a note on the surface volume change of shallow earthquakes summary this note presents an exact analytical formula for determining the magnitude of coseismic surface volume change v of earthquake faults in a half space for a poisson solid the formula is remarkably simple v mzz 8 where mzz is one of the moment tensor elements of the source maximum v values derive from dip slip on faults plunging 45 for these events surface volume changes of 0 0001 and 4 3 km3 can be expected for magnitude 5 and 8 earthquakes respectively all of the coseismic surface volume change is recovered in the interseismic period through relaxation of the earth and rebound of the surface a useful rule of thumb for estimating the magnitude of vertical rebound in 45 dip slip events is hp s 24 where s is the coseismic slip on the fault

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

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

source complexity of the october 1 1987 whittier narrows earthquake long and short period body waveform data recorded at regional and teleseismic distances are modeled to obtain the source parameters of the whittier narrows earthquake of october 1 1987 although the long period data can be satisfied by a simple thrust earthquake on a plane striking 280 at a depth of 14 km we require a double source solution to satisfy both the long and short period teleseismic data the first subevent has a focal mechanism of strike 250 dip 30 n and a slip vector 90 the second is 280 40 n and 98 the second event occurred 1 0 s after the first was 5 times larger 1 2 1025 dyn cm for the second 0 23 1025 for the first in terms of seismic moment and was also shallower 12 km whereas the first subevent was at 15 km the two subevents probably occurred on a north dipping listric fault but may have occurred on two subparallel faults these features were determined by studying a variety of phases including teleseismic p and sh and regional pnl the whittier narrows earthquake was a high stress drop event but a single value for stress drop cannot be obtained because of the complex nature of the source if we define the fault area as the aftershock area we obtain a stress drop of about 750 bars

dislocation modeling of blind thrusts in the eastern los angeles basin california 1 the east and west coyote hills in the eastern los angeles basin are the surface expression of uplift accompanying blind reverse faulting folded quaternary strata indicate that the hills are growing and that the faults underlying them are active detailed subsurface mapping in the east coyote oil field shows that a previously mapped reverse separation fault is predominantly an inactive left lateral strike slip fault that is not responsible for the uplift of the east coyote hills the fault responsible for folding and uplift of the coyote hills does not cut wells in either the east or west coyote oil fields to characterize the geometry of the blind fault responsible for folding we employ dislocation modeling the dip and upper fault tip depths obtained from modeling suggest that the thrust fault beneath the coyote hills may be an extension of the puente hills blind thrust fault that continues westward beneath the santa fe springs oil field modeling results suggest that the segment of the thrust fault responsible for folding the coyote hills would have accumulated 1500 m of reverse displacement over the last 1 2 myr yielding an average slip rate of 1 3 0 5 mm yr the santa fe springs segment of the fault has a slip rate of 1 5 0 4 mm yr for the last 1 2 myr the estimated moment magnitude for a reverse displacement earthquake on the puente hills blind thrust ranges from 6 6 to 7 2 depending on the length of the rupture the estimated average recurrence interval for these earthquakes is 1700 3200 years

quantifying ground deformation in the los angeles and santa ana coastal basins due to groundwater withdrawal we investigate complex surface deformation within the los angeles and santa ana coastal basins due to groundwater withdrawal and subsequent aquifer compaction expansion we analyze an 18 year interferometric synthetic aperture radar insar time series of 881 interferograms in conjunction with global positioning system gps data within the groundwater basins the large data set required the development of a distributed time series analysis framework able to automatically decompose both the insar and gps time series into short term and long term signals we find that short term seasonal oscillations of ground elevations due to annual groundwater withdrawal and recharge are unsteady due to changes in seasonal withdrawal by major water districts the spatial pattern of seasonal ground deformation near the center of the basin corresponds to a diffusion process with peak deformation occurring at locations with highest groundwater production long term signals occur over broader areas and are ultimately caused by long term changes in groundwater production comparison of the geodetic data with hydraulic head data from major water districts suggests that different regions of the groundwater system are responsible for different temporal components in the observed ground deformation short term seasonal ground deformation is caused by compaction of shallower aquifers used for the majority of groundwater production whereas long term ground deformation is correlated with delayed compaction of deeper aquifers and potential compressible clay layers these results demonstrate the potential for geodetic analysis to be an important tool for groundwater management to maintain sustainable pumping practices

simulation of barotropic and baroclinic tides off northern british columbia abstract the tidal response of northern british columbia coastal waters is studied through simulations with a three dimensional prognostic primitive equation model the model is forced at the boundaries with the leading semidiurnal and diurnal constituents and experiments with stratified and homogeneous fluid are compared the barotropic response shows good agreement with previously published studies of tides in the region a comparison with tide gauge measurements indicates that average relative rms differences between observations and the model surface elevation field are less than 5 for the largest constituents an internal tide is generated in cases where the model is initialized with a vertical stratification diagnostic calculations of the baroclinic energy flux are used to identify regions of generation and propagation of internal tidal energy with a representative summer stratification the integrated offshore flux is about 0 5 gigawatts higher than previously estimated from theoretical model

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

tectonics of the san gabriel basin and surroundings southern california the sangabriel basin is a pliocene pleistocene transverse ranges structure bounded by the san gabriel mountains on the north the san jose and puente hills on the east and the repetto and montebello hills on the west a basement boundary between gneissoid rocks of san gabriel mountains aspect and peninsular ranges rocks strikes east across the basin and connects with the san jose fault this boundary may represent the eastward continuation of the canton fault a middle miocene precursor of the san gabriel fault system with up to 30 km of right slip canton fault displacement is largely canceled by 22 5 km of left slip on a precursor to the raymond fault such that an oligocene dacite in the san jose hills and clasts derived from it show little evidence of large horizontal displacement between the san gabriel mountains and puente hills clockwise rotation in the middle miocene probably influenced the distribution of lateral slip the fill in the northeast trending pliocene pleistocene san gabriel basin is a basal shallow marine sequence overlain by the nonmarine duarte conglomerate in contrast to the deep water fernando formation of the los angeles basin the paleoslope trends south in the direction of the repetto and puente hills evidence that uplift of these hills postdated deposition the triangular san gabriel basin is currently a stable block bounded by the right slip northwest striking east montebello fault on the west and the left slip northeast striking walnut creek fault on the east farther east the san jose and puente hills are underlain by an east trending active fold belt including the san jose reverse fault and anticline the walnut anticline and the puente hills anticline adjacent to the right lateral whittier fault the east montebello fault is bounded on the west by the elysian park anticlinorium a fold belt in downtown los angeles the left slip holocene raymond fault joins the sierra madre reverse fault at its segment boundary marked by the clamshell sawpit fault the triangular stable block between east trending fold belts is bounded on the south by the puente hills blind thrust and on the north by the sierra madre fault documenting north south quaternary convergence east west extension is also indicated although not confirmed by geodetic measurements

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

tectonic contraction across los angeles after removal of groundwater pumping effects after the 1987 whittier narrows and 1994 northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan los angeles a network of 250 continuously recording global positioning system gps stations was deployed to monitor displacements associated with deep slip on both blind and surface faults here we augment this gps data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike slip faulting after removing these non tectonic signals we are left with 4 4 mm yr 1 of uniaxial contraction across the los angeles basin oriented n 36 degrees e perpendicular to the major strike slip faults in the area this indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast trending strike slip faults we have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults in the 40 km long santa ana basin groundwater withdrawal and re injection produces 12 mm yr 1 of long term subsidence accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally

coseismic folding earthquake recurrence and the 1987 source mechanism at whittier narrows los angeles basin california static deformation associated with the october 1 1987 whittier narrows california m 6 0 earthquake was detected by geodetic elevation changes the earthquake uplifted a 1 5 km high quaternary fold the santa monica mountains anticlinorium by 50 mm but caused no fault rupture at the ground surface this suggests that folding and faulting of the los angeles basin sediments are coincident and continuing after correction for surveying errors and nontectonic subsidence we model the 214 geodetic observations with a simple dislocation in an elastic half space a thrust fault with reverse slip of 1 1 0 3 m and dipping 30 4 n with an upper edge at a depth of 12 1 km and a lower edge at 17 1 km fits the geodetic data best and is consistent with the main shock hypocenter fault plane solution and initial aftershock distribution the upper limit on the static stress drop is 17 5 5 0 mpa 175 50 bar the lower limit on the geodetic moment is 1 0 0 2 1025 dyne cm in accord with seismic estimates indicating that most of the slip took place during seismic rupture if earthquakes of m 6 characterize the blind thrust fault on which the earthquake occurred we estimate a 200 year repeat time at whittier narrows and a 5 18 year rate of earthquake occurrence within a 150 km long band in the northern los angeles basin and santa monica bay to the west the 25 year rate of historical occurrence since 1860 is less than this prediction the deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here

spatial and temporal distribution of deformation at the front of the andean orogenic wedge in southern bolivia new observations from an active orogenic wedge help link the seismotectonic behavior of individual faults to wedge deformation rates and patterns over multiple timescales we provide the first detailed constraints on the distribution and timing of deformation at the front of the andean orogenic wedge in southern bolivia where a recent study suggests that great mw 8 earthquakes could rupture the master fault underlying the wedge we use stratigraphic relationships across fault related folds and elastic dislocation modeling of seismic reflection horizons to obtain probabilistic estimates of wedge front fault ages and slip rates our analyses reveal that at least half of the previously determined gps based wedge loading and quaternary whole wedge shortening rates are absorbed across a 20 40 km wide wedge front zone consisting of 1 4 en echelon and partially to fully overlapping faults and folds associated with blind thrust faults the difference between our slip rates and the geodetic geologic observations combined with evidence for activity across internal wedge structures supports the notion that nonsteady state mass balance conditions coupled with elevated erosional efficiency result in distributed wedge deformation the orogenic wedge in southern bolivia behaves in a similar fashion to the taiwanese and himalayan ranges slip accumulates at downdip locations along the master fault and is released incrementally by earthquakes that rupture the wedge front fault zone the faults and folds comprising this zone pose a major source of seismic hazard accumulating slip is also released in the wedge interior and older internal wedge faults must be considered in any future assessment of regional earthquake risk

using vertical rock uplift patterns to constrain the three dimensional fault configuration in the los angeles basin comparison of geologic uplift patterns with results of three dimensional mechanical models provides constraints on the fault geometry compiled by the south ern california earthquake center community fault model in the northern los angeles basin california the modeled uplift matches well the geologic pattern of uplift as sociated with the santa fe springs and coyote hills segments of the puente hills thrust fault but does not match structures to the west of the san gabriel river to better match the geologic patterns in this area alternative fault configurations were tested the best match to geologic uplift is attained with a model incorporating 1 a steep blind thrust fault at the location of the los angeles segment of the puente hills thrust system following interpretations of the las cienegas fault geometry at this location 2 removal of an inferred linking fault between the raymond and holly wood faults and 3 lateral continuation of the lower elysian park fault a blind low angle detachment at 10 km depth along strike to the northwest these geometric revisions alter the connectivity of northern los angeles basin faults and significantly improve the match of model uplift pattern to geologic data model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area the preferred model alters slip rates by 0 2 mm yr for the upper elysian park hollywood lower elysian park raymond sierra madre west and verdugo faults additionally the preferred model alters the surface area of several faults in the northern los angeles basin such as the puente hills thrust and the lower elysian park fault which may have important implications for seismic hazard assess ment in the northern los angeles basin

geochemistry and petrology of the jequie granulitic complex brazil an archean basement complex the jequie granulitic complex is part of the extensive high grade metamorphic terrain located within the sao francisco craton of northeastern brazil some jequie rocks appear to have been formed in the middle archean 3 1 ga from preexisting sialic crust the dominant mineral composition of these rocks is quartz microcline plagioclase hyperstene and occurs over an extensive area 2 000 km2

k rb sr ba u and th geochemistry of the lapland granulites fennoscandia lile fractionation controlling factors the lile geochemical patterns of the three main lithological units graywacke shale metasedimentary sequence tholeiitic metaigneous rocks and migmatitic rocks of the lapland granulite belt are described k ba sr and th concentrations in metasediments are nearly similar to average continental crust whereas rb and u are unevenly impoverished in particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium tholeiitic metaigneous rocks comprises metavolcanics which present k rb ratios similar to metasediments and metaplutonics with lile abundances close to those of the low k tholeiites migmatites show wide range in lile content metatexites and diatexites have higher k rb th and u concentrations and similar k rb ratios with respect to equivalent unmobilized rocks potassic pegmatoids are strongly enriched k rb ba and th but moderately in sr and u plagioclasic pegmatoids and ferromagnesian restites are rich in sr and poor in other lil elements

revisions to the age of the brunhes matuyama boundary and the pleistocene geomagnetic polarity timescale the currently referenced age of 730 ka for the brunhes matuyama b m geomagnetic polarity boundary mankinen and dalrymple 1979 is based largely on k ar and paleomagnetic data from postcollapse rhyolites in the valles caldera new mexico doell and dalrymple 1966 doell et al 1968 dating of sanidine from valles rhyolites by the 40a 39ar laser fusion technique indicates that samples critical to defining the b m boundary are up to 90 ka older than previously thought the new data combined with earlier results yield an age for the b m boundary of 780 10 ka 40ar 39ar ages from other samples suggest the jaramillo normal polarity subchron extended from 1010 to 915 ka

four dimensional analysis of the inversion of a half graben to form the whittier fold fault system of the los angeles basin abstract analysis of structural and stratigraphic data from hundreds of oil wells surface geologic maps and earthquake focal mechanisms reveals a three phase structural evolution of the whittier fold fault system that evolution progressed from an extensional phase 16 ma accommodated by the formation of a half graben through an episode of graben filling 14 8 ma to lithospheric shortening which began ca 8 ma with contractional reactivation of the graben forming fault as a blind reverse fault and accompanying anticlinal uplift about 1 ma the blind reverse fault broke through onto the sea floor initiating the activity of the present whittier fault system horizontal contraction estimated from reconstructed cross sections ranges from 1 4 to 3 2 km and has been at an average rate of 0 3 mm year in the p direction n s of the 1987 whittier narrows earthquake for the past ca 8 my that rate may have accelerated after breakthrough of the blind reverse fault to as much as 1 0 mm year the ratio of strike slip to dip slip displacement on the whittier fault over the past ca 8 my is about 0 4

puente hills blind thrust system los angeles california we describe the three dimensional geometry and quaternary slip history of the puente hills blind thrust system pht using seismic reflection profiles petroleum well data and precisely located seismicity the pht generated the 1987 whittier narrows moment magnitude m w 6 0 earthquake and extends for more than 40 km along strike beneath the northern los angeles basin the pht comprises three north dipping ramp segments that are overlain by contractional fault related folds based on an analysis of these folds we produce quaternary slip profiles along each ramp segment the fault geometry and slip patterns indicate that segments of the pht are related by soft linkage boundaries where the fault ramps are en echelon and displacements are gradually transferred from one segment to the next average quaternary slip rates on the ramp segments range from 0 44 to 1 7 mm yr with preferred rates between 0 62 and 1 28 mm yr using empirical relations among rupture area magnitude and coseismic displacement we estimate the magnitude and frequency of single m w 6 5 6 6 and multisegment m w 7 1 rupture scenarios for the pht manuscript received 16 november 2001

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

crystal chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the newer volcanics of victoria australia a clinopyroxene suite from lherzolite inclusions associated with the victorian australia newer volcanics has been investigated with the aim of understanding the clinopyroxene crystal chemical response to increasing temperature e g a melting model and or crystallization processes prevailing at high pressure

radioactive element distribution in the archean granulite terrane of jequie bahia brazil the radioactive element distribution in the archean granulite facies rocks of jequie brazil has been determined the k rb ratios show a large spread with majority of the values falling within the normal field k rb 300 the th u ratios fall into two groups one group with values in the range of 1 7 to 2 6 whereas the other group show th u values greater than 7 7 to 25 these distributions can be explained on the basis of the presence of accessory minerals apatite and zircon the relatively high concentrations of the radioactive and other elements in jequie granulites probably represents the relict primary geochemical character little modified during metamorphism the radiogenic heat production data for the area are calculated and using the measured regional heat flow value from nearby area an estimate of the relative thickness of the continental lithosphere is made using the model developed by oxburgh 1981

the growth of geological structures by repeated earthquakes 1 conceptual framework in many places earthquakes with similar characteristics have been shown to recur if this is common then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures following this paradigm we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault in these models an elastic layer containing the fault overlies a viscous half space with a different density fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion in this paper the parameters that control the growth of dip slip structures are identified we find that the flexural rigidity of the crust or the apparent elastic thickness provides the main control of the width of a structure the loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault the flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character we find that in general real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities

recognition of paleoearthquakes on the puente hills blind thrust fault california borehole data from young sediments folded above the puente hills blind thrust fault beneath los angeles reveal that the folding extends to the surface as a discrete zone 145 meters wide buried fold scarps within an upward narrowing zone of deformation which extends from the upward termination of the thrust ramp at 3 kilometers depth to the surface document the occurrence of at least four large moment magnitude 7 2 to 7 5 earthquakes on this fault during the past 11 000 years future events of this type pose a seismic hazard to metropolitan los angeles moreover the methods developed in this study can be used to refine seismic hazard assessments of blind thrusts in other metropolitan regions

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

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

magma genesis in a late proterozoic proto oceanic rift ree and other trace element data from the keweenawan mamainse point formation ontario canada abstract the keweenawan mamainse point formation of ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes with trace element characteristics similar to other proto oceanic rift sequences stratigraphic variation of geochemistry allows the subdivision of the sequence into five series each of which has suffered shallow level fractional crystallization differences in incompatible element ratios and ree patterns however can only be explained by variations in the melting process ree modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and lree depleted sources however the systematic evolution of ree patterns and other trace elements are better explained by dynamic partial melting within two subsequent rising mantle diapirs

long term fault slip rates distributed deformation rates and forecast of seismicity in the western united states from joint fitting of community geologic geodetic and stress direction data sets 1 the long term average velocity field of the western united states is computed with a kinematic finite element code community data sets include fault traces geologic offset rates geodetic velocities principal stress directions and euler poles there is an irreducible minimum amount of distributed permanent deformation which accommodates one third of pacific north america relative motion in california much of this may be due to slip on faults not included in the model all data sets are fit at a common rms level of 1 8 datum standard deviations experiments with alternate weights fault sets and euler poles define a suite of acceptable community models in pseudoprospective tests fault offset rates are compared to 126 additional published rates not used in the computation 44 are consistent another 48 have discrepancies under 1 mm a 1 and 8 have larger discrepancies updated models are then computed novel predictions include dextral slip at 2 3 mm a 1 in the brothers fault zone two alternative solutions for the mendocino triple junction slower slip on some trains of the san andreas fault than in recent hazard models and clockwise rotation of some domains in the eastern california shear zone long term seismicity is computed by assigning each fault and finite element the seismicity parameters coupled thickness corner magnitude and spectral slope of the most comparable type of plate boundary this long term seismicity forecast is retrospectively compared to instrumental seismicity the western united states has been 37 below its long term average seismicity during 1977 2008 primarily because of temporary reduced activity in the cascadia subduction zone and san andreas fault system

basalt geochemistry of the explorer ridge area northeast pacific ocean forty two fragments of young fresh basalts dredged from the explorer ridge paul revere ridge fracture zone and dellwood knolls have been analysed for 34 major minor and trace elements and 87sr 86sr ratios have been determined in seven of the fragments the explorer ridge basalts have major element compositions similar to most mid ocean ridge basalts morb particularly to the iron rich basalts of the southern juan de fuca ridge the basalts exhibit variability in trace element content largely attributable to crystal fractionation at low pressure with respect to morb the incompatible minor and trace elements are weakly to strongly enriched in the explorer samples and are most strongly enriched in basalts from explorer deep adjacent ridge segments erupt basalts with variable incompatible element concentrations and ratios this could be an indicator of a chemically heterogeneous mantle source or may be the result of intermittent injection of enriched magmas from a hotspot beneath explorer deep

estimation of fault propagation distance from fold shape implications for earthquake hazard assessment a numerical grid search using the trishear kinematic model can be used to extract both slip and the distance that a fault tip line has propagated during growth of a fault propagation fold the propagation distance defines the initial position of the tip line at the onset of slip in the santa fe springs anticline of the los angeles basin we show that the tip line of the underlying puente hills thrust fault initiated at the same position as the 1987 magnitude 6 0 whittier narrows earthquake

structural evolution of the harts range area and its implication for the development of the arunta block central australia abstract a division of the high grade metamorphics of the arunta block in the harts range area into two main groups on the basis of lithology structure metamorphism and igneous activity is suggested as a result of detailed mapping the lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss minor amphibolite and mafic granulite while the structurally overlying group is regarded as a cover sequence irindina supracrustal assemblage and comprises predominantly metasediments pelitic and semi pelitic gneiss calc silicate and quartzite and amphibolite anorthosite and ultramafic rock of the harts range meta igneous complex the cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre existing basement the two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover the basement s early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early precambrian terrains and developed spatially and or temporally separated from the cover cover deformation closely resembles continental margin cordillera type overthrusting and thus thin skinned tectonics associated with large scale crustal shortening during the later harts range remobilization the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation recumbent contraction fault mylonites repeatedly developed along the dislocation which was also isoclinally folded intruded by a major megacrystic granitoid bruna gneiss sheet and repeatedly rejuvenated the history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

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

a basaltic ferrobasaltic granulite association oonagalabi gneiss complex central australia magmatic variation in an early proterozoic rift extremely fractionated basaltic to ferrobasaltic amphibolites and granulites comprise two spatially associated mafic tholeiitic suites deformed sills within the early proterozoic oonagalabi basement gneiss complex harts range central australia the metatholeiites are characterised by high to very high feo tio2 and p2o5 contents and variable depletion in cao and al2o3 despite similar zr nb ratios the rocks from the two suites show different degrees of enrichment in lree and other immobile incompatible elements the basaltic melts which were parental to the two mafic suites were not comagmatic and the rocks cannot be related simply by fractionation of realistic assemblages of low pressure fractionating phases

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

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

interseismic strain accumulation on faults beneath los angeles california geodetic data show that the los angeles metropolitan area is undergoing 8 9 mm year of north south tectonic shortening associated with the big bend of the san andreas fault this shortening has been linked to multiple damaging twentieth century thrust earthquakes as well as possible mw 7 0 holocene thrust events beneath central los angeles to better characterize this seismic hazard we assess how this shortening is being accommodated by interseismic strain accumulation on subsurface faults incorporating detailed seismology and geology based models of fault geometry and the low stiffness los angeles sedimentary basin we find that strain accumulation on local strike slip faults likely contributes no more than 1 2 mm year of the shortening we formally invert the geodetic data for the pattern of interseismic strain accumulation on the north dipping sierra madre puente hills and compton thrust faults and a master decollement we explore the impact of the assumed material model strain accumulation on faults to the west and east and other model assumptions we infer that the three faults slip at 3 4 mm year over the long term and are currently partially or fully locked and accruing interseismic strain on their upper sections this locking implies an annual deficit of seismic moment 1 6 1 3 0 5 10 17 nm year in total which is presumably balanced over the long term average by the moment released in earthquakes the depth distribution of moment deficit accumulation rate matches that of seismicity rates in los angeles to first order in part because the models incorporate the blind nature of the puente hills and compton faults

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years