source rupture plane determination from directivity doppler effect for small earthquakes recorded by local networks abstract fault plane solutions from first motion polarity of p waves give two nodal planes among which it is not possible to distinguish the fault plane from the auxiliary one we found that for earthquakes recorded by a dense local network it is possible to identify the rupture direction from the doppler effect produced by directivity this principle was successfully applied using three different simple techniques direct inspection and time and frequency domains analyses to one m l 3 6 and six smaller 1 2 m l 2 0 well recorded earthquakes from northern baja california mexico the results indicate activity in faults conjugate to those currently mapped in this region

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

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

the tectonic crustal stress field and style of faulting along the pacific north america plate boundary in southern california we invert for the state of stress in the southern california crust using a catalogue of high quality earthquake focal mechanisms 1981 2010 the stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region from the stress field we determine the maximum horizontal compressive stress s hmax orientations and the style of faulting across southern california the trend of s hmax exhibits significant regional and local spatial heterogeneities the regional trend of s hmax varies from north along the san andreas system to nne to the east in the eastern california shear zone as well as to the west within the continental borderland and the western transverse ranges the transition zones from one state of stress to the other occur over a distance of only a few kilometres following a trend from yucca valley to imperial valley to the east and the western edge of the peninsular ranges to the west the local scale heterogeneities in the s hmax trend include nnw trends along the san andreas fault near cajon pass tejon pass and the cucapah range as well as nne trends near the northern san jacinto fault and the wheeler ridge area the style of faulting exhibits similar complexity ranging from predominantly normal faulting in the high sierra nevada to strike slip faulting along the san andreas system to three consecutive bands of thrust faulting in the wheeler ridge area and the western transverse ranges the local variations in the style of faulting include normal faulting at the north end of the san jacinto fault and scattered areas of thrust faulting the regional variations in the shmax trends are very similar to the pattern of the gps measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper mantle deformation field the mean trend of s hmax departs about approximately 14 to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle

tectonic implications of paleomagnetic data from sills and dykes in the sverdrup basin canadian arctic mesozoic intrusives 71 sills and 12 dykes from different structures in the folded eastern sverdrup basin contain stable well grouped paleomagnetic directions of both normal and reverse polarity the mean direction is 274 79 k 173 95 3 7 with a corresponding pole position at 157 67 n k 61 a95 6 6 the paleomagnetic fold test shows that the major deformation involving tilting of strata around strike postdated the emplacement of sills and dykes minor preintrusion deformation may also have occurred but more importantly an offset of the paleopole from the cretaceous section of the polar wander path permits a postintrusion anticlockwise rotation of eastern sverdrup basin of about 15 20 with respect to stable north america if the intrusives are cretaceous the degree of rotation is close to that 11 estimated for greenland during the opening of the labrador sea no positive evidence has been found to support models of distributed shear with block rotations to accommodate the contentious northeast translation of greenland along the nares strait

changes in vp vs with depth implications for appropriate velocity models improved earthquake locations and material properties of the upper crust properties associated with the arrival times of shallow microearthquakes are determined for three tectonically different sites soviet central asia the central united states and southern california simple graphical presentations of the earthquake data including wadati and riznichenko diagrams are used to resolve the trade off in origin time and focal depth estimates of the average half space velocities for p and s waves and of the travel time ratio ts tp are also obtained that are relatively free of any model constraints used to initially locate the earthquake in all cases p and s arrivals exhibit a systematic decrease in ts tp with depth in the upper few kilometers this decrease is significant and matches similar variations observed in situ for the velocity ratio vp vs based on laboratory and theoretical studies such observations are consistent with the closing of saturated microcracks with increasing confining pressure independent determinations of velocity structure and rock properties from both refraction studies and borehole experiments show analogous results by specifying separate velocity models for both p and s waves that reflect this variation rather than assuming a constant value of vp vs we find improved stability in hypocenter determination and increased resolution of shallow seismicity patterns

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

miocene pliocene tectonic evolution of the slovenian periadriatic fault implications for alpine carpathian extrusion models the periadriatic line pal is a remarkable several hundred kilometer long fault system of the alpine orogen its dextral character was documented by several authors using diverse criteria but detailed kinematics and timing of movements had not been investigated along its whole length structural and paleomagnetic measurements mapping and stratigraphic and sedimentological studies have helped to unravel the miocene pliocene evolution of the slovenian segment of the pal brittle deformation was characterized by nw se to n s compression and perpendicular tension deformation has resulted in dextral strike slip faulting folding and tilting of beds the first transpressional event corresponds to the first phase of lateral extrusion of the east alpine western carpathian northern pannonian block in the early miocene 24 17 5 ma after a short period of transtension during the karpatian 17 5 16 5 ma dextral transpression reoccurred during the middle miocene to pliocene and lasted up to the quaternary middle miocene dextral slip can be connected to the second phase of extrusion the highly deformed rocks within the dextral shear zones show variable clockwise sometimes counterclockwise rotations the mechanism of rotation seems to be complex ranging from regional rotation to local folding due to pure or simple shear domino type rotation

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

palaeoseismic investigations along the kachchh mainland fault a comprehensive review and new insights of the past earthquakes in the kachchh basin western india abstract the intraplate kachchh rift basin has been hit by several devastating earthquakes in the historic past including the 1819 allah bund earthquake and the 2001 bhuj earthquake the source region of these earthquakes within the basin have been studied by several workers in the last two decades to understand their potential for earthquake recurrence however very little information is available on the palaeoseismic and geomorphic characterization of kachchh mainland fault kmf therefore in the present study six trenches were excavated across the kmf between lakhpat and nirona to understand ground deformation pattern and timing of the historic earthquakes based on geomorphic and palaeoseismic investigations five palaeoearthquakes of early to late holocene have been identified between 10 000 890 yrs apart from the holocene an earthquake of late quaternary period was also identified which possibly occurred around 19 800 yrs bp fault related parameters were analyzed to understand the geometry of the active fault scarp along kmf the results of the analyzed fault geometric parameters show that the vertical displacement along the kmf is higher than the horizontal displacement the slip rate of the kmf from the western portion to the middle portion decreases from 0 08 to 0 04 mm yr and increases towards east from 0 22 mm yr to 0 36 mm yr as the kachchh district of the gujarat state is rapidly developing in terms of infrastructural development the outcome of this research might provide significant inputs for micro zonation studies and also in the evaluation of the seismic hazard in the kachchh region

late mesozoic and cenozoic tectonic history of south central california the late mesozoic cenozoic history of south central california is punctuated by at least five major tectonic events the oldest of these involves late cretaceous drag folding and thrusting of epizonal shelf sediments westward over and to the north along the rising eastern margin of the peninsular ranges batholith these overthrusts with associated drag folds and nappes form imbricate stacks of metasediments slices of plutons and mylonites which overlie a regional core of cretaceous granitic plutons of the peninsular ranges batholith a second distinct kind of epizonal low temperature crustal displacement includes an episode of mid tertiary detachment faulting and folding this represents the westerly extension of the detachment terrane of eastern california and western arizona and utah superimposed upon the cretaceous structural features the mid tertiary motions involve major e w extension and n s compressive strain with formation of east trending antiformal core complexes remarkably parallel to a pervasive set of fold axes and b lineations formed in the cretaceous thrusting and folding all of these features are repeatedly segmented and displaced by step faulting as sial in the salton trough is fragmented and depressed 5 to 10 km below sea level at intervals throughout the cenozoic the irregular step faulted interface of the peninsular ranges batholith and salton trough is accentuated by a series of ne to se striking left lateral and dipslip faults with displacements of several to 10 kilometers these appear to both predate and occur during movements along the san andreas system pliocene pleistocene right lateral faulting a younger extension of the san andreas displacements north of the transverse ranges has broken and displaced all older structural features with total net slip in south central california of perhaps 300 km total right lateral slip along the several strands of the san andreas system are measurable via reconstruction of the mosaic of numerous older predominantly e w trending linear elements mid tertiary antiforms and cretaceous and older rocks right lateral strain on the san andreas appears to either alternate with or to be a byproduct of n s compressional strain from the pleistocene to the present a resultant effect is a large family of e w trending folds and fault scarps in plio pleistocene sediments

paleomagnetism of the ocotillo badlands southern california and implications for slip transfer through an antidilational fault jog abstract the right lateral coyote creek fault is the southernmost segment of the san jacinto fault zone in california at the ocotillo badlands an anticlinorium of locally updomed material is exposed at a step over between echelon segments of the fault here uplift and deformation most probably result from multiple slip increments transferred across the antidilational fault jog under the assumption that material away from the jog region deforms by rigid body translation within the 2 km wide fault jog contraction has occurred by folding of the quaternary sedimentary strata about east west trending hinge lines structural analysis of this deformation shows that folding has accommodated 800 m of fault slip transferred through the antidilational jog by comparison total right slip on the coyote creek fault is 2 5 km measured 25 km to the northwest at coyote ridge a magnetostratigraphic study of the deformed strata was undertaken to determine the longevity of this fault discontinuity at the ocotillo badlands the eroded core of the updomed material at the ocotillo badlands exposes a 325 m thick sequence of sedimentary rock made up by the lacustrine borrego formation 200 m and the overlying conglomeratic ocotillo formation 125 m stepwise thermal demagnetization of samples from the exposed strata suggests that the formation boundary also marks a magnetic polarity reversal of reversed field to normal field as one travels upwards through the composite section the inferred primary magnetization is probably produced by a detrital remanent magnetization drm and is often overprinted with a weak viscous present field component comparison with magnetostratigraphy of the borrego badlands 10 km to the northwest indicates that the reversal sampled is the onset of the jaramillo event 0 97 ma the presence of a 20 angular unconformity within the upper portion of the exposed stratigraphy shows that deformation within the ocotillo badlands began during deposition of the ocotillo formation shortly after the time of the field reversal if total slip on the coyote creek fault at the ocotillo badlands is greater than 800 m it appears that the fault jog has been a transitory feature within the fault zone with slip alternately bypassing it or being transferred across it such switchyard behavior of slip transfer through the fault jog has important implications for understanding structural controls on earthquake rupture

sensitivity of the southern san andreas fault system to tectonic boundary conditions and fault configurations the southern big bend of the san andreas fault saf accommodates transpression along numerous non vertical non planar and intersecting active surfaces using three dimensional boundary element method bem models we test the sensitivity of fault slip rates to a range of tectonic boundary conditions constrained by global positioning system gps studies of the region 45 50 mm yr and 320 325 using this approach our models provide a range of distributed fault slip rates associated with both spatial variations in geometry and present day uncertainties in plate motion model slip rates match most of the available geologic slip rates and discrepancies may owe to inaccurate fault geometries more northerly plate velocity 325 produces greater transpression along the saf system associated with greater uplift of the san bernardino mountains greater reverse slip rates along range bounding reverse thrust faults lower strike slip rates along the san andreas and san jacinto faults and greater strike slip rates along the eastern california shear zone ecsz and garlock fault these results suggest that the degree of regional transpression controls the partitioning of deformation between uplift and slip along both the saf system and the ecsz the insensitivity of modeled slip rates to applied velocity along the southern san andreas and san jacinto faults suggests that fault geometry greatly influences slip rates within the san bernardino mountains region a northerly shift in plate velocity orientation could account for the abandonment of the mill creek strand of the saf 95 ka and development of the present day active geometry which accommodates greater uplift

three dimensional modeling of pull apart basins implications for the tectonics of the dead sea basin we model the three dimensional 3 d crustal deformation in a deep pull apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the dead sea basin the brittle upper crust is modeled by a boundary element technique as an elastic block broken by two en echelon semi infinite vertical faults the deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress free shear zone in the vicinity of the fault zone the bottom displacement represents the regional relative plate motion results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin bounding faults as the width of the shear zone increases the depth of the basin decreases the rotation around a vertical axis near the fault tips decreases and the basin shape the distribution of subsidence normalized by the maximum subsidence becomes broader in contrast two dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone our models also predict full graben profiles within the overlapped region between bounding faults and half graben shapes elsewhere increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin we suggest that the observed structure of the dead sea basin can be described by a 3 d model having a large overlap more than 30 km that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km

mesoscale and regional kinematics of the northwestern yalakom fault system major paleogene dextral faulting in british columbia canada the northwestern yalakom fault system yfs lies on the eastern margin of the coast belt in southwestern british columbia the yalakom fault is the most significant structure in the fault system with at least 115 km of dextral slip mostly of early tertiary age the nw yfs is up to 25 km wide and is bounded on the southwest by the tchaikazan fault zone which had 5 km or more of dextral slip kinematic and paleostress analysis of mesofaults and map scale structures shows that secondary faults within the nw yfs had both dextral and sinistral oblique slip three large synclines had increasing amounts of clockwise vertical axis rotation from southeast to northwest block rotation between two synthetic strike slip faults with large offset yalakom and moderate offset tchaikazan is similar to current kinematics between the san andreas and san jacinto faults in southern california the nw yfs was active from latest cretaceous to mainly eocene time and postdates early late cretaceous thrust faulting to the southwest early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike slip faulting commenced the abrupt northwest termination of the yfs may be related to extension within the adjacent tatla lake metamorphic complex some dextral slip may have been transferred farther northwest through the coast belt but these structures have not been documented in west central british columbia most of the 120 km of dextral slip on the yfs was transferred to the north via a mosaic of normal and strike slip faults in central british columbia thus the yfs forms the southwestern margin of a broad zone of transtensional structures that extends to the tintina rocky mountain trench fault zone these structures accommodated northwestward displacement of the northwestern cordillera in paleogene time

slip line model for southern california crustal kinematics we propose a simple slip line model to explain the faulting and seismicity patterns of southern california we assume the sierra nevada great valley sngv block to be rigid in view of its low seismicity and other regions to undergo plastic deformation from this point of view the complex deformation patterns observed in southern california including the borderland region result from the plowing of the eastern margin of the pacific plate by the nondeforming sngv block a simple analogy can then be made between regional tectonics in southern california and two dimensional flow solutions obtained in metallurgy for the extrusion of rigid plastic metals we use a simplified geometry based on the geology and seismicity of southern california to calculate a two dimensional slip line field which predicts the shape of the san andreas fault near the big bend surprisingly well the largely aseismic high topography western mojave can then be interpreted as an analogue to the nondeforming dead metal zone which arises for certain metal extrusion geometries the slip field agrees quite well with the strikes and slip orientation of major dextral faults in southern california including the san jacinto elsinore and newport inglewood faults it also predicts correctly the strikes and slip orientation of sinistral cross faults found in some areas of the region in the transverse ranges where thrust faulting dominates the slip lines are no longer potential fault planes but a simple argument predicts that they should lie at 45 on the strikes of thrust faults in fair agreement with the observations

neogene tectonic rotation of the san gabriel region california suggested by paleomagnetic vectors previous paleomagnetic work in the western transverse ranges and the islands offshore of california has suggested that these regions have experienced large clockwise rotations since middle miocene time this study is aimed at testing how far east these rotations extend into the continent neogene volcanic rocks from the region bounded by the san gabriel and san andreas faults were selected for paleomagnetic study the sampled units are lavas from the late oligocene to early miocene continental vasquez formation and water laid tuff beds from the middle to late miocene mint canyon formation these rocks possess a stable remanent magnetization which was acquired soon after deposition and is believed to be primary seventy four lava flows from eleven locations in the vasquez formation were studied the average direction for seven locations near the center of the study area suggests that this area has rotated a net clockwise amount of 37 1 12 2 since the early miocene there is no suggestion of north or south translation a location west of the central region is rotated almost 90 clockwise although this is believed to be a local effect detailed magnetic stratigraphy at the western and central location can be correlated over 6 km locations in the east near the san andreas fault yielded highly dispersed directions which cannot now be interpreted in terms of rotation the mean paleomagnetic direction from six sites four different beds in the mint canyon formation west of the vasquez study area is deflected in a counterclockwise sense by 16 30 these data are consistent with a proposed model in which the san gabriel region first was rotated clockwise 53 37 16 in the early miocene and later rotated 16 counterclockwise as it slid northward into the leftward bend region of the san andreas fault the clockwise rotation is believed to be due to dextral shear within the pacific north american plate shear zone counterclockwise rotation in the mojave region is believed to be responsible for the bend in the san andreas fault

block rotation and deformation by strike slip faults 2 the properties of a type of macroscopic discontinuous deformation study of the general features of deformation by systems of subparallel strike slip faults shows that the governing constraints are kinematic the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings as a result fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip spacing and orientation areas where left and right lateral faults move simultaneously are usually divided into domains that contain faults of one kind only deformation of such multidomain areas is nonhomogeneous but the differences between domains can compensate for each other so that the regional deformation is quite simple strike slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents which may often greatly exceed the results of other types of coeval deformation paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data block rotation changes the initial angles between right and left lateral faults and also their orientation relative to the stress field the faults probably tend to rotate away from the axis of maximum compression rotated faults continue to move because they are weak surfaces since the deformation depends on geometric factors such as fault orientation and domain geometry it is not simply related to the stress field these properties including rotation of material units characterize discontinuous deformation in general

transform deformation of tertiary rocks along the tjornes fracture zone north central iceland significant fracture zone deformation has been imparted to tertiary lavas on the peninsula flateyjarskagi located between eyjafjordur and skjalfandi in north central iceland the region lies immediately south of the flatey fault one of a series of wnw trending left stepping faults comprising the right lateral oblique slip husavik flatey fault system hffs the hffs defines the present southern margin of the 70 to 80 km wide tjornes fracture zone tfz lava bedding dikes and faults in the western half of flateyjarskagi display a progressive 0 110 clockwise change of trend and steepening of lava dip angles to 15 45 over an 11 km wide zone nearest the flatey fault accompanying this structural curvature is an increase in fault and dike frequency and amygdule vein mineralization northeast striking faults develop predominant downthrows to the east approaching the tfz accommodating the steep lava dips via repetition of stratigraphic section and normal fault bounded block rotations although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear maximum on land shear strain has been crudely estimated at 3 5 estimates of shear displacement suggest that the tfz was active for at least 2 m y sometime prior to 6 7 ma at a strain rate of approximately 5 5 10 14 s 1 antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished wnw extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear paleomagnetic and structural evidence suggest that rocks of tertiary age on tjornes peninsula immediately to the east also have been tectonically rotated implying an original shear zone width of at least 25 km

great earthquakes and great asperities san andreas fault southern california the big bend region of the southern san andreas fault consists of two great asperities that rupture infrequently in great earthquakes the eastern knot near san gorgonio pass which has not ruptured historically in a large event is the main locus of plate motion appears to break in great events every few hundred years and is more advanced in the cycle of strain accumulation than the western knot the large size of these asperities results in the properties of these earthquakes being nearly invariant over many cycles of great shocks an unusual sequence of moderate size shocks within the eastern knot from 1940 to 1948 is an example of the type of precursory phenomenon that might precede a large earthquake

rates of late cenozoic tectonism in the vallecito fish creek basin western imperial valley california the kinetics of continental rifting are recorded in the late neogene sediments of the western imperial valley california the gulf of california opened in its present form about 4 m y ago capturing the colorado river some 5 km of deltaic and fluvial sediments accumulated in the vallecito fish creek area during the interval 4 3 0 9 m y ago initial sedimentation rates were 5 5 mm yr 1 which diminished in an exponential fashion to 0 5 mm yr 1 when isostatic adjustments are accounted for this implies tectonic subsidence rates of 1 5 mm yr 1 decreasing systematically to 0 1 mm yr 1 after 0 9 m y ago right lateral shearing along the elsinore fault zone cut across the basin causing it to be tilted and uplifted at a mean rate of 5 9 mm yr 1 the basin was concurrently rotated as a unit 35 clockwise the dissected pediments and badlands that now characterize the area were formed and eroded in the past 0 9 m y

fault plane determinations for three small earthquakes along the san jacinto fault california search for cross faults using rupture directivity and aftershock locations three earthquakes ml 4 2 4 3 were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane all three events occurred near the san jacinto fault zone during december 1989 and february 1990 and had a large component of strike slip motion with nodal planes oriented in the northeast and northwest directions smaller earthquakes were used as empirical green functions to deconvolve the path and site complications from the short period p waveforms the resultant far field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data the results of the waveform inversion showed that the earthquake near the anza gap december 2 1989 and the earthquake near san bernardino december 28 1989 ruptured northwest trending planes the earthquake near buck ridge february 18 1990 appeared to rupture a northeast trending plane but the waveform results for this event were not considered as reliable available aftershock data for the three events were also examined using master event relocations the aftershock sequence following the buck ridge event showed a clear lineation trending toward the northeast the results of this study indicate that the anza and san bernardino earthquakes ruptured areas on or parallel to the san jacinto fault while the buck ridge event ruptured a northeast trending cross fault in addition to determining the fault plane the waveform analysis resolved details of the slip distribution for these relatively small earthquakes

new evidence on the slip rate renewal time and late holocene surface displacement southernmost san andreas fault mecca hills california we estimate a renewal time for large earthquakes on the southernmost san andreas fault saf of 260 100 years based on new paleoseismological data and a 34 millennia slip rate of 5 8 mm yr our mean recurrence interval 260 100 is longer and its variance is smaller than the 160 240 93 estimate for a m 7 earthquake used by the working group on california earthquake probabilities wgcep 1995 a smaller creep rate than on contiguous 12 km fault segments is explained by global positioning system data that reveals 2 4 mm yr of creep on the recently discovered witbaard fault that bypasses the 12 km long north shore strand of the saf geomorphic features near an archaeological site that cross the fault allow us to measure displacements of 155 25 cm based on right lateral gully offsets by the most recent earthquake e g 1690 an indian stone ring straddling the fault trace is currently offset 142 12 cm the ring lies on the 12 km long north shore segment of the saf that has accumulated a minimum of 30 mm of triggered slip and aseismic creep since 1969 contrasting slip rates of 23 35 mm yr near indio and 5 14 mm yr on the southernmost saf suggest that the coachella valley strand may be comprised of two distinct segments smaller long term strain accumulation along the southernmost segment probably reduces earthquake potential for that area

geodetic measurement of crustal deformation on the san andreas hayward and calaveras faults near san francisco california analysis of a geodetic network of 115 lines crossing the san andreas hayward and calaveras faults in the vicinity of san francisco bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area the most striking result is that the deformation is not uniformly distributed across the area in the east bay along the hayward and calaveras faults all motion appears to take place as slip directly on the fault with no accumulation of strain in the adjacent crust on both the calaveras and the hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades the hayward fault slip rate is 7 1 mm yr the calaveras fault slip rate is 7 1 mm yr with perhaps half of this slip distributed across a zone a few kilometers wide probably as inelastic deformation of weak near surface material the absence of strain accumulation in the east bay is surprising since the hayward and calaveras faults have been the site of large earthquakes in the past a block located east of the calaveras fault and south of the las positas fault has been rotating clockwise at a rate of 0 3 0 1 rad yr with very little internal deformation along the san francisco peninsula no detectable slip occurs less than 1 5 mm yr at the surface but appreciable strain is accumulating near fault shear strain rates are 0 6 0 1 strain yr engineering with direction n47 w 9 the slip rate near the san andreas fault is 12 2 3 9 mm yr distributed across a broad zone the relative motion across the whole region during the period 1970 1980 is 32 1 7 4 mm yr supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 011 1 00 payment must accompany order

kinematic analysis of rotated fractures and blocks in simple shear deformation in part of the surface rupture of the imperial valley california earthquake of 15 october 1979 provides a geometric and mechanical analog for the model of tectonic evolution of southern california in miocene time as proposed by luyendyk et al 1980 the n35w striking fault broke as an irregular series of left stepping en echelon r shears across a field that prior to the earthquake had been scored by n s oriented furrows between overlapping segments of the r shears elongate blocks of soil separated along the furrows and rotated up to 70 clockwise the furrows functioned as planar discontinuities which guided and accommodated shear strain in those parts of the mole track between the r shears the relatively rigid blocks detached on an irregular horizontal decollement from the less rigid subsoil triangular gaps opened between blocks where rotation was variable blocks buckled or were thrust out of the shear zone where they were constrained by the walls of the shear zone the geometry and kinematics of structures in the mole track virtually duplicate the larger scale structures and requisite movements in the regional tectonic model and show that the regional model is plausible

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

transtensional tectonics along the south scotia ridge antarctica multichannel seismic reflection profiles recorded aboard b o hesperides during the austral summer of 1991 1992 were used to identify the tectonic style of the south scotia ridge along the scotia antarctica plate boundary the ridge is composed of continental crustal fragments transported eastward from the south america antarctic isthmus 28 to 6 ma during the opening of drake passage it is made up of two highs north and south branches of the south scotia ridge separated by a central depression that contains four narrow deeps fragmentation of the ridge during and since its transport to its present position is due to transtensional sinistral motion along the scotia antarctic plate boundary this fragmentation of the ridge appears to have been in two phases during an early phase of transtension which probably took place in the oligocene a half graben fronted by a high along its northern edge was formed along the southern flank of the ridge concurrent with this transtension episode an extensive sediment prism was deposited north of the ridge the second tectonic episode during which the present plate boundary was established in its current location along the central depression may have begun about 4 ma transtensional tectonics along the sinistral transform fault plate boundary during this phase led to the creation of the present tectonic geomorphology of the south scotia ridge extension during this phase is characterized by listric faults dipping both north and south which root into a northerly dipping basal detachment surface motion along these faults caused the block above the detachment surface upper block north branch of the south scotia ridge to undergo some degree of tilting differences in morphology along the north branch suggest that this block tilting varies along strike being the least on its eastern and western ends and maximum in the center this suggests that continuity of the listric faults parallel to the plate boundary is disrupted by transverse structures structures which may have been produced by bends along the plate boundary as the blocks were transported laterally along the transtensional sinistral plate boundary they experienced some degree of counterclockwise rotation along the right lateral transverse structures creating local zones of compression and extension at their corners it is this compression created by the rotating blocks which led to the deformation of the sediments north of the eastern end of the north branch of the south scotia ridge and within the central depression itself

bending within the axial tectonic belt of new zealand in the last 9 myr from paleomagnetic data abstract tongaporutuan stage late miocene 11 6 myr sedimentary rocks at two localities within the axial tectonic belt of new zealand have a direction of magnetisation in which the declination is 020 11 and 030 8 d 95 the directions of magnetisation in rocks of similar age outside the belt have a declination like that of the earth s present axial dipole 360 the clockwise deviation of the declination is inferred to indicate bending of the rocks within the zone of deformation between the pacific and indian plates due to the oblique convergence of these plates

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

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

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

recent and long term behavior of the brawley fault zone imperial valley california an escalation in slip rate the brawley fault zone bfz and the brawley seismic zone constitute the principal transfer zone accommodating strain between the san andreas and im perial faults in southernmost california the bfz ruptured along with the imperial fault in the 1940 mw 6 9 and the 1979 mw 6 4 earthquakes although in each case only minor slip apparently occurred on the bfz several other episodes of slip and creep have been documented on the bfz historically until this study it has been unclear whether the past few decades reflect average behavior of the fault two trenches were opened and a series of auger holes were bored across three strands of the bfz at harris road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record evidence is presented across the westernmost strand of the bfz and across the entire bfz at harris road to show that both the average vertical slip rate observed in modern times since 1970 and the vertical creep rate excluding coseismic slip observed during the 1970s are sig nificantly higher than the long term average across the westernmost strand the long term vertical rate is 1 2 1 5 0 5 mm yr and the average rate since about a d 1710 is determined to be no greater than 2 0 mm yr in contrast the average vertical rate between 1970 and 2004 across that strand was at least 4 3 mm yr and the 1970s vertical aseismic creep rate was 10 mm yr likewise across the entire bfz the long term vertical rate is 2 8 4 1 1 4 mm yr whereas the rate between 1970 and 2004 was at least 7 2 mm yr and the 1970s aseismic creep rate was 10 mm yr the long term strike slip rate cannot be determined across any strands of the bfz but may be significant in contrast to the commonly accepted higher sedimentation rates inferred for the entire imperial valley we find that the average sedimentation rate on the downthrown side of the bfz adjacent to mesquite basin in the millennium preceding the onset of agricultural influences was at most 3 5 mm yr finally a creep event occurred on the bfz during our study in 2002 and is documented herein online material appendices 1 and 2 trench logs and color versions of figures 7 8 and 10

numerical and theoretical analyses of in plane dynamic rupture on a frictional interface and off fault yielding patterns at different scales s u m m a r y we perform numerical simulations of in plane ruptures with spontaneous mohr coulomb yielding in the bulk and analyse properties of the ruptures and yielding zones at different scales using a polar coordinate system we show that the overall shapes and patterns of the simulated yielding zones can be well explained by combining the slip induced coulomb stress change and the background stress although there is no apparent mechanism for preferring synthetic versus antithetic shearing at a scalemuch smaller than the yielding zone size this is not the case at larger scales for shallow angles between the maximum background compressive stress and the fault representing thrust faulting large scale off fault synthetic fractures are dominant but there are two conjugate sets of fractures with a typical size comparable to the yielding zone thickness for smooth rupture propagation with moderate to high values representing large strike slip faults most of the off fault fractures that grow across the entire yielding zone are of the synthetic type the less preferred antithetic set may become more pronounced for rupture propagation encountering fault heterogeneities in particular a strong fault barrier promotes antithetic fractures with a comparable size to those of the synthetic type around the barrier where very high permanent strain is also observed a consideration of non local properties of the stress field in space or time can explain the above differences our results provide an alternative way of understanding riedel shear structures and the potentially preferred synthetic shear fractures suggested in previous studies the examined dynamic processes may be distinguished from quasi static patterns by the timing location and inclination angle of characteristic fracture elements in agreement with other studies we propose that backward or orthogonally inclined antithetic shear fractures on strike slip faults and very high permanent strain could be used as signals that reflect abrupt rupture deceleration on the other hand relative lack of off fault yielding at given locations may indicate abrupt rupture acceleration

neotectonic intersection of the aegean and cyprus tectonic arcs extensional and strike slip faulting in the isparta angle sw turkey abstract the isparta angle and adjacent antalya bay areas constitute an important segment of the eastern mediterranean region located at the intersection of the southward convex aegean and cyprus arcs some recent tectonic maps show the isparta angle as a nw se compressional lineament extending eastwards into the kyrenia range of northern cyprus however fault data from the onshore isparta angle together with offshore shallow seismic reflection data show that the present morpho tectonic setting is dominated by extension the last phase of compression to affect the area studied in the late miocene was accompanied by regional nappe emplacement lycian nappes onshore fault planes measured from fault zones bounding both the limbs and the core of the isparta angle are oriented predominantly ne sw nw se and n s superimposed slickenfibres show that reverse faults were succeeded in turn by right lateral faults then by normal faults the fault phases are dated by stratigraphical and geomorphological evidence reverse faults date from the late miocene or earlier compressional deformation whereas the right lateral faults mainly developed during latest miocene early pliocene normal faulting dominated from the late pliocene recent an interpretation of shallow seismic reflection data shows that antalya bay is characterised by a nw se trending asymmetrical graben system that has continued to be active during the late miocene early pliocene right lateral strike slip resulted from shear along the eastern termination of a zone of extension and rotation that characterises the western aegean this shear was focused in a n s direction by inherited zones of structural weakness in the basement antalya complex the switch to ne sw extension in the late pliocene quaternary relates to a regional change in stress direction throughout the aegean region and was accompanied by strong uplift of the bey da lari region of the taurus mountains bordering the isparta angle in the west the isparta angle is the link between a the extensional province of western turkey bounded to the south by the actively subducting hellenic arc and b the uplifted anatolian plateau bounded to the south by the cyprus subduction zone understanding the miocene to recent tectonic development helps elucidate the kinematics of the region the new structural data presented lend no support for recent suggestions that the isparta angle and antalya bay represent parts of a regional compressional zone related to plate collision

does the great glen fault really disrupt moho and upper mantle structure many previous interpretations of deep seismic reflection profiles across the great glen strike slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the moho discontinuity such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the british isles reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the moho step interpretation diffraction analysis and seismic migration applied to reflection data north of shetland across the northward continuation of the great glen fault walls boundary fault indicate a narrow synform or keel developed on the moho directly beneath the fault this keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault north and west of ireland analyses of amplitudes frequencies and the geometrical behavior of reflections upon migration show that structures previously interpreted as moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin in one location north of ireland where some three dimensional control exists the vertically downward projection of the great glen fault intersects without disrupting dipping structure in the upper mantle this observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp or blind thrust in the uppermost mantle somewhat analogous to models developed for the san andreas fault that indicate displacement along the fault to be laterally offset within the middle crust one of the principal conclusions of this study that major vertical steps on the moho beneath the great glen fault are difficult to support from the available seismic data is consistent with theologically based studies which predict that moho topography such as vertical steps is unlikely to be preserved over long periods of geologic time

sawtooth segmentation and deformation processes on the southern san andreas fault california five contiguous 12 13 km fault segments form a sawtooth geometry on the south ernmost san andreas fault the kinematic and morphologic properties of each segment depend on fault strike despite differences of strike between segments of as little as 3 degrees oblique slip transpression of fault segments within the indio hills mecca hills and durmid hill results from an inferred 8 1 ratio of dex tral slip to convergence across the fault zone triggered slip and creep are confined almost en tirely to transpressive segments of the fault durmid hill has been formed in the last 28 n 6 ka by uplift at an average rate of 3 1 mm a

confluence and intersection of interacting conjugate faults a new concept based on analogue experiments abstract examination of the interaction area of conjugate faults yields evidence of a yet unknown structural pattern analogue experiments revealed the evolution of this pattern during the interaction process a fault propagating towards the highly active section of another conjugate fault becomes increasingly deflected towards the obtuse angle side of the conjugate system while approaching until joining it during this process the tip of the curved propagating fault retains its shear sense until immediately before joining with the highly active fault a process called confluence in this report under constant remote stress the deflected fault now becomes dominant a fault branch splits off at the point where deflection commences propagating straight ahead of the original direction and may dissect the former master fault alternating activity at both of the faults finally produces the known intersection patterns further studies of the proposed concept on natural examples are recommended due to the close geometric similarity of the evolutionary structural stages and natural fault patterns

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

modeling of neogene crustal block rotation case study of southeastern california numerical modeling using a two dimensional distinct element method dem was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the pacific north america transform plate boundary paleomagnetic and structural investigations have shown that large 30 50 horizontal rotations of crustal blocks occurred in the eastern transverse zone and the northeast mojave block southeastern california since the end of early miocene the main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not the simulations were performed using different stress boundary conditions and different parameters of the fault strength applying an assumed ratio of 2 for the horizontal stress components as boundary condition block rotations in the order of 40 only occur if 1 the major horizontal stress sh is oriented n20 e with a maximal deviation of 15 to the east or west and 2 the fault strength described by the friction coefficient is smaller than 0 12 or 14 only of the value 0 85 for intact rocks byerlee s law for a weakening model of the fault strength an initial friction coefficient of 0 85 allows also rotations of about 40 if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0 12 or smaller for the same amount of rotation 40 a decreasing stress ratio requires a lowering of the friction coefficient used whereas an increase of the ratio allows a higher friction coefficient the simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation especially the cumulative slip values discrepancies in the observations and modeling results for the western parts of the mojave desert are the result of a neglect of the three dimensional deformation in our two dimensional modeling

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

foreshocks 1966 1980 in the san andreas system california the spatial and temporal distributions of seismicity preceding moderate ml 5 0 main shocks in the san andreas fault system in california have been analyzed to recognize and characterize the patterns of foreshock occurrence of 20 main shocks in the san andreas system 7 or 35 per cent have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks a possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike slip earthquakes had foreshocks some enhanced seismic activity was also observed at relatively large distances from the main shock 13 to 30 km 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity of the seven immediate foreshock sequences only two had the swarm like appearance of the class ii foreshocks defined by mogi the other foreshock sequences appear to be single events sometimes with their own aftershocks preceding the respective main shocks four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock and similar discontinuities may also be associated with the other sequences the durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km which has been interpreted as a dependence on stress to account for this stress dependence of the duration and the presence of discontinuities a model for foreshocks occurrence is presented this model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

kinematic evolution of the olinghouse fault and the role of a major sinistral fault in the walker lane dextral shear zone nevada usa abstract the olinghouse fault is a highly oblique sinistral fault in the northern walker lane dextral shear zone in this study we use geologic mapping and structural analysis of 56 fault surfaces and several folds to constrain the evolution and kinematic role of the olinghouse fault within the walker lane we document a two stage development of the olinghouse fault with an early phase of extension between 12 and 10 ma followed by left lateral shear with a component of extension beginning at 8 to 5 ma the sinistral olinghouse fault has played a relatively complex kinematic role in the evolution of the walker lane including acting as a boundary for clockwise vertical axis block rotation a strain transfer zone between loci of extension and a structural domain boundary redistributing strain between the carson and pyramid lake domains the fault has accumulated 2 5 3 km of left lateral offset at an average rate of 0 3 0 6 mm yr the complexity of this sinistral normal fault zone within a broad zone of dextral shear was likely influenced by the propagation of walker lane dextral shear into a zone of pre existing crustal structure

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

magnetotelluric study to characterize kachchh mainland fault kmf and katrol hill fault khf in the western part of kachchh region of gujarat india abstract the kachchh mainland fault kmf is a major e w trending fault in the kachchh region of gujarat extending 150 km from lakhpat village in the west to the bhachau town in the east the katrol hill fault khf is an e w trending intrabasinal fault located in the central region of kachchh basin and the south of kmf the western parts of both of the faults are characterized and the sediment thickness has been estimated in the region using a magnetotelluric mt survey at 17 sites along a 55 km long north south profile with a site spacing of 2 3 km the analysis reveals that the maximum sediment thickness is 2 3 km quaternary tertiary and mesozoic in the region out of which the mesozoic sediments feature a maximum thickness of 2 km the estimated sediment thickness is found consistent with the thickness suggested by a deep borehole depth approx 2 5 km drilled by oil and natural gas corporation ongc at nirona northern part of the study area from 2 d inversion of the mt data three conductive zones are identified from north to south the first conductive zone is dipping nearly vertical down to 7 8 km depth it becomes north dipping below 8 km depth and is inferred as kmf the second conductive zone is found steeply dipping into the southern limbs near manjal village 28 km south of nirona which is inferred as the khf a vertical dipping down to 20 km depth conductive zone has also been observed near ulat village located 16 km north of manjal village and 12 km south of nirona village this conductive zone becomes listric north dipping beyond 20 km depth it is reported first time by a geophysical survey in the region

satellite and hydrographic observations of eddy induced shelf slope exchange in the northwestern gulf of alaska 1 satellite and hydrographic observations of oceanographic conditions in the northwestern gulf of alaska indicate that anticyclonic eddies propagating adjacent to the continental shelf alter the structure of the shelf break front and in doing so influence the shelf slope exchange of biota and water mass properties eddies typically form in the northern gulf of alaska during fall and winter and propagate southwestward within an 200 km wide corridor along and above the continental slope eddy activity within this corridor diminishes in the downstream direction the trajectories of faster propagating 4 km day eddies tend to lie closer to the shelf break than do the trajectories of slower propagating 1 5 km day eddies the interaction between azimuthal eddy currents and the shelf break frontal jet 1 establishes an upwelling zone associated with the leading flank of the eddy that strengthens cross slope gradients 2 weakens cross slope gradients and promotes shelf slope exchange where the eddy is adjacent to the shelf and 3 reestablishes a weak upwelling zone associated with the trailing flank of the eddy

strain localization along the san andreas fault consequences for loading mechanisms the orientation of the near fault strain accumulation associated with the relative motion between the pacific and north american plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion this observation follows from consideration of shear strain results from geodetic studies ranging from the imperial valley to shelter cove along the san andreas fault system this set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other both the localization of the strain associated with pacific north american motion and the orientation of the associated strain can be simply caused by failure of the lithosphere as opposed to localization of plate driving forces two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature 1 viscoelastic models which are dominated by horizontal layering of the crust and 2 elastic half space models with geometry which is dominantly vertical horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes vertical models produce strains which are parallel to the local structure at all times if the horizontal detachments of the viscoelastic models exist they must be at depths 2 3 times the seismogenic thickness geologic studies also indicate that major transcurrent features extend as ductile shear zones to depths much larger than would be expected in horizontal models these considerations lead us to favor models with vertical geometries the vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain softening processes associated with flow of composite materials such as quartzo feldspathic rocks independent of the horizontal rheological layering in the starting low strain material progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material thus shear zones such as the san andreas system develop vertical tabular geometries which extend far below the brittle plastic transition and which are mechanically persistent

behavior of the superstition hills fault during the past 330 years we have investigated the recent prehistoric behavior of the superstition hills fault by examining its effect on the beach deposits of ancient lake cahuilla excavation of these sediments in three dimensions where they are cut by the fault has enabled determination of total offset since the latest highstand of the lake about 330 years ago as of 3 march 1988 total dextral offset was 1106 50 mm about 609 mm of this amount can be attributed to one or more slip events before 1987 the remaining slip occurred during the moderate earthquake of 24 november 1987 and as subsequent aftercreep additional aftercreep here might produce a total of 1210 100 mm for that event thus if afterslip continues according to our prediction slip associated with the penultimate seismic slip event on the superstition hills fault was only about half of the co seismic slip and predicted afterslip associated with the 1987 earthquake this difference established at a single site on the fault may reflect a larger size for the 1987 event than for its predecessor we calculate that the slip rate of the superstition hills fault zone averaged over the past 330 years is between about 2 and at least 6 mm yr at this site during this time the average interval between large surface slip events on the superstition hills fault has been between about 150 and 300 years the pre 1987 slip event documented in our excavations could have occurred at any time between a d 1660 and about a d 1915 these results are significant for understanding earthquake recurrence and patterns of earthquakes in the imperial and coachella valleys

quaternary flake tectonics of the california transverse ranges p residual studies suggest that the pacific american plate boundary in the upper mantle lies in the central mojave desert northeast of its crustal position at the san andreas fault focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north dipping thrust faults resulting in a mid crustal detachment in which the central transverse ranges move south relative to the mantle this makes sense geologically if the central transverse ranges and western mojave desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile pelona schist or catalina schist which makes up the zone of decollement such ductile schists would occur today at depths comparable to those at which they were metamorphosed the detachment breaks through to the surface at the point dume malibu coast fault zone and the red mountain san cayetano santa susana sierra madre cucamonga fault zone the latter coinciding with the southern edge of the palmdale bulge the transverse ranges flake now moves west northwest relative to the western mojave flake south southwest relative to the pacific plate and northwest relative to the great valley sierra nevada the mantle boundary may rejoin the san andreas fault north of a southwest trending zone of high seismicity in the tehachapi mountains which may be produced by convergence the age of the present tectonic system may be the same as the age of the break through thrust faults these are probably no older than 2 m y and may be even younger

geochemical characteristics of tertiary saline lacustrine oils in the western qaidam basin northwest china abstract based on the systematic analyses of light hydrocarbon saturate aromatic fractions and c isotopes of over 40 oil samples along with related tertiary source rocks collected from the western qaidam basin the geochemical characteristics of the tertiary saline lacustrine oils in this region was investigated the oils are characterized by bimodal n alkane distributions with odd to even c11 c17 and even to odd c18 c28 predominance low pr ph mostly lower than 0 6 high concentration of gammacerane c35 hopane and methylated mttcs reflecting the high salinity and anoxic setting typical of a saline lacustrine depositional environment mango s k1 values in the saline oils are highly variable 0 99 1 63 and could be associated with the facies dependent parameters such as pr ph and gammacerane indexes compared with other tertiary oils the studied tertiary saline oils are marked by enhanced c28 sterane abundance 30 or more of c27 c29 homologues possibly derived from halophilic algae it is noted that the geochemical parameters of the oils in various oilfields exhibit regular spatial changes which are consistent with the depositional phase variations of the source rocks the oils have uncommon heavy c isotopic ratios 24 to 26 and a flat shape of the individual n alkane isotope profile and show isotopic characteristics similar to marine organic matter the appearance of oleanane and high 24 24 27 norcholestane ratios 0 57 0 87 in the saline oils and source rocks confirm a tertiary organic source

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

contemporary block tectonics california and nevada well determined fault plane solution and the gross pattern of late cenozoic faulting in california and nevada show a systematic relation between the orientation of fault planes and slip directions in general normal faults have northerly strikes reverse faults have easterly strikes and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes respectively kinematically this relation is consistent with the response of clusters of fault bounded crustal blocks to a regional stress field generated by the relative motion between the pacific and north american plates in this stress field the greatest and least principal compressive stresses are restricted to northerly and easterly striking vertical planes respectively clusters of crustal blocks bound by faults having the above attitudes form three basic regimes 1 a spreading regime with a combination of normal and conjugate strike slip faults 2 a converging regime with a combination of reverse and conjugate strike slip faults and 3 a wrench regime with sets of subparallel throughgoing northwest striking dextral faults or northeast striking sinstral faults these three regimes are typified by faulting patterns in the basin and range province spreading the transverse ranges converging and the san andreas garlock fault systems wrench respectively the gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north south shortening and east west extension with the ratio of extensional to shortening strains and the areal dilitation decreasing systematically from spreading to wrench to compressional regimes the wrench regime involves a component of net rotational deformation clockwise for dextral slip and counter clockwise for sinstral slip while deformation of the spreading and converging regimes is irrotational local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength byerlee s law along preexisting fractures and that pore pressure in the brittle crust is laterally uniform a minimum strength difference between active block bounding faults and block interiors is 15 30 simple arrangements of such block clusters mimic the gross kinematic pattern of quaternary faulting in california and nevada some implications for contemporary tectonics emphasized by this model involve the westward displacement of the sierra nevada block with respect to the stable interior of the north american plates oblique thrusting of the salinian block over the pacific plate and a progressive increase in the offset of the san andreas fault represented by the big bend through the transverse ranges