evidence from precise earthquake hypocenters for segmentation of the san andreas fault in san gorgonio pass we use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike slip fault at depth connecting the san bernardino strand of the san andreas fault with the coachella valley segment of the banning fault we inverted 560 000 arrival times from 23 000 earthquakes 1981 1993 for high quality hypocenters and three dimensional p wave velocity structure in a 1 by 2 area centered on the san gorgonio pass cross sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters the step riser defines a near vertical locally curved surface that extends westerly more than 60 km from the coachella valley segment of the san andreas fault to the san jacinto fault a hypothetical continuous vertical san andreas fault through san gorgonio pass would cross the step at an oblique angle we suggest that the step is the expression of the contact between different basement rock types juxtaposed by large scale right slip motion on the ancestral san andreas fault south of the step in peninsular ranges type basement intrusives brittle failure occurs down to about 20 km depth while north of the step in san bernardino type basement pelona schist brittle failure occurs to only about 13 km depth the step provides a piercing plane that should be offset about 3 km right laterally by an active continuous vertical san andreas fault within the resolution of our mapping the step is not offset in this manner implying either that there has not been a throughgoing vertical fault at depth that a throughgoing fault has not experienced enough slip to offset the step or that a throughgoing fault is not vertical and dips north over the top of the step hypocentral patterns and focal mechanisms indicate distributed deformation thrust normal and strike slip faulting over a large volume in the san gorgonio pass region there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing continuous near vertical san andreas fault in summary we find no evidence indicating a continuous fault at seismogenic depth connecting the san bernardino strand and coachella valley segment of the san andreas fault zone we speculate that this is because the 3 km of right slip on the san bernardino stand of the san andreas fault and coachella valley segment of the banning fault has not been sufficient to form a single new structure through the 15 to 20 km gap between the two previously unconnected segments this implies that large earthquake rupture on the san andreas fault may be inhibited from propagating through san gorgonio pass thus limiting the maximum magnitudes on the southern san andreas fault

microbial fabrics from neogene cold seep carbonates northern apennine italy abstract a number of hydrocarbon seep carbonate masses are hosted within miocene and pliocene siliciclastic units along the apennine range and sicily and are mainly composed of micritic and microsparitic textures geomicrobiological analysis of some of these deposits shows that preserved microbial processes are few and the contribution of microbe derived textures to their construction is volumetrically very limited in the carbonate groundmass of two northern apennine miocene and pliocene seep carbonate deposits uncommon and well preserved features suggest direct and indirect evidence of microbial activity some of these features are interpreted as the mineralization products of microbial textures and include complex alveolar textures and fossilized biofilms that line and fill microconduits pores and other voids similar to living bacterial aggregations recovered in modern seep carbonates other mineral structures such as spheroids and rhombs with concentric fabrics composed of calcite dolomite alternations abound as cement components and suggest a nucleation via bacterial sulfate reduction processes a similar origin is also assumed for the acicular aragonite bundles that in the miocene seep carbonate mass enclose dense clusters of millimeter size filamentous morphologies of biological origin

accelerated stress buildup on the southern san andreas fault and surrounding regions caused by mojave desert earthquakes a sequence of four mw 6 earthquakes including the 1992 mw 5 7 3 landers and mw 5 7 1 hector mine earthquakes occurred in the mojave desert in the 1990s in close proximity to the southern san andreas fault inducing stress changes on several of its segments we calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and or upper mantle has led to a coulomb stress increase of 2 3 3 5 bar on the san bernardino mountain segment of the southern san andreas fault between 1992 and 2001 with a projected increase of 3 6 4 9 bar by the year 2020 in comparison the calculated coseismic stress increase is 1 8 bar for this segment this accelerated buildup of stresses is predicted to bring the san bernardino mountain segment which last ruptured more than 190 yr ago closer to a potentially major rupture meanwhile we project a net stress decrease of as much as 23 5 bar between 1992 and 2020 for the western coachella valley segment if the fault is governed by low effective friction or an increase of 1 5 bar if the fault is governed by high effective friction coulomb stresses are calculated to decrease on the mojave segment by as much as 21 bar between 1992 and 2020 accelerated stress buildup is also predicted to occur on parts of the san jacinto elsinore and calico faults the pattern of the observed post landers aftershock clustering and the calculated coulomb stress buildup on the calico fault is similar to that noted in the hector mine region prior to the 1999 mw 5 7 1 earthquake these results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

quaternary landscape evolution in the san jacinto fault zone peninsular ranges of southern california transient response to strike slip fault initiation well constrained case studies of transient landscape response to external forcing are needed to improve our understanding of erosion processes in tectonically active mountain belts the peninsular ranges portion of the san jacinto fault zone sjfz is an excellent location for such a study because it displays pronounced geomorphic disequilibrium resulting from initiation of a major strike slip fault in the past 1 0 to 2 5 million years we recognize two geomorphic domains in this region 1 a relict low relief upland domain consisting of broad flat valleys and low gradient streams and 2 very steep rough topography with deeply incised canyons and retreating erosional knickpoints pleistocene sediments exposed along and near the sjfz include fluvial conglomerate sandstone and mudstone with weak paleosols and west to nw directed paleocurrents these sediments accumulated in a lowgradient stream system represented by domain 1 during an early phase of slip in the sjfz prior to the modern phase of erosion and degradation domain 2 late pliocene or early pleistocene initiation of the sjfz triggered a wave of headward erosion and stream capture that is still migrating nw along the fault zone using the total distance that capture points have migrated along the fault zone and a range of possible ages for fault initiation the rate of knickpoint retreat is estimated at 12 to 44 km my to explore the signal of transient geomorphic response to fault initiation we analyzed 23 tributaries along an 20 km portion of the main fault valley within domain 2 the analysis reveals three zones with distinctive morphologies 1 strongly convex longitudinal profiles in the nw 2 a large ca 5 6 km 2 landslide in the central zone and 3 concave tributaries in the se with profile complexity decreasing and catchment area increasing from nw to se the distribution of these zones suggests close spatial and temporal association of active fault slip bedrock incision deep seated landslides and erosional modification the fundamental driving force behind these processes is profound geomorphic disequilibrium resulting from initiation of the sjfz we suggest that landslides may have played a significant role in shaping the morphology of this fault zone and that the influence of landslides may be underestimated in areas where characteristic landforms and deposits are obscured by later erosion and faulting d 2005 elsevier b v all rights reserved

miocene chemoherms of the northern apennines italy in the miocene of the northern apennines methane derived limestones from various geologic settings represent important examples of fossil chemoherms and provide a basis for more accurate interpretations of seepage systems the systematic study of fossil chemoherms has allowed the definition of new field and compositional criteria for the recognition of these deposits which were previously based almost exclusively on negative carbon isotope composition and peculiar chemosynthetic communities this has constrained the relations between seepage and sedimentary and tectonic instability processes we suggest that the peculiar brecciated structures associated with fossil chemoherms are related to diapirism of overpressured pelitic sediments due to fluid venting fluidization of sediments increases the instability of mudstones in which chemoherms are contained thus favoring gravity slumping processes and the reworking of many chemoherms

characterization of the san jacinto fault zone near anza california by fault zone trapped waves we installed three 350 m long seismic arrays each array consisting of 12 three component stations across the coyote creek fault ccf clark valley fault cvf and buck ridge fault brf of the san jacinto fault zone sjfz near anza california to record fault zone trapped waves from microearthquakes we observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone the coda normalized amplitude spectra of trapped waves showed peaks at 4 7 hz which decreased sharply with the distance from the fault trace observations and three dimensional finite difference simulations of trapped waves revealed low velocity and low q waveguides on these active faults with the width of 75 100 m in which shear velocities are reduced by 25 30 from wall rock velocities and q values are 40 90 at depths between the surface and 18 km the locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the sjfz in a region with complicated slip planes near anza the low velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities the waveguide on the brf dips south westward to connect the waveguide on the cvf which dips northeastward this waveguide extends at the seismogenic depth through anza slip gap to another low velocity waveguide on the casa loma fault clf which has been delineated in our previous study of the sjfz using trapped waves li et al 1997 the waveguide on the ccf in coyote mountain is nearly vertical and disconnected from the clf at the south edge of anza gap we interpret the low velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the anza region

small scale stress heterogeneity in the anza seismic gap southern california focal mechanism inversions reveal significant lateral variations in stress orientations along the anza segment of the san jacinto fault zone the most notable stress anomaly is within the 20 km aseismic seismic gap portion of the fault zone where 1 the maximum compressive stress is nearly horizontal and is oriented at 74 13 relative to the fault strike this contrasts with orientations ranging from 62 11 to 49 7 along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap regional stress results found by inverting all focal mechanisms simultaneously indicate that 1 is horizontal and trends north south while 3 is horizontal and trends east west approximately 15 km west of the seismic gap in the off fault cahuilla swarm area 1 and 3 solutions are rotated clockwise by about 25 relative to the regional model roughly 10 km southeast of the seismic gap near the buck ridge fault 1 and 3 are rotated counterclockwise by about 10 relative to the regional solution northwest of the seismic gap along the fault zone 3 plunges about 30 from the horizontal correlating with a local increase in reverse faulting between the hot springs and san jacinto faults southeast of the seismic gap 1 plunges about 45 from the horizontal correlating with a local increase in normal faulting in the trifurcation region of the buck ridge clark and coyote creek faults we propose a simple mechanical model in which a block rotation superimposed on the dominant right lateral strikeslip motion of the fault zone satisfies the first order observations of stress orientation faulting and horizontal surface strain under this model the anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap

the detection and interpretation of hydrogen in fault gases hydrogen gas can be released by chemical and mechanical changes in crustal rocks once released it is highly mobile buoyant and almost insoluble in groundwater a fault system may act as a conduit allowing hydrogen to accumulate in soil gases near a surface expression since hydrogen is scarce in ambient air its presence at elevated levels in soil gases may be a tool for fault mapping in order to evaluate this tool we surveyed eleven different faults by measuring the concentration of hydrogen and methane in 2 to 21 soil gas samples that were collected near each of them the sense of motion at four of those faults is normal western united states greece at five it is strike slip or dip slip california colorado japan and at two it is thrusting california at four of these faults hebgen lake yellowstone yamasaki burro mountain maximum concentrations of hydrogen ranged from 80 ppm to 70 and methane from 300 ppm to 5 all other sites showed ambient levels of both gases except for one sample taken at mt borah idaho that was 2 methane from this preliminary study it is not clear whether the presence of hydrogen is correlated uniquely to the location of faults or whether it occurs randomly the conditions required to produced and accumulate hydrogen are also not clear excess hydrogen may well be produced by different mechanisms in different geological regimes for example if ferrous hydroxide is present in local rocks it may react to produce hydrogen detailed and extensive studies are needed to clarify the connection between hydrogen and tectonic faulting

the central and southern elsinore fault zone southern california we examine hypocenter patterns geomorphic features total slip determinations and slip rate information to define the active elements of the southern and central portions of the right lateral elsinore fault zone in southern california in addition to the elsinore fault the seismicity patterns define a continuous fault between about 33 n and 33 5 n 7 to 12 km east of the elsinore fault that corresponds to the aguanga agua tibia and earthquake valley faults this part of the fault zone is double stranded total slip and current slip rate are apparently distributed between the two strands slip transfer between the strands occurs at the ends of the eastern strand over two steps with topographic signatures reflecting the step geometries in a right lateral fault zone a contractional left step occupied by palomar mountain and an extensional right step within the vallecito fish creek basin we suggest that the two strands of the double stranded portion of the fault zone have different ages and the proportion of slip carried by each strand may have changed through time hypocenter patterns define several northeast striking faults within the fault zone we attribute a 1940 ml 5 1 earthquake to one of them

plate boundary segmentation by stress directions southern san andreas fault california we propose a new method for defining segmentation of plate boundaries and faults based on the directions of the stress tensor estimates for these directions are obtained by minimizing the average misfit between the theoretical and observed slip directions on fault planes of earthquake focal mechanisms the misfit f for an individual earthquake is the parameter we use for defining the segmentation of plate boundaries we hypothesize that the stress directions along plate boundaries and faults are uniform within segments but different from other segments if this is true a cumulative plot of f as a function of space along strike will show constant but different slopes for each segment the significance of the difference between segment slopes can be estimated by the standard deviate z test applying this method to the san andreas fault from the carrizo plains to its southern end we identify quantitatively the same four boundaries between segments as proposed based on non quantitative tectonic considerations plus one additional segment boundary we interpret the relatively uniform but segmented distribution of stress directions as due to the changing strike and possibly changing fault surface properties whether great earthquake ruptures or their major asperities may terminate at segment boundaries should be determined along faults that recently generated large earthquakes this method of defining fault segmentation also allows identification of volumes with uniform stress directions suitable for inversion for stress orientations with a minimum of computing time and finally the method affords an alternative estimate of the significance of differences in stress directions

a 21 event 4 000 year history of surface ruptures in the anza seismic gap san jacinto fault and implications for long term earthquake production on a major plate boundary fault paleoseismic work completed at hog lake on the san jacinto fault sjf near anza california indicates that at least 21 surface ruptures have occurred in the anza seismic gap over the past 4 000 years the ages of the ruptures are constrained by 111 radiocarbon dates 97 of which fall in stratigraphic order the average recurrence interval for all ruptures for this period is about 185 105 years although some ruptures such as occurred in the april 1918 earthquake caused only minor displacement we rate the expression of each interpreted event in each of the twelve developed field exposures presented in this work by assigning numeric values for the presence of different criteria that indicate rupture to a paleo ground surface weakly expressed ruptures for example the deformation we interpret to be the result of the historical 1918 earthquake received low scores and are interpreted as smaller earthquakes from this analysis we infer that at least fifteen of the identified ruptures are indicative of large earthquakes similar to the penultimate earthquake inferred to be the m w 7 3 22 november 1800 earthquake the adjusted recurrence interval for large earthquakes lengthens to approximately 254 years comparison with the rupture history at the mystic lake paleoseismic site on the claremont strand indicates that it is plausible that several of the large ruptures identified at hog lake could have jumped the hemet step over at mystic lake and continued on the claremont strand or vice versa but most of the event ages do not match between the two sites indicating that most ruptures do not jump the step finally comparison with san andreas fault ruptures both to the north and south of its juncture with the sjf suggest that some northern sjf ruptures identified at mystic lake may correlate with events identified at wrightwood but that these northern ruptures have no match at hog lake and can not indicate rupture of the entire sjf onto the saf

geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the san jacinto fault zone southern california we examine the role of preexisting crustal structure within the peninsular ranges batholith on determining the location of the san jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data a 1000 km long boundary within the peninsular ranges batholith separating relatively mafic dense and magnetic rocks of the western peninsular ranges batholith from the more felsic less dense and weakly magnetic rocks of the eastern peninsular ranges batholith strikes north northwest toward the san jacinto fault zone modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km the anomalies do not cross the san jacinto fault zone but instead trend northwesterly and coincide with the fault zone a 75 km long gradient in initial strontium ratios sr i in the eastern peninsular ranges batholith coincides with the san jacinto fault zone here rocks east of the fault are characterized by sr i greater than 0 706 indicating a source of largely continental crust sedimentary materials or different lithosphere we argue that the physical property contrast produced by the peninsular ranges batholith boundary provided a mechanically favorable path for the san jacinto fault zone bypassing the san gorgonio structural knot as slip was transferred from the san andreas fault 1 0 1 5 ma two historical m6 7 earthquakes may have nucleated along the peninsular ranges batholith discontinuity in san jacinto valley suggesting that peninsular ranges batholith crustal structure may continue to affect how strain is accommodated along the san jacinto fault zone

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

effects of biological activity on the entrainment of marine sediments nowell a r m jumars p a and eckman j e 1981 effects of biological activity on the entrainment of marine sediments mar geol 42 133 153 the effects of animal tracks and fecal pellet production on the critical entrainment velocity of marine sediments were examined experimentally laboratory measurements in a free surface seawater flume were made using three sediment sizes effects of three species of polychaetes and two species of bivalves were tested boundary shear velocity was calculated from the mean velocity profile in the logarithmic region of the boundary layer measurements were made with a hot film anemometer tracking doubled the boundary roughness and decreased the critical entrainment velocity by 20 ambient or free sediments were more easily entrained than fecal mounds which were restrained from movement by mucous adhesion between the fecal coils isolated pellets such as those egested by amphicteis scaphobranchiata transported readily as bedload over a cohesive sediment surface

delineation of intrabasin structure in a dilational jog of the san jacinto fault zone southern california three high resolution seismic reflection lines were acquired in the northern part of the san jacinto graben the graben a pull apart basin formed by a dilational right step of the san jacinto fault zone has been previously interpreted as a simple rhombochasm the reflection survey located at least one significant and previously unidentified intrabasin fault referred to here as the farm road strand this fault lies approximately halfway between the claremont and casa loma strands of the san jacinto fault zone at the north end of the basin the southwestern boundary of the graben is interpreted to be the newly identified farm road strand and not the casa loma strand as was previously thought the identification of this intrabasin fault allows us to infer that the san jacinto basin comprises coalescing subbasins and is not a simple pull apart basin with an unusually large length width ratio the distances between the en echelon casa loma farm road and claremont strands are between 1 and 2 km this close spacing would likely permit an earthquake rupture to jump between strands and thus propagate through the san jacinto basin

infrequent triggering of tremor along the san jacinto fault near anza california we examine the conditions necessary to trigger tremor along the san jacinto fault sjf near anza california where previous studies suggest triggered tremor occurs but observations are sparse we investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near anza california we examine 44 mw 7 4 teleseismic events between 2001 and 2011 these events occur at a wide range of back azimuths and hypocentral distances in addition we included one smaller magnitude regional event the 2009 mw 6 5 gulf of california earthquake because it induced extremely high strains at anza we find the only episode of triggered tremor occurred during the 3 november 2002 mw 7 8 denali earthquake the tremor episode lasted 300 s was composed of 12 tremor bursts and was located along sjf at the northwestern edge of the anza gap at approximately 13 km depth the tremor episode started at the love wave arrival when surface wave particle motions are primarily in the transverse direction we find that the denali earthquake induced the second highest stress 35 kpa among the 44 teleseismic events and 1 regional event the dominant period of the denali surface wave was 22 8 s at the lower end of the range observed for all events 20 40 s similar to periods shown to trigger tremor in other locations the surface waves from the 2009 mw 6 5 gulf of california earthquake had the highest observed strain yet a much shorter dominant period of 10 s and did not trigger tremor this result suggests that not only the amplitude of the induced strain but also the period of the incoming surface wave may control triggering of tremors near anza in addition we find that the transient shear stress 17 35 kpa required to trigger tremor along the sjf at anza is distinctly higher than what has been reported for the well studied san andreas fault

structural properties of the san jacinto fault zone at blackburn saddle from seismic data of a dense linear array we image the san jacinto fault zone at blackburn saddle using earthquake waveforms recorded by a 2 km across fault linear array with 108 three component sensors separated by 10 30 m the length and spatiotemporal sampling of the array allow us to derive high resolution information on the internal fault zone structure with spatial extent that can be merged with regional tomography models across fault variations in polarization amplitude and arrival time of teleseismic p waves indicate abrupt changes in subsurface structure near the surface trace of the fault sensor bs55 and 270 m to the northeast sensor bs34 analysis of fault zone head waves from local events reveals the existence of a deep bimaterial interface that extends from the array to at least 50 km southeast and has a section with 10 velocity contrast this analysis also corroborates the teleseismic results and indicates a broad damage zone primarily northeast of the fault bounded by a shallow bimaterial interface near bs34 that merges with the deep interface detection and waveform inversions of love type fault zone trapped waves generated by local events indicate a trapping structure within the broader damage zone with width of 150 m velocity reduction of 55 from the surrounding rock and depth extent of 2 km the performed analyses provide consistent results on the subsurface location of the main seismogenic fault and properties of a major bimaterial interface and damage structure the imaged fault zone properties are consistent with preferred propagation direction of earthquake ruptures in the area to the northwest

segmentation of the northern san jacinto fault zone southern california accurate forecasting of large earthquakes in the san jacinto fault zone depends on the correct determination of fault segmentation we have searched for discontinuities in the fault zone using as data geological maps of the fault traces relocated earthquake hypocenters focal mechanisms slip rates and historic large earthquake rupture zones we identify nine principal discontinuities in the northern san jacinto fault zone these are characterized as structural discontinuities bends steps branches intersections geodetic discontinuities slip rate and lithologic discontinuities most of the discontinuities have more than one defining characteristic we also analyzed the local strain field orientation along the fault zone significant lateral discontinuities in the strain field orientation are coincident with many of the structural and geodetic discontinuities we identify segment lengths defined from the data vary from about 7 to 35 km this range of segment lengths is consistent with the lengths of large historical earthquake ruptures however evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rupture multiple adjacent segments in these cases maximum magnitudes may be larger than those observed historically because of the short historic record and the sparse paleoseismic record probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments ours and previous delineations of segment lengths have varied considerably indicating that assumed segment length is a major source of uncertainty in the recurrence calculations this source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates

deep creep as a cause for the excess seismicity along the san jacinto fault earthquakes are far more common along the san jacinto fault in california than the nearby southern san andreas fault an analysis of seismic and geodetic data suggests that the excess seismicity along the san jacinto fault is due to a creeping motion along the deeper parts of the fault

shallow seismic trapping structure in the san jacinto fault zone near anza california summary we analyse fault zone trapped waves generated by 500 small earthquakes for high resolution imaging of the subsurface structure of the coyote creek clark valley and buck ridge branches of the san jacinto fault zone near anza california based on a small number of selected trapped waves within this data set a previous study concluded on the existence of a lowvelocity waveguide that is continuous to a depth of 15 20 km in contrast our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3 5 km this is based on the following lines of evidence 1 earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones 2 a traveltime analysis of the difference between the direct s arrivals and trapped wave groups shows no systematic increase moveout with increasing hypocentral distance or event depth estimates based on the observed average moveout values indicate that the propagation distances within the low velocity fault zone layers are 3 5 km 3 quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low velocity fault zone layers the results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones the waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch this may result from a preferred propagation direction of large earthquake ruptures on the san jacinto fault

paleoseismic evidence for the 21 april 1918 mw 6 9 surface rupture of the northern clark strand of the central san jacinto fault california abstract in this study we present the results of a new paleoseismic investigation in the south fork wilderness 7 km southeast of blackburn canyon 5 km northwest of hog lake near anza california that demonstrate that the northern clark fault has ruptured twice in the past 300 yrs these new results corroborate previously made interpretations that were based solely on surficial geomorphology that the northern clark fault with a recurrence interval of 100 yrs ruptured in both the november 1800 and april 1918 earthquakes and has a more frequent recurrence interval than the central and southern clark fault

straightening of the northern san jacinto fault california as seen in the fault structure evolution of the san jacinto valley stepover abstract we investigate a releasing stepover between the casa loma and claremont strands of the northern san jacinto fault zone to evaluate the late quaternary structural evolution of the fault zone and to assess the likelihood of a rupture jumping across the stepover our new cone penetration test cpt and trench observations along the claremont fault at mystic lake indicate that the main strand of the claremont fault has jumped nearly a half kilometer westward into the san jacinto releasing stepover during the late quaternary multiple faults are inferred from the cpt data within a small sag at the northeast side of the stepover that cuts through younger stratigraphy to the west of the basin bounding fault near mystic lake previous seismic reflection data also suggest the presence of a young fault that cuts basin strata beneath the middle of mystic lake farther west of our study area numerous tectono geomorphic features observed in satellite and light detection and ranging digital elevation model lidar dem imagery are interpreted to delineate the location of the currently active faults including a zone of faults that cut across the basin from the northern end of the casa loma fault to the claremont fault seismicity observations suggest the presence of many faults within the stepover zone finally new paleoseismic data from the mystic lake site suggest that some late holocene earthquakes may have jumped the stepover all of these observations suggest that the san jacinto stepover which has been used as the primary basis for segmenting the northern san jacinto fault zone is being bypassed and that the fault zone may now be capable of larger earthquakes than previously expected

san jacinto fault zone guided waves a discrimination for recently active fault strands near anza california we deployed three 350 m long eight element linear seismic arrays in the san jacinto fault zone sjfz near anza california to record microearthquakes starting in august through december 1995 two arrays were deployed 18 km northwest of anza across the casa loma fault clf and the hot springs fault hsf strands of the sjfz the third array was deployed across the san jacinto fault sjf in the anza slip gap we observed fault zone guided waves characterized by low frequency large amplitudes following s waves at the clf array and the sjf array for earthquakes occurring within the fault zone however we did not observe guided waves at the hsf array for any events the amplitude spectra of these guided waves showed peaks at 4 hz at the clf and 6 hz at the sjf which decreased sharply with the distance from the fault trace in contrast no spectral peaks at frequency lower than 6 hz were registered at the hsf array we used a finite difference method to simulate these guided modes as 5 waves trapped in a low velocity waveguide sandwiched between high velocity wall rocks the guided mode data are adequately fit by a waveguide on the clf with the average width of 120 m and s velocity of 2 5 km s about 25 reduced from the s velocity of the surrounding rock this waveguide becomes 40 to 60 m wide with the 5 velocity of 2 8 km s in the anza slip gap on the other hand there is not a continuous waveguide on the hsf at depth locations of the events with guided modes suggest that the fault plane waveguide extends along the clf between the towns of san jacinto and anza dipping northeastward at 75 80 to a depth of about 18 km it becomes nearly vertical in the anza gap we speculate that the existence of a continuous low velocity waveguide on the clf can be caused by the rupture of the magnitude 6 9 earthquake on april 21 1918 occurring near the towns of san jacinto and hemet further the lack of a clear waveguide on the hsf suggests that it was not ruptured in this event

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

in the shadow of 1857 the effect of the great ft tejon earthquake on subsequent earthquakes in southern california the great 1857 fort tejon earthquake is the largest earthquake to have hit southern california during the historic period we investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake when post 1857 earthquakes with unknown focal mechanisms were assigned strike slip mechanisms with strike and rake determined by the nearest active fault 13 of the 13 southern california m 5 5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture when post 1857 earthquakes in the transverse ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike slip 11 of the 13 earthquakes were encouraged by the 1857 earthquake these results show significant correlations between static stress changes and seismicity patterns the correlation disappears around 1907 suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20th century

structural model of the san bernardino basin california from analysis of gravity aeromagnetic and seismicity data 1 the san bernardino basin is an area of quaternary extension between the san jacinto and san andreas fault zones in southern california new gravity data are combined with aeromagnetic data to produce two and three dimensional models of the basin floor these models are used to identify specific faults that have normal displacements in addition aeromagnetic maps of the basin constrain strike slip offset on many faults relocated seismicity focal mechanisms and a seismic reflection profile for the basin area support interpretations of the gravity and magnetic anomalies the shape of the basin revealed by our interpretations is different from past interpretations broadening its areal extent while confining the deepest parts to an area along the modern san jacinto fault west of the city of san bernardino through these geophysical observations and related geologic information we propose a model for the development of the basin the san jacinto fault related strike slip displacements started on fault strands in the basin having a stepping geometry thus forming a pull apart graben and finally cut through the graben in a simpler bending geometry in this model the san bernardino strand of the san andreas fault has little influence on the formation of the basin the deep central part of the basin resembles classic pull apart structures and our model describes a high level of detail for this structure that can be compared to other pull apart structures as well as analog and numerical models in order to better understand timing and kinematics of pull apart basin formation

microbialites in a modern lagoonal environment nature and distribution tikehau atoll french polynesia abstract microbialites including stromatolitic structures gelatinous masses and mats develop in tikehau lagoon tuamotu french polynesia on the flanks and slopes of numerous pinnacles and motu islets between 0 and 25 m deep on coral colonies algal tufts or on lagoonal sediments individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive phormidium species symploca and possibly to one new species of schizothrix gelatinous masses hanging from the ceiling of caverns are produced by phormidium laysanense microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations the study of micro and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation in addition to phototrophic biomineralization pervasive carbonate precipitation occurred in association with decaying organic matter the basic constructional elements of the precipitated carbonate consist of 0 1 0 2 m rounded bodies that form very fine anhedral to subhedral micrite their surfaces comprise extremely small carbonate granules 20 50 nm gradual changes ageing in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities possibly induced by temporary and local nutrient pulses nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients

lithospheric imaging via teleseismic scattering tomography summary the coda of the teleseismic p phase consists largely of energy scattered by small inhomogeneities in the receiver side lithosphere given large collections of teleseismic data from dense permanent networks previous workers have successfully back propagated coda energy back to scattering source points using various kinematic migration schemes as well as by inverting using an inverse scattering radon transform approach under the born approximation seismic scattering is a linear process therefore it is possible to approach coda scattering as a linear waveform inversion problem mathematically similar to transmission based tomography assuming ray theoretical propagation and rayleigh scattering we pose the inverse scattering problem in tomographic form and recover perturbations in density and p and s velocities from pp and ps scattered data the method is applied to data from the southern california seismic network scsn covering the san jacinto anza region the results show a considerable correlation between seismicity and velocity perturbation structure particularly in the region between the mission creek and banning fault branches features connecting the coyote creek and elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression

probabilities of occurrence of large plate rupturing earthquakes for the san andreas san jacinto and imperial faults california 1983 2003 the san andreas san jacinto and imperial faults in california are divided into 19 segments conditional probabilities are calculated that a particular segment will be the site of a large plate rupturing earthquake i e an event that breaks the entire down dip extent of the seismogenic zone during the next 20 years the sizes of such events which account for most of the slip that occurs seismically appear to vary greatly for different segments of these faults repeat time of large shocks coseismic displacement moment release rupture length and seismic magnitude appear to correlate with one another and to be a function of the tectonic style of different parts of those fault zones tectonic inhomogeneities on a scale of about 1 to 100 km are much larger than displacement in any single seismic event and may be regarded as being invariant in their effects upon earthquake generation over many cycles of large shocks it is this invariance that appears to lead to a given segment of a fault rupturing repeatedly in events of nearly the same size since repeat time varies however for a given segment of a fault a simple probabilistic approach is used to forecast the likelihood of large future earthquakes for each segment using as input the time of the last large shock the average recurrence time and the standard deviation of time intervals between events dates of the last large shocks are available for most of the segments investigated repeat times are estimated from times of historic and prehistoric events tectonic similarity and times calculated from coseismic displacement in the last large shock divided by a rate of fault motion or strain buildup the standard deviation of time intervals divided by the average repeat time is close to 1 3 for three fault segments in california for which data are available on historic and prehistoric earthquakes that normalized value is adopted for all calculations of conditional probability magnitudes and seismic moments of future plate rupturing events are estimated for each fault segment several of the segments have well constrained probabilities for rupture during the next 20 years whereas others have large uncertainties eight of the segments constituting about 40 of the total lengths of the three faults have moderate to high probability and thus should be instrumented and studied extensively so as to provide shorter time warnings in some cases two adjacent segments may be nearly in phase in their loading history and may break in a single event the segment of the san andreas fault from opposite san jose to san juan bautista which ruptured less than 1 5 m in 1906 and which probably also broke in 1838 is calculated to have a moderate to high probability of an earthquake of magnitude 6 to during the next 20 years a 325 km section of the southern san andreas between tejon pass and the salton sea has about a 25 chance of rupturing in a shock of magnitude near 8 during the next 20 years all the other segments have only a small chance i e less than 5 to 10 of rupturing in an event of magnitude near 8 during that interval

a comprehensive approach to the study of methane seep deposits from the lincoln creek formation western washington state usa a comprehensive approach using palaeontology petrography stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane seep carbonate deposits these deposits are in the oligocene part of the lincoln creek formation exposed along the canyon and satsop rivers in western washington each deposit preserves invertebrate fossils many representing typical seep biota authigenic carbonates with 1 3 c values as low as 51 pdb reveal that the carbon is predominately methane derived carbonates contain the irregular isoprenoid hydrocarbons 2 6 11 15 tetramethylhexadecane crocetane and 2 6 10 15 19 pentamethylicosane pmi lipid biomarkers diagnostic for archaea these lipids are strongly depleted in 1 3 c 1 3 c values as low as 120 pdb indicating that archaea were involved in the anaerobic oxidation of methane small filaments preserved in the carbonate may represent methanotrophic archaea archaeal methanogenesis induced the formation of a late diagenetic phase brownish calcite consisting of dumbbell shaped crystal aggregates that exhibit 1 3 c values as high as 7 pdb clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation downward directed carbonate aggregation in the seeps produced inverted stromatactoid cavities large filaments interpreted as green algae based on their size shape arrangement and biomarkers imply that deposition occurred in places in water no deeper than 210 m

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

chapter 5 extension and contraction within an evolving divergent strike slip fault complex the san andreas and san jacinto fault zones at their convergence in southern california

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

lidar and field observations of slip distribution for the most recent surface ruptures along the central san jacinto fault we measured offsets on tectonically displaced geomorphic features along 80 km of the clark strand of the san jacinto fault sjf to estimate slip per event for the past several surface ruptures we identify 168 offset features from which we make over 490 measurements using b4 light detection and ranging lidar imagery and field observations our results suggest that lidar technology is an exemplary supplement to traditional field methods in slip per event studies displacement estimates indicate that the most recent surface rupturing event mre produced an average of 2 5 2 9 m of right lateral slip with maximum slip of nearly 4 m at anza a m w 7 2 7 5 earthquake average multiple event offsets for the same 80 kms are 5 5 m with maximum values of 3 m at anza for the penultimate event cumulative displacements of 9 10 m through anza suggest the third event was also similar in size paleoseismic work at hog lake dates the most recent surface rupture event at ca 1790 a poorly located large earthquake occurred in southern california on 22 november 1800 we relocate this event to the clark fault based on the mre at hog lake we also recognize the occurrence of a younger rupture along 15 20 km of the fault in blackburn canyon with 1 25 m of average displacement we attribute these offsets to the 21 april 1918 m w 6 9 event these data argue that much or all of the clark fault and possibly also the casa loma fault fail together in large earthquakes but that shorter sections may fail in smaller events online material topographic contour maps and hillshades generated from b4 lidar data corresponding field photographs and data tables comparing lidar based and field based slip measurements for individual geomorphic features along the clark fault

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

stratigraphic record of pleistocene faulting and basin evolution in the borrego badlands san jacinto fault zone southern california sedimentary rocks in the borrego badlands southern california contain a record of pleistocene crustal deformation during initiation and evolution of the san jacinto fault zone we used detailed geologic stratigraphic and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault controlled sedimentation in this area the base of the 300 to 500 m thick ocotillo formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the borrego formation at 1 05 0 03 ma this coincides with regional scale progradation of ocotillo formation sand and gravel and appears to record initiation of strike slip faults in the southwestern salton trough at ca 1 1 ma thickness trends clast compositions paleocurrents and distribution of paleosols provide evidence for initiation of the east coyote mountain fault at ca 1 05 ma followed by onset of nne ward basin tilting obliquely toward the santa rosa segment of the clark fault at ca 1 0 ma stratigraphic omission of the ocotillo formation and progressively older units southwest of the coyote creek fault beneath the fonts point sandstone provides evidence that tilting to the northnortheast was related in part to growth of the san felipe anticline during deposition of the ocotillo formation map and stratigraphic data suggest that the coyote creek fault in the western borrego badlands postdates ocotillo deposition and thus appears to have propagated southeast into the study area at ca 0 6 ma the fonts point sandstone is a thin sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the borrego badlands the base of the fonts point sandstone changes from a conformable contact in a narrow belt southeast of the inspiration point fault where it is dated at 0 6 0 02 ma to an angular unconformity on the folded ocotillo formation northwest of the fault the pattern of stratal truncation records initiation of the inspiration point fault at ca 0 6 ma this coincides with a major structural reorganization in the san jacinto fault zone that initiated the modern phase of north south shortening and erosion in the southwestern salton trough

rupture and ground motion models on the northern san jacinto fault incorporating realistic complexity we use the 3d finite element method to conduct dynamic models of rupture and resulting ground motion on the claremont casa loma stepover of the northern san jacinto fault we incorporate complex fault geometry from the u s geological survey usgs quaternary faults database see data and resources 1 a realistic velocity structure the southern california earthquake center community velocity model s a realistic regional stress field with an orientation taken from seismicity relocation literature and several stochastic self similar shear stress distributions as we incorporate more types of complexity the specific effects of any individual factor become less apparent within the overall rupture behavior we also find that the distribution of high and low shear stress that arises from combining regional and stochastic stress fields has the strongest control over where the rupture terminates using a regional stress field alone as well as with the combined regional and stochastic stress realization we find that the stepover presents a significant barrier to rupture regardless of our choice of initial nucleation point and that it is difficult for rupture to propagate the full length of either fault segment greater heterogeneity of stresses tends to produce shorter ruptures within this result we find that the claremont strand is more favorable for long ruptures than the casa loma clark strand low frequency ground motion intensity and distribution are controlled largely by the velocity structure rather than by stress heterogeneity the strongest motions produced in these models are in the san bernardino basin although directivity effects do contribute to the low frequency ground motion distribution particularly in the near field they are secondary to the effects of the velocity structure online material figures of ground motions from models used to calibrate the stress conditions for dynamic rupture propagation 1 sec 18

microbial signatures in peritidal siliciclastic sediments a catalogue a catalogue of microbial structural signatures is presented based upon the coupling of fundamental biogeochemical microbial processes and local morphogenetic determinants it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones temperate humid mellum island southern north sea subtropical arid coast of southern tunisia textural geometries reveal a high structural diversity but their determinants are primarily based upon six major parameters 1 intrinsic biofactors structural diversification of sedimentary microbial films and mats inherent in the organisms i e their construction morphology growth taxis and behaviour and local abundance of specific morphotypes most prominent are the ensheathed filamentous cyanobacteria microcoleus chthonoplastes and lyngbya aestuarii and the sheathless filamentous cyanobacterium oscillatoria limosa 2 biological response to physical disturbances sediment supply erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats 3 trapping binding effects physicobiological processes give rise to grain orientations and wavy to lenticular lamina lamina specific grain arrangements and sucrose calcium carbonate accumulations 4 secondary physical deformation of biogenic build ups mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures 5 post burial processes textural fabrics that evolve from mechanical effects of gas formation from decaying mats and features related to the formation of authigenic minerals calcium carbonates calcium sulphates pyrite 6 bioturbation and grazing post depositional structures such as skolithos type dwellings traces of burrowing insects gastropod grazing traces and faecal pellets in synopsis the catalogue firstly comprises a sound set of ubiquitous signatures this uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings the catalogue secondly documents signatures that are extremely sensitive to tidal position hydrodynamic regime and overall climatic conditions these kinds of signature indicate narrow facies zones which often coincide with the activity or dominance zones of certain organisms an overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings

spheroidal dolomites origin by hydrocarbon seepage spheroidal dolomite cements are a pervasive and distinctive feature in five exposed lithologies of different geological age in kuwait the spheroids range in diameter from 5 350 m have concentric zones and nuclei of fluid inclusions with or without a radial fabric and compositions varying from near stoichiometric to ca56 mg44 all spheroids appear to be primary in origin with no evidence of replacement some concentric zones show selective calcitization and or leaching most of these features are confined to within 30 m of the surface consequently spheroidal dolomites recognizable at depth in ancient sequences may be indicative of a local unconformity the dolomite distributions are areally correlative with known areas of hydrocarbon seepage the latter being confined to prominent oil field anticlinal trends it is proposed that the dolomitization was caused by groundwaters carrying hydrocarbons to the surface which were then oxidized to carbon dioxide the gas bubbles may have acted as nuclei for spheroid growth with a possible inducement by bacteria which have an affinity for hydrocarbons the implication is that the regional distribution of spheroidal dolomites may be an indicator of a potential reservoir facies at depth

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

interaction of the san jacinto and san andreas fault zones southern california triggered earthquake migration and coupled recurrence intervals two lines of evidence suggest that large earthquakes that occur on either the san jacinto fault zone sjfz or the san andreas fault zone safz may be triggered by large earthquakes that occur on the other first the great 1857 fort tejon earthquake in the safz seems to have triggered a progressive sequence of earthquakes in the sjfz these earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1 7 kilometers per year along the sjfz after the 1857 earthquake second the similarity in average recurrence intervals in the sjfz about 150 years and in the mojave segment of the safz 132 years suggests that large earthquakes in the northern sjfz may stimulate the relatively frequent major earthquakes on the mojave segment analysis of historic earthquake occurrence in the sjfz suggests little likelihood of extended quiescence between earthquake sequences

strike slip fault geometry in turkey and its influence on earthquake activity the geometry of turkish strike slip faults has been reviewed and described from this data set it appears that fault geometry the distribution of discontinuities such as bends and stepovers along the main fault trace plays an important role in controlling the location of large earthquake rupture segments along the fault zones large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 more important than the size of each discontinuity however is the total geometric pattern i e the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size certain geometric patterns restraining single or double bends are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior for example large earthquake epicenters often occur near restraining bends or double bends furthermore aftershocks and swarm activity can often be associated with releasing areas areas along the strike slip fault that are undergoing a relatively large amount of extension

historic earthquakes 1918 to 1923 and an assessment of source parameters along the san jacinto fault system body waveform modeling is used to determine the source parameters of two earthquakes that occurred along the northern san jacinto fault system in 1918 and 1923 the 1918 earthquake parameters are consistent with a 20 to 25 km rupture along the claremont fault at a depth of 7 5 km limited data for the 1923 event are more consistent with rupture on the san jacinto fault than rupture along the san andreas fault or a buried cross fault although the greatest damage associated with the earthquake occurred closer to the san andreas fault using the results of this study and previous studies of historic earthquakes along the southern san jacinto fault the relationships between rupture length and magnitude focal depth and heat flow and other parameters are examined in an effort to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone one important observation is that rupture lengths along the san jacinto fault are shorter for an equivalent moment magnitude than those along the imperial valley mexicali fault system the higher heat flow of the imperial valley region may be responsible for this difference

internal structure of the san jacinto fault zone at blackburn saddle from seismic data of a linear array local and teleseismic earthquake waveforms recorded by a 180 m long linear array bb with seven seismometers crossing the clark fault of the san jacinto fault zone northwest of anza are used to image a deep bimaterial interface and core damage structure of the fault delay times of p waves across the array indicate an increase in slowness from the southwest most bb01 to the northeast most bb07 station automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves the observed fault zone head waves imply that the clark fault in the area is a sharp bimaterial interface with lower seismic velocity on the southwest side the moveout between the head and direct p arrivals for events within 40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3 2 per cent a constant moveout for events beyond 40 km to the southeast is due to off fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance the lack of head waves from events beyond 20 km to the northwest is associated with structural complexity near the hemet stepover events located in a broad region generate fault zone trapped waves at stations bb04 bb07 waveform inversions indicate that the most likely parameters of the trapping structure are width of 200 m s velocity reduction of 30 40 per cent with respect to the bounding blocks q value of 10 20 and depth of 3 5 km the trapping structure and zone with largest slowness are on the northeast side of the fault the observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest this inference is consistent with results of other geological and seismological studies

mapping asperities by minima of local recurrence time san jacinto elsinore fault zones minima of the local recurrence time tl m are correlated with the locations of historical main shocks in the san jacinto sj elsinore e fault zones of southern california we define tl m as the recurrence time calculated from the local b and a values of the frequency magnitude distribution earthquakes within 6 and 10 km from each point in the sj and e fault zones respectively we propose that anomalously low tl m values map asperities out of six historic main shocks 1899 1968 five ruptured substantial parts of four asperities mapped by our method which uses solely the modern earthquake catalog m 1 2 1981 0 1998 8 we estimate the probability for this correlation to occur at random as 10 2 to 10 3 depending on the scenario chosen it appears that the minima in tl m are tied to minima in b values more than to maxima in a values thus locations of low b values can also be used as an indication where asperities may be located and where main shocks are most likely the anza gap a segment of the san jacinto fault with low a values is mapped as an asperity by our method the heterogeneity in b values is strong throughout the study area the b values range from 0 5 to 1 8 over distances of 10 km volumes with approximately constant b values have radii of r 8 6 km our results suggest that detailed mapping of b values and local recurrence times in volumes with a few kilometers radius reveals important information about the local seismic hazard hitherto untapped by conventional seismicity analyses