a large refined catalog of earthquake relocations and focal mechanisms for the island of hawai i and its seismotectonic implications we present high quality focal mechanisms based on a refined earthquake location catalog for the island of hawai i focusing on mauna loa and k lauea volcanoes the relocation catalog is based on first arrival times and waveform data of both compressional and shear waves for about 180 000 events on and near the island of hawai i between 1986 and 2009 recorded by the seismic stations at the hawaiian volcano observatory we relocate all the earthquakes by applying ray tracing through an existing three dimensional velocity model similar event cluster analysis and a differential time relocation method the resulting location catalog represents an expansion of previous relocation studies covering a longer time period and consisting of more events with well constrained absolute locations the focal mechanisms are obtained based on the compressional wave first motion polarities and compressional to shear wave amplitude ratios by applying the hash program to the waveform cross correlation relocated earthquakes overall the good quality defined by the hash parameters focal solutions are dominated by normal faulting in our study area especially in the active ka iki and h lea seismic zones k lauea caldera is characterized by a mixture of approximately equal numbers of normal strike slip and reverse faults whereas its south flank has slightly fewer strike slip events our relocation and focal mechanism results will be useful for mapping the seismic stress and strain fields and for understanding the seismic volcanic tectonic relationships within the magmatic systems

strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

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

a reinterpretation of seismicity associated with the january 1983 dike intrusion at kilauea volcano hawaii in january 1983 a dike intrusion fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16 km segment of kilauea s middle east rift zone we searched the hawaiian volcano observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm consistent with generation near the propagating dike tip while others were active for several days the two multiplets nearest the origin of the seismic swarm include events from the preceding days and months most multiplets span only 50 to 100 m following relocation are located at about 3 to 4 km depth and appear to deepen downrift the catalog depths of those earthquakes in multiplets and those not in multiplets are similar suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface by analogy with a mechanical model used to explain a similar clustering of background seismicity in the upper east rift in 1991 we infer that the earthquakes are generated in regions of high stress concentration immediately above kilauea s deforming deep rift body this conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1

an estimate of the absolute stress tensor in kaoiki hawaii the stress tensor orientation was calculated by inversion of 81 fault plane solutions of m 3 5 0 6 earthquakes located in an area 6 by 16 km at depths of 7 2 km this crustal volume is situated on a straight line between the summits of the active volcanoes kilauea and mauna loa the orientation of the greatest principal stress was found to be near horizontal and in the line connecting the two volcanoes this is further evidence supporting the model that magmatic expansion within the feeding conduits of these volcanoes is the source of stress that causes earthquakes in southern hawaii these earthquakes are tectonic earthquakes in the sense that they occur in a brittle elastic crust at distances of a few to several tens of kilometers from the volcanoes and that they are not directly associated with the opening of cracks by intrusions we propose that this model leads to the corollary that the shear stresses responsible for earthquake failure in the shallow hawaiian crust are approximately 3 2 mpa and that the pore pressure of ground water p in the hypocentral volume must be under near lithostatic pressure this result is obtained by arguing that the greatest principal stress 1 is equal to the magma pressure in the volcanic feeder pipe but cannot exceed the least principal stress 3 by more than the tensile strength of the crust because otherwise hydrofracture would occur opening a crack against the least principal stress given the difference between the greatest and least principal stress the fact that the overburden at the 7 km hypocentral depth equals the intermediate principal stress and the ratio r 1 2 1 3 0 4 which is obtained in the inversion of fault plane solutions we estimate that 1 202 mpa 2 200 mpa 3 196 mpa 3 mpa and n 199 mpa with uncertainties on the order of several mpa it follows that for any coefficients of friction larger than 0 15 the pore fluid pressure necessary for faulting on the observed fault planes has to be larger than 90 of the lithostatic pressure these considerations suggest that many earthquakes represent fault ruptures under low ambient shear stresses in the order of the average observed stress drop of 3 mpa

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

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

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

relocation of seismicity preceding the 1984 eruption of mauna loa volcano hawaii delineation of a possible failed rift abstract waveform cross correlation based repicking of p arrival times and subsequent relocation of 187 earthquakes that occurred near the summit of mauna loa hawaii prior to the march 1984 eruption has illuminated a previously obscured structure beneath the northwestern flank simultaneous inversion for hypocenters and velocity model parameters using the refined arrival times resulted in well constrained relative earthquake locations with very low arrival time misfits average rms 0 041 s pre eruption seismicity from this time period occurred in two groups a shallow group located near mauna loa s summit at depths of 1 3 km and a deeper group 5 10 km located 4 6 km northwest of the summit after relocation we found that most of the northwest flank earthquakes occurred along a 3 km long planar feature striking about 60 west of north in a thin band about 1 km thick there is a temporal migration of epicenters from the northwestern edge of the zone migrating towards the summit focal mechanisms in this zone reveal a change in faulting from strike slip in the southeast to a mix of strike slip and normal faulting in the northwest this feature we interpret to be related to a rift zone that was stunted by the buttressing of the adjacent volcanoes hualalai and mauna kea previous gravity and magnetic studies provide supporting evidence for the existence of a rift zone there was only a modest increase in clustering of the summit events after relocation hypocenters that were previously located within a diffuse zone 4 5 km were reduced to a 2 5 km wide zone they extend from a depth of 6 km to about sea level trending toward the summit the focal mechanisms from the summit events are mix of faulting types without a consistent pattern

source properties of two microearthquakes at kilauea volcano hawaii abstract the source time functions of two microearthquakes with magnitudes 2 1 and 2 3 at kilauea volcano hawaii are determined by using the deconvolution technique p wave seismograms of a smaller event with magnitude 1 6 in a nearby location are treated as empirical green9s functions and deconvolved from the waveforms of the larger earthquakes by spectral division from the time domain analysis of the source time functions some of the source properties such as the complexity and directivity and some of the source parameters such as pulse width rise time and source dimension are determined with a good azimuthal distribution of stations the direction and magnitude of rupture velocity are estimated for one of the events the rupture direction is about n85 w and rupture velocity is 0 75 v s assuming v p 1 73 v s the result for direction agrees with one of the nodal planes of the focal mechanism our results indicate that there exist obvious variations in rupture durations for nearby small events with similar seismic moments suggesting significant differences in the static stress drops the stress drops of two events 1 5 and 16 5 bar estimated from the rise times differ by an order of magnitude implying heterogeneity of the stress distribution over this small area

location and size of the shallow magma reservoir beneath kīlauea caldera constraints from near source vp vs ratios we present high resolution compressional wave to shear wave velocity ratios vp vs beneath k lauea s summit caldera by applying an in situ estimation method using waveform cross correlation data for three similar earthquake clusters we observe high vp vs ratios 1 832 and 1 852 for two event clusters surrounded by the low background vp vs value of 1 412 at 2 1 km depth below the surface these high and low vp vs ratios can be explained by melt and co2 filled cracks respectively based on a theoretical crack model the event cluster with the highest vp vs ratio consists of long period events that followed the 1997 east rift zone eruption indicating their association with fluid and magma movement the depths of the two clusters with high vp vs ratios are consistent with the magma reservoir location inferred from geodetic observations their locations east and north of halema uma u crater suggest a horizontal extent of a few kilometers for the reservoir

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

long period source characteristics of the 1975 kalapana hawaii earthquake the 1975 kalapana hawaii earthquake occurred under the highly mobile south flank of kilauea volcano it has been interpreted variously as a normal faulting earthquake a thrust faulting earthquake and a landslide primary evidence for the landslide model has been the failure of previous faulting models to explain the observed love wave radiation pattern and the tsunami amplitudes generated by the kalapana event here we present a reanalysis of the long period digital seismic data for this event centroid moment tensor analysis shows that the seismic radiation pattern can be explained well by thrust faulting on a plane dipping shallowly landward the seismic moment of 3 8 10 20 n m m w 7 7 that we determine is approximately twice as large as earlier estimates the geometry and seismic moment of the focal mechanism determined here are consistent with the observed tsunami amplitudes inversion of long period body wave waveforms shows that the earthquake source duration 72 sec is unusually long for an earthquake of this size

stress control of deep rift intrusion at mauna loa volcano hawaii mauna loa volcano hawaii deforms by a combination of shallow dike intrusions in the rift zones and earthquakes along the base of the volcano but it is not known how the spreading is accommodated in the lower part of the volcanic edifice we present evidence from interferometric synthetic aperture radar data for secular inflation of a dike like magma body at intermediate depth in the southwest rift zone during 2002 to 2005 magma accumulation occurred in a section of the rift zone that was unclamped by previous dikes and earthquakes suggesting that stress transfer plays an important role in controlling subsurface magma accumulation

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

patterns of co seismic strain computed from southern california focal mechanisms summary geometrical properties of an earthquake population can be described by summation of seismic potency tensors that provide a strain based description of earthquake focal mechanisms we apply this method to 170 000 potency tensors for 0 ml 5 southern california earthquakes recorded between january 1984 and june 2003 we compare summed tensors for populations defined by faulting region and earthquake magnitude to investigate the relation between earthquake characteristics tectonic domains and fault related length scales we investigate spatial scales ranging from 1 700 km and use the results to identify systematic differences between seismic behaviour for different faults and different regions our results show features that are indicative of both scale invariant and scale dependent processes on the largest scale the overall potency tensor summation for southern california 0 ml 5 earthquakes over 20 yr corresponds closely to a double couple dc mechanism with slip direction parallel to relative plate motion the summed tensors and derived quantities for the different regions show clear persistent variations that are related to the dominant tectonic regime of each region significant differences between the non dc components of the summed tensors which we relate to fault heterogeneity indicate systematic differences in deformation associated with earthquake populations from different fault zones or different magnitude ranges we find an increase of heterogeneity for populations of smaller earthquakes and for regions where faulting deviates strongly from the overall sense of deformation even when corrected for quality the results imply an overall organization of earthquake characteristics into domains that are controlled to first order by geometrical properties of the largest faults and the plate motion smaller scale characteristics are related to local variations in the orientation complexity and size of faults

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

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

applying a three dimensional velocity model waveform cross correlation and cluster analysis to locate southern california seismicity from 1981 to 2005 we compute high precision earthquake locations using southern california pick and waveform data from 1981 to 2005 our latest results are significantly improved compared to our previous catalog by the following 1 we locate events with respect to a new crustal p and s velocity model using three dimensional ray tracing 2 we examine six more years of waveform data and compute cross correlation results for many more pairs than our last analysis and 3 we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation these results build on the relocated catalogs of hauksson and shearer 2005 and shearer et al 2005 and provide additional insight regarding the fine scale fault structure in southern california and the relationship between the san andreas fault saf and nearby seismicity in particular we present results for two regions in which the seismicity near the southern saf seems to align on dipping faults

three dimensional seismic velocity structure of mauna loa and kilauea volcanoes in hawaii from local seismic tomography we present a new three dimensional seismic velocity model of the crustal and upper mantle structure for mauna loa and kilauea volcanoes in hawaii our model is derived from the first arrival times of the compressional and shear waves from about 53 000 events on and near the island of hawaii between 1992 and 2009 recorded by the hawaiian volcano observatory stations the vp model generally agrees with previous studies showing high velocity anomalies near the calderas and rift zones and low velocity anomalies in the fault systems the most significant difference from previous models is in v p vs structure the high vp and high v p vs anomalies below mauna loa caldera are interpreted as mafic magmatic cumulates the observed low vp and high v p vs bodies in the kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments the high vp and moderate to low vp vs anomalies beneath kilauea caldera can be explained by a combination of different mafic compositions likely to be olivine rich gabbro and dunite the systematically low vp and low vp vs bodies in the southeast flank of kilauea may be caused by the presence of volatiles another difference between this study and previous ones is the improved vp model resolution in deeper layers owing to the inclusion of events with large epicentral distances the new velocity model is used to relocate the seismicity of mauna loa and kilauea for improved absolute locations and ultimately to develop a high precision earthquake catalog using waveform cross correlation data 2014 american geophysical union all rights reserved

analysis of similar event clusters in aftershocks of the 1994 northridge california earthquake 1 we perform waveform cross correlation on over 14 000 aftershocks of the 1994 northridge mw 6 7 earthquake in southern california as recorded by short period stations of the southern california seismic network scsn approximately 10 30 of the events belong to similar event clusters depending upon the similarity criteria that are applied we relocate events within 218 of these clusters to a relative location accuracy of about 30 m using the differential times obtained from the cross correlation these relocated event clusters often show planar features suggestive of faults at depth and we apply principal parameter analysis to characterize the shape of each cluster and to compute best fitting planes in several cases these planes are parallel to the main shock fault plane however more generally the seismicity planes exhibit a wide range of orientations suggesting complexity in the aftershock faulting composite focal mechanisms can be obtained for each cluster by combining the p polarity data from individual events a comparison of polarity measurement differences within similar event clusters provides constraints on the error rate in the individual focal mechanisms for some clusters we are able to resolve the primary versus auxiliary fault plane ambiguity by comparing the computed focal mechanisms with the best fitting seismicity planes individual event focal mechanisms are in general agreement with the composite focal mechanisms for the similar event clusters events occurring along the main shock rupture plane are mainly thrust whereas events in the hanging wall are predominately strike slip

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

evidence for water filled cracks in earthquake source regions 1 we identify lowered vp vs ratios near earthquake source regions in southern california using observations from a seismic tomography model and high resolution local vp vs estimates using waveform cross correlation data from within similar event clusters the median tomographic vp vs ratio is 1 716 0 008 at all the relocated crustal earthquake locations compared to the background median value of 1 729 0 007 for the tomography model although the error estimates overlap slightly the median in situ vp vs ratio of 1 673 0 022 within the similar event clusters suggests that tomographic studies are overestimating vp vs at source regions interpretation of vp vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute vp and vs velocities in the tomography model however the low vp vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water filled cracks with several percent porosity in earthquake source regions in southern california which likely has an effect on faulting and earthquake activity

seismic evidence for a crustal magma reservoir beneath the upper east rift zone of kilauea volcano hawaii an anomalous body with low vp compressional wave velocity low vs shear wave velocity and high vp vs anomalies is observed at 8 11 km depth beneath the upper east rift zone of kilauea volcano in hawaii by simultaneous inversion of seismic velocity structure and earthquake locations we interpret this body to be a crustal magma reservoir beneath the volcanic pile similar to those widely recognized beneath mid ocean ridge volcanoes combined seismic velocity and petrophysical models suggest the presence of 10 melt in a cumulate magma mush this reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 m7 2 kalapana earthquake

mechanism of the 1975 kalapana hawaii earthquake inferred from tsunami data we investigated the source mechanism of the 1975 kalapana hawaii earthquake m s 7 2 by modeling the tsunamis observed at three tide gauge stations hilo kahului and honolulu we computed synthetic tsunamis for various fault models the arrival times and the amplitudes of the synthetic tsunamis computed for ando s fault model fault length 40 km fault width 20 km strike n70 e dip 20 se rake 90 fault depth 10 km and slip 5 6 m are 10 min earlier and 5 times smaller than those of the observed respectively we tested fault models with different dip angles and depths models with a northwest dip direction yield larger tsunami amplitudes than those with a southeast dip direction models with shallower fault depths produce later first arrivals than deeper models we also considered the effects of the hilina fault system but its contribution to tsunami excitation is insignificant this suggests that another mechanism is required to explain the tsunamis one plausible model is a propagating slump model with a 1 m subsidence along the coast and a l m uplift offshore this model can explain the arrival times and the amplitudes of the observed tsunamis satisfactorily an alternative model is a wider fault model that dips 10 nw with its fault plane extending 25 km offshore well beyond the aftershock area of the kalapana earthquake these two models produce a similar uplift pattern offshore and kinematically are indistinguishable as far as tsunami excitation is concerned the total volume of displaced water is estimated to be 2 5 km 3 from the comparison of slump model and the single force model suggested earlier from seismological data we prefer a combination of faulting and large scale slumping on the south flank of kilauea volcano as the most appropriate model for the 1975 kalapana earthquake two basic mechanisms have been presented for explaining the deformation of the south flank of kilauea 1 pressure and density variation along the rift zone caused by magma injection and 2 gravitational instability due to the steep topography of the south flank of kilauea in either mechanism large displacements on the south flank are involved that are responsible for the observed large tsunamis

swarms of similar long period earthquakes in the mantle beneath mauna loa volcano abstract we present analyses of two swarms of long period lp earthquakes at 30 km depth that accompanied the geodetically observed 2002 2005 mauna loa intrusion the first lp earthquake swarm in 2002 consisted of 31 events that were precursory and preceded the start of mauna loa inflation the second lp swarm of two thousand events occurred from 2004 2005 the rate of lp earthquakes slowed significantly coincident with the occurrence of the december 26 2004 m w 9 3 sumatra earthquake suggesting that the seismic waves from this great earthquake may have had a dynamic triggering effect on the behavior of mauna loa s deep magma system using waveform cross correlation and double difference relocation we find that a large number of earthquakes in each swarm are weakly similar and can be classified into two families the relocated hypocenters for each family collapse to compact point source regions almost directly beneath the mauna loa intrusion we suggest that the observed waveform characteristics are compatible with each family being associated with the resonance of a single fluid filled vertical crack of fixed geometry with differences in waveforms between events being produced by slight variations in the trigger mechanism if these lp earthquakes are part of the primary magma system that fed the 2002 2005 intrusion as indicated by the spatial and temporal associations between mantle seismicity and surface deformation then our results raise the possibility that this magma system may be quite focused at these depths as opposed to being a diffuse network it is likely that only a few locations of mauna loa s deep magma system met the geometric and fluid dynamic conditions for generating lp earthquakes that were large enough to be recorded at the surface and that much of the deep magma transfer associated with the 2002 2005 intrusion occurred aseismically

earthquake mechanisms from linear programming inversion of seismic wave amplitude ratios the amplitudes of radiated seismic waves contain far more information about earthquake source mechanisms than do first motion polarities but amplitudes are severely distorted by the effects of heterogeneity in the earth this distortion can be reduced greatly by using the ratios of amplitudes of appropriately chosen seismic phases rather than simple amplitudes but existing methods for inverting amplitude ratios are severely nonlinear and require computationally intensive searching methods to ensure that solutions are globally optimal searching methods are particularly costly if general moment tensor mechanisms are allowed efficient linear programming methods which do not suffer from these problems have previously been applied to inverting polarities and wave amplitudes we extend these methods to amplitude ratios in which formulation on inequality constraint for an amplitude ratio takes the same mathematical form as a polarity observation three component digital data for an earthquake at the hengill grensdalur geothermal area in southwestern iceland illustrate the power of the method polarities of p sh and sv waves unusually well distributed on the focal sphere cannot distinguish between diverse mechanisms including a double couple amplitude ratios on the other hand clearly rule out the double couple solution and require a large explosive isotropic component

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

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

lithospheric flexure under the hawaiian volcanic load internal stresses and a broken plate revealed by earthquakes several lines of earthquake evidence indicate that the lithospheric plate is broken under the load of the island of hawai i where the geometry of the lithosphere is circular with a central depression the plate bends concave downward surrounding a stress free hole rather than bending concave upward as with past assumptions earthquake focal mechanisms show that the center of load stress and the weak hole is between the summits of mauna loa and mauna kea where the load is greatest the earthquake gap at 21 km depth coincides with the predicted neutral plane of flexure where horizontal stress changes sign focal mechanism p axes below the neutral plane display a striking radial pattern pointing to the stress center earthquakes above the neutral plane in the north part of the island have opposite stress patterns t axes tend to be radial the m6 2 honomu and m6 7 kiholo main shocks both at 39 km depth are below the neutral plane and show radial compression and the m6 0 kiholo aftershock above the neutral plane has tangential compression earthquakes deeper than 20 km define a donut of seismicity around the stress center where flexural bending is a maximum the hole is interpreted as the soft center where the lithospheric plate is broken kilauea s deep conduit is seismically active because it is in the ring of maximum bending a simplified two dimensional stress model for a bending slab with a load at one end yields stress orientations that agree with earthquake stress axes and radial p axes below the neutral plane a previous inversion of deep hawaiian focal mechanisms found a circular solution around the stress center that agrees with the model for horizontal faults the shear stress within the bending slab matches the slip in the deep kilauea seismic zone and enhances outward slip of active flanks

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

magmatically triggered slow slip at kilauea volcano hawaii we demonstrate that a recent dike intrusion probably triggered a slow fault slip event sse on kilauea volcano9s mobile south flank our analysis combined models of advanced land observing satellite interferometric dike intrusion displacement maps with continuous global positioning system gps displacement vectors to show that deformation nearly identical to four previous sses at kilauea occurred at far field sites shortly after the intrusion we model stress changes because of both secular deformation and the intrusion and find that both would increase the coulomb failure stress on possible sse slip surfaces by roughly the same amount these results in concert with the observation that none of the previous sses at kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation suggest that both extrinsic intrusion triggering and intrinsic secular fault creep fault processes can lead to sses

the relationship between earthquake swarms and magma transport kilauea volcano hawaii a detailed investigation of earthquake locations and focal mechanisms for swarms associated with intrusive events at kilauea volcano hawaii further illuminates the relationships among stress state faulting and magma transport we determine the earthquake locations and mechanisms using a three dimensional crustal model to improve their accuracy and consistency swarms in kilauea s upper east and southwest rift zones from the years 1980 through 1982 provide clear evidence for the propagation and or dilation of dikes focal mechanisms are predominantly strike slip and the faulting and inferred dike orientations can be interpreted quite consistently in terms of the model ofhill 1977 stresses induced by the summit magma reservoir system strongly control faulting and magma transport in the rift zones close to the summit

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

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

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

stress field interactions between overlapping shield volcanoes borehole breakout evidence from the island of hawai i usa acknowledgments this pta2 borehole investigation was funded by the international continental scientific drilling program icdp and by vmapp volcanic margin petroleum prospectivity project vbpr dougalearth tgs in collaboration with the humu ula groundwater research project d a j and s p are partly funded through a norwegian research council centres of excellence project project number 223272 ceed we thank marco groh for the logging operations we thank two anonymous reviewers for the comments and suggestions we are particularly grateful to the associate editor mike poland for his valuable comments and his critical review that greatly improved the manuscript

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

detailed fault structure of the 2000 western tottori japan earthquake sequence we investigate the faulting process of the aftershock region of the 2000 western tottori earthquake m w 6 6 by combining aftershock hypocenters and moment tensor solutions aftershock locations were precisely determined by the double difference method using p and s phase arrival data of the japan meteorological agency unified catalog by combining the relocated hypocenters and moment tensor solutions of aftershocks by broadband waveform inversion of freesia f net we successfully resolved very detailed fault structures activated by the mainshock the estimated fault model resolves 15 individual fault segments that are consistent with both aftershock distribution and focal mechanism solutions rupture in the mainshock was principally confined to the three fault elements in the southern half of the zone which is also where the earliest aftershocks concentrate with time the northern part of the zone becomes activated which is also reflected in the postseismic deformation field from the stress tensor analysis of aftershock focal mechanisms we found a rather uniform stress field in the aftershock region although fault strikes were scattered the maximum stress direction is n107 e which is consistent with the tectonic stress field in this region in the northern part of the fault where no slip occurred during the mainshock but postseismic slip was observed the maximum stress direction of n130 e was possible as an alternative solution of stress tensor inversion