomena including earthquakes landslides and volcanic events mechanisms that suggested to explain seismoelectrical signals include triboelectricity piezoelectricity streaming potentials and the migration of electron holes yet the origin of such phenomena remains poorly understood we present results from laboratory experiments regarding the relationship between electrical and mechanical signals for frictional stick slip events in sheared soda lime glass bead layers the results are interpreted in the context of lattice defect migration and granular force chain mechanics during stick slip events we observe two distinct behaviors delineated by the attainment of a frictional stick slip steady state during initial shear loading layers charge during stick slip events and the potential of the system rises after steady state stick slip behavior is attained the system begins to discharge coseismic signals are characterized by potential drops superimposed on a longer term trend we suggest that the observed signal is a convolution of two effects charging of the forcing blocks and signals associated with the stress state of the material the long term charging of the blocks is accomplished by grain boundary movement during the initial establishment of force chain networks short term signals associated with stick slip events may originate from produced electron holes applied to tectonic faults our results suggest that electrical signals generated during frictional failure may provide a way to monitor stress and the onset of earthquake rupture potential changes could produce detectable signals that may forecast the early stages of failure providing a modest warning of the event

electromagnetic radiation induced by mining rock failure abstract anticipating roof fall in mine workings has been a problem for centuries the focus in the search for early warning indicators has been on observing seismic acoustic events prior to the fall these precursors have been studied in great detail at many places but none has been fully successful so far no valid effective early warning system based on low frequency seismic precursors has been established in this paper we investigated a promising new technique which is not yet completely understood or been widely tested in mines the new method is the sensing of the embryonic stages of roof fall by detection of high frequency electromagnetic radiation emr emitted from rock microcracks two examples of combined observations of emr and low frequency acoustic emission prior to roof fall at moonee colliery are presented anomalously high emr was detected more than 1 h before roof fall giving a significant time advantage over the first indicators of low frequency acoustic emission analysis of benioff strain release diagrams of emr emanating from developing medium scale failure in the mine enabled us to fill the gap between previously known microscale rock fracture in lab and macroscale earthquake emr observations and to conclude that indeed a common fundamental relationship must lie behind this multi scale phenomenon

microseismic monitoring and stability evaluation for the large scale underground caverns at the houziyan hydropower station in southwest china abstract the stability of underground caverns and the excavatability of rock masses are important for geotechnical engineering practices during the design and construction stages the risks associated with underground caverns at the houziyan hydropower station in southwest china are growing due to excavation induced unloading to assess the instability of underground caverns and resolve the complex subsurface conditions of the highly fractured rock mass a high resolution microseismic monitoring system was established in deep underground caverns this system was used to determine the relationship between the measured microseismic activities and the excavation damage zones of the surrounding rock mass the excavation damage zones and potential risk regions in the underground caverns were identified by analyzing the tempo spatial distribution of microseismic activities in addition the correlation between microseismic activities and pre existing geological structures was determined and traditional monitoring results were analyzed to validate the correlation between seismicity and the excavation damaged zones of the underground caverns a numerical model was employed to further evaluate the deformation and stability of the surrounding rock mass the monitoring results demonstrate that microseismic events mainly occurred in high stress concentration regions corresponding with the results obtained from the numerical analysis therefore this comprehensive method which incorporates microseismic monitoring numerical analysis traditional monitoring and field observations is promising for predicting the deformation and instability of surrounding rock masses in the underground caverns subjected to excavation

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

the seismic noise wavefield is not diffuse passive seismology is burgeoning under the apparent theoretical support of diffuse acoustics however basic physical arguments suggest that this theory may not be applicable to seismic noise a procedure is developed to establish the applicability of the diffuse field paradigm to a wavefield based on testing the latter for azimuthal isotropy and spatial homogeneity this procedure is then applied to the seismic noise recorded at 65 sites covering a wide variety of environmental and subsoil conditions considering the instantaneous oscillation vector measured at single triaxial stations the hypothesis of azimuthal isotropy is rejected in all cases with high confidence which makes the spatial homogeneity test unnecessary and leads directly to conclude that the seismic noise wavefield is not diffuse however such a conclusion has no practical effect on passive imaging which is also possible in non diffuse wavefields

earthquake source mechanisms and transform fault tectonics in the gulf of california we have determined the source parameters of 19 earthquakes in the gulf of california from an inversion of long period p and sh waveforms fifteen of the earthquakes on transforms between the cerro prieto fault and the tamayo fracture zone are characterized by right lateral strike slip faulting and all but one of these likely reflect the relative motion between the north american and pacific plates slip vectors from these events constitute an improved data set for determination of north american pacific motion most centroid depths are poorly resolved because of trade offs between depth and source time function both the ranges in acceptable centroid depths and the average fault width estimated from cumulative moment release during this century are compatible with the hypothesis that seismic slip does not extend below about 10 km depth or the approximate depth of the nominal 800 c isotherm along the principal oceanic transforms of the gulf two normal faulting earthquakes during a swarm in the northern gulf have centroid depths of 6 3 and 3 2 km consistent with earthquake centroid depths in other settings arctic northern red sea where plate separation has carried continental rifting nearly to completion a thrust faulting earthquake near the tres marias escarpment has a centroid depth of 11 2 km and probably reflects relative motion between the rivera and north american plates the transition from oceanic transform faulting to continental transform faulting in the northern gulf is marked by a broadening of the zone of deformation and a 10 change in the strike of the principal fault accommodating plate motion fault kinematics in the transition zone may be understood in terms of an unstable fff triple junction with the agua bianca and san miguel faults acting to transfer a portion of the plate motion to faults west of the san andreas system and with the delfin and wagner basins forming as a direct result of slip on the three fault systems meeting in the junction region

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

pliocene change in pacific plate motion the absolute motion of the pacific plate has been nearly constant during the past 15 myr but epp1 recognized that some change took place between 15 myr and the present and tentatively chose 1 myr for the time of the change cox and engebretson2 found a change in pacific absolute motion based on hotspot tracks and a change in pacific antarctic relative motion the pliocene deformation in california and the juan de fuca rotations led them to postulate a 5 myr age for the change the manner and timing of the change are explored more thoroughly here and we put the time for the change in motion of the pacific plate at between 5 and 3 2 myr basing the younger limit on the correlation of computed hotspot tracks with pacific and nazca hotspot traces the change may explain several observations on the neighbouring continental and oceanic plate boundaries including direct adjustments at the pacific spreading ridges a likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the kurile trench where a fossil spreading ridge is inferred to have been subducted

quantifying the effects of nondiffuse noise on ballistic and coda wave amplitude from variances of seismic noise interferometry in southern california we develop a new approach based on the variance of noise cross correlation to characterize the noise source and wave propagation under the influence of a non diffuse noise field based on the rando

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

pole of rotation analysis of present day juan de fuca plate motion convergence rates between the juan de fuca and north america plates are calculated by means of their relative present day pole of rotation subduction rates as measured normal to the strike of the washington oregon trench are predicted to range from about 40 mm yr under washington to 20 mm yr under oregon an integral part of this analysis is the formulation of a numerical inversion scheme which calculates both the pole of rotation between two plates and the associated errors this method is used to determine the euler pole for pacific juan de fuca this pole can then be vectorially added to previously published poles for north america pacific and hot spot pacific to obtain north america juan de fuca and hot spot juan de fuca respectively the errors associated with these resultant poles are also determined by propagating the errors of the two summed angular velocity vectors with the assumption that hot spots are fixed with respect to a mantle reference frame if is computed that the average absolute velocity of the juan de fuca plate is approximately 15 mm yr thereby making it the slowest moving of the oceanic plates