investigating the semiannual oscillation on mars using data assimilation abstract a martian semiannual oscillation sao similar to that in the earth s tropical stratosphere is evident in the mars analysis correction data assimilation reanalysis dataset macda version 1 0 not only in the tropics but also extending to higher latitudes unlike on earth the martian sao is found not always to reverse its zonal wind direction but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes singular system analysis ssa is further applied on the zonal mean zonal wind in different latitude bands to reveal the characteristics of sao phenomena at different latitudes the second pair of principal components pcs is usually dominated by a sao signal though the sao signal can be strong enough to manifest itself also in the first pair of pcs an analysis of terms in the transformed eulerian mean equation tem is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal mean zonal wind the zonal mean meridional advection is found to correlate strongly with the observed oscillations of zonal mean zonal wind and supplies the majority of the westward retrograde forcing in the sao cycle the forcing due to various non zonal waves supplies forcing to the zonal mean zonal wind that is nearly the opposite of the forcing due to meridional advection above 3 pa altitude but it also partly supports the sao between 40 pa and 3 pa some distinctive features occurring during the period of the mars year my 25 global scale dust storm gds are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of my 25 and stronger forcing due to vertical advection transient waves and thermal tides mars mars atmosphere mars climate atmospheres dynamics atmospheres structure meteorology

equatorial jets in the dusty martian atmosphere we investigate the production of equatorial jets which demonstrate strong local superrotation in an atmospheric general circulation model of mars these westerly jets are driven by diurnal thermal tides and their strength is shown to be closely related to the amount of dust in the atmosphere the superrotating jets are strongest near to equinox and under conditions of high atmospheric dust loading if there is sufficient dust in amounts corresponding to dust storm conditions the westerly equatorial jets can occur at any time of year and reach speeds of over 40 m s peaking between 10 and 20 km altitude for more moderate dust amounts typical of background levels on mars the jets are still strong when the subsolar point is close to the equator and latitudinally symmetric tidal modes are forced strong easterly retrograde winds are also found high above the equator and it is shown that the thermal tides play a major role in their formation this process is especially relevant close to equinox when the cross equatorial meridional circulation is weak

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

using repeating earthquakes to correct high precision earthquake catalogs for time dependent station delays waveform cross correlation allows one to measure the relative arrival times of similar microearthquakes with errors of less than 1 10 of 1 sample location algorithms based on these measurements have greatly improved images of earthquake distribution for the northern california seismic network catalog however the relative location errors implied by the scatter of relocated repeating earthquakes sometimes 10s of meters is often considerably larger than the errors estimated from monte carlo simulations meters using a priori estimates of the cross correlation errors i find that most of this discrepancy arises from unmodeled time varying station delays by identifying many clusters of repeating earthquakes presumed to rupture the same source and assuming the measured arrival time differences between event pairs to be due to the difference in origin time plus the difference in station delay it is possible to invert for time dependent station corrections the method is robust in that these corrections are consistent over many kilometers of fault and greatly reduce the scatter and residuals when relocating earthquake clusters not used in the station correction determination many of the largest delay changes some approaching 2 samples 20 ms are associated with changing station electronics at several stations an annual cycle is also clearly seen with amplitudes up to 0 5 samples some m 4 7 earthquakes have produced comparable transient delay changes after applying the station corrections the scatter within clusters of repeating earthquakes is reduced to a few meters and the apparent seismogenic thickness of many kilometer long sections of faults is only 10 20 m manuscript received 31 may 2001

effects of loading velocity stiffness and inertia on the dynamics of a single degree of freedom spring slider system numerical simulation of the nonlinear dynamics of a single degree of freedom spring slider system can provide useful insight into the effects of loading velocity friction environment elastic stiffness and inertia on the stress drop and dynamic behavior of earthquakes in our numerical simulations using a rate and state dependent friction law we varied the loading velocity by 7 orders of magnitude the stiffness by 2 orders of magnitude the mass by 5 orders of magnitude and the velocity weakening parameter b a by a factor of 160 four stability regimes were identified in the load point velocity stiffness space when inertia was taken into account for cyclic stick slip instabilities the stress drop amplitude increases with decreasing load point velocity with decreasing stiffness with increasing velocity weakening parameter b a and with decreasing mass simple scaling relations for the static stress drop dynamic stress drop static friction and shear fracture energy with the natural logarithm of load point velocity were observed using the scaling relation for static stress drop we infer the normalized velocity weakening parameter b a with respect to normal stress to range from 0 0011 to 0 0039 for triaxial experiments on saw cut westerly granite specimens sandwiched with a simulated gouge layer of ultrafine quartz at pressures from 10 mpa to 100 mpa unless we take b a throughout the seismogenic layer to be much higher than experimental measurements the effect of loading velocity by itself is not sufficient to account for the increase of stress drop with increasing earthquake recurrence time inferred from seismological data to the extent that our simple system can be treated as an analog for earthquakes our study shows that the velocity weakening parameter b a controls the magnitude of the static stress drop far more than the loading velocity if velocity weakening increases systematically with increasing healing time then the coupled effect of loading velocity variation and change in friction environment can account for the seismological observations

intraseasonal oscillations in the global atmosphere part ii southern hemisphere abstract in part ii of this two part article we complete the systematic examination of oscillatory modes in the global atmosphere by studying 12 years of 500 mb geopotential heights in the southern hemisphere as in part i for the tropics and northern hemisphere extratropics the data were band pass filtered to focus on intraseasonal is phenomena and spatial eofs were obtained the leading principal components were subjected to singular spectrum analysis ssa in order to identify nonlinear is oscillations with high statistical confidence in the southern hemisphere the dominant mode has a period of 23 days with spatial patterns carried by the second and third winter eof of the is band it has a zonal wavenumber four structure the 40 day mode is second and dominated by wavenumbers three and four while a 16 day mode is too weak to separate its spatial behavior from the previous two the is dynamics in the southern hemisphere is more complex and dominated by shorter wavenumbers than the northern

effects of vertically propagating thermal tides on the mean structure and dynamics of mars lower thermosphere 1 a general circulation model of mars atmosphere is used to elucidate the effects of vertically propagating thermal tides excited near the surface on the zonal mean density temperature and wind structure of mars atmosphere between 90 and 160 km the effects are substantial and amount to order 10 40 10 50 k and 50 160 m s 1 respectively at these altitudes under low dust conditions dust visible optical depth 0 3 these induced changes in the zonal mean structure are dominated by the non migrating tides of particular note are the mean density perturbations which represent an important consideration for modeling of the aerobraking region 100 170 km

semiannual oscillations in the atmosphere of mars 1 we report on the first detection of the semiannual oscillation sao in the martian atmosphere the semiannual periodicity is found in the difference between day and night time atmospheric temperatures a good proxy for solar tides measured from mars global surveyor simulations with a general circulation model proved that this modulation of tidal amplitudes is a manifestation of the sao of zonal winds in martian tropics our numerical experiments revealed significant differences in driving mechanisms of the sao between mars and earth on mars unlike on earth equatorial kelvin waves supply only small retrograde torque to the mean circulation instead thermal tides and quasi stationary planetary waves induced by martian topography contribute strongly to the prograde super rotation acceleration the existence of the sao on mars suggests that this phenomenon is not a result of the unique terrestrial environment but a more general consequence of wave mean flow interactions in atmospheres of fast rotating planets

systematic search for repeating earthquakes along the haiyuan fault system in northeastern tibet repeating earthquakes have been found at many faults around the world and they provide valuable information on diverse faulting behavior at seismogenic depth the haiyuan fault is a major left lat

spatial delineation of groundwater salinity using deep time domain electromagnetic geophysical measurements a feasibility study 1 the objective of this study was to delineate the spatial distribution of fresh brackish and saline groundwater bodies as well as brine beneath the judea desert israel we used the deep time domain electromagnetic tdem geophysical system cycle 5m which provides the maximum exploration capability to depths 1 5 2 km below land surface the study was conducted at 21 locations at the desert plateau where fresh groundwater flows through the upper aquifers from the replenishment area at the judea mountains toward the dead sea springs in deeper aquifers brackish and saline groundwater bodies exist which originated from lakes and lagoons that existed at the dead sea rift valley during the pleistocene and pliocene respectively it was found that within carbonate formations fresh groundwater c 103 ppm total dissolved solids tds is characterized by a resistivity of more than 15 m brackish groundwater 103 c 104 ppm tds is characterized by 5 15 m saline groundwater 104 c 105 ppm tds is characterized by 0 5 5 m and brine 105 c ppm tds is characterized by less than 0 5 m

the configuration of the fresh saline groundwater interface within the regional judea group carbonate aquifer in northern israel between the mediterranean and the dead sea base levels as delineated by deep geoelectromagnetic soundings a combined high resolution short offset transient electromagnetic shotem and deep sounding long offset lotem survey has been carried out along two traverses between the mediterranean sea and the jordan dead sea rift dsr the dsr is located in the study area some 200 250 m below sea level the measurements detected a deep conductor the top of which exhibited a regular behavior along the both traverses declining from the mediterranean to the dsr base level the geometry of this geoelectric boundary coincides fairly well with the configuration of a supposed fresh saline groundwater interface as also obtained by both numerical and physical modeling for the known hydrogeological conditions in the study area therefore the detected geoelectric boundary is identified with the interface supporting the hypothesis of current seawater intrusion into the deep regional aquifers between the mediterranean and the dsr base levels the intrusion causes the salination of fresh groundwater within the aquifers as well as the salination of the sea of galilee

strain rate effect on frictional strength and the slip nucleation process abstract kato n yamamoto k yamamoto h and hirasawa t 1992 strain rate effect on frictional strength and the slip nucleation process in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 269 282 the strain rate dependence of frictional strength is investigated in relation to sliding behaviour by biaxial compression tests of large scale granite samples frictional sliding is generated by increasing shear strain at a constant rate under a constant normal stress on the fault the ram displacement is held constant for a prescribed time interval between two successive sliding events so as to have the same time effect of stationary contact between sliding surfaces we obtained the following results as the strain rate increases 1 sliding becomes more unstable 2 the nucleation zone size of stick slip decreases and 3 frictional strength logarithmically increases result 3 is consistent with the previous study by dieterich 1979a and 1 and 3 are similar to the strain rate effects on failure characteristics of intact rocks reported in the literature result 3 can be explained by a rate and state dependent friction law our results suggest that the strain rate dependence of the fault strength plays an important role in controlling the nucleation process of faulting

faulting style controls for the space time aftershock patterns abstract to understand the complexity and thus to predict earthquake occurrence in size time and space seismicity patterns are characterized by two robust empirical laws the exponential distribution of magnitude and the power law decrease of aftershock rate over time these laws are known as the gutenberg richter law and the omori law respectively using global earthquake catalogs we resolve that on average the k aftershock productivity and the p value exponent of the power law decrease of aftershock rate over time are dependent on the mainshock faulting style strike slip events have a lower aftershock rate n and k values and a larger p value than thrust and normal events respectively within the epidemic type aftershock sequence model strong k n values are driven by a high branching ratio value n within the same framework a relatively higher n value for the thrust events also predicts the lower p value we observe for thrust events as compared to strike slip and normal faulting events respectively furthermore we observed that earthquake interactions through time and space are a function of the faulting style when measured by t the exponent of the power law decrease of earthquake density over space the t values of thrust events for different time windows always remain smaller than those of the strike slip events when changes in faulting styles are driven by stress pattern the anderson faulting theory predicts thrust faulting that requires somewhat larger stresses in absolute magnitude than do normal and strike slip faulting within the framework of rate and state friction law changes in the stress heterogeneity patterns reproduce the p value changes we observe our results suggest that only stress perturbations associated with mainshock rupture affect the productivity and decay rate over time of aftershocks

seismicity rate immediately before and after main shock rupture from high frequency waveforms in japan 1 we analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the hi net borehole array in japan by scrutinizing high frequency signals we detect 5 times as many aftershocks in the first 200 s as in the japan meteorological agency catalogue after correcting for the changing completeness level immediately after the main shock the aftershock rate shows a crossover from a slower decay with an omori s law exponent p 0 58 0 08 between 20 and 900 s after the main shock to a faster decay with p 0 92 0 04 after 900 s the foreshock seismicity rate follows an inverse omori s law with p 0 73 0 08 from several tens of days up to several hundred seconds before the main shock the seismicity rate in the 200 s immediately before the main shock appears steady with p 0 35 0 50 these observations can be explained by the epidemic type aftershock sequence etas model and the rate and state model for a heterogeneous stress field on the main shock rupture plane alternatively nonseismic stress changes near the source region such as episodic aseismic slip or pore fluid pressure fluctuations may be invoked to explain the observation of small p values immediately before and after the main shock

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

triggering effect of m 4 5 earthquakes on the earthquake cycle of repeating events at parkfield california stress perturbations influence earthquake recurrence and are of funda mental importance to understanding the earthquake cycle and determining earthquake hazard the large population of repeating earthquakes on the san andreas fault at parkfield california provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes using 187 m 0 4 to 1 7 repeating earthquake sequences from the high resolution seismic network cat alog we find that the time to recurrence of repeating events subsequent to nearby m 4 5 earthquakes is shortened suggesting triggering by major events the triggering effect is found to be most evident within a distance of 5 km corresponding to static coseismic stress changes of 0 6 26 6 kpa and decays with distance we also find coherently reduced recurrence intervals from 1993 to 1998 this enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s online material figures of event chronologies of res estimates of dt and coseismic static stress changes

do earthquakes talk to each other triggering and interaction of repeating sequences at parkfield 1 knowledge of what governs the timing of earthquakes is essential to understanding the nature of the earthquake cycle and to determining earthquake hazard yet the variability and controls of earthquake recurrences are not well established the large population of small characteristically repeating earthquakes at parkfield provides a unique opportunity to study how the interaction of earthquakes affects their recurrence properties we analyze 112 m 0 4 3 0 repeating earthquake sequences ress to examine the triggering effect from nearby microseismicity we find that the repeating events with a smaller number of neighboring earthquakes in their immediate vicinity tend to recur in a more periodic manner i e the coefficient of variation in recurrence intervals is less than 0 3 the total static stress perturbation from close by earthquakes however does not seem to strongly influence res regularity the uneven distribution of stress changes in time has a modest but significant impact on recurrence intervals a significant reduction of recurrence intervals occurs in the case of very high stress changes from neighboring events close by events influence res timing in a matter of several days or less by short term triggering events that occurred within less than 1 day of an res often imposed or experienced high stress changes a static stress increment of 30 kpa can be enough to produce such short term triggering we find that the triggered repeating events are often near the end of their average earthquake cycle but some events occur following a substantially shortened interval when comparing the accelerated occurrence at the time of res events following neighboring events with varying magnitudes we find that the distance of short term triggering increases from 1 km to 4 km for m1 to m4 events

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

high resolution image of calaveras fault seismicity 1 by measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35 km section of the calaveras fault this represents 92 of all seismicity since 1984 and includes the rupture zone of the m 6 2 1984 morgan hill california earthquake the relocated seismicity forms highly organized structures that were previously obscured by location errors there are abundant repeating earthquake sequences as well as linear clusters of earthquakes large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30 year time interval suggesting that these portions of the fault are either locked or creeping the area of greatest slip in the morgan hill main shock coincides with the most prominent of these voids suggesting that this part of the fault may be locked between large earthquakes we find that the calaveras fault at depth is extremely thin with an average upper bound on fault zone width of 75 m given the location error however this width is not resolvably different from zero the relocations reveal active secondary faults which we use to solve for the stress field in the immediate vicinity of the calaveras fault we find that the maximum compressive stress is at a high angle only 13 from the fault normal supporting previous interpretations that this fault is weak

distribution of postseismic slip on the calaveras fault california following the 1984 m6 2 morgan hill earthquake abstract repeating earthquakes res are sequences of events that have virtually identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth to investigate the postseismic deformation after the 1984 m6 2 morgan hill earthquake we identify re sequences occurring on the central calaveras fault between 1984 and 2005 using a combination of cross correlation and spectral coherence techniques both the accelerated slip transients due to the earthquake as well as the return to interseismic background creep rates can be imaged from our dataset a comparison between the regions of the fault that ruptured coseismically and the locations of the res show that res preferentially occur in areas adjacent to the coseismic rupture using calculated re derived subsurface slip distributions at 6 months and 18 months after the mainshock we predict surface electronic distance meter edm line length changes between stations near the morgan hill rupture area the re derived slip model underpredicts a subset of the observed line length changes inclusion of transient aseismic slip below the seismogenic zone is needed to better match the measured surface deformation

a numerical study on the influence of fractured regions on lake groundwater interaction the lake kinneret sea of galilee case increased lake salinity is a growing problem in arid and semi arid regions operational management which is based on a reliable hydrological understanding has the potential to reduce the lake salinity this is the case of the salinity in lake kinneret sea of galilee where saline water flows into the lake through on shore and off shore springs here we present a time dependent flow and transport numerical model that successfully reproduces the monitored groundwater level discharge and salinity of the lake springs the model utilizes a continuum approach and describes the flow through a confined saline carbonate aquifer which interacts with the discharge lake through fractures and faults in particular the model investigates the hydrology around two groups of springs fuliya and tabgha along the western shore of the lake based on seasonal characteristics of the springs and measured boundary conditions the two springs groups were defined as lake dominated springs lds fuliya and aquifer dominated springs ads tabgha the models of the two groups differ from each other in the distribution of fractured regions that link between the lake and the underlying aquifers the numerical solution shows how the fractured regions affect groundwater head spring discharge and spring salinity the different behavior of the lds and ads was reproduced with an excellent agreement between measured and calculated patterns differences in salinity within a group of springs are shown to be dependent on the depth of the fractured region it was revealed that in addition to the appropriate distribution of fractured regions some adjustment in the up stream boundary conditions is necessary to fully reproduce the system hydrology the model has improved our understanding and provides a prediction tool for future management strategies

thermal tides during the 2001 martian global scale dust storm the 2001 mars year 25 global dust storm radically altered the dynamics of the martian atmosphere using observations from the thermal emission spectrometer onboard the mars global surveyor spacecraft and marswrf general circulation model simulations we examine the changes to thermal tides and planetary waves caused by the storm we find that the extratropical diurnal migrating tide is dramatically enhanced during the storm particularly in the southern hemisphere reaching amplitudes of more than 20 k the tropical diurnal migrating tide is weakened to almost undetectable levels the diurnal kelvin waves are also significantly weakened particularly during the period of global expansion at ls 200 210 in contrast the westward propagating diurnal wavenumber 2 tide strengthens to 4 8 k at altitudes above 30 km the wavenumber 1 stationary wave reaches amplitudes of 10 12 k at 50 70 n far larger than is typically seen during this time of year the phase of this stationary wave and the enhancement of the diurnal wavenumber 2 tide appear to be responses to the high altitude westward propagating equatorial wavenumber 1 structure in dust mixing ratio observed during the storm in previous works this work provides a global picture of dust storm wave dynamics that reveals the coupling between the tropics and high latitude wave responses we conclude that the zonal distribution of thermotidal forcing from atmospheric aerosol concentration is as important to understanding the atmospheric wave response as the total global mean aerosol optical depth

interseismic coupling and refined earthquake potential on the hayward calaveras fault zone interseismic strain accumulation and fault creep is usually estimated from gps and alignment arrays data which provide precise but spatially sparse measurements here we use interferometric synthetic aperture radar to resolve the interseismic deformation associated with the hayward and calaveras faults hf and cf in the east san francisco bay area the large 1992 2011 sar data set permits evaluation of short and long wavelength deformation larger than 2 mm yr without alignment of the velocity field to a gps based model our time series approach in which the interferogram selection is based on the spatial coherence enables deformation mapping in vegetated areas and leads to refined estimates of along fault surface creep rates creep rates vary from 0 2 mm yr on the northern cf to 14 2 mm yr on the central cf south of the hf surface junction we estimate the long term slip rates by inverting the long wavelength deformation and the distribution of shallow slip due to creep by inverting the remaining velocity field this distribution of slip reveals the locations of locked and slowly creeping patches with potential for a m6 8 0 3 on the hf near san leandro a m6 6 0 2 on the northern cf near dublin a m6 5 0 1 on the hf south of fremont and a m6 2 0 2 on the central cf near morgan hill with cascading multisegment ruptures the hf rupturing from berkeley to the cf junction could produce a m6 9 0 1 the northern cf a m6 6 0 1 the central cf a m6 9 0 2 from the junction to gilroy and a joint rupture of the hf and central cf could produce a m7 1 0 1

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

rescaled earthquake recurrence time statistics application to microrepeaters slip on major faults primarily occurs during characteristic earthquakes the recurrence statistics of characteristic earthquakes play an important role in seismic hazard assessment a major problem in determining applicable statistics is the short sequences of characteristic earthquakes that are available worldwide in this paper we introduce a rescaling technique in which sequences can be superimposed to establish larger numbers of data points we consider the weibull and log normal distributions in both cases we rescale the data using means and standard deviations we test our approach utilizing sequences of microrepeaters microearthquakes which recur in the same location on a fault it seems plausible to regard these earthquakes as a miniature version of the classic characteristic earthquakes microrepeaters are much more frequent than major earthquakes leading to longer sequences for analysis in this paper we present results for the analysis of recurrence times for several microrepeater sequences from parkfield ca as well as ne japan we find that once the respective sequence can be considered to be of sufficient stationarity the statistics can be well fitted by either a weibull or a log normal distribution we clearly demonstrate this fact by our technique of rescaled combination we conclude that the recurrence statistics of the microrepeater sequences we consider are similar to the recurrence statistics of characteristic earthquakes on major faults

on the salt water intrusion into the durusu lake istanbul a joint central loop tem and multi electrode ert field survey durusu lake is the biggest and most important freshwater source supplying drinking water to the european side of istanbul in this study electrical resistivity tomography ert and transient electromagnetic tem measurements were applied to detect a possible salt water intrusion into the lake and to delineate the subsurface structure in the north of durusu lake the ert and tem measurements were carried out along six parallel profiles extending from the sea coast to the lake shore on the dune barrier tem data were interpreted using different 1 d inversion methods such as occam marquardt and laterally constrained inversion lci ert data were interpreted using 2 d inversion techniques the inversion results of ert and tem data were shown as resistivity depth sections including topography the sand layer spreading over the basin has a resistivity of 150 400 omega m with a thickness of 5 10 m the sandy layer with clay silt and gravel has a resistivity of 15 100 omega m and a thickness of 10 40 m followed by a clay layer of a resistivity below 10 omega m when the inversion of these data is interpreted along with the hydrogeology of the area it is concluded that the salt water intrusion along the dune barrier is not common and occurs at a particular area where the distance between lake and sea is very close using information from boreholes around the lake it was verified that the common conductive region at depths of 30 m or more consists of clay layers and clay lenses

detection of northern hemisphere transient eddies at gale crater mars abstract the rover environmental monitoring station rems on the curiosity rover is operating in the southern hemisphere of mars and is detecting synoptic period oscillations in the pressure data that we attribute to northern hemisphere transient eddies we base this interpretation on the similarity in the periods of the eddies and their seasonal variations with those observed in northern midlatitudes by viking lander 2 vl 2 18 mars years earlier further support for this interpretation comes from global circulation modeling which shows similar behavior in the transient eddies at the grid points closest to curiosity and vl 2 these observations provide the first in situ evidence that the frontal systems often associated with flushing dust storms do cross the equator and extend into the southern hemisphere

laboratory study of fault healing and lithification in simulated fault gouge under hydrothermal conditions abstract seismic data geologic observations and laboratory friction studies suggest that faults lithify and strengthen heal during the interseismic period of the earthquake cycle we report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults layers of m sized quartz powder were used to simulate granular fault gouge gouge layers were sheared within sioux quartzite in a triaxial pressure vessel at elevated pressures p c 250 mpa elevated temperatures 230 636 c and in the presence of water p h 2 o 75 mpa we performed hold slide experiments in which samples were subjected to a period of healing under hydrostatic load followed by shear deformation healing times ranged from a few hundred s to 10 5 s to isolate the effects of lithification and temperature on friction we ran two types of hold slide experiments 1 samples were subjected to healing and shear deformation at elevated temperature and 2 following healing at elevated temperature temperature was reduced prior to shear we also ran slide hold slide experiments in which samples were healed under shear load at elevated temperature 636 c experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time samples healed and deformed at higher temperature showed lower peak strengths and within the scatter of our data did not exhibit time dependent strengthening these data are interpreted to result from enhanced lithification rates at elevated temperature samples healed under shear load exhibited a log linear decrease in friction with hold time modeling using rate and state dependent friction laws indicates velocity strengthening behavior with friction a b values of 0 05 to 0 08 our data indicate that at high temperature the effective state evolution term b is negative moreover the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non hydrostatic load conditions our measurements of healing are consistent with seismic estimates of fault healing rates if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 mpa

mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of co2 abstract we present direct observations of mars zonal wind velocities around northern spring equinox ls 336 ls 355 ls 42 during martian year 27 and 29 data was acquired by means of infrared heterodyne spectroscopy of co2 features at 959 3917 cm 1 10 4232 m and 957 8005 cm 1 10 4405 m using the cologne tuneable heterodyne infrared spectrometer this at the mcmath pierce telescope of the national solar observatory on kitt peak in arizona and the nasa infrared telescope facility on mauna kea hawaii between 2005 and 2008 winds were measured on the dayside of mars with an unprecedented spatial resolution allowing sampling of up to nine independent latitudes over the martian disk retrieved wind velocities depend strongly on latitude and season with values ranging from 180 m s prograde to 94 m s retrograde a comparison of the observational results to predicted values from the mars climate database yield a reasonable agreement between modeling and observation

time dependent friction of granite implications for precursory slip on faults frictional forces were measured during sliding between saw cut cylinders of barre granite deformed in a servo controlled triaxial loading machine two different effects occur depending on the type of loading when steady shortening of a stably sliding sample is halted suddenly slip continues at a diminishing rate which depends on the logarithm of the initial sliding velocity thus the frictional strength of the surface is decreasing with time when shortening at a constant rate resumes after the holding period however the frictional resistance temporarily rises to a peak proportional to the logarithm of time of no driving showing a well known increase in friction with time the subsequent decrease in resistance from this peak is continuous the observed frictional resistance of a surface thus results from the interaction of several processes and is not a constant in general but depends on the immediate and preceding deformation when shortening is resumed after a period of holding slip rate across the friction surface temporarily accelerates above the driving rate the slip rate can then either decrease to the driving rate and remain stable or accelerate further and lead to stick slip motion at stress levels below the peak value there is no apparent difference in the frictional resistance until the unloading rate of the loading machine is exceeded between slip episodes which settle stably to the constant driving rate and those which become unstable and result in stick slip the stick slip failure criterion is not a peak stress but rather loss of strength more rapidly that the unloading rate of the loading machine the evidence that accelerating motion before stick slip is due to the time variable strength of rock suggests that accelerating slip will occur before earthquakes unfortunately for easy observation experiments indicate that slip need not be uniform over a fault surface also to be established is whether slip will be sufficiently large in the earth to be detectable nevertheless a plausible mechanism for generating observable phenomena prior to earthquakes is suggested by the laboratory observations

strain rate effect on fault slip and rupture evolution insight from meter scale rock friction experiments abstract we conduct meter scale rock friction experiments to study strain rate effect on fault slip and rupture evolution two rock samples made of indian metagabbro with a nominal contact dimension of 1 5 m long and 0 1 m wide are juxtaposed and loaded in a direct shear configuration to simulate the fault motion a series of experimental tests under constant loading rates ranging from 0 01 mm s to 1 mm s and under a fixed normal stress of 6 7 mpa are performed to simulate conditions with changing strain rates load cells and displacement transducers are utilized to examine the macroscopic fault behavior while high density arrays of strain gauges close to the fault are used to investigate the local fault behavior the observations show that the macroscopic peak strength strength drop and the rate of strength drop can increase with increasing loading rate at the local scale the observations reveal that slow loading rates favor generation of characteristic ruptures that always nucleate in the form of slow slip at about the same location in contrast fast loading rates can promote very abrupt rupture nucleation and along strike scatter of hypocenter locations at a given propagation distance rupture speed tends to increase with increasing loading rate we propose that a strain rate dependent fault fragmentation process can enhance the efficiency of fault healing during the stick period which together with healing time controls the recovery of fault strength in addition a strain rate dependent weakening mechanism can be activated during the slip period which together with strain energy selects the modes of fault slip and rupture propagation the results help to understand the spectrum of fault slip and rock deformation modes in nature and emphasize the role of heterogeneity in tuning fault behavior under different strain rates

the magnitude frequency distribution of earthquakes recorded with deep seismometers at cajon pass southern california abstract the cumulative b value the slope of the gutenberg richter relationship between earthquake occurrence rate and magnitude is commonly found to be constant 1 network catalogues however reveal a decrease at small magnitudes b value is not just an artifact of catalogue incompleteness but that small earthquakes are really not as numerous as a constant b value extrapolated from larger events would predict in the cajon pass area southern california the b value of seismicity recorded by the local network scsn appears to decrease below about m l 1 6 in order to investigate whether this decrease is real or simply represents the network detection threshold we use seismicity recorded by the deep 1 5 and 2 5 km seismometers deployed in the cajon pass scientific drillhole between april 1992 and october 1994 the maximum amplitudes recorded downhole are compared to scsn magnitudes for events recorded by the network to determine the relationship between amplitude and m l as a function of hypocentral distance from the borehole magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole magnitude frequency curves are calculated for those events within 18 km of the borehole and a constant b value is observed to m l 0 5

repeating aftershocks of the great 2004 sumatra and 2005 nias earthquakes abstract we investigate repeating aftershocks associated with the great 2004 sumatra andaman mw 9 2 and 2005 nias simeulue mw 8 6 earthquakes by cross correlating waveforms recorded by the regional seismographic station psi and teleseismic stations we identify 10 and 18 correlated aftershock sequences associated with the great 2004 sumatra and 2005 nias earthquakes respectively the majority of the correlated aftershock sequences are located near the down dip end of a large afterslip patch we determine the precise relative locations of event pairs among these sequences and estimate the source rupture areas the correlated event pairs identified are appropriately referred to as repeating aftershocks in that the source rupture areas are comparable and significantly overlap within a sequence we use the repeating aftershocks to estimate afterslip based on the slip seismic moment scaling relationship and to infer the temporal decay rate of the recurrence interval the estimated afterslip resembles that measured from the near field geodetic data to the first order the decay rate of repeating aftershocks as a function of lapse time t follows a power law decay 1 t p with the exponent p in the range 0 8 1 1 both types of observations indicate that repeating aftershocks are governed by post seismic afterslip

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

emergence and disappearance of interplate repeating earthquakes following the 2011 m9 0 tohoku oki earthquake slip behavior transition between seismic and aseismic depending on the loading rate we investigated spatiotemporal change in the interplate seismic activity following the 2011 tohoku oki earthquake m9 0 in the region where interseismic interplate coupling was relatively weak and large postseismic slip was observed we classified earthquakes by their focal mechanisms to identify the interplate events and conducted hypocenter relocation to examine the detailed spatiotemporal distribution of interplate earthquakes in the mostly creeping area the results show that many interplate earthquakes including m 6 events emerged immediately after the tohoku oki earthquake in areas where very few interplate earthquakes had been observed in the 88 previous years the emergent earthquakes include repeating sequences and the extremely long quiescence of small to moderate earthquakes before the tohoku oki earthquake suggests that the source areas for the post m9 events slipped aseismically during the quiescence the repeaters magnitudes decayed over time following the tohoku oki earthquake and some sequences disappeared within a year the emergence of interplate earthquakes suggests that areas where aseismic slip had been dominant before the tohoku oki earthquake started to cause seismic slip after the earthquake probably due to the increased loading rate from the afterslip the magnitude decrease and disappearance of repeaters can be interpreted as shrinkage in seismic areas around the repeaters sources as the loading rate decreased due to the afterslip decay over time these observations suggest that changes in the loading rate can cause slip behavior transition between seismic and aseismic this indicates that such loading rate dependent slip behavior plays an important role in the spatiotemporal distribution of earthquakes in interplate seismogenic zones

on behaviour and scaling of small repeating earthquakes in rate and state fault models with abundant seismic data for small repeating earthquakes it is important to construct a dynamic model that can explain various aspects of related observations in this work we study small repeating earthquakes on a fault governed by rate and state dependent friction laws the earthquakes occur on a velocity weakening patch surrounded by a much larger velocity strengthening region the whole fault is subject to long term tectonic loading the model with a circular patch and the aging form of rate and state dependent friction laws has been shown to reproduce the scaling of recurrence time versus seismic moment for small repeating earthquakes in a previous study here we investigate the behaviour of small repeating earthquakes in related models under different scenarios including several forms of the state evolution equations in rate and state dependent friction laws rectangular velocity weakening patch geometries quasi dynamic versus fully dynamic representation of inertial effects and 2 d versus 3 d simulations we find that the simulated scalings between the recurrence time and seismic moment for these different scenarios is similar while differences do exist we propose a theoretical consideration for the scaling between the recurrence time and seismic moment of small repeating earthquakes for patch radii smaller than or comparable to the full nucleation size the scaling is explained by the increase of seismic to aseismic slip ratio with magnitude for patch radii larger than the full nucleation size the scaling is explained by the model in which the recurrence time is determined by the earthquake nucleation time which is in turn determined by the time for aseismic slip to penetrate the distance of the full nucleation size into the patch the obtained theoretical insight is used to find the combinations of fault properties that allow the model to fit the observed scaling and range of the seismic moment and recurrence time

damage to the shallow landers fault from the nearby hector mine earthquake crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes which allows for the relative motion between adjacent crustal blocks although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present day crustal stresses and fault motions to geological structures formed in the past fault zone damage and healing have been documented quantitatively in only a few cases we have been monitoring the healing of damage on the shallow johnson valley fault after its rupture in the 1992 magnitude 7 3 landers earthquake and here we report that this healing was interrupted in 1999 by the magnitude 7 1 hector mine earthquake rupture which occurred 20 30 km away the hector mine earthquake both strongly shook and permanently strained the johnson valley fault adding damage discernible as a temporary reversal of the healing process the fault has since resumed the trend of strength recovery that it showed after the landers earthquake these observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking and may be involved in friction reduction during faulting

velocity contrast along the calaveras fault from analysis of fault zone head waves generated by repeating earthquakes 1 we systematically investigate the velocity contrast along the calaveras fault that ruptured during the 1984 morgan hill earthquake using fault zone head waves fzhw that refract along the fault interface we stack waveforms in 353 sets of repeating clusters and align the peaks or troughs of the direct p waves assuming rightlateral strike slip focal mechanisms the obtained velocity contrasts are 2 3 and 12 14 nw and se of station cco respectively the fzhw and the fault plane outlined by the relocated seismicity se of cco are more complicated than those nw of cco the results can be explained by a relatively simple and sharp fault interface in the nw and a complicated fault structure with a presence of a low velocity zone in the se the along strike variations in the strength of the velocity contrast are consistent with surface geological mapping and recent 3d tomography studies in this region citation zhao p and z peng 2008 velocity contrast along the calaveras fault from analysis of fault zone head waves generated by repeating earthquakes geophys res lett 35 l01303 doi 10 1029 2007gl031810

observations of changes in waveform character induced by the 1999 m w7 6 chi chi earthquake 1 we observe changes in the waveforms of repeating earthquakes in eastern taiwan following the 1999 mw7 6 chi chi earthquake while their recurrence intervals appear to be unaffected there is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the chi chi event these changes are limited to stations in and paths that cross the 70 100 km region surrounding the chi chi source area the area where seismic intensity and co seismic surface displacements were largest this suggests that damage at the near surface is responsible for the observed waveform changes delays are largest in the late s wave coda reaching approximately 120 ms this corresponds to a path averagedswave velocity reduction of approximately 1 there is also evidence that damage in the fault zone caused changes in waveform character at sites in the footwall where source receiver paths propagate either along or across the rupture the reduction in waveform similarity persists through the most recent repeating event in our study november 15 2007 indicating that the subsurface damage induced by the chi chi earthquake did not fully heal within the first 8 years following the chi chi earthquake

detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 mw 7 6 nicoya earthquake we apply a waveform matching technique to obtain a detailed earthquake catalog around the rupture zone of the 5 september 2012 moment magnitude 7 6 nicoya earthquake with emphasis on its aftershock sequence starting from a preliminary catalog we relocate 7900 events using tomodd to better quantify their spatiotemporal behavior relocated aftershocks are mostly clustered in two groups the first is immediately above the major coseismic slip patch partially overlapping with shallow afterslip the second one is 50 km se to the main shock nucleation point and near the terminus of coseismic rupture in a zone that exhibited little resolvable afterslip using the relocated events as templates we scan through the continuous recording from 29 june 2012 to 30 december 2012 detecting approximately 17 times more than template events we find 190 aftershocks in the first half hour following the main shock mostly along the plate interface later events become more scattered in location showing moderate expansion in both along trench and downdip directions from the detected catalog we identify 53 repeating aftershock clusters with mean cross correlation values larger than 0 9 and indistinguishably intracluster event locations suggesting slip on the same fault patch most repeating clusters occurred within the first major aftershock group very few repeating clusters were found in the aftershock grouping along the southern edge of the peninsula which is not associated with substantial afterslip our observations suggest that loading from nearby afterslip along the plate interface drives spatiotemporal evolution of aftershocks just above the main shock rupture patch while aftershocks in the se group are to the se of the observed updip afterslip and poorly constrained

systematic variations in recurrence interval and moment of repeating aftershocks 1 the recurrence intervals for 194 repeating clusters on the calaveras fault follow a power law decay relation with elapsed time after the 1984 m6 2 morgan hill california mainshock the decay rates of repeating aftershocks in the immediate vicinity of a high slip patch that failed during the mainshock systematically exceed those that are farther away the trend between relative moment and recurrence interval which is a measure of the fault healing rate varies systematically with depth and changes from negative to positive value as the distance between the repeating aftershock and the mainshock slip patch increases we speculate that high strain rate in the early postseismic period may cause transient embrittlement and strengthening of the deep repeating clusters in the areas adjacent to the mainshock slip patch resulting in large moments that decrease with time as the strain rate diminishes our observations suggest that systematic behavior of repeating aftershocks reflect variations in the fault zone rheology

the solsticial pause on mars 1 a planetary wave reanalysis large scale planetary waves are diagnosed from an analysis of profiles retrieved from the thermal emission spectrometer aboard the mars global surveyor spacecraft during its scientific mapping phase the analysis is conducted by assimilating thermal profiles and total dust opacity retrievals into a mars global circulation model transient waves are largest throughout the northern hemisphere autumn winter and spring period and almost absent during the summer the southern hemisphere exhibits generally weaker transient wave behaviour a striking feature of the low altitude transient waves in the analysis is that they show a broad subsidiary minimum in amplitude centred on the winter solstice a period when the thermal contrast between the summer hemisphere and the winter pole is strongest and baroclinic wave activity might be expected to be strong this behaviour here called the solsticial pause is present in every year of the analysis this strong pause is under represented in many independent model experiments which tend to produce relatively uniform baroclinic wave activity throughout the winter this paper documents and diagnoses the transient wave solsticial pause found in the analysis a companion paper investigates the origin of the phenomenon in a series of model experiments

depth extent of damage zones around the central calaveras fault from waveform analysis of repeating earthquakes summary we systematically investigate spatial variations of temporal changes and depth extent of damage zones along the calaveras fault that ruptured during the 1984 morgan hill earthquake by the waveform analysis of 333 sets of repeating earthquakes we use a sliding window waveform cross correlation technique to measure traveltime changes in waveforms generated by each repeating cluster we find clear traveltime delays in the s and early s coda waves for events immediately after the morgan hill main shock the amplitudes of the time delays decrease logarithmically with time since the main shock indicating a time dependent recovery healing process following the abrupt coseismic temporal changes the largest temporal changes are observed at station cco that is the closest to the rupture zone of the morgan hill main shock the time delays at this station are larger for clusters in the top 6 km and decrease systematically at larger depth in comparison the time delays observed at other five stations are much smaller and do not show clear relationship with hypocentral depth we suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near surface layers while the temporal changes at station cco are likely associated with the damage zone around the calaveras fault that is well developed in the top few kilometres of the upper crust our results are consistent with the inference of a widespread damage and non linearity in the near surface layers associated with strong ground motions of nearby large earthquakes and localized damages and flower type structures around active faults based on previous studies of fault zone structures and recent 3 d numerical simulations

remote triggering of fault strength changes on the san andreas fault at parkfield a 20 year seismological record of from the parkfield area of the san andreas fault has been used to identify two occasions when long term changes in fault strength seem to have been induced remotely by large seismic events the 2004 sumatra andaman and the 1992 landers earthquakes in both cases two changes occurred that are consistent with variations in fault strength variation in the properties of seismic scatterers with time reflecting stress induced migration of fluids and systematic variation in the characteristics of repeating earthquake sequences these findings suggest that the largest earthquakes have a global influence on the strength of the earth s fault systems in support of that hypothesis an unusually large number of m 8 earthquakes occurred in the three years following the 2004 sumatra andaman earthquake changes in fault strength over time can increase or decrease the likelihood of failure and the ultimate triggering of seismic events observation of the parkfield area at the san andreas fault over the years 1987 2008 now reveals two occasions the 2004 sumatra andaman earthquake and the 1992 landers earthquake where long term changes in fault strength have probably been induced remotely by large seismic events fault strength is a fundamental property of seismogenic zones and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events although changes in fault strength have been suggested to explain various phenomena such as the remote triggering of seismicity1 there has been no means of actually monitoring this important property in situ here we argue that 20 years of observation 1987 2008 of the parkfield area at the san andreas fault have revealed a means of monitoring fault strength we have identified two occasions where long term changes in fault strength have been most probably induced remotely by large seismic events namely the 2004 magnitude m 9 1 sumatra andaman earthquake and the earlier 1992 m 7 3 landers earthquake in both cases the change possessed two manifestations temporal variations in the properties of seismic scatterers probably reflecting the stress induced migration of fluids and systematic temporal variations in the characteristics of repeating earthquake sequences that are most consistent with changes in fault strength in the case of the 1992 landers earthquake a period of reduced strength probably triggered the 1993 parkfield aseismic transient2 3 4 5 as well as the accompanying cluster of four m 4 earthquakes at parkfield the fault strength changes produced by the distant 2004 sumatra andaman earthquake are especially important as they suggest that the very largest earthquakes may have a global influence on the strength of the earth s fault systems as such a perturbation would bring many fault zones closer to failure it should lead to temporal clustering of global seismicity this hypothesis seems to be supported by the unusually high number of m 8 earthquakes occurring in the few years following the 2004 sumatra andaman earthquake

recurrence time and frequency slip statistics of slip events on the creeping section of the san andreas fault in central california summary an important question in seismic hazard assessments is the frequency size and recurrence interval statistics at a point on a fault does a point on a fault obey the same statistics as earthquakes in a region do this is a difficult question to answer because the number of repetitive earthquakes on a particular fault that have been observed is small in order to overcome this difficulty we consider slip events on the creeping section of the san andreas fault in central california sequences of up to 100 events are obtained from creepmeter records we compare the statistical distribution of recurrence times with the brownian passage time lognormal and weibull distributions and using goodness of fit tests find that the weibull is the preferred distribution we also consider the frequency amplitude distribution of slip events we find that the data clearly do not obey a gutenberg richter distribution instead there is a uniform distribution of event sizes for a large fraction of events

groundwater salinization processes and reversibility of seawater intrusion in coastal carbonate aquifers summary seawater intrusion swi has led to salinization of fresh groundwater reserves in coastal areas worldwide and has forced the closure of water supply wells there is a paucity of well documented studies that report on the reversal of swi after the closure of a well field this study presents data from the coastal carbonate aquifer in northeast china where large scale extraction has ceased since 2001 after salinization of the main well field the physical flow and concomitant hydrogeochemical processes were investigated by analyzing water level and geochemical data including major ion chemistry and stable water isotope data seasonal water table and salinity fluctuations as well as changes of 2 h 18 o values of groundwater between the wet and dry season suggest local meteoric recharge with a pronounced seasonal regime historical monitoring testifies of the reversibility of swi in the carbonate aquifer as evidenced by a decrease of the cl concentrations in groundwater following restrictions on groundwater abstraction this is attributed to the rapid flushing in this system where flow occurs preferentially along karst conduits fractures and fault zones the partially positive correlation between 18 o values and tds concentrations of groundwater as well as high no 3 concentrations 39 mg l suggest that irrigation return flow is a significant recharge component therefore the present day elevated salinities are more likely due to agricultural activities rather than swi nevertheless seawater mixing with fresh groundwater cannot be ruled out in particular where formerly intruded seawater may still reside in immobile zones of the carbonate aquifer the massive expansion of fish farming in seawater ponds in the coastal zone poses a new risk of salinization cation exchange carbonate dissolution and fertilizer application are the dominant processes further modifying the groundwater composition which is investigated quantitatively using hydrogeochemical models

spatial temporal distribution of early aftershocks following the 2016 ms 6 4 menyuan qinghai china earthquake abstract the lenglongling fault located in the north qilian fold zone along the northeastern margin of the tibetan plateau plays an important role in accommodating the tectonic deformation between the tibetan plateau and the alxa block on 21 january 2016 the ms 6 4 menyuan earthquake occurred near the lenglongling fault in this study we utilize the waveform matched filter technique and double difference earthquake location algorithm to examine the spatial temporal evolution of aftershocks following the menyuan mainshock events from both china earthquake networks center cenc routine catalog and relocated catalog are used as templates to scan through the continuous waveform data 21 january 2016 to 31 december 2016 we identify three and eleven times more events than listed in cenc routine catalog and relocated catalog respectively we find that the earliest aftershocks e g in the first 10 min or so mostly illuminated a steeply dipping plane while aftershocks at later times outlined a near vertical dipping plane with mostly strike slip focal mechanisms the relocated aftershocks also suggest that the menyuan mainshock occurred in a secondary fault rather than the major lenglongling fault which is consistent with geodetic and geological observations moreover we find that aftershocks systematically expand with time along the fault strike and 16 repeating earthquake clusters occur in the rupture zone which are likely driven by afterslip of the mainshock

frictional heterogeneities on carbonate bearing normal faults insights from the monte maggio fault italy observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and or spatial heterogeneity of fault frictional behavior such complex slip patterns have been observed for earthquakes on normal faults throughout central italy where many of the mw 6 to 7 earthquakes in the apennines nucleate at depths where the lithology is dominated by carbonate rocks to explore the relationship between fault structure and heterogeneous frictional properties we studied the exhumed monte maggio fault located in the northern apennines we collected intact specimens of the fault zone including the principal slip surface and hanging wall cataclasite and performed experiments at a normal stress of 10 mpa under saturated conditions experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 mpa stress drop as the fault surface fails then velocity neutral frictional behavior and significant frictional healing overall our results suggest that 1 earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface 2 postseismic slip is more likely to occur in areas of the fault where gouge is present and 3 high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop

fault healing inferred from time dependent variations in source properties of repeating earthquakes we analyze two sets of repeating earthquakes on the calaveras fault to estimate in situ rates of fault strengthening healing earthquake recurrence intervals tr range from 3 to 803 days variations in relative moment and duration are combined to study changes in stress drop rupture dimension rupture veloc ity and particle velocity as a function of t healing rates and source variations are compared with predictions of laboratory derived friction laws two interpretations of event duration z are used one in which is given by the ratio of slip to particle ve locity and one in which it scales as rupture dimension divided by rupture velocity our data indicate that faults strengthen during the interseismic period we infer that source dimension de creases with t r due to aseismic creep within the region surround ing the repeating events stress drop increases 1 3mpa per dec ade increase in twhich represents an increase of a factor of 2 3 relative to events with tbetween 10 and 100 days this rate of fault healing is consistent with extrapolations of laboratory meas urements of healing rates if fault strength is high on order of 60mpa and stress drop is roughly 10 of this value

scaling relations of strike slip earthquakes with different slip rate dependent properties at depth abstract empirical observations suggest that earthquake stress drop is generally constant to investigate the effect of rupture width on earthquake scaling relations we analyze synthetic seismicity produced by a 3d vertical strike slip fault model using two different profiles of frictional slip rate behavior below the seismogenic zone within the rate and state framework a relatively abrupt transition of the a b profile from velocity weakening to strengthening at the base of the seismogenic crust produces increasing slip and stress drop with increasing event size choosing a smoother transition allows large earthquakes to propagate deeper leading to similar slip length scaling but constant stress drop scaling our numerical experiments support the idea that the maintenance of constant stress drop across the entire range of observed earthquake magnitudes may be achieved by allowing coseismic slip to rupture to depths below the seismogenic layer

quasi dynamic modeling of seismicity on a fault with depth variable rate and state dependent friction 1 neither the omori type of clustering prior to and following large earthquakes nor the gutenberg richter distribution are reproducible by present continuous models discrete models on the other hand give rise to more complex and closer to realistic earthquake clustering the objective of this study is twofold to explore the consequences of spatial discreteness on the distribution in time and space of earthquake activity on a fault governed by rate and state friction and to examine the effect of interaction between seismic slip and aseismic creep on aftershock sequences to that end we model a long vertical two dimensional strike slip fault that is governed by rate and state dependent friction and is embedded in a three dimensional elastic half space quasi dynamic motion on the fault is driven by steady displacement applied below the fault and at distance w 2 on either side of the fault plane the model is said to be spatially discrete in that the computational cells are oversized with respect to the critical length scale that is implied by the friction law the model reproduces some of the characteristics of natural seismicity including the nonperiodic recurrence times and the omori type of clustering prior to and following large earthquakes it also produces a wide range of earthquake magnitudes but with a ratio of small to large earthquakes that is in excess with respect to what is inferred form natural catalogs we examined the effect of a stress step applied on the creeping portions of the model and confirmed that aftershock sequences resulting from stress relaxation on creeping segments decay asymptotically to 1 time finally we discuss fundamental differences between seismicity models employing rate and state friction and those employing static kinetic friction

the effect of loading rate on static friction and the rate of fault healing during the earthquake cycle the seismic cycle requires that faults strengthen heal between earthquakes and the rate of this healing process plays a key role in determining earthquake stress drop1 2 3 4 rupture characteristics5 6 and seismic scaling relations2 3 4 7 frictional healing as evidenced by increasing static friction during quasi stationary contact between two surfaces1 8 9 10 11 12 is considered the mechanism most likely to be responsible for fault strengthening2 3 13 14 previous studies however have shown a large discrepancy between laboratory and seismic field estimates of the healing rate2 3 4 14 15 in the laboratory rock friction changes by only a few per cent per order of magnitude change in slip rate whereas seismic stress drop increases by a factor of 2 to 5 per order of magnitude increase in earthquake recurrence interval but in such comparisons it is assumed that healing and static friction are independent of loading rate here i summarize laboratory measurements showing that static friction and healing vary with loading rate and time as expected from friction theory16 17 18 applying these results to seismic faulting and accounting for differences in laboratory seismic and tectonic slip rates i demonstrate that post seismic healing is expected to be retardedfor a period of several hundred days following an earthquake in agreement with recent findings from repeating earthquakes13 14 19 20

salinization and dilution history of ground water discharging into the sea of galilee the dead sea transform israel abstract the mechanism governing salinization of ground water discharging into the sea of galilee in israel has been the subject of debate for several decades because the lake provides 25 of the water consumed annually in israel correct identification of the salt sources is essential for the establishment of suitable water management strategies for the lake and the ground water in the surrounding aquifers existing salinization models were evaluated in light of available and newly acquired data including general chemistry and o h c and cl isotopes based on the chemical and isotopic observations the proposed salt source is an ancient intensively evaporated brine 21 to 33 fold seawater which percolated through the valley formations from a lake which had formed in the rift valley following seawater intrusion during the late miocene low na cl and high br cl values support the extensive evaporation whereas high ca cl and low mg cl values indicate the impact of dolomitization of the carbonate host rock on the residual solution based on radiocarbon and other isotope data the dilution of the original brine occurred in two stages the first took place 30 000 a ago by slightly evaporated fresh to brackish lake water to form the sea of galilee brine the second dilution phase is associated with the current hydrological regime as the sea of galilee brine migrates upward along the rift faults and mixes with the actively circulating fresh ground water to form the saline springs the spatially variable chemical and isotopic features of the saline springs suggest not only differential dilution by fresh meteoric water but also differential percolation timing of the original brine into the tectonically disconnected blocks registering different evaporation stages in the original brine consequently various operations to reduce the brine contribution to the lake may be differentially effective in the various areas

does aftershock duration scale with mainshock size 1 it has been claimed that aftershock duration scales with the mainshock recurrence interval dieterich 1994 this implies a scaling between aftershock duration and mainshock magnitude to see if such a scaling exist we compare properties of aftershock sequences associated with mainshocks whose size span several magnitude units earthquake properties were examined along two northern california fault segments one is the calaveras fault another is a northern segment of the san andreas fault the first study area contains the rupture area of the m6 2 1984 morgan hill and the latter is adjacent the southern end of the m7 1 1989 loma prieta rupture both the morgan hill and the loma prieta aftershock activities lasted 1 3 108 seconds on the other hand aftershock activities of small to moderate mainshocks lasted about 1 2 106 seconds nevertheless there is no evidence for scaling between mainshock size and aftershock duration for moderate to small mainshocks it thus seems that large earthquakes because they break the entire seismogenic depth and interact with deeper parts of the crust trigger a relaxation mechanism that is inactive during aftershock activity due to moderate to small quakes and whose characteristic relaxation time is much longer than that of the brittle crust current seismicity models do not provide explanation for these observations

transient electromagnetic sounding for groundwater the feasibility of using the transient electromagnetic sounding ts or tdem method for groundwater exploration can be studied by means of numerical models as examples of its applicability to groundwater exploration we study four groundwater exploration problems 1 mapping of alluvial fill and gravel zones over bedrock 2 mapping of sand and gravel lenses in till 3 detection of salt or brackish water interfaces in freshwater aquifers and 4 determination of hydrostratigraphy these groundwater problems require determination of the depth to bedrock location of resistive high porosity zones associated with fresh water determination of formation resistivity to assess water quality and determination of lithology and geometry respectively the ts method is best suited for locating conductive targets and has very good vertical resolution unlike other sounding techniques where the receiver transmitter array must be expanded to sound more deeply the depth of investigation for the ts method is a

fixed recurrence and slip models better predict earthquake behavior than the time and slip predictable models 1 repeating earthquakes 1 the behavior of individual events in repeating earthquake sequences in california taiwan and japan is better predicted by a model with fixed inter event time or fixed slip than it is by the time and slip predictable models for earthquake occurrence given that repeating earthquakes are highly regular in both inter event time and seismic moment the time and slip predictable models seem ideally suited to explain their behavior taken together with evidence from the companion manuscript that shows similar results for laboratory experiments we conclude that the short term predictions of the time and slip predictable models should be rejected in favor of earthquake models that assume either fixed slip or fixed recurrence interval this implies that the elastic rebound model underlying the time and slip predictable models offers no additional value in describing earthquake behavior in an event to event sense but its value in a long term sense cannot be determined these models likely fail because they rely on assumptions that oversimplify the earthquake cycle we note that the time and slip of these events is predicted quite well by fixed slip and fixed recurrence models so in some sense they are time and slip predictable while fixed recurrence and slip models better predict repeating earthquake behavior than the time and slip predictable models we observe a correlation between slip and the preceding recurrence time for many repeating earthquake sequences in parkfield california this correlation is not found in other regions and the sequences with the correlative slip predictable behavior are not distinguishable from nearby earthquake sequences that do not exhibit this behavior

similar microearthquakes observed in western nagano japan and implications for rupture mechanics 1 we have applied a waveform cross correlation technique to study the similarity and the repeatability of more than 21 000 microearthquakes 0 m 4 5 in the aftershock zone of the 1984 western nagano earthquake in central japan we find that the seismicity in this particular intraplate fault essentially consists of no repeating earthquakes that occurred on the same patch of the fault in a quasiperiodic manner in the study period between 1995 and 2001 on the other hand we identify a total of 278 doublets and 62 multiplets 807 events that occurred consecutively within seconds to days on the basis of the relative arrival times of the p and s waves we have obtained precise relative locations of these consecutive events with an error between several meters to a few tens of meters there is a clear lower bound on the distances measured between these consecutive events and the lower bound appears to be proportional to the size of the first events this feature is consistent with what rubin and gillard 2000 have observed near the san juan bautista section of the san andreas fault shear stress increases at the edge of an earthquake rupture and the rupture edge becomes the most likely place where the second events are initiated the observed minimum distance thus reflects the rupture size of the first events the minimum distance corresponds to the rupture size calculated from a circular fault model with a stress drop of 10 mpa we found that using different time windows results in a slight difference in the delay time estimates and the subsequent projection locations which may reflect the finite size nature of earthquake ruptures

numerical simulation of the kamaishi repeating earthquake sequence change in magnitude due to the 2011 tohoku oki earthquake abstract we conducted numerical simulations of a repeating earthquake sequence on the plate boundary off the shore of kamaishi northeastern japan assuming rate and state dependent friction laws uchida et al 2014 reported that the kamaishi repeating earthquakes showed an increase in magnitude and a decrease in recurrence interval after the 2011 tohoku oki earthquake m 9 but an approximately constant magnitude m 4 9 and a regular recurrence interval 5 5 years before the tohoku oki earthquake a m 5 9 event occurred just after the m 9 event and was followed by a m 5 5 event we considered a fault patch of velocity weakening friction with frictional parameters leading to seismic slip confined in its central part of the patch afterslip due to the m 9 event was involved in the model to increase the loading rate on the patch the simulation successfully reproduced increasing magnitude and decreasing recurrence time caused by the afterslip a m 6 class event in which seismic slip spread over the entire area of the patch occurred just after the m 9 event for the aging law and the nagata law when we assumed the aging law with frictional parameters which lie near the boundary between leading to slip on the entire patch and slip confined in its central part the m6 class event was followed by a m 5 5 class event furthermore we examined a conditionally stable large patch that contained a small unstable patch this model also reproduced a m 6 class event after the m 9 event in these models stress outside the confined area of the path is released before a dynamic event under a constant low loading rate whereas the stress perturbation due to afterslip within the seismic cycle induces a dynamic event on the entire patch

postseismic response of repeating aftershocks the recurrence intervals of repeating earthquakes on the san andreas fault in the loma prieta aftershock zone follow the characteristic 1 t decay of omori s law a model in which these earthquakes occur on isolated patches of the fault that fail in stick slip with creep around them can explain this observation in this model the recurrence interval is inversely proportional to the loading rate due to creep logarithmic velocity strengthening friction predicts 1 t decay in creep rate following the mainshock the time dependence of recurrence is inconsistent with a simple viscous constitutive relationship which predicts an exponential decay of loading rate thus our observations imply postseismic slip at seismogenic depth under a power law rheology the time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths

variations in rupture process with recurrence interval in a repeated small earthquake in theory and in laboratory experiments friction on sliding surfaces such as rock glass and metal increases with time since the previous episode of slip1 this time dependence is a central pillar of the friction laws widely used to model earthquake phenomena2 3 on natural faults other properties such as rupture velocity4 5 porosity and fluid pressure6 11 may also vary with the recurrence interval eighteen repetitions of the same small earthquake separated by intervals ranging from a few days to several years allow us to test these laboratory predictions in situ the events with the longest time since the previous earthquake tend to have about 15 larger seismic moment than those with the shortest intervals although this trend is weak in addition the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact

radiography of a normal fault system by 64 000 high precision earthquake locations the 2009 l aquila central italy case study 1 we studied the anatomy of the fault system where the 2009 l aquila earthquake mw 6 1 nucleated by means of 64 k high precision earthquake locations spanning 1 year data were analyzed by combining an automatic picking procedure for p and s waves together with cross correlation and double difference location methods reaching a completeness magnitude for the catalogue equal to 0 7 including 425 clusters of similar earthquakes the fault system is composed by two major faults the high angle l aquila fault and the listric campotosto fault both located in the first 10 km of the upper crust we detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops we observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays the width of the l aquila fault zone varies along strike from 0 3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution up to 1 5 km at the fault tips with an increase in the geometrical complexity these characteristics similar to damage zone properties of natural faults underline the key role of aftershocks in fault growth and co seismic rupture propagation processes additionally we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological i e creeping discontinuity explaining how normal faults detach at depth