observation and scaling of microearthquakes from the taiwan chelungpu fault borehole seismometers summary microearthquakes with magnitude down to 0 3 were detected by the taiwan chelungpu fault drillingproject borehole seismometers tcdpbhs despitethelargecoseismic slipof12m atthedrillsiteduringthe1999chi chiearthquake ourstudiesshowverylittleseismicitynear the tcdpbhs drill site 6 yr after the chi chi main shock the microearthquakes clustered at a depth of 9 12 km where the chelungpu thrust fault turns from a 30 dipping into the horizontal decollement of the taiwan fold and thrust tectonic structure continuous gps surveys did not observe post slip deformation at the larger slip region and no seismicity was observed near the drill site therefore we suggest that the thrust belt above the decollement is locked during this interseismic period we further investigated source parameters of 242 microearthquakes by fitting 2 shaped brune source spectra to our observation data using a frequency independent q model we find that the static stress drop increases significantly with increasing seismic moment however due to the intense debate on this topic of scalingrelations and the related self similarity of earthquakes we further improve the data analysis and correct for path and site effects using the projected landweber deconvolution pld method for events within some clusters the pld method analyses the source time functions of the larger and the smaller event by an iterative technique as a result we received source dimensions and stress drops of larger events including path and site effect corrections the results from the pld method are less scattered and also show a positive relation between static stress drop and seismic moment we find a similar positive trend for the apparent stress scaling with seismic moment

san andreas fault geometry through the san gorgonio pass california three dimensional numerical models are needed to investigate the role of nonvertical strike slip fault segments on the deformation within restraining bends numerical models simulate geologic deformation of two alternative three dimensional present day configurations for the san andreas fault through the restraining bend within the san gorgonio pass region sgpr in southern california both models produce decreasing strike slip rates southward along the san bernardino strand of the san andreas fault similar to geologic data the north dipping san andreas fault model better matches the available strike slip data as well as the geologic uplift data for the southern san bernardino mountains than the vertical san andreas fault model we conclude that a north dipping fault configuration is preferred for models of the san andreas fault in the sgpr the complexity of the active fault geometry at the sgpr promotes the transfer of strike slip from the san andreas fault to the nearby but unconnected san jacinto fault slip rates and uplift patterns are sensitive to fault geometry within strike slip restraining bends

seismic reflection character of the cameroon volcanic line evidence for uplifted oceanic crust deep imaging multifold seismic fines across submarine parts of the cameroon volcanic line west africa gulf of guinea show asymmetric uplift of oceanic crust associated with extensive magmatism the main pulse of uplift occurred after creation of a regional sequence boundary believed to be miocene in age the apparent synchroneity of uplift argues against the cameroon line being a simple hotspot trace as previously inferred one plausible theory of origin for the seaward part of the cameroon volcanic line and its asymmetric uplift geometry combines regional asthenospheric upwelling with restriction of magmatic egress to regularly spaced weak spots corresponding to fracture zone crossings horizontal motion and buckling also may have occurred along the cameroon volcanic line

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

the structure of composite volcanoes unravelled by analogue modelling a review abstract volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures a documentation of these structures facilitates the assessing of the various hazards associated with volcanoes e g flanks instabilities and eruptive events however structures are not easily assessed in the field as volcanoes are regularly covered by eruptive products and are rapidly eroded in the case of recent and active volcanoes internal structures are generally extrapolated from the external morphology of the edifice which can be documented using digital elevation models dem and geodetic data interpretation of such data greatly benefits from analogue models which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice this short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones the other volcanic morphologies e g caldera are excluded from this review as they have distinct structure and evolution that have already been thoroughly reviewed the reviewed models were used to investigate the impact that regional faults magma injections and ductile substratum among other processes have on the internal structure and surface morphology of many volcanic edifices these models were also used to locate unstable flanks and eruption sites

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

transtension driving volcano edifice anatomy insights from andean transverse to the orogen tectonic domains abstract dykes and minor eruptive vents are not randomly distributed within tectonically controlled volcanic arcs which is reflected in the resulting morphology of stratovolcanoes as a first order approach regional stress field controls the spatial distribution and geometry of dykes and minor eruptive vents however favourably oriented pre existing weakness zones such as inherited fault systems parallel or close to parallel to the orogen scale shortening axis may reactivate and be the loci for magma migration within the upper crust in intra arc regions field mapping and satellite image analyses were used to establish the callaqui stratovolcano csv structural setting in the andean southern volcanic zone central southern chile geometrical and morphometric analyses of intravolcanic dykes and minor eruptive vents respectively yields a consistent model of dykes propagating and erupting into either 1 n 60 e 5 oriented inherited pre existing fractures in central portions of the volcanic edifice and or 2 regional stress controlled n 70 e 10 self propagating fractures at distal zones a wing crack like structural array of the intra volcanic dyke swarm documents a local right lateral transtensional setting driving the late pleistocene to holocene volcano building up process the latter plays a fundamental role in the progressive shaping of the csv elongated anatomy in space and time parallel dyking and fissural eruptions contribute to successive volcanic output in a predominant ene orientation on the other hand a prominent elongated relief enhances normal to the volcano elongation flank instability developing steeper flanks that could trigger sector collapses this may promote an increase in the extensional component reinforcing further parallel dyking and subsequent eruptions these results improve the understanding of the tectonic role in magma migration processes within the upper crust which is crucial to understand geothermal systems and to minimize hazards related to volcano instability

morphometry and evolution of arc volcanoes volcanoes change shape as they grow through eruption intrusion erosion and deforma tion to study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs central america and the southern central andes using shuttle radar topography mission srtm digital elevation models we compute and defi ne parameters for plan ellipticity irregularity and profi le height width summit basal width slope shape as well as size height width volume we classify volcanoes as cones sub cones and massifs and recognize several evolutionary trends many cones grow to a critical height 1200 m and volume 10 km 3 after which most widen into sub cones or massifs but some grow into large cones large cones undergo sector collapse and or gravitational spreading without sig nifi cant morphometry change other smaller cones evolve by vent migration to elliptical sub cones and massifs before reaching the critical height the evolutionary trends can be related to magma fl ux edifi ce strength structure and tectonics in particular trends may be controlled by two balancing factors magma pressure versus lithostatic pressure and conduit resistance versus edifi ce resistance morphometric analysis allows for the long term state of individual or volcano groups to be assessed morphological trends can be integrated with geological geophysical and geochemical data to better defi ne volcano evolution models

complexities of the san andreas fault near san gorgonio pass implications for large earthquakes 1 geologic relationships and patterns of crustal seismicity constrain the three dimensional geometry of the active portions of san andreas fault zone near san gorgonio pass southern california within a 20 km wide contractional stepover between two segments of the fault zone the san bernardino and coachella valley segments folds and dextral reverse and dextral normal faults form an east west belt of active structures the dominant active structure within the stepover is the san gorgonio pass garnet hill faults a dextral reverse fault system that dips moderately northward within the hanging wall block of the san gorgonio pass garnet hill fault system are subsidiary active dextral and dextral normal faults these faults relate in complex but understandable ways to the strike slip faults that bound the stepover the pattern of crustal seismicity beneath these structures includes a 5 8 km high east west striking step in the base of crustal seismicity which corresponds to the downdip limit of rupture of the 1986 north palm springs earthquake we infer that this step has been produced by slip on the linked san gorgonio pass garnet hill coachella valley banning sgp gh cvb fault this association enables us to construct a structure contour map of the fault plane the large step in the base of seismicity downdip from the sgp gh cvb fault system probably reflects a several kilometers offset of the midcrustal brittle plastic transition u th he thermochronometry supports our interpretation that this south under north thickening of the crust has created the region s 3 km of topographic relief we conclude that future large earthquakes generated along the san andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1 20 km two tasks in seismic hazard evaluation may now be attempted with greater confidence first the construction of synthetic seismograms that make useful predictions of ground shaking and second theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures

stress drop heterogeneity within tectonically complex regions a case study of san gorgonio pass southern california in general seismic slip along faults reduces the average shear stress within earthquake source regions but stress drops of specific earthquakes are observed to vary widely in size to advance our understanding of variations in stress drop we analysed source parameters of small magnitude events in the greater san gorgonio area southern california in san gorgonio the regional tectonics are controlled by a restraining bend of the san andreas fault system which results in distributed crustal deformation and heterogeneous slip along numerous strike slip and thrust faults stress drops were estimated by fitting a brune type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra the estimates have large scatter among individual events but the median of event populations shows systematic statistically significant variations we identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting changes in average tectonic slip rates mineralogical composition of the host rocks as well as the hypocentral depths of seismic events we observed anomalously high stress drops 20 mpa in a small region between the traces of the san gorgonio and mission creek segments of the san andreas fault furthermore the estimated stress drops are higher below depths of 10 km and along the san gorgonio fault segment but are lower both to the north and south away from san gorgonio pass showing an approximate negative correlation with geologic slip rates documenting controlling parameters of stress drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes

determination of earthquake energy release and ml using terrascope we estimated the energy radiated by earthquakes in southern california using on scale very broadband recordings from terrascope the method we used involves time integration of the squared ground motion velocity and empirical determination of the distance attenuation function and the station corrections the time integral is typically taken over a duration of 2 min after the p wave arrival the attenuation curve for the energy integral we obtained is given by q r cr n exp kr r 2 2 h ref 2 with c 0 49710 n 1 0322 k 0 0035 km 1 and h ref 8 km where is the epicentral distance a similar method was used to determine m l using terrascope data the station corrections for m l are determined such that the m l values determined from terrascope agree with those from the traditional optical wood anderson seismographs for 1 5 6 5 m l saturates the ratio e s m 0 m 0 seismic moment a measure of the average stress drop for six earthquakes the 1989 montebello earthquake m l 4 6 the 1989 pasadena earthquake m l 4 9 the 1990 upland earthquake m l 5 2 the 1991 sierra madre earthquake m l 5 8 the 1992 joshua tree earthquake m l 6 1 and the 1992 landers earthquake m w 7 3 are about 10 times larger than those of the others that include the aftershocks of the 1987 whittier narrows earthquake the sierra madre earthquake the joshua tree earthquake and the two earthquakes on the san jacinto fault the difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times the aftershocks which occurred on the fault plane where the mainshock slippage occurred had a very short time to heal hence a low stress drop the repeat time of the major earthquakes on the frontal fault systems in the transverse ranges in southern california is believed to be very long a few thousand years hence the events in the transverse ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times such as the san andreas fault and the san jacinto fault the observation that very high stress drop events occur in the transverse ranges and the los angeles basin has important implications for the regional seismic potential the occurrence of these high stress drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events characterization of earthquakes in terms of the e s m 0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the los angeles basin and the transverse ranges

experimental study of the interplay between magmatic rift intrusion and flank instability with application to the 2001 mount etna eruption mount etna volcano is subject to transient magmatic intrusions and flank movement the east flank of the edifice in particular is moving eastward and is dissected by the timpe fault system the relationship of this eastward motion with intrusions and tectonic fault motion however remains poorly constrained here we explore this relationship by using analogue experiments that are designed to simulate magmatic rift intrusion flank movement and fault activity before during and after a magmatic intrusion episode using particle image velocimetry allows for a precise temporal and spatial analysis of the development and activity of fault systems the results show that the occurrence of rift intrusion episodes has a direct effect on fault activity in such a situation fault activity may occur or may be hindered depending on the interplay of fault displacement and flank acceleration in response to dike intrusion our results demonstrate that a complex interplay may exist between an active tectonic fault system and magmatically induced flank instability episodes of magmatic intrusion change the intensity pattern of horizontal flank displacements and may hinder or activate associated faults we further compare our results with the gps data of the mount etna 2001 eruption and intrusion we find that syneruptive displacement rates at the timpe fault system have differed from the preeruptive or posteruptive periods which shows a good agreement of both the experimental and the gps data therefore understanding the flank instability and flank stability at mount etna requires consideration of both tectonic and magmatic forcing

propagation linkage and interaction of caldera ring faults comparison between analogue experiments and caldera collapse at miyakejima japan in 2000 the formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash flow eruptions caldera collapse occurred in 2000 at miyakejima island japan in response to a lateral intrusion based on geophysical data it is inferred that a set of caldera ring faults was propagating upward to understand the kinematics of ring fault propagation linkage and interac tion we describe new laboratory sand box experiments that were analyzed through digital image correlation dic and post processed using 2d strain analysis the results help us gain a better understanding of the processes occurring during caldera subsidence at miyakejima we show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards then a set of normal faults initiates from tension fractures at the surface propagating downwards to link with the reverse faults at depth with increasing amounts of subsidence interaction between the reverse and normal fault segments results in a deactivation of the reverse faults while displacement becomes focused on the outer normal faults modeling results show that the area of faulting and collapse migrates successively outward as peak displace ment transfers from the inner ring faults to later developed outer ring faults the final structural architecture of the faults bounding the subsiding piston like block is hence a consequence of the amount of subsidence in agreement with other caldera structures observed in nature the experimental simulations provide an analogy to the obser vations and seismic records of caldera collapse at miyake jima volcano but are also applicable to caldera collapse in general

radiated seismic energy from coda measurements and no scaling in apparent stress with seismic moment 1 the seismic coda consists of scattered waves that leave the earthquake source in a variety of directions the averaging of source radiation that results leads to stable ground motion spectra that we use as the basis for a robust measurement of radiated wave energy we apply an empirical green s function egf method to the seismic coda in order to investigate scaling of the radiated seismic energy we correct for path effects in the spectra of earthquakes by using a stack of closely located small earthquakes as an egf we apply this approach to four earthquake sequences in western north america that span a magnitude range from mw 3 0 mw 7 1 our estimates of scaled energy are consistent with independent measurements where available we find no dependence in individual seismic energy estimates on source station distance which validates the egf approximation we find that a constant scaled energy of 3 5 10 5 provides a reasonable fit to the data with no dependence of the scaled energy on seismic moment

dating lava flows of tropical volcanoes by means of spatial modeling of vegetation recovery the age of past lava flows is crucial information for evaluating the hazards and risks posed by effusive volcanoes but traditional dating methods are expensive and time consuming this study proposes an alternative statistical dating method based on remote sensing observations of tropical volcanoes by exploiting the relationship between lava flow age and vegetation cover first the factors controlling vegetation density on lava flows as represented by the normalized difference vegetation index ndvi were investigated these factors were then integrated in pixel based multi variable regression models of lava flow age to derive lava flow age maps the method was tested at a pixel scale on three tropical african volcanoes with considerable recent effusive activity nyamuragira democratic republic of congo mt cameroon cameroon and karthala the comoros due to different climatic and topographic conditions the parameters of the spatial modeling are volcano specific validation suggests that the obtained statistical models are robust and can thus be applied for estimating the age of unmodified undated lava flow surfaces for these volcanoes when the models are applied to fully vegetated lava flows the results should be interpreted with caution due to the saturation of ndvi in order to improve the accuracy of the models when available spatial data on temperature and precipitation should be included to directly represent climatic variation

gravitational spreading and formation of new rift zones on overlapping volcanoes large volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading to better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age and examined the formation of rift zones the results allow distinction of two main rift geometries i spreading edifices of similar age that partly overlap tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge ii partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes such a rift zone causes two volcanoes to structurally separate the results are widely applicable for large volcanoes subject to rifting and flank spreading which we demonstrate for reunion island and for southern hawaii

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

lithostratigraphic geophysical and morpho tectonic studies of the south cameroon shelf abstract a geological and structural sketch of the southern cameroon shelf was drawn on the basis of a high resolution seismic survey dredgings and kullenberg core sampling of the outcropping rocks at sea bed palynological and mineralogical data x ray diffractometry supported by occasional foraminiferal evidence revealed four main lithostratigraphic sets resting on a precambrian basement upper cretaceous albian to senonian paleocene lower eocene miocene pliocene and pleistocene three successive deformational types characterise the structure of this margin 1 n s trending fractures parallel to the onshore precambrian structural directions formed during the rifting period and crosscut by n60 e trending cross faults induced by the transform movement of the south atlantic 2 compressive structures governed by local stresses during the eocene transcurrent reactivation along the n60 e transform faults excluding a salt controlled deformation 3 a set of approximately n s faults affecting the unfolded sequence of the miocene pliocene or the quaternary and indicative of distensive conditions the geodynamic evolution is placed in the general framework of the central and south atlantic rifting and passive margin history interestingly the age of the compressive deformation varies from late albian to mid eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain

a new perspective on the geometry of the san andreas fault in southern california and its relationship to lithospheric structure the widely held perception that the san andreas fault saf is vertical or steeply dipping in most places in southern california may not be correct from studies of potential field data active source imaging and seismicity the dip of the saf is significantly nonvertical in many locations the direction of dip appears to change in a systematic way through the transverse ranges moderately southwest 55 75 in the western bend of the saf in the transverse ranges big bend vertical to steep in the mojave desert and moderately northeast 37 65 in a region extending from san bernardino to the salton sea spanning the eastern bend of the saf in the transverse ranges the shape of the modeled saf is crudely that of a propeller if confirmed by further studies the geometry of the modeled saf would have important implications for tectonics and strong ground motions from saf earthquakes the saf can be traced or projected through the crust to the north side of a well documented high velocity body hvb in the upper mantle beneath the transverse ranges the north side of this hvb may be an extension of the plate boundary into the mantle and the hvb would appear to be part of the pacific plate

field evidence for flank instability basal spreading and volcano tectonic interactions at mt cameroon west africa the mt cameroon volcano is the highest and most active volcano of the cameroon volcanic line little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics after reviewing the tectonic evolution of the region the analysis of a digital elevation model and results from a field campaign dedicated to mapping geological structures in the summit area and at the se base of mt cameroon are presented mt cameroon is a lava dominated volcano with long steep over 30 flanks it is elongate parallel to its well defined rift zone the summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults geological profiles were measured along rivers cutting through a topographic step at the se base of mt cameroon this step is associated with deformed miocene sediments from the douala basin that are overlain by volcanic products weak sediments of this area are deformed by 050 060 and 020 030 trending asymmetrical folds verging toward the se and thrusts faults related to the spreading of the volcano over its mechanically weak substratum combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice repeated dyke intrusions the weak sedimentary substratum and tectonic structures

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

vertical and lateral collapse of tharsis tholus mars tharsis tholus a more than 3 9 ga old composite shield volcano to the east of the major tharsis montes has experienced a complex history of growth and destruction on the basis of new high resolution images we analysed the morphology as well as the tectonic structures of the tharsis tholus volcano in detail from morphological data cross cutting relations of the surface structures and crater modelling ages we propose a chronostratigraphy for the volcano tectonic history of tharsis tholus the strongly faulted volcano reveals two large scale landslide events followed by two subsequent shield re growth phases between 3 8 and 1 7 ga and two caldera collapses tharsis tholus was also affected by regional extensional tectonics between 1 7 ga and 0 4 ga recorded by sub parallel sets of ne trending graben structures the steep and up to 5 4 km high landslide scarps on tharsis tholus suggest deep faulting of the edifice in order to confirm this hypothesis we used analogue sand box models in which we demonstrated that gravitational flank movement on top of weak basal substrata may have produced the deformation structures as observed on tharsis tholus c 2011 elsevier b v all rights reserved

repeating earthquakes as low stress drop events at a border between locked and creeping fault patches the source of repeating earthquakes on creeping faults is modeled as a weak asperity at a border between much larger locked and creeping patches on the fault plane the x 1 2 decrease in stress concentration with distance x from the boundary is shown to lead directly to the observed scaling t m 1 6 between the average repeat time and average scalar moment for a repeating sequence the stress drop in such small events at the border depends on the size of the large locked patch for a circular patch of radius r and representative fault parameters 7 6 m r 3 5 mpa which yields stress drops between 0 08 and 0 5 mpa 0 8 5 bars for r between 2 km and 100 m these low stress drops are consistent with estimates of stress drop for small earthquakes based on their seismic spectra however they are orders of magnitude smaller than stress drops calculated under the assumption that repeating sources are isolated stuck asperities on an otherwise creeping fault plane whose seismic slips keep pace with the surrounding creep rate linear streaks of microearthquakes observed on creeping fault planes are trivially explained by the present model as alignments on the boundaries between locked and creeping patches

landslides and spreading of oceanic hot spot and arc shield volcanoes on low strength layers lsls an analogue modeling approach during their growth and evolution oceanic hot spot and arc shield volcanoes are destabilized and often dismantled by landslides the origin of these phenomena can be found in the structure of volcanic islands marked by several low strength horizons such as a marine substratum hyaloclastites hydrothermally altered rocks or volcaniclastic deltas that weaken the edifice we have tested the role of these low strength layers lsls with analogue modeling we conclude that destabilization on these levels can lead to landslides or gravitational spreading in our experiments slides generated are only slump type events but they can evolve in nature into debris avalanches through mechanisms that could not be modeled in the laboratory the role of fan shaped volcaniclastic deltas included in the edifice structure is critical to slump formation this process is more efficient if deltas are associated with another upper lsl such as weak hyaloclastites or hydrothermally altered rocks in this case recurrent events generating collapses with increasing width are observed gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum or includes an intra edifice unconstrained hyaloclastic layer 1 3 as thick as the height of the subaerial edifice spreading is characterized by edifice stretching and subsidence and does not generate major landslides we also show that gravitational spreading on a marine substratum tends to inhibit slide generation especially when the thickness of the sedimentary layer is larger than 1 10 of the edifice total height however if this value is lower spreading occurs later and slumps can develop initially we describe natural examples of oceanic hot spot or arc shield volcanoes affected by both spreading and landslides such as reunion island hawaii island james ross island antarctica and martinique island antilles the main structural features on these volcanoes can be explained solely by deformation on lsls we therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield like edifices

a lithospheric instability origin for the cameroon volcanic line the cameroon volcanic line cvl is an enigmatic structure that defies common dynamic models of melt generation and volcanic activity on earth there magma generation and intrusion has been sustained for more than 70 myr over a 1600 km long chain straddling the ocean continent boundary with no detectable spatial age progression the chain is nearly perpendicular to the coastline and terminates in a y shaped structure that has not been affected by absolute plate motions implying that the mantle upwelling that feeds magmatic activity is attached to the continent we propose that this form of volcanism is due to a new type of instability that may develop within the subcontinental lithospheric mantle at the edge of a continent laboratory experiments document how lithosphere beneath a continental block of finite size can become unstable due to cooling from above the instability pattern is made of linear upwellings and downwellings that converge radially towards the center of the continent in an outer region and an array of polygonal cells in a central region the pattern is characterized by branching structures that are reminiscent of the strike and y shaped outline of the cvl the instability develops over long timescales with small rates of upwelling and melting and is attached to the continent by construction downwellings adjacent to upwellings induce compression in the crust which may account for deformation in the benue trough just before the onset of cvl magmatism

potential signatures of damage related radiation from aftershocks of the 4 april 2010 mw 7 2 el mayor cucapah earthquake baja california mexico abstract we observe enhanced high frequency radiation of p waves from aftershocks of the 4 april 2010 m w 7 2 el mayor cucapah baja california earthquake that may reflect isotropic radiation generated by rock damage in the source regions to eliminate path radiation pattern and site effects we use spectral ratios of four pairs of collocated events with similar size and focal mechanism that occurred within short time intervals less than 24 hrs and were recorded at the same stations the p p and the s s spectral ratios calculated for the selected pairs of events show that events with similar size may have differences of high frequency radiation up to a factor of 4 at 10 hz and up to a factor of 10 at 4 hz for p and s waves respectively to evaluate the differences between p and s wave energy radiated at high frequencies we calculate the p p s s ratio of the ratios at high frequencies f 1 5 hz in a band for which the signal to noise ratio is adequate since the pairs of used events have approximately the same size the ratio of ratios is expected to be near unity we observed high p p s s spectral ratios at high frequencies f 6 hz up to a factor of 9 which may reflect isotropic radiation associated with rock damage to find additional possible signatures of rock damage we evaluate changes of coda q using the same data the results show small change of q for a doublet located on the laguna salada fault however for the doublets located south of the sierras el mayor cucapah where there was no surface rupture before the sequence changes of q at 5 hz by a factor of about 3 indicate significant temporal variations of rock properties along the source receiver path

palaeomagnetism of the continental sector of the cameroon volcanic line west africa summary measurement of samples from 154 sites in the continental sector of the cameroon volcanic line yielded six palaeomagnetic poles at 243 6 e 84 6 n a 95 6 8 224 3 e 81 2 n a 95 8 4 176 1 e 82 0 n a 95 8 5 164 3 e 86 4 n a 95 3 4 169 4 e 82 6 n a 95 4 6 and 174 7 e 72 8 n a 95 9 5 belonging to rocks which have been dated by the k ar method at 0 4 0 9 ma 2 6 ma 6 5 11 ma 12 17 ma 20 24 ma and 28 31 ma respectively the results are in general agreement with other palaeomagnetic poles from oligocene to recent formations in africa the first three poles for rocks formed between 0 4 and 11 ma are not significantly diverent from the present geographical pole together with other african poles for the same period this suggests that the african continent has moved very little relative to the pole since 11 ma the other three poles for rocks dated between 12 and 31 ma are significantly diverent from the present geographical pole showing a 5 polar deviation from the present pole in the miocene and 13 in the middle oligocene

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

buttressing and fractional spreading of tenerife an experimental approach on the formation of rift zones 1 the island of tenerife is composed of three miocene shields which are centered by the canadas volcano since the pliocene tenerife sits on more than 2 km thick oceanic sediments quaternary volcanism of the canadas series and giant landslides were principally confined to triaxial rift zones the mechanism of triaxial rifting however has remained unclear physical analog models show that these rift zones may have formed by gravity driven lateral escape of island segments induced by loading of the deformable substratum for experiments scaled to tenerife three adjacent sand cones were mounted onto viscous pdms substratum gravitational spreading caused circumferential expansion of each cone until a large edifice canadas was constructed in their center the older cones now acted each as a buttress radial fractures were overprinted by fractional spreading of the canadas edifice this resulted in formation of three main extensional zones resembling the triaxial rifting configuration of tenerife

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

observations of premonitory acoustic emission and slip nucleation during a stick slip experiment in smooth faulted westerly granite 1 to investigate laboratory earthquakes stick slip events were induced on a saw cut westerly granite sample by triaxial loading at 150 mpa confining pressure acoustic emissions ae were monitored using an innovative continuous waveform recorder the first motion of each stick slip was recorded as a large amplitude ae signal these events source locate onto the saw cut fault plane implying that they represent the nucleation sites of the dynamic failure stick slip events the precise location of nucleation varied between events and was probably controlled by heterogeneity of stress or surface conditions on the fault the initial nucleation diameter of each dynamic instability was inferred to be less than 3 mm a small number of ae were recorded prior to each macro slip event for the second and third slip events premonitory ae source mechanisms mimic the large scale fault plane geometry

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

edifice and substrata deformation induced by intrusive complexes and gravitational loading in the mull volcano scotland it is likely that the structure of a volcanic edifice can be significantly modified by deformation caused by large shallow intrusions such deformation may interact with that caused by volcano loading we explore such intrusion related and loading related deformation with field evidence and analogue models to do this we have chosen the eroded palaeogene mull volcano scotland that had a major edifice has well exposed intrusions and significant deformation there are thin mesozoic sedimentary rocks forming ductile layers below the volcano but their thickness is insufficient to allow the gravitational spreading of the volcanic edifice especially when considering that a thick lava pile covers them thus intrusive push may have been the driving force for deformation the mull activity migrated toward the northwest forming three successive intrusive complexes centres 1 2 and 3 our detailed fieldwork reveals that deformation due to these was accommodated on three levels along thrust planes in lava sequences along a decollement located in a thin clay rich sediment succession and in basement schists a relative chronology has been established between different groups of structures using dyke and sill cross cutting relationships centre 1 is surrounded by a fold and thrust belt leading to radial expansion in contrast centre 2 and 3 are connected to thrusts located to the south and east bounded by strike slip faults leading to expansion to the southeast the migration of centres and the directed sliding of the edifice may be related to the presence to the southeast of low resistance dalradian basement that failed significantly during growth of centres 2 and 3 to study the observed relationships we have carried out scaled analogue models models are made with fine powder intruded by a viscous magma analogue the models show an intimate relationship between intrusion growth uplift of the volcano and subsequent flank sliding the structures produced can be compared with mull and suggest that the centre 1 thrust belt probably formed following edifice gravitational sliding as a consequence of the uplift associated with centre 1 formation centre 2 and 3 are responsible for the sector sliding of the edifice flank toward the southeast as the magmatic complex became more asymmetric the features observed at mull and in the models are similar to those seen on active volcanoes such as etna providing a structural framework for their deformation and evolution

source duration scales with magnitude differently for earthquakes on the san andreas fault and on secondary faults in parkfield california we used a comparison of source time function pulse widths to show that a group of earthquakes on the san andreas fault near parkfield have a constant duration over a magnitude range of 1 4 3 7 earthquakes on secondary faults have an increase in duration with magnitude which is the expected relationship for the usual observation of constant stress drop the constant duration suggests that fault area is the same regardless of magnitude and that variations in stress drop are due entirely to variations in slip calculated stress drop values on secondary faults range from 0 31 to 14 mpa and stress drop values on the san andreas fault range from 0 18 to 63 mpa the observation of constant duration on the san andreas fault is consistent with a model of a locked asperity in a creeping fault the differences in durations between the events on the san andreas fault and on secondary faults suggest that earthquakes on the san andreas fault are inherently different we speculate that faults with more cumulative displacement have earthquakes that may rupture differently furthermore the differences in source properties between the two populations might be explained by differences in fault surface roughness online material station subsets used to evaluate cross fault seismic velocity contrasts

acoustic emissions document stress changes over many seismic cycles in stick slip experiments 1 the statistics of large earthquakes commonly involve large uncertainties due to the lack of long term robust earthquake recordings small scale seismic events are abundant and can be used to examine variations in fault structure and stress we report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes those generated in the laboratory we investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory created fault zones we show that b values mirror periodic stress changes that occur during series of stick slip events and are correlated with stress over many seismic cycles moreover the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop consequently b value variations can be used to approximate the stress state on a fault a possible tool for the advancement of time dependent seismic hazard assessment citation goebel t h w d schorlemmer t w becker g dresen and c g sammis 2013 acoustic emissions document stress changes over many seismic cycles in stick slip experiments geophys res lett 40 2049 2054 doi 10 1002 grl 50507

variations of strain drops of aftershocks of the 1999 izmit and duzce earthquakes around the karadere duzce branch of the north anatolian fault summary we estimate the strain drops of 7498 aftershocks of the 1999 u and d uzce earthquakes using p waveforms recorded by a local seismic network along the karadere d uzce branch of the north anatolian fault in the 6 months following the u main shock and evaluate the site effects of the various recording stations the method is associated with separation of source traveltime and station spectral terms and stacking results at several stages to enhance the signal to noise ratio the strain drops are obtained by fitting iteratively the separated source spectra of 201 nearest neighbouring events in different amplitude bins to the 2 source spectral model the obtained strain drops generally increase with depth between 3 and 10 km and remain approximately constant for the deeper section a local shallow patch of seismicity north of the karadere segment has relatively low strain drop values along the relatively straight karadere segment the strain drops are lower than along the geometrically complex bounding regions in some sections the range of strain drop values decay with time from the u and d uzce main shocks whereas in others the values fluctuate within about constant ranges the observed spatial variations of strain drops can be explained by increasing normal stress with depth along with the degree of geometrical complexity of different fault sections and the expected slip deficit at different depth sections the seismic energy in the separated station spectra decreases overall with distance from the rupture zone and the spectra of various stations have three peaks at 6 14 and 25 hz the spectral peak at 6 hz is also observed in trapped waves studies and may be related to the damaged fault zone layer

the seismogenic thickness of the southern california crust the average seismogenic thickness measured from the surface down to maximum depth of earthquake rupture for the southern california crust is 15 0 km 1 2 1 1 km we determine the seismogenic thickness using the depth distribution of the seismic moment release of 19 years of seismicity we calibrate the depth distribution of moment release from background seismicity by comparing the maximum depth of rupture during moderate to large magnitude earthquakes to the premainshock background seismicity of the respective mainshock region the calibration shows that the depth above which 99 9 of the moment release of background seismicity occurs reliably estimates the maximum depth of rupture during moderate to large earthquakes locally the seismogenic thickness is highly variable ranging from less than 10 km in the salton trough to greater than 25 km at the southwestern edge of the san joaquin valley similarly the seismogenic thickness along the major strike slip faults can vary significantly along strike changes in seismogenic thickness along strike do not correspond to the mapped surface segmentation of the major southern california strike slip fault systems in the future such estimates of the seismogenic thickness can be used to refine existing seismic hazard estimates for southern california

a sagging spreading continuum of large volcano structure gravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes using scaled analog models we characterize a range of structural architectures produced by volcano sagging and volcano spreading these arise from the interplay of variable basement rigidity and volcano basement de coupling from comparison to volcanoes on earth la reunion and hawaii and mars elysium and olympus montes the models highlight a structural continuum in which large volcanoes throughout the solar system lie

evolution of microseismicity during frictional sliding we have done frictional sliding experiments on inada granite in double shear and monitored the acoustic emission ae produced and temporal changes in the microseismic b value where b is defined as the log linear slope of the ae frequency amplitude distribution using both rough and smooth ground simulated fault surfaces we have found i the maximum amplitudes of ae events during stable sliding are strongly dependent on the surface roughness with smooth ground surfaces giving smaller maximum ae amplitudes ii b values are related to the surface topographic fractal dimensions so that in steady state stable sliding smooth surfaces exhibit lower b values than rough surfaces iii the b value falls before stick slip instability the change of b with slip we interpret in terms of evolving fractal crack damage during frictional sliding of the fault surfaces

directional flank spreading at mount cameroon volcano evidence from analogue modeling mount cameroon is characterized by an elongated summit plateau steep flanks and topographic terraces around its base although some of these features can be accounted for by intrusion induced deformation we here focus on the contribution of edifice scale gravitational spreading in the structure of mount cameroon we review the existing geological and geophysical data and morphostructural features of mount cameroon and surrounding sedimentary basins volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation three sets of models are presented i models recorded in cross section type i ii models recorded from above with a uniform type iia and iii nonuniform ductile layer type iib type i models illustrate the formation of faults accommodating summit subsidence and lower flank spreading type iia models favor displacement perpendicular to the long axis with formation of a summit graben and basal folds but fail to reproduce the steep flanks type iib models investigate the effect of spatial variations in sediment thickness and or properties consistent with geological evidence directional spreading of the volcano s central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis the later model closely reproduces key features observed at mount cameroon steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit the relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted

seismic radiation from regions sustaining material damage we discuss analytical results for seismic radiation during rapid episodes of inelastic brittle deformation that include in addition to the standard moment term a damage related term stemming from changes of elastic moduli in the source region the radiation from the damage related term is associated with products of the changes of elastic moduli and the total elastic strain components in the source region order of magnitude estimates suggest that the damage related contribution to the motion in the surrounding elastic solid which is neglected in standard calculations can have appreciable amplitude that may in some cases be comparable to or larger than the moment contribution a decomposition analysis shows that the damage related source term has an isotropic component that can be larger than its double couple component

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

broadband simulations for mw 7 8 southern san andreas earthquakes ground motion sensitivity to rupture speed using the high performance computing resources of the southern california earthquake center we simulate broadband 0 10 hz ground motions for three m w 7 8 rupture scenarios of the southern san andreas fault the scenarios incorporate a kinematic rupture description with the average rupture speed along the large slip portions of the fault set at 0 96 0 89 and 0 84 times the local shear wave velocity consistent with previous simulations a southern hypocenter efficiently channels energy into the los angeles region along the string of basins south of the san gabriel mountains however we find the basin ground motion levels are quite sensitive to the prescribed rupture speed with peak ground velocities at some sites varying by over a factor of two for variations in average rupture speed of about 15 these results have important implications for estimating seismic hazards in southern california and emphasize the need for improved understanding of earthquake rupture processes

identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics 1 seismicity clusters within fault zones can be connected to the structure geometric complexity and size of asperities which perturb and intensify the stress field in their periphery to gain further insight into fault mechanical processes we study stick slip sequences in an analog laboratory setting analysis of small scale fracture processes expressed by acoustic emissions aes provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events the present work connects x ray computer tomography ct scans of faulted rock samples with spatial maps of b values slope of the frequency magnitude distribution seismic moments and event densities our current experimental setup facilitates the creation of a series of stick slips on one fault plane thus allowing us to document how individual stick slips can change the characteristics of ae event populations in connection to the evolution of the fault structure we found that geometric asperities identified in ct scan images were connected to regions of low b values increased event densities and moment release over multiple stick slip cycles our experiments underline several parallels between laboratory findings and studies of crustal seismicity for example that asperity regions in lab and field are connected to spatial b value anomalies these regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes

on the characteristics of earthquake stress release variations in japan abstract the amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process as such it represents a key parameter for improving our understanding of earthquake source physics and for reliable ground motion prediction large earthquake populations usually show variations in stress release as large as three orders of magnitude but the underlying mechanisms have remained largely elusive and particularly the dependence of stress release on earthquake size is still a matter of debate here i use a unique dataset from japan encompassing earthquakes of a wide magnitude range and spatial coverage to show that stress release variations of crustal earthquakes are strongly correlated with heat flow variations indicating that they are thermally controlled in contrast subcrustal events depict highest stress release in regions of strong subduction interface coupling and overall less pronounced variability as compared with crustal earthquakes stress release is overall only weakly dependent on earthquake size but at local scales i e within individual earthquake sequences the dependence can be very strong and apparently varies with stress regime accounting for these systematic variations reduces the stress release variability on local scale by a factor of two to three as compared with the full earthquake population a finding that is of key significance in the endeavor to reduce the uncertainties in future ground motion predictions

growth and persistence of hawaiian volcanic rift zones hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms the intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike trapping stress field within the prism possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis faulting and gravitational sagging of the topography in extreme models with very steep slopes and high poisson s ratio corresponding to the gelatin models of rift zones by fiske and jackson 1972 results of finite element calculations indicate that gravity induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by fiske and jackson however the mechanism does not work for gently sloping flanks or a more acceptable poisson s ratio of about 0 25 additionally trapping stresses in the gravity loading and density stratification models will not persist after a few dike injection episodes therefore in mature hawaiian rift zones with possibly thousands of dikes additional processes must act to control the stresses that permit continued dike intrusion and rift persistence it is proposed that accommodation to dike emplacement occurs by slip on deep faults possibly of the type proposed for the 1975 kalapana hawaii earthquake as suggested by others for this earthquake the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis other possible faulting mechanisms such as shallow gravity slides and normal faulting of the flanks do not appear to favor rift zone persistence in this model the horizontal stress generated by a standing column of magma at the time of dike emplacement the stresses in the ridges and the fault strength are coupled this results in a feedback between the maximum height that magma can rise along the rift fault friction and fault width through this feedback the slope of the volcano flank is controlled by the fault friction when applied to kilauea volcano the model yields an estimate for the coefficient of fault friction as high as 0 39 assuming normal hydrostatic pore fluid pressure an implication of this model supported by other studies is that rift intrusion and lateral spreading could be major contributors to volcano growth

importance of gravitational spreading in the tectonic and volcanic evolution of mount etna the interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude from small cones to the oceanic crust1 3 although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth it is rarely recognized as such here we describe a striking example of this process at etna volcano in italy the volcano and its clay rich substratum are slowly spreading towards the east and south driven by gravity spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity as gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions an appreciation of its effects could become an essential element of future volcanic hazard evaluation our model proposed here for etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

simulating the recent evolution of the southern big bend of the san andreas fault southern california 1 the southern big bend of the san andreas fault has been interpreted to have successively abandoned two strands the mission creek and mill creek strands of the san andreas fault during the past 1 ma before taking up activity along its present day configuration this evolution is simulated within three dimensional models to explore the hypothesis that fault systems evolve to increase mechanical efficiency the three dimensional boundary element method models are validated by comparison of modeled fault slip rates and uplift rates with geologic data the model results match well the geologic strike slip rates and uplift patterns overestimation of reverse slip rates along the northern san bernardino mountains may reflect model assumptions of fault geometry the partitioning of slip among active faults changes between the three phases of southern san andreas fault evolution revealing 1 a trade off in strike slip rates between the san jacinto and san andreas faults 2 a trade off between strike slip rates on the san andreas fault and reverse slip along faults associated with uplifting the san bernardino mountains and 3 a trade off in strike slip rates along the san andreas and strike slip rates along the eastern california shear zone mechanical efficiency of the entire model increases from the mission creek to mill creek fault configuration and decreases from the mill creek configuration to the present day configuration the decrease in mechanical efficiency with the transition to the present day fault geometry may reflect gaps in our understanding of tectonic loading and or fault geometry

source parameters of picoseismicity recorded at mponeng deep gold mine south africa implications for scaling relations abstract we investigate the source parameters of picoseismic and nanoseismic events m w 4 1 recorded with a high sensitivity seismic network at the mponeng gold mine in south africa to gain new insights into the scaling of small seismic events the japanese german underground acoustic emission research in south africa jaguars network composed of one three component 3c accelerometer sensitivity 50 hz to 25 khz and 8 acoustic emission ae sensors sensitivity 1 khz to 180 khz is located at a depth of 3268 m and covers the limited volume of approximately 300 300 300 m the ae sensors are calibrated with respect to the 3c accelerometer in the frequency band 400 hz 17 khz the waveform data of two datasets are analyzed 1 the aftershock sequence of an m w 1 9 event that occurred approximately 30 m from our network and 2 the postblasting activity recorded during working days located at a distance 90 m from the network at the exploitation level for the analysis we applied spectral fitting and spectral ratio methods the calculated values of m w range from 0 8 down to 4 1 with corner frequencies 0 8 khz 13 6 khz source sizes from 8 cm to 1 3 m we observe static stress drops ranging from 1 mpa to 10 0 mpa with apparent stresses of 0 01 mpa 1 00 mpa stress drops are independent of the moment suggesting self similar behavior in the analyzed magnitude range 4 1 m w

analogue benchmarks of shortening and extension experiments we report a direct comparison of scaled analogue experiments to test thereproducibility of model results among ten different experimental modelling laboratories we present results for two experimen

post eruptive seismic activity of mount cameroon cameroon west africa a statistical analysis abstract recent seismological studies of the cameroon volcanic line show that mt cameroon is the most active centre so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986 the network was reinforced with temporary stations up till 1987 and the local seismicity was studied here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations four swarms lasting 9 to 14 months are identified at intervals of 2 3 years most earthquakes are felt intensity and magnitude respectively less than vi mm and 5 during the first three swarms and a few during repose periods the main focal regions are the northwest and southeast flanks the bimbia and bioko regions in the south of the volcano hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities the subcrustal events are observed only in the southeast flank they are the most regular earthquakes with a monthly frequency of 9 to 15 events they are characteristic earthquakes with magnitude 2 8 0 1 between 1984 and 1992 their yearly mean time interval between successive events range from 50 to 86 hours for that period their occurrence can be modelled as a stationary renewal process with a 3 day period but the analysis of variance shows possible significant differences among yearly means a weibull s distribution confirms that the time intervals between successive deep events are not independent and in 1993 a swarm of deep earthquakes is recorded hence a non loglinear magnitude frequency relation the deep seismicity is thought to be associated with a zone of weakness perhaps a magmatic conduit and may have some close relationship with the magmatic activity

waveform relocated earthquake catalog for southern california 1981 to june 2011 we determine a new relocated catalog hys catalog 2011 for southern california from 1981 through june 2011 about 75 3 of the hypocenters are calculated with absolute and differential travel time picks and 24 7 could be relocated only by using absolute travel time picks with 3d or 1d velocity models the total catalog consists of more than 502 000 earthquakes in the region extending from baja california in the south to coalinga and owens valley in the north the catalog consists of three m 7 1 m 7 2 and m 7 3 mainshocks their foreshocks and aftershocks and background seismicity caused by tectonic and other processes in the southern california crust hypocenters in the new relocated catalog exhibit tighter spatial clustering of seismicity than does the routinely generated catalog and the depth distribution is tighter and reflects the thickness of the seismogenic zone more accurately compared to the standard catalog the relocated hypocenters are more easily related to other data sets such as mapped late quaternary faults

evidence for vertical partitioning of strike slip and compressional tectonics from seismicity focal mechanisms and stress drops in the east los angeles basin area california we have synthesized the characteristics of the seismogenic zone in the east los angeles basin by analyzing earthquake data recorded during the past 30 years 1981 2010 the seismicity is distributed along the whittier fault with the majority of earthquakes located adjacent to the south side in the depth range from 0 to 9 km with b value of 1 1 0 05 and mostly normal and strike slip faulting within the depth range of 9 12 km the seismicity is scattered uniformly across the region the b value is 1 0 0 05 and all three faulting styles are present at the deepest depths 12 18 km seismicity is sparse and primarily limited to a few clusters striking north these deeper earthquakes primarily have reverse fault motion and the b value is 0 78 0 04 inversion of high quality focal mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from n12 w at shallow depth to due north at the bottom of the seismogenic zone similarly a depth dependence is observed in stress drops calculated from p wave source spectra which indicate stress drop generally increases from 7 mpa at shallow depth 3 km to 53 mpa at the base of the seismogenic zone 17 km overall our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east los angeles basin

geophysical evidence for wedging in the san gorgonio pass structural knot southern san andreas fault zone southern california geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the san gorgonio pass region this picture serves as the basis for inferring past fault movements within the san andreas system which are fundamental to understanding the tectonic evolution of the san gorgonio pass region interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of san gorgonio pass beneath basement rocks of the southeast san bernardino mountains subtle long wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of san gorgonio pass and south under peninsular ranges basement and has a southern edge that is roughly parallel to but 5 6 km south of the surface trace of the banning fault this deep magnetic body is composed either of upper plate rocks of san gabriel mountains basement or rocks of san bernardino mountains basement or both we suggest that transpression across the san gorgonio pass region drove a wedge of peninsular ranges basement and its overlying sedimentary cover northward into the san bernardino mountains during the neogene offsetting the banning fault at shallow depth average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data seismicity suggests a deeper detachment surface beneath the deep magnetic body this interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross sectional view given the multilayered nature of deformation it is unlikely that the san andreas fault will rupture cleanly through the complex structures in san gorgonio pass

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

southern california hypocenter relocation with waveform cross correlation part 2 results using source specific station terms and cluster analysis we obtain precise relative relocations for more than 340 000 southern california earthquakes between 1984 and 2002 by applying the source specific station term ssst method to existing p and s phase picks and a differential lo cation method to about 208 000 events within similar event clusters identified with waveform cross correlation the entire catalog is first relocated by using existing phase picks a reference 1d velocity model and the ssst method of richards dinger and shearer 2000 we also perform separate relocations of imperial valley events by using a velocity model more suited to this region next we apply cluster analysis to the waveform cross correlation output to identify similar event clusters we re locate earthquakes within each similar event cluster by using the differential times alone keeping the cluster centroid fixed to its initial ssst location we estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs these errors are often as small as tens of meters in many cases the relocated events within each similar event cluster align in planar features suggestive of faults we observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends in general the fine scale details of the seismicity reveal a great deal of structural com plexity in southern california fault systems

a new tectonic model for the cameroon line central africa abstract the cameroon line a major geological feature in central africa has been considered successively as a series of horsts and grabens a continental rift and a mega shear zone it is marked out by about 60 anorogenic complexes and a dozen volcanic centres all of which have alkaline affinity remote sensing allows us recognition of the main lineament trends n70 n s n135 and e w while autocorrelation analysis reveals a major fault zone striking n30 in western cameroon and n15 in the northern region a mega left lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes syntectonic leucogranites marking out older shear zones and vein dykes the n70 adamawa fault zone a pan african fracture reworked during albian aptian times is the only shear zone of continental scale that could have initiated en echelon mega tension gashes within the cameroon line during a cainozoic left lateral transcurrent movement

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

morphometry of terrestrial shield volcanoes abstract shield volcanoes are described as low angle edifices built primarily by the accumulation of successive lava flows this generic view of shield volcano morphology is based on a limited number of monogenetic shields from iceland and mexico and a small set of large oceanic islands hawaii galapagos here the morphometry of 158 monogenetic and polygenetic shield volcanoes is analyzed quantitatively from 90 meter resolution srtm dems using the morvolc algorithm an additional set of 24 lava dominated shield like volcanoes considered so far as stratovolcanoes are documented for comparison results show that there is a large variation in shield size volumes from 0 1 to 1000 km3 profile shape height basal width h wb ratios mostly from 0 01 to 0 1 flank slope gradients average slopes mostly from 1 to 15 elongation and summit truncation although there is no clear cut morphometric difference between shield volcanoes and stratovolcanoes an approximate threshold can be drawn at 12 average slope and 0 10 h wb ratio principal component analysis of the obtained database enables to identify four key morphometric descriptors size steepness plan shape and truncation hierarchical cluster analysis of these descriptors results in 12 end member shield types with intermediate cases defining a continuum of morphologies the shield types can be linked in terms of growth stages and shape evolution related to 1 magma composition and rheology effusion rate and lava pyroclast ratio which will condition edifice steepness 2 spatial distribution of vents in turn related to the magmatic feeding system and the tectonic framework which will control edifice plan shape and 3 caldera formation which will condition edifice truncation

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

the development of magmatism along the cameroon volcanic line evidence from teleseismic receiver functions 1 the cameroon volcanic line cvl in west africa is a chain of cenozoic volcanism with no clear age progression the reasons for its existence are unclear and the nature of its magmatic plumbing system is poorly understood specifically whether or not the cvl crust presently contains melt and or mafic intrusions as is often observed at hot spots and rifts elsewhere is presently unknown to address this issue we present a receiver function study of crustal structure using earthquakes recorded by the cameroon broadband seismic experiment in regions of the cvl unaffected by cretaceous extension associated with the breakup of gondwana e g the garoua rift vp vs ratios are markedly low network average 1 74 compared to hot spots elsewhere providing no evidence for either melt or cooled mafic crustal intrusions due to cvl magmatism the character of p to s conversions from beneath the cvl also indicates that lower crustal intrusions often termed underplate are not present beneath the region our observations thus corroborate earlier petrological studies that show cvl alkaline magmas fractionate in the mantle not the crust prior to eruption hypotheses for the formation of the cvl should not include markedly elevated upper mantle potential temperatures or large volumes of partial melt both of which can explain observations at hot spots and rifts worldwide the protracted yet sporadic development of small volume alkali melts beneath the cvl may instead be explained better by lower melt volume mechanisms such as shear zone reactivation or lithospheric delamination

how to reveal unknown hidden faults and historical earthquake damage applying multidisciplinary methods in archaeological sites the case of mid third century ce mt etna earthquake eastern sicily italy abstract archaeoseismological evidence of a strong earthquake which probably occurred a year before the 252 ce mt etna eruption has been uncovered at santa venera al pozzo site located near to acireale along the lower eastern flank of the volcano the site known for its remarkable and peculiar hydrothermal fluids emissions exploited for thermal uses since the greek and roman age has been investigated through a multidisciplinary approach by integrating archaeoseismological observations with geological and geochemical surveys analysis of the observed damage and deformation at the site suggest a capable fault zone through the area geochemical and geological surveys were performed in and around the site and have revealed anomalies of soil co2 efflux along an known active fault and likely also along some unmapped hidden faults the chemical and isotopic composition of the emitted fluids suggests that these have not a volcanic origin but they could be related with a deep tectonic system the data presented in this paper suggest the occurrence of both exposed and hidden active faults in the area of santa venera al pozzo which could be linked to the seismic activity that damaged the historical site

uncertainties in slip rate estimates for the mission creek strand of the southern san andreas fault at biskra palms oasis southern california this study focuses on uncertainties in estimates of the geologic slip rate along the mission creek strand of the southern san andreas fault where it offsets an alluvial fan t2 at biskra palms oasis in southern california we provide new estimates of the amount of fault offset of the t2 fan based on trench excavations and new cosmogenic 10be age determinations from the tops of 12 boulders on the fan surface we present three alternative fan offset models a minimum a maximum and a preferred offset of 660 m 980 m and 770 m respectively we assign an age of between 45 and 54 ka to the t2 fan from the 10be data which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface and with ages from u series geochronology on pedogenic carbonate from t2 described in a companion paper by fletcher et al this volume these new constraints suggest a range of slip rates between 12 and 22 mm yr with a preferred estimate of 14 17 mm yr for the mission creek strand of the southern san andreas fault previous studies suggested that the geologic and geodetic slip rate estimates at biskra palms differed we find however that considerable uncertainty affects both the geologic and geodetic slip rate estimates such that if a real discrepancy between these rates exists for the southern san andreas fault at biskra palms it cannot be demonstrated with available data

eruptive and earthquake activities related to the 2000 eruption of mount cameroon volcano west africa abstract mount cameroon is an active volcano located in the gulf of guinea west of central africa after the march april 1999 eruption on the sw flank another eruption of the volcano occurred in 2000 it took place from three sites on the southwest flank and near the summit the first eruptive site was located 500 m to the southwest of the summit at 3900 m altitude activity on this site was mainly explosive with no lava flow the second site was located between 3220 and 3470 m altitude lava was emitted along nne sse fissures from this site and flew towards buea the main city of the area stopping 4 km from the first houses the last site was located in the south western flank at 2750 m altitude the lava ejected from an old cone near the first 1999 eruptive site was divided into two branches for a total length of around 1 km the location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics the pre eruptive period was characterized by a seismic swarm which may be a precursor recorded in march 2000 by an analogue seismic station the main shock was a magnitude 3 2 event and was felt by the population in ekona town located on the eastern flank it had a modified mercalli intensity of iii iv when the eruption started a temporary network of short period 3 component seismic stations was set up around the volcano to improve the monitoring of seismic activity the co eruptive period from late may to september was characterized by sequences of earthquake swarms volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in august and early september some of the earthquakes were felt by the population in buea and its environments the largest seismic event recorded had a magnitude of 4 during the post eruptive period from mid september to december seismicity returned to its background level of 1 3 earthquakes per 3 days hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano the frequency magnitude relationship revealed a b value of 1 43 higher than those previously determined but more representative of volcanic media seismic energy release was gradual after the 2000 eruption started

morphology and structure of the 1999 lava flows at mount cameroon volcano west africa and their bearing on the emplacement dynamics of volume limited flows the morphology and structure of the 1999 lava flows at mount cameroon volcano are documented and discussed in relation to local and source dynamics structures are analysed qualitatively and more detailed arguments are developed on the processes of levee formation and systematic links between flow dynamics and levee channel interface geometry the flows have clear channels bordered by four main types of levees initial accretionary rubble and overflow levees thermally immature pahoehoe lava units with overflow drapes define the proximal zone whereas rubble and accretionary levees are common in the distal region bordering thermally mature aa clinker or blocky aa flow channels pressure ridges squeeze ups and pahoehoe ropes are the prevalent compressive structures standlines displayed on clinkery breccias are interpreted to represent levee channel interactions in response to changing flow levels these data complement previous knowledge on lava flow morphology thus far dominated by etnean and hawaiian examples

slope failure and volcanic spreading along the submarine south flank of kilauea volcano hawaii 1 new multichannel reflection data and high resolution bathymetry over the submarine slopes of kilauea volcano provide evidence for current and prior landsliding suggesting a dynamic interplay among slope failure regrowth and volcanic spreading disrupted strata along the upper reaches of kilauea s flank denote a coherent slump correlated with the active hilina slump the slump comprises mostly slope sediments underlain by a detachment 3 5 km deep extension and subsidence along the upper flank is compensated by uplift and folding of the slump toe which surfaces about midway down the submarine flank uplift of strata forming papa u seamount and offset of surface features along the western boundary of kilauea indicate that the slump has been displaced 3 km in a south southeast direction this trajectory matches coseismic and continuous ground displacements for the hilina slump block on land and contrasts with the southeast vergence of the rest of the creeping south flank to the northeast slope sediments are thinned and disrupted within a recessed region of the central flank demonstrating catastrophic slope failure in the recent past debris from the collapsed flank was shed into the moat in front of kilauea building an extensive apron seaward sliding of kilauea s flank offscraped these deposits to build an extensive frontal bench a broad basin formed behind the bench and above the embayed flank uplift and back tilting of young basin fill indicate recent and possibly ongoing bench growth the hilina slump now impinges upon the frontal bench this buttress may tend to reduce the likelihood of future catastrophic detachment

the influence of edifice slope and substrata on volcano spreading gravitational volcano spreading is caused by flow of weak substrata due to volcanic loading and is now a process known to affect many edifices the process produces extension in the upper edifice evidenced by graben and normal faults and compression at the base seen in strike slip faults and thrusts where spreading is identified host volcanoes have a range of fault densities variable rift and graben shapes and different degrees of structural asymmetry previous studies have suggested a link between edifice shape and structure and the proportion of brittle to ductile material in the substrata or lower edifice we study this link using refined sand cone analogue models standing on a brittle ductile sand silicone substrata two scenarios have been investigated the first mainly represents oceanic volcanoes with a ductile layer within the edifice type i where there is an outer ductile free surface the second represents most continental volcanoes that have ductile substrata type ii we apply the model results to natural examples and develop quantitative relationships between slope brittle ductile ratio fault density spreading rate and structural style displacement fields calculated from stereophotogrammetry show significant differences between different slope models we find that more faults are produced when the cone is initially steeper or when the brittle substratum is thinner however the effect of the brittle layer dominates over that of slope the strike slip movements are found to be an essential feature in the spreading mechanism and the graben are in fact transtensional features strike slip and graben faults make a conjugate flower pattern the structures produced are well organised for type ii edifices but they are poorly organised for type i models type i models represent good analogues for oceanic volcanoes that are commonly affected by large slumps bounded by an extensional zone and lack of well formed sector graben the well observed connection between oceanic volcano rifts and large landslide slumps is confirmed to be a consequence of spreading

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

volcanic spreading and lateral variations in the structure of olympus mons mars the olympus mons volcano on mars is notable not only for its immense height and width but also for substantial asymmetries in its structure the gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank furthermore the northwest flank exhibits lower flank extensional faults whereas the southeast shows upper flank compressional terraces and lower flank upthrust blocks however both the northwest and southeast flanks exhibit characteristic concave upward profiles and steep bounding scarps in contrast to other sectors the nw se asymmetries are aligned with the regional slope from the tharsis rise but an understanding of the underlying causes has remained elusive we use particle dynamics models of growing spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying olympus mons we find that basal slopes alone are insufficient to produce the observed concave upward slopes and asymmetries in flank extent and deformation style that are observed at olympus mons instead lateral variations in basal friction are required these variations are most likely related to the presence of sediments transported and preferentially accumulated downslope from the tharsis rise such sediments likely correspond to ancient phyllosilicates clays recently discovered by the mars express mission

lack of continuity of the san andreas fault in southern california three dimensional fault models and earthquake scenarios 1 the 1200 km long san andreas fault loses its apparent continuity in southern california near san gorgonio pass allen 1957 which raises significant questions given the dominant role of this fault in active california tectonics what is the fundamental three dimensional 3 d geometry and kinematic behavior of the san andreas fault system in this complex region is a throughgoing if complex san andreas rupture from the mojave desert to the coachella valley possible we have explored the issue of 3 d continuity by mapping over 60 faults in this region to depths of 15 20 km from hypocenter locations and focal mechanisms we were able to constrain the 3 d geometry of the san andreas fault zone saf near san gorgonio pass from the 3 d geometry of the fault network surrounding it the most likely configuration is for the san andreas fault to merge into the shallow dipping san gorgonio pass thrust northwest of indio we concluded that there is no direct continuity at present but rather a network of faults and the only kind of rupture possible for the saf in this region is a complex rupture involving both strike slip and reverse faulting gps measurements also suggest that despite the fact that large motions must have occurred in the past based on offset geologic markers only minor motion is occurring today in this area applying our findings about the fault geometry we explored several simple earthquake scenarios to determine the most favorable conditions for a throughgoing rupture of the san andreas fault system from the mojave desert to the coachella valley

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

analogue modelling of volcano flank terrace formation on mars of the features that characterize large shield volcanoes on mars flank terraces remain the most enigmatic several competing mechanisms have been proposed for these laterally expansive topographically subtle landforms here we test the hypothesis that horizontal contraction of a volcano in response to the down flexing of its underlying basement leads to flank terracing we performed a series of analogue models consisting of a conical sand plaster load emplaced on a basement comprising a layer of brittle sand plaster atop a reservoir of viscoelastic silicone our experiments consistently produced a suite of structures that included a zone of concentric extension distal to the conical load a flexural trough adjacent to the load base and convexities terraces on the cone s flanks the effects of variations in the thickness of the brittle basal layer as well as in the volume slope and planform eccentricity of the cone were also investigated for a given cone geometry we find that terrace formation is enhanced as the brittle basement thickness decreases but that a sufficiently thick brittle layer can enhance the basement s resistance to loading such that terracing of the cone is reduced or even inhibited altogether for a given brittle basement thickness terracing is reduced with decreasing cone slope and or volume our experimental results compare well morphologically to observations of terraced edifices on mars and so provide a framework with which to understand the developmental history of large shield volcanoes on the red planet

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

computing a large refined catalog of focal mechanisms for southern california 1981 2010 temporal stability of the style of faulting using the method developed by hardebeck and shearer 2002 2003 termed the hash method we calculate focal mechanisms for earthquakes that occurred in the southern california region from 1981 to 2010 when available we use hypocenters refined with differential travel times from waveform cross correlation using both the p wave first motion polarities and the s p amplitude ratios computed from three component seismograms we determine mechanisms for more than 480 000 earthquakes and analyze the statistical features of the whole catalog we filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high quality catalog with approximately 179 000 focal mechanisms as more s p amplitude ratios become available after 2000 the average nodal plane uncertainty decreases significantly compared with mechanisms that include only p wave polarities in general the parameters of the focal mechanisms have been stable during the three decades the dominant style of faulting is high angle strike slip faulting with the most likely p axis centered at n5 e for earthquakes of m 2 5 events using the 210 moment tensor solutions in tape et al 2010 as benchmarks we compare the focal plane rotation angles of common events in the catalog seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty the common events with relatively large rotation angles are either located around the edge of the scsn network or poorly recorded

numerical experiments on the dynamics of channelised lava flows at mount cameroon volcano with the flowgo thermo rheological model abstract as for many other effusive volcanoes mount cameroon mc is a volcano for which only limited information exists on lava flow properties and emplacement dynamics for recent eruptions this study provides new quantitative constraints for rheological and dynamic characteristics of lava flow effusion for the 1982 and 2000 eruptive events used to calibrate the flowgo thermo rheological model for these lava flows the flowgo 1 d physical model is used to simulate down flow evolution of the geometry and rheology of channel contained cooling limited lava flows morphometric data from historical lava flows were acquired from the field e g channel geometry levee and background slope in order to estimate lava yield strength velocity and effusion rate lava density and viscosity were also estimated from compositional data and laboratory methods to account for uncertainty in the input rheological and geometrical data three end member scenarios were used to bracket the potential range in lava channel initial dimension initial lava temperature and phenocryst content for each of these scenarios two crustal growth models were used one assuming strong insulation due to lava flow surface crusting the other a much lower crusting rate twelve numerical simulations were made per flow and the results were compared against the channel geometry microlite content yield strength and viscosity estimates from field and laboratory investigations best fit models where obtained for both the 1982 and 2000 lava flows using a low rate of surface crusting high initial temperature and low phenocryst content model predicted lengths were within 5 of the actual lengths modelled mean effusion rates for the 1982 52 64 m 3 s 1 and 2000 10 m 3 s 1 flows closely matched field data derived estimations 26 68 and 9 5 m 3 s 1 respectively flowgo model results are highly sensitive to initial channel dimensions phenocryst content and the flowgo model is unable to reproduce the observed rapid near vent increase in microlite content near vent rapid cooling induced by lava degassing is yet to be fully documented and integrated into numerical lava flow models such as flowgo but obtained results already provide valuable insights for lava flow hazard assessment at mount cameroon