magnitude scales for very local earthquakes application for deception island volcano antarctica abstract different magnitude scales are calculated for a set of volcano tectonic earthquakes recorded in deception island volcano antarctica the data set includes earthquakes recorded during an intense seismic series that occurred in january february 1999 with hypocentral distances that range between 0 5 and 15 km this data set is enlarged to include some regional earthquakes with hypocentral distances up to 200 km the local magnitude scale m l fixed at a hypocentral distance of 17 km is used as the reference for the other magnitude scales studied in the present work m l is determined on a standard wood anderson simulated trace assuming a gain of 2080 maximum peak to peak amplitudes are measured on the vertical components of a short period sensor the m w scale is calculated in the vertical component both for p and s waves the attenuation correction of the ground motion displacement spectra is introduced using data from coda waves studied in the area the comparison between m l values and m w estimations indicates severe discrepancies between both values a magnitude duration scale is calibrated from the comparison between coda durations of the recorded events and their assigned local magnitude scales in order to investigate the causes of the discrepancy between the m l and m w values we analyze two possible error sources a wrong coda q value or the effects of the near surface attenuation that initially are not taken into account in the correction of the ground displacement spectra the analysis reveals that the main cause of this discrepancy is the effect of the near surface attenuation the near surface attenuation is also the cause of the determination of an anomalous spectral decay slope after the corner frequency and the determination of this corner frequency value this near surface attenuation represented by is estimated over the data set obtaining an average value of 0 025 with this value the m w scale is recalculated using an automatic algorithm the new m w values are more consistent with the m l values obtaining a relationship of m w 0 78 m l 0 02

roller bearing tectonic evolution of the juan fernandez microplate a combined gloria sidescan and hydrosweep multi narrow beam sonar survey shows striking evidence for fan shaped spreading systems bounding pseudofaults and compression ridges associated with 90 of clockwise rotation of the juan fernandez microplate in the past 4 myr a quantitative analysis of this evolution suggests that the microplate has rotated as a rigid roller bearing for most of its history driven at its edges by shear between the surrounding major plates the remarkable similarity between the juan fernandez and easter microplates despite their different tectonic settings suggests that this is a general model for microplate tectonics

evolution of the juan fernandez microplate during the last three million years during the spring of 1983 the r v thomas washington surveyed an area located north of the antarctic nazca pacific triple junction at 35 s magnetic and seabeam bathymetric data collected during the survey confirmed the existence of the juan fernandez microplate this paper presents an analysis of the magnetic anomaly data the western boundary of the microplate is a fast spreading center which has existed since 2 ma and where the accretion rate has been about 14 5 cm yr for the last 0 7 ma the eastern boundary of the microplate is characterized by a slow spreading center which separates the juan fernandez and nazca plates the accretion rate has been about 7 0 cm yr between 0 7 and 0 4 ma and about 1 6 cm yr for the last 0 4 ma the two spreading centers are connected in the north and south by transform faults between the juan fernandez and rapanui microplates the east pacific rise is well defined between 30 and 32 s in this region the axis displays a record accretion of about 17 2 cm yr south of the juan fernandez microplate one magnetic profile oceano 7008 indicates that the opening rate is about 12 0 cm yr on the antarctic pacific ridge the birth of the microplate is dated at about 2 ma when the western boundary started to accrete the evolution of the microplate corresponds to a transfer of accretion from the eastern boundary to the western axis this is revealed by the net decrease of the opening rate from 7 0 cm yr to 1 6 cm yr observed at the eastern ridge where a small jump occurred at 0 4 ma

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

seismic quantification of the explosions that destroyed the dome of volcan de colima mexico in july august 2003 in july august 2003 the andesitic lava dome at volcan de colima mexico was destroyed by a sequence of explosions that replaced the 2 106 m3 dome with a crater 200 m across and 30 m deep the two strongest explosions occurred on july 17 and august 28 the initial low frequency impulses that they produced which were recorded on broadband seismic records allowed an estimation of the counter forces of the initiating process as being equal to 0 3 1011 n and 1 1011 n for the july and august events respectively the seismic characteristics follow the nishimura hamaguchi scaling law for volcanic explosions reflecting self similarity in the processes initiating explosive events the results also show that counter forces can discriminate between the sizes of explosive eruptions that are assigned the same magnitude by conventional methods of classification such as the volcanic explosivity index the increasing use of broadband seismometers may therefore provide the basis for using counter forces to determine the magnitude of explosive eruptions

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

magnitude scale for lp events a quantification scheme for volcanic quakes summary the peculiar source characteristics of long period seismic events time persistency of the source low frequency peaks in the source spectrum absence of high frequency radiation prevent the formation of a definite high frequency coda in the seismograms in contrast this is well formed in volcano tectonic quakes for this reason the widely used duration magnitude scale that is based on the proportionality between the energy and the coda duration cannot be used for long period estimation in observatory practice the long period magnitude is sometimes estimated using the same duration magnitude scale leading to confusing results in this report we show a new method to estimate the magnitude of long period events that generally occur for volcanoes with some application examples from data for mt etna italy colima volcano mexico and campi flegrei italy

a seismically constrained mass discharge rate for the initiation of the may 18 1980 mount st helens eruption we calculate the vertical mass discharge rate from mount st helens for the first few minutes of the may 18 1980 cataclysmic eruption using a new method based on seismic constraints the observed seismic waves indicate that the seismic source is a series of single forces we model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet the momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model only two parameters are necessary for the calculation mach number and sound velocity the calculated mass ejected in the first 100 s is 1 6 10 11 4 6 10 11 kg since the total blast deposit is 3 2 10 11 4 1 10 11 kg one possible interpretation is that the directed blast had a significant 30 vertical component

source mechanism of vulcanian degassing at popocatepetl volcano mexico determined from waveform inversions of very long period signals 1 the source mechanism of very long period vlp signals accompanying volcanic degassing bursts at popocatepetl is analyzed in the 15 70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium the waveforms of two eruptions 23 april and 23 may 2000 representative of mild vulcanian activity are well reproduced by our inversion which takes into account volcano topography the source centroid is positioned 1500 m below the western perimeter of the summit crater and the modeled source is composed of a shallow dipping crack sill with easterly dip of 10 intersecting a steeply dipping crack northeast striking dike dipping 83 northwest whose surface extension bisects the vent both cracks undergo a similar sequence of inflation deflation and reinflation reflecting a cycle of pressurization depressurization and repressurization within a time interval of 3 5 min the largest moment release occurs in the sill showing a maximum volume change of 500 1000 m3 pressure drop of 3 5 mpa and amplitude of recovered pressure equal to 1 2 times the amplitude of the pressure drop in contrast the maximum volume change in the dike is less 200 300 m3 with a corresponding pressure drop of 1 2 mpa and pressure recovery equal to the pressure drop accompanying these volumetric sources are single force components with magnitudes of 108 n consistent with melt advection in response to pressure transients the source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds such source behavior is consistent with the opening of a pathway for escape of pent up gases from slow pressurization of the sill driven by magma crystallization the opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles assuming a penny shaped crack at ambient pressure of 40 mpa the observed pressure and volume variations can be modeled with the following attributes crack radius 100 m crack aperture 5 m bubble number density 1010 1012 m 3 initial bubble radius 10 6 m final bubble radius 10 5 m and net decrease of gas concentration in the melt 0 01 wt

seismological constraints on the position of the fragmentation surfaces in the volcano conduit abstract the durations of the pre explosion low frequency impulses of the broadband seismic records of 286 vulcanian explosions of volcan de colima and 15 vulcanian explosions of popocatepetl volcano mexico were analyzed these pre explosion impulses are thought to be produced during the upward movement of fragmented magma in the volcano conduit before an explosion it is shown that three different magma fragmentation surfaces acted simultaneously during the 2004 september november episode at volcan de colima associated with the andesitic block lava extrusion the optimal apparent depth for the fragmentation surface that generated the largest with the energy 10 11 j 2004 2005 explosions at volcan de colima and the 1998 2000 popocatepetl explosions is constrained by pre explosion low frequency impulse durations between 3 and 7 s

the role of fluvial and glacial erosion in landscape evolution the ben ohau range new zealand a morphometric comparison of valleys has been made for the ben ohau range in the central southern alps of new zealand the range is undergoing rapid tectonic transport and uplift the humid north of the range is a glacial trough and arete landscape with a temperate glacial climate the dry south has rounded divides and plateau remnants dissected by fluvial valleys assuming that space time substitution allows today s spatial valley form transition to represent evolutionary stages in valley development the tectonic history allows time constraints to be placed on the rate of transition to an alpine glacial landscape morphometric change has been quantified using hypsometric curves and distance elevation plots of cirque and valley floor altitudes ancestral fluvial valleys have less concave long profiles but are stepped at altitude owing to the presence of high level cirques and remnant plateau surfaces and possess a low proportion of land area at low elevation increasing glacial influence is manifest as smoother more deeply concave long profiles and u shaped cross profiles associated with a higher proportion of the land area at lower elevation the full morphological transition has involved up to 2 4 km of vertical denudation over the 4 ma lifetime of the mountain range of which 80 per cent would have occurred by preglacial fluvial erosion combining the trajectory of tectonic transport with reconstructed glaciation limits and climatic history it is indicated that about 200 ka of temperate glacial erosion produces recognizable trough and areete topography mean and modal relief increase where glacial activity is confined to cirques but decrease when trough incision by ice becomes established as a dominant process in the landscape 1997 by john wiley sons ltd

inversion for p and s wave attenuation structure long valley caldera california p and s wave amplitude data from digital three component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three dimensional attenuation structure in the upper crust of long valley caldera california differential attenuation q 1 values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios the spectral ratios are inverted for q 1 using the lsqr algorithm zones of high positive p wave q 1 greater p wave attenuation are observed at a depth of 4 5 km beneath the east flank of mammoth mountain and at 6 8 km beneath the southwestern edge of the resurgent dome a region of positive s wave q 1 is seen at 7 8 km beneath the southern edge of the resurgent dome whereas at 6 7 km the s wave results are near average the region of positive s wave q 1 at 7 8 km beneath the resurgent dome may indicate the roof of a magmatic system the positive p wave q 1 at 4 5 km beneath mammoth mountain and 6 8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water these zones may represent some of the source regions for the long valley geothermal system

crustal imaging of western michoacan and the jalisco block mexico from ambient seismic noise abstract detailed crustal imaging of western michoacan and the jalisco block is obtained from ambient noise tomography results show a deep and well delineated volcanic system below the colima volcano complex rooting up to 22 km depth with a shallow magmatic chamber constrained to the first 7 km a shallow low velocity system to the south of the chapala rift and west of the michoacan guanajuato volcanic field merges underneath the colima rift with the colima volcano system at about 20 km depth honoring the geometry of the trans mexican volcanic belt for depths greater than 30 km low velocity features become parallel to the slab strike right beneath the mascota ayutla and tapalpa volcanic fields suggesting the presence of the mantle wedge above the rivera plate all mentioned low velocity bodies are spatially correlated with the superficial volcanic activity suggesting their magmatic origin so that the shallower these bodies the younger are the associated volcanic deposits along the coast different depths of the uppermost layer of the rivera and the cocos plates suggest that the latter plate subducts with an angle 9 steeper than the former

analysis of long period seismic waves excited by the may 18 1980 eruption of mount st helens a terrestrial monopole long period 100 to 260 s love and rayleigh waves excited by the eruption of mount st helens on may 18 1980 and recorded by ida sro and asro stations were analyzed to determine the mechanism of the eruption the amplitude radiation patterns of both rayleigh and love waves are two lobed with a nodal direction in e5 s for rayleigh waves and in n5 e for love waves these radiation patterns preclude any double couple mechanism the radiation pattern the initial phase the relatively large amplitude ratio of love to rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher mode rayleigh waves suggest that the source is represented by an almost horizontal less than 15 from the horizontal single force pointed toward s5 w the surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process although the details of the source time history could not be determined a smooth bell shaped time function f o s t 1 2 f o l cos t for 0 t 2 and f o s t 0 for t 2 with 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum the peak value of the force f 0 is about 10 18 dynes the tailing end of the source time function could not be resolved and some overshoot may be added the magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption two distinct events about 110 s apart can be identified on body wave and short period surface wave records the first event may correspond to the earthquake which triggered the landslide and the lateral blast the second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

very long period signals associated with vulcanian explosions at popocatepetl volcano mexico very long period vlp seismic signals associated with large summit eruptions of popocatepetl volcano in the last four years are investigated using data from a broadband seismometer 0 04 120 s deployed on the north flank of the volcano at a radial distance of 5 km from the active crater the vlp signals associated with individual eruptions share similar waveforms discrete vlp signals accompanying long period lp events also share similar signatures and have dominant periods that are nearly identical to those observed in the vlp waveforms of explosions the vlp particle motions for eruption onsets consistently point to a source located a few km beneath the crater the vlp ground displacement response to each explosion is marked by a compression followed by a dilatation and terminating with another compression suggesting a sequence of pressurization depressurization repressurization of the conduit the repetitive nature of the waveforms points to a non destructive source process which has remained active in the magmatic system of popocatepetl at least since april 1997

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

a simplified equation of state for the density of silicate hydrous magmas an application to the popocatepetl buoyancy driven dome growth process abstract we present a simplified equation of state eos for calculating the density of hydrous siliceous magmas in a range of pressures temperatures and dissolved water content of interest to study several magma conduit processes a simpler linear form and a more precise non linear yet simple form of the equation both valid in the range 0 to 0 25 gpa 800 to 1000 k and 0 to 7 wt h2o are proposed for the dacitic and andesitic popocatepetl magma melt compositions we apply these equations to explain some aspects of the dome forming effusive activity that has been observed at popocatepetl volcano since 1996 this model poses a density reduction dependent mainly on the volatile contents of the magma filling the volcano conduit system as the driving buoyancy force causing magma extrusions and dome growth

attenuation tomography based on strong motion data case study of central honshu region japan three dimensional frequency dependent s wave quality factor q f value for the central honshu region of japan has been determined in this paper using an algorithm based on inversion of strong motion data the method of inversion for determination of three dimensional attenuation coefficients is proposed by hashida and shimazaki j phys earth 32 299 316 1984 and has been used and modified by joshi curr sci 90 581 585 2006 nat hazards 43 129 146 2007 and joshiet al j seismol 14 247 272 2010 twenty one earthquakes digitally recorded on strong motion stations of kik net network have been used in this work the magnitude of these earthquake ranges from 3 1 to 4 2 and depth ranging from 5 to 20 km respectively the borehole data having high signal to noise ratio and minimum site effect is used in the present work the attenuation structure is determined by dividing the entire area into twenty five three dimensional blocks of uniform thickness having different frequency dependent shear wave quality factor shear wave quality factor values have been determined at frequencies of 2 5 7 0 and 10 hz from record in a rectangular grid defined by 35 4 n to 36 4 n and 137 2 e to 138 2 e the obtained attenuation structure is compared with the available geological features in the region and comparison shows that the obtained structure is capable of resolving important tectonic features present in the area the proposed attenuation structure is compared with the probabilistic seismic hazard map of the region and shows that it bears some remarkable similarity in the patterns seen in seismic hazard map

size of popocatepetl volcano explosions 1997 2001 from waveform inversion several volcanic explosions have been recorded since april 1997 at broadband seismic stations located around the popocatepetl volcano mexico we have inverted waveforms of ten of these explosions to estimate the following source parameters depth duration magnitude and direction of the single force f the crustal structure used in generating green s function at nearest stations is derived from the inversion of teleseismic receiver functions at the broadband permanent station ppig located 5 km north of the volcano this inversion reveals a low velocity zone at 8 km beneath the summit with high poisson ratio possibly related to the magma chamber we find that f scales with the duration of the source time function as f tau based on this relationship we determine an impulse magnitude scale mk this magnitude is tied to the mount saint helens initial explosive phase of may 18 1980 whose magnitude is estimated as 4 6 mk of the ten popocatepetl explosions ranges between 1 8 and 3 2 finally we also propose an equivalent formula for rapid estimation of magnitude of future popocatepetl explosions which requires filtered amplitudes at ppig

the methodology of quantification of volcanic explosions from broad band seismic signals and its application to the 2004 2005 explosions at volcan de colima mexico summary a methodology is proposed for the quantification of volcanic explosions based on three parameters derived from broad band seismic signals the counter force of the eruption f the power of the explosion p and the duration of the upward movement of the gas slug in the conduit to the free surface of magma d this methodology was applied to the 2004 2005 sequence of

source mechanism of vulcanian eruption at tungurahua volcano ecuador derived from seismic moment tensor inversions source mechanisms of explosive volcanic eruptions are critical for understanding magmatic plumbing systems and magma transport tungurahua is a large andesitic stratovolcano where seismoacoustic data has been recorded over several years in may 2010 an energetic eruption cycle began with a midsize vulcanian explosion followed by swarms of explosive eruptions the five station seismoacoustic network recorded significant seismic and infrasonic signals from the explosions source mechanisms of 50 explosion earthquakes associated with vulcanian explosions during this eruptive period are investigated here the source centroid locations and geometries of explosive signals in the 10 2 s band were determined by full waveform moment tensor inversion the observed waveforms are well explained by a combination of volumetric moment tensor components and a single vertical downward force component the source centroids are positioned about 1 5 km below and about 320 m north of the active crater eigenvalue and eigenvector analysis indicates that the source geometries can be described by a subhorizontal thin ellipsoid representing a sill like magma accumulation resultant source time histories show a repetitive sequence of inflation and deflation from event to event indicating identical source processes frequently occurred over the period the inflation deflation in the deep source region may be the result of crack opening volatile bubble growth at depth opens a pathway for gases to escape and triggers shallow explosions at the summit crater the associated downward single force is interpreted as an exchange of linear momentum between the source and the surrounding region during the escaping gas flow

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

quantification of the 1998 1999 explosion sequence at popocatepetl volcano mexico abstract the sequence of large vulcanian explosions occurring at the andesitic popocatepetl volcano mexico during november 1998 to april 1999 was studied the size of 26 largest explosions was estimated from broadband seismic records at the distance of 4 km from the crater the sequence began with the largest explosion e 2 6 10 12 j occurring on 25 november at 08 05 and following largest daily explosions were characterized by gradual decrease in the energy the energy of 20 large e 10 11 j explosions was distributed as student s t distribution with a geometrical mean log e 11 81 j

analysis of source characteristics of experimental gas burst and fragmentation explosions generated by rapid decompression of volcanic rocks gas burst and fragmentation explosions induced by rapid decompression of volcanic rocks in a high pressure autoclave have been analyzed we performed experiments from 4 to 20 mpa on 1 loose pyroclasts and 2 pumice from recent eruptions of popocatepetl volcano our aim is to characterize the source mechanism distinguishing the physical processes that operate in the conduit which determine the dynamics of explosive volcanoes for this aim we have analyzed experimentally the parameters that indicate the state and behavior of the conduit i e decompression time fragmentation threshold and speed forces and the partitioning of energy using microseismic monitoring the decompression time curves and the sample rheological properties the initial available potential energy in the system and its partitioning into different types of energies are correlated in space and time with specific stages of the explosive phenomenon such correlations taken together with the energy distribution enable the distinction of the individual physical processes involved and their causal sequence and relationships our observations suggest that in volcanic conduits with regular explosive activity a source mechanism may operate whereby a causal sequence of processes results in a system that undergoes both rapid and stable transitions such behavior may persist over long periods of time

characteristics and source modeling of broadband seismic signals associated with the hydrothermal system at satsuma iwojima volcano japan abstract the broadband seismic data observed at satsuma iwojima volcanic island in southwestern japan show a wide variety of interesting features one of them is the periodic and frequency dependent amplitude modulation of volcanic tremors another interesting feature is the occurrence of very long period vlp seismic pulses synchronized with the tremor amplitude modulation both the tremor amplitude modulation and the vlp pulses have fairly regular intervals of 46 50 min suggesting a spatially fixed non destructive repetitive source we performed waveform inversions to reveal the source mechanism of the vlp pulses assuming a point source embedded in a medium that realistically represented the topography of the island numerical tests showed that the green s functions used in the inversion were significantly affected by the topography the observed vlp pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater which had a seismic moment of 3 10 10 nm both the characteristic amplitude modulation and the vlp pulses are explained systematically by a model consisting of a water filled crack and a network of gas paths the water filled crack was heated above the boiling temperature of the water in it a sudden break of the unstable superheated state caused sudden vaporization and a pressure increase which was observed as the vlp pulse the network of gas paths consisted of swollen portions and constricted portions which worked as tremor generators when vapor from the water filled crack passed through

seismological analysis of conduit dynamics in fragmentation experiments we simulate gas burst and volcanic explosions under controlled laboratory conditions inducing fragmentation of volcanic rocks by rapid depressurization a series of experiments were performed in a shock tube apparatus at room temperature and a pressure range of 4 to 20 mpa using argon ar gas and particles or pumice samples of different porosities the instrumentation of this system with high precision piezoelectric sensors enabled us to capture elastic waves and to recognize their characteristic signatures by relating these signals to physical processes in the wave field we have been able to characterize the conduit mechanism and the source dynamics we compare and discuss conspicuous features of the waveforms and frequency spectra of these experimental signals with those of volcanic origin despite the fact that these signals are different in amplitude resulting from different scale conditions our observations indicate that the physical processes that occur during simulated explosions and those that occur during volcanic eruptions yield comparable signatures in their respective records the effects of the source receiver configuration and resonance also have significant implications all this suggests that the physical processes e g pressurization and depressurization of a system involve a system response that causes similar distinctive effects independent of the system size reflecting its intrinsic dynamics these similarities imply that powerful constraints on the source mechanisms of volcanic seismicity can emerge from seismic investigations of experimental simulations of volumetric sources such constraints may yield significant advances in the understanding of volcanic conduit dynamics and in the interpretation of seismic unrest at volcanic centers

evidence of low q below popocatepetl volcano and its implication to seismic hazard in mexico city seismograms recorded in mexico city reveal that the amplitudes of seismic waves whose wavepaths pass below popocatepetl presently an volcano before reaching the city are diminished by a factor of about one third at frequencies greater than 1 hz as compared to those which do not cross the volcano the high attenuation of seismic waves below the volcano may be attributed to the presence of magma and partial melting of rocks q of shear waves below the volcano is roughly estimated as 60 a consequence of the large attenuation of high frequency seismic waves is a decrease in the seismic hazard to low rise buildings in mexico city from intraplate earthquakes whose wavepaths cross the volcano

the three dimensional structure beneath the popocatepetl volcano mexico based on local earthquake seismic tomography abstract this paper presents a new seismic velocity model p and s wave velocities and vp vs ratio beneath the popocatepetl volcano to a depth of 4 km below sea level bsl this model is based on the tomographic inversion of p and s wave arrival time data from earthquakes in the region of the volcano these data were recorded by the 15 stations of a temporary seismic network that was deployed on the volcano in 1999 and 2000 the recording period was during a relatively quiet period between two strong eruptions which occurred before and after the experiment this period is characterized by low levels of volcano related seismicity most of the recorded events occurred across an area much larger than the network in this study we conducted several synthetic tests which validate the use of the out of network events to improve the resolution of the tomographic inversion beneath the stations in the resulting model we see that the main volcanic edifice is associated with high velocities that have a mushroom shape and that these high velocities are most prominent in the p wave model this feature may indicate the presence of overpressured solidified igneous rocks which comprise the edifice of popocatepetl below the summit of the volcano we observe a prominent high vp vs anomaly which reaches a value of 1 9 this anomaly probably indicates the existence of cracks and pores filled with melts and fluids and it may represent a fracture zone that serves as a conduit feeding the volcano this model characterizes the interior structure of the popocatepetl volcano prior to the strong september 2000 eruption which occurred a few months after the termination of the experiment

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

quantification of volcanic explosions from tilt records volcan de colima mexico abstract during the 2004 2005 explosive sequence at volcan de colima mexico impulses with a duration of several minutes were recorded by a two component tiltmeter installed at a distance of 1 6 km from the crater for 46 vulcanian explosions the correlation between the amplitudes of tilt impulses a a rad 2 a tang 2 and the energy of explosions e estimated from the broadband seismic records is significant at the 99 confidence level the regression equation can be used to estimate the energy of explosions at volcan de colima from the tilt records

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

seismic tomography at popocatepetl volcano mexico abstract we performed the first 3d seismic tomography of the popocatepetl volcano mexico from the inversion of the p wave arrival times of nearly 1500 earthquakes recorded by up to 11 seismic stations of the permanent cenapred ssn network we used the bayesian tomographic algorithm developed by monteiller et al 2005 to perform this p wave travel time tomography and carefully choose optimal regularization hyper parameters sensitivity tests show that the inversion is correctly constrained under the volcano from 1 to 4 km depth b s l at these depths the optimal tomographic model mainly shows low velocity volumes surrounded by higher velocity volumes lateral heterogeneity is strong the variation of the p wave velocity may reach up to 20 of the average value at a given depth low velocity volumes correlate well with results from former geological and geophysical studies

scaling relationship for vulcanian explosions derived from broadband seismic signals 1 a proposed conceptual model of vulcanian explosions describes the volcanic explosion process as consisting of two stages the movement of the fragmented magma to the surface and a subsequent explosion and is constrained by four source parameters inferred from broadband seismic records the counter force of the eruption f acting in the conduit before an explosion and governing the fragmented magma movement to the surface the time d1 of the movement of fragmented magma in the conduit before an explosion the duration of the explosion in the conduit d2 and the energy e of the explosion our paper discusses within the framework of the proposed conceptual model the source scaling relationship inferred from the parameters of 135 vulcanian explosions occurring at two mexican andesitic volcanoes volcan de colima and popocatepetl during 1996 2006 two groups of explosions are discriminated small e 1011 j and large e 1011 j characterized by a different type of scaling and origin the scaling of the counter force f on the source duration of the single force is different for large and small explosions but is characterized by the same tendency of f 2 according to the dependence e versus d1 large explosions are generated within the narrow depth zone in the conduit corresponding to d1 from 3 5 to 7 5 s while small explosions are generated within a wide depth zone corresponding to d1 from 0 to 13 s the counter force of the eruption strongly governs the energy of explosions the duration of the explosion at the explosion surface within the conduit d2 has a tendency to negatively correlate with the energy of the explosion

crustal structure below the valley of mexico estimated from receiver functions abstract receiver functions rfs from two permanent stations cuig and zaig of the mexican broadband network were calculated to infer the crustal structure at both sites the data were stacked to allow uncertainty estimations and inverted using two global optimization algorithms simulated annealing and genetic algorithms strong and systematic azimuthal dependence was observed in the stacked functions at cuig site which is located in the mexican volcanic belt mvb assuming a one dimensional five layer model per azimuthal stack these variations were explained with the same upper crustal structure including a superficial 2 km thick low velocity layer associated with the mvb and a well defined conrad interface around 15 km depth different depths for the moho discontinuity and variations in both the s wave velocity and poisson s ratio for the upper mantle our inversion of observed rfs at cuig and theoretical considerations suggest an eastward crustal thickening of at least 5 km below the valley of mexico we tested the sensitivity of the modeling procedure by applying it to the recordings at zaig station in this case no azimuthal dependence was noticed in the stacked functions after a simultaneous inversion of three different azimuthal stacks a single four layer model satisfactorily explained all data thus revealing a flat one dimensional crustal model below the mexican mesa central where zaig station is located these results support the resolution and robustness of the applied method at both sites

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

crustal structure below popocatepetl volcano mexico from analysis of rayleigh waves an array of ten broadband stations was installed on the popocatepetl volcano mexico for five months between october 2002 and february 2003 26 regional and teleseismic earthquakes were selected and filtered in the frequency time domain to extract the fundamental mode of the rayleigh wave the average dispersion curve was obtained in two steps firstly phase velocities were measured in the period range 2 50 s from the phase difference between pairs of stations using wiener filtering secondly the average dispersion curve was calculated by combining observations from all events in order to reduce diffraction effects the inversion of the mean phase velocity yielded a crustal model for the volcano which is consistent with previous models of the mexican volcanic belt the overall crustal structure beneath popocatepetl is therefore not different from the surrounding area and the velocities in the lower crust are confirmed to be relatively low lateral variations of the structure were also investigated by dividing the network into four parts and by applying the same procedure to each sub array no well defined anomalies appeared for the two sub arrays for which it was possible to measure a dispersion curve however dispersion curves associated with individual events reveal important diffraction for 6 s to 12 s periods which could correspond to strong lateral variations at 5 to 10 km depth

analysis of seismic body waves excited by the mount st helens eruption of may 18 1980 seismic body waves which were excited by the may 18 1980 eruption of mount st helens and recorded by the global digital seismographic network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption the polarity of teleseismic p waves period 20s is identical at six stations which are distributed over a wide azimuthal range this observation together with a very small s to p amplitude ratio at 20 s suggests that the source is a nearly vertical single force a simple model shows that for seismic radiation a volcanic eruption can be represented by a single force applied in the direction opposite to the blast direction the time history of the vertical force suggests two distinct groups of events about 2 min apart each consisting of several subevents with a duration of about 25 s the magnitude of the force is approximately 2 6 10 12 n this vertical force is in contrast with the long period 150 s southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide an m s 5 2 earthquake initiated the eruption sequence the direction of the p wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide the ground motions observed at station lon 67 km are dominated by rayleigh waves i e lamb pulse and provide tight constraints on the time sequence of the events

the mathematician paleoplate the pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations one of the clearest examples of such a reorganization is documented in the mathematician seamounts region where a distinctive geomorphology and well developed magnetic anomalies are present this reorganization involved a short lived microplate between the failing mathematician ridge and a new propagating spreading center the east pacific rise it produced a transfer of a fragment of lithosphere from the farallon to the pacific plate and also created a number of landforms and magnetic patterns within and on the margins of the captured fragment these make up the mathematician paleoplate in many cases two sides of a microplate are active spreading ridges a microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge we define a paleoplate as the area of the seafloor from the axis of a failed rift to the boundary of resumed full spreading it includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading prior to complete abandonment of the failed rift the mathematician paleoplate has the following boundaries and components from west to east the axis of the mathematician failed rift the fragment of captured farallon plate a complex rift initiation site at the moctezuma trough a zone of slow spreading and an as yet ill defined eastern boundary where dual spreading stopped and full spreading resumed the northern boundary of the paleoplate is the rivera fracture zone its southeastern boundary a now inactive transform fault the west o gorman fracture zone in this case as well as in other more poorly documented ones relict landforms and magnetic patterns are carried on the aging lithosphere away from the spreading ridge recording a former geometry

intraplate thrusting near the easter microplate gloria long range sidescan sonar images from a region of the nazca plate immediately to the north of the easter microplate have revealed a prominent series of anomalous ridges these extend from lat 22 s long 114 w near the east pacific rise to 23 s 112 w near pito deep at the northeast corner of the microplate they are 20 to 50 km long and trend east west subparallel to the pacific nazca spreading direction swath bathymetric data indicate that the ridges dip steeply to the south and gently to the north and in places reliefs exceed 1 km earthquake focal mechanisms and predicted plate motions point to thrust faulting as the most likely mechanism for producing these ridges and their morphology suggests that the crust to the north has over ridden that to the south the nazca easter euler pole predicts a maximum present day convergence of about 50 km m y at the western end of the easter microplate9s northern boundary and we estimate that about one third of this has been taken up by the intraplate thrusts during the past 2 5 m y

scaling law of volcanic explosion earthquake the aim of the present study is to clarify a scaling law for volcanic explosion earthquakes excited by a counter force of eruption based on the data of explosion earthquakes at three volcanoes we derived the following results the peak amplitude of the single force is proportional to the square of the pulse width of source time function the initial pressure stored in a shallow region beneath a volcano before eruption is 1 mpa with a perturbation of one order the kinetic energy of explosion earthquake is proportional to the cube of crater radius the scaling law of the explosion earthquake is quite similar to that of the earthquake caused by a faulting it is possible to predict the strength of explosion earthquake by measuring the crater radius beforehand