deep magma body beneath the summit and rift zones of kilauea volcano hawaii a magnitude 7 2 earthquake in 1975 caused the south flank of kilauea volcano hawaii to move seaward in response to slippage along a deep fault since then a large part of the volcano s edifice has been adjusting to this perturbation the summit of kilauea extended at a rate of 0 26 meter per year until 1983 the south flank uplifted more than 0 5 meter and the axes of both the volcano s rift zones extended and subsided the summit continues to subside these ground surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones kilauea s magmatic system is therefore probably deeper and more extensive than previously thought the summit and both rift zones may be underlain by a thick near vertical dike like magma system at a depth of 3 to 9 kilometers

focal mechanisms of earthquakes related to the 29 november 1975 kalapana hawaii earthquake the effect of structure models initial focal mechanism determinations for the 29 november 1975 kalapana hawaii earthquake indicated discrepancy between the mechanism determined from teleseismic data by ando and the mechanism determined using data from the local u s geological survey network surrounding the epicenter region the resolution of this difference is crucial to correctly understand this earthquake as well as to understand the tectonics of the south flank of kilauea volcano when a model with a low velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms the discrepancy vanishes to further investigate this result focal mechanisms were determined using several contrasting models for a set of well recorded earthquakes a large number of these earthquakes have mechanisms identical to the main shock when the low velocity layer model is used dispersion of p and t axes is also minimized by use of this model a low angle slip direction favored for the main shock and typical of most other solutions exhibits remarkable stability normal to the east rift zone of kilauea our results suggest a tectonic model similar in nature to that proposed by ando in which the south flank of kilauea consists of a mobile block of crust which is relatively free to move laterally on a low strength zone at about 10 km depth forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak horizontal layer in a cycle of perhaps 100 yr

stresses in the vanuatu subducting slab a test of two hypotheses 1 stress inversions of intermediate depth focal mechanisms in vanuatu were used to test two hypotheses that the slab acts as a stress guide and whether intermediate focus earthquakes occur on preexisting faults although the strike of the vanuatu slab varies by over 70 the maximum compression 1 is everywhere slab normal and the least compression 3 is everywhere within the plane of the slab this provides strong confirmation of the stress guide hypothesis in the depth range 61 120 km the solution space for 3 forms a girdle pattern in the plane of the slab indicating bi axial tension this gradually changes to down dip tension at greater depth we find that preexisting faults may become reactivated by intermediate depth earthquakes but only when the slab stresses favor it otherwise new faults are formed this contradicts the idea that the mechanism of intermediate focus earthquakes requires serpentinite dehydration on preexisting faults

prehistoric fault offsets of the hilina fault system south flank of kilauea volcano hawaii historical accounts of earthquakes on the island of hawaii date only to 1823 but lava flows as old as 1500 3000 years b p contain fault offsets from prehistoric earthquakes the m7 2 1975 kalapana earthquake produced over 25 km of fault rupture along the hilina fault system we compare fault offsets in prehistoric lava flows with kalapana earthquake fault offsets in neighboring 1969 1974 mauna ulu lava flows to estimate the frequency of prehistoric major earthquakes on the hilina fault system horizontal and vertical fault offset rates across the hilina fault system are 4 0 to 12 0 and 2 0 to 20 0 mm yr respectively based on lava flows 750 years b p age assuming prehistoric earthquakes produced similar fault offsets compared to the kalapana earthquake three to five events are recorded in 400 750 years b p old lava flows yielding recurrence intervals of 260 80 years vertical fault offsets in prehistoric lava flows suggest that hanging wall rotation of the hilina fault system contributes to fault offset the assumption that past faulting resulted from prehistoric earthquakes with similar magnitude and fault offset to the kalapana earthquake is an oversimplification large south flank earthquakes most likely do not have uniform recurrence intervals

deformation of kilauea volcano during 1982 and 1983 a transition period a major transition in the style of deformation of kilauea volcano occurred during the period january 1982 through november 1983 prior to 1983 the deformation was dominated by magma intrusions rapid extension of the summit and seaward movement of the south flank the pattern of activity changed to magma eruption and slowed deformation of the volcanic edifice in january 1983 an earthquake swarm in the southwest rift zone in june 1982 and an intrusion of a large dike in the east rift zone in january 1983 prepared the volcano for the eruptive sequences that continue today we estimate the sources of surface displacements associated with these events using a least squares nonlinear inversion technique and planar dislocations the models are constrained with measurements of line length changes and of surface height changes from tide gauges water wells and spirit level tilt data deformation associated with the 1982 earthquake swarm is best modeled as the emplacement of a steeply dipping dike in the upper and middle southwest rift zone extending from 2 3 km to 10 11 km depth with a width of 1 2 m tilt measurements gathered at the time of the earthquake swarm record a small deflation of kilauea s summit however trilateration data indicate the summit was rapidly extending in 1982 the extension and our models are consistent with seaward slip of the south flank on a low angle basal thrust fault dislocation models of the january 1983 dike intrusion yield a 14 to l5 km long dike along the zone of observed ground rupture the dike extends from just below the surface to the region of the most intense rift zone seismicity at 4 5 km depth a strong nonlinear relationship among height dip and width results in a suite of models that explain the data equally well tests of parameter sensitivity indicate that the dike width and height are the least constrained parameters the best fit average width ranges from 1 7 to 2 8 m our models also include a source of contraction at 5 km depth that produces 40 cm of ground subsidence near makaopuhi the point of initiation of the intrusion this location is consistent with geochemical and seismic evidence that indicate magma is stored in the rift zone residual errors for all models indicate that substantial line length changes are unexplained seaward motions of the south flank during 1982 and 1983 are not well modeled by simple dikes and summit deflations however attempts to constrain low angle faults in the south flank were only partly successful due to the limited extent of the geodetic networks

evidence for a weak and overpressured decollement beneath sediment dominated accretionary prisms we present structural data from several underthrust sequences in southwest alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical 1 and regionally extensive layer parallel shearing these results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where 1 plunges gently seaward or toward the deformation front on the basis of mohr circle diagrams we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak decollement and calculate coefficient of friction values that range from 0 25 to 0 08 these values are significantly lower than coefficient of friction values calculated for the basal decollement in the taiwan fold and thrust belt e g 0 2 barr and dahlen 1989a and they are also lower than laboratory friction measurements for most rocks e g 0 6 0 8 byerlee 1978 consequently these low values suggest the presence of rocks with unusual material properties e g montmorillonite logan and rauenzahn 1987 or anomalous fluid pressures along the decollement on the basis of the occurrence of shale diapirs the distribution of early veins and the absence of montmorillonite in the underthrust sequences we propose that there was an anomalously high fluid pressure along the decollement fault instead of there being such a low coefficient of friction these results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement which in turn determines how the accretionary prism grows in essence it is the development of a weak overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes

a linear gradient crustal model for south hawaii a new crustal model with linear velocity gradients within layers does as good a job of locating earthquakes on south hawaii as any model yet published incorporating linear gradients means the model can be simpler and free of artificial velocity discontinuities using travel time residuals from local earthquakes and consistency of focal mechanism solutions as tests it is seen that a low velocity zone at the base of the crust is not required

block tectonics of the island of hawaii from a focal mechanism analysis of basal slip the southern third of the island of hawaii is composed of at least three rigid blocks kilauea volcano s south flank mauna loa volcano s southeast flank and mauna loa volcano s west flank analysis of earthquake focal mechanisms suggests that kilauea s south flank the most mobile of these blocks is moving seaward perpendicular to its east rift zone along a southeasterly course at an azimuth of 155 5 mauna loa s southeastern flank is also moving in a southeasterly direction although at a slightly more easterly azimuth of 140 5 although data are sparse on the western flank of mauna loa due to low seismicity and inadequate seismic network station density the available data suggest that this region is translating seaward along a generally westerly azimuth relative differences in these translational azimuths may represent oblique opening across the boundaries between these regions the active rift zones of kilauea and mauna loa volcanoes

deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of kilauea dense microearthquake swarms occur in the upper south flank of kilauea providing multiplets composed of hundreds of events the similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters a simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically relocation of these events greatly improves the definition of the seismic image of the fault that generates them indeed relative relocations define a plane dipping about 6 northward although corresponding absolute locations are widely dispersed in the swarm a composite focal mechanism built from events providing a correct spatial sampling of the multiplet also gives a well constrained northward dip of about 5 to the near horizontal plane this technique thus collapses the clouds of hypocenters of single event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies we cannot conclude that all south flank earthquakes collapse to a single plane there may locally be several planes perhaps with different dips and depths throughout the south flank volume the 6 northward dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of hawaii this plane is probably an important feature that characterizes the basal slip layer below the upper south flank of kilauea volcano differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane the present work illustrates how high precision relative relocations of similar events in dense swarms combined with the analysis of geodetic measurements can help to describe deep fault plane geometry systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well instrumented highly seismic areas to provide reliable basic information especially useful for understanding of earthquake generation processes

precursors to the kalapana m 7 2 earthquake the kalapana hawaii earthquake of november 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano kilauea the source mechanism was dip slip normal faulting on a plane dipping 20 degrees to the se with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into kilauea s rifts the source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano we studied the distribution of epicenters for small earthquakes from 1962 to 1975 seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914 we found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns in the larger outer anomalous area the seismicity rate was decreased by 50 during the 3 8 years before the mainshock in addition several geodetic lines indicated anomalous strain release during this time within two inner areas the seismicity remained high then increased shortly before the mainshock in one of the inner areas a p wave travel time delay of 0 2 s could be detected which began about 3 5 years before the mainshock within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3 5 10 4 strain 35 bars was released aseismically by contrast the outer anomalous volume was experiencing strain softening from 1970 1971 on we interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities locked portions of the fault a velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other implying that dilatancy of the crust probably occurred our model is qualitatively supported by the independent evidence of strong motion records which show that the kalapana earthquake was a complex multiple rupture we conclude that the kalapana earthquake was preceded by a preparatory process which lasted 3 8 0 3 years and which had dimensions of 45 10 km covering approximately the aftershock area

mechanics of wedge shaped fault blocks 2 an elastic solution for extensional wedges listric planar low angle and high angle normal faults are common in hanging walls of detachment faults an elastic model has been developed to evaluate the role of basal friction wedge geometry pore fluid pressure within the wedge and boundary conditions applied along the wedge rear in controlling the stress distribution in an extensional fault wedge this model assumes a stress free condition on the top and fractional sliding on the base of the wedge respectively a linear variation of stress components as a function of depth along the wedge rear and a uniform horizontal stress applied on the wedge toe the model predicts that 1 for the surface slope equal to zero a thin wedge favors development of high angle and planar faults whereas a thick wedge favors development of low angle and listric normal faults variation of pore fluid pressures along the basal detachment fault hardly affects the predicted fault geometry pore fluid pressure within the wedge is critical in controlling the state of stress in the wedge higher values of the internal pore fluid pressure promote low angle normal faulting and locally high angle reverse faulting 2 variation of surface slope and the uniform horizontal normal stress applied at the wedge toe does not affect the predicted fault geometry appreciably although the distribution of deviatoric stress magnitude changes for different cases and 3 listric normal faults are predicted in all computations and the fault curvature increases as the vertical gradient of the horizontal normal stress along the wedge rear the wedge angle surface slope dip angle and the internal pore fluid pressure increase the model provides a simple conceptual guide to deciphering the formation of complex fault geometries and cross cutting relationships as a function of mechanical parameters related to geologic processes and settings for example it provides an explanation for why low angle normal faults cut high angle normal faults in hanging walls of some detachment fault systems in the u s cordillera

inversion for source parameters from sparse data sets test of the method and application to the 1951 m 6 9 kona hawaii earthquake we have tested a method originally developed by beisser et al 1990 to retrieve the source parameters strike dip rake and depth of sparsely recorded earthquakes from the inversion of teleseismic waveform data the complete wave train of all body waves is modeled using the reflectivity method the parameter space of strike dip and rake is searched to find the source orientation which leads to the minimum misfit between the observed and the synthetic seismogram the 1983 kaoiki hawaii earthquake ms 6 6 was chosen as a test case the inversion of the full data set 16 stations gave a fault plane solution similar to the best double couple moment tensor solution of harvard university and national earthquake information centre these three solutions were averaged to create a standard solution sparse data sets were simulated by decimating the full data set and the resulting fault plane solutions were compared with the standard we found that as few as one to three stations were sufficient to retrieve the focal mechanism of the 1983 kaoiki event we applied this technique to the 1951 kona hawaii earthquake ms 6 9 a total of four stations and nine components were used to model the source parameters of this earthquake the depth was estimated at 13 3 km the fault plane solution was a decollement type with a near horizontal plane dipping at about 15 to the southwest and a near vertical plane striking nw se this observation supports a tectonic model for the kona coast similar to that of kilauea s south flank the upper crust is pushed away from the center of hawaii slipping westward along a near horizontal plane of weakness

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

evidence of multiple strain fields beneath the eastern flank of mt etna volcano sicily italy deduced from seismic and geodetic data during 2003 2004 we carried out a study of the seismicity and ground deformation occurring on mt etna volcano after the end of the 2002 2003 eruption and before the onset of the 2004 2005 eruption data were recorded by the permanent local seismic network run by istituto nazionale di geofisica e vulcanologia sezione di catania and by geodetic surveys carried out in july 2003 and july 2004 on the gps network most earthquakes were grouped in two main clusters located in the northeastern and southeastern sectors of the volcano the areal distribution of seismic energy associated with the recorded earthquakes allowed us to highlight the main seismogenic areas of mt etna in order to better understand the kinematic processes of the volcano 3d seismic locations were used to compute fault plane solutions and a selected dataset was inverted to determine stress and strain tensors the focal mechanisms in the northeastern sector show clear left lateral kinematics along an e w fault plane consistent with events occurring along the pernicana fault system the fault plane solutions in the southeastern sector show mainly right lateral kinematics along a nne and ene fault plane and left lateral kinematics along nw fault planes that together suggest roughly e w oriented compression surface ground deformation affecting mt etna measured by gps surveys highlighted a marked inflation during the same period and exceptionally strong seawards motion of its eastern flank the 2d geodetic strain tensor distribution was calculated and the results show mainly ene wsw extension coupled with wnw ese contraction indicating right lateral shear along a nw se oriented fault plane the different deformation of the eastern sector of the volcano as measured by seismicity and ground deformation must be interpreted by considering the different depths of the two signals seismic activity in the southeastern sector of volcano is located between 3 and 8 km b s l and can be associated with a very strong additional e w compression induced by a pressurizing source just westwards and at the same depth located by inverting gps data ground deformation in contrast is mainly affected by the shallower dynamics of the fast moving eastern flank which produces a shallower opposing e w extension the entire dataset shows that two different processes affect the eastern flank at the same time but at different depths the boundary is clearly located at a depth of 3 km b s l and could represent the decollement surface for the mobile flank

stress field in friuli ne italy from fault plane solutions of activity following the 1976 main shock abstract fault plane solutions of 78 earthquakes with magnitude larger than 3 2 which occurred in friuli between may 1976 and august 1979 have been used to estimate the orientations of the principal stress axes by an inversion technique the analysis of the confidence limits and the average angular misfit of the solution found when inverting the entire data set revealed that faulting was not produced by a uniform stress distribution subdivision of the study area into smaller volumes did not lead to significant improvements in terms of stress uniformity whereas temporal subdivision of the sample reduced significantly misfit and confidence areas of the solution for some data sets therefore we conclude that the stress directions changed as a function of time a nearly uniform stress direction compatible with tectonic models proposed for the study area ca n s convergence of the adriatic and south alpine units was found between the main event of 6 may 1976 m l 6 4 and the strong aftershocks of mid september 1976 5 1 m l 6 1 the analysis of the individual misfit cumulative pattern reveals that some degree of stress heterogeneity appeared in july 1976 which is explained here in terms of heterogeneous local responses of the stress field to the seismic activity superimposed to the regional compressive factor the degree of perturbation of the principal stress directions by local effects increased with time and showed in particular a clear variation after the strong earthquakes of mid september 1976 the perturbations appear to be reflected by low magnitude quakes whereas the faulting in strong events is compatible with the regional stress field

rapid deformation of the south flank of kilauea volcano hawaii the south flank of kilauea volcano has experienced two large magnitude m 7 2 and m 6 1 earthquakes in the past two decades global positioning system measurements conducted between 1990 and 1993 reveal seaward displacements of kilauea9s central south flank at rates of up to about 10 centimeters per year in contrast the northern side of the volcano and the distal ends of the south flank did not displace significantly the observations can be explained by slip on a low angle fault beneath the south flank combined with dilation deep within kilauea9s rift system both at rates of at least 15 centimeters per year

a seismotectonic model for western hawaii based on stress tensor inversion from fault plane solutions for 57 fault plane solutions of earthquakes with magnitude ml 3 0 located west of the southwest rift zone of mauna loa volcano in hawaii which occurred between 1972 and 1988 the dominant focal mechanisms 44 events are decollement type with one nodal plane nearly horizontal dip 30 the average slip vector of the upper crust on the decollement plane points in an azimuth of 260 toward the ocean away from mauna loa s southwest rift zone at an angle of about 150 with respect to the ne sw oriented rift two other types of focal mechanisms are present normal faults four events and strike slip faults with normal component nine events the orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism after subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west hawaii the orientation for the stress tensor is best resolved in the area located between latitudes 19 27 n and 19 4 n and longitudes 155 7 and 155 9 w where the greatest principal stress directions within the 95 confidence limits are nearly vertical with some spreading to the west their plunge varies between 53 and 85 the intermediate and least principal stresses are mostly horizontal and have similar magnitudes their plunge varies between 1 and 36 the area between latitudes 19 4 and 19 6 n and longitudes 155 7 and 156 w is characterized partly by near vertical and partly by east west oriented greatest principal stresses the least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses in the two areas where the stress tensor was well resolved the principal strain directions differed from the principal stress directions by 30 in the plunge suggesting that the faulting in western hawaii takes place on a weak plane but this result could not be established at the 95 confidence level the tectonic model proposed for west hawaii is similar to the model for the kalapana region the strain is accumulated through magmatic intrusions in the southwest rift zone of mauna loa and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth the west flank of mauna loa slips in the direction away from the rift

intrusive mechanism of the 2008 2009 mt etna eruption constraints by tomographic images and stress tensor analysis abstract since january 2008 several geophysical parameters have evidenced a recharging phase at mt etna volcano culminating with an effusive eruption that began on may 13 2008 seismic activity recorded at mt etna from january 2007 to may 2008 was analyzed in order to provide seismological constraints to the volcano dynamics leading to the eruption a total of 336 selected earthquakes with m l 1 5 were used as data source for this study specifically we calculated 3d velocity and attenuation tomography including a 3d relocation of the events and we computed 53 selected fault plane solutions fpss that were used for stress tensor inversion the most important result obtained from the joint analysis of v p v p v s and p wave attenuation is an anomalous zone with normal to high v p values between 3 5 and 4 5 km s and low v p v s values 1 64 which partially overlaps with a low q p values 50 volume located along a ns trending channel beneath the central crater this can be interpreted as a shallow volume characterized by high temperature where the magma is located with the presence of supercritical fluids the analysis of seismic stress tensor evidenced an extensional regime in the depth range 3 13 km with a vertically oriented 1 this finding may suggest an extensional stress regime probably related to the kinematic response of the volcanic edifice to both a deep magmatic intrusion and a condition of decreased regional compressive stress facilitated by sliding processes of the eastern flank of the volcano

a fault model for the 1989 kilauea south flank earthquake from leveling and seismic data the geometry of the fault that ruptured during the m6 1 south flank earthquake on kilauea volcano in 1989 is determined from leveling data the elastic dislocation in a homogeneous elastic half space that best fits the data is found using a nonlinear inversion procedure the best fitting model is a gently dipping thrust fault that lies at 4 km depth this is significantly shallower than the 9 km hypocentral depth determined from the local seismic network two dimensional finite element calculations indicate that at least part of this discrepancy can be attributed to the focusing of the surface deformation by the upper few kilometers of compliant low density lavas we conclude that it is important to include realistic elastic structure to estimate source geometry from geodetic data

comparison of orientations of stress and strain tensors based on fault plane solutions in kaoiki hawaii the stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly m 3 5 0 6 located in the 10 km radius kaoiki crustal volume separate inversions for subvolumes containing 20 50 events yielded the same results in several adjacent volumes suggesting that the stress tensor is homogeneous in those parts of the kaoiki area and that the inversion results are stable and meaningful five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20 80 and at confidence levels exceeding 95 the volcano summits of kilauea and mauna loa and their rift systems are identified as the source of stress in the kaoiki crust because the greatest principal stress points to kilauea and mauna loa in addition the strain tensor due to energy released by these 238 earthquakes was computed for the kaoiki area and several subvolumes of it by summing the moment tensors the moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment magnitude relationship a comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike slip faulting in these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes these observations are interpreted to show that in the strike slip regime of the upper part of the crust neither of the nodal planes is preferred for faulting rupture probably occurs along the nw and along the nh striking nodal planes in separate earthquakes subvolumes with more decollement faulting showed significant differences of 30 40 between the principal strain and stress directions in these volumes the near horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions these facts are interpreted to indicate that the decollement plane is weak allowing slip on it even if the principal stresses are inclined at a large angle to it it is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength no well developed fault plane and regimes in which a fault with low frictional strength dominates as a function of time significant rotations of the strain tensor by approximately 45 can be observed which seem to be related to the occurrences of kaoiki mainshocks during three background periods of about 7 years each the average strain tensor showed an orientation typical for predominant decollement faulting while two premainshock periods of 2 5 years each showed an orientation closer to strike slip faulting it is proposed that this pattern may be repeated before the next kaoiki mainshock the strain released seismically is less than the geodetically observed strain by approximately an order of magnitude stress strain fault plane solutions hawaii

new insights into kilauea s volcano dynamics brought by large scale relative relocation of microearthquakes 1 we investigated the microseismicity recorded in an active volcano to infer information concerning the volcano structure and long term dynamics by using relative relocations and focal mechanisms of microearthquakes there were 32 000 earthquakes of the mauna loa and kilauea volcanoes recorded by more than eight stations of the hawaiian volcano observatory seismic network between 1988 and 1999 we studied 17 000 of these events and relocated more than 70 with an accuracy ranging from 10 to 500 m about 75 of these relocated events are located in the vicinity of subhorizontal decollement planes at a depth of 8 11 km however the striking features revealed by these relocation results are steep southeast dipping fault planes working as reverse faults clearly located below the decollement plane and which intersect it if this decollement plane coincides with the pre mauna loa seafloor as hypothesized by numerous authors such reverse faults rupture the pre mauna loa oceanic crust the weight of the volcano and pressure in the magma storage system are possible causes of these ruptures fully compatible with the local stress tensor computed by gillard et al 1996 reverse faults are suspected of producing scarps revealed by kilometer long horizontal slip perpendicular lineations along the decollement surface and therefore large scale roughness asperities and normal stress variations these are capable of generating stick slip large magnitude earthquakes the spatial microseismic pattern observed in the south flank of kilauea volcano and hilina type instabilities rupture intersecting the decollement surface causing its large scale roughness may be an important parameter controlling the growth of hawaiian volcanoes

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

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

joint analysis of geodetic and earthquake fault plane solution data to constrain magmatic sources a case study from kīlauea volcano abstract a joint analysis of geodetic and seismic datasets from k lauea volcano during a period of magmatic unrest in 2006 demonstrates the effectiveness of this combination for testing and constraining models of magma dynamics for a complex multi source system at the end of 2003 k lauea s summit began a four year long period of inflation due to a surge in magma supply to the volcano in 2006 for the first time since 1982 k lauea s southwest rift zone swrz also experienced inflation to investigate the characteristics of active magma sources and the nature of their interactions with faults in the swrz during 2006 we integrate through coulomb stress modeling contemporary geodetic data from insar and gps with a new catalogue of double couple fault plane solutions for volcano tectonic earthquakes we define two periods of inflation during 2006 based on the rate of deformation measured in daily gps data spanning february to 15 march 2006 period 1 and 16 march to 30 september 2006 period 2 insar data for these two periods are inverted to determine the position change in size and shape of inflation sources in each period our new models are consistent with microseismic activity from each period they suggest that during period 1 deformation in the swrz can be explained by pressurization of magma in a spherical reservoir beneath the south caldera and that during period 2 magma was also aseismically intruded farther to the southwest into the swrz along a sub horizontal plane our coulomb stress analysis shows that the microseismicity recorded in the swrz is induced by overpressurization of the south caldera reservoir and not by magma intrusion into the swrz this study highlights the importance of a joint analysis of independent geophysical datasets to fully constrain the nature of magma accumulation

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

focal mechanisms and locations of earthquakes in the vicinity of the 1975 kalapana earthquake aftershock zone 1970 1979 implications for tectonics of the south flank of kilauea volcano island of hawaii the november 29 1975 kalapana earthquake of magnitude 7 1 on the south flank of kilauea volcano hawaii represents a major rift tectonic event it is generally understood that the earthquake resulted from movement on a nearly horizontal fault plane with the crustal block south of kilauea s east rift zone moving south southeastward up to several meters well recorded earthquakes on the south flank of kilauea from 1970 to 1979 were analyzed and focal mechanisms determined to obtain a better understanding of the details of south flank tectonics included are a large number of earthquakes from the aftershock sequence of the 1975 earthquake hypocenters are distributed in a subplanar zone that dips 2 to 3 to the west at a depth of about 8 9 km depth we interpret this pattern to coincide with a portion of the slip plane of the kalapana earthquake and to be consistent with the notion of the slip plane occurring in a weak layer at the top of the old oceanic crust focal mechanisms are remarkably constant throughout this time period both before and after the 1975 event and closely agree with the mainshock mechanism p axis azimuths and slip vectors for the low angle fault planes are normal to the middle and lower east rift zones of kilauea there is suggestion of a change in the orientation of p axes from shallower to steeper plunge at the time of the kalapana earthquake such a change is consistent with a general reduction in the horizontal compressional stress normal to the rift zone at the time of the large earthquake no other evidence of precursory reorientation of the stress field was found and the precise predictability of this event remains in doubt however a general model of the tectonics of the volcanic rift system is now available that satisfies a number of geological and geophysical observations

comment on a single force model for the 1975 kalapana hawaii earthquake by holly k eissler and hiroo kanamori

inverse solution of surface mass balance of midtre lovenbreen svalbard we investigate the temporal evolution and spatial distribution of mass balance on the glacier midtre lovenbreen svalbard running a diagnostic high resolution full stress ice flow model with geometries obtained from five digital elevation models dems in the period 1962 2005 we compute velocity fields and linearly interpolated volume change of the glacier we evaluate the kinematic free surface equation using these model outputs to solve the surface mass balance smb monitoring data on midtre lovenbreen allows model results to be compared with point measurements from the glacier over several decades this method allows us to estimate the mass balance over the entire glacier surface beyond the spatially limited field measurements and to derive past smb over an extended time period

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

flexure and seismicity beneath the south flank of kilauea volcano and tectonic implications we present a refined model for the tectonic behavior of kilauea volcano s south flank including flexure calculations and microearthquake data supportive of the model in our model the south flank moves seaward over the downwardly flexed ocean crust in a manner analogous to an accretionary prism in a subduction zone the flank is driven seaward by the gravitational stresses inherent in its shape this driving force is augmented by high density material in kilauea s rift zones elastic flexure calculations predict a configuration for the downwardly flexed crust which agrees with previous seismic refraction and gravity modeling this configuration in turn is consistent with the hypothesis that many earthquakes occur along the volcanic pile ocean crust interface focal mechanisms for these events predominantly indicate southeastward directed overthrusting we also propose a relationship between rift zone intrusion and south flank displacement including a suggestion that the decollement configuration controls the relative activity of kilauea s east and southwest rift zones

transtensional origin of the ne sw simitli basin along the strouma strymon lineament sw bulgaria several depocentres have developed along the nnw sse striking strouma strymon lineament since the miocene forming the strouma strymon graben system some of these basins including the dzherman and simitli strike at high angles to the lineament the rhomboid shaped simitli basin is limited in the south by the ne sw striking seismically active kroupnik fault and contains terrestrial clastic sediments which have been deposited since the sarmatian its formation is attributed to wrench dominated transtension d b event which activated the basin9s ne sw striking boundary faults as left lateral strike slip structures during the early middle miocene this deformation was associated with nne ssw contraction and wnw ese extension and succeeded the nne ssw contraction caused by pure shear dominated transpression d a event which governed the region in late oligocene early miocene times during the middle late miocene the d b event was followed by wnw ese pure shear dominated extension d 1 event which caused further widening of the basin the transtensional origin of the simitli basin is explained by lateral extrusion of the crustal mass squeezed between the apulian adriatic microplate and the european foreland towards the north aegean sea the subduction retreat of the hellenic orogen possibly enhanced this lateral extrusion and from the late miocene onwards the ne sw oriented back arc extension balanced the continuing retreat

an elastic wedge model for the development of coeval normal and thrust faulting in the mauna loa kilauea rift system in hawaii a long standing enigma of the mauna loa kilauea rift system in hawaii is the coeval development of normal and thrust faults that are vertically partitioned to address this question we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that 1 the pore fluid pressure ratio within the wedge and along the basal decollement b must be exceedingly high i e b 0 90 0 95 and 2 a tensile stress of the order of 10 30 mpa must have existed in the very top part of the rift zone at the back side of the wedge shaped rift flank the high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water saturated sediments between the volcanic pile above and the oceanic floor below although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope our results show that this is not a prerequisite therefore we attribute occurrence of tensile stress to either upward bending of the hawaiian volcanic pile due to emplacement of magma or inflation of a shallow magma chamber several kilometers beneath the surface in any case the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process while the deep rift zone experiences forceful emplacement i e active rifting via magma push

incipient extension along the active convergent margin of nubia in sicily italy cefalu etna seismic zone 1 recent geodetic data are compatible with nne ssw tectonic extension at a rate of 5 mm yr in sicily southern italy within a broader region of net active compression along the nubian plate margin northern africa the structures that accommodate such extensional regime and its cause are still unknown from field structural surveys and seismological analyses the geometry kinematics structural architecture and seismic potential of an extensional seismic zone linking cefalu and mount etna in central eastern sicily are defined the zone includes high angle wnw striking normal and right lateral strike slip faults and subordinate north and nne striking strike slip faults either right or left lateral the occurrence of small discontinuous faults and the absence of related depressions and sedimentary basins suggest that the extensional regime is still in an incipient stage the ongoing seismic activity possibly reactivates preexisting faults instrumentally and historically recorded earthquakes are lower than about 6 in magnitude and destructive events are historically unknown since at least 1300 a d this apparent upper bound of earthquake magnitudes is consistent with the maximum magnitude values estimated from the length of the longest mapped faults and sources of seismic swarms which all together suggest a value between 6 and 6 5 as the maximum expected magnitude that can be proposed at the present stage of investigation for earthquakes in the study area lateral extension on preexisting faults and upwelling of melt mantle material beneath mount etna are considered viable processes to explain at least in part the active extensional tectonics along the cefalu etna seismic zone strike slip seismic faulting beneath mount etna may be part of a previously proposed diffuse transfer zone affecting northeastern sicily and including the tindari fault

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

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

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

moderate magnitude earthquakes induced by magma reservoir inflation at kīlauea volcano hawai i 1 although volcano tectonic vt earthquakes often occur in response to magma intrusion it is rare for them to have magnitudes larger than m4 on 24 may 2007 two shallow m4 earthquakes occurred beneath the upper part of the east rift zone of k lauea volcano hawai i an integrated analysis of geodetic seismic and field data together with coulomb stress modeling demonstrates that the earthquakes occurred due to strike slip motion on pre existing faults that bound k lauea caldera to the southeast and that the pressurization of k lauea s summit magma system may have been sufficient to promote faulting for the first time we infer a plausible origin to generate rare moderate magnitude vts at k lauea by reactivation of suitably oriented pre existing caldera bounding faults rare moderate to large magnitude vts at k lauea and other volcanoes can therefore result from reactivation of existing fault planes due to stresses induced by magmatic processes

volcanic unrest leading to the july august 2001 lateral eruption at mt etna seismological constraints abstract a close relationship between earthquake swarms volcanic eruptions and ground deformation at mt etna was well documented shortly before the beginning of the july august 2001 eruption past experiences at this volcano suggest how magma dike intrusion in the shallow crust or in the upper part of the volcanic pile normally occurs after several years months of internal recharging since seismic investigations provide a means to study the scale and origin of stress perturbations at active volcanoes allowing to better investigating the preparation phase of an eruption in this paper we performed a close examination of the seismic activity recorded at mt etna in the months preceding the 2001 eruption and in particular between november 2000 and july 2001 after integrating data recorded by the two networks operating during that time and run by the istituto internazionale di vulcanologia and sistema poseidon we relocated 522 earthquakes by using the tomodd code in a 3d velocity model and then we computed their fault plane solutions the application of different selection criteria enabled obtaining a good quality revised data set consisting of 111 fault plane solutions the high precision locations identified well defined seismic clusters in different periods suggesting a link with the magma migration from a depth of 8 13 km b s l towards shallower zones moreover the computed maximum compressive stress axis as inferred from earthquake focal mechanisms indicated a roughly w e oriented 1 this findings reflect an overpressure of the mid to shallow crust due to the progressive magma uprising in central portion of the volcano and also highlighted a rotation of the local stress field with respect to the regional one n s trending in addition p axis distribution pointed out the presence of a center of pressure located to the south of the central craters these results provide particularly compelling evidence for a direct causal link between pressurization of the mid level volcanic plumbing system by ascending magma and precursory local stress field reorientations demonstrating that seismological analysis can be used to detect subtle local stress changes that herald eruptive activity

an increment of volcano collapse kinematics of the 1975 kalapana hawaii earthquake abstract earthquakes on kilauea volcano can be even more hazardous than volcanic eruptions the most recent large earthquake was the 1975 kalapana quake which generated a locally damaging tsunami over 8 m of horizontal surface displacement and 3 m of subsidence at the coast the seismic magnitude estimates for this earthquake range from ms 7 2 to mw 7 7 the complexity of the seismic data is matched by the complex surface deformation that was observed with a combination of leveling tilt and electronic distance measurements edm the geodetic data shows evidence of collapse of the caldera due to magma withdrawal localized subsidence over the rift zone normal faulting associated with shallow slumping along the hilina pali and displacement on the basal detachment fault we model the summit caldera rift zones and basal detachment fault as dislocations in an elastic half space to better quantify the sources of deformation related to this devastating event using inversion techniques that allow us to let the data constrain the geometry and magnitude of these sources we find 0 04 km3 of magma withdrawal at the summit between 3 5 m of opening along the rift zone and 7 1 m of slip along the basal detachment at 8 3 km depth models that allow for finer spatial resolution of slip on the detachment show that the largest slip occurred west of the seismically recorded earthquake hypocenter along the region of the fault below the coast and that the majority of the fault slip occurred south of the region of microseismicity our best fitting model has a geodetic moment of 4 1 1020 nm mw 7 7 which is consistent with tsunami models and recent analysis of long period seismic data the residual displacements of sites located in the hanging wall of the hilina pali slump system which were not used in the inversion of the other sources suggest that triggered shallow slumping contributed several meters of horizontal displacement to the coseismic displacement at these sites

systematic variations in stress state in the southern san joaquin valley inferences based on well bore data and contemporary seismicity analysis of stress induced well bore breakouts in 35 wells from 10 production fields in the southern san joaquin valley ssjv indicates systematic spatial variations in the direction of the maximum horizontal stresses at three different scales first the regional northeast southwest compressional stress direction seen along the western margin of the san joaquin valley in the elk hills kettleman hills and coalinga areas gradually changes to approximately north south compression over a distance of 10 20 km in the ssjv this major excursion in the stress field seen in the yowlumne yowlumne north paloma and rio viejo production fields represents an approximately 40 counterclockwise rotation in the direction of the maximum horizontal stress shmax th s systematic reorientation is consistent with approximately north south convergence as seen in the local fold axes and reverse faults of pliocene age and younger second at the extreme southern reaches of the ssjv in the san emidio los lobos pleito wheeler ridge and north tejon fields another systematic but localized reorientation in the stress field indicates an abrupt change to an approximately east northeast west southwest compression over a distance of a few kilometers this latter reorientation of shmax stress direction which is inconsistent with the local east west trending fold axes and thrust faults represents a 40 50 clockwise rotation in the stresses this reorientation appears to be limited to production fields located within the inferred hanging w ll of the white wolf fault that ruptured during the 1952 ms 7 8 kern county earthquake inversion of earthquake focal mechanisms of events located below the perturbed stress field indicates approximately north south compression the stress drop associated with the 1952 earthquake may have been responsible for rotating the shmax stress direction from the regional approximately north south to approximately east northeast west southwest in the southernmost ssjv implying that the remote horizontal stresses are comparable in magnitude finally localized stress perturbations observed at the scale of a few meters in several wells may reflect local perturbations in the stress field resulting from slip along small faults penetrated by the borehole knowledge of the nonun formity in the ssjv stress field is important for designing development programs for production fields in terms of borehole stability propagation of hydraulic fractures and fluid transport properties

a possible triggering mechanism for large hawaiian earthquakes derived from analysis of the 26 june 1989 kilauea south flank sequence abstract examination of short period seismic data from the m l 6 1 kilauea south flank earthquake and aftershock sequence indicates that the rupture process in large hawaiian earthquakes is more complex than previously modeled in contrast to the low angle thrust solution determined for the mainshock from long period teleseismic body waves by other workers i find an intermediate to high angle reverse solution i find however that focal mechanisms for coastal aftershocks of m l 3 0 are similar to the teleseismic mechanism for the mainshock a difference in focal mechanisms determined from short period local network seismic data and from long period teleseismic data has been noted for other recent large hawaiian earthquakes both the mapping of surface cracks and the focal mechanism derived from short period seismic data for the m l 6 6 1983 kaoiki earthquake show strike slip motion whereas the centroid moment tensor solution shows low angle thrusting the focal mechanism calculated from short period seismic data for the m l 7 2 1975 kalapana mainshock shows low angle thrusting according to some workers but intermediate to high angle reverse faulting according to others whereas focal mechanisms calculated from long period seismic data show low angle thrusting this result suggests that rupture initiation in large hawaiian earthquakes as represented by the short period focal mechanisms differs significantly from the overall rupture process as represented by the teleseismic mechanisms i propose that small earthquakes trigger the large scale energy release at the bases of the volcanic edifices the type of energy release often observed in large hawaiian earthquakes these triggering events may occur along rupture surfaces that differ from those along which the long period moment release occurs and thus may represent release of a local stress concentration superposed upon the regional stress field

seismicity and deformation induced by magma accumulation at three basaltic volcanoes we analyzed the evolution of volcano tectonic vt seismicity and deformation at three basaltic volcanoes kilauea mauna loa piton de la fournaise during phases of magma accumulation we observed that the vt earthquake activity displays an accelerating evolution at the three studied volcanoes during the time of magma accumulation at the same times deformation rates recorded at the summit of kilauea and mauna loa volcanoes were not accelerating but rather tend to decay to interpret these observations we propose a physical model describing the evolution of pressure produced by the accumulation of magma into a reservoir this variation of pressure is then used to force a simple model of damage where damage episodes are equivalent to earthquakes this model leads to an exponential increase of the vt activity and to an exponential decay of the deformation rate during accumulation phases seismicity and deformation data are well fitted by such an exponential model the time constant deduced from the exponential increase of the seismicity is in agreement with the time constant predicted by the model of magma accumulation this vt activity can thus be a direct indication of the accumulation of magma at depth and therefore can be seen as a long term precursory phenomenon at least for the three studied basaltic volcanoes unfortunately it does not allow the prediction of the onset of future eruptions as no diverging point i e critical time is present in the model

type of faulting and orientation of stress and strain as a function of space and time in kilauea s south flank hawaii earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of kilauea volcano hawaii are used to infer the state of stress and strain as a function of time and space we have determined 870 fault plane solutions from p wave first motion polarities for events with magnitudes m l 2 5 and depth ranging between 6 and 12 km faulting is characterized by a mixture of decollement reverse and normal faults most large earthquakes with magnitude m 7 rupture the decollement plane since it is the only surface large enough to generate magnitude 7 or larger earthquakes the percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972 1983 the percentage of decollement type focal mechanisms becomes dominant after 1983 this pattern of faulting activity suggests that pressure was building up within kilauea s rift zone prior to the 1983 puu oo eruption overall a single stress orientation with the maximum compressive stress oriented se perpendicular to the rift and dipping at 45 is compatible with the coeval existence of decollement reverse and normal faults however in a crustal volume east of longitude 155 10 w we find a change of the orientation of 1 from nearly horizontal to plunging 45 se occurring in 1979 this stress rotation suggests magma movements within the aseismic part of kilauea s east rift zone the strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed we observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements while stress stays constant or by a rotation of stress while strain remains constant assuming that the model of a noncohesive coulomb wedge is appropriate for kilauea s south flank we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0 85

comparison of strain and stress tensor orientation application to iran and southern california strain and stress tensors along plate boundaries in iran and southern california are oriented coaxially if they are estimated based on small to moderate earthquakes if large earthquakes m 7 are included in the analysis the estimate of stress directions does not change but the orientation of the released strain rotates significantly thus the direction of the total released strain is different from the stress by about 20 in both iran and southern california and by about 45 in central california we interpret this observation to indicate that the smaller earthquakes occur on faults favorably oriented for failure in the prevailing stress field in which they are possibly generated the larger events rupture near vertical strike slip faults that are unfavorably oriented but weak this weakness causes failure under low ratio of shear to normal stress this view is supported by the fact that in the southern california data set the average angle between the fault planes and the greatest principal stress is about 40 whereas it forms an angle of 70 with the san andreas fault nearly 90 in central california for estimating the seismic strain release by kostrov s method along about a 10 km wide section of the san andreas fault zone we used the data set from which jones determined the stress directions for earthquakes in the range of 2 6 m 4 3 we estimated the directions of principal strain for iran based on 111 focal mechanisms and the scalar moment for earthquakes in the range of 4 4 m 8 we inverted regional subsets of these data for volumes with dimensions of a few hundred kilometers for the stress directions using the method of gephart and forsyth the average misfits for these inversions ranged from 2 to 5 based on test inversions of synthetic data with known misfits we found that these misfits can be attributed solely to inaccuracies of less than 10 in the fault plane solutions therefore the results for iran are most likely obtained for volumes with uniform stress directions we propose that stress inversions with misfits smaller than 6 can be accepted as meaningful as an approximate general guideline because our tests for iran agree with those for hawaiian data sets performed by wyss

seismological evidence for a slab detachment in the tonga subduction zone abstract the tonga wadati benioff zone is characterized by a large seismicity gap beneath the lau basin that raises the question of the slab continuity between the shallow to intermediate part 60 300 km and the deep part 400 700 km to address this problem we investigated the wadati benioff zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor using the global seismicity engdahl e van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seism soc am 88 722 743 and the centroid moment tensor solutions cmt catalogs dziewonski a m chou t woodhouse j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 the stress tensors were computed using the gephart s program gephart j w forsyth d 1984 an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence j geophys res 89 9305 9320 the stress inversion results indicate that between 21 s and 27 s and depths down to 700 km the slab is under homogeneous down dip compressional stress regime while north of 21 s we found strong variations of the stress orientations between the intermediate and deep portions of the slab we also show that between 14 and 19 s the stresses at intermediate depth 60 300 km can be resolved into two slab parallel domains a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension this pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670 km depth boundary these results together with the large seismicity gap within the slab argue for a slab detachment

a global database and statistical analyses of 4 vesta craters abstract in this paper we present a global database of craters larger than 0 7 km in diameter on the surface of 4 vesta which we created based on a global mosaic of the dawn spacecraft s high altitude mapping orbit hamo images at a resolution of 60 m pixel and cross checked using the images from the low altitude mapping orbit lamo mission phase the method to produce the crater database consists of visual crater identification and mathematical shape determination to give precise measurements topographic corrections have also been considered by combining the digital elevation model dem data in this global survey we calculated the morphometric parameters of the craters which consist of diameter depth depth to diameter ratio ellipticity and azimuth of the major axis our results show that the craters on 4 vesta are very shallow with a mean depth to diameter ratio of 0 065 which is similar to that of the craters on 1 ceres and 25143 itokawa but lower than for craters on other solid bodies the results also show that the fractions of craters with an ellipticity greater than 1 1 or 1 2 are smaller than those of craters on the martian surface which is probably because few secondaries exist on the surface of 4 vesta in addition the azimuths of the major axis are almost uniformly distributed in different directions our globally consistent crater database has many potential applications for future studies such as geologic mapping and surface age dating