waveform classification of volcanic low frequency earthquake swarms and its implication at soufriere hills volcano montserrat low frequency volcanic earthquakes are indicators of magma transport and activity within shallow conduit systems at a number of volcanoes these events exhibit a high degree of waveform similarity providing a criterion for classification using cross correlation techniques to quantify the degree of similarity we develop a method to sort events into families containing comparable waveforms events within a family have been triggered within one small source volume from which the seismic wave has then travelled along an identical path to the receiver this method was applied to a series of 16 low frequency earthquake swarms well correlated with cyclic deformation recorded by tiltmeters at soufriere hills volcano montserrat in june 1997 nine waveform groups were identified containing more than 45 events each the families are repeated across swarms with only small changes in waveform indicating that the seismic source location is stable with time the low frequency seismic swarms begin prior to the point at which inflation starts to decelerate suggesting that the seismicity indicates or even initiates a depressurisation process a major dome collapse occurred within the time window considered removing the top 100 m of the dome this event caused activity within some families to pause for several cycles before reappearing this shows that the collapse did not permanently disrupt the source mechanism or the path of the seismic waves

waveforms and spectra of preshocks and aftershocks of the 1979 imperial valley california earthquake evidence for fault heterogeneity we have compared digitally recorded waveforms of m l 2 0 2 8 earthquakes that occurred in two small areas along the imperial fault before and after it broke in the ml 6 6 imperial valley earthquake on october 15 1979 eight preshocks 1977 1979 from a 4 by 1 km area centered 4 km se of the mainshock epicenter have strikingly similar waveforms over the entire record length 30 s with an average peak cross correlation between seismograms of 0 74 the seismograms are well correlated at frequencies up to at least 4 hz this implies similar source mechanisms and hypocenters within of the 4 hz wavelengths i e 200 400 m five aftershocks from the same area show an average peak cross correlation between seismograms of only 0 23 any associated changes in mechanism must be small because they are not reflected in the first motion data analysis of frequency content of these events using bandpass filtering techniques showed no systematic temporal changes in spectral shape ten preshocks and 24 aftershocks from a 1 by 2 km source area centered along the fault 16 km nw of the mainshock epicenter were also studied first motion data suggest that all of the aftershocks and a swarm of six preshocks on december 7 9 1978 were associated with the main fault but that four earlier preshocks were not the six preshocks on december 7 9 1978 were tightly clustered as evidenced by the strong similarity of the waveforms most peak cross correlations 0 6 during this swarm the 8 to 16 hz spectral amplitude increased relative to the 1 to 2 hz spectral amplitude over the whole record length by about a factor of 3 suggesting a systematic increase in stress drop groups of like events are also present among the aftershocks in this data set the average peak correlation for pairs of aftershocks 0 43 is almost the same as that for pairs of preshocks 0 45 if all 10 preshocks are included however several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations 0 6 the highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes

mount hood earthquake activity volcanic or tectonic origins on 29 june 2002 a m d 4 5 earthquake occurred 4 6 km south of mount hood oregon more than 200 small aftershocks m d 3 8 occurred between 29 june and 15 august 2002 by which time seismicity returned to background levels we analyze well constrained earthquakes from the summer 2002 sequence near mount hood recorded by the pacific northwest seismic network pnsn and well constrained small earthquakes m d 3 5 recorded by the pnsn between 1986 and 2002 we apply waveform cross correlation to selected events establish a one dimensional p wave velocity model and relocate the entire catalog by using the double difference algorithm of waldhauser and ellsworth 2000 we find that earthquakes before 2002 occur in four distinct clusters a a well defined linear feature striking n15e about 5 km south of the summit b a linear feature trending northwest southeast a few kilometers south of group a c a few tight clusters located about 9 km south southwest of the summit and d a small cluster beneath the summit earthquakes in group d do not appear to lie on a fault and may relate to volcanic activity however almost all earthquakes in the 2002 swarm occur in group a south of the summit at depths similar to earthquakes from previous swarms first motion fault plane solutions from the mainshock and largest aftershocks have normal mechanisms and solutions from well constrained aftershocks have normal to oblique normal mechanisms this is consistent with the apparent strike of the feature south of mount hood s summit and is similar to focal mechanisms of older earthquakes in group a b and p values for the summer 2002 earthquakes resemble a tectonic mainshock aftershock sequence we interpret these earthquakes as the northernmost example of basin and range seismicity yet recorded and conclude that the mount hood earthquakes do not yet suggest potential for volcanic unrest

improving local earthquake locations using the l1 norm and waveform cross correlation application to the whittier narrows california aftershock sequence experiments with different earthquake location methods applied to aftershocks of the october 1 1987 whittier narrows earthquake in california ml 5 9 suggest that local event locations can be greatly improved through the use of the l1 norm station corrections and waveform cross correlation the whittier narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the southern california seismic network scsn a telemetered network of several hundred short period seismographs scsn travel time picks and waveforms obtained through the southern california earthquake center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock using a smoothed version of the standard southern california velocity model and the existing travel time picks improved location accuracy is obtained through use of the l1 norm rather than the conventional least squares l2 norm approach presumably due to the more robust response of the former to outliers in the data a large additional improvement results from the use of station terms to account for three dimensional velocity structure outside of the event cluster to achieve greater location accuracy waveforms for these events are resampled and low pass filtered and the p and s wave cross correlation functions are computed at each station for every event pair for those events with similar waveforms differential times may be obtained from the cross correlation functions these times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked locations obtained from the adjusted picks show a further improvement in accuracy location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick improvements in location accuracy are indicated by the reduced scatter in the residuals smaller estimated location errors and the increased tendency of the locations to cluster along well defined fault planes median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location although the relative locations within localized clusters of similar events are better constrained seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a ml 5 3 aftershock these fault planes appear to cross and activity began on the secondary fault plane prior to the large aftershock

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

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

ability of a global three dimensional model to locate regional events 1 we assess the ability of a global three dimensional 3d seismic model of the crust and upper mantle cub2 0 to locate seismic events using regional travel time data alone epicentral distances 20 assessments are based on a ground truth gt database comprising nearly 1000 uncommonly well located events which occur in 23 event clusters across eurasia and north africa groomed p and pn arrival time data observed at regional distances from individual events and 1000 empirical phase path anomalies produced from event clusters evidence is presented that supports the conversion of the shear velocities vs in cub2 0 to compressional velocities vp in the upper mantle using the thermoelastic properties of an assumed mantle composition rather than traditional empirical scaling relationships two principal results are presented first this 3d vp model fits the empirical p and pn phase path anomalies very well in both pattern and absolute level second location assessments demonstrate that intrinsic regional location accuracy is 5 km using the 3d vp model and 10 km using the 1d model ak135 these findings highlight the importance of gt databases in assessing regional travel time models and establish that a global 3d vs model together with vp inferred from it provides a solid basis on which to build improvements in regional location capabilities

magma system recharge of mount st helens from precise relative hypocenter location of microearthquakes 1 four hundred forty seven well recorded earthquakes at mount st helens during the late 1990s are relocated with high precision using a combination of a cross correlation technique and then a procedure to obtain a one dimensional velocity structure solution together with station corrections the resulting high resolution pattern of the spatial temporal evolution of the earthquake activity along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5 5 10 km connected to the surface by a thin vertical conduit moreover the distribution of the events below 5 5 km yielded a much clearer view of the structures that generate the seismicity at that depth some of the deeper events occur along a sequence of faults under the influence of a radial stress field however a large number of the events occur on a nne ssw striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault

stability of coda wave attenuation during the loma prieta california earthquake sequence the loma prieta california earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the october 1989 mainshock this affords an unprecedented opportunity to detect changes in seismic wave propagation in the earth s crust associated with a major earthquake in this study we use pairs of nearly identical earthquakes doublets to search for temporal changes of coda attenuation in the vicinity of the loma prieta earthquake we analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic coseismic and postseismic intervals and measure the change in coda q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 hz this method provides an estimate of changes in coda q that is insensitive to other factors that influence coda amplitudes our observations place an upper bound of about 5 on preseismic coseismic and postseismic changes of coda q in the epicentral region of the loma prieta earthquake even at this low level the changes are neither spatially coherent nor correlated between adjacent frequency bands the only hint of a signal is in the preseismic data where there is a possible precursory increase in coda q of approximately 5 in the two years before the mainshock the stability of coda q throughout the loma prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda q for other earthquakes

the rise collapse and compaction of mt mantap from the 3 september 2017 north korean nuclear test surveillance of clandestine nuclear tests relies on a global seismic network but the potential of spaceborne monitoring has been underexploited we used satellite radar imagery to determine the complete surface displacement field of up to 3 5 meters of divergent horizontal motion with 0 5 meters of subsidence associated with north korea s largest underground nuclear test combining insight from geodetic and seismological remote sensing we found that the aftermath of the initial explosive deformation involved subsidence associated with subsurface collapse and aseismic compaction of the damaged rocks of the test site the explosive yield from the nuclear detonation with best fitting source parameters for 450 meter depth was 191 kilotonnes of tnt equivalent our results demonstrate the capability of spaceborne remote sensing to help characterize large underground nuclear tests

local crustal extension at mount st helens washington seismicity and the orientation of fault planes from focal mechanisms indicate that mount st helens is located at a dextral offset along the st helens seismic zone shz earthquake swarms occurring in this offset are related to volcanic eruptions because motion on the shz is in a right lateral strike slip sense this dextral offset creates extension within a volume of the crust between the offset fault segments this offset geometry is similar to that of geothermal areas along the san andreas fault system we apply a model derived from these geothermal areas to mount st helens and find that the major differences between mount st helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments l to the seismogenic depth h at mount st helens this ratio is 1 whereas in the geothermal areas the ratio is 1 we propose that when l h 1 as at mount st helens the regional minimum principal stress does not completely dominate the small volume under extension and as a consequence the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l h 1 late quaternary volcanic vents near mount st helens strike northeast similar to the strike of a set of pre quaternary faults and intrusive rocks that are mapped north of the volcano in addition the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress we infer that the spatial position of mount st helens is controlled by the junction of the right stepping offset of the shz with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust the sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right lateral shear measured from geodetic lines around mount st helens during june and july 1980

finite element analyses of an in situ wall propped at formation level the use of formation level props to support an in situ retaining wall can result in a structural system which is stiff and remote from rotational failure however the interaction between the wail the soil and a continuous prop slab cannot be analysed using simple techniques finite element analyses have been carried out to investigate some of the factors affecting the behaviour of an in situ wall propped at formation level retaining 9 m of stiff overconsolidated boulder clay this paper describes the results of a parametric study in which the effects of soil wall prop stiffness and the pre excavation earth pressure coefficient were investigated it is found that because the wall is very stiff computed deformations are governed by the assumed stiffness of the soil rather than the flexura rigidity of the wall bending moments in the wall are influenced significantly by the assumed pre excavation lateral earth pressures and to a lesser extent by the nature of the structural connection between the wa

using repeating volcano tectonic earthquakes to track post eruptive activity in the conduit system at redoubt volcano alaska in the year following the end of the 2009 eruption of redoubt volcano alaska four signifi cant swarms of low frequency lowmagnitude m l 0 1 earthquakes occurred at shallow depths beneath the summit because swarms of low frequency lf earthquakes preceded eruptions in 1989 and 2009 the posteruption swarms caused considerable concern and prompted the alaska volcano observatory to raise the monitoring levels on three occasions none of these swarms led to eruptions however and most observers including us initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers new observations reveal that the shallow lf swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level where the magma storage region is thought to be these mid crustal volcano tectonic vt type earthquakes were identical to earthquakes recorded during the 2009 redoubt eruption more than 6 months earlier focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike slip nature of regional faulting based on these observations we conclude that they are generated through processes occurring within the magma storage region the concurrence of the repeating vt earthquakes with the shallow lf swarms indicates that the shallow lf earthquakes were also magmatically driven our results emphasize that even brief episodes of low amplitude earthquake activity such as the lf swarms observed at redoubt following the 2009 eruption can be indicative of magmatic activity perhaps more signifi cant however is the demonstration that the conduit system at redoubt remained active intact and capable of transporting heat and fl uids to the surface months after the eruption was considered over

multiplets their behavior and utility at dacitic and andesitic volcanic centers 1 multiplets or groups of earthquakes with similar waveforms are commonly observed at volcanoes particularly those exhibiting unrest using triggered seismic data from the 1980 1986 mount st helens msh eruption we have constructed a catalog of multiplet occurrence our analysis reveals that the occurrence of multiplets is related at least in part to the viscosity of the magma we also constructed catalogs of multiplet occurrence using continuous seismic data from the 2004 eruption at msh and 2007 eruption at bezymianny volcano russia prior to explosions at msh in 2004 and bezymianny in 2007 the multiplet proportion of total seismicity mpts declined while the average amplitudes and standard deviations of the average amplitude increased the life spans of multiplets time between the first and last event were also shorter prior to explosions than during passive lava extrusion dome forming eruptions that include a partially solidified plug like msh 1983 1986 and 2004 2008 often possess multiplets with longer life spans and mpts values exceeding 50 conceptually the relatively unstable environment prior to explosions is characterized by large and variable stress gradients brought about by rapidly changing overpressures within the conduit we infer that such complex stress fields affect the number of concurrent families mpts average amplitude and standard deviation of the amplitude of the multiplets we also argue that multiplet detection may be an important new monitoring tool for determining the timing of explosions and in forecasting the type of eruption

infrasound from volcanic rockfalls proximal infrasound arrays can robustly track rapidly moving gravity driven mass wasting which occurs commonly at erupting volcanoes this study reports on detection localization and quantification of frequent small rockfalls and infrequent pyroclastic density currents descending the southeast flanks of santiaguito s active caliente dome in january of 2014 such activities are identified as moving sources which descend several hundred meters at bulk flow speeds of up to 10 m s which is considerably slower than the descent velocity of individual blocks infrasound rockfall signal character is readily distinguishable from explosion infrasound which is manifested by a relatively fixed location source with lower frequency content in contrast the rockfalls of santiaguito possess higher frequencies dominated by 7 5 to 20 hz energy during our observation periods typical rockfall signals occurred 10 times per hour and lasted tens of seconds or more array beamforming permitted detection of rockfall transients with amplitudes of only a few tens of millipascals that would be impossible to distinguish from noise using a single sensor conjoint time synchronized video is used to corroborate location and to characterize various gravity driven events

earthquake swarm activity revealed from high resolution relative hypocenters clustering of microearthquakes abstract waveform similarity in an earthquake swarm enables precise arrival time differences of p and s waves through cross spectral methods to be obtained a numerical simulation guarantees the time resolution of the order of 1 ms for the time differences this precise arrival time difference is used to determine relative hypocenters of microearthquakes occurring in a swarm source region with a spatial resolution of the order of 10 m this precise hypocenter determination millisecond hypocenter determination is applied to the data using temporary seismic observation in the ashio region japan where swarm activities are especially high relative hypocenters of 153 events are determined with high spatial resolution it is revealed that the swarm activities consist of several clusters of events a cluster is a group of events concentrated within a small area in a short time interval the cluster activities in one swarm never overlap one another sharing some part of the swarm source region of about 100 m 1 km in length one cluster forms a fault like planar area with a length of about 40 150 m the event cluster swarm activity would be a manifestation of the hierarchy structure of earthquake sources

evidence of possible induced seismicity in the wabash valley seismic zone from improved microearthquake locations to evaluate the spatial structure of seismicity in the wabash valley of southern indiana and illinois we analyzed data from a temporary seismic network that included 10 three component short period seismometers and a 10 component dense phased array we produced the first comprehensive catalog of local earth quakes recorded during the 211 day deployment from november 1995 through june 1996 the results are dominated by a cluster of 534 microearthquakes with magni tudes ranging from 0 6 to 1 8 near the town of new harmony indiana that were detected and recorded only by the phased array the remarkable similarity of the cluster events in terms of waveform magnitude and temporal distribution suggests a tightly spaced grouping from a single seismic source we relocated the earthquakes using improved relative arrival times of p and s waves computed by time domain cross correlation of the vertical beam traces and by complex cross correlation of the horizontal beam traces respectively additional constraints on absolute locations were applied using a graphical method of arrival time difference analysis for six earthquakes recorded by two network stations outside the phased array the resulting locations define a tight spatial grouping about a zone of post paleozoic faulting just west of the indiana illinois border in white county illinois average source depths from phased array location methods and waveform modeling with synthetic seis mograms indicate in contrast to larger events in the region that the earthquakes occurred at depths less than 4 km within the sedimentary section of the illinois basin we propose that these earthquakes are artificially induced events likely related to water injection for the purpose of secondary recovery of petroleum in the illinois basin the primary evidence for this includes 1 tight spatial clustering of earth quakes 2 unusually shallow earthquake depths 3 good spatial correlation of the relocated hypocenters to existing wells and oil fields and 4 highly repetitive events with only a small range in magnitude

seismic slip on a low angle normal fault in the gulf of corinth evidence from high resolution cluster analysis of microearthquakes the gulf of corinth in western greece is one of the most active extensional zones in the aegean region it is still an open question whether extension can be actively accommodated on low angle faulting or if those faults as seen in geological records have been rotated whilst numerous fault plane solutions obtained from a dense temporary network deployed in the western part of the gulf in july august of 1991 showed one of the nodal planes as a subhorizomal plane slip on the high angle conjugate plane is equally probable from the focal mechanism data rigo et al 1996 since part of the activity occurred in spatial clusters with similar focal mechanisms we used a high resolution cluster analysis to determine the most likely active plane exploiting the waveform similarity of these events relative onset times of p and s waves could be determined at subsample accuracy less than 0 01 s the cluster analyzed here contains 12 evems among these 8 have a well con strained normal faulting fault plane solution with a shal low 12 20 o north dipping plane and a steeply south dipping plane a master evem relocation shows that the relocated 12 hypocenter cemroids are aligned along the low angle plane showing clear evidence for active low angle normal faulting

location of long period events below kilauea volcano using seismic amplitudes and accurate relative relocation 1 we present methods for improving the location of long period lp events deep and shallow recorded below kilauea volcano by the permanent seismic network lp events might be of particular interest to understanding eruptive processes as their source mechanism is assumed to directly involve fluid transport however it is usually difficult or impossible to locate their source using traditional arrival time methods because of emergent wave arrivals at kilauea similar lp waveform signatures suggest the existence of lp multiplets the waveform similarity suggests spatially close sources while catalog solutions using arrival time estimates are widely scattered beneath kilauea s summit caldera in order to improve estimates of absolute lp location we use the distribution of seismic amplitudes corrected for station site effects the decay of the amplitude as a function of hypocentral distance is used for inferring lp location in a second stage we use the similarity of the events to calculate their relative positions the analysis of the entire lp seismicity recorded between january 1997 and december 1999 suggests that a very large part of the lp event population both deep and shallow is generated by a small number of compact sources deep events are systematically composed of a weak high frequency onset followed by a low frequency wave train aligning the low frequency wave trains does not lead to aligning the onsets indicating the two parts of the signal are dissociated this observation favors an interpretation in terms of triggering and resonance of a magmatic conduit instead of defining fault planes the precise relocation of similar lp events based on the alignment of the high energy low frequency wave trains defines limited size volumes

using earthquake doublets to study inner core rotation and seismicity catalog precision 1 we report our search for doublets in the south sandwich islands ssi region among 906 earthquakes from january 1982 to december 1998 event pairs with source separation less than 40 km which was calculated using pde locations were checked using vertical component short period and broadband digital records cross correlation coefficients of p or pkp waveforms were computed for similarity comparison whole waveforms of those event pairs with correlation coefficients larger than 0 8 were checked by eye by this process 17 doublets were found some of which form multiplets however only one doublet event pair 14 august 1995 and 28 march 1987 has clear pkp df phases on both records at college alaska col the pkp df travel time change measured by cross correlation is a decrease by 0 15 s with standard deviation 0 02 s over a time period of about 8 years this result strongly supports differential rotation of the earth s inner core relative to the mantle and the crust the estimated rotation rate ranges from 0 4 to 1 per year eastward the special case of a locked inner core can then be ruled out we also checked the precision of pde and isc locations using the 17 doublets results show that both catalogs have relatively high location precision standard deviations in latitude longitude and depth are less than 4 km 6 km and 10 km respectively supporting prior claims of detection of inner core rotation song and richards 1996

estimation of an optimum velocity model in the calabro peloritan mountains assessment of the variance of model parameters and variability of earthquake locations summary accurate earthquake locations are of primary importance when studying the seismicity of a given area they allow important inferences on the ongoing seismo tectonics both for standard as well as for earthquake relative location techniques the velocity parameters are kept fixed to a priori values that are assumed to be correct and the observed traveltime residuals are minimized by adjusting the hypocentral parameters however the use of an unsuitable velocity model can introduce systematic errors in the hypocentre location precise hypocentre locations and error estimate therefore require the simultaneous solution of both velocity and hypocentral parameters we perform a simultaneous inversion of both the velocity structure and the hypocentre location in ne sicily and sw calabria italy since the density of the network is not sufficient for the identification of the 3 d structure with a resolution of interest here we restrict ourselves to a 1 d inversion using the well known code velest a main goal of the paper is the analysis of the stability of the inverted model parameters for this purpose we carry out a series of tests concerning the initial guesses of the velocity structure and locations used in the inversion we further assess the uncertainties which originate from the finiteness of the available data set carrying out resampling experiments from these tests we conclude that the data catalogue is sufficient to constrain the inversion we note that the uncertainties of the inverted velocities increases with depth on the other hand the inverted velocity structure depends decisively on the initial guess as they tend to maintain the overall shape of the starting model in order to obtain an improved starting model we derive a guess for the probable depth of the moho for this purpose we exploit considerations of the depth distribution of earthquake foci and of the shear strength of rock depending on its rheological behaviour at depth in a second step we derive a smooth starting model and repeated the inversion strong discontinuities tend to attract hypocentre locations which may introduce biases to the earthquake location using the smooth starting model we obtaine again a rather smooth model as final solution which gives the best traveltime residuals among all models discussed in this paper this poses severe questions as to the significance of velocity discontinuities inferred from rather vague a priori information besides this the use of those smooth models widely avoids the problems of hypocentre locations being affected by sudden velocity jumps an effect which can be extremely disturbing in relative location procedures the differences of the velocity structure obtained with different starting models is larger than those encountered during the bootstrap test this underscores the importance of the choice of the initial guess fortunately the effects of the uncertainties discussed here on the final locations turned out as limited that is less than 1 km for the horizontal coordinates and less than 2 km for the depth

an analysis of the earthquake time series at mount st helens washington in the framework of recent volcanic activity abstract the analysis of the earthquake time distribution at mount st helens reveals a good correlation between the physical state of the volcano and statistical parameters of earthquake sequence there are three main seismic phases in the whole 1980 1986 period the first one precedes the main eruption of may 18 1980 it begins with a sudden increase of the seismicity level in late march and continues with an utsu 1961 type decay of the seismic occurrence rate characterized by a small value of the decay coefficient the second phase lasts from the cataclysmic eruption on may 18 1980 until the continuous dome building episode in 1983 and is characterized by a very slow exponential increase of the background level of seismicity the third phase covers the remaining part of the sample and is characterized by a stationary earthquake clustering process episodically interrupted by peaks of activity related to eruptions the trends in seismic occurrence rate within each phase as well as the statistical parameter variations at each transition are analyzed and discussed in the framework of volcanic activity this leads to the conclusion that statistical techniques may give a significant contribution in understanding changes in volcanic processes such as those at mount st helens

monitoring a temporal change of seismic velocity in a volcano application to the 1992 eruption of mt merapi indonesia multiplets i e events with similar waveforms are selected from shallow earthquakes recorded on merapi volcano indonesia before the eruption of february 2nd 1992 two multiplet families are found with their sources close to the summit their seismograms are analyzed using the moving window cross spectrum technique which measures the precise time delay between seismic phases in the entire seismogram for both families of multiplets a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption coda waves are becoming progressively faster up to 1 2 per cent as the time interval to the eruption shortens this observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano the increase in velocity started in may 1991 and was observed until september 1991 4 months before the eruption this velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks

dome growth behavior at soufriere hills volcano montserrat revealed by relocation of volcanic event swarms 1995 1996 abstract we have relocated a subset of events from the digital waveform catalogue of 17 000 volcanic microearthquakes recorded between july 1995 and february 1996 at soufriere hills volcano shv montserrat using a cross correlation based phase repicking technique with a joint location method hypocenters were estimated for 3914 earthquakes having five or more corrected p wave picks the seismic source region collapsed to a volume of 1 km 3 from an initial 100 km 3 relocated events represent 36 swarms each containing nearly identical waveforms having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less each swarm occurred over a span of several hours to a few days triggered data appear to miss between 65 and 98 of the events that occur within these swarms based on review of helicorder records visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma although dome observations are too sparse to make a direct comparison for this time period the limited depth range over which dome growth related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit governed by magma supply rate overpressure buildup and physical properties of the magma and conduit geometry seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

observations of hybrid seismic events at soufriere hills volcano montserrat july 1995 to september 1996 swarms of small repetitive events with similar waveforms and magnitudes are often observed during the emplacement of lava domes over 300 000 such events were recorded in association with the emplacement of the lava dome at soufriere hills volcano montserrat from august 1995 through august 1996 these events originated 2 3 km deep they exhibited energy ranging over 1 5 4 5 hz and were broader band than typical long period events we term the events hybrid between long period and volcano tectonic the events were more impulsive and broader band prior to compared with during and after periods of inferred increased magma flux rate individual swarms contained up to 10 000 events often exhibiting very similar magnitudes and waveforms throughout the swarm swarms lasted hours to weeks during which inter event intervals generally increased then decreased often several times long duration swarms began about every two months starting in late september 1995 we speculate that the events were produced as the magma column degassed into adjacent cracks

repeating earthquakes in the yellowstone volcanic field implications for rupture dynamics ground deformation and migration in earthquake swarms abstract we evaluated properties of yellowstone earthquake swarms employing waveform multiplet analysis thirty seven percent of the earthquakes in the yellowstone caldera occur in multiplets and generally intensify in areas undergoing crustal subsidence outside the caldera in the hedgen lake tectonic area the clustering rate is higher up to 75 the yellowstone seismicity follows a succession of two phases of earthquake sequence the first phase is defined between swarms it is characterized by a decay of clustering rate and by foreshock aftershock sequences the second phase is confined to swarms and is characterized by an increase in clustering rate and dominant aftershock sequences this phase reflects tectonic swarms that occur on short segments of optimally oriented faults for example the largest recorded swarm in yellowstone occurred in autumn 1985 on the northwest side of the yellowstone plateau which was initiated as a tectonic source sequence fitting experimental dependence of fluid injection with intrusion migration suggests that the 1985 swarm involved after 10 days hydrothermal fluids flowing outward from the caldera the 2008 2009 yellowstone lake swarm exhibited a high migration rate of 1 km day a decrease in clustering rate without a main shock and appears to be associated with magma injection of 1 to 5 m 3 s in a succession of migrating magma fronts that incrementally solidify and fracture at its brittle edges the 2010 madison plateau earthquake swarm on the west side of the caldera initiated as a tectonic sequence but the expansion of the swarm front was associated with hydrothermal fluid migration

effect of support characteristics on the earth pressure in a jointed rock mass this study examines the magnitude and distribution of earth pressures against a support system in a jointed rock mass according to the support characteristics strut stiffness and spacing differe

evidence for a nimbostratus uncinus in a convectively generated mixed phase stratiform cloud shield abstractthe structure of a melting layer associated with a mesoconvective system is examined using a combination of in situ aircraft measurements and a unique doppler radar operated by the u s navy that has a range resolution as fine as 0 5 m interest in this case was motivated by ground based all sky camera images that captured the transient development of midlevel billow cloud structures within a precipitating trailing stratiform cloud shield associated with a passing deep convective system a sequence of high fidelity time height radar measurements taken of this storm system reveal that the movement of the billow cloud structure over the radar site corresponded with abrupt transitions in the observed low level precipitation structure of particular note is an observed transition from stratiform to more periodic and vertically slanted rain shaft structures that both radar and aircraft measurements indicate have the same temporal periodicity determined to arise visually between successive billow cloud

installation effects of a bored pile wall in overconsolidated clay in situ embedded retaining walls are used to form basements and cuttings in urban areas throughout the world particularly where space is restricted it is generally accepted that the process of installing such a wall may influence the stress state of the soil and hence the post construction displacements wall bending moments and prop loads however wall installation is a complex three dimensional problem and its effects are not well understood this paper describes a study carried out on a section of contiguous bored pile retaining wall in an overconsolidated clay in kent england using instrumentation to monitor total horizontal stresses and pore water pressures before during and after wall installation reductions in horizontal stress were recorded during wall installation bringing the ratio of horizontal to vertical effective stress k from an initial value of about 1 to about 0 8 following wall installation there was no further change in horizontal stress over an additional period of 10 month

fault reactivation by stress pattern reorganization in the hyblean foreland domain of se sicily italy and seismotectonic implications abstract between the october 2011 and the july 2012 several seismic swarms occurred in the hyblean foreland domain of se sicily italy along the cavagrande canyon one of the most impressive fluvial incisions of sicily despite the low magnitude of the events main shock with m 3 7 they represent the biggest strain release of the hyblean area over the last 10 years a careful waveform analysis of the earthquakes revealed that most of them form a family of multiplets these findings allow us to reconstruct the attitude of the accountable fault plane by interpolating their high precision 3d location parameters into a gis platform a detailed morpho structural analysis performed at the ideal updip projection of the modeled plane showed that during the middle late pleistocene the epicentral area has been deformed by a belt of extensional faults a segment of which matches well with the computer generated surface despite the field evidence computed focal solutions support contrasting strike slip kinematics on the same fault plane clearly indicating a dextral shearing on this pre existing normal fault the seismic swarms nucleated on a small rupture area along a 10 km long nw se trending fault segment that could be able to generate m 6 earthquakes following our analysis and looking at seismicity distribution in the se portion of hyblean area we assess that a stress pattern reorganization occurred all over the hyblean foreland between the late pleistocene and present day change in the trajectory of the max stress axes from vertical to horizontal seems to have involved a pre existing large scale fault configuration with considerable seismotectonic implications

a double difference method for high resolution acoustic tracking using a deep water vertical array ray tracing is typically used to estimate the depth and range of an acoustic source in refractive deep water environments by exploiting multipath information on a vertical array however mismatched array inclination and uncertain environmental features can produce imprecise trajectories when ray tracing sequences of individual acoustic events double difference methods have previously been developed to determine fine scale relative locations of earthquakes along a fault waldhauser and ellsworth 2000 bull seismolog soc am 90 1353 1368 this technique translates differences in travel times between nearby seismic events recorded at multiple widely separated stations into precise relative displacements here this method for acoustic multipath measurements on a single vertical array of hydrophones is reformulated changes over time in both the elevation angles and the relative arrival times of the multipath are converted into relative changes in source position this approach is tested on data

waveform analysis of two preshock main shock aftershock sequences in utah we use waveform cross correlation to study clustering of small earthquakes that occurred from 1981 to 1983 within 20 km of 1 an ml 4 3 main shock near salt lake city utah on 8 october 1983 and 2 an ml 4 0 main shock near richfield utah on 24 may 1982 no clusters of earthquakes were observed prior to the salt lake city event during the time period studied however a cluster of four nearly identical preshocks within 80 m of each other possibly representing the failure of a critical asperity occurred 5 km northwest of the richfield event during a 4 hour period 10 months before the main shock we found no similar clusters among the other preshocks within the richfield study area all of which were located more than 10 km from the main shock the aftershocks in both study regions during the first 17 days after the respective main shocks occupied areas smaller than the rupture areas estimated for the main shocks from initial pulse width measurements both aftershock sequences showed a spatial migration of the initial events that could only be discerned from the waveform data one possible explanation for this migration could be propagating stress changes caused by the occurrence of the aftershocks themselves or by postseismic creep

relocation of microearthquake swarms in the peloritani mountains implications on the interpretation of seismotectonic patterns in ne sicily italy summary the peloritani mountains in northeastern sicily make part of the appennine maghrebian chain which forms the highly deformed southern margin of the european continent in this zone the nw se striking aeolian tindari giardini system atg separates two areas of seismicity to the west of the atg fault system seismic activity below a depth of 40 km is essentially absent to the east of this fault system we note a significant presence of intermediate depth and deep events which mark the subduction zone in the calabrian arc between 1994 and 2003 300 microearthquakes could be located with fair accuracy near the atg fault system their depths range from less than 5 to 40 km with greater depths occurring to the east of the atg we examined the resolution capability of the standard location by applying a grid search location for typical events the distribution of the residuals shows trends recognizable in the standard locations that are in part an artefact of a non ideal conditions of the standard locations such as the station configuration the use of an unsuitable velocity model and inconsistencies of arrival time pickings by applying relative location techniques the double difference method and a master event technique we were able to reduce the scatter of hypocentres significantly we focused in particular on earthquake families with similar waveforms and estimated the geometrical extent of hypocentre clusters compared to the standard location the dispersion of hypocentres decreased by an amount of over 90 per cent and the volume occupied by the foci contracted to 1 per cent the significance of these geometries was tested with monte carlo experiments and by interchanging the master events the cluster geometries are consistent with the dislocation patterns as inferred from fault plane solutions but do not show a simple relation to the atg the role of fluid flow of plutonic origin may be invoked as a possible trigger mechanism this hypothesis is supported by the presence of geothermal anomalies in the vicinity as well as by an upward migration trend in foci

emergence and disappearance of interplate repeating earthquakes following the 2011 m9 0 tohoku oki earthquake slip behavior transition between seismic and aseismic depending on the loading rate we investigated spatiotemporal change in the interplate seismic activity following the 2011 tohoku oki earthquake m9 0 in the region where interseismic interplate coupling was relatively weak and large postseismic slip was observed we classified earthquakes by their focal mechanisms to identify the interplate events and conducted hypocenter relocation to examine the detailed spatiotemporal distribution of interplate earthquakes in the mostly creeping area the results show that many interplate earthquakes including m 6 events emerged immediately after the tohoku oki earthquake in areas where very few interplate earthquakes had been observed in the 88 previous years the emergent earthquakes include repeating sequences and the extremely long quiescence of small to moderate earthquakes before the tohoku oki earthquake suggests that the source areas for the post m9 events slipped aseismically during the quiescence the repeaters magnitudes decayed over time following the tohoku oki earthquake and some sequences disappeared within a year the emergence of interplate earthquakes suggests that areas where aseismic slip had been dominant before the tohoku oki earthquake started to cause seismic slip after the earthquake probably due to the increased loading rate from the afterslip the magnitude decrease and disappearance of repeaters can be interpreted as shrinkage in seismic areas around the repeaters sources as the loading rate decreased due to the afterslip decay over time these observations suggest that changes in the loading rate can cause slip behavior transition between seismic and aseismic this indicates that such loading rate dependent slip behavior plays an important role in the spatiotemporal distribution of earthquakes in interplate seismogenic zones

rates of similar micro earthquakes comparison between campi flegrei southern italy and other volcanic areas abstract the precise relocation of micro earthquakes using waveform cross correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas to be applied and to provide meaningful results the technique requires however the presence of sufficient amounts of similar events we examined about 870 earthquakes recorded at the campi flegrei cf caldera by a temporary digital network during the end of the 1982 1984 bradiseismic crisis the classification of these earthquakes indicates a surprising low amount of similar events especially according to the clustered aspect of the seismicity the relocation technique allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera to determine up to which point this low similarity rate is unusual we examined the rates for several data sets coming from various volcanic areas hawaii mount pinatubo philippines and piton de la fournaise la reunion island the results show for all areas percentages of similar events significantly higher than for cf and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics at cf most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures the low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress

joint relative location of earthquakes without a pre defined velocity model an example from a peculiar seismic cluster on katla volcano s south flank iceland relative location methods are commonly used to precisely locate earthquake clusters consisting of similar waveforms repeating waveforms are often recorded at volcanoes where however the crust s

seismological studies at parkfield iii microearthquake clusters in the study of fault zone dynamics abstract more than half of the microearthquakes that occur near parkfield california when located with high resolution methods are seen to define some 80 small clusters of 2 to 12 similar events each cluster occupies a patch typically 100 to 200 m in length within the fault zone cluster members have nearly identical waveforms correlation coefficient of 0 9 or greater to frequencies of 50 to 100 hz as recorded by the borehole installed seismographic network the clusters are distributed throughout the fault zone around the presumed nucleation region in the locked section to the se and in the creeping part of the nw of the previous m 6 hypocenter they are also found in the salinian block several kilometers sw of the fault zone the total area occupied by all of the clusters constitutes only a small fraction 1 to 2 of the fault zone approaching failure at parkfield such clusters provide insight into the dynamics of the failure process through their spatial temporal characteristics and their mechanisms they also serve as highly repetitive sources distributed throughout the fault zone suitable for monitoring the nucleation zone for possible precursory changes in physical properties that affect wave propagation in this article we demonstrate these applications with a detailed analysis of selected clusters

reconstruction of seismogenetic structures by multiplet analysis an example of western liguria italy abstract this work investigates the spatial distribution of three seismic swarms that occurred in western liguria nw italy in 1990 and 1993 the largest events recorded on july and december 1993 exceeded magnitude 4 0 the routine locations of hypocenters were not able to depict the geometry of the active zone due to the limitations of the regional seismic network also the adoption of different location techniques forward probability estimation relative locations did not furnish any particular information on the real spatial distribution of these very closely spaced events the analysis of the recorded signals demonstrated the strong coherency among seismograms and suggested the use of doublet multiplet techniques in order to evaluate the relative position of events for every swarm the relative locations obtained with estimated errors of the order of a few meters depict clearly the geometry of the very limited area involved in this seismic activity of the order of 1 km 2 all the analyzed events lie within a few meters on a plane that is in agreement with one of the planes of the focal mechanism of the mainshocks obtained by first motion polarities a strike slip rupture with a compressive component on planes with strike around 80 and dip ranging from 56 to 74 is in agreement with all the available information moreover using some similar events belonging to the different periods of activity bridge events the relative position of the three groups of earthquakes was also determined showing that the more energetic activity of 1993 reactivated a shallower area contiguous to the sector mobilized in 1990 and characterized by a slightly different dip

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

numerical analysis of a multipropped excavation in stiff clay this paper presents the procedures and the results of numerical analyses of a multipropped deep excavation at lion yard cambridge using the nonlinear brick model the computed results are compare

source characterization and fault plane determination for mblg 1 2 to 4 4 earthquakes in the charlevoix seismic zone quebec canada two earthquake doublets and two multiplets recorded by the charlevoix telemetered network cltn in the charlevoix seismic zone csz of southern quebec canada have been analyzed using an empirical green s function egf method to derive the relative source time functions rstf s of seven master events with mblg 1 2 to 4 4 we identified the doublets and multiplets using a waveform cross correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations three component s waveforms recorded by the high dynamic range 126 db instrumentation of the cltn were used to extract the rstf s the rstf s reveal that six of the seven events are simple with single source pulses having durations of 0 05 to 0 2 sec another earthquake 920310 0545 m 3 3 appears to be a double event with two episodes of rupturing azimuthal variations of the rstf pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes m 1 2 to 4 4 the azimuthal variations in the rstf pulse amplitudes were used to estimate the rupture directions and rupture velocities lower bound estimates of the rupture velocity range from 0 5 to 0 7 vs estimates of the rupture direction were combined with p wave focal mechanisms for the four largest events m 3 3 to 4 4 to identify the fault plane for these earthquakes source parameters were measured for the rstf s of the master events including seismic moments of 3 5 1018 to 5 3 1021 dyne cm fault radii of 100 to 330 m and static stress drops of 2 to 90 bars the fault radii and stress drop estimates for m 3 events agree well with estimates obtained by other researchers for m 3 to 4 5 earthquakes in the csz we also observed apparent scaling between the stress drop and the earthquake size which has been reported in other studies of stress drop in northeastern north america

precise relocations and stress change calculations for the upland earthquake sequence in southern california we relocate earthquakes that occurred near the 1988 ml 4 7 and the 1990 ml 5 5 upland california earthquakes to map the fault geometry of the poorly defined san jose fault and to test the static stress triggering hypothesis for this sequence we adopt the l1 norm waveform cross correlation method of shearer 1997 to obtain precise relocations for 1573 events between 1981 and 1997 in the upland area to limit computation time we only perform waveform cross correlation on 60 of the nearest neighbors of each relocated event our final relocations show two linear features the first is imaged by the locations of the initial month of aftershocks of the 1988 upland earthquake which delineate a fault with a dip angle of 45 between 7 and 9 km depth consistent with the mainshock focal mechanism the second linear feature is a plane dipping at about 74 from 2 to 9 km depth which is illuminated by both the 1988 and 1990 upland sequences in agreement with the inferred location of the san jose fault at depth however below 9 km the event locations become more diffuse giving rise to two different interpretations of the fate of the san jose fault at depth one possibility is that the fault shallows at depth consistent with our relocations but not with the focal mechanism of a ml 4 7 deep aftershock alternatively the fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake using these inferred fault geometries we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the hypothesis that more aftershocks occur where the change in static coulomb failure stress is positive on faults optimally oriented for failure this requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation we find that patterns of change in coulomb failure stress differ little between the different fault geometries all are nearly symmetric about the fault and so do not match the aftershock distribution in which most of the off fault events occur to one side of the fault plane

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

estimation of arrival times in intense seismic sequences using a master events methodology based on waveform similarity summary intense seismic sequences involve a large number of earthquakes densely clustered in space and time their detailed analysis is important for the geometry of the activated faults which contributes to studies of the seismotectonic characteristics of an area the sheer number of small events as well as their low energy content renders their processing problematic in this work we have developed and applied a methodology to automatically pick the arrival times of p and s waves using a correlation detector event detection is performed using the waveform recordings of a reference station located close to the epicentral area of an intense seismic sequence such as aftershocks or swarms cross correlation matrices are constructed followed by nearest neighbour clustering and the formation of multiplets a master event is chosen from each cluster and its arrival times are picked manually the automatic algorithm uses the p o rs wave of each master event as a correlation detector searches the waveforms of the other events of the same multiplet and imposes the corresponding arrival time when the bestfit is achieved the picks are characterized by observation weights which derive from the quality of thefit the type of the available waveform components and the consistency between multiple measurements the proposed methodology was applied to an important seismic sequence that occurred between 2010 january 18 and 26 near the city of efpalio greece this procedure has the potential to increase 10 fold the amount of information and provide sufficient detail for a subsequent analysis of the spatiotemporal distribution of a seismic series

comparison of measured and calculated temporary prop loads at canada water station the loads developed in tubular steel temporary props during the construction of the london underground jubilee line extension jle station at canada water were measured using vibrating wire strain gauges prop temperatures were monitored and their influence on the prop loads assessed wall movements were also measured by means of inclinometers in this paper the temperature normalized prop loads are compared with the results of modified limit equilibrium calculations prop loads and wall movements are compared with the results of a series of parametric finite element analyses carried out using the program crisp on the basis of the finite element analysis results the design assumptions giving the closest correlation between measured and calculated prop loads and wall movements are identified

rupture geometry from high precision relative hypocentre locations of microearthquake clusters summary in 1987 two microearthquake sequences with a duration of about a week each and consisting of 37 and 46 events respectively occurred in the upper crust below the jura mountains of northern switzerland the seismograms within each sequence exhibit a high degree of similarity indicating tight clustering of hypocentres and similar focal mechanisms using a cross correlation technique applied to the seismograms in the time domain together with a new least squares adjustment procedure it was possible to determine relative hypocentre locations of the earthquakes in each cluster with a precision of a few tens of metres the results show that hypocentres of events with the same focal mechanism lie on a plane which coincides exactly with one of the nodal planes of the fault plane solution and that consequently swarm like sequences of similar earthquakes are due to repeated slip on the same fault in one case slip occurred as right lateral motion on a steeply dipping plane striking in wnw ese direction whereas the other case corresponds to left lateral slip on two almost vertical planes striking roughly n s in 1988 an additional earthquake triplet occurred on a wnw ese oriented normal fault nearby all three mechanisms are consistent with a general nnw sse oriented direction of maximum crustal shortening and a corresponding wsw ene oriented extension repeated slip on the same fault is indicative of a large degree of heterogeneity and of short term temporal variability of both frictional resistance and stress distribution on the fault an explanation for the occurrence of such swarm like seismic activity in terms of barriers or asperities would require that shear stress on the unbroken patches increase from one event to the next since however the overall shear stress would be expected to decrease as a consequence of the stress released in each event a more plausible mechanism involves pore pressure fluctuations caused by fluids under suprahydrostatic pressures migrating upward through pre existing zones of weakness in the crust

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

evidence from a cluster of small earthquakes for a fault at 18 km depth beneath oak ridge southern california abstract a swarm of about 50 small earthquakes m 1 5 occurred for a month during 1989 beneath oak ridge southern california location accuracy using conventional analysis of arrival time picks is limited for these events by the weak emergent nature of arrivals on the available seismograms however waveform cross correlation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations the relocated events form a small cluster about 1 km across at a depth of 18 km and are aligned along a plane that dips 35 to the northwest estimated standard errors for the locations are generally less than 50 m the time evolution of the sequence shows a gradual migration of activity away from its initiation point three additional events occurred several months later these align along the same plane but are displaced about 500 m to the southeast from the main swarm reliable fault plane solutions are difficult to obtain for these events due to the small number of station records available the limited range of takeoff angles and the weak initial arrivals on many of the seismograms stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average p first motions analysis of these first motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity the seismicity alignment may represent the extension of the simi fault in which case some shallowing of the fault dip would be required to match the observed 35 dip at 18 km

high precision relocation of microearthquakes at mt etna 1991 1993 eruption onset a tool for better understanding the volcano seismicity abstract the beginning of the 1991 1993 large eruption at mt etna was marked by a seismic swarm and a sudden increase of volcanic tremor amplitude several earthquakes were recognized as clusters of shallow events with nearly identical waveforms multiplets we have analyzed these signals with a cross spectral technique that allows us to locate the relative position of the events within a multiplet obtaining a precision higher than with traditional routines the relocated shocks occurred some kilometers away from the propagating eruptive fracture east of both the summit craters and the modeled eruptive dykes they clustered in a relatively small volume elongated in the nnw sse direction the depth of which ranged from about 1100 to 2000 m below sea level these earthquakes are interpreted as the passive brittle response of the volcanic basement to the intrusion of the eruptive dyke rather than being directly linked to the magma transport

fracture mapping in the soultz sous forets geothermal field using microearthquake locations 1 in 2003 a massive hydraulic fracturing experiment was carried out at the european geothermal hot dry rock site at soultz sous forets france the 2 week injection of water generated a high level of microseismic activity triggering about 90 000 microearthquakes during and after the fluid injection of these 21 000 events detected at all stations were located individually with a grid search algorithm to characterize the extent of the seismic zones and ultimately of the fracture network the accuracy of these initial locations was around 70 m not sufficient to map detailed fracture patterns a precise relocation effort was undertaken to reduce the location uncertainties using three different techniques joint hypocenter determination jhd collapsing and multiplet analysis the collapsing method was added to the jhd results to further consolidate the hypocenters on the other hand a multiplet analysis was performed on the initial data set for identifying microearthquakes with similar waveforms and hence relocating relatively the correlated events the delays in traveltime were computed by wavelet analysis and the events were relocated using a grid search algorithm we found 7463 events whose seismograms correlated with a correlation coefficient of 0 8 or higher most of which were doublets multiplets are horizontal tube like structures with lengths from a hundred to several hundred meters striking along the direction of the maximum compressive horizontal stress we interpret these structures as the preferential paths where fluid flow is largely confined this hypothesis is reinforced by the good correlation in depth between the events and the fluid outflows observed in the well therefore we believe that these relocated events highlight the zones where the permeability of the reservoir is increased

precursors to a magnitude 4 8 earthquake in the virgin islands spatial clustering of small earthquakes anomalous focal mechanisms and earthquake doublets abstract waveforms of small earthquakes that preceded a magnitude 4 8 earthquake in the virgin islands were studied using digital recordings from a local seismic network during the 10 months prior to the main shock a spatial cluster of five events 1 4 m 2 5 occurred within 3 km of the impending shock variations in the p wave first motions at one station as well as p sv amplitude ratios indicate that the focal mechanisms of the events within the cluster changed 2 months before the main shock the focal mechanism of the later events and of two foreshocks that occurred the day of the main event differed from the typical focal mechanism observed for the area the main event and most of its aftershocks exhibited the normal set of first motions recorded for events in this region the cluster events and the foreshocks occurred in pairs each pair with very similar waveforms cross correlations of these waveforms reveal that the earthquakes in each pair are located within about 110 m of each other each pair can be interpreted as an earthquake doublet that is two similarly sized events with adjacent rupture zones that are related in time the occurrence of doublets is unusual for events in this region and may be indicative of high stress in the vicinity of the impending shock no significant differences were observed between the spectra of the cluster events the foreshocks or the aftershocks

seismic precursors to the mount st helens eruptions in 1981 and 1982 six categories of seismic events are recognized on the seismograms from stations in the vicinity of mount st helens two types of high frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers medium and low frequency earthquakes occur at shallow depths less than 3 kilometers within the volcano and increase in number and size before eruptions temporal changes in the energy release of the low frequency earthquakes have been used in predicting all the eruptions since october 1980 during and after eruptions two types of low frequency emergent surface events occur including rockfalls and steam or gas bursts from the lava dome

structures and evolution of the plumbing system of piton de la fournaise volcano inferred from clustering of 2007 eruptive cycle seismicity analysis of seismic activity associated with the eruptions of 2007 which led to the collapse of the dolomieu crater on april 5th reveals the link between the seismicity and the magma transfers at piton de la fournaise three eruptive phases occurred on february 18th march 30th and april 2nd 2007 at the summit 2 000 m and 600 m high on the south east flank respectively illustrating the three types of eruptions defined for the current piton de la fournaise activity we use cross correlation of seismic waveforms and clustering to improve the earthquake locations and determine the best constrained focal mechanisms with an average of 78 p phase polarities the pre eruptive seismicity of the february and march eruptions is composed of time extended clusters that also preceded other distal eruptions from 2000 to 2007 our analysis shows that the seismic swarm prior to the february eruption initiated the intrusion that led to the april 2007 eruption the seismicity preceding the dolomieu crater collapse consists of numerous but time limited clusters and specific seismic activity that accompanied the dolomieu crater collapse from april 1st to april 5th earthquakes with clvd mechanisms combined with normal faulting sources occurred between 0 8 and 0 km asl until the complete rupture of the shallow magma storage roof this collapse induced the propagation of a de pressurization front and triggered a migration of seismicity from 0 to 8 km along a very narrow path

deep earthquakes beneath mount st helens evidence for magmatic gas transport small magnitude earthquakes began beneath mount st helens 40 days before the eruption of 20 march 1982 unlike earlier preeruption seismicity for this volcano which had been limited to shallow events less than 3 kilometers many of these earthquakes were deep between 5 and 11 kilometers the location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 march eruption than prior to any of the earlier dome building eruptions the depth time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir

characterization of fracture systems using precise array locations of earthquake multiplets an example at deception island volcano antarctica 1 volcano tectonic earthquakes are common seismic events in active volcanic areas the stress produced by volcanic processes is released through fracturing of the shallow crust very often these earthquakes occur in multiplets with similar waveforms a fact which indicates common source characteristics in this work we introduce a method that uses array techniques to calculate precise relative locations of earthquake multiplets we use the relative slowness estimate method to determine accurately the apparent slownesses and propagation azimuths of the earthquakes relative to a selected master event we also obtain precise estimates of the s p delays this information is used to calculate precise relative locations by ray tracing in an earth model we applied this method to determine the characteristics of the fractures activated during the 1999 seismic series at the deception island volcano antarctica we selected a set of 17 earthquake multiplets initially located in a small 4 4 km region a few km ne of the array site we estimated precise locations for 14 of the clusters in most cases hypocenters were distributed in well defined planar geometries we found the best fitting planes which we interpreted as fractures in the medium for two clusters the method spatially separated the earthquakes into two subgroups thus we obtained two planes for each of these clusters resulting in a total of 16 fracture planes this is the first time that the orientations of fracture planes related to a seismic series have been obtained using a seismic array we performed several tests to check various aspects in relation to the stability of the method and concluded that the results were robust the dip angles indicate that the planes are mostly subvertical while the strike angles clearly show a nw se trend for most of the planes and a few planes with ne sw trends the geometry and position of these planes suggest that the 1999 seismic series was influenced by regional tectonics although the origin of the destabilization of the system may be related to the reactivation of a shallow magma chamber

differential analysis of coda q using similar microearthquakes in seismic gaps part 2 application to seismograms recorded by the parkfield high resolution seismic network abstract nearly two thirds of the microseismicity occurring at parkfield california consists of repeating clusters of closely located events exhibiting waveform cross correlation values exceeding 0 95 this observation coupled with a high probability for an m 6 earthquake within the next decade working group on california earthquake probabilities 1988 make this segment of the san andreas fault system a prime location to search for temporal variations in coda q associated with crustal changes that may occur during the earthquake cycle the coda comparison analysis method described in part 1 of this study aster et al 1996 is applied to data from four borehole stations of the parkfield high resolution seismic network hrsn for the period january 1987 through february 1994 we examine 21 of the nearly 300 repeating clusters observed to date the restriction of the data set to only the most highly similar sources is observed to be critical small differences in the source process even for event pairs with centroid locations within approximately 20 m significantly increase the spurious noise level in the parameter estimates absolute 68 confidence bound estimates obtained on the repeatability of coda q for all microearthquake pairs are 10 for frequencies below 20 hz tighter constraints are obtainable by reconstructing the temporal history of coda q from the first difference measurements between microearthquake pairs these functional reconstructions show that the mean value of coda q has not varied by more than approximately 5 from 3 to 30 hz at the 1 level during the study period however other observables such as an increased level of microseismicity suggest that a preparatory stage for the next parkfield earthquake has begun indicating that coda q is so far showing no sign of sensitivity to preparatory processes at parkfield

magma intrusion near volcan tancitaro evidence from seismic analysis abstract between may and june 2006 an earthquake swarm occurred near volcan tancitaro in mexico which was recorded by a temporary seismic deployment known as the mars network we located 1000 events from this seismic swarm previous earthquake swarms in the area were reported in the years 1997 1999 and 2000 we relocate and analyze the evolution and properties of the 2006 earthquake swarm employing a waveform cross correlation based phase repicking technique hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0 75 90 of the earthquakes provide at least sixteen phase picks we used the single event location code hypo71 and the p wave velocity model used by the jalisco seismic and accelerometer network to improve hypocenters based on the correlation adjusted phase arrival times we relocated 121 earthquakes which show clearly two clusters between 9 10 km and 3 4 km depth respectively the average location error estimates are

the may 20 mw 6 1 and 29 mw 6 0 2012 emilia po plain northern italy earthquakes new seismotectonic implications from subsurface geology and high quality hypocenter location abstract this study presents new geological and seismological data that are used to assess the seismic hazard of a sector of the po plain northern italy a large alluvial basin hit by two strong earthquakes on may 20 mw 6 1 and may 29 mw 6 0 2012 the proposed interpretation is based on high quality relocation of 5369 earthquakes emilia sequence and a dense grid of seismic profiles and exploration wells the analyzed seismicity was recorded by 44 seismic stations and initially used to calibrate new one dimensional and three dimensional local vp and vs velocity models for the area considering these new models the initial sparse hypocenters were then relocated in absolute mode and adjusted using the double difference relative location algorithm these data define a seismicity that is elongated in the w nw to e se directions the aftershocks of the may 20 mainshock appear to be distributed on a rupture surface that dips 45 ssw and the surface projection indicates an area 10 km wide and 23 km long the aftershocks of the may 29 mainshock followed a steep rupture surface that is well constrained within the investigated volume whereby the surface projection of the blind source indicates an area 6 km wide and 33 km long multichannel seismic profiles highlight the presence of relevant lateral variations in the structural style of the ferrara folds that developed during the pliocene and pleistocene there is also evidence of a mesozoic extensional fault system in the ferrara arc with faults that in places have been seismically reactivated these geological and seismological observations suggest that the 2012 emilia earthquakes were related to ruptures along blind fault surfaces that are not part of the pliocene pleistocene structural system but are instead related to a deeper system that is itself closely related to re activation of a mesozoic extensional fault system

moment tensor analysis of near field broadband waveforms observed at aso volcano japan abstract locations and focal mechanisms of long period volcanic events observed at aso volcano japan are determined by waveform inversion near field broadband three component seismograms of four to seven stations are simultaneously inverted in the time domain in order to find the six component seismic moment tensor a linear inversion is performed at each point of a 3d grid located under the volcano in order to constrain the centroid position and the focal mechanism of the source the complete displacements including near field waves in a homogeneous half space for a general point source are taken into account inversions of 43 long period tremors and six phreatic eruptions exhibit mainly an isotropic mechanism with a minor deviatoric component that may originate from a north south trending crack these events are all located in a small region at about 1 3 km depth beneath the active crater the high accuracy both of locations a few hundred meters and of focal mechanisms given by the inversion is partly attributed to the rapid variation of the amplitude of the static displacement seen in the long period part of the near field seismograms