empirical observations regarding reverse earthquakes blind thrust faults and quaternary deformation are blind thrust faults truly blind active thrust faults pose a significant seismic hazard worldwide many of these faults include blind thrusts where the propagating fault tip does not reach the earth s surface and buried faults where the geomorphic expression of the fault is obscured by subsequent sedimentation and or erosion this raises the issue of whether conventional geologic geomorphic and paleoseismic methods can be used to identify and characterize thrust faults for the assessment of seismic hazards or whether these faults sometimes are truly blind we compiled a data base of 148 worldwide moderate to large magnitude thrust reverse earthquakes to evaluate whether or not the event occurred on a fault that could have been identified prior to the earthquake on the basis of recognizable quaternary surface deformation i e a pre existing fault or fold analysis of the data shows that interplate reverse earthquakes almost always are associated with pre existing quaternary deformation that was or could have been recognized prior to the earthquake in particular most interplate reverse earthquakes are associated with an active reverse fault at the surface and or an active anticline in contrast intraplate reverse earthquakes seldom occur on faults associated with pre existing recognizable surface deformation we conclude that thrust faults can be detected in interplate regions with careful quaternary geologic and geomorphic mapping furthermore the absence of quaternary surface deformation can be used to infer the absence of an underlying active blind thrust fault in interplate tectonic settings however the data show that quaternary geologic mapping techniques alone likely are insufficient to characterize blind thrusts in intraplate regions in these areas inclusion of a floating or random earthquake may be necessary to assess earthquake hazards

prospects for larger or more frequent earthquakes in the los angeles metropolitan region far too few moderate earthquakes have occurred within the los angeles california metropolitan region during the 200 year long historic period to account for observed strain accumulation indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries long interseismic period between much larger moment magnitude m w 7 2 to 7 6 events geologic slip rates and relations between moment magnitude average coseismic slip and rupture area show that either of these hypotheses is possible but that the latter is the more plausible of the two the average time between m w 7 2 to 7 6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years

supergene and hypogene halloysite in a porphyry epithermal environment at cerro la mina chiapas mexico halloysite a phyllosilicate clay mineral chemically similar to but structurally different from kaolinite occurs in a variety of mineral deposit types it is however difficult to identify and dehydrates quite readily when identified in a porphyry or epithermal environment halloysite is often interpreted to be a low temperature polymorph of kaolinite and an indication of supergene processes the occurrence of halloysite in the cerro la mina au cu mo prospect chiapas mexico was investigated to determine if halloysite in porphyry epithermal deposits might also be the product of hypogene processes the prospect consists of quaternary volcaniclastic breccias intruded by monzodiorites and trachyandesites crosscut by a volcanic hydrothermal breccia pipe gold copper molybdenum mineralization at cerro la mina is structurally and lithologically controlled by matrix rich breccia pipes hosting all of the significant alteration and mineralization drilled to date average grades within the prospect are 0 4 ppm au 0 16 cu and 131 ppm mo a total of 100 samples were obtained from 22 diamond drill holes over a range of stratigraphic levels x ray diffraction xrd and swir techniques were employed to identify the different clay minerals found within these samples including halloysite kaolinite dickite and illite identification of halloysite and kaolinite were facilitated by the use of formamide intercalation to produce distinctive peaks for the two minerals in x ray diffractograms halloysite and kaolinite were found to occur from the current erosional surface to depths of over 800 m halloysite occurs as both the hydrated 10a and dehydrated 7a forms within the deposit the morphology of the halloysite present is that of well developed tubes and spheroids results of the study show that halloysite associated with gypsum and jarosite is of supergene origin whereas halloysite occurring with quartz alunite dickite kaolinite and pyrite is of hypogene origin

reply to comment by r j shlemon j e slosson and j t barnhart on late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures by j n baldwin k i kelson and c e randolph this reply addresses the three main comments by shlemon et al 2001 namely that our article 1 provided inadequate reference to a large body of earlier work 2 yielded a conclusion that had already been made and 3 estimated a range in slip rate uncertainty that is too large we address these three issues individually first we acknowledge that the northridge hills fault was identified many years ago and that it was previously interpreted as a probable active fault however shlemon et al 2001 implied that baldwin et al 2000 failed to acknowledge the earlier work performed by others barnhart and slosson 1973 saul 1975 and johnson et al 1996 they also suggest that earlier geomorphic and geologic observations by barnhart slosson and saul are sufficient to define the seismogenic potential of the northridge hills fault we disagree as stated in our article the northridge park trench site was selected on the basis of detailed geologic and geomorphic mapping of surficial deposits in the northern san fernando valley hitchcock and kelson 1996 wills and hitchcock 1999 p 631 in addition to the earlier work performed by others e g barnhart and slosson 1973 smith 1977 saul 1979 dibblee 1992 we also referenced johnson et al 1996 throughout the manuscript the detailed 1 24 000 scale mapping of surficial deposits summarized by hitchcock and kelson 1996 and wills and hitchcock 1999 has recently been published by the california division of mines and geology as map sheet 50 1 48 000 scale hitchcock and wills 2000 second we acknowledge that the northridge hills fault was interpreted as a possible active fault barnhart and slosson 1973 but also note barnhart and slosson 1973 state proper credence to the

the triple junction of the north america cocos and caribbean plates seismicity and tectonics the triple junction of the north america cocos and caribbean plates is ambiguously defined mainly because the north america caribbean plate boundary does not clearly continue beyond its known surface trace the motagua fault zone in western guatemala to intersect the middle america trench well located regional shallow earthquakes h 70 km show that there is no intermediate or large magnitude seismic activity associated with a presumed extension of the north america caribbean plate boundary to the west beyond its well defined surface trace there is however a clear zone of shallow seismic activity from the western section of the fault system through southern mexico fault plane solutions for these events indicate a left lateral strike slip displacement which is in good agreement with surface faulting we suggest that these strike slip faults together with the salina cruz fault in the isthmus of tehuantepec mark the boundaries of a broad zone of deformation in southern mexico and northern central america which takes up the interactions of the three plates in this sense no single point constitutes the triple junction the geologic record suggests that the motagua fault zone developed because the westernmost portion of the caribbean plate became locked against north america

structural evolution of the northern los angeles basin california based on growth strata subsurface structure contour maps and cross sections show that the northern los angeles basin is underlain by a south facing monocline complicated by secondary faults and folds the monocline and associated structures form a structural high the northern los angeles shelf which marks the northern margin of the los angeles central trough analysis of growth strata sediments deposited during structural growth shows that during the miocene the predominant structural style was extension growth strata show that at approximately the beginning of the pliocene 5 ma contraction reactivated the miocene normal faults as reverse faults and formed the monocline its secondary structures and the los angeles central trough lensing out of earliest pliocene delmontian stage growth sandstones onto the flanks of the monocline and secondary structures the south salt lake the east beverly hills and the las cienegas anticlines indicates that these contractional structures all began to form at this time all of the secondary structures became largely inactive prior to the deposition of upper pico member of early pleistocene age 1 2 ma however thick accumulations of growth strata of the upper pico member of the fernando formation within the los angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive the growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline the los angeles fault can be calculated in the east beverly hills area the dip of the los angeles fault is 61 and at las cienegas the dip is 62 these are maximum values based on the assumption that the growth strata bed lengths record all shortening the pliocene pleistocene average fault slip rates for the los angeles fault are 1 1 1 3 mm yr in the east beverly hills and 1 3 1 5 mm yr at las cienegas the resulting pliocene pleistocene horizontal convergence rates are 0 5 0 6 mm yr and 0 6 0 7 mm yr respectively the pliocene pleistocene growth strata show increased dip with age indicating that the monocline grew by progressive limb rotation rather than by kink band migration therefore fault bend and fault propagation fold models based on kink band migration are inadmissible solutions to explain the growth of the monocline we suggest a basement involved shear zone to explain the geometry of the northern los angeles shelf

deformation rates across the placerita northridge mw 6 7 aftershock zone and hopper canyon segments of the western transverse ranges deformation belt abstract aftershocks of the 1994 northridge m w 6 7 earthquake provide insights into the geometry of the seismic source faults in the san fernando valley and the east ventura basin and allow the calculation of deformation rates for the region the northridge thrust and santa susana faults dip in opposite directions with the northridge thrust entirely beneath the santa susana fault these opposing reverse faults interact resulting in a folded active santa susana fault and uplift in the footwall block of that reverse fault two balanced cross sections suggest thick skinned deformation of the western transverse ranges the western section across the modelo lobe segment of the north dipping san cayetano fault and the easternmost surface trace of the south dipping oak ridge fault is west of any aftershocks of the northridge earthquake and has been termed the hopper canyon segment of the deformation belt structural modeling predicts a dip of 46 s on the oak ridge fault at seismogenic depths horizontal shortening rates are calculated by adding the products of the dip slip displacements and the cosines of the dips of both faults the eastern cross section shows the northridge mainshock with a 45 south dipping nodal plane at a depth of 18 km aftershocks reach a depth of 20 km in a thin skinned paradigm a hinge should occur at the surface near the santa monica mountains due to rocks moving from a decollement at the brittle plastic transition and changing dip as they move up the ramp no hinge of that magnitude occurs there calculation of horizontal short ening rates across this part of the western transverse ranges must take into account the displacement on both the northridge thrust eastern extension of the oak ridge fault and the santa susana fault placerita segment horizontal shortening rates are 8 2 2 4 mm yr across the modelo lobe segment of the san cayetano fault and the oak ridge fault and 5 7 2 5 mm yr across the northridge thrust and the santa susana fault these rates are consistent with those based on tectonic geodesy using gps dip slip displacement rates on the faults are 1 7 mm yr for the northridge thrust since 2 3 ma 4 1 0 4 mm yr for the oak ridge fault since 500 ka 5 9 3 9 3 8 mm yr for the santa susana fault since 600 to 2300 ka and 7 4 3 0 mm yr for the modelo lobe segment of the san cayetano fault since 500 ka this indicates that the slip rates on the north dipping the eastern san cayetano and the santa susana faults are comparable but the slip rate on the south dipping faults decreases eastward the slip rate on the oak ridge fault in the ventura basin is more than double that of the northridge thrust

regional analysis of tertiary volcanic calderas western u s using landsat thematic mapper imagery abstract the basin and range province of southern nevada is an excellent setting to study landsat imagery over volcanic rock assemblages the area is semiarid sparsely vegetated and contains relatively youtful 6 14 ma volcanic calderas with compositionally diverse volcanic rock assemblages investigation of rock spectra whole rock geochemistry desert varnish and landsat imagery indicates that the longer wavelength visible and near infrared thematic mapper tm bands 3 5 and 7 provide more effective lithologic discimination than the shorter wavelength bands due partly to deeper penetration of lower frequency energy and greater primary rock compositional response spectral contamination of vegetation degrades lithologic information in tm band 4 thus ratios color composites and intensity saturation hue images using tm bands 3 5 and 7 generally lead to superior lithologic contrast shorter wavelength tm bands 1 and 2 are affected more by surficial weathering products including desert varnish which may or may not provide an indirect link to lithologic identity these relationships lead to a rock varnish albedo difference that aids identification of desert varnish on leucocratic rocks using tm band 2 5 and 5 2 ratio images principles component color composites provide best overall lithologic contrast but not specific lithologic identification pc2 tends to contain lithologically dominant variance and combines favorably with other images influenced strongly by lithology late magmatic differentiates that are evolved with respect to alkalies and the incompatible elements exhibit steep spectral curves throughout the tm bands 5 7 interval leading generally to low 5 7 ratios and dark contrast on 5 7 ratio images guidelines for lithologic analysis of volcanic terrain using landsat tm imagery are outlined

using mineral chemistry to aid exploration a case study from the resolution porphyry cu mo deposit arizona the giant high grade resolution porphyry cu mo deposit in the superior district of arizona is hosted in proterozoic and paleozoic basement and in an overlying cretaceous volcaniclastic breccia and sandstone package resolution has a central domain of potassic alteration that extends more than 1 km outboard of the ore zone overlapping with a propylitic halo characterized by epidote chlorite and pyrite that is particularly well developed in the laramide volcaniclastic rocks and proterozoic dolerite sills the potassic and propylitic assemblages were overprinted in the upper parts of the deposit by intense phyllic and advanced argillic alteration the district was disrupted by tertiary basin and range extension and the fault block containing resolution and its cretaceous host succession was buried under thick mid miocene dacitic volcanic cover obscuring the geologic geophysical and geochemical footprint of the deposit to test the potential of propylitic mineral chemistry analyses to aid in the detection of concealed porphyry deposits a blind test was conducted using a suite of epidote chlorite pyrite altered laramide volcaniclastic rocks and proterozoic dolerites collected from the propylitic halo with samples taken from two domains located to the north and south and above the resolution ore zone laser ablation inductively coupled plasma mass spectrometry la icp ms data of epidote provided indications of deposit fertility and proximity competition for chalcophile elements as sb pb between coexisting pyrite and epidote grains led to a subdued as sb fertility response in epidote consistent with epidote collected between 0 7 and 1 5 km from the center of a large porphyry deposit temperature sensitive trace elements in chlorite provided coherent spatial zonation patterns implying a heat source centered at depth between the two sample clusters and application of chlorite proximitor calculations based on la icp ms analyses provided a precisely defined drill target in this location in three dimensions drilling of this target would have resulted in the discovery of resolution confirming that epidote and chlorite mineral chemistry can potentially add value to porphyry exploration under cover

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

overlapping fault planes of the 1971 san fernando and 1994 northridge california earthquakes aftershocks of the 1971 san fernando and 1994 northridge earthquakes were relocated using a three dimensional velocity model that was derived from inverting p wave travel time data the hypocenters show clear orientations of the dipping fault planes the san fernando aftershocks form a plane extending from a depth of 15 km to the surface dipping toward the northeast at about 40 the northridge aftershocks delineate a fault extending from a depth of 18 km up to about 5 km dipping toward the southwest at about 40 in the region the aftershocks overlap in map view the san fernando plane cuts off the northridge plane at a depth of 5 to 8 km preventing it from reaching the surface the similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast southwest compression

stratigraphic relations and source areas of ash flow sheets of the black mountain and stonewall mountain volcanic centers nevada recent work has resulted in major changes in the stratigraphic correlations and source area assignments of ash flow sheets of the late miocene black mountain and stonewall mountain volcanic centers southern nevada ash flow tuffs exposed around stonewall mountain have different trace element patterns and thermoremanent magnetization directions than do ash flow sheetsof the thirsty canyon tuff erupted from the black mountain center with which they were previously correlated hafnium concentrations are particularly useful in distinguishing the megascopically and petrographically similar tuffs ofthe two centers the ash flow sheet overlyingthe spearhead member in the stonewall mountainarea herein named the civet cat canyon member can be traced northward continuously from outflow facies south of stonewall mountain to near vent facies on the southern flank of stonewall mountain this in conjunction with distribution thickness and facies relations show that these units are the outflow sheets of the stonewall mountain volcanic center they are herein assigned to a new formation the stonewall flat tuff of late miocene age tuffs previously mapped near black mountain as the labyrinth canyon member herein abandoned are correlated on the basis of trace element paleomagnetic petrographic and radiometric age data with the type spearhead member at goldfield tuffs previously assigned to the spearhead member in the black mountain frea are reassigned to the pahute mesa member new of the thirsty canyon tuff the stonewall mountain volcanic center is thus slightly younger than the black mountain center and the period of time over which the black mountain center was active probably was appreciably shorter than was previously inferred recognition of the stone wall flat tuff as a separate stratigraphic and genetic entity enables us to explain a number of geological inconsistencies including the marked asymmetric distribution previously attributed to the thirsty canyon tuff moreover the inferred original volume of the thirsty canyon tuff is significantly reduced increasing the importance of lava relative to tuff in the eruptive history of the black mountain center

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

paleomagnetic constraints on the initiation of uplift on the santa susana fault western transverse ranges california the plio pleistocene nonmarine saugus formation is widely exposed in the east ventura basin and the northern san fernando valley north and south of the santa susana fault in the east ventura basin the saugus is overlain by the pacoima formation magnetostratigraphy of the fine grained saugus and pacoima strata and the presence of the 0 76 ma bishop ash are used to calculate their average sedimentation rates 0 9 km m y in the east ventura basin and 1 1 km m y in the northern san fernando valley extrapolation of the saugus and pacoima sedimentation rates shows that they were deposited from about 2 3 to 0 5 ma both north and south of the santa susana fault the shift from remote to locally derived clasts marks the initiation of uplift of the santa susana mountains the extrapolated age of this boundary is about 0 7 0 6 ma

tectonic setting of the 1971 sylmar and 1994 northridge earthquakes in the san fernando valley california the san fernando valley lies above the north dipping 1971 sylmar and south dipping 1994 northridge earthquake faults to understand the tectonic setting of these two earthquakes we mapped subsurface geology of the san fernando valley down to a depth of 3 km using industry oil well and seismic data the 1994 northridge earthquake did not rupture the surface and the south dipping aftershock zone terminated against the north dipping 1971 aftershock zone at a depth of 5 8 km however the blind northridge fault has a near surface geologic expression fault propagation folding related to the northridge fault has preserved a thick forelimb sequence of plio pleistocene saugus formation in the sylmar basin and merrick syncline which are located on the hanging wall side of north dipping reverse faults the north dipping mission hills verdugo and northridge hills reverse faults are interpreted to be potential seismic sources because fault propagation folds above these faults have tectonic geomorphic expression these north dipping reverse faults were initiated during the deposition of the saugus formation between 2 3 and 0 5 ma and have minimum dip slip rates of 0 35 to 1 1 mm yr based on the oldest possible age of the initiation of faulting the northridge hills and mission hills faults are interpreted to merge at depth and are located at the updip extension of the 1971 aftershock zone even though these faults did not rupture during the 1971 earthquake surface breaks appeared north of these faults mostly along north dipping bedding planes and are interpreted as secondary features related to flexural slip folding rather than a direct extension of the 1971 seismogenic fault surface and subsurface geology together with seismological data of the 1971 and 1994 earthquakes suggests that the north and south dipping deformation zones in the san fernando valley are divided into multiple segments separated by northeast trending structural discontinuities

the gold flat tuff nevada insights into the evolution of peralkaline silicic magmas the gold flat tuff is the youngest 9 15 ma ash flow sheet erupted from the black mountain volcanic centre southwest nevada usa this paper explores some aspects of the very complex nature of the tuff s magmatic plumbing system the main body of the deposit is a mixed magma product comprising pantelleritic and comenditic melts derived from independently evolving reservoirs and antecrysts and enclaves derived from a range of basic to intermediate sources metre scale cognate xenoliths point to the presence of alkali feldspar accumulation zones the pantellerite contains phenocrysts of fluorite and chevkinite ce the inferred intermediate magma component contains perrierite ce phenocrysts the pantellerite has unusually high contents of f 2 2 wt f cl 2 9 wt and zro 2 1 04 wt the high halogen contents may have influenced the evolution of the strongly peralkaline magma the crystallization conditions are poorly constrained but those for the pantelleritic magma may have been close to water saturation 4 wt melt water at temperatures 740 c and fo 2 around fmq

strike slip faults and k alkaline volcanism at el chichon volcano southeastern mexico abstract el chichon volcano is located in the northern part of the state of chiapas in southeastern mexico the volcano is situated within the transcurrent fault province characterized by a series of rising and sinking blocks bounded by sinistral strike slip faults the basement around el chichon is a sequence of jurassic evaporates cretaceous limestones and tertiary terrigenous rocks folded into open and an echelon structures the main structures in the area are the la union and caimba anticlines and the buena vista syncline the region is cross cut by two conjugate fault systems a dextral strike slip n s trending set and a sinistral strike slip e w set the most significant fault of the latter system is the san juan fault which controlled the emplacement of a basaltic dike near the village of chapultenango some 1 1 ma ago and the activity of el chichon during the last 0 2 ma additionally a series of n45 e trending faults chapultengo fault system have produced a half graben geometry of blocks on top of which el chichon has been emplaced microstructures such as slickensides tension fractures layer parallel slip stylolites and macro and meso folds of the sedimentary rocks indicate that during the late miocene the el chichon area was affected by a maximum principal stress 1 oriented n70 e a minimum principal stress 3 oriented n20 w and a vertical intermediate principal stress this stress pattern indicates that the area underwent a strike slip motion that produced widespread deformation the occurrence of crustal earthquakes

gps observations of fault afterslip and upper crustal deformation following the northridge earthquake global positioning system gps observations indicate that significant aseismic deformation occurred in the year following the january 17 1994 northridge earthquake the postseismic observations show the same sense of motion as the coseismic compared with the coseismic displacements the far field postseismic motions 1 2 fault dimensions away are proportionally larger than those seen in the near field the postseismic data are best modeled with two faults one on the rupture plane and one located in the shallow crust the upper crustal fault may represent an actual fault or may be indicative of viscous relaxation occurring in the upper crust the inferred afterslip and or relaxation moment is approximately 2 9 1018 n m or 22 of the mainshock moment release we expect that the moment release due to the afterslip and relaxation effectively reduce the earthquake hazard locally it is not clear from this study how the postseismic deformation loads the surrounding faults or alters the state of stress on those faults

geology and genesis of the cerro la mina porphyry high sulfidation au cu mo prospect mexico the cerro la mina au cu mo porphyry high sulfidation prospect is located in chiapas state southeastern mexico outside of the major metallogenic provinces of mexico the prospect is hosted by pleistocene alkaline volcanic rocks of the chiapanecan volcanic arc that formed in a complex triple junction tectonic setting cerro la mina s stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1 04 0 04 ma u pb zircon and that were overlain by debris flows and synvolcanic trachyandesite domes the volcanic stratigraphy of cerro la mina is dominated by pyroclastic flows and rare basalts that are cut by the cerro la mina breccia pipe a matrix rich granular vertically oriented downward tapering polymict lithic rock unit that is host to all of the significant alteration and mineralization a nw trending sinistral wrench fault which was active throughout the history of cerro la mina is responsible for dismembering the prospect after mineralization the magmatic hydrothermal system was composed of early porphyry style potassic veins quartz k feldspar biotite and stage 1 pyrite that are preserved in clasts within the breccia pipe suggesting that brecciation disrupted an embryonic porphyry system late potassic alteration occurred after the formation of the breccia pipe as its matrix is strongly k feldspar altered hydrothermal fluids then produced phyllic alteration composed of quartz muscovite illite illite smectite and chlorite that is associated with stage 2 pyrite chalcopyrite molybdenite quartz veins an unusual zoned pattern of advanced argillic argillic alteration overprinted potassic and phyllic alteration this zoning included a low temperature 120 c quartz dickite kaolinite pyrophyllite alunite that occurs from 250 m to the present day surface the advanced argillic argillic altered rocks host the most significant au cu mineralization which is associated with stage 3 marcasite sphalerite galena and barite and stage 4 arsenian pyrite enargite covellite the magmatic hydrothermal system at cerro la mina began sometime between monzodiorite emplacement 1 04 0 04 ma zircon u pb and the precipitation of porphyry stage 2 molybdenite at 780 10 ka re os 40ar 39ar dating of biotite 689 13 ka records the age at which the hydrothermal system cooled below the biotite closure temperature of 300 c and provides a maximum estimate for the onset of advanced argillic argillic alteration sulfur isotope results of sulfides 2 5 to 4 9 mean 0 7 n 20 and a sulfate barite 10 5 n 1 suggest a magmatic source of sulfur for all four stages of mineralization the lack of residual quartz rare alunite and anomalous halloysite kaolinite alteration may be explained by the high acid buffering capacity of alkaline volcanic host rocks high co2 contents of the alkaline magma and or potentially by a highly reduced magmatic hydrothermal fluid at the regional metallogenic scale the cerro la mina prospect along with the nearby santa fe mine and campamento deposit represent parts of a porphyry copper system specifically a porphyry high sulfidation proximal skarn and intermediate sulfidation deposit respectively the characteristics of cerro la mina i e anomalous halloysite kaolinite alteration broaden the window for additional discoveries to be made in the porphyry epithermal environment

paleoseismic features as indicators of earthquake hazards in north coastal san diego county california usa abstract new road cut and mass grading excavations in the north coastal area of san diego county california expose heretofore generally unrecognized probable late holocene tsunami deposits and paleoseismically deformed sediments remnant tsunami deposits occur up to 100 m in elevation around the margins of modern coastal lagoons and estuaries and combined with local mima mounds of possible sand blow origin provide indirect but compelling evidence for the late quaternary activity of onshore and offshore faults in the immediate study area probable paleoliquefaction features are regionally widespread and range from fissures filled with sediments derived from overlying marine terrace sand and soil to the more traditional sand filled injection dikes and sills lateral spreads and filled craterlets the source of most liquefied sediment is underlying tertiary bedrock sand and local quaternary marine terrace deposits a paleoseismic liquefaction origin rather than soft sediment loading is deduced for these features based on morphology internal stratigraphy field setting and near proximity to known seismogenic sources some paleoseismic events impacted late holocene indian middens and burial sites the last seismic event probably occurred within the past 1 to 3 ka and possibly even records the historic earthquakes of either november 22 1800 or may 27 1862 the liquefaction features also affect marine terrace sediments tens of meters above modern regional water levels inferentially recording paleoseismic events in this mediterranean type climatic region during winter rains when high level perched water saturates the several meter thick source sediments based on their regional extent the paleoseismic features were likely caused by m 7 tectonic events inferentially generated by the nearby offshore newport inglewood rose canyon fault system or possibly by smaller recently exposed related and localized faults accordingly the seismic hazard of the north coastal area of san diego county may be substantially higher than previously assumed and hence of concern owing to the rapid ongoing and projected population increase

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

evidence for large earthquakes in metropolitan los angeles the sierra madre fault along the southern flank of the san gabriel mountains in the los angeles region has failed in magnitude 7 2 to 7 6 events at least twice in the past 15 000 years restoration of slip on the fault indicated a minimum of about 4 0 meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10 5 meters for the past two prehistoric earthquakes large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes within the los angeles region are not yet considered in most seismic hazard and risk assessments

an elusive blind thrust fault beneath metropolitan los angeles seismic reflection profiles petroleum wells and relocated earthquakes reveal the presence of an active blind thrust fault beneath metropolitan los angeles a segment of this fault likely caused the 1987 whittier narrows magnitude 6 0 earthquake mapped sizes of other fault segments suggest that the system is capable of much larger magnitude 6 5 to 7 and more destructive earthquakes

comment on late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures by j n baldwin k i kelson and c e randolph the 1994 northridge earthquake in southern california spawned literally hundreds of investigations and their resulting publications many investigations are now deemed as paleoseismic a term now widely used for good old fashioned trenching and fault dating baldwin et al 2000 continue this tradition by emplacing boreholes a trench and test pits across the face of a 2 m high escarpment that marks the south side of the nw se trending northridge hills in the northern san fernando valley the northridge hills fault zone underlies the northridge hills and interest in its ground rupture potential was set forth over 50 yr ago by hazzard 1944 and later by slosson and barnhart 1967 barnhart and slosson 1973 and saul 1975 who basically concluded that fault movement occurred during late quaternary time and that therefore the fault was likely to be active in their article baldwin et al p 637 asked the rhetorical question is the northridge fault active they answer in the affirmative based mainly on interpretations of leveling data on the construction of regional structural models and on the inferred age of sediments exposed in trenches

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

paleomagnetic and cooling constraints on the duration of the pahute mesa trail ridge eruptive event and associated magmatic evolution black mountain volcanic center southwestern nevada two successive ash flow cooling units of the black mountain volcanic center the pahute mesa and trail ridge members of the late miocene thirsty canyon tuff possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178 declination and 10 inclination these are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition or possibly from a field oriented near the limit of secular variation regardless of the type of geomagnetic field behavior the pahute mesa member was apparently overlain by the trail ridge member before appreciable shift of the anomalous geomagnetic field this strongly suggests that the time between eruption of the two ash flow sheets was very short perhaps less than 100 years a conclusion that is independently supported by the local absence of a complete cooling break between the two units the interval between eruption of the pahute mesa and trail ridge members was too brief for reestablishment of chemical stratification in the black mountain magma system nevertheless a small volume of highly evolved comendite magma developed and was erupted as lava within the black mountain caldera and as a component of the trail ridge member the short repose interval and rapid development of the comendite together with evidence for mixing of magmas of silicic to near mafic composition vogel et al this issue and the presence of air fall tuff at the base of the trail ridge lead us to conclude that eruption of the trail ridge member was probably triggered by injection of hot relatively unevolved magma into the black mountain magma body

rates and processes of soil development on quaternary terraces in cajon pass california field and laboratory analyses of soils on 11 well dated fluvial terraces spanning the past 0 5 m y demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils correlations with respect to time among iron species soil morphology and soil silt and clay demonstrate that the chronosequence at cajon pass reflects primarily an evolutionary largely time dependent trend and does not reflect differences in external factors such as climate most of the soil development on holocene terraces of the cajon pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels this process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering latest pleistocene and older pleistocene soils have developed clay and authigenic iron oxide rich b horizons at the expense of organic matter rich a horizons and color b horizons as the extent of chemical weathering has increased this conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time 4 000 yr and is herein defined as a type of pedologic threshold in the cajon pass area the attainment of the threshold and subsequent development of the argillic b horizon of soils on latest pleistocene terraces occurred during the holocene thus the absence of argillic horizons in soils on holocene terraces is attributable to simply their younger age rather than to the pleistocene to holocene climatic change the threshold is a function of several variables including influx rate of eolian dust and initial soil permeability therefore the time required to attain the threshold will vary in chronose quences characterized by geomorphic or geographic settings that are different from conditions found in cajon pass

fault slip rates and interseismic deformation in the western transverse ranges california to better constrain fault slip rates and patterns of interseismic deformation in the western transverse ranges of southern california we present results from analysis of gps and interferometric synthetic aperture radar insar data and three dimensional mechanical and kinematic models of active faulting anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous gps site locations gps measures contraction rates across the ventura basin of 7 mm yr oriented west northwest with rates decreasing to the west and east the santa barbara channel is accommodating 6 5 mm yr in the east and 2 5 mm yr in the western portions of n s contraction inversion of horizontal gps velocities highlights a zone of localized fast contraction rates following the ventura basin using a mechanical model driven by geodetically calculated strain rates we show that there are no significant discrepancies between short term slip rates captured by geodesy and longer term slip rates measured by geology mechanical models reproduce the first order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the ventura basin due to the inadequate homogeneous elastic properties of the model existing two dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the gps data mechanical models predict zones of fast contraction in the santa barbara channel and offshore near malibu suggesting that offshore faults represent a significant seismic hazard to the region furthermore many active faults throughout the region may produce little to no interseismic deformation making accurate seismic hazard assessment challenging 2013 american geophysical union all rights reserved

active parasitic folds on the elysian park anticline implications for seismic hazard in central los angeles california we characterize the seismic hazard of the elysian park fault a blind reverse fault beneath central los angeles by analysis of the elysian park anticline which overlies it new shallow subsurface geotechnical data combined with other surficial stratigraphy and geomorphology reveal that the elysian park anticline is an active 20 km long structure from the style and rates of deformation of parasitic folds on the southern limb of the anticline we estimate a contraction rate of 0 6 1 1 mm yr this rate provides a basis for estimating a rate of contraction of the entire elysian park anticline which in turn allows us to estimate a 0 8 2 2 mm yr time averaged rate of slip on the underlying fault at this rate of slip rupture of the elysian park fault could produce a nominal mw 6 2 to 6 7 earthquake every 500 to 1300 yr on average although this elysian park earthquake would recur infrequently its size and recurrence interval may be similar to those estimated for the sources of the destructive 1971 san fernando and 1994 northridge earthquakes

the 1987 whittier narrows earthquake in the los angeles metropolitan area california the whittier narrows earthquake sequence local magnitude m l 5 9 which caused over 358 million damage indicates that assessments of earthquake hazards in the los angeles metropolitan area may be underestimated the sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east west length of the northern margin of the los angeles basin peak horizontal accelerations from the main shock which were measured at ground level and in structures were as high as 0 6g where g is the acceleration of gravity at sea level within 50 kilometers of the epicenter the distribution of the modified mercalli intensity vii reflects a broad north south elongated zone of damage that is approximately centered on the main shock epicenter

the 3 december 1988 pasadena california earthquake evidence for strike slip motion on the raymond fault the pasadena earthquake m l 4 9 occurred on 3 december 1988 at a depth of 16 km the hypocenters of the earthquake and its aftershocks define a east northeast striking steeply northwest dipping surface that projects up to the active surficial trace of the raymond fault one of the nodal planes of the focal mechanism of the earthquake parallels the raymond fault with left lateral strike slip movement on that plane and is consistent with geomorphic and paleoseismic evidence that the raymond fault is dominantly a left lateral strike slip fault the existence of a component of sinistral slip along the raymond fault had been suspected prior to the earthquake but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse

stonewall mountain volcanic center southern nevada stratigraphic structural and facies relations of outflow sheets near vent tuffs and intracaldera units directly south and southeast of stonewall mountain nevada a depression and north facing caldera scarp were formed during and or after eruption of the spearhead member of the late miocene stonewall flat tuff abundant large lithic and juvenile blocks are present in the spearhead member within 0 5 km of this topographic margin but absent elsewhere in the ash flow sheet consistent with eruption from vents in the stonewall mountain area within about 100 000 years comendite tuff of the overlying civet cat canyon member of the stonewall flat tuff buried the depression and associated scarp the civet cat canyon member is traceable continuously to the north from an outflow sheet capping northwestern pahute mesa into near vent tuff on the southeastern flank of stonewall mountain proximal outflow sheet tuff locally exhibits strong rheomorphic disruption and is overlain without a cooling break by surge flow and fall deposits of trachytic composition much of stonewall mountain is composed of welded tuff and megabreccia interpreted as intracaldera tuff of the civet cat canyon member strongly suggesting that the vent area of the member was largely within stonewall mountain welded tuff of trachytic composition comprises an important part of the intracaldera civet cat canyon member which was intruded by dikes and plugs of trachyte and rhyolite juvenile inclusions of basalt dispersed in near vent facies trachyte tuff provide direct evidence for the high level involvement of basaltic magma in the evolution of the highly potassic stonewall mountain center complex discordant compaction foliations and the widespread presence of megabreccia within the intracaldera tuff suggest following foley 1978 cauldron subsidence by piecemeal collapse during eruption of the civet cat canyon member the elevation of intracaldera tuff and intrusions in stonewall mountain above the surrounding ashflow sheet suggests a significant amount of magmatic uplift perhaps involving the emplacement of plugs dikes and small stocks within the intracaldera tuff prism

wilshire fault earthquakes in hollywood the wilshire fault is a potentially seismogenic blind thrust fault inferred to underlie and cause the wilshire arch a quaternary fold in the hollywood area just west of downtown los angeles california two inverse models based on the wilshire arch allow us to estimate the location and slip rate of the wilshire fault which may be illuminated by a zone of microearthquakes a fault bend fold model indicates a reverse slip rate of 1 5 1 9 mm yr whereas a three dimensional elastic dislocation model indicates a right reverse slip rate of 2 6 3 2 mm yr the wilshire fault is a previously unrecognized seismic hazard directly beneath hollywood and beverly hills distinct from the faults under the nearby santa monica mountains

a geophysical geological transect of the silent canyon caldera complex pahute mesa nevada revision of lithological logs for boreholes penetrating the volcanic center at pahute mesa nevada has led to a thorough review of the volcanic stratigraphy and geologic structure we have combined this review with a compilation of old and newly acquired gravity and seismic travel time data producing a unified interpretation along a northwest to southeast profile the analysis supports a new interpretation of the silent canyon caldera complex the caldera is found to be more asymmetric than previously suggested with the southeastern boundary formed by linear high angle normal faults and a more gently sloping northwestern boundary the total thickness of volcanic units within the caldera complex does not appear to exceed 5 km the shallow structure at pahute mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site the silent canyon caldera complex is actually a set of nested calderas first filled by thick 1 km postcaldera lavas and subsequently buried by outflow sheets of the timber mountain caldera to the south thick postcaldera lavas filled a half graben structure formed west of the west greeley fault dropping the tops of the youngest caldera forming units to depths in excess of 2 km therefore the western boundary of the caldera complex is poorly defined east of the west greeley fault two overlapping calderas are defined and stratigraphic data suggest the presence of even older calderas the youngest caldera the calc alkaline area 20 caldera is well defined from drill hole data the area 20 caldera overlaps the 13 6 ma peralkaline grouse canyon caldera which is less well defined but apparently collapsed in trap door style along the almendro fault for both these calderas collapse continued after the main caldera forming eruption concurrent with the accumulation of thick 1 km lavas within the peripheral collapse zones the geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the nne trending scrugham peak and almendro faults which offset the pre tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 ma age drill hole data show that offsets along these faults increase systematically within older up to 15 ma units which are commonly rotated eastward in a style similar to units at the surface abrupt changes in the subsurface thickness of the caldera forming units occur across the faults indicating that these linear features served as caldera boundaries

escape tectonics in the los angeles metropolitan region and implications for seismic risk recent damaging earthquakes in california including the 1971 san fernando1 1983 coalinga2 1987 whittier narrows3 and 1994 northridge4 events have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern california consequently many geological studies5 6 have concluded that thrust faults in southern california pose the greatest seismic hazard and also account for most of the estimated 5 7 mm yr 1 of contraction across the greater los angeles metropolitan area7 8 indicated by global positioning system geodetic measurements9 our study demonstrates however that less than 50 of the geodetically observed contraction is accommodated on the principal thrust systems across the los angeles region we integrate the most recent geological geodetic and seismological data to assess the spatial distribution of strain across the los angeles metropolitan region we then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east west crustal escape extrusion along known strike slip and oblique slip faults

response of high rise and base isolated buildings to a hypothetical mw 7 0 blind thrust earthquake high rise flexible frame buildings are commonly considered to be resistant to shaking from the largest earthquakes in addition base isolation has become increasingly popular for critical buildings that should still function after an earthquake how will these two types of buildings perform if a large earthquake occurs beneath a metropolitan area to answer this question we simulated the near source ground motions of a m w 7 0 thrust earthquake and then mathematically modeled the response of a 20 story steel frame building and a 3 story base isolated building the synthesized ground motions were characterized by large displacement pulses up to 2 meters and large ground velocities these ground motions caused large deformation and possible collapse of the frame building and they required exceptional measures in the design of the base isolated building if it was to remain functional

coseismic deformation during the 1989 loma prieta earthquake and range front thrusting along the southwestern margin of the santa clara valley california damage patterns caused by the 1989 loma prieta earthquake along the southwestern margin of the santa clara valley california form three zones that coincide with mapped and inferred traces of range front thrust faults northeast of the san andreas fault damage in these zones was largely contractional consistent with past displacement associated with these faults the damage zones coincide with gravity and aeromagnetic anomalies modeling of the anomalies defines a southwest dipping thrust fault that places the franciscan complex over cenozoic sedimentary rocks to minimum depths of 2 km diffuse loma prieta earthquake aftershocks encompass the downward projection of this modeled thrust to depths of 9 km our results indicate that in this region the potential for concentrated damage arising from either primary deformation along the thrust faults themselves or by sympathetic motion triggered by earthquakes on the san andreas fault may be higher than previously recognized

active tectonic influence on the evolution of drainage and landscape geomorphic signatures from frontal and hinterland areas along the northwestern himalaya india abstract the kangra re entrant in the nw himalaya is one of the most seismically active regions falling into seismic zone v along the himalaya in 1905 the area experienced one of the great himalayan earthquakes with magnitude 7 8 the frontal fault system the himalayan frontal thrust hft associated with the foreland fold janauri anticline along with other major as well as secondary hinterland thrust faults provides an ideal site to study the ongoing tectonic activity which has influenced the evolution of drainage and landscape in the region the present study suggests that the flat uplifted surface in the central portion of the janauri anticline represents the paleo exit of the sutlej river it is suggested that initially when the tectonic activity propagated southward along the hft the janauri anticline grew along two separate fault segments north and south faults the gap between these two fault and the related folds allowed the sutlej river to flow across this area later the radial propagation of the faults towards each other resulted in an interaction of the fault tips which caused the rapid uplift of the area rapid uplift resulted in the disruption and longitudinal deflection of the sutlej river channel fluvial deposits on the flat surface suggest that an earlier fluvial system flowed across this area in the recent past geomorphic signatures like the sharp mountain fronts along the hft in some places as well as along various hinterland subordinate faults like the nalagarh thrust nat the barsar thrust bat and the jawalamukhi thrust jmt the change in the channel pattern marked by a tight incised meander of the beas channel upstream of the jmt indicate active tectonic movements in the area the prominent v shaped valleys of the beas and sutlej rivers flowing across the thrust fronts with vf values ranging from

the magnitude 6 7 northridge california earthquake of january 17 1994 the most costly american earthquake since 1906 struck los angeles on 17 january 1994 the magnitude 6 7 northridge earthquake resulted from more than 3 meters of reverse slip on a 1 5 kilometer long south dipping thrust fault that raised the santa susana mountains by as much as 70 centimeters the fault appears to be truncated by the fault that broke in the 1971 san fernando earthquake at a depth of 8 kilometers of these two events the northridge earthquake caused many times more damage primarily because its causative fault is directly under the city many types of structures were damaged but the fracture of welds in steel frame buildings was the greatest surprise the northridge earthquake emphasizes the hazard posed to los angeles by concealed thrust faults and the potential for strong ground shaking in moderate earthquakes

locating distributed faulting contributions from insar imaging to probabilistic fault displacement hazard analysis pfdha abstract prediction of location and amount of slip for distributed faulting associated to strong earthquakes is a recently explored issue with major implications in terms of hazard assessment currently the best practice involves the applications of probabilistic fault displacement hazard analysis pfdha whose results fit sufficiently well real data in the near range of the primary fault but show considerable inaccuracies in the far range we believe that this inaccuracy descends from the biased earthquake databases used for regression analysis whose data relative to old earthquakes were largely collected only by field surveys focused close to the primary fault i e near range remotely sensed data i e interferometric radar imaging insar offer the opportunity to precisely measure the surface deformation induced by strong earthquakes and thus to explore its possible relation with distributed faulting we analyze the l aquila earthquake case study 29th april 2009 mw 6 3 and explore the correlation between location and slip on distributed faulting and insar derived deformation field we find a significant correlation between occurrence of distributed faulting and profile curvature of the dislocation field in spite of the distance from the primary fault moreover distributed faults tend to occur within the area deformed by the earthquake as imaged by insar data and whose extent is directly proportional to the earthquake magnitude mw according to a dataset of 30 recent earthquakes we then propose that these observations have to be incorporated into the present pfdha practice as limit boundaries to possible scenarios of probabilistic analysis and that an integrated use of field based and remotely data collection have to be implemented following strong earthquakes

effects of nonplanar fault topology and mechanical interaction on fault slip distributions in the ventura basin california to assess the control of fault geometry and mechanical interactions on fault slip distributions in a complex natural system we present results from three dimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the ventura basin region of southern california we find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip rate data than models utilizing rectangular planar fault surfaces the model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions additionally we find that slip rates at most locations along the surface trace of ventura faults are not likely to rep resent average values for the entire fault surface we propose that results from three dimensional mechanical models using realistic i e nonplanar fault geometry can be used to both predict slip rates at specific locations and determine whether existing site specific slip rate estimates are representative of average fault slip rates although geo metric irregularities along fault surfaces should resist slip planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations this suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation we assert that detailed knowledge of three dimensional fault shape as well as the geometry and con figuration of deep fault intersections is essential for accurate seismic hazard char acterization of regions of complex faulting such as the ventura basin of southern california

late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures paleoseismic investigation of the northridge hills fault in the northern san fernando valley california helps assess the timing and style of near surface late quaternary deformation in the epicentral area of the 1994 northridge earthquake the northridge hills fault a 15 km long north dipping reverse fault exhibits geomorphic evidence of late quaternary surface deformation including topographic scarps across late quaternary fluvial terraces and aligned alluvial fan apices on the footwall block we excavated one 40 m long trench and six test pits and drilled nine boreholes across a 2 m high scarp developed on a probable holocene fluvial terrace adjacent to aliso canyon wash a continuous clayey gravel identified in the trench test pits and boreholes defines a south facing monocline with 6 1 m of vertical separation across the northridge hills fault based on pedochronology the clayey gravel ranges in age from 6 to 30 ka the borehole data also suggest that an unconformity developed on the plio pleistocene saugus formation is warped into a monocline that has 13 2 m of vertical separation across the fault these preliminary data yield a dip slip rate of 1 0 0 7 mm yr for the northridge hills fault the absence of distinct scarp derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small incremental uplifts rather than large uplifts that generate distinct scarp relief we postulate that such uplift could be produced via moderate magnitude earthquakes m w 6 on the northridge hills fault or secondary deformation induced by earthquakes on other faults for instance evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the northridge hills fault produced by movement on the underlying northridge blind reverse fault or other nearby large structures based on our geologic investigations the distribution of aftershocks following the 1994 earthquake and pre and post 1994 leveling and geodetic surveys we interpret that the northridge hills fault underwent triggered slip during the 1994 earthquake

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

halloysite characteristics and formation in the northern golan heights the mineralogy of alteration products of late pleistocene basaltic scoriae and that of a paleosol from the northern golan heights was examined these products had relatively high specific surface areas ssas and cation exchange capacities cecs that were attributed to kaolinite smectite mixed layers 10a halloysite traces of smectite and illite the latter is attributed to aeolian deposition using specific surface area data from 18 to 59 clay were calculated to have formed in the weathered scoria the 10a halloysite was particularly well preserved in the paleosol the absence of significant amounts of short rage ordered minerals allophane ferrihydrite is attributed to the long dry summers and to the relatively old age of the parent materials sem observations of weathered rock and of mildly dispersed clay revealed two types of clay particles a hexagonal or prism like idiomorphic suggesting kaolinite and b rounded composite of a diameter approx 0 5 um appearing to be made up of spherical segments joined together by one or more flat faces hrtem of the 10a particles in the paleosol showed elongated ellipsoids tapered at one end and with internal layering or spiraling and spheroids with spiraling layering with a diameter 130 150 nm less common are sheet like crinkly films sio2 al2o3 molar ratios vary between 1 9 and 3 0 the particles have high fe up to 5 5 and ti up to 3 1 contents ti contents were higher in the peripheral parts of the spheroidal and in the crinkly sheet like particles sem observations suggest that while spheroidal and ellipsoidal 10a halloysite formed by solid phase transformation of volcanic glass the crinkly sheet like particles formed by precipitation on vesicle walls morphological features also suggest transformation of halloysite particles into kaolinite the absence of significant amounts of 10a halloysite in the modern soils indicates their xeric moisture regime

exploration tools for linked porphyry and epithermal deposits example from the mankayan intrusion centered cu au district luzon philippines the mankayan mineral district of northern luzon philippines hosts several significant ore deposits and prospects of various types within an area of similar to 25 km 2 including the far southeast porphyry cu au deposit the lepanto high sulfidation epithermal cu au deposit the victoria intermediate sulfidation epithermal au ag vein deposit the teresa epithermal au ag vein deposit the guinaoang porphyry cu au deposit and the buaki old palidan porphyry cu au prospects all having formed in a period of about 2 m y from similar to 3 ma the geologic units include 1 a basement composed of late cretaceous to middle miocene metavolcanic rocks volcaniclastic rocks 2 the miocene 12 to 13 ma tonalitic bagon intrusive complex 3 the pliocene similar to 2 2 to 1 8 ma imbanguila dacite porphyry and pyroclastic rocks and 4 postmineralization cover rocks including die similar to 1 2 to 1 0 ma bato dacite porphyry and pyroclastic rocks and the similar to 0 02 ma lapangan tuff extensive advanced argillic alteration crops out for similar to 7 kin along the inconformity between the basement rocks and the imbanguila dacite formation consists of quartz alunite pyrophyllite or diaspore with local zones of silicic alteration and a halo of dickite kaolinite the alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps the northwest striking portion is similar to 4 km long and hosts the lepanto enargite au ore deposit also controlled by the lepanto fault the lepanto epithermal deposit is related to the underlying far southeast porphyry the quartz alunite alteration halo of lepanto is contemporaneous with the similar to 1 4 ma potassic alteration of the porphyry there are also silicic advanced argillic alteration patches similar to 600 m above the far southeast orebody at the present surface these are interpreted to be perched alteration there is no systematic mineralogical or textural zoning in the lepanto lithocap that indicates direction to the intrusive source most surface samples of the lithocap contain less than 50 ppb au despite many being less than a few hundred meters from underground cu au ore this study found that several characteristics of the lepanto lithocap change systematically with distance from the causative intrusion the alunite absorption peak at similar to 1 480 nm in the short wavelength infrared swir spectrum shifts to higher wavelengths where the sample is closer to the intrusive center due to higher na and lower k content in the alunite published experimental studies indicate that high na na k is related to higher formation temperature high ca alunite including huangite also occurs at locations proximal to the intrusive center alunite mineral composition analyzed by laser ablation inductively coupled plasma mass spectrometry la icp ms indicates that the pb content decreases toward the intrusive center whereas sr la sr ph and la pb increase markedly whole rock compositions using only nonmineralized taken as cu similar to 350 m above the upper extent of the veins an airborne geophysics survey indicates that the far southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite destructive phyllic alteration such low magnetic anomalies on the margin of a large lithocap elsewhere may deserve attention the directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry epithermal districts

variability of site response in the los angeles urban area this article addresses the variability of site response in the los angeles area and possible structural causes for the observations aftershock records from 231 sites in the san fernando and los angeles basins and the surrounding mountains are used in this study spectral ratios taken with respect to a low amplitude reference site are used to document the variation in site amplification in the frequency range 2 to 6 hz both spatially and with backazimuth to the source at higher frequencies 6 to 10 hz spectral ratios are shown to have greater spatial variability interstation spectral ratios are used to measure the standard deviation among source as a function of station separation an increase in the variation in ground motion is shown to take place at a station separation of 1 km relative site response estimates between nearby stations are used to demonstrate that preferred directions of motion can exist even in areas with no surface topographic effects significant variations in site response exist over short baselines up to a factor of 2 over 200 m that are not explained by differences in surficial geology or shallow shear wave velocity a variety of investigative approaches is used including spectral ratios arrival time variations 1d and 2d waveform modeling and comparison with seismic reflection lines to determine the most likely causes of these observations a correlation is demonstrated between late arrival times of p and s waves and larger site amplification in sherman oaks and northridge this observation in conjunction with waveform modeling and seismic reflection profiles is used to infer that sedimentary structures in the upper 1 to 2 km and topography on the sediment basement interface play an important role in determining site amplification these structures in the form of folds and buried basins focus energy in spatially restricted areas at the surface comparison of displacement waveforms at nearby stations having disparate site amplifications complemented by known shallow shear wave velocities at selected sites is used to support the argument that these structures in some cases can be the dominant factor in the modification of local ground motions

kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 northridge california earthquake geodetic observations of surface deformation associated with the 1994 northridge southern california earthquake generally are reproduced by simple models of a large scale elastic dislocation on a blind or buried thrust fault the smaller scale aftershocks of the northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock these aftershocks averaged over volumes that are large relative to their rupture radii reflect a distributed permanent deformation that is accommodated by local brittle fracture we use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain aftershock deformation that occurred at depths below about 6 km and which is associated with the primary rupture zone is consistent with slow continuation of the southwest side up reverse slip on the blind northridge thrust fault in contrast aftershock deformation from the upper 5 7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal ne sw shortening and horizontal nw se i e fault parallel extension this pattern of deformation is similar to that associated with the mainshock as observed geodetically and as calculated from the elastic dislocation model we interpret that the aftershock activity in the hanging wall represents the quasi ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block the aftershocks along the mainshock rupture zone are interpreted as resulting from either 1 the time dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or 2 the continued slip in the weakened fault zone driven by the deformation of a ductile elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust in either case the aftershock activity suggests that the crust undergoes quasi ductile flow as a brittle elastic material and is not a strictly elastic material

the quaternary uplift history of central southern england evidence from the terraces of the solent river system and nearby raised beaches abstract we have used fluvial solent river system and marine terraces to reconstruct the uplift history of central southern england in the case of the former we make the assumption that fluvial incision has been a direct response to surface uplift with its precise timing controlled by climatic forcing of fluvial activity such that height of terrace gravel above modern river is a consequence of uplift since deposition in the case of the marine sequence we take the height of interglacial raised beaches above a calculated contemporaneous sea level as a measure of uplift the calculation involving an adjustment from modern sea level using the deep oceanic oxygen isotope signal as an indication of global ice volume at the time of deposition this exercise requires some degree of dating constraint which is problematic for both environments the solent terraces have yielded little biostratigraphical evidence whereas the south coast raised beaches have either been poorly exposed in recent years or their ages have been controversial because of disputes between biostratigraphy and geochronological data we have supplemented the evidence available from these sources by using key aspects of the archaeological record as dating constraints in particular the first appearances of levallois technique a marker for mis 9 8 and of bout coupe handaxes mis 3 the first of these has been particularly useful in modelling of the middle pleistocene parts of the river terrace staircases of the solent system in undertaking this reappraisal we have noted several inconsistencies and disagreements between past correlation schemes for the terraces of the solent and its various tributaries we find that versions involving shallower downstream gradients in the main solent river are most likely to be correct and that revisions on this basis solve a number of problems in interpretation encountered previously our results show that most of this region has uplifted by 70 m since the late early pleistocene and by 150 m since the middle pliocene there being a high degree of consistency between uplift histories inferred for river terraces and marine terraces uplift rates increase gradually westward such that along the river frome at the western end of the hampshire basin 80 m of uplift since the late early pleistocene is indicated this variation is interpreted as a consequence of a regional scale variation in crustal properties about 80 m of uplift is also indicated on this timescale by raised beaches in the portsdown area and adjacent terraces of the river test and solent in the vicinity of the portsdown anticline to the north of southampton we interpret this as a consequence of 10 m of vertical slip in the past million years on the blind reverse fault beneath this anticline this dataset thus provides the first clear indication of measurable quaternary structural development in crustal basement in the onshore uk the solent has formed more than a single terrace per 100 ka milankovitch cycle leading us to attribute terraces to isotopic substages potentially improving upon the resolution available from sequences in which terraces formed once per cycle athough the first appearence of levallois technique was initially considered to date from the mis 9 8 transition based on evidence from the thames we found that there was a better modelling fit if this was taken as having occurred slightly earlier in mis 9b perhaps in association with the post mis 9e or 9c marine regression which could have permitted immigration into a previously insular britain of people versed in levallois technology

first indications of high slip rates on active reverse faults nw of damascus syria from observations of deformed quaternary sediments implications for the partitioning of crustal deformation in the middle eastern region recent research on rates of crustal shortening within the palmyra fold belt pfb in syria has drawn attention to the possibility that reverse faults near the city of damascus which adjoins the sw pfb have significant slip rates we infer that the damascus fault directly adjacent to the city has developed a throw of 2500 m and report the discovery of the en echelon bassimeh fault with a throw of 1000 m this fault being revealed by warping of the local bedrock and of a terrace of inferred late pleistocene age of the river barada we estimate that this set of faults became active circa 0 9 ma synchronous with changes to the pattern of faulting previously reported farther southwest in the northern jordan valley vertical slip rates on the bassimeh and damascus faults of 1 1 and 2 8 mm a 1 respectively are thus estimated we also infer that large historical earthquakes previously attributed to left lateral faulting farther west on the dead sea fault zone dsfz probably occurred on this set of reverse faults these faults thus represent a significant hazard to the city of damascus our observations indicate that as much as half of the northward motion of the arabian plate relative to the african plate may be absorbed by crustal shortening within the pfb potentially explaining the low slip rate recently measured geodetically on the northern dsfz in western syria

groundwater geochemistry and composition of hardpans in southwestern australian regolith the mineralogy and geochemistry of pedogenic and groundwater silcretes and ferricretes and associated groundwater geochemistry were investigated for deeply weathered highly kaolinised lateritic regolith in southwesternaustralia the silcretes and ferricretes form subsurface hardpans and impede movement of the highly saline acidic shallow groundwaters in lower valleys of the subdued ancient landscape these valleys are associated with the discharge of groundwater and the formation of saline seeps which host a variety of secondary minerals including amorphous silica opal ct goethite feooh hematite fe 2 o 3 akaganeite feooh jarosite kfe 3 so 4 2 oh 6 barite baso 4 gypsum caso 4 2h 2 o and halite nacl deep groundwater silcretes are commonly composed of silicified in situ lateritic pallid zone materials and saprolite that have developed from granitic rocks the matrix consists of amorphous silica diffused through a porous kaolin groundmass these materials were initially highly porous but the voids are now mostly occupied by amorphous silica and secondary anatase acid groundwater weathering has resulted in the dissolution of most primary silicates including feldspars and micas which has greatly elevated the concentration of dissolved silicon in groundwater isovolumetric weathering of feldspar initially produced highly porous assemblages of fine grained kaolin and minor halloysite which now contain high concentrations of amorphous silica up to about 50 wt occupying most of the pores alkali feldspars alter to kaolin pseudomorphs that are much more porous than vermicular kaolin pseudomorphs replacing mica minerals consequently these pseudomorphs contain much less amorphous silica geochemical modeling using phreeqc enabled prediction of the saturation state of minerals and indicated the dissolution and precipitation reactions occurring in the regolith groundwater environment acid moderately saline groundwaters were in near equilibrium with respect to amorphous silica while near neutral ph saline groundwaters corresponded to undersaturated conditions for extremely acid groundwaters ph 3 5 phreeqc predicted dissolution of kaolin goethite and hematite which is reflected in the elevated concentrations of iron up to 169 mg l and aluminium up to 389 mg l in the groundwater this study is important for the understanding of how groundwater geochemistry influences the precipitation of cementing minerals that form various hardpans in the southwest australian landscape

inferring a thrust related earthquake history from secondary faulting a long rupture record of la laja fault san juan argentina abstract trenches excavated across the surface rupture of the 15 january 1944 m w 7 san juan argentina earthquake show evidence for repeated rupture of la laja fault in the late pleistocene and holocene the 1944 rupture extended for about 7 km striking n35 e and dipping 42 e parallel to neogene bedding with a maximum east side up vertical displacement of 30 cm we interpret nine discrete colluvial wedges each capped by a soil as evidence for nine surface ruptures in the past 32 35 ka our topographic profile of the t3 terrace surface projected across the fault suggests these nine events collectively produced 10 3 m of dip displacement we used radiocarbon and optically stimulated luminescence dating to define the timing of past events yielding an average recurrence interval of 3 9 0 3 ka the average estimate for displacement per event is about 1 1 m compared with the 0 5 m of dip slip that occurred in 1944 using a fault related fold model that infers 28 m of shortening based on the axial surface migration for the t 3 terrace along with the 35 ka age of the terrace yields a shortening rate of 0 8 mm yr by the 1944 source thrust fault which is about 20 of the global positioning system constrained total shortening rate across the precordillera using this shortening rate and an assumed fault dip of 30 implies a fault slip rate of about 1 1 mm yr which combined with the average recurrence interval implies 4 m of average slip per event on the causative fault at depth online material unit descriptions from the trenches stepwise reconstruction from trench 1 and annotated photographs of the la laja region

petrologic evolution of divergent peralkaline magmas from the silent canyon caldera complex southwestern nevada volcanic field the silent canyon volcanic center consists of a buried miocene peralkaline caldera complex and outlying peralkaline lava domes its location has been corroborated by geophysical data and more than 50 drill holes two widespread ash flow sheets the tub spring and overlying grouse canyon members of the miocene belted range tuff were erupted from the caldera complex and have volumes of 60 100 km3 and 200 km3 respectively eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex lava flows and tuffs were also deposited between the two major ash flow sheets rocks of the silent canyon center vary significantly in silica content and peralkalinity the most mafic rocks are precollapse and postcollapse trachytes 65 69 sio2 low silica comendites 69 73 sio2 were erupted as the mafic upper part of the chemically zoned grouse canyon member and as postcollapse lavas the lower part of the grouse canyon member and the underlying rhyolite of split ridge are moderately peralkaline comendite pi is molar ratio na k al is 1 17 1 26 these comendites have major element characteristics and trace element enrichments approaching those of pantellerites the tub spring member by contrast is a weakly peralkaline chemically unzoned silicic comendite 75 76 sio2 ash flow tuff weakly peralkaline silicic comendites pi 1 0 1 1 are the most abundant precaldera lavas postcollapse lavas range from trachyte to silicic comendite some have anomalous light rare earth element lree enrichments silent canyon rocks follow a common petrologic evolution from trachyte to low silica comendite above 73 sio2 compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites these contrasting differentiation paths are shown in the behavior of fe and other transition metals al na k the trace elements ba zr nb and probably f and cl weakly peralkaline silicic comendites show a lree heavy ree crossover in early erupted late erupted rocks moderately peralkaline comendites are enriched in all ree the development of divergent peralkaline magmas toward both pantelleritic and weakly peralkaline compositions is unusual in a single volcanic center

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

eruptive history of the rhyolitic kane springs wash volcanic center nevada the 19 13 km kane springs wash caldera formed 14 ma ago following eruption of the three youngest members of the kane wash tuff two older cooling units of the kane wash tuff are dated 15 7 0 2 ma and 14 7 0 2 ma and apparently erupted from source areas west of the exposed caldera volcanic eruptions from the kane springs wash center began about 14 ma ago with extrusion of trachyte lavas now preserved north of the caldera member v1 of the kane wash tuff overlies the trachytes and is zoned from rhyolite to trachyte in contrast overlying members v2 and v3 are slightly peralkaline rhyolites comendite eruption of these tuffs 14 1 0 2 ma ago initiated caldera collapse following collapse trachyte lavas indistinguishable in age from the ash flow tuffs erupted within the caldera forming a 5 m diameter cumulodome whose interior portions crystallized to syenite this magmatic resurgence formed an extrusive complex within the caldera rather than doming the cauldron block as in classic resurgent calderas metaluminous ferroedenite rhyolite lavas and associated ash flow tuffs then erupted in the moat surrounding the central trachyte syenite complex pyroclastic deposits from the domes were covered by trachyandesite and basalt lava flows about 13 4 ma ago biotiterhyolite domes dated 13 4 0 2 ma continued to erupt in the northeastern moat and appear to be the last silicic eruptions from the kane springs wash magma system a 13 4 ma metaluminous biotite rhyolite dome containing vapor phase topaz overlies the trachyandesite lavas and appears to be the youngest biotite rhyolite dome these intracaldera units filled the moat resulting in a caldera filled with its own eruptive products local basalt flows covered both intracaldera units and outflow sheets between 12 7 and 11 5 ma ago prior to basin and range faulting the dominant volume of the system was trachytic as trachyte lavas erupted both prior to and following caldera collapse and trachyte inclusions occur as scoriaceous bombs at the top of member v2 fractionation of this trachyte magma produced the slightly peralkaline rhyolitic magma that erupted as the kane wash tuff the shift from an anhydrous mineral assemblage in the tuffs to hydrous assemblages in the later intracaldera rhyolites shows that the system was cooling and was possibly more hydrous in its late stages petrographic characteristics of trachyandesite lavas erupted late in the history of the center suggest that they represent basaltic magmas contaminated with silicic material

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

paleomagnetism and rotation constraints for the middle miocene southwestern nevada volcanic field middle miocene rocks of the southwestern nevada volcanic field swnvf lie across the projection of the walker lane belt within the basin and range province and thus provide an interesting opportunity to test for late cenozoic vertical axis rotation paleomagnetic data from individual ash flow sheets document no significant relative vertical axis rotation among localities within central swnvf an area of relatively low stratal tilts and widely spaced faults a time averaged mean paleomagnetic direction d 351 4 i 52 7 95 4 5 calculated from data from numerous separate rock units suggests that the central swnvf underwent minimal counterclockwise vertical axis rotation r 7 1 6 6 with respect to the north american craton no clockwise vertical axis rotation is found to support projection of dextral faults of the walker lane beneath the central swnvf clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field these clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central swnvf block and more strongly extended areas to the south and southwest of the field negligible rotation of 11 45 ma to 13 25 ma tuffs relative to the central swnvf was found at the southeast margin of the field where 90 clockwise rotation at the northwest termination of the las vegas valley shear zone had been postulated any clockwise rotation in this area must predate 13 25 ma and thus dextral shear within this part of the walker lane belt was not synchronous or connected across the southern margin of the field small counterclockwise vertical axis rotation relative to the craton as found for the central swnvf block might be a regional feature in the western great basin

growth of late quaternary folds in southwest santa clara valley san francisco bay area california implications of triggered slip for seismic hazard and earthquake recurrence we combine analysis of geologic and geomorphic data with observations of deformation during the 1989 loma prieta earthquake in northern california to evaluate the contribution of aseismic triggered slip and creep processes to cumulative late cenozoic deformation along the northeastern santa cruz mountains range front deformed late pleistocene alluvial fans and terraces provide evidence for localized late quaternary uplift above range bounding reverse faults within the southwestern santa clara valley adjacent to the range front on the basis of offset of late quaternary surfaces the long term average slip rate on the primary range bounding structure monte vista fault is estimated to be 0 2 mm yr northeast of the range front are several discontinuous northwest trending folds indicated by alignment of late pleistocene alluvial fan apices anomalous stream channel convexities and topographic and vegetation lineaments within a 3 5 km wide northwest trending corridor subsurface geologic and geophysical data support the interpretation that the surface folds are a result of blind reverse faulting along the cascade fault beneath the santa clara valley from stream incision rates we estimate an average uplift rate of 0 2 0 05 mm yr for the cascade fault measurements of triggered and postseismic slip following the 1989 loma prieta earthquake combined with estimates of the average return period for loma prieta type events suggest a long term average deformation rate along the range front of 0 25 0 4 mm yr associated with these aseismic processes this range of values is comparable to rates of late quaternary deformation on the range front faults derived from geologic and geomorphic data and it suggests that growth of the overlying folds is at least partially the result of triggered slip and postseismic creep associated with nearby loma prieta type earthquakes if this inference is correct then the return period for independent events on the range front faults probably is greater than that suggested by the long term average geologic slip rates

northridge earthquake rupture models based on the global positioning system measurements abstract we use global positioning system gps data to study the rupture mechanism of the 1994 northridge earthquake in southern california we include data from 62 observation sites of which two palos verdes and jet propulsion lab are permanent gps geodetic array pgga sites we use a grid search scheme to study the range of single and dual plane uniform and varied slip models consistent with the data we find that in order to fit the geodetic data with a fault model whose primary fault patch is confined to a plane through the aftershocks a secondary fault plane is required above the primary fault plane the moment release of the secondary fault can be as large as 1 9 10 18 n m 14 of the moment release of the primary fault this result implies significant deformation in the shallow crust associated with the mainshock our preferred model has a 14 14 array of dislocation patches on a plane through the main aftershock cluster and a 5 6 array of patches in the hanging wall west of the epicenter we estimate the displacements on the patches by linear inversion with a first order smoothness constraint the estimated displacements on the main fault for this model are confined to a simple region between depths of 5 and 18 km in the interior of the modeled fault surface the mainshock lies at the bottom of the aftershock zone near which about 1 m slip is shown on our modeled fault surface the maximum slip on the fault surface is about 2 2 m located at 34 28 n 118 55 w and 12 4 km at depth the seismic moment release estimate of 1 34 0 15 10 19 n m on the main fault at the 95 confidence is consistent with the estimate from strong motion studies

geochemistry of the volcano hydrothermal system of el chichon volcano chiapas mexico the 1982 eruption of el chichon volcano ejected more than 1 km3 of anhydrite bearing trachyandesite pyroclastic material to form a new 1 km wide and 300 m deep crater and uncovered the upper 500 m of an active volcano hydrothermal system instead of the weak boiling point temperature fumaroles of the former lava dome a vigorously boiling crater spring now discharges 20 kg s of cl rich 15 000 mg kg and sulphur poor 200 mg kg of so4 almost neutral ph up to 6 7 water with an isotopic composition close to that of subduction type magmatic water d 15 18o 6 5 this spring as well as numerous cl free boiling springs discharging a mixture of meteoric water with fumarolic condensates feed the crater lake which compared with values in 1983 is now much more diluted 3000 mg kg of cl vs 24 030 mg kg less acidic ph 2 6 vs 0 56 and contains much lower amounts of s 200 mg kg of so4 vs 3550 mg kg with 34s 0 5 4 2 17 in 1983 agua caliente thermal waters on the southeast slope of the volcano have an outflow rate of approximately 100 kg s of 71 c na ca cl water and are five times more concentrated than before the eruption b r molina unpublished data relative n2 ar and he gas concentrations suggest extensional tectonics for the el chichon volcanic centre the 3he 4he and 4he 20ne ratios in gases from the crater fumaroles 7 3ra 2560 and agua caliente hot springs 5 3ra 44 indicate a strong magmatic contribution however relative concentrations of reactive species are typical of equilibrium in a two phase boiling aquifer sulphur and c isotopic data indicate highly reducing conditions within the system probably associated with the presence of buried vegetation resulting from the 1982 eruption all cl rich waters at el chichon have a common source this water has the appearence of a partially matured magmatic fluid condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in s due to anhydrite precipitation shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits red waters are ca so4 rich and have a negative oxygen isotopic shift probably due to ongoing formation of clay at low temperatures

origin of the matauri bay halloysite deposit northland new zealand at the matauri bay halloysite deposit economically valuable halloysite rich clays are hosted by a sanidine rhyolite dome ar ar dated at 10 1 0 03 ma the rhyolite dome intrudes an older basalt and is overlain by alluvial sediments and a younger basalt 4 0 0 7 ma a blanket like halloysite rich zone is restricted to depths of 10 30 m from the present day erosion surface primary sanidine and plagioclase phenocrysts in rhyolite are completely leached out in the halloysite rich zone but are only partially leached out at greater depth halloysite was formed by hydrolysis and cation leaching of sanidine and plagioclase phenocrysts and groundmass glass in the rhyolite resulting in loss of k ca na and si and enrichment in oh loi 6 10 and al2o3 20 30 relative to least altered rhyolite with 1 8 loi and 14 5 al2o3 oxygen and hydrogen isotope data indicate the halloysite is supergene rather than hydrothermal in origin which is consistent with the absence of pyrite alunite and other acid sulphate type hydrothermal minerals and with the blanket like alteration profile the dominance of halloysite over kaolinite was favoured by water saturated weathering conditions during the late miocene pliocene subtropical weathering regime in northland

active tectonics paleoseismology and seismic hazards of the hollywood fault northern los angeles basin california data from geotechnical boreholes and trenches in combination with geomorphologic mapping indicate that the hollywood fault is an oblique reverse left lateral fault that has undergone at least one surface rupturing earthquake during latest pleistocene to middle or late holocene time geomorphologic observations show that the fault extends for 14 km along the southern edge of the eastern santa monica mountains from the los angeles river westward through downtown hollywood to northwestern beverly hills where the locus of active deformation steps 1 2 km southward along the west beverly hills lineament to the santa monica fault rupture of the entire hollywood fault by itself could produce a m w 6 6 earthquake similar in size to the highly destructive 1994 northridge earthquake but even closer to more densely urbanized areas assuming a 0 35 mm yr minimum fault slip rate consistent with available geologic data we calculate an average maximum recurrence interval for such moderate events of 4000 yr although occurrence of such moderate events is consistent with the elapsed time since the poorly constrained age of the most recent surface rupture the data do not preclude a longer quiescent interval suggestive of larger earthquakes if earthquakes much larger than m w 6 6 occurred in the past we speculate that they may have been generated by the hollywood fault together with other faults in the transverse ranges southern boundary fault system

mapping advanced argillic alteration at cuprite nevada using imaging spectroscopy mineral maps based on airborne visible infrared imaging spectrometer aviris data were used to study late miocene advanced argillic alteration at cuprite nevada distributions of fe bearing minerals clays micas sulfates and carbonates were mapped using the tetracorder spectral shape matching system the al content of white micas increases toward altered areas and near intrusive rocks alunite composition varies from pure k to intimate mixtures of na k endmembers with subpixel occurrences of huangite the ca analogue of alunite intimately mixed na k alunite marks areas of relatively lower alteration temperature whereas co occurring na alunite and dickite may delineate relict hydrothermal conduits the presence of dickite halloysite and well ordered kaolinite but absence of disordered kaolinite is consistent with acidic conditions during hydrothermal alteration partial lichen cover on opal spectrally mimics chalcedony limiting its detection to lichen free areas pods of buddingtonite are remnants of initial quartz adularia smectite alteration thus spectral maps provide a synoptic view of the surface mineralogy and define a previously unrecognized early steam heated hydrothermal event faulting and episodes of hydrothermal alteration at cuprite were intimately linked to upper plate movements above the silver peak lone mountain detachment and growth collapse and resurgence of the nearby stonewall mountain volcanic complex between 8 and 5 ma isotopic dating indicates that hydrothermal activity started at least by 7 61 ma and ended by about 6 2 ma spectral and stable isotope data suggest that cuprite is a late miocene low sulfidation adularia sericite type hot spring deposit overprinted by late stage steam heated advanced argillic alteration formed along the margin of the stonewall mountain caldera