local observations of the onset of a large earthquake 28 june 1992 landers california the landers earthquake mw 7 3 of 28 june 1992 had a very emergent onset the first large amplitude arrivals are delayed by about 3 sec with respect to the origin time and are preceded by smaller scale slip other large earthquakes have been observed to have similar emergent onsets but the landers event is one of the first to be well recorded on nearby stations we used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release and to determine the slip function of the 3 sec nucleation process relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2 5 sec three station array analysis of the p wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals indicating that the large amplitude source was several kilometers deeper than the initial onset an ml 2 8 foreshock located close to the hypocenter was used as an empirical green s function to correct for path and site effects from the first 3 sec of the mainshock seismogram the resultant deconvolution produced a slip function that showed two subevents preceding the main energy release an mw 4 4 followed by an mw 5 6 these subevents do not appear anomalous in comparison to simple moderate sized earthquakes suggesting that they were normal events which just triggered or grew into a much larger earthquake if small and moderate sized earthquakes commonly detonate much larger events this implies that the dynamic stresses during earthquake rupture are at least as important as long term static stresses in causing earthquakes and the prospects of reliable earthquake prediction from premonitory phenomena are not improved

a multiple fracture model of pre seismic electromagnetic phenomena abstract among three main stages of the process of earthquake nucleation long middle and short term the final stage is characterized by abrupt activation of the process of stress strain state variation the strain rates increase by orders of magnitude throughout the whole region of preparation which increases the signal to noise ratio and therefore the fracture is easier to detect through monitoring geophysical parameters the near critical stresses existing in the earth s crust and near the surface make it necessary to take into account the real rheology of the rocks which as is well known is characterized by clearly pronounced plastic properties the condition is favorable for developing creep processes in general and in its avalanche phase immediately before the fracture the process is completely nonlinear but not chaotic based on the assumption that the attenuation of the stress strain field is proportional to r 3 a possible scenario of the final stage of earthquake nucleation process is proposed on the basis of the slip weakening mechanism in the source and the associated mosaic pattern of precursors on the earth s surface the formulas for estimating the ultimate distance of precursor detection and the duration of precursor events depending of normalized epicentral distance at the final stage of inelastic deformation preceding brittle failure of rocks are proposed the data of electromagnetic precursors are interpreted in terms of a multiple fracture model the results of in situ observations of electric field anomalies in the atmosphere are analyzed in relation to seismic activity the possibility of constructing a physical model for the generation of a quasi stable field and electromagnetic emission on the basis of the deformation process prior to an earthquake is discussed numerical estimates are obtained for polarization density and life time of an ensemble of electric dipoles generated in surface layers of the crust due to opening of microcracks and other dislocation mechanisms the origin of quasi constant electric field along a deformation region on a scale of the preparation zone is advantageous for the propagation of seismic anomalous atmospheric electric fields from the earth s surface into the ionosphere the field can be regarded as a physical agent of lithosphere ionosphere connection

limits on earthquake nucleation and other pre seismic phenomena from continuous strain in the near field of the 2009 l aquila earthquake 1 two laser strainmeters that operate at 1400 m depth about 20 km ne of the epicenter of the 2009 04 06 magnitude 6 3 l aquila italy earthquake have produced records of strain since 1995 during the two years before the event no anomalous signal larger than a few tens of nanostrains is visible limiting the volume of the possible earthquake preparation zone to less than 100 km3 moreover earth tidal response is stable within 0 5 in amplitude and 0 5 in phase thus reality of large scale precursory phenomena seems unlikely during the last few days there is some evidence of dilatancy of satured rock over the earthquake causative fault maybe related to the foreshocks seconds before the event strain is stable at the 10 12 level and prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 1012 nm i e 0 00005 of the main shock seismic moment

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

continuous monitoring of seismic energy release associated with the 1994 northridge earthquake and the 1992 landers earthquake we have developed a method to detect long period precursors for large earthquakes observed in southern california if they occur the method allows us to continuously monitor seismic energy radiation over a wide frequency band to investigate slow deformation in the crust e g slow earthquakes especially before large earthquakes we used the long period records 1 sample sec from terrascope a broadband seismic network in southern california the method consists of dividing the record into a series of overlapping 30 min long windows computing the spectra over a frequency band of 0 00055 to 0 1 hz and plotting them in the form of a time frequency diagram called spectrogram this procedure is repeated daily over a day long record we have analyzed the 17 january 1994 northridge earthquake m w 6 7 and the 28 june 1992 landers earthquake m w 7 3 no slow precursor with spectral amplitude measured over a duration of 30 min larger than that of a magnitude 3 7 was detected prior to either event in other words there was no precursor whose moment was larger than 0 003 of the mainshock

co and postseismic characteristics of indian sub continent in response to the 2004 sumatra earthquake abstract the m w 9 3 sumatra earthquake of december 26 2004 caused extensive coseismic displacements globally measurements of which were made essentially using modern geodetic techniques this earthquake induced considerable perturbation in stress distribution as far as 8000 km away from the epicenteral region which is tending to relax to its normal rates as seen from postseismic transient deformation the monitoring of crustal displacements from strategically located sites using gps provides coseismic as well as postseismic deformation that facilitates the understanding of the fault geometry elastic thickness postseismic relaxation mechanisms rheology and earthquake recurrence time interval we investigated coseismic and postseismic gps derived displacements in indian region together with the gps data collected from andaman and sumatra region it is found that while ew displacements are significantly large in peninsular india those in the region to the north of central india tectonic zone citz are relatively small we could delineate the postseismic transients from position time series and interpreted them in terms of viscoelastic relaxation it is inferred that the postseismic deformation is characterized by a power law viscoelastic flow in the mantle in indian peninsula region the timescale parameter of the exponential decay 250 days would require an extremely low viscosity for the upper mantle relying on the prevailing coseismic and postseismic displacement fields the present study also reflects upon the contemporary litho tectonics of the indian sub continent

transient creep of a composite lower crust 2 a polymineralic basis for rapidly evolving postseismic deformation modes postseismic horizontal strain and displacement following the june 28 1992 landers california earthquake mw 7 3 is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace both the observed strain at pinon flat observatory pfo and observed global positioning system receiver velocities evolve rapidly after the landers big bear earthquake sequence the observed exponential decay of these motions with timescales of 4 34 days may reflect a soft creep rheology in the lower crust and brittle ductile transition zone or even within the seismogenic crust itself here a simple model of a two dimensional screw dislocation in a layered maxwell viscoelastic earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes 4 34 days are consistent with the behavior of a biviscous lower crust the lowest viscosity of this system is derivable from laboratory experimental data on the long term creep of natural quarztites and the highest viscosity is consistent with isostasy related lower crustal flow in a continental extensional tectonic environment the model predicts significant stress relaxation at the base of the seismogenic crust near the base of the seismogenic zone and about 4 km away from the mainshock the rate of predicted relaxation is of the order of 0 01 mpa d 1 during the first 20 days of postseismic flow oblate spheroidal inclusions at 5 concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3 4 1015 pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at pfo after the landers mainshock it is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are in part the cause of such rapid postseismic evolution in southeastern california unmylonitized quartz rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes

fault slip seismicity and deformation in mexicali valley baja california mexico after the m 7 1 1999 hector mine earthquake a local small deformation network in mexicali valley baja california mexico recorded fault slip seismicity and deformation produced by the 16 october 1999 m 7 1 hector mine earthquake 260 km away the network consists of two crackmeters two biaxial surface tiltmeters and one broadband seismograph all recording continuously these instruments are complemented by other seismological instruments within the valley local microearthquakes with clear p and s phases began 2 hours after arrival of the hector mine p wave and earthquakes with m 2 5 large enough to be located started about 33 hours after the earthquake local seismicity continued with decreasing frequency for about 2 weeks after the hector mine earthquake vertical and horizontal slip and tilt began on the imperial fault simultaneously with the hector mine earthquake seismic wave arrival which is clearly recorded by the tiltmeters deformation recorded by a tiltmeter on the cerro prieto geothermal field began about 30 hours after the main event and ended 13 hours later after the occurrence of the largest local earthquake m 4 1 in this episode the temporal pattern of seismicity and deformation suggests that deformation is the main process triggered by the hector mine earthquake waves in the study area and the transient local seismicity increase is a response to the triggered deformation

evidence for aseismic deformation rate changes prior to earthquakes abstractfor ten earthquakes in nonmagmatic settings there are credible published accounts of pre earthquake deformation rate changes lasting hundreds of seconds to more than a decade although most m 7 5 earthquakes have occurred without detectable pre earthquake deformation the detection threshold for aseismic deformation remains high in that aseismic slip with moment equivalent to an m5 earthquake would in most although not all cases have been missed interseismic deformation rates vary without being followed by earthquakes and a strain rate change prior to the 1989 mw6 9 loma prieta california earthquake is shown to be similar in size to many other rate changes that have occurred since that time most examples of pre earthquake aseismic deformation lasting hundreds of seconds or more probably originate adjacent to or downdip of the seismic rupture plane rather than within the zone that undergoes seismic failure

analysis of deformation data at parkfield california detection of a long term strain transient analysis of more than a decade of high quality data particularly those from the two color electronic distance meter edm in the parkfield california area reveals a significant transient in slip rate along the san andreas fault this transient consists of an increase in fault slip rate of 3 3 0 9 mm yr during 1993 0 to 1998 0 the most reliable fault creep instruments show a comparable increase in slip rate suggesting that the deformation is localized to the fault which breaks the surface there was also an increase in precipitation around 1993 it is unlikely however that this anomaly is due directly to hydrology as its spatial distribution is what would be expected for increased slip on the san andreas fault the increase in slip rate corresponds temporally to a dramatic increase in seismicity including the four largest earthquakes in the period 1984 1999 that occurred along a 6 km segment of the fault just to the north of the edm network there was also a previously reported anomaly in borehole shear strain gwyther et al 1996 that closely corresponds temporally to the transient in edm data solely on the basis of edm data the transient can be modeled as a slip event on a 10 km long segment of the fault the calculated shear strains from this model however are not consistent with the observed ones a compatible model can be found if there is increased aseismic slip to the northwest in conjunction with the four earthquakes support for this northwestern slip is provided by a recent study of slip rate based on microearthquake activity we speculate that this northwestern event served to load the fault to the southeast with the stress being partially released by the observed slip

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

middle miocene gypsum gulf of suez marine or nonmarine abstract middle miocene gypsum on the eastern coast of the gulf of suez west central sinai includes massive and bedded selenite these selenites are interpreted as primary deposits because of their textures and fabrics such as vertically oriented crystals primary fluid inclusions in gypsum are used to interpret depositional settings and parent water chemistries the final melting temperatures of ice in fluid inclusions nearly all fall between 0 0 c and 8 0 c fluid inclusions with ice melting temperatures above 1 9 c melting temperature of ice in seawater show that some of the gypsum must have formed from a relatively dilute non seawater parent water the nearly total absence of fluid inclusions with final ice melting temperatures below 7 0 c melting temperature of ic in evaporated seawater at gypsum saturation indicates that the gypsum did not form from simple evaporative concentration of seawater rather the fluid inclusion data suggest that gypsum grew from 1 recycled seawater enriched in caso4 whereby seawater dissolved surrounding gypsum or anhydrite deposit or 2 mixed seawater nonmarine waters sulfur and oxygen isotope analyses 34s values of 21 9 to 23 6 cdt and 18o values of 10 1 to 12 7 smow suggest a marine sulfate source but do not rule out any of the above parent waters given the rift tectonic setting the most likely interpretation is that the parent waters were seawater with input of marginal nonmarine groundwaters and surface waters all of which may have recycled caso4 via dissolution of gypsum or anhydrite miocene gypsum deposits formed in a subsidiary closed ba in that may have been separated from the main gulf of suez trough during periods of low sea level or active faulting the marginal closed basin may have been cut off from the gulf of suez and thus completely nonmarine the same basin would have become a marine fed lagoon if it had been connected to the gulf of suez and flooded by seawater during high sea level stands or breaching of tectonic barriers

a shear strain anomaly following the loma prieta earthquake borehole tensor strain instruments deployed along the san andreas fault for the past ten years have provided sufficient resolution and stability to sample regional tectonic processes potentially enhancing earthquake prediction capability data obtained from the instrument at san juan bautista in the near field region of the 17 october 1989 loma prieta earthquake ms 7 1 provide the first opportunity to observe shear strain processes associated with a large earthquake we previously reported1 a prominent shear strain anomaly in those data for more than a year before the earthquake this anomaly ceased immediately after the earthquake but as we report here a new and higher rate of fault parallel shear accumulation 2 microstrain per year was established about four months later and has continued to the present associated changes in creep rate are apparent at a number of sites on the surface trace of the fault within 30 km of the strain meter we propose that the observed strain accumulation results from increased slip around a nearby locked section of the fault this slip arising from loading by the failed loma prieta source region to the north this model is consistent with suggestions of an increased probability of a moderate earthquake near san juan bautista9 10 and with evidence12 that interactions between fault regions are important in earthquake processes

gps constraints on africa nubia and arabia plate motions summary we use continuously recording gps cgps and survey mode gps sgps observations to determine euler vectors for relative motion of the african nubian arabian and eurasian plates we present a well constrained eurasia nubia euler vector derived from 23 igs sites in europe and four cgps and three sgps sites on the nubian plate 0 95 4 8 n 21 8 4 3 e 0 06 0 005 myr 1 we see no significant 1m m yr 1 internal deformation of the nubian plate the gps nubian eurasian euler vector differs significantly from nuvel 1a 21 0 4 2 n 20 6 0 6 e 0 12 0 015 myr 1 implying more westward motion of africa relative to eurasia and slower convergence in the eastern mediterranean the arabia eurasia and arabia nubia gps euler vectors are less well determined based on only one cgps and three sgps sites on the arabian plate the preliminary arabia eurasia and arabia nubia euler vectors are 27 4 1 0 n 18 4 2 5 e 0 40 0 04 myr 1 and 30 5 1 0 n 25 7 2 3 e 0 37 0 04 myr 1 respectively the gps arabia nubia euler vector differs significantly from nuvel 1a 24 1 1 7 n 24 0 3 5 e 0 40 0 05 myr 1 but is statistically consistent at the 95 per cent confidence level with the revised euler vector reported by chu gordon based on a re evaluation of magnetic anomalies in the red sea 31 5 1 2 n 23 0 2 7 e 0 40 0 05 myr 1 the motion implied in the gulf of aqaba and on the dead sea fault dsf by the new gps nubia arabia euler vector i e ignoring possible sinai block motion and possible internal plate deformation grades from pure left lateral strike slip in the gulf and on the southern dsf with increasing compression on the central and northern dsf with relative motion increasing from 5 6 to 7 5 mm yr 1 1m m yr 1 from south to north along the northern dsf i e north of the lebanon restraining bend motion is partitioned between 6 1m m yr 1 left lateral motion parallel to the fault trace and 4 1m m yr 1 faultnormal compression relative motions on other plate boundaries including the anatolian and aegean microplates derived from the gps euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes m 6 where data are available on fault slip rates on plate bounding faults north anatolian fault east anatolian fault dead sea fault red sea rift they are generally lower than but not significantly different from the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures

subsurface structural trends of the offshore nile delta area egypt evidences from gravity and magnetic data subsurface structural trends and tectonics affecting the offshore nile delta area egypt have been studied through the interpretations of gravity and magnetic data reduced to the pole regional residual separation tilt derivative and euler deconvolution techniques are applied for the processing and interpretations of the magnetic and gravity data the average depth of the sedimentary cover estimated from the two dimensional power spectrum technique ranges between 8 km and 13 km the interpretation of the gravity and magnetic data indicates that the study area is affected by many subsurface structural trends the nw se is the major trend related to el temsah and misfaq bardwil trend the ne sw direction is the second dominant trend related to the rosetta trend other trends defined through the interpretation of gravity and magnetic data include the n s direction related to the baltim fault trend the e w direction related to the neogene hinge line and the nne ssw related to the gulf of aqaba accessory trends include the ene wsw wnw ese and finally the nnw ssw

earthquake geodesy and hazard monitoring catastrophic events such as major earthquakes and volcanic eruptions result when the earth s crust fails in response to accumulated deformation the accumulation of deformation results from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motions or the ascent of magma through a volcanic plumbing system geodetic measurements document the crustal deformation leading to these failures and the deformation resulting from them providing unique insight into the physical processes involved by virtue of the space and time domains of ground deformation detected with geodetic measurements geodesy is a useful tool in monitoring active tectonic and volcanic regimes for a wide range of natural events including earthquakes aseismic fault motions and volcanic events geodetic measurements constrain physical models of the processes that cause such events because these geodetic studies probe the underlying processes leading to and culminating in catastrophic natural events many such studies are directly applicable to earthquake and volcano hazard assessment

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

detection of crustal deformation from the landers earthquake sequence using continuous geodetic measurements the measurement of crustal motions in technically active regions is being performed increasingly by the satellite based global positioning system gps 1 2 which offers considerable advantages over conventional geodetic techniques3 4 continuously operating gps arrays with ground based receivers spaced tens of kilometres apart have been established in central japan5 6 and southern california to monitor the spatial and temporal details of crustal deformation here we report the first measurements for a major earthquake by a continuously operating gps network the permanent gps geodetic array pgga 7 9 in southern california the landers magnitude afw of 7 3 and big bear mw 6 2 earthquakes of 28 june 1992 were monitored by daily observations ten weeks of measurements centred on the earthquake events indicate significant coseismic motion at all pgga sites significant post seismic motion at one site for two weeks after the earthquakes and no significant preseismic motion these measurements demonstrate the potential of gps monitoring for precise detection of precursory and aftershock seismic deformation in the near and far field

widespread neogene remagnetization in jurassic limestones of the south iberian palaeomargin western betics gibraltar arc abstract palaeomagnetic investigations in upper jurassic carbonate rocks from the western subbetics betic cordillera southern spain testify to a widespread neogene remagnetization progressive thermal and alternating field demagnetization analyses reveal the presence of two stable components of the natural remanent magnetization nrm the nrm is dominated by a pervasive neogene overprint which always shows normal polarity and maximum unblocking temperatures of 450 c throughout the whole region studied the primary jurassic component however can also be isolated in many outcrops this component has low intensity values and maximum unblocking temperatures of about 550 c showing both normal and reversed polarities palaeomagnetic and rock magnetic analyses indicate that both primary jurassic and secondary neogene components are carried by magnetite the incremental fold test performed in all sampling localities demonstrates that the remagnetization occurred during some stage of the neogene deformation of the subbetics being pre syn or post folding in the various folds studied

tidal calibration of borehole strain meters removing the effects of small scale inhomogeneity we investigate the estimation of earth strain from borehole strain meter data in a study of tidal calibration of the gladwin borehole tensor strain meter btsm at pinon flat small scale geological inhomogeneity is one of several effects examined that cross couple remote areal shear strain into measured areal shear strain a methodology is developed for incorporating cross coupling into the strain meter calibration using the measured strain tides from the colocated laser strain meter lsm as a reference we show that calibration of the btsm with cross coupling removes systematic errors of up to 30 in the borehole strain meter tides this calibration accurately relates the btsm measurements to strains at the scale length of the lsm about 1 km the calibration technique provides a solution to a major criticism of all short baseline strain measurements namely that tectonic strains are not representatively sampled due to small scale inhomogeneities the technique removes errors potentially greater than 50 in observed strain offsets from fault slip permitting improved constraint of slip mechanisms we show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross coupled calibration comparison of theoretical tides with measurements from the lsm suggest that at least half of the error is in the ocean load tide estimates

comparison of fluid tiltmeter data with long period seismograms surface waves and earth s free oscillations we compare observations of long period seismic surface waves and free oscillations recorded by high resolution long base fluid tube tiltmeters and by nearby broadband seismometers after large earthquakes the quality of the tiltmeter data is comparable to that of the best horizontal component seismic data recording some of the gravest free oscillations of the earth as well as successive passages of seismic surface waves circling the globe we compare the observations with theoretical seismograms and with theoretical tilt the predicted and observed surface wave tilt waveforms are very similar provided that we take into account horizontal acceleration effects on the tiltmeter phase and amplitude anomalies between the waveforms are well explained by the theoretical transfer function of the instrument likewise observed horizontal seismograms converted into tilt match the tiltmeter data very well long base fluid tube tiltmeters could potentially contribute to obtain high quality measurements of the long period seismic spectrum

southern california permanent gps geodetic array continuous measurements of regional crustal deformation between the 1992 landers and 1994 northridge earthquakes the southern california permanent gps geodetic array pgga was established in 1990 across the pacific north america plate boundary to continuously monitor crustal deformation we describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes we compare displacement rates at four site locations with those reported by feigl et al 1993 which were derived from an independent set of global positioning system gps and very long baseline interferometry vlbi measurements collected over nearly a decade prior to the landers earthquake the velocity differences for three sites 65 100 km from the earthquake s epicenter are of order of 3 5 mm yr and are systematically coupled with the corresponding directions of coseismic displacement the fourth site 300 km from the epicenter shows no significant velocity difference these observations suggest large scale postseismic deformation with a relaxation time of at least 800 days the statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets two possible noise models fit the pgga data equally well as described in the companion paper by zhang et al this issue the pre landers data are too sparse and heterogeneous to derive a reliable noise model under a fractal white noise model for the pgga data we find that the velocity differences for all three sites are statistically different at the 99 significance level a white noise plus flicker noise model results in significance levels of only 94 43 and 88 additional investigations of the pre landers data and analysis of longer spans of pgga data could have an important effect on the significance of these results and will be addressed in future work

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

postseismic relaxation following the 1992 m7 3 landers and 1999 m7 1 hector mine earthquakes southern california 1 postseismic relaxation measured postseismic deformation less the deformation that would have occurred at the preseismic rate has been monitored at the same 10 monuments over 6 years following both the 1992 landers and the 1999 hector mine earthquakes for both earthquakes the displacement components of the observed relaxation are well described by i if1 t where i and i are constants peculiar to each component at each monument t is the time after the earthquake and f1 t is a temporal function common to all components and all monuments for that earthquake the temporal function f1 t can be approximated by bt c loge 1 t where 38 7 15 2 days and 25 6 7 7 days for the landers and hector mine relaxations respectively because the estimated values of do not differ significantly the transient term loge 1 t in the temporal function may be the same for both earthquakes the asymptotic t relaxation rates ib are only a few mm a and do not appear to be significantly different following the two earthquakes the asymptotic deformation rates appear to be slightly greater than the preseismic deformation rates but the preseismic rates were not measured directly thus the deformations of the landers array measured over the first 5 6 years following the landers earthquake and over the first 6 4 years following the hector mine earthquake are generally consistent with a simple model in which the transient postearthquake relaxation depends upon time as loge 1 t with 28 5 days and the asymptotic postseismic speeds of the monuments exceed the preseismic speeds by at most only a few millimeters per annum

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

a structural and palaeomagnetic study of a section through the eastern subbetic southern spain a well exposed 2 km section through the eastern subbetic immediately to the south of the crevillente wrench fault zone can be divided into an upper thrust sheet of triassic rocks a middle thrust sheet of cenomanian turonian basinal sediments and a lower thrust sheet of jurassic limestones unconformably overlain by upper cretaceous palaeocene sediments structural relationships suggest that the middle sheet overthrust the lower sheet and both units were subsequently folded and imbricated this sequence was then overthrust by the upper sheet incorporating a footwall duplex of cenomanian turonian sediments these structures can be related to the complex deformation associated with a single overthrusting event the dominant thrust transport direction is to the east strongly oblique to the overall trend of the orogen however a palaeomagnetic study isolates stable components which indicate no cumulative vertical axis rotation of the thrust sheets the section is cut by strike slip faults whose displacements appear to be relatively minor the triassic rocks of the upper thrust sheet form a highly disrupted thrust sheet extending over much of the subbetic rather than being local to the crevillente fault zone this section through the subbetic can be interpreted as the result of a major east directed thrust of basinal sediments of the median subbetic over marginal shelf facies of the internal subbetic strongly oblique to the general ese trend of the orogen the shortened sequence is cut by a later set of strike slip faults which are probably related to movement on the crevillente fault

the optical anchor a geophysical strainmeter an instrument has been developed to monitor the horizontal displacement of near surface monuments so as to reduce the noise of observatory based strain measurements the device measures the shear strain in the upper 24 m of the earth s crust using an equal path length michelson interferometer the magnitude of the observations 50 m indicates that such measurements are needed to interpret the records produced by precision strainmeters

nature and origin of authigenic k feldspar in precambrian basement rocks of the north american midcontinent authigenic k feldspar occurs in alteration profiles in uppermost precambrian igneous and metamorphic basement rocks of the midcontinent the k feldspar is widespread and has been identified in six states the profiles occur directly below the cambrian precambrian unconformity and range from about 1 to 8 m in thickness authigenic k feldspar occurs throughout the profile the k feldspar is monoclinic or triclinic by x ray diffraction of end member composition and may compose 63 of rock volume much of the k feldspar formed by replacement of primary feldspar a sample of wholly authigenic k feldspar from altered basement in southern illinois gives a k ar data of 549 plus minus 18 ma early cambrian this data is in agreement with early cambrian rb sr dates for potassic alteration of uppermost precambrian basement in ohio dated authigenic k feldspars from both ohio and illinois give identical delta sup 18 o values of 17 5 suggesting formation from a very similar fluid concordancy of both dates and delta sup 18 o values suggests that the k feldspar formed during an episode of potassic alteration during early cambrian time that affected much of midcontinent north america the dates and delta sup 18 o values for k feldspar when considered together do not fit any of the hypotheses presented here

displacement of surface monuments horizontal motion measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth s crust existing techniques capable of resolving the signals of interest strain rates of 10 n yr are mainly observatory based the limitation in the baselength of these instruments 1 km results in a requirement that the reference monuments be exceptionally stable 10 tzm yr however records from pition flat observatory california show that the actual horizontal displacements for massive near surface monuments eraplaced in competent weathered granite are of the order of 50 tzm yr the low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small until high accuracy geodetic techniques are developed sophisticated monuments or monument monitoring devices will be necessary to record faithfully continuous crustal deformation

measurements of coseismic deformation in southern california 1972 1982 since 1972 a number of static strain changes have been measured at pinon flat observatory southern california as a result of regional faulting as recorded by a collection of long baselength strainmeters and tiltmeters these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half space in contrast colocated short baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration calculations show that the root mean square rms of the coseismic signals from several different instruments at one site or from several sites at known hypocentral distances provide a good estimate of the seismic moment a result confirmed by the observations static moment estimates for the four largest events in this period are imperial valley 1979 m0 9 0 1018nm mexicali valley 1980 m0 6 1 1018nm westmorland 1981 m0 5 3 1017nm and brawley 1979 m0 2 3 1017 nm these estimated moments generally exceed those determined seismically by about 25 which is as expected since they include some postseismic deformation results from the mexicali valley earthquake are particularly interesting indicating an average fault strike approximately 9 different from the historic fault trace several small to moderate earthquakes ml 3 7 5 2 were also well enough recorded to estimate source parameters for the only other detectable events in the period 1972 1982 a total of three others for which the strain change erms was greater than 0 4 ne high accelerations caused most of the instruments at the site to malfunction no precursory signals were evident for any of the earthquakes recorded at this site

a medium term precursor to the loma prieta earthquake the loma prieta earthquake ms 7 1 17 october 1989 provided unique near field borehole tensor strain observations the medium term data at the san juan bautista site showed a clear anomalous change in the fault parallel shear strain rate beginning about a year before the event the anomaly ultimately reached 30 of the coseismic offset the signal shows similarities with changes in strain rates reported from the geodetic record suggesting a broad regional anomaly the limited spacial sampling available prevents determination of a causal link useful for prediction between these data and the earthquake

transient stress coupling between the 1992 landers and 1999 hector mine california earthquakes a three dimensional finite element model fem of the mojave block region in southern california is constructed to investigate transient stress coupling between the 1992 landers and 1999 hector mine earthquakes the fem simulates a poroelastic upper crust layer coupled to a viscoelastic lower crust layer which is decoupled from the upper mantle fem predictions of the transient mechanical be havior of the crust are constrained by global positioning system gps data inter ferometric synthetic aperture radar insar images fluid pressure data from water wells and the dislocation source of the 1999 hector mine earthquake two time dependent parameters hydraulic diffusivity of the upper crust and viscosity of the lower crust are calibrated to 10 2 m 2 sec 1 and 5 10 18 pasec respectively the hydraulic diffusivity is relatively insensitive to heterogeneous fault zone permeabil ity specifications and fluid flow boundary conditions along the elastic free surface at the top of the problem domain the calibrated fem is used to predict the evolution of coulomb stress during the interval separating the 1992 landers and 1999 hector mine earthquakes the predicted change in coulomb stress near the hypocenter of the hector mine earthquake increases from 0 02 to 0 05 mpa during the 7 yr interval separating the two events this increase is primarily attributed to the recovery of decreased excess fluid pressure from the 1992 landers coseismic undrained strain field coulomb stress predictions are insensitive to small variations of fault plane dip and hypocentral depth estimations of the hector mine rupture

geoelectric signals in china and the earthquake generation process we present original apparent resistivity records for stations located in the chinese provinces of hebei and sichuan along with seismicity patterns fault maps and limited hydrologic data these records are part of a 30 year database of geoelectric signals that have been recorded in china by a nationwide network of more than 100 stations set up by the state seismological bureau of the people s republic of china the database includes the time period during which the m 7 8 1976 tangshan and m 7 2 1976 songpan earthquakes occurred resistivity data recorded in and around the epicentral region of the tangshan event show coherent changes both temporal and in areal distribution years and months before and after the earthquakes a regional change in water table occurred more than 3 years prior to the tangshan earthquake the resistivity and water table signals appear to correlate in the 2 to 3 year period prior to the earthquake rainfall records from the region around tangshan extending to the cities of beijing and tianjin do not appear to account for the resistivity changes regional seismicity and active fault patterns in north china indicate that deformation occurs mainly along the borders of large tectonic blocks the source region for the geoelectric signals appears to lie within a large shear zone at the edge of a tectonic block undergoing rotation the tangshan epicentral area is thus only part of a larger zone of deformation we suggest that dilational strain associated with the opening of fractures may lower resistivity and water table for shallow crustal depths such as those monitored by the chinese geoelectric network dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension the chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes

correction to transient creep of a composite lower crust 2 a polymineralic basis for rapidly evolving postseismic deformation modes by erik r ivins

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

geodetic predictions of a strike slip fault model implications for intermediate and short term earthquake prediction mechanical models of faulting during an earthquake cycle now exist that are based on laboratory derived constitutive laws for rock friction such models allow calculation of geodetic changes as a function of depth distance from the fault and time and are useful for gaining insight on the magnitude and timing of geodetic changes that might occur prior to an earthquake we have used the tse and rice 1986 model of a crustal strike slip fault to study the sensitivity of surface deformation to changes in the values of the friction constitutive parameters and the amount of dynamic overshoot during an earthquake models were examined with values of the characteristic decay distance dc in the constitutive law ranging from 5 to 80 mm with near surface values of the steady state velocity dependence a b of 0 0015 and 0 003 and with either zero or 30 dynamic overshoot the time variations of the predicted displacement velocity strain and strain rate distributions are strongly affected by changes in dc and a b and only slightly affected by the amount of dynamic overshoot the larger values of dc and the less negative value of a b i e large ratios of dc to b a produce larger and earlier precursory signals the most useful signals for predicting the model earthquakes are strain changes near the fault because for nearly all model parameters studied the predicted strain changes are considerably larger than the detectability limits of existing borehole strain meters in the presence of earth noise detectable changes occur a few minutes to a month prior to the earthquake depending on parameter values premonitory increases in velocity also occur a pattern of larger velocities at distances a few kilometers away from the fault trace is a diagnostic indication of accelerating slip at depth that would be useful for predictive purposes unfortunately these velocity changes are detectable with creep meters and two color laser electronic distance measuring devices only for the largest ratios of dc to b a in agreement with recent measurements for several earthquakes the predicted amount of preseismic moment release is 0 5 of the earthquake moment for most simulations and the postseismic moment release is a significant fraction of the earthquake moment this two dimensional model suggests that earthquakes of magnitude 6 or greater on strike slip faults that cut the earth s surface could be predicted using existing instrumentation unless the ratio of dc to b a for natural faults is equal to or smaller than the smallest values investigated by tse and rice 1986 study of three dimensional models is needed since premonitory signals will be smaller if along strike variation is considered

palaeomagnetic results from upper miocene and pliocene rocks from the internal zone of the eastern betic cordilleras southern spain palaeomagnetic and rock magnetic studies were carried out on samples from thirteen volcanic and nine sedimentary sites of late miocene to pliocene age from the internal zone of the eastern betic cordilleras after comparing palaeomagnetic results with the expected pliocene miocene direction rotated and unrotated areas can be recognized rotations being clockwise and counter clockwise some rotations are of great magnitude unlike deformation in the external betics late miocene to present block rotations in the internal betic zone are non systematic and related to the movement of faults as local responses to the late miocene present regional stress field due to the n140 convergence of africa and the iberian peninsula

borehole tensor strain measurements in california two continuous borehole plane strain monitoring sites have been operational in california since late 1983 using borehole tensor strain monitors implanted at a depth of 150 m shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals at pinon flat observatory data indicate a constant shear strain accumulation of 0 6 microstrain per annum with the axis of maximum compression oriented 50 5 west of north this result differs significantly from regional geodetic estimates the amplitude being dominated by continued viscoelastic response of the hole variations of measured strain rate exceeding 100 nstrain yr would be significant at this instrument site at san juan bautista measurements of the angle of maximum compression 10 3 west of north agree closely with previous geodetic and hydrofracture estimates after 2 years of operation the residual shear strain rate is 1 strain yr preliminary analysis of strain steps observed at san juan bautista during the morgan hill earthquake of april 24 1984 show good agreement with calculations from seismically determined source parameters for this event

dynamics of fault creep observations along the surface traces of active faults in california and elsewhere indicate that tectonic displacement can occur as seismic slip or as aseismic fault creep fault creep occurs as secular displacement and as displacement episodes called creep events which last a few hours to days and include from a millimeter or less to a few tens of millimeters of displacement instrumental measurements of displacement versus time during creep events show that many events have a characteristic simple form including a steep beginning followed by a gradual decay in the rate of displacement some creep events display small amounts of precursory slip and many seem to be composed of several discrete simple events propagation of creep events along the san andreas calaveras and imperial faults in california has been inferred from the nearly simultaneous observation of creep events at nearby creepmeters and from kinematic modeling of continuous measurements of strain during creep events fault creep can be viewed as arising from the interplay of three factors stress applied to the fault from external sources stress caused by the geometry and distribution of displacement on the fault arising from the elastic response of the surrounding medium to the displacements within the anelastic fault zone itself and the constitutive relations characterizing the resistance to slip on the fault analytic solutions are presented for a simple rectangular dislocation quasi static model of nonpropagating fault creep assuming viscous and quasi plastic power law creep fault zone rheology and for a multi element composed of several rectangular or striplike dislocation surfaces quasi static model assuming a viscous rheology viscous rheology implies solutions composed of exponentials that decay with time the simple one element quasi plastic model is in agreement with the form of creep events observed at melendy ranch california a matrix formulation is presented to calculate dislocation stresses on a fault zone in a half space or plate the formulation involves the division of the dislocation surface into strips or rectangles and the use of published solutions to calculate the stress resulting from displacement on each individual element results from the matrix method agree with analytic results for equilibrium displacements on stress free cracks the matrix formulation is then used as the basis for a numerical method to simulate one and two dimensional propagating creep assuming a quasi plastic rheology on the fault zone results from this method agree reasonably well with observations of afterslip from the 1966 parkfield 1975 oroville and 1979 imperial valley california earthquakes and of a propagating creep event observed along the southern calaveras fault central california during july 1977 this method of analysis and the assumption that creep at each point on the fault zone follows a power law rheology after the applied stress reaches a spatially varying yield stress can explain the secular and episodic nature of fault creep afterslip the simple shape of some individual events and the multiple composite shape of others and the propagation of creep events on the southern calaveras fault observations of propagating creep indicate a yield stress near zero at the surface and throughout much of the fault zone however they also indicate the presence of a zone at a depth of about 0 5 km and about 5 km long with a yield stress of about 15 bars 1 5 mpa the methods of solution presented here should be applicable to many problems in the dynamics of faulting

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

strainmeters and tiltmeters despite steady effort over the last century continuously recording tiltmeters and strainmeters have not yet been successful except for earth tide measurements this article reviews the techniques and instrument designs that have been developed in this field the continuous measurement of true tectonic motions which have rates of 10 14 s 1 requires extremely high instrument stability but more rapid motions tides and seismic waves apart are even smaller and not much more easily detectable much past effort has been spent improving the transducers used in these instruments which usually measure small displacements the commonest choices are optical the most accurate capacitive the most sensitive and inductive the most rugged nowadays building a transducer is the easiest part a much harder task equally important to a good design is attaching the instrument to the ground so that it can detect the signal wanted this part of the design includes choosing a location a large underground opening is thermally stable but distorting a surface installation is easy to build but noisy because of soil weathering and a borehole is inaccessible to weather but also to the operator all such installations and indeed the whole field of strain and tilt measurement of course rely on the assumption that the deformations of interest can be measured over baselengths of a few hundred meters or less this is true for tides and seismic waves but unproven for tectonic motions appropriate attachment techniques differ for each location and many mundane details such as cementing techniques for borehole instruments remain to be worked out the goal of measuring tectonic and tidal tilt has created many tiltmeter designs most are relatively small instruments that use some form of pendulum or bubble level a few use a liquid surface to measure tilts over a long baseline repeated experience has shown the latter type when well built to be superior to the former proving end monument motions to be a dominant source of noise the same appears to hold true for strainmeters the best results coming from long base laser instruments though strainmeters using rigid or flexible material length standards are useful for special purposes and borehole instruments show promise because achieving a good design has been so difficult it is important to select the goals an instrument must meet and be prepared to compromise on others the difficulty of validating results from this type of instrument makes comparison tests essential such tests as have been made so far show that good results cannot be gotten at low cost or without careful attention to details

remagnetization in southeast china and the collision and suturing of the huanan and yangtze blocks abstract nearly all paleomagnetic studies in south china have encountered problems with remagnetization the region underwent widespread tectonic deformation during the mesozoic and the warm humid climate causes rapid weathering of surficial sediments paleomagnetic results from three carboniferous formations from three provinces in southeast china reveal two possible overprinting ages one is probably a post folding cretaceous remagnetization and the other appears to be a jurassic overprint which may be associated with the collision and suturing of the yangtze and the huanan or north china blocks the jurassic overprint is found in sediments with both magnetite and hematite as primary magnetic remanence carriers the remagnetization pre dates deformation in at least one of the formations further constraining the overprint to the jurassic or late triassic

new constraints on the bending of the gibraltar arc from palaeomagnetism of the ronda peridotites betic cordilleras spain

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

palaeomagnetic study of the ronda peridotites betic cordillera southern spain abstract a palaeomagnetic study of the ronda peridotites southern spain has been carried out on 301 samples from 20 sites spread along the three main outcrops of the ultrabasic complex ronda ojen and carratraca massifs different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed rock magnetic experiments have been carried out on a representative set of samples these measurements include curie curves hysteresis cycles isothermal remanent magnetization irm acquisition curves thermal demagnetization of irm imparted along three orthogonal axes and magnetic bulk susceptibility results indicate that magnetite is the main magnetic mineral present in the samples stepwise thermal and alternating field af demagnetization of the natural remanent magnetization nrm reveals the presence of a characteristic remanent magnetization chrm carried by magnetite and in some sepentinized samples a northward component with variable unblocking temperatures up to 250 575 c the appearance and the relative intensity of this northward component are strongly related to serpentinization degree taking into account the geological history of the peridotites the chrm has been considered as a thermo chemical remanent magnetization acquired during the first serpentinization phase associated to the post metamorphic cooling of this unit on the basis of radiometric and fission track analysis the chrm is proposed to have been acquired between 20 and 17 18 ma the inclination of the mean direction of the chrm statistically coincides with the expected inclination for stable iberia during the oligocene miocene the declination of the chrm differs from the expected declination indicating clockwise block rotations of 41 12 about vertical axes since the cooling of the peridotites when applying a compositional layering correction the chrm directions fail to pass this kind of fold test thus the compositional layering was not a palaeohorizontal during chrm acquisition time normal and reversed polarities of the chrm are reported showing that at least one reversal of the earth s magnetic field took place during chrm acquisition process a tentative polarity zonation within the peridotitic outcrops is also suggested no evidence is found from these data for the previously proposed simultaneity between post metamorphic cooling and rotation of the peridotites

on the mechanics of earthquake afterslip we propose a model for earthquake afterslip based on rate and state variable friction laws in the model afterslip is attributed to the interaction of a velocity weakening region at depth within which earthquakes nucleate with an upper region of velocity strengthening frictional behavior the existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments in our model afterslip is the result of relaxation of a stress perturbation within the velocity strengthening region which arises when an earthquake propagates into that region from below we derive the stress perturbation and its decay from the friction constitutive law using a simple 1 degree of freedom approximation for the elastic interaction between the fault and its surroundings this approximation is based on thickness averaged displacements and slip velocities within the velocity strengthening region which is assumed to slip as a rigid block coseismic and postseismic slip are coupled through the thickness averaged stiffness k of the velocity strengthening region we assume k to be inversely proportional to the thickness of this region which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip we model the afterslip time histories of the 1966 parkfield and 1987 superstition hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults in accord with field observations our model predicts 1 that afterslip on some faults scales with the thickness of the unconsolidated sedimentary cover and 2 that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth velocity strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones about 500 m within their shallow regions 3 5 km combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip

multiple precursors to the landers earthquake we present evidence for several temporally related premonitory movements preceding the landers earthquake of 28 june 1992 premonitory movements were measured at alignment arrays spanning the northeast painted canyon and san andreas faults in mecca hills california a transitory fault normal extension of 30 3 mm to eight targets was measured twice on 8 and 9 march 1992 and 7 and 8 june 1992 across the northeast and painted canyon faults at the hidden spring wash site hsws four kilometers northwest of the hsws simultaneous fault normal extensions of 24 21 and 6 6 mm were measured across a previously locked 12 km segment of the san andreas fault on 6 june 1992 22 days prior to the landers earthquake a linear regression fit of horizontal displacement at hsws showed a distinct increase in horizontal angles for those targets spanning active faults on the same dates as the fault normal extension for the first time other than a few small events on the north shore creepmeter dextral slip was measured on a nontranspressive parallel slip segment of the san andreas on 6 june 1992 we measured offsets of 14 8 and 3 arc sec 3 sec at the indian stone ring site isrs on the san andreas fault which translates into movement of 20 18 and 9 mm triggered slip was not observed on the main trace of the san andreas fault after the joshua tree earthquake but 17 mm of dextral slip possibly coseismic with the landers earthquake was measured along cracks on 5 july 1992 this amount of slip supports the magnitude of premonitory movement taken together substantial simultaneous fault normal extension and lateral slip at three alignment arrays across three faults followed by a large triggered slip event provide strong evidence for the occurrence of precursors to the landers earthquake

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

continuous measurements of crustal deformation for the 1992 landers earthquake sequence abstract we describe and attempt to interpret continuous measurements of strains and tilts made at pinon flat observatory pfo before during and after the landers and joshua tree earthquake sequences these data show substantial transient deformation following the landers mainshock with a total amplitude of several percent of the co seismic deformation and a decay time of at least several days comparing data from the many types of instruments at pfo allows us to infer possible sources for this deformation the immediate postseismic transient was nearly the same size on three long base strainmeters suggesting either broadscale deformation or local motion near one part of the observatory the latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source possible mechanisms for broad scale deformation include postseismic fault slip time dependent creep in near surface rocks and elastic or thermal responses to water table changes the first two agree best with the observations from pfo but if postseismic fault slip is the source it must have been distributed differently than the co seismic slip and may have included faults other than those that ruptured seismically if one of the other mechanisms is the main source the pfo data imply that the postseismic slip must have been very much smaller than the seismic slip perhaps 2 or less no significant preseismic deformation was observed at a level of 2 10 3 of the co seismic deformation for the days to minutes before the earthquake

measurements and interpretations of tilt strain gauges in seismically active areas abstract an overview of different aspects of deformation measurements is given an introduction to the general theory of point crustal deformations with an application to the stress strain relations at the earth surface is made the instrumentation used in crustal deformation measurements is briefly explained emphasizing different constructions of tilt and strainmeters the problems connected with the housing of the instruments i e the cavity and topography effects and with the ambient noise factors as the hydrological and pressure effects are discussed the last part of the paper is concerned with the relation between deformation measurements and seismic events both from a theoretical and observational standpoint a number of examples are shown that report on pre co and post seismic and the secular term deformation in different areas of the world

earthquake prediction a critical review summary earthquake prediction research has been conducted for over 100 years with no obvious successes claims of breakthroughs have failed to withstand scrutiny extensive searches have failed to find reliable precursors theoretical work suggests that faulting is a non linear process which is highly sensitive to unmeasurably fine details of the state of the earth in a large volume not just in the immediate vicinity of the hypocentre any small earthquake thus has some probability of cascading into a large event reliable issuing of alarms of imminent large earthquakes appears to be effectively impossible key word earthquake prediction

time dependent deformation of the eastern flank of mt etna after slip or viscoelastic relaxation abstract major results of post intrusive gps monitoring of the 5 months encompassing and following the onset of 2002 03 mt etna eruption are presented and discussed here the overall ground deformation pattern is characterized by a time dependent relaxation function we evaluated two different parametrizations of relaxation functions each one linked to a different relaxation process i viscoelastic relaxation and ii after slip mechanism the former indicated that the process occurred within a weak layer characterized by viscosity values ranging between 7 1 1014 pa s and 1 3 1015 pa s the latter evidenced that frictional processes occurred beneath a layer of velocity strengthening having a thickness with values ranging between 0 4 km and 1 7 km either model may explain the observed time dependent deformation both from the statistical point of view and the comparison with geologic and seismic information although we are unable to favour one model rather than the other these results indicate that the mechanism responsible for the observed post intrusive deformation occurs within a sliding zone located inside the clayey sedimentary basement

origin of the western subbetic arc south spain palaeomagnetic and structural evidence abstract an integrated structural and palaeomagnetic study in late cretaceous eocene rocks of the western subbetic arc in the betic cordillera of southern spain shows that the change in fold trend around the arc is significantly greater than the variation in primary magnetic declination this suggests that the arcuate geometry is not primarily a consequence of differential rotation the area has experienced a regional clockwise rotation of at least 40 which was most probably imposed during oblique wnw directed convergence along the southern iberian margin in early miocene time before the arcuate structure was formed kinematic indicators associated with the arcuate folds and thrust traces arc for the most part consistently wnw directed suggesting that the arc is not primarily a result of variations in the direction of tectonic transport the arcuate structure was probably initiated without significant rotation by the wnw directed indentation of the curved western end of the alboran domain into the iberian margin and was then tightened involving up to about 35 of further rotation in different segments of the arc finally one sector in the arc was rotated anticlockwise by about 40 with respect to the rest of the region reorientating the magnetic declinations the fold trends and the kinematic indicators this rotation was associated with local sinistral shear

preliminary palaeomagnetic results on oligocene early miocene mafic dykes from southern spain abstract a preliminary palaeomagnetic study has been carried out on oligocene early miocene mafic dykes intruding the malaguide and alpujarride complexes of the internal betic zone southern spain the aims of the study were to ascertain if relative perpendicular orientations ne sw and nw se shown by different dyke families are of primary or secondary origin and to find out if palaeomagnetic rotations can be detected in that area ten sites with dykes showing both orientations were sampled in addition to palaeomagnetic analysis susceptibility versus temperature curves of several representative samples were measured and ore microscopic studies were carried out palaeomagnetic behaviour of samples was characterised by low intensities of magnetisation and alterations occurring during the demagnetisation procedure and often also by overlapping of palaeomagnetic components besides an initial viscous component two different palaeomagnetic components named m and h were recognised component m is carried by pyrrothite and magnetite and seems to record a primary palaeomagnetic direction component h is difficult to isolate and seems to be related to the presence of magnetite and hematite nine of ten studied sites offer consistent palaeomagnetic results although in two cases statistical parameters are relatively weak strong clockwise rotations approximately 90 140 are observed these rotations are in accordance with other block rotations observed in the internal and external zones of the central and western betics although their magnitudes are much stronger on the other hand both dyke families show similar rotations independently of their original direction thus their relative perpendicular orientations seem to be of primary intrusive origin

design parameters for borehole strain instrumentation the response of a borehole strain meter to hydrostatic and shear deformations in an isotropic medium is calculated to facilitate optimum instrument design and produce instrument response factors for parameters typically encountered in installed instruments results for an empty borehole are first compared with results for an instrument in intimate contact with the surrounding rock the effects of the grout used to install the instrument are then examined where possible analytic forms for the response factors are given results for typical installations are then presented in graphical form for optimizing instrument design in an environment of known elastic parameters alternatively the results may be applied in the measurement of unknown strain signals to correct for instrument response or to provide in situ estimates of the elastic properties of the environment by examination of observed strain response to known strain signals

continuous gps observations of postseismic deformation following the 16 october 1999 hector mine california earthquake mw 7 1 rapid field deployment of a new type of continuously operating global positioning system gps network and data from southern california integrated gps network scign stations that had recently begun operating in the area allow unique observations of the postseismic deformation associated with the 1999 hector mine earthquake innovative solutions in fieldcraft devised for the 11 new gps stations provide high quality observations with 1 year time histories on stable monuments at remote sites we report on our results from processing the postseismic gps data available from these sites as well as 8 other scign stations within 80 km of the event a total of 19 sites from these data we analyze the temporal character and spatial pattern of the postseismic transients data from some sites display statistically significant time variation in their velocities although this is less certain the spatial pattern of change in the postseismic velocity field also appears to have changed the pattern now is similar to the pre landers pre 1992 secular field but laterally shifted and locally at twice the rate we speculate that a 30 km 50 km portion of crust near twentynine palms which was moving at nearly the north american plate rate to within 3 5 mm yr of that rate prior to the 1992 landers sequence now is moving along with the crust to the west of it as though it has been entrained in flow along with the pacific plate as a result of the landers and hector mine earthquake sequence the inboard axis of right lateral shear deformation at lower crustal to upper mantle depth may have jumped 30 km farther into the continental crust at this fault junction that comprises the southern end of the eastern california shear zone

southern california permanent gps geodetic array spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 landers earthquake the june 28 1992 m w 7 3 landers california earthquake was the first earthquake to be surveyed by a continuously operating global positioning system gps array the coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake we employ a new spatial filtering technique that removes common mode errors from the coordinate time series this approach provides precise estimates of site specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations all sites indicate significant coseismic horizontal displacements of 5 65 mm with uncertainties of 1 2 mm horizontal displacements are in general agreement with elastic dislocation models in particular for sites closer to the epicenter vertical displacements range from 13 to 7 mm with uncertainties of 2 4 mm the observed vertical displacements in all cases show 5 10 mm more subsidence than expected from geodetic and seismic geologic models significant postseismic horizontal displacements totaling 6 2 mm 10 20 of the coseismic displacement are detected at the three sites closest to the epicenter these displacements are modeled as a short term exponential relaxation with a decay time of 22 10 days superimposed on a longer term linear interseismic trend scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain which agrees well with the direction and magnitude determined from more precise laser strain meter data the time series do not show any detectable preseismic displacements

testing a model of earthquake nucleation abstract some laboratory models of slip find that a critical amount or velocity of slow slip is required over a nucleation patch before dynamic failure begins typically such patch sizes when extrapolated to earthquakes have been thought to be very small and the precursory slip undetectable ohnaka 1992 1993 has proposed a model in which foreshocks delineate a growing zone of quasi static slip that nucleates the dynamic rupture and suggests that it could be large enough 10 km across to be detectable and thus useful for short term earthquake prediction the 1992 landers earthquake m 7 3 had a distinctive foreshock sequence and initiated only 70 km from the strain meters at the pinon flat observatory pfo we use this earthquake to investigate the validity and usefulness of ohnaka9s model the accurate relocations of dodge et al 1995 show that the foreshock zone can be interpreted as expanding from an area of 800 m along strike by 900 m in depth to 2000 by 3200 m in the 6 5 hr before the mainshock we have calculated the deformation signals expected both at pfo and 20 km from the foreshock zone assuming either constant slip or constant stress drop on a circular patch expanding at 5 cm sec over 6 5 hr we find the slips or stress drops would have to have been implausibly high meters or kilobars to have been detectable on the strain meters at pfo slightly better limits are possible only 20 km from the source even though the distance from landers to pfo is small compared with the average spacing of strain meters in california we are unable to prove or disprove ohnaka9s model of earthquake nucleation this suggests that even if the model is valid it will not be useful for short term prediction

investigation of nearfield postseismic slip following the mw 7 3 landers earthquake sequence of 28 june 1992 california repeated precise surveys of six nearfield trilateration arrays reveal that minor horizontal displacements occurred across the several fault ruptures in the five months following the landers earthquake sequence the displacements range from 0 2 mm on the camp rock and emerson faults to 3 2 mm on the landers fault to 9plusmn 2 mm on the johnson valley fault to 40 5 mm on the eureka peak fault consistent in magnitude and duration with gps and creepmeter measurements by other investigators the relative lack of afterslip at all sites except across the eureka peak fault is probably due to the lack of a thick alluvial cover concomitant precise resurveys of level lines across the san andreas fault in coachella valley and of a trilateration array at durmid hill south easternmost san andreas fault failed to reveal vertical or horizontal displacements greater than the allowable uncertainty 1 mm for leveling 2 mm 2 ppm for trilateration therefore the landers earthquake sequence did not perturb the san andreas in any way that was evident from our measurements of near surface strain

transient creep of a composite lower crust 1 constitutive theory a composite model is proposed to describe the time dependent response of the earth s lower crust the motivation for such a model is twofold first new observations of widespread postseismic deformation indicate that the deep continental crust responds viscoelastically having both long and short term decay times second by any number of observationally based rationales the lower crust is compositionally and structurally heterogeneous over many length scales for heterogeneities that have much smaller characteristic lengths than the minimum deformation wavelength of interest the aggregate rheology can be described by composite media theory for wavelengths of the order of the thickness of the lower crust 25 40 km and larger composite theory may be applied to heterogeneities that are smaller than about several hundred meters or equivalent to the vertical extent of a thick lower crustal mylonitic shear zone the composite media theory developed here is constructed using both eshelby mori tanaka theory for aligned generalized spheroidal inclusions and a generalized self consistent method the inclusions and matrix are considered to be maxwellian viscoelastic a rheology that is consistent with past homogeneous models of postseismic stress relaxation the composite theory presented here introduces a transient response to a suddenly imposed stress field which does not appear in homogeneous maxwell models analytic expressions for the amplitude and duration of the transient and for the effective long and short term viscosities of the composite are given which describe the sensitivity to inclusion concentration to shape and to ratio of inclusion to matrix viscosity r

occurrence patterns of foreshocks to large earthquakes in the western united states observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake unfortunately the apparent randomness of foreshock occurrence they precede some events and not others has severely hampered their use in reliable earthquake prediction understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short term prediction will ever be possible1 here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western united states the incidence of foreshocks decreases with increasing depth of the mainshock and also depends on the mainshock slip orientation this pattern of occurrence may be explained by a decrease in small scale crustal heterogeneity with increasing depth and suggests that increasing normal stress both regional tectonic stress and lithostatic load inhibits the occurrence of foreshocks no relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process or that foreshocks are not part of this process

water table effects on the measurement of earth strain abstract forced changes in the water head within a granite penetrating borehole were found to induce anomalously large free surface strains and tilts in the vicinity of the hole this deformation is shown to be due to fluid pressure induced changes in the aperture of a compliant hydraulically conductive fracture at a depth of 100 m and a quantitative analysis of the deformation data is performed to recover fracture characteristics the importance of the effect for secular earth strain measurements is discussed in the light of the ubiquitous nature of both water table fluctuations and fractures in the shallow crust

postseismic rebound in fault step overs caused by pore fluid flow near field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation synthetic aperture radar sar interferometry revealed several centimeters of postseismic uplift in pull apart structures and subsidence in a compressive jog along the landers california 1992 earthquake surface rupture with a relaxation time of 270 45 days such a postseismic rebound may be explained by the transition of the poisson s ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium

magnetotelluric imaging of the creeping segment of the san andreas fault near hollister 1 fluids may play a significant role in the earthquake rupture process since fluids can dramatically change the electrical resistivity of crustal rocks electromagnetic exploration methods can image in situ fault zone fluids magnetotelluric mt data has been collected on segments of the san andreas fault saf in central california exhibiting a range of seismic behavior from locked to creeping this paper describes mt data from the creeping segment of the saf near hollister california inversion models image a zone of enhanced conductivity beneath the surface trace of the saf that has similar geometry and greater depth extent than the fault zone conductor at parkfield comparison with geologic and seismic tomographic studies show this zone is bounded by the san andreas and calaveras faults and coincident with a zone of low seismic p wave velocities combined with previous mt studies this indicates a correlation between seismicity and the presence of fault zone fluids

near surface velocities and attenuation at two boreholes near anza california from logging data to investigate near surface site effects in granite rock we drilled 300 m deep boreholes at two sites which are collocated with stations from the digital array at anza california the first borehole was sited at station knw keenwild fire station which is located along a ridge line about 8 7 km east of the san jacinto fault zone station pfo pinon flat observatory chosen for the second site is another 6 km further to the east of station knw and is located on a gently sloping hillside we logged each borehole for p and s wave velocities as well as for crack density and orientation p waves were generated by striking a plate with a hammer at the surface a tool consisting of weighted anvils driven by compressed air against end plates along a 3 5 m beam was used to generate shear waves signals were recorded downhole with a three component sensor package at 2 5 m intervals from the surface to 50 m depth and at 5 m intervals from 50 m depth to the bottom of the hole velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth travel times were computed for the first breaks at shallow depths however below about 100 m depth times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable the knw site yielded a shear velocity of 1 9 km sec by only 30 m in depth and reached close to 2 6 km sec at the bottom of the hole p wave velocities at knw were also high at 5 4 km sec starting at 120 m depth the pfo site had similar but slightly higher shear wave velocities the bottom hole shear wave velocity reached 3 0 km sec and the p wave velocity was 5 4 km sec shear wave attenuation was computed using both the pulse rise time and spectral ratio methods at station knw attenuation was significant only in an interval between 17 5 and approximately 40 m in depth over the top 50 m attenuation corresponding to a q of about 8 was obtained a total t of 0 004 sec was measured for this interval pulse rise times also increased rapidly in this zone the spectral ratio data for station pfo yields two peaks in attenuation above 50 m similar to the attenuation found for station knw the peak in attenuation corresponds to a q of about 11 averaged over the top 50 m spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 hz significant motion perpendicular to the polarizations of the first shear wave arrival was recorded within a few meters of the surface apparently the rock structure is sufficiently complicated that body waves are being converted sh to sv at oblique incidence very close to the surface the presence of these elliptical particle motions within a mere few m of the pure shear wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not by itself a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth s crust crack densities and orientations were determined from televiewer records these records showed cracks with a preferred direction at station knw and of a greater density than at station pfo at station pfo crack densities were smaller and more diffuse in orientation

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

the 1987 superstition hills earthquake sequence strains and tilts at pinon flat observatory abstract we describe observations at pinon flat observatory of preseismic co seismic and postseismic strain and tilt changes associated with the superstition hills earthquake sequence of 24 november 1987 the data come from two long base strainmeters one long base tiltmeter and a borehole dilatometer co seismic offsets were seen for the two largest earthquakes these offsets roughly agree with those from a dislocation in a half space though with disagreements at the 30 per cent level suggesting some error in one or more of the co seismic observations for the main shock the moment deduced from the offsets is 5 to 6 10 25 dyne cm less than that found from the seismic data no precursive strain changes were seen for the 8 hr before the main shock any strain changes were less than the instrument noise level 1 per cent of the co seismic strain during the last 1000 sec before this earthquake any precursory strains must be less than 0 5 per cent of the co seismic amount postseismic strain during the 2 hr after the main shock was about 10 per cent of the co seismic strain

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

continuous borehole strain and pore pressure in the near field of the 28 september 2004 m 6 0 parkfield california earthquake implications for nucleation fault response earthquake prediction and tremor near field observations of high precision borehole strain and pore pres sure show no indication of coherent accelerating strain or pore pressure during the weeks to seconds before the 28 september 2004 m 6 0 parkfield earthquake minor changes in strain rate did occur at a few sites during the last 24 hr before the earth quake but these changes are neither significant nor have the form expected for strain during slip coalescence initiating fault failure seconds before the event strain is stable at the 10 11 level final prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 10 12 nm m 2 2 and a source size less than 30 m ground displacement data indicate similar constraints localized rupture nucleation and runaway precludes useful prediction of damaging earthquakes coseismic dynamic strains of about 10 microstrain peak to peak were superimposed on volumetric strain offsets of about 0 5 microstrain to the northwest of the epicenter and about 0 2 microstrain to the southeast of the epicenter consistent with right lateral slip observed strain and global positioning system gps offsets can be simply fit with 20 cm of slip between 4 and 10 km on a 20 km segment of the fault north of gold hill m 0 7 10 17 n m variable slip inversion models using gps data and seismic data indicate similar moments observed postseismic strain is 60 to 300 of the coseismic strain indicating incomplete release of accumulated strain no measurable change in fault zone compliance preceding or following the earthquake is indicated by stable earth tidal response no indications of strain change accompany nonvolcanic tremor events reported prior to and following the earthquake

mantle lithosphere bodies in the alboran crustal domain ronda peridotites betic rif orogenic belt abstract geological and geophysical data are used to demonstrate the existence of intracrustal high density high p wave velocity bodies in the western betics these bodies appear to correspond to buried peridotites similar to those that outcrop in the ronda area a gravity study shows how the gravity field is mainly the result of a combined effect of crustal thinning and the presence of ultramafic bodies the size of the buried high density body as interpreted from gravity and seismic results shows maximum dimensions of about 40 km in length nnw sse about 8 km in thickness and a lateral extension ene wsw of about 70 km the thinning of the crust from 32 35 km to 20 22 km takes place in a narrow area less than 35 km wide our results are compatible with an interpretation in terms of an unrooted peridotite slab dismembering of an initial slab of ultramafic rocks is a possible consequence of the extensional regime that originated the alboran basin

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

postseismic deformation following the 1906 san francisco earthquake we have reevaluated triangulation data from northern california following the 1906 san francisco earthquake thereby increasing the temporal and spatial resolution of postseismic deformation following that event we have calculated uniform shear strain rates and average station velocities at point arena using data from 1906 1907 1929 1930 and 1973 1975 and for the point reyes petaluma arc using data from 1929 1930 1938 1939 and 1960 1961 with the addition of recent geodetic data we infer an effective relaxation time for long term postseismic deformation following the 1906 earthquake of 36 16 years the point arena data are satisfactorily fit with accelerated afterslip at depth along the san andreas fault plane for a 10 km deep 25 km wide afterslip zone the average slip rate between 1906 and 1930 is 11 2 cm yr between 1929 and 1975 it is 4 6 cm yr deformation in the point reyes petaluma arc is clearly asymmetric with respect to the trace of the san andreas fault especially between 1929 and 1939 after inverting for the range of acceptable accelerated deep afterslip and horizontal detachment models a detailed analysis using geologically reasonable geometries revealed that both model types have trouble explaining the spatial variations in the deformation field through time in particular accelerated deep afterslip models cannot reproduce the breadth of the observed deformation field to the northeast of the san andreas fault as a result more complex time varying deformation mechanisms are required to explain the point reyes petaluma arc observations

continuous borehole strain in the san andreas fault zone before during and after the 28 june 1992 mw 7 3 landers california earthquake abstract high precision strain was observed with a borehole dilational strainmeter in the devil9s punchbowl during the 11 58 ut 28 june 1992 m w 7 3 landers earthquake and the large big bear aftershock m w 6 3 the strainmeter is installed at a depth of 176 m in the fault zone approximately midway between the surface traces of the san andreas and punchbowl faults and is about 100 km from the 85 km long landers rupture we have questioned whether unusual amplified strains indicating precursive slip or high fault compliance occurred on the faults ruptured by the landers earthquake or in the san andreas fault zone before and during the earthquake whether static offsets for both the landers and big bear earthquakes agree with expectation from geodetic and seismologic models of the ruptures and with observations from a nearby two color geodimeter network and whether postseismic behavior indicated continued slip on the landers rupture or local triggered slip on the san andreas we show that the strain observed during the earthquake at this instrument shows no apparent amplification effects there are no indications of precursive strain in these strain data due to either local slip on the san andreas or precursive slip on the eventual landers rupture the observations are generally consistent with models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake finally there are some indications of minor postseismic behavior particularly during the month following the earthquake

delineation the active subsurface structures at fayoum cairo district egypt using magnetic and magnetotelluric tools in this work we present a reconnaissance study to elucidate and delineate subsurface fault structures for an active tectonics area that lies between cairo and el fayoum provinces and consider major sources of seismicity in egypt well logging aeromagnetic land magnetic and magnetotelluric data have been used the well logging data were used for several drilled wells along w e direction the magnetic data were analyzed using trend analysis 3d magnetic modeling and werner deconvolution techniques the magnetotelluric data were interpreted using 2d tm te modeling techniques the results show that there are eight major fault structures having e w n s and nw se directions these faults extend downward for about 20 km at the dahshour and qatrani areas the epicenter sources of the earthquakes are clustering around the intersections of these structures the kattaniya horst structure has been interpreted as a regional structure that exceeds the limits previously determined by geologists the depth to this horst reaches about 1 8 km at the nw and more than 4 3 km at the southern parts the interpreted values of magnetic susceptibility at the horst zone indicate that they are ultrabasic basic intrusion bodies

extremal bounds on earthquake movement from geodetic data application to the landers earthquake we present a technique to place quantifiable bounds on the moment of an earthquake from geodetic data assuming known fault geometry application of this technique to the 1992 landers earthquake shows that the moment must have been between 0 84 and 1 15 10 20 nm with 90 confidence m 7 25 to 7 34 we also find that to satisfy the data to this same level of confidence the slip on the fault must have exceeded 7 m in at least one location in good agreement with field mapping of the surface rupture

postseismic deformation following the landers earthquake california 28 june 1992 accelerated strain followed the landers and big bear earthquakes returning to the normal rate only after a period of several months we observed this strain throughout most of southern california using the global positioning system gps three gps receivers operating continuously in fixed positions at pinyon flat jet propulsion laboratory pasadena and goldstone all recorded postseismic deformation in a relative sense in addition we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault we modeled the displacements using dislocation theory as due to variable slip on the faults that were displaced at the times of the earthquakes the model suggests that the postseismic strain released the equivalent of about 15 of the seismic moment of the mainshock while the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks the major contribution of strain release comes from the lower layer below 10 km depth significant afterslip or viscous relaxation must have occurred below 10 km depth to explain the observed deformation more than 100 km from the fault one interpretation is that high stress on the margin of the co seismic rupture zone drives the rupture to extend itself into urbroken rock below and along the initial rupture zone