mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

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

variety and spatial heterogeneity of tectonic tremor worldwide 1 deep tectonic tremor occurs at various sites worldwide and the source characteristics are heterogeneous even at small scales in this study tremor sources were determined using data sets of seismic waveforms from various locations some of which are not well recognized as being a site of tremor activity the regions of interest are subduction zones at nankai and kyushu japan cascadia in western north america mexico southern chile and new zealand tremor locations are consistent with the geometry of subducting plates and tremor depth tends to be shallower where younger plate is subducting tremor duration shows a negative correlation with tidal sensitivity as reported previously in western shikoku in the nankai subduction zone the duration is also related to the width of the tremor zone in the dip direction of the subducting plate linear structures resembling striations are observed in hypocenter distributions in western shikoku and in the probability of short term migration direction these spatial heterogeneities in tremor activity may be controlled by the heterogeneous frictional properties of the downgoing slab due to long term plate subduction thereby providing information on the maturity of the plate interface

holocene paleoclimate inferred from salinity histories of adjacent lakes in southwestern sicily italy marked uncertainties persist regarding the climatic evolution of the mediterranean region during the holocene for instance whether moisture availability gradually decreased remained relatively constant or increased during the last 7000 years remains a matter of debate to assess holocene limnology hydrology and moisture dynamics the coastal lakes lago preola and gorgo basso located in southwestern sicily were investigated through several stratigraphic analyses of ostracodes including multivariate analyses of assemblages transfer functions of salinity and biochemical analyses of valves sr ca 18o and 13c during the early holocene the gorgo basso and lago preola ostracode records are similar after an initial period of moderate salinity 1690 6100 mg l from ca 10 000 8190 cal yr bp syndepositional or diagenetic dissolution of ostracode valves suggests that salinity declined to 250 mg l from ca 8190 to 7000 cal yr bp at both sites after ca 6250 cal yr bp the ostracode records are strikingly different lago preola became much more saline with paleosalinity values that ranged from 2270 to about 24 420 mg l we suggest that lago preola s change from a freshwater to mesosaline lake at about 6250 cal yr bp was related to sea level rise and resulting intrusion of seawater influenced groundwater in contrast gorgo basso remained a freshwater lake the salinity of gorgo basso declined somewhat after 6250 cal yr bp in comparison to the early holocene ranging from about 550 to 1680 mg l cypria ophtalmica a species capable of rapid swimming and flourishing in waters with low dissolved oxygen levels became dominant at approximately the time when greek civilization took root in sicily 2600 cal yr bp and it completely dominates the record during roman occupation roughly 2100 to 1700 cal yr bp these freshwater conditions at gorgo basso suggest high effective moisture when evergreen olive oak forests collapsed in response to increased greco roman land use and fire ostracode valve geochemistry sr ca 18o suggests significant changes in early vs late holocene hydrochemistry either as changes in salinity or in the seasonality of precipitation harmonizing the autecological and geochemical data from gorgo basso suggests the latter was more likely with relatively more late holocene precipitation falling during the spring summer and fall than winter compared to the early holocene our ostracode inferred paleosalinity data indicate that moisture availability did not decline during the late holocene in the central mediterranean region instead moisture availability was lowest during the early holocene and most abundant during the late holocene

long period long duration seismic events during hydraulic stimulation of shale and tight gas reservoirs part 2 location and mechanisms abstractlong period long duration lpld seismic events that have been observed during hydraulic stimulation of shale gas and tight gas reservoirs appear to represent slow shear slip on relatively large faults within the limitations of the recording geometry we determine the areas in the reservoirs where the events are located in two case studies in the barnett shale in one data set lpld events appear to occur in the region where the density of natural fractures as well as the fluid pressure during pumping were highest in the other data set the lpld events are observed to occur between two wells and seem to establish a hydraulic connection between them in both data sets the lpld events occur in areas with very few located microearthquakes a combination of factors such as high fluid pressure and or high clay content is potentially responsible for the slowly slipping faults the lpld events appear to be occurring only on faults large enough to produce a sequence of slow slip events we suggest that

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

non volcanic tremor resulting from the combined effect of earth tides and slow slip events swarms of non volcanic tremor in southeastern japan are associated with slow slip events and tend to occur with a periodicity of 12 or 24 h this periodicity can be reproduced by a combination of stresses due to earth tides and transient stress changes caused by slow slip events non volcanic tremors may therefore be useful for understanding stress relaxation at the subduction zone interface slow slip events1 are accompanied by swarms of non volcanic tremor2 3 along the subduction zone of the philippine sea plate in southwest japan the swarms often occur with a periodicity of about 12 or 24 h refs 4 5 these episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface1 6 7 here we analyse seismic data to record the locations and durations of non volcanic tremor swarms that occurred in may 2005 and february 2006 we evaluate the magnitude of stress changes produced by slow slip events as well as the effective normal stress on the plate interface where slow slip events occur we find that the observed periodicity in tremor occurrence originates from a combined effect of the periodic stress due to earth tides and the transient stress due to slow slip events our calculations show that non volcanic tremor is sensitive to stress change this phenomenon can therefore be effective in monitoring the process of stress relaxation at subducting plate interfaces

cascadia tremor located near plate interface constrained by s minus p wave times nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network an alternative approach to identifying specific phases is to measure the lag between the s and p waves we cross correlate vertical and horizontal seismograms to reveal signals common to both but with the horizontal delayed with respect to the vertical this lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves measurements of this interval combined with location techniques resolve the depth of tremor sources within 2 kilometers for recent cascadia tremor the sources locate near or on the subducting slab interface strong correlations and steady s p time differences imply that tremor consists of radiation from repeating sources

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

array measurements of deep tremor signals in the cascadia subduction zone 1 preliminary analysis of deep tremor recorded during july 2004 in the cascadia subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution data were collected by three dense arrays of short period seismometers specifically deployed in the puget sound area under an us italy canada cooperative effort slowness analyses at the three arrays indicate that the 2 4 hz tremor wave field is composed of waves propagating with apparent velocities higher than 4 km s combining this with polarisation analysis show these waves to be transverse sh waves however p waves though smaller in amplitude can be detected by different slowness values obtained for the radial and transverse components the intersection of wave vectors determined by the back azimuth and slowness values measured at the three arrays provides a preliminary estimate of source location for a sample of the recorded deep tremor

long period long duration seismic events during hydraulic stimulation of shale and tight gas reservoirs part 1 waveform characteristics abstractlong period long duration lpld seismic events are relatively low amplitude signals that have been observed during hydraulic fracturing in several shale gas and tight gas reservoirs these events are similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries lpld events are predominantly composed of s waves but weaker p waves have also been identified in some cases microearthquakes are observed during the events based on the similarity with tectonic tremors and our observations of several impulsive s wave arrivals within the lpld events we interpret the lpld events as resulting from the superposition of slow shear slip events on relatively large faults most large lpld waveforms appear to start as a relatively slower low amplitude precursor lacking clear impulsive arrivals we estimate the energy carried by the larger lpld events to be 1000 times greater than a mw 2 microseismic event that is typical of the events that occur during hyd

long period long duration seismic events during hydraulic fracture stimulation of a shale gas reservoir we report here a series of long period and long duration lpld seismic events observed during hydraulic fracturing in a shale gas reservoir these unusual events 10 100 s in duration are observed most clearly in the frequency band of 10 80 hz and are remarkably similar in appearance to tectonic tremor sequences first observed in subduction zones these complex but coherent wave trains have finite moveouts obtained from cross correlation the moveout direction of the events confirms that they originate in the reservoir from the area where the fracturing is going on clear p and s wave arrivals cannot be resolved within the lpld episodes but in some cases small micro earthquakes occur in the sequences whether these micro earthquakes are causal or coincidental is not known it has also been observed that in three contiguous frac stages all lpld events appear to come from two distinct places along one of two hypothetical fracture planes interestingly the stages which have the largest number of lpld

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets

automated detection and location of cascadia tremor 1 a new autonomous seismic location and detection methodology enables real time opportunities for high resolution spatio temporal monitoring of non volcanic tremor combining a unique cross correlation technique with epicenter clustering analysis in northern cascadia automatically yields thousands of tremor epicenters from the may 2008 episodic tremor and slip ets event while routinely detecting and locating inconspicuous inter ets tremor bursts although ets events in this area produce about two weeks of continuous tremor we find a nearly equal amount of tremor during the last 15 month inter ets period the resulting ets and inter ets epicenters occur in the slow slip region where the plate interface is 30 45 km deep and have a sharp well resolved updip boundary about 75 km east of the downdip edge of the seismogenic megathrust zone this ability to track tremor with high spatio temporal resolution facilitates automatic tremor monitoring and the mapping of the transition zone and regions of locked zone stress accumulation

testing small aperture array analysis on well located earthquakes and application to the location of deep tremor we have here analyzed local and regional earthquakes using array tech niques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals we have applied the zero lag cross correlation method to earthquakes recorded by three dense arrays in puget sound and vancouver island to estimate the slowness and back azimuth of direct p waves and s waves the results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the pacific northwest seismic network pnsn this comparison has allowed a quanti fication of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data the statistical analysis gives bp 10 and bs 8 as standard deviations for the back azimuth and sp 0 021 sec km and ss 0 033 sec km for the slowness results of the p and s phases respec tively these values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties we have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel time information then we applied the probabilistic method to the deep nonvolcanic tremor recorded by the arrays during july 2004 the results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques thewide depth range of between 10 and 70 km and the source migration with time are evident in our results the method is useful for locating the source of signals characterized by the absence of pickable seismic phases

tidal triggering of low frequency earthquakes near parkfield california implications for fault mechanics within the brittle ductile transition 1 studies of nonvolcanic tremor nvt have established the significant impact of small stress perturbations on nvt generation here we analyze the influence of the solid earth and ocean tides on a catalog of 550 000 low frequency earthquakes lfes distributed along a 150 km section of the san andreas fault centered at parkfield lfe families are identified in the nvt data on the basis of waveform similarity and are thought to represent small effectively co located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km we calculate the sensitivity of each of these 88 lfe families to the tidally induced right lateral shear stress rlss fault normal stress fns and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity lfe occurrence is most strongly modulated by fluctuations in shear stress with the majority of families demonstrating a correlation with rlss at the 99 confidence level or above producing the observed lfe rate modulation in response to shear stress perturbations requires low effective stress in the lfe source region there are substantial lateral and vertical variations in tidal shear stress sensitivity which we interpret to reflect spatial variation in source region properties such as friction and pore fluid pressure additionally we find that highly episodic shallow lfe families are generally less correlated with tidal stresses than their deeper continuously active counterparts the majority of families have weaker or insignificant correlation with positive tensile fns two groups of families demonstrate a stronger correlation with fault normal tension to the north and with compression to the south of parkfield the families that correlate with fault normal clamping coincide with a releasing right bend in the surface fault trace and the lfe locations suggesting that the san andreas remains localized and contiguous down to near the base of the crust the deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth synthesizing our observations with those of other lfe hosting localities will help to develop a comprehensive understanding of transient fault slip below the seismogenic zone by providing constraints on parameters in physical models of slow slip and lfes

regional seismicity a potential pitfall for identification of long period long duration events long period long duration lpld events are tremorlike signals that have been observed during monitoring of hydraulic fracture treatment programs lpld events have been interpreted to reflect slow deformation processes on fractures or faults that are misoriented for reactivation with respect to the present day stress field regional earthquakes could easily be mistaken for lpld events because both are characterized by similar frequency content 100 hz and duration 1 min using data from a 10 5 month continuous downhole deployment of a 15 hz geophone array in a tight sand gas field in western canada we compared recordings of small earthquakes with previously published lpld events we determined that regional earthquakes can show similar waveform characteristics to lpld events underscoring the importance of distinguishing regional earthquake signals from lpld events to ensure robust interpretation of reservoir deformation processes

hydraulic fractures how far can they go the maximum reported height of an upward propagating hydraulic fracture from several thousand fracturing operations in the marcellus barnett woodford eagle ford and niobrara shale usa is 588 m of the 1170 natural hydraulic fracture pipes imaged with three dimensional seismic data offshore of west africa and mid norway it is 1106 m based on these empirical data the probability of a stimulated and natural hydraulic fracture extending vertically 350 m is 1 and 33 respectively constraining the probability of stimulating unusually tall hydraulic fractures in sedimentary rocks is extremely important as an evidence base for decisions on the safe vertical separation between the depth of stimulation and rock strata not intended for penetration

distribution and state of h2o in the high latitude shallow subsurface of mars abstract a quantitative model of the state distribution and migration of water in the shallow martian regolith is presented reported results are confined to the region of the planet greater than 40 lat the calculations take into account 1 expected thermal variations at all depths latitudes and times resulting from seasonal and astronomically induced insolation variations 2 variations in atmospheric ph2o and pco2 resulting from polar insolation variations and regolith adsorptive equilibria 3 feedback effects related to latent heat and albedo variations resulting from condensation of atmospheric constituents 4 two possible regolith mineralogies 5 variable total h2o content of the regolith 6 kinetics of h2o transport through the martian atmosphere and regolith and 7 equilibrium phase partitioning of h2o between the condensed adsorbed and vapor phases results suggest that the adsorptive capacity of the regolith is important in controlling the state and distribution of high latitude h2o unweathered mafic silicates favor the development of shallow ground ice at all temperate and polar latitudes while heavily weathered clay like regolith materials leads to a deeper ground ice interface and far more extensive quantities of adsorbed h2o the capacity of the high latitude regolith for storage of h2o and the total mass of h2o exchanged between the atmosphere polar cap and subsurface over an obliquity cycle is found to be relatively independent of mineralogy the maximum exchanged volume is found to be 3 0 104 km3 of ice per cycle implications for the history of the polar caps and the origin of the layered terrain are discussed results also suggest that seasonal thermal waves act to force adsorbed h2o into the solid phase over a wide variety of latitude obliquity conditions seasonal phase cycling of regolith h2o is most common at high latitudes and obliquities such phase behavior is highly dependent on regolith mineralogy in a highly weathered regolith adsorbed h2o is annually forced into the solid phase at all latitudes 40 at obliquities greater than approximately 25 seasonal adsorption freezing cycles which are predicted here may produce geomorphologic signatures not unlike those produced by terrestrial freeze thaw cycles