a geochemical perspective and assessment of leakage potential for a mature carbon dioxide enhanced oil recovery project and as a prototype for carbon dioxide sequestration rangely field colorado measurements of co2 and ch4 soil gas concentrations and gas exchange with the atmosphere at a large scale co2 enhanced oil recovery eor operation at rangely colorado united states allowed assessment of the microseepage potential shallow and deep soil gas concentrations and direct transport of co2 and ch4 complemented by carbon isotopes have demonstrated an estimated microseepage rate to the atmosphere of approximately 400 t ch4 yr from the 78 km2 area of the rangely field preliminary estimates of deep sourced co2 losses are in the range of 170 to less than 3800 t yr several holes as much as 9 m deep for nested sampling of gas composition stable carbon isotopic ratios for co2 and ch4 and carbon 14 measurements on co2 indicate that deep sourced co2 microseepage loss was detected methanotrophic oxidation of microseeping ch4 to co2 in soils demonstrates significant contribution to soil gas co2 in high ch4 flux areas necessitating a revision of the estimated direct co2 microseepage rate to less than 170 t yr over the rangely field an evaluation of produced water quality from pre co2 eor to 1999 demonstrates an increase in some parameters particularly of ca2 and hco3 indicating dissolution of ferroan calcite and ferroan dolomite in the weber formation inverse computer modeling suggested carbonate mineral sequestration was possible within the constraints of evolution of produced water quality x ray analyses of well scales however do not support the presence of significant mineral sequestration instead modeling indicates that the bulk of the co2 that has been injected since 1986 is sequestered as dissolved co2

the mw 8 3 illapel earthquake chile preseismic and postseismic activity associated with hydrated slab structures the accumulated stress in subduction zones is discharged with earthquake and aseismic activity the latter is hosted in rheological complex regions characterized by high pore fluid pressure and is often accompanied by repeated earthquakes and earthquake swarms the spatiotemporal analysis of seismic activity can reveal the presence of aseismic transients associated with large earthquakes here we study 20 years of seismicity prior to and after the m w 8 3 earthquake that occurred in a d 2015 in central chile we identified several earthquake swarms before the main shock and repeating aftershocks at the border of the main slip area spatial clustering of the seismic activity shares similar orientation with the main fracture zones observed on the outer rise of the subducting nazca plate our findings suggest that the fracture zones enclosing the rupture are playing a major role in accommodating the pre and post main shock stress evolution we further recognize how fracture regions have acted as barriers to the propagation of large earthquakes in the region

automatic approach for increasing the location accuracy of slow moving landslide endogenous seismicity the apoloc method seismological observations offer valuable insights on the stress strain states the physical mechanisms and the possible precursory signs of activation of various earth surface processes i e volcanoes glaciers and landslides comprehensive catalogues of the endogenous landslide seismicity that is corresponding to seismic sources generated by the unstable slope from either mechanical or hydrological origins should include the typology and an estimate of the source parameters location magnitude of the event these advanced catalogues constitute a strong basis to better describe the slope deformation and its time evolution and better understand the controlling factors because the number of seismic events in landslide catalogues is generally large automatic approaches must be considered for defining both the typology and the location of the sources we propose here a new location approach called automatic picking optimization and location method apoloc for locating landslide endogenous seismic sources from seismological arrays located at close distance the approach is based on the automatic picking of the p waves arrivals by optimizing the intertrace correlations the method is tested on calibration shots realized at the super sauze landslide southeast french alps and compared to other location approaches by using a realistic velocity model obtained from a seismic tomography campaign apoloc reduces the epicentre errors to 23 m on average compared to ca 40 m for the other approaches apoloc is then applied for documenting the endogenous seismicity i e slopequakes and rockfalls at the landslide

using surface waves recorded by a large mesh of three element arrays to detect and locate disparate seismic sources author posting the authors 2018 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 215 2018 942 958 doi 10 1093 gji ggy316

experimental study on water rock interactions during co2 flooding in the tensleep formation wyoming usa abstract the conditions for mineral alteration and formation damage during co2 treatment of tensleep sandstone reservoirs in northern wyoming usa were examined through core flooding laboratory experiments carried out under simulated reservoir conditions 80 c and 166 bars subsurface cores from the tensleep sandstone which were cemented by dolomite and anhydrite and synthetic brines were used the brines used were ca mg na so4 nacl solution 9 69 g l total dissolved solids for run 1 and a 0 25 mol l nacl solution for run 2 the solution used in run 1 was saturated with respect to anhydrite at run conditions which is characteristic of tensleep formation waters three major reactions took place during flooding including 1 dissolution of dolomite 2 alteration of k feldspar to form kaolinite and 3 precipitation in run 1 or dissolution in run 2 of anhydrite all sample solutions remained undersaturated with respect to carbonates the permeability of all the cores except one used in run 2 decreased during the experiments despite the dissolution of authigenic cement kaolinite crystal growth occurring in pore throats likely reduced the permeability application of the experimental results to reservoirs in the tensleep formation indicates that an injection solution will obtain saturation with respect to dolomite and anhydrite in the immediate vicinity of the injection well the injection of nacl type water which can be obtained from other formations causes a greater increase in porosity than the injection of tensleep formation waters because of the dissolution of both dolomite and anhydrite cements

crustal structure of the peninsular ranges batholith from magnetic data implications for gulf of california rifting 1 a 70 km wide belt of magnetic highs extends 1200 km northwest from the southern tip of the baja california peninsula into southern california the anomalies are caused by the mafic western belt of the peninsular ranges batholith which is exposed extensively along the northern 800 km length of the magnetic belt modeling indicates that the source of the anomalies extends to mid to lower crustal depths the linearity and undisrupted nature of the magnetic belt support the hypothesis that the peninsula has behaved as a rigid block and has resisted significant 50 km strike slip deformation during the tertiary the absence of a corresponding magnetic belt along the coast of mainland mexico to the south suggests that the location of rifting in the gulf of california was influenced by the mafic western peninsular ranges batholithic crust perhaps because it is thermally and mechanically more resistant to deformation than the surrounding crust

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

chemical alteration and mineral growth under high pco2 conditions insights from the mineral chemistry of carbonate phases in the caprese reservoir northern apennines central italy abstract carbon cycle is one of the most complex geochemical cycles of the earth as it involves atmosphere hydrosphere biosphere and lithosphere among these different geochemical spheres mechanisms and reaction rates governing migration and fixation of carbon i e fluxes such as the stability of the different carbonates minerals under specific conditions in crustal reservoirs are still matter of detailed studies in this work sem and xrd investigations and emp analyses were combined to study the mineralogical and chemical compositions of carbonate phases occurring in strongly altered triassic volcanic rocks hosting a co 2 reservoir namely the caprese reservoir located in eastern tuscany central italy the caprese crustal fluid reservoir lying at 3300 m depth beneath the quaternary san cassiano basin hosts a supercritical co 2 n 2 rich phase and a na cl rich saline up to 81 5 g l brine the mineralogical assemblage consists of authigenic microcrystalline quartz ca fe mg carbonates na k clay minerals and fe oxides the primary minerals being almost completely chemically altered by secondary processes scanning electron microscope images and cation ca fe mg contents in carbonates highlighted that zoned ankerite was re grown on calcite crystals due to interactions processes with co 2 rich fluids the geochemical and mineralogical data provided useful insights for understanding both the processes that formed the c bearing minerals and their growth in deep saline reservoirs

postseismic variations in seismic moment and recurrence interval of repeating earthquakes in laboratory experiments longer stationary contact time leads to larger seismic moment during repeated ruptures however not all observations in natural fault systems agree with the prediction we analyze a subset of 34 m 0 4 2 1 repeating earthquake sequences res from 1987 to 2009 at parkfield to examine the variation of their recurrence properties in space and time following a 2004 m6 earthquake many of the repeating events have greatly reduced recurrence intervals tr that systematically increase with time in addition to this change in timing we also find systematic changes in seismic moment mo where most sequences experienced an immediate increase in mo and subsequent decay as tr approached pre quake durations the res at shallower depth tend to have a larger range in both tr and mo whereas deeper res show smaller variation the shallowest res with the greatest magnitude m1 8 2 1 among the events we studied reveal a large variation in tr but small variation in mo these observations are qualitatively consistent with earthquake simulations in 3d continuum fault models with rate and state dependent friction in the models res are produced on velocity weakening patches surrounded by velocity strengthening fault areas the models show that the degree of postseismic variation in mo and tr is a function of radius r and nucleation zone size h of the velocity weakening patch a ratio of r h 1 produces negative mo tr slopes whereas larger ratios of r h yield weak positive slopes given the same nucleation size h i e the same frictional properties and effective normal stress smaller radii and hence smaller seismic moments result in negative mo tr slopes whereas larger radii and hence larger moments lead to weak positive mo tr slopes which are consistent with observations conversely with only a small percentage of its slip accumulated seismically a small asperity appears to grow in mo under high loading rate which is contrary to the view that mo should decrease due to a reduced strength recovery time

hydrogeochemical modeling of a thermal system and lessons learned for co2 geologic storage abstract geological storage of carbon dioxide is presently considered to be one of the main strategies to mitigate the impact of the emissions of this gas on global warming among the various alternatives considered for co 2 geological storage one of the main geological candidates for hosting injected co 2 in the long term are deep porous reservoir rock formations saturated with brackish or saline solutions although valuable information on the expected hydrogeochemical processes involved in the co 2 storage in such deep saline aquifers can be obtained in laboratory or modeling studies the only direct source of information about the long term behavior of geological storages for co 2 in deep aquifers is natural analogues in this work a classical and simple geochemical methodology is successfully applied to the study of the features and hydrogeochemical processes determining the evolution of a spanish thermal system the alhama jaraba complex which can be considered as a natural analogue for deep geological co 2 storage in carbonate rocks the geology structure depth and hydrogeochemistry of the alhama jaraba thermal system are very similar to the expected features of a potential co 2 reservoir in carbonate materials the processes determining the hydrogeochemical evolution in the alhama jaraba thermal system have been successfully identified and quantified with the assistance of ion ion plots speciation solubility calculations and mass balance calculations furthermore the feasibility of the proposed conceptual hydrogeochemical model for this system has been verified by using reaction path calculations mass balance calculation results have indicated that the observed hydrogeochemical evolution between springs is mainly due to halite dissolution and dedolomitization triggered by gypsum or anhydrite dissolution co 2 g mass transfer has been estimated to be negligible which suggests that the main processes responsible for the variation in the tic and the co 2 g pressure during deep circulation are dissolution and precipitation reactions for carbonate minerals all the processes identified in the alhama jaraba thermal system are relevant for the long term evolution of a deep co 2 storage site hosted by carbonate rocks as shown in this study the application of classical geochemical tools provides an excellent starting point for understanding the behavior of prospective storage systems moreover the existence of dedolomitization is very relevant for the hydraulic properties of carbonate aquifers potentially used for co 2 geological storage because of the effects on porosity and therefore permeability during the long term evolution of such systems furthermore dedolomitization may represent a mechanism of mineral trapping for co 2 sequestration under certain conditions

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

earthquake source nucleation a physical model for short term precursors abstract ohnaka m 1992 earthquake source nucleation a physical model for short term precursors in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 149 178 this paper deals with a quasistatic leading to a quasidynamic at a later time nucleation process that precedes the earthquake dynamic rupture and models of the earthquake source nucleation are put forward based on physical principles a quasistatic to quasidynamic rupture nucleation process is an intrinsic part of the ensuing earthquake dynamic rupture in other words the nucleation process itself is an short term or immediate precursor that occurs in a localized zone during the earthquake nucleation premonitory slip proceeds in the localized nucleation zone and shear stress also decreases gradually in the zone since slip weakening occurs during the nucleation immediate foreshock activity is a part of the nucleation process leading to the mainshock dynamic rupture therefore hypocentral locations of foreshocks are necessarily restricted to lie near the mainshock hypocentre onset of the mainshock rupture whether or not foreshocks occur during the mainshock nucleation depends on how the rupture growth resistance varies on a local to small scale since patches of greater rupture growth resistance are considered to prevail on a local to small scale in the fault zone carrying immediate foreshocks would be one of the major characteristics of earthquakes that nucleate within the brittle seismogenic layer when a mainshock earthquake nucleates within the brittle seismogenic layer and its hypocentre is located near the base of the seismogenic layer immediate foreshocks for this mainshock are necessarily restricted to lie within a localized region shallower than hypocentral depth of the mainshock by contrast the nucleation process below the base of the seismogenic layer is aseismic in nature so that carrying no conspicuous foreshocks would be a common characteristic of interplate earthquakes that nucleate below the base of the brittle seismogenic layer the breakdown strength p the breakdown stress drop b and the critical slip displacement dc are indicative of the rupture growth resistance to estimate depth variations of these parameters the effects of the normal stress n and temperature t on those parameters are examined using available data so far published the combined effects of n and t on p in the brittle to semibrittle regime are found to be represented empirically by p n t po n 1 cosh 50 t 40 sinh 50 t exp 2000 t where po n 135 7 0 750 n and t is measured in k and n in mpa it is found that dc increases sharply with t but is insensitive to n above 300 c while dc depends on n but is insensitive to t below 300 c on the basis of these results variations in p dc and b at mid crustal depths are estimated for quartzo feldspathic rocks for a given geothermal gradient

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

the impact of diagenetic fluid rock reactions on rotliegend sandstone composition and petrophysical properties altmark area central germany in the framework of the german r d joint project clean co2 large scale enhanced gas recovery in the altmark natural gas field rotliegend reservoir sandstones of the altensalzwedel block in the altmark area saxony anhalt central germany have been studied to characterise litho and diagenetic facies mineral content geochemical composition and petrophysical properties these sands have been deposited in a playa environment dominated by aeolian dunes dry to wet sand flats and fluvial channel fills the sediments exhibit distinct mineralogical geochemical and petrophysical features related to litho and diagenetic facies types in sandstones of the damp to wet sandflats their pristine red colours are preserved and porosity and permeability are only low rocks of the aeolian environment and most of the channel fill deposits are preferentially bleached and exhibit moderate to high porosity and permeability although geochemical element whole rock content in these rocks is very similar element correlations are different variations in porosity and permeability are mainly due to calcite and anhydrite dissolution and differences in clay coatings with fe bearing illitic chloritic composition exposed to the pore space moreover mineral dissolution patterns as well as compositions of clays and carbonate and morphotypes of authigenic minerals chlorite illite are different in red and bleached rocks comparison of the geochemical composition and mineralogical features of diagenetically altered sandstones and samples exposed to co2 bearing fluids in laboratory batch experiments exhibit similar character experiments prove an increase in wettability and water binding capacity during co2 impact

intense foreshocks and a slow slip event preceded the 2014 iquique mw 8 1 earthquake the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 the moment magnitude mw 8 1 iquique earthquake of 1 april 2014 broke a highly coupled portion of this gap to understand the seismicity preceding this event we studied the location and mechanisms of the foreshocks and computed global positioning system gps time series at stations located on shore seismicity off the coast of iquique started to increase in january 2014 after 16 march several mw 6 events occurred near the low coupled zone these events migrated northward for 50 kilometers until the 1 april earthquake occurred on 16 march on shore continuous gps stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

seismic tomography constrained by bouguer gravity anomalies applications in western washington tomographic inversions for velocity variations in western washington indicate a high correlation with surface geology and geophysical measurements including gravity observations by assuming a simple linear relationship between density and velocity birch s law it is possible to calculate the gravity field predicted from the velocity perturbations obtained by local tomographic inversion while the predicted gravity matches observations in parts of the model the overall correlation is not satisfactory in this paper we suggest a method of constraining the tomographic inversion to fit the gravity observations simultaneously with the seismic travel time data the method is shown to work well with synthetic data in 3 dimensions where the assumption of birch s law holds strictly if the sources of the gravity anomalies are assumed to be spatially localized integration can be carried out over a relatively small volume below the observation points and sparse matrix techniques can be applied we have applied the constrained inversion method to western washington using 4 387 shallow earthquakes to depths of 40 0 km 36 865 raypaths convering a 150 250 km region and found that the gravitational constraints may be satisfied with minor effect on the degree of misfit to the seismic data

carbon dioxide injection and resultant alteration of the weber sandstone rangely field colorado 1 geologic interpretations made during the current enhanced oil recovery project at rangely field rio blanco county colorado have shown the relationships between alteration of reservoir mineralogy and texture changes in produced water composition and increased problems with oil production the bottom hole ph of weber brine has decreased from approximately 7 5 to 4 5 since the initiation of co 2 injection into the pennsylvania permian weber sandstone in the central portion of the field much of the weber is partially cemented with carbonate minerals including ferroan calcite and ferroan dolomite with authigenic clay minerals predominately illite and mixed layer smectite illite also present these minerals are affected by the lowered ph of the co 2 flood changes in the chemistry of produced water are associated with alteration of reservoir mineralogy the co 2 flood has caused substantial increases in the concentrations of iron calcium and magnesium in the produced brine from the dissolution of carbonate cements and perhaps authigenic clays and detrital feldspars produced water from some wells has experienced large changes in barium and sulfate concentrations an increase in bromide concentration indicates that previously segregated portions of the reservoir which contained very saline original formation brine have been accessed by the carbonated water hypotheses based on the water chemistry changes were confirmed in core flood experiments these experiments also indicated that there was no net change in permeability following co 2 injection

average stress drops of southern california earthquakes in the context of crustal geophysics implications for fault zone healing to understand how fault healing processes affect earthquake stress drops we search for a possible dependency of stress drops on crustal conditions and geophysical parameters we reanalyze the stress drop values of 60 000 earthquakes in southern california which were originally determined by sheareret al j geophys res 111 b06303 2006 using a spectral method we modify the dataset to include only stress drops that are derived from at least 10 spectra and with corner frequencies between 3 and 30 hz and correct the rupture velocity for increasing s wave speed with depth we see no dependence of stress drop on moment magnitude or depth except for a small poorly determined increase from 15 to 25 km we use six crustal geophysics parameters to search for obvious correlations that may explain changes in the mean values of the stress drops 1 crustal thickness 2 isostatic gravity 3 heat flow 4 shear strain rate 5 crustal stress regime and 6 style of faulting none of the variables reduce the scatter but most can explain up to 10 20 variations in the mean stress drops the geographical distribution of the grouped mean stress drops includes very high stress drops near ridgecrest eastern california as well as near fault jogs within the san andreas fault system low stress drops dominate in trans tensional regions heat flow and gps based shear strain rate estimates have the largest influence on stress drop variations in the range of low to medium heat flow the stress drops increase with increasing heat flow in contrast at high heat flow in thin crust the stress drops decrease systematically with increasing heat flow increasing shear strain rate systematically correlates with decreasing stress drops the crustal stress regime and style of faulting also influence the stress drops as demonstrated by lower stress drops for north northeast trending principal horizontal stress and in areas of dip slip faulting the mean variations in stress drops with heat flow stress regime crustal thickness and density can be explained in the context of fault healing grain boundary growth and corresponding increase in fault zone strength on time scales modulated by the tectonic shear strain rate

detection and assessment of a large and potentially tsunamigenic periglacial landslide in barry arm alaska the retreat of glaciers in response to global warming has the potential to trigger landslides in glaciated regions around the globe landslides that enter fjords or lakes can cause tsunamis which endanger people and infrastructure far from the landslide itself here we document the ongoing movement of an unstable slope total volume of 455 106 m3 in barry arm a fjord in prince william sound alaska the slope moved rapidly between 2010 and 2017 yielding a horizontal displacement of 120 m which is highly correlated with the rapid retreat and thinning of barry glacier should the entire unstable slope collapse at once preliminary tsunami modeling suggests a maximum runup of 300 m near the landslide which may have devastating impacts on local communities our findings highlight the need for interdisciplinary studies of recently deglaciated fjords to refine our understanding of the impact of climate change on landslides and tsunamis

evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation when fault slip velocity is slow 0 001 cm s and dynamic rupture when fault slip rates exceed centimeters per second the application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding v 0 001 cm s to dynamic stick slip v 1 cm s the full range of fault slip modes was achieved by controlling the ratio k k k c where k is the elastic loading stiffness and k c is the fault zone critical rheologic stiffness we show that k controls the transition from slow and silent slip k 0 9 to fast and audible k v 3 cm s slip duration 0 003 s slip events microstructural observations show that with accumulated strain deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains which favor the development of frictional instabilities once this fabric is established fault fabric does not change significantly with slip velocity and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form

carbonate dissolution in mesozoic sand and claystones as a response to co2 exposure at 70 c and 20mpa abstract the response to co2 exposure of a variety of carbonate cemented rocks has been investigated using pressurised batch experiments conducted under simulated reservoir conditions 70 c and 20 mpa and with a durations of up to14 months calcite dolomite ankerite and siderite cement were present in the unreacted reservoir rocks and caprocks core plugs of the reservoir rocks were used in order to investigate the alterations in situ crushing of the caprock samples was necessary to maximise reactions within the relatively short duration of the laboratory experiments synthetic brines were constructed for each batch experiment to match the specific formation water composition known from the reservoir and caprock formations in each well chemical matched synthetic brines proved crucial in order to avoid reactions due to non equilibra of the fluids with the rock samples for example observations of the dissolution of anhydrite which were not associated with the co2 injection but rather caused by mismatched brines carbonate dissolution as a response to co2 injection was confirmed in all batch experiments by both petrographical observations and geochemical changes in the brines increased ca and mg concentrations after 1 month reaction with co2 and crushed caprocks are ascribed to calcite and dolomite dissolution respectively though not verified petrographically ankerite and possible siderite dissolution in the sandstone plugs are observed petrographically after 7 months reaction with co2 and are accompanied by increased fe and mn contents in the reacted fluids clear evidence for calcite dissolution in sandstone plugs is observed petrographically after 14 months of reaction with co2 and is associated with increased amounts of ca and mg in the reacted fluid dolomite in sandstones shows only minor dissolution features which are not clearly supported by increased mg content in the reacted fluid silicate dissolution cannot be demonstrated either by chemical changes in the fluids as si and al concentrations remain below the analytical detection limits nor by petrographical changes as partly dissolved feldspar grains and authigenic analcime are present in the sediments prior to the experiments it is noteworthy that authigenic k feldspar and authigenic albite in sandstones show no signs of dissolution and consequently seem to be stable under the experimental conditions

an experimental study of co2 oil brine rock interaction under in situ reservoir conditions to understand the mineralogical and chemical changes in oil bearing reservoirs e g depleted oil reservoirs during massive co2 injection we have carried out a core flooding experimental study of co2 oil brine rock interactions under a simulated reservoir condition of 100 c and 24 mpa the experimental condition is based on field data from a co2 eor project in the southern songliao basin this oil bearing co2 flooding experiment used the same experimental setup reservoir conditions and workflow as the oil free experiment reported by yu et al 2012 the sandstone core samples used in the experiment have similar mineralogical compositions as that used in the previous experiment compared with the oil free experiment the presence of oil appears to substantially reduce the reaction degree between the co2 fluid and some sensitive minerals the dissolution rates of the k feldspar and carbonate minerals for the oil bearing experiment are 1 5 and 1 4 of that for the oil free experiments respectively for the silicate minerals represented by the k feldspar the presence of oil mainly delays the dissolution during the experiment and reduces the equilibrium dissolution rate for the carbonate minerals the presence of oil appears to primarily affect the dissolution at the beginning of the experiments and reduce the maximum dissolution rate attained the core permeabilities for the oil free and oil bearing cases are both reduced after experiments the reduction in permeability is probably due to the precipitation of fine siliceous mineral and clay particles released by the dissolution of the carbonate cement which may clog some pore throats the results provide some new insights on the fluid rock interaction during co2 injection in depleted oil reservoirs or during co2 eor

geophysical monitoring of moisture induced landslides a review author s whiteley js chambers je uhlemann s wilkinson pb kendall jm abstract 2018 the authors geophysical monitoring of landslides can provide insights into spatial and temporal variations of subsurface properties associated with slope failure recent improvements in equipment data analysis and field operations have led to a significant increase in the use of such techniques in monitoring geophysical methods complement intrusive approaches which sample only a very small proportion of the subsurface and walk over or remotely sensed data which principally provide information only at the ground surface in particular recent studies show that advances in geophysical instrumentation data processing modeling and interpretation in the context of landslide monitoring are significantly improving the characterization of hillslope hydrology and soil and rock hydrology and strength and their dynamics over time this review appraises the state of the art of geophysical monitoring as applied to moisture induced landslides here we focus on technical and practical uses of time lapse methods in geophysics applied to monitoring moisture induced landslide the case studies identified in this review show that several geophysical techniques are currently used in the monitoring of subsurface landslide processes these geophysical contributions to monitoring and predicting the evolution of landslide processes are currently underrealized hence the further integration of multiple parametric and geotechnically coupled geophysical monitoring systems has considerable potential the complementary nature of certain methods to map the distribution of subsurface moisture and elastic moduli will greatly increase the predictive and monitoring capacity of early warning systems in moisture induced landslide settings

comparison of short term seismic precursors and explosion parameters during the 2016 2017 bogoslof eruption in december 2016 the unmonitored bogoslof volcano unexpectedly began erupting producing at least 70 explosions over the next 8 5 months roughly half of the explosions were preceded by observable short term precursory activity defined here as seismic activity ending within 30 min of the explosion onset or an earthquake swarm ending within a few hours before the explosion onset five different types of precursors were noted during the bogoslof eruption non swarm earthquakes earthquake swarms rapid rate events tremor and chromatic tremor most of the precursors that were detectable by the nearby seismic networks 45 km away occurred during the first 3 months of the eruption with nearly all precursory earthquake swarms occurring during this period a hydrophone deployed 7 km northeast of the summit for the last 3 months of the eruption detected weaker activity that was not recorded by the more distant seismic networks we describe the observed precursors and discuss their inter relations overall the precursors seem indicative of processes occurring throughout the volcanic system and suggest possible changes in the system during the eruption we then compare the short term precursors with explosion parameters such as plume height duration and occurrence of lightning to look for relations that may provide more insight into the eruption dynamics and subsurface processes and help to further improve forecasting we find no obvious relations between any of the precursor or explosion parameters suggesting that their use in forecasting eruption characteristics at bogoslof is limited

comparison between crustal density and velocity variations in southern california we predict gravity from a three dimensional v p model of the upper crust and compare it to the observed isostatic residual gravity field in general this comparison shows that the isostatic residual gravity field reflects the density variations in the upper to middle crust both data sets show similar density variations for the upper crust in areas such as the peninsular ranges and the los angeles basin both show similar variations across major faults such as the san andreas and garlock faults in the mojave desert the difference between the two data sets in regions such as the salton trough the eastern california shear zone and the eastern ventura basin where depth to moho is 30 km however suggests high density middle to lower crust beneath these regions hence the joint interpretation of these data sets improves the depth constraints of crustal density variations

seismic and acoustic signatures of surficial mass movements at volcanoes abstract surficial mass movements such as debris avalanches rock falls lahars pyroclastic flows and outburst floods are a dominant hazard at many volcanoes worldwide understanding these processes cataloging their spatio temporal occurrence and detecting tracking and characterizing these events would advance the science of volcano monitoring and help mitigate hazards seismic and acoustic methods show promise for achieving these objectives many surficial mass movements generate observable seismic and acoustic signals and many volcanoes are already monitored significant progress has been made toward understanding modeling and extracting quantitative information from seismic and infrasonic signals generated by surficial mass movements however much work remains in this paper we review the state of the art of the topic covering a range of scales and event types from individual rock falls to sector collapses we consider a full variety of volcanic settings from submarine to subaerial shield volcano to stratovolcano finally we discuss future directions toward operational seismo acoustic monitoring of surficial mass movements at volcanoes

geochemical monitoring for potential environmental impacts of geologic sequestration of co2 carbon dioxide sequestration is now considered an important component of the portfolio of options for reducing greenhouse gas emissions to stabilize their atmospheric levels at values that would limit global temperature increases to the target of 2 c by the end of the century pacala and socolow 2004 ipcc 2005 2007 benson and cook 2005 benson and cole 2008 iea 2012 romanak et al 2013 increased anthropogenic emissions of co2 have raised its atmospheric concentrations from about 280 ppmv during pre industrial times to 400 ppmv today and based on several defined scenarios co2 concentrations are projected to increase to values as high as 1100 ppmv by 2100 white et al 2003 ipcc 2005 2007 eia 2012 global ccs institute 2012 an atmospheric co2 concentration of 450 ppmv is generally the accepted level that is needed to limit global temperature increases to the target of 2 c by the end of the century this temperature limit likely would moderate the adverse effects related to climate change that could include sea level rise from the melting of alpine glaciers and continental ice sheets and from the ocean warming increased frequency and intensity of wildfires floods droughts and tropical storms and changes in the amount timing and distribution of rain snow and runoff ipcc 2007 sundquist et al 2009 iea 2012 rising atmospheric co2 concentrations are also increasing the amount of co2 dissolved in ocean water lowering its ph from 8 1 to 8 0 with potentially disruptive effects on coral reefs plankton and marine ecosystems adams and caldeira 2008 schrag 2009 sundquist et al 2009 sedimentary basins in general and deep saline aquifers in particular are being investigated as possible repositories for the large volumes of

relation between the dissolution rates of single minerals and reservoir rocks in acidified pore waters abstract a series of kinetic experiments has been carried out to investigate the rates of dissolution release of al and si of common sandstone minerals in response to acidification of pore waters ph 3 using an experimental procedure designed to maximise the proportion of solid to fluid and to minimise possible damage from agitation the results have then been compared with those from experiments using disaggregated sandstones from two north sea reservoirs experiments were carried out at 25 c and 80 c and in 0 01 0 1 and 1 m nacl solutions with a ph of 3 hydrochloric acid was used as the source of acidity and rate constants were determined based on both release of al and si mineral dissolution rates were closely comparable to literature values despite the different experimental technique except in the case of smectite where particle aggregation appears to have inhibited reaction the dissolution rates calculated for reservoir sandstones based on their modal mineralogy and surface areas agree within a factor of 2 with the measured vales based on the reaction rates measured here reservoir rocks rich in feldspar illite and or smectite are likely to react most rapidly with acidified pore waters

interpretation of crustal seismic velocities in the san gabriel mojave region southern california the increased concern over seismic hazards in the los angeles region has created interest in the crustal structure and mechanical properties associated with faults like the sierra madre and san andreas this study presents the seismic properties of major lithologies found in the vicinity of the san andreas fault zone in order to correlate with the los angeles region seismic experiment lars e results scientists from the larse group have recently acquired high quality seismic data along three transects thirty nine rock samples were collected along the larse line i which trends ne sw across the central mojave desert and the san gabriel mountains these rock samples represent the following lithologies precambrian gneisses san gabriel anorthosite mesozoic granitic intrusives pelona schist mendenhall granulite gneisses and san andreas fault zone cataclasites the seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure average compressional and shear wave velocities at 150 mpa show a wide range of values the mendenhall gneiss and the san gabriel anorthosite complex have high velocities 6 5 km s and 6 4 km s respectively the pelona schist the mojave granitic intrusives and the san andreas cataclasitic rocks have average velocities of 5 8 km s 5 2 km s and 5 1 km s respectively average densities range from 2320 kg m3 for fault zone cataclasites to 3100 kg m3 for a mendenhall granulite gneiss gneisses and schists show significant seismic anisotropy 5 20 due to preferred mineral orientation while most other rocks are nearly isotropic poisson s ratios range from a high of 0 29 in the san gabriel anorthosite to a low of 0 22 in the san gabriel gneiss comparison of the measured seismic properties with the larse velocity model allows interpretation of crustal structure in terms of rock compositions the upper crust southwest of the san andreas fault contains granitic intrusives gneisses and schists while under the mojave desert the upper crust is dominated by gneisses high velocities under the san gabriel mountains represent either lower crustal granulites or anorthosite the velocity decrease beneath the san andreas fault zone originates from minor amounts of cataclasite or the presence of low velocity lithologies such as pelona schist 1998 elsevier science b v all rights reserved

extended nucleation of the 1999 mw 7 6 izmit earthquake laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture we report here that one of the best recorded large earthquakes to date the 1999 moment magnitude mw 7 6 izmit turkey earthquake was preceded by a seismic signal of long duration that originated from the hypocenter the signal consisted of a succession of repetitive seismic bursts accelerating with time and increased low frequency seismic noise these observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust this slip accelerated slowly initially and then rapidly accelerated in the 2 minutes preceding the earthquake

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

attenuation models qp and qs in three dimensions of the southern california crust inferred fluid saturation at seismogenic depths we analyze high dynamic range waveform spectra to determine t values for both p and s waves from earthquakes in southern california we invert the t values for three dimensional 3 d frequency independent q p and q s regional models of the crust the models have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth and extend from the u s mexico border to the coast ranges in the south and sierra nevada in the north in general qp and qs increase rapidly with depth consistent with crustal densities and velocities the 3 d q p and q s models image the major tectonic structures and to a much lesser extent the thermal structure of the southern california crust the near surface low q p and q s zones coincide with major sedimentary basins such as the san bernardino chino san gabriel valley los angeles ventura and santa maria basins and the salton trough in contrast at shallow depths beneath the peninsular ranges southern mojave desert and southern sierras we image high q p and q s zones which correspond to the dense and high velocity rocks of the mountain ranges several clear transition zones of rapidly varying q p and q s coincide with major late quaternary faults and connect regions of high and low q p and q s at midcrustal depths the q p and q s exhibit modest variation in slightly higher and lower qp or qs zones which is consistent with reported crustal reflectivity in general for the southern california crust q s q p is greater than 1 0 suggesting partially fluid saturated crust a few limited regions of q s q p less than 1 0 correspond to areas mostly outside the major sedimentary basins including areas around the san jacinto fault suggesting a larger reduction in the shear modulus compared to the bulk modulus or almost complete fluid saturation

investigating uncertainties in empirical green s function analysis of earthquake source parameters i use a well recorded earthquake sequence to investigate the uncertainties of earthquake stress drops calculated using an empirical green s function egf approach the earthquakes in the largest m 2 1 repeating sequence targeted by the san andreas observatory at depth safod parkfield california are recorded by multiple borehole stations and have simple sources well constrained stress drops and abundant smaller earthquakes to use as egfs i perform three tests to estimate quantitatively the likely uncertainties to arise in less optimal settings i use egf earthquakes with a range of cross correlation values and separation distances from the main earthquakes the stress drop measurements decrease by a factor of 3 as the quality of the egf assumption decreases a good egf must be located within approximately one source dimension of the large earthquake with high cross correlation i subsample the large number of measurements for the main earthquakes to investigate the expected stress drop uncertainties in studies where fewer stations or egfs are available if only one measurement is available the uncertainties are likely to be at least 50 the uncertainties decrease to 20 with five or more measurements using multiple egfs is a good alternative to multiple stations to investigate the effects of limited frequency bandwidth i recalculate the corner frequencies after progressively decimating the sample rate decreasing the high frequency limit of the bandwidth decreases the estimate of the corner frequency and stress drop the corner frequency may be underestimated if it is within a factor of 3 of the maximum frequency of the signal

crustal structure and tectonics from the los angeles basin to the mojave desert southern california a seismic refraction and low fold reflection survey known as the los angeles region seismic experiment larse was conducted along a transect line 1 extending from seal beach california to the mojave desert crossing the los angeles and san gabriel valley basins and san gabriel mountains the chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern california that have been debated for decades and have important implications for earthquake hazard assessment the results or constraints are as follows 1 the maximum depth of the los angeles basin along line 1 is 8 9 km 2 the deep structure of the sierra madre fault zone in the northern san gabriel valley is as follows the duarte branch of the sierra madre fault zone forms a buried 2 5 km high moderately north dipping buttress between the sedimentary and volcanic rocks of the san gabriel valley and the igneous and metamorphic rocks of the san gabriel mountains for deeper structure see following 3 there are active crustal decollements in southern california at middle crustal depths the sierra madre fault zone appears to sole into a master decollement that terminates northward at the san andreas fault and projects southward beneath the san gabriel valley to the puente hills blind thrust fault 4 the dip and depth extent of the san andreas fault along line 1 dips steeply 83 northward and extends to at least the moho 5 the subsurface lateral extent of the pelona schist in southern california is as follows along line 1 the pelona schist underlies much if not all of the san gabriel mountains south of the san andreas fault to middle crustal depths north of the san andreas fault it is apparently not present along the transect

an autocorrelation method to detect low frequency earthquakes within tremor 1 recent studies have shown that deep tremor in the nankai trough under western shikoku consists of a swarm of low frequency earthquakes lfes that occur as slow shear slip on the down dip extension of the primary seismogenic zone of the plate interface the similarity of tremor in other locations suggests a similar mechanism but the absence of cataloged low frequency earthquakes prevents a similar analysis in this study we develop a method for identifying lfes within tremor the method employs a matched filter algorithm similar to the technique used to infer that tremor in parts of shikoku is comprised of lfes however in this case we do not assume the origin times or locations of any lfes a priori we search for lfes using the running autocorrelation of tremor waveforms for 6 hi net stations in the vicinity of the tremor source time lags showing strong similarity in the autocorrelation represent either repeats or near repeats of lfes within the tremor we test the method on an hour of hi net recordings of tremor and demonstrates that it extracts both known and previously unidentified lfes once identified we cross correlate waveforms to measure relative arrival times and locate the lfes the results are able to explain most of the tremor as a swarm of lfes and the locations of newly identified events appear to fill a gap in the spatial distribution of known lfes this method should allow us to extend the analysis of shelly et al 2007a to parts of the nankai trough in shikoku that have sparse lfe coverage and may also allow us to extend our analysis to other regions that experience deep tremor but where lfes have not yet been identified

dedolomitization as an analogue process for assessing the long term behaviour of a co2 deep geological storage the alicun de las torres thermal system betic cordillera spain abstract carbon dioxide deep geological storage especially in deep saline aquifers is one of the preferred technological options to mitigate the effects of greenhouse gases emissions thus in the last decade studies characterising the behaviour of potential co 2 deep geological storage sites along with thorough safety assessments have been considered essential in order to minimise the risks associated with these sites the study of natural analogues represents the best source of reliable information about the expected hydrogeochemical processes involved in the co 2 storage in such deep saline aquifers in this work a comprehensive study of the hydrogeochemical features and processes taking place at the natural analogue of the alicun de las torres thermal system betic cordillera has been conducted thus the main water co 2 rock interaction processes occurring at the thermal system have been identified quantified and modelled and a principle conclusion is that the hydrogeochemical evolution of the thermal system is controlled by a global dedolomitization process triggered by gypsum dissolution this geochemical process generates a different geochemical environment to that which would result from the exclusive dissolution of carbonates from the deep aquifer which is generally considered as the direct result of co 2 injection in a deep carbonate aquifer therefore discounting of the dedolomitization process in any co 2 deep geological storage may lead to erroneous conclusions this process will also influence the porosity evolution of the co 2 storage formation which is a very relevant parameter when evaluating a reservoir for co 2 storage the geothermometric calculation performed in this work leads to estimate that the thermal water reservoir is located between 650 and 800 m depth which is very close to the minimum required to inject co 2 in a deep geological storage it is clear that the proper characterisation of the features and hydrogeochemical processes taking place at a natural system analogous to a man made deep geological storage will provide useful conceptual semi quantitative and even quantitative information about the processes and consequences that may occur at the artificial storage system

seismic reconstruction of the 2012 palisades rockfall using the analytical solution to lamb s problem abstract we investigate a large rockfall that occurred along the cliffs of the hudson river palisades united states on 12 may 2012 using seismic signals recorded at a distance of about 2 km a rockfall involves a combination of rock detachment acceleration impact and settling motion each of which involves forces that load and unload the earth and generate seismic waves we characterize different phases in the seismograms and associate them with specific stages of the rockfall using the analytical solution to lamb s problem we simulate the seismic wave propagation between the event and seismic station taking into account the elastic properties of the crust in the palisades region the dynamics and the source history of the palisades rockfall are reconstructed by analyzing the characteristics of the seismic signal from the modeled force history we infer that the bulk of the mass detached from about 30 m above the highest part of the riverbank and that lower bounds of the mobilized mass and volume are 3 1 10 4 metric tons and 1 1 10 4 m 3 respectively

uncertainties of geochemical modeling during co2 sequestration applying batch equilibrium calculations one of the uncertainties in the field of carbon dioxide capture and storage ccs is caused by the parameterization of geochemical models the application of geochemical models contributes significantly to calculate the fate of the co2 after its injection the choice of the thermodynamic database used the selection of the secondary mineral assemblage as well as the option to calculate pressure dependent equilibrium constants influence the co2 trapping potential and trapping mechanism scenario analyses were conducted applying a geochemical batch equilibrium model for a virtual co2 injection into a saline keuper aquifer the amount of co2 which could be trapped in the formation water and in the form of carbonates was calculated using the model code phreeqc thereby four thermodynamic datasets were used to calculate the thermodynamic equilibria furthermore the equilibrium constants were re calculated with the code supcrt92 which also applied a pressure correction to the equilibrium constants varying the thermodynamic database caused a range of 61 in the amount of trapped co2 calculated simultaneously the assemblage of secondary minerals was varied and the potential secondary minerals dawsonite and k mica were included in several scenarios the selection of the secondary mineral assemblage caused a range of 74 in the calculated amount of trapped co2 correcting the equilibrium constants with respect to a pressure of 125 bars had an influence of 11 on the amount of trapped co2 this illustrates the need for incorporating sensitivity analyses into reaction pathway modeling

experimental simulation of plagioclase diagenesis at p t conditions of 3 5 km burial depth dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments the diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a bridgeman type pressure vessel the experiments each of two weeks duration simulated pressure temperature conditions approximating 3 5 km burial depth by using a double acting pore fluid reservoir solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz 90 10 wt respectively

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

the chemical dissolution and physical migration of minerals induced during co2 laboratory experiments their relevance for reservoir quality the characterization of the quality and storage capacity of geological underground reservoirs is one of the most important and challenging tasks for the realization of carbon capture and storage ccs projects one approach for such an evaluation is the upscaling of data sets achieved by laboratory co2 batch experiments to field scale sub microscopic petrophysical tomographic and chemical analytical methods were applied to reservoir sandstone samples from the altmark gas field before and after static autoclave batch experiments at reservoir specific conditions to study the relevance of injected co2 on reservoir quality these investigations confirmed that the chemical dissolution of pore filling mineral phases carbonate anhydrite associated with an increased exposure of clay mineral surfaces and the physical detachment and mobilization of such clay fines illite chlorite are most appropriate to modify the quality of storage sites thereby the complex interplay of both processes will affect the porosity and permeability in opposite ways mineral dissolution will enhance the rock porosity and permeability but fine migration can deteriorate the permeability these reactions are realized down to m scale and will affect the fluid rock reactivity of the reservoirs their injectivity and recovery rates during co2 storage operations

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

microslips as precursors of large slip events in the stick slip dynamics of sheared granular layers a discrete element model analysis 1 we investigate the stick slip behavior of a granular system confined and sheared by deformable solid blocks using three dimensional discrete element method simulations our modeling results show that large slip events are preceded by a sequence of small slip events microslips whose occurrence accelerates exponentially before the large slip event onset microslips exhibit energy release several orders of magnitude smaller than the large slip events the microslip event rate is proposed as a measure of slip activity in the granular gouge layer a statistical analysis shows that microslip event rate correlates well with large slip event onset and that variations in it can be used to predict large slip events the emergence of microslips and their duration are found to be controlled by the value of the slipping contact ratio and are therefore related to the jamming unjamming transition of frictional granular packings citation ferdowsi b m griffa r a guyer p a johnson c marone and j carmeliet 2013 microslips as precursors of large slip events in the stick slip dynamics of sheared granular layers a discrete element model analysis geophys res lett 40 doi 10 1002 grl 50813

crustal geophysics and seismicity in southern california the geographical distribution of the 1981 2005 seismicity in southern california forms a 150 km broad zone adjacent to the pacific north america plate boundary ranging from depths of 1 30 km with the bulk of the focal depths in the range of 2 12 km the distribution of the seismicity that includes both mainshock aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust the static parameters include heat flow topography crustal density v p v s ratio hypocentral fault distance and crustal thickness from receiver functions the tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field and the dilatational strain rate field in our analysis we normalize the seismicity relative to the areal density of the range of values of each of the parameters most of the seismicity occurs in areas of average heat flow low to intermediate topography average vp vs and high late quaternary fault density and forms seismogenic zones that extend through the brittle crust the location of late quaternary faults often described as zones of weakness influences the geographical distribution of seismicity more than any other parameter although above or below average crustal properties such as high heat flow thin crust or very low v p v s values exist these properties only influence the spatial distribution of seismicity in a minor way as an example the salton trough area of low topography high heat flow high v p v s high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone also somewhat surprisingly areas of high topography low heat flow low v p v s low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone we determine an empirical relationship between heat flow and crustal thickness to show how the 400 c temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km for crustal thickness ranging from 37 to 43 km the 250 c isotherm forms the base of the seismogenic zone suggesting that seismic faulting in these regions is confined to the top of the upper crust 12 14 km and thus does not accommodate plate motion

creep and slip seismic precursors to the nuugaatsiaq landslide greenland precursory signals to material s failure are predicted by numerical models and observed in laboratory experiments or using field data these precursory signals are a marker of slip acceleration on weak regions such as crustal faults observation of these precursory signals of catastrophic natural events such as earthquakes and landslides is necessary for improving our knowledge about the physics of the nucleation process furthermore observing such precursory signals may help to forecast these catastrophic events or reduce their hazard i report here the observation of seismic precursors to the nuugaatsiaq landslide in greenland time evolution of the detected precursors implies that an aseismic slip event is taking place for hours before the landslide with an exponential increase of slip velocity furthermore time evolution of the precursory signals amplitude sheds light on the evolution of the fault physics during the nucleation process

empirical constraints on magnitude distance relationships for seismically induced submarine tsunamigenic landslides abstract seismically induced submarine landslide tsunamis here called silts attract growing attention due to their unique setting damage potential and short warning time a question of key importance especially for tsunami early warning is how far can an earthquake trigger a tsunamigenic slump here we investigate the relationships between earthquake magnitude and the maximal distance between the seismogenic fault and the tsunamigenic slumps rf and between the earthquake epicenter and the slump re and propose empirical constraints on these magnitude distance m d relations the 49 identified silts are spread in various seismotectonic and bathymetric settings in different seas around the globe we found that the maximal distance of the tsunamigenic submarine landslide from the parent earthquake grows with the magnitude and is bounded by an envelope constrained by mw as low as 4 9 for several kilometers the 1968 liguria coast italy and as far as 250 km for mw 7 7 the 1819 rann of kachchh india higher earthquake magnitudes also produce silts seemingly distances lower than 250 km however it is suspected that the available data is limited and unable to reflect the true nature of silt s generation generalized empirical relations between the earthquake magnitude mw and the maximal distance from the seismogenic fault to its generated silt rf in km are log rf 1 17 0 49 mw r2 0 78 the empirical distance between the earthquake epicenter and the corresponding tsunamigenic slump re in km is log re 0 87 0 45 mw r2 0 83 although many earthquakes within the m d bounding envelope did not produce tsunamis the successful events still enable the delineation of the m d zone vulnerable to tsunami generation which in turn can be applied to tsunami early warning procedures this is particularly important for near coast inland earthquakes as their tsunamigenic potential might be overlooked

an experimental study of co2 brine rock interaction at in situ pressure temperature reservoir conditions abstract a detailed investigation of co 2 brine rock interactions through a core flooding laboratory experiment was carried out under simulated reservoir conditions 100 c and 24 mpa changes in the ionic chemistry of the outlet solution combined with core scanning electron microscopy sem and bulk rock x ray diffraction xrd analysis of the core pre and post experiment reveal new insights into co 2 brine rock interactions minerals such as potassium k feldspar albite calcite and ankerite are variably dissolved after the experiments calcite is the mineral most affected by dissolution followed by ankerite whereas dissolution of feldspar minerals is minimal small amounts of kaolinite and solid phases were generated as a result of the experiment the solid phases are mainly comprised of c o na cl al and si and are presumed to be the transitional products in the formation of carbonate minerals the very low fluid penetration rate in the experiment resulted in significantly reduced contact area between the acidic co 2 fluid and minerals in the cores resulting in low rates of feldspar dissolution core permeability decreased substantially throughout the experiment although core porosity remained unchanged the permeability reduction is the result of precipitation of new mineral phases e g kaolinite and solid phases and potentially also the presence of clay particles released by the dissolution of carbonate cement which have then been transported in the fluid flow path and accumulated at pore throats the results provide new insights into co 2 trapping mechanisms in depleted oil and gas reservoirs and into the potential formation damage that may result from massive injections of co 2 into reservoirs during enhanced oil recovery programs

significance of carbonate buffers in natural waters reacting with supercritical co2 implications for monitoring measuring and verification mmv of geologic carbon sequestration 1 successful geologic sequestration of carbon in deep saline aquifers requires accurate predictive models of rock brine co2 interaction often overlooked in siliciclastic hosted saline reservoirs is the carbonate buffering of the groundwater carbonate minerals are ubiquitous even in siliciclastic host rocks resulting in some carbonate buffering geochemical modeling of rock brine co2 systems often do not accurately predict the geochemical evolution of the system leading to significant doubts in predicting the performance of carbon repositories new data from a simple nacl brine plagioclase hydrothermal experiment tests carbon sequestration in dawsonite and sensitivity to carbonate buffering this is contrasted to a nacl brine siliciclastic rock system containing some initial bicarbonate buffering analogous to most saline aquifer sequestration targets and show that critical errors are caused by incomplete or inaccurate characterization of the in situ geochemistry we provide a methodology that accurately predicts the in situ condition using samples collected from brine rock co2 experiments or well heads in a carbon sequestration monitoring scenario

precursory changes in seismic velocity for the spectrum of earthquake failure modes temporal changes in seismic velocity during the earthquake cycle have the potential to illuminate physical processes associated with fault weakening and connections between the range of fault slip behaviors including slow earthquakes tremor and low frequency earthquakes1 laboratory and theoretical studies predict changes in seismic velocity prior to earthquake failure2 however tectonic faults fail in a spectrum of modes and little is known about precursors for those modes3 here we show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults we systematically altered the stiffness of the loading system to reproduce the transition from slow to fast stick slip and monitored ultrasonic wave speed during frictional sliding we find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure which suggests that real time monitoring of active faults may be a means to detect earthquake precursors

transferability of a calibrated numerical model of rock avalanche run out application to 20 rock avalanches on the nuussuaq peninsula west greenland long run out rock avalanches are one of the most hazardous geomorphic processes and risk assessments of the potential threat they pose are often reliant on numerical modelling of their potential run out distance the development of such models requires a thorough understanding of past flow behaviour inferred from deposits emplaced by previous events despite this few records exist of multiple rock avalanches that occurred in conditions sufficiently consistent to develop a set of more generalised and hence transferrable rules we conduct field and imagery based mapping and use numerical modelling to investigate the emplacement of 20 adjacent rock avalanches on the southern flanks of the nuussuaq peninsula west greenland the rock avalanches run out towards the vaigat strait and are sourced from a range of coastal mountains of relatively uniform geology we calibrate a three dimensional continuum dynamic flow code volcflow with data from a modern well constrained event that occurred at paatuut ad 2000 the best fit model assumes a constant retarding stress with a collisional stress coefficient simulating run out to within 0 3 of that observed this calibration was then used to model the emplacement of deposits from five other neighbouring rock avalanches before simulating the general characteristics of a further 14 rock avalanche deposits on simplified topography our findings illustrate that a single calibration of volcflow can account for the observed deposit morphology of a uniquely large collection of rock avalanche deposits emplaced by a series of events spanning a large volume range although the prevailing approach of tuning models to a specific case may be useful for detailed back analysis of that event we show that more generally applied models even using a single pair of rheological parameters can be used to model potential rock avalanches of varied volumes in a region and therefore to assess the risks that they pose

supercritical carbon dioxide and sulfur in the madison limestone a natural analog in southwest wyoming for geologic carbon sulfur co sequestration article i nfo the madison limestone on the moxa arch southwest wyoming usa contains large volumes 65 95 of supercritical co2 that it has stored naturally for 50 million years this reservoir also contains supercritical h2s aqueous sulfur complexes so4 2 and hs and sulfur bearing minerals anhydrite and pyrite although so2 is not present these sulfur bearing phases are known products of so2 disproportionation in other water rock systems the natural co occurrence of so4 2 s 2 supercritical co2 and brine affords the opportunity to evaluate the fate of a carbon sulfur co sequestration scenario mineralogic data was obtained from drill core and aqueous geochemical data from wells outside and within the current supercritical co2 sulfur brine rock system in addition to dolomite calcite and accessory sulfur bearing minerals the madison limestone contains accessory quartz and the aluminum bearing minerals feldspar illite and analcime dawsonite naalco3 oh 2 predicted as an important carbon sink in sequestration modeling studies is not present after confirming equilibrium conditions for the madison limestone system reaction path models were constructed with initial conditions based on data from outside the reservoir addition of supercritical co2 to the madison limestone was simulated and the results compared to data from inside the reservoir the model accurately predicts the observed mineralogy and captures the fundamental changes expected in a madison limestone brine system into which co2 is added ph decreases from 5 7 to 4 5 at 90 c and to 4 0 at 110 c as expected from dissolution of supercritical co2 creation of carbonic acid and buffering by the carbonate rock the calculated redox potential increases by 0 1 v at 90 c and 0 15 v at 110 c due to equilibrium among co2 anhydrite and pyrite final calculated eh and ph match conditions for the co existing sulfur phases present in produced waters and core from within the reservoir total dissolved solids increase with reaction progress mostly due to dissolution of calcite with an accompanying increase in dissolved bicarbonate the madison limestone is a natural example of the thermodynamic end point that similar fluid rock systems will develop following emplacement of a supercritical co2 sulfur mixture and is a natural analog for geologic carbon sulfur co sequestration

rapid porosity and permeability changes of calcareous sandstone due to co2 enriched brine injection reservoir injectivity and storage capacity are the main constraints for geologic co2 sequestration subject to safety and economic considerations brine acidification following co2 dissolution leads to fluid rock interactions that alter porosity and permeability thereby affecting reservoir storage capacity and injectivity thus we determined how efficiently co2 enriched brines could dissolve calcite in sandstone cores and how this affects the petrophysical properties during computerized tomography monitored flow through reactor experiments calcite dissolved at a rate largely determined by the rate of acid supply even at high flow velocities which would be typical near an injection well the porosity increase was accompanied by a significant increase in rock permeability larger than that predicted using classical porosity permeability models this chemically driven petrophysical change might be optimized using injection parameters to maximize injectivity and storage

upper crustal structure from the santa monica mountains to the sierra nevada southern california tomographic results from the los angeles regional seismic experiment phase ii larse ii in 1999 the u s geological survey and the southern california earthquake center scec collected refraction and low fold reflection data along a 150 km long corridor extending from the santa monica mountains northward to the sierra nevada this profile was part of the second phase of the los angeles region seismic experiment larse ii chief imaging targets included sedimentary basins beneath the san fernando and santa clarita valleys and the deep structure of major faults along the transect including causative faults for the 1971 m 6 7 san fernando and 1994 m 6 7 northridge earthquakes the san gabriel fault and the san andreas fault tomographic modeling of first arrivals using the methods of hole 1992 and lutter et al 1999 produces velocity models that are similar to each other and are well resolved to depths of 5 7 5 km these models together with oil test well data and independent forward modeling of larse ii refraction data suggest that regions of relatively low velocity and high velocity gradient in the san fernando valley and the northern santa clarita valley north of the san gabriel fault correspond to cenozoic sedimentary basin fill and reach maximum depths along the profile of 4 3 km and 3 km respectively the antelope valley within the western mojave desert is also underlain by low velocity high gradient sedimentary fill to an interpreted maximum depth of 2 4 km below depths of 2 km velocities of basement rocks in the santa monica mountains and the central transverse ranges vary between 5 5 and 6 0 km sec but in the mojave desert basement rocks vary in velocity between 5 25 and 6 25 km sec the san andreas fault separates differing velocity structures of the central transverse ranges and mojave desert a weak low velocity zone is centered approximately on the north dipping aftershock zone of the 1971 san fernando earthquake and possibly along the deep projection of the san gabriel fault modeling of gravity data using densities inferred from the velocity model indicates that different velocity density relationships hold for both sedimentary and basement rocks as one crosses the san andreas fault the larse ii velocity model can now be used to improve the scec community velocity model which is used to calculate seismic amplitudes for large scenario earthquakes