repeating earthquakes associated with the wfsd 1 drilling site abstract we cross correlated three component seismic waveforms to find repeating events among the aftershocks of the 2008 wenchuan earthquake centered at the wfsd 1 drilling site during and after the drilling the aftershocks selected occurred from 2008 10 01 to 2011 09 15 with a magnitude range of ml 1 4 5 5 using the 1 10 hz bandpass filtered waveforms recorded by the regional and mobile seismic stations 518 repeating events are identified with the criterion that for at least one station the three components of seismograms all have cross correlation coefficients larger than 0 8 using 12 repeating multiplets defined by the criterion that each pair of events in the multiplet must be repeating pairs fault slip rates are estimated slip rate was averaged at about 5 mm year being within the range of the deformation rate before the wenchuan earthquake obtained by repeating earthquakes with a weak attenuating trend and a suspected disturbance by a strong aftershock

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

deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of kilauea dense microearthquake swarms occur in the upper south flank of kilauea providing multiplets composed of hundreds of events the similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters a simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically relocation of these events greatly improves the definition of the seismic image of the fault that generates them indeed relative relocations define a plane dipping about 6 northward although corresponding absolute locations are widely dispersed in the swarm a composite focal mechanism built from events providing a correct spatial sampling of the multiplet also gives a well constrained northward dip of about 5 to the near horizontal plane this technique thus collapses the clouds of hypocenters of single event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies we cannot conclude that all south flank earthquakes collapse to a single plane there may locally be several planes perhaps with different dips and depths throughout the south flank volume the 6 northward dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of hawaii this plane is probably an important feature that characterizes the basal slip layer below the upper south flank of kilauea volcano differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane the present work illustrates how high precision relative relocations of similar events in dense swarms combined with the analysis of geodetic measurements can help to describe deep fault plane geometry systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well instrumented highly seismic areas to provide reliable basic information especially useful for understanding of earthquake generation processes

crustal rheology from focal depths in the north china basin abstract in order to investigate earthquakes in the lower crust of north china basin ncb we develop a new method of resolving very accurately the focal depth for local earthquakes in the sedimentary regions by using p and s to p converted wave sp at the sedimentary interface theoretical analysis shows clearly that the travel time difference between the sp and p wave almost linearly correlates with the focal depth this finding provides tight constraints on the depth with this method we obtain well constrained depths of 44 events in the ncb with uncertainties in the depth of about 2 km such a fine resolution can have great potential in asking questions regarding the crustal rheology the depth distribution shows abundance of earthquakes in depth interval of 20 km with some events in the lower crust but also reveals the absence of seismicity deeper than 25 km we find a good fit between the depth frequency distribution in this region and the yield strength envelope yse in the baikal rift systems brs we infer that the seismogenic thickness is 25 km in the ncb and the main deformation mechanism is brittle fracture we further hypothesize that 1 the rheological layering of dominant rheology in the ncb is similar to that of the brs which can be explained with a quartz rheology at 0 10 km depth and a diabase rheology at 10 35 km depth 2 the temperature is moderate in the seismogenic zone of crust we emphasize that many accurately resolved earthquake locations can shed light on the local nature of the crustal rheology and this strategic method can be employed in other sedimentary regions which are seismically active

late holocene episodic displacement on fault scarps related to interstratal dissolution of evaporites teruel neogene graben ne spain abstract the mio pliocene formations of the turolian type section teruel graben spain have sagged due to dissolution of the underlying triassic evaporites generating a 1 7 km long monocline and accompanying synform with 130 m of structural relief the crest of the monocline is affected by a graben that counterbalances the shortening caused by passive bending in the adjacent syncline this graben is controlled by a master synthetic fault and a swarm of antithetic and synthetic faults with a conspicuous geomorphic expression after acquiring ground penetrating radar gpr profiles three trenches were excavated in depressions associated with uphill facing fault scarps to our knowledge these are the first trenches excavated across faults caused by deep seated dissolution of evaporites the geometrical relationships observed in two trenches indicate late holocene episodic displacement as many as 3 events some of the parameters estimated for the investigated faults are clearly different from those expectable for tectonic faults in this intraplate area they include high apparent vertical slip rates 0 6 1 mm yr low average recurrence of faulting events 1 2 2 ka and very high displacement per event values 65 cm for surface ruptures less then 200 m long our findings suggest that considering evaporite collapse faults as creeping structures is not a reliable criterion to differentiate between tectonic faults seismogenic and dissolution induced gravitational faults nonseismogenic

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

an earthquake model with interacting asperities summary a model is presented that treats an earthquake as the failure of asperities in a manner consistent with modern concepts of sliding friction the mathematical description of the model includes results for elliptical and circular asperities oblique tectonic slip static and dynamic solutions for slip on the fault stress intensity factors strain energy and second order moment tensor the equations that control interaction of asperities are derived and solved both in a quasi static tectonic mode when none of the asperities are in the process of failing and a dynamic failure mode when asperities are failing and sending out slip pulses that can trigger failure of additional asperities the model produces moment rate functions for each asperity failure so that given an appropriate green function the radiation of elastic waves is a straightforward calculation the model explains an observed scaling relationship between repeat time and seismic moment for repeating seismic events and is consistent with the properties of pseudo tachylites treated as fossil asperities properties of the model are explored with simulations of seismic activity that results when a section of the fault containing a spatial distribution of asperities is subjected to tectonic slip the simulations show that the failure of a group of strongly interacting asperities satisfies the same scaling relationship as the failure of individual asperities and that realistic distributions of asperities on a fault plane lead to seismic activity consistent with probability estimates for the interaction of asperities and predicted values of the gutenberg richter a and b values general features of the model are the exterior crack solution as a theoretical foundation a heterogeneous state of stress and strength on the fault dynamic effects controlled by propagating slip pulses and radiated elastic waves with a broad frequency band

earth science microseismicity data forecast rupture area on 28 september 2004 there was an earthquake of magnitude 6 0 at parkfield california here we show that the size distribution of the micro earthquakes recorded in the decades before the main shock occurred allowed an accurate forecast of its eventual rupture area applying this approach to other well monitored faults should improve earthquake hazard assessment in future

cross correlation coefficients for the study of repeating earthquakes an investigation of two empirical assumptions conventions in seismological interpretation practice for the identification and analysis of repeating earthquakes there are two empirical concepts the first is the assumption that the cross correlation coefficient of the filtered seismograms of closely spaced repeaters depends exponentially on the inter event separation distance the second is the convention that in processing regional seismograms a 0 5 5 0 hz band pass filter is used in this article using a simple layered structure model we investigated the cross correlation coefficient of the filtered synthetic seismograms of two closely located events that is a doublet we investigated the relation between the cross correlation coefficient and the inter event separation distance simulation shows that in the 0 5 5 0 hz frequency band even if for simple synthetic seismograms without considering lateral heterogeneity or scattering the exponential dependence is only a first order approximation concept to check the frequency dependence of the cross correlation coefficient we analyzed a group of seismograms of a multiplet in xiuyan liaoning northeast china recorded by the regional seismographic network of liaoning province the cross correlation coefficients were observed to be relatively stable against frequency for the 0 5 5 0 hz frequency band

the tangshan aftershock sequence aftershocks of the tangshan china earthquake sequence occurred on strike slip normal and thrust faults in an intensely faulted region the generally northeast trending aftershock zone outlines an area 150 km long and 30 km wide except for a northwest southeast trend of aftershocks which is about 60 km wide the main shock fault plane inferred from the locations of the aftershocks varies in strike from n30 e at the southern end to n50 e at the northeastern end with the change in strike occurring near the epicenter of the main shock fault plane solutions of the aftershocks indicate that strike slip faulting occurred along the tangshan fault and thrust faulting occurred on smaller transverse faults south and west of the tangshan fault although faulting during the main shock included right lateral strike slip and thrust movement perturbations to the prevailing stress field due to slip during the main shock and the configuration of existing faults encouraged normal faulting in the northeast part of the zone the largest aftershock occurred about 45 km northeast of the main shock and was associated with normal faulting at shallow depths two other large aftershocks both associated with large strike slip components occurred southwest of the main shock fault plane solutions and p wave and s wave forms of these two aftershocks indicate primarily left lateral motion along a northwest striking fault plane the aftershocks range in depth from near the surface to about 20 km fault plane solutions of 61 of the aftershocks indicate that the overall deformation consists of north south extension accommodated by both normal and strike slip faults such that some east west crustal shortening and possibly some crustal thinning took place

deep slip rates along the longmen shan fault zone estimated from repeating microearthquakes 1 we investigated deep slip rates along the longmen shan fault zone where the devastating 2008 m7 9 wenchuan earthquake occurred using repeating earthquake data recorded by two digital seismic networks near the fault using cross correlation analysis of waveform data we identified a total of 231 doublets and 224 multiplets that have highly similar waveforms most of these sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days for each multiplet we first constructed a reference seismogram at each station and used it to measure the delay time of the p and s waves of each event within the cluster we used the delay times to determine the relative distances between each event and the cluster centroid the relative distances were used in screening repeating events in each multiplet we identified a total of 12 repeating earthquake clusters in the source region of the m7 9 wenchuan earthquake most of the clusters are located at the edge of locked areas where large coseismic slips were observed during the 2008 wenchuan earthquake suggesting a close spatial relationship between microearthquakes and impending large devastating earthquakes the measured in situ deep slip rates appeared to increase with depth and varied from 3 5 to 9 6 mm yr at depth range of 4 18 km they are approximately twice as large as those inferred from surface gps and geological data our results here have significant implications to the understanding of faulting as well as risk assessment of seismic hazards

an attempt to detect temporal variations of crustal structure in the source area of the 2006 wen an earthquake in north china abstract lei et al 2008 revealed a low velocity low v anomaly in the lower crust under the source area of the 4 july 2006 wen an earthquake m 5 1 in north china in this work we tried to investigate the temporal variations of the crustal structure by using a number of p and pmp moho reflected wave arrival times recorded by 107 digital seismic stations from earthquakes that occurred separately in 2002 2003 2004 and 2005 2006 to determine p wave velocity structures in and around the source area of the wen an earthquake in different periods our results show that tomographic images inferred from the data sets in different years are all dominated by a low v anomaly in the lower crust under the wen an source area however there exist some differences in the p wave velocity image in the wen an source area inferred from the p and p pmp data sets suggesting that the pmp data have improved the tomographic images in the middle and lower crust but the results from the 2005 2006 p and p pmp data sets all show a relatively lager increase of the low v anomaly under the wen an source area in the amplitude and extent as compared with those from the 2003 and 2004 p and p pmp data sets incorporating the previous results if this low v anomaly may indicate the existence of fluids then our results suggest that the occurrence of the wen an earthquake is not only related to the long term influence of fluids that decrease the effective normal stress on the fault plane but also closely associated with the drastic increase of such influence however this study is just an experimental work and the results are still preliminary because the resolution scale of the present tomographic model is much larger than the wen an source area and our extensive tests show that different samplings of seismic rays from different data sets have affected the details of the tomographic images suggesting that the present sparse data coverage can hardly detect reliably any temporal variations of the velocity anomalies in the wen an source area in future studies it is necessary to improve the resolution of crustal tomography to the size of the rupture zone and utilize identical seismic ray paths from the same pairs of sources and receivers in order to detect any temporal variations of the crust structure in the source area of a large earthquake

evolution of fluid rock interaction in the reykjanes geothermal system iceland evidence from iceland deep drilling project core rn 17b abstract we describe the lithology and present spatially resolved geochemical analyses of samples from the hydrothermally altered iceland deep drilling project iddp drill core rn 17b the 9 3 m long rn 17b core was collected from the seawater dominated reykjanes geothermal system located on the reykjanes peninsula iceland the nature of fluids and the location of the reykjanes geothermal system make it a useful analog for seafloor hydrothermal processes although there are important differences the recovery of drill core from the reykjanes geothermal system as opposed to drill cuttings has provided the opportunity to investigate evolving geothermal conditions by utilizing in situ geochemical techniques in the context of observed paragenetic and spatial relationships of alteration minerals the rn 17b core was returned from a vertical depth of 2560 m and an in situ temperature of 345 c the primary lithologies are basaltic in composition and include hyaloclastite breccia fine grained volcanic sandstone lithic breccia and crystalline basalt primary igneous phases have been entirely pseudomorphed by calcic plagioclase magnesium hornblende chlorite titanite albitized plagioclase vein epidote and sulfides despite the extensive hydrothermal metasomatism original textures including hyaloclastite glass shards lithic clasts chilled margins and shell fragment molds are superbly preserved multi collector la icp ms strontium isotope ratio 87 sr 86 sr measurements of vein epidote from the core are consistent with seawater as the dominant recharge fluid epidote hosted fluid inclusion homogenization temperature and freezing point depression measurements suggest that the rn 17b core records cooling through the two phase boundary for seawater over time to current in situ measured temperatures electron microprobe analyses of hydrothermal hornblende and hydrothermal plagioclase confirm that while alteration is of amphibolite grade it is in disequilibrium and the extent of alteration is dependent upon protolith type and water rock ratio alteration in the rn 17b core bares many similarities to that of type ii basalts observed in mid atlantic ridge samples

lithospheric rheology and moho upheaval control the generation mechanism of the intraplate earthquakes in the north china basin abstract many devastating intraplate earthquakes such as the 1966 xingtai earthquake ms 7 2 and the 1976 tangshan earthquake ms 7 8 occurred in the north china basin ncb this study aims to investigate the generation mechanism of the large intraplate earthquakes in the ncb and the spatial distribution of earthquake activity through numerical experiments in order to simulate the interseismic stress accumulation process in the ncb we set up several 3d finite element models based on different lithospheric rheological structure and apply boundary conditions of horizontal compression we find that stress concentration with high rate in the regions where moho upheaves is responsible for the large earthquakes in the ncb during the interseismic period large stress rate is located nearly around the bottom of the brittle upper crust where stress accumulates fast to reach fault strength and active the main shocks aftershocks in the seismogenic layers could be triggered by the main shocks two factors are critical to the crustal stress accumulation process 1 the first is moho upheaval in the seismic zones 2 the second is viscosity contrast among the crustal layers our results support the lithospheric rheological structure in the ncb as following the brittle upper crust brittle ductile transition in the middle crust the ductile lower crust and the ductile lithospheric upper mantle

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

the tangshan earthquake sequence and its implications for the evolution of the north china basin the 1976 tangshan earthquake sequence that occurred on the northern margin of the north china sedimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long period seismograph networks we have studied the source process of the six largest events of this earthquake sequence by formally inverting long period p and sh wave seismograms from the world wide standardized seismographic network our analysis shows that the main shock was caused by motion on at least three fault segments each with a different orientation the initial and dominant faulting occurred on two segments of a north northeast trending right lateral strike slip fault system the last stage of faulting had a significant component of thrusting and occurred on an east northeast trending subsidiary fault at the southern end of the main fault the largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east west striking nodal planes this event occurred in a pull apart region where the north northeast trending main strike slip fault system steps to the right this region subsided by more than 1 m during the earthquake sequence to the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by and large consistent with right lateral slip on this segment the large event in 1945 m 6 3 probably occurred on this part of the fault a region of substantial subsidence up to 1 5 m associated with the earthquake sequence was also observed in the south where the main fault system takes another step to the right two of the large aftershocks occurred nearby and had strike slip mechanisms the overall faulting process of the tangshan sequence is well characterized by right lateral slip on a set of right stepping faults the mechanisms and the points of nucleation of the strongest shocks and the extent of the ruptures appear to be closely tied to preexisting fault geometries a compilation of the mechanisms of large earthquake sequences in the north china basin strongly suggests that the present deformation of the entire basin is dominated by motions on right lateral strike slip systems trending north northeast the basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large scale strike slip system the deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately selfsimilar fashion as evidenced by the deformation during the tangshan sequence this interpretation provides an explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values as well as the overall shape of the north china basin it also provides an explanation for occasional thrust faulting earthquakes that occur apparently as a result of geometrical complexities of the strike slip faults transpression these observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence

spatial clustering and repeating of seismic events observed along the 1976 tangshan fault north china 1 spatial and temporal features of the seismicity occurring along the tangshan fault in 2001 2006 were investigated with data recorded by the beijing metropolitan digital seismic network the relocated seismicity with the double difference method clearly exhibits a dextral bend in the middle of the fault more than 85 of the earthquakes were found in the two clusters forming the northern segment where relatively small coseismic slips were observed during the 1976 m7 8 earthquake the b values calculated from the seismicity occurring in the northern and southern segment are 1 03 0 02 and 0 85 0 03 respectively the distinct seismicity and b values are probably the collective effect of the fault geometry and the regional stress field that has an ene wsw oriented compression using cross correlation and fine relocation analyses we also identified a total of 21 doublets and 25 multiplets that make up 50 of the total seismicity most of the sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days based on a quasi periodic sequence we obtained a fault slip rate of 2 6 mm yr at 15 km which is consistent with surface gps measurements

spatial changes of inter plate coupling inferred from sequences of small repeating earthquakes in japan 1 we extract sequences of small repeating earthquakes to clarify inter plate coupling of subducting plates over a large area of the japanese islands as a result many sequences arc detected at the philippine sea plate subducting from the ryukyu trench and pacific plate subducting from the kuril japan trench the average slip rates and standard deviations estimated from the sequences show substantial spatial changes of inter plate coupling the large deviations of slip rates correspond to the occurrence of episodic slips such as after slips following large earthquakes constant slip rates approaching the relative plate motion indicate weak coupling areas slip deficits and sparse distributions of repeating groups suggest locked areas in the nankai trough deep low frequency earthquakes in the transition zone and burst type repeating sequences within plates have not been located in the downdip direction of groups with slow slip rates this suggests that the space time characteristics of inter plate coupling affected these seismic events

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

electrical conductivity of icelandic deep geothermal reservoirs up to supercritical conditions insight from laboratory experiments electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth up to this point technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs in this study we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus this allows us to perform resistivity measurement at temperatures up to 700 c and at effective pressures up to 100 mpa i e a confining pressure of 130 mpa and an equilibrium pore pressure of 30 mpa using cm scale plugs rock samples originate from five boreholes located in the icelandic geothermal fields of reykjanes rn 17b hyaloclastite rn 19 rn 30 dolerites and hengill nj 17 basalt and nj 17b hyaloclastite these samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole facies i e temperature of 250 c and 400 c respectively and their wide range of porosities from 3 to 20 to determine the effects of surface mineral and electrolytic conductions on bulk electrical conduction experiments were performed under dry and saturated conditions using three different fluid salinities at temperatures ranging from 25 to 350 c electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction then under supercritical conditions i e temperature from 374 c to 600 c electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction at higher temperatures 500 c 700 c the rock conductivities lie within the range of dry rock electrical conductivity values suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction amphibole rich samples show an irreversible increase in conductivity at temperature above 500 c 600 c which can be attributed to amphibole dehydration comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large scale surveys our results provide a general trend that helps interpreting electrical conductivity surveys in the icelandic crust

on finding and using repeating seismic events in and near china 1 a comprehensive search for repeating seismic events was performed using cross correlation on waveforms for 17 898 seismic events in and near china reported in the annual bulletin of chinese earthquakes from 1985 to 2005 we define a repeat as an event pair having a high cross correlation 0 8 for a particular bandwidth 0 5 to 5 hz and time window 5 s before p arrival to 40 s after lg arrival such events typically have hypocenters separated by no more than about 1 km this result enables a determination of median relative location error of around 15 km for global and regional catalogs that locate events in this study one at a time using phase picks the maximum location error is on the order of hundreds of kilometers we use nonnormalized cross correlation values to measure relative amplitudes of event pairs the standard deviation is about 0 06 magnitude units much better than the precision of relative magnitudes for doublet events which we estimate for the chinese catalog to have a standard deviation of 0 36 magnitude units two thousand three hundred and seventy nine events out of the 17 898 or 13 of the events in this catalog are classified as repeats with later years showing an increase in the percentage about half the repeats occur as isolated doublets the rest as multiplets of three or larger up to a 92 event multiplet most of the events appear to be triggered or are due to some earthquake interaction since their recurrence intervals are significantly shorter than would be estimated from tectonic loading

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

repeating events as estimator of location precision the china national seismograph network repeating earthquakes identified by waveform cross correlation with inter event separation of no more than 1 km can be used for assessment of location precision assuming that the network measured apparent inter epicenter distance x of the repeating doublets indicates the location precision we estimated the regionalized location quality of the china national seismograph network by comparing the repeating events in and around china by schaff and richards science 303 1176 1178 2004 j geophys res 116 b03309 2011 and the monthly catalogue of the china earthquake networks center the comparison shows that the average x value of the china national seismograph network is approximately 10 km the mis location is larger for the tibetan plateau west and north of xinjiang and east of inner mongolia as indicated by larger x values mis location is correlated with the completeness magnitude of the earthquake catalogue using the data from the beijing capital circle region the dependence of the mis location on the distribution of seismic stations can be further confirmed

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