linking rock physics and basin history filling gaps between wells in frontier basins a method has been developed to link present day well or seismic measurements through rock physics temperature and net erosion variations caused by past tectonic events within a sedimentary basin an ambiguous and intricate link among physical rock properties temperature history and burial history is demonstrated by modeling examples net erosion estimates presuppose an estimate about temperature history and vice versa net erosion and temperature history normally cannot be estimated independently when using elastic rock properties although net erosion in principle can be estimated independently of temperature for shallow unconsolidated sediments for rocks that have been subjected to thermal diagenesis either net erosion or paleotemperature must be known by consensus while estimating the other or both must be estimated simultaneously well data demonstrate how rock physics is used to predict the maximum historical temperature and net erosion based on well log velocities from these parameters sediment cooling can be derived these estimates can be performed away from well positions based on seismic velocities in shale temperature and erosion estimates are given for a 2d line and large lateral variations are estimated normally if well data are sparse there will be a high uncertainty in net erosion estimates the new method reduces uncertainty in net erosion and temperature estimates in areas away from well control the method is used in an area with good estimations of uplift and erosion from wells and gives results consistent with observations

seismic evidence for magma in the vicinity of mt katmai alaska p wave traveltime delays of as much as 0 9 sec are consistently observed at one seismic station from local and regional earthquakes 70 to 150 km deep this station is on the southwest flank of mt trident the most recently active volcano within katmai national park alaska delays from local shallow earthquakes are typically less than 0 3 sec suggesting that most of the major delay results from anomalous material at depths of more than a few kilometers this station is located near the center of a bowlshaped low in the bouguer gravity field that is approximately 15 km in diameter and more than 25 mgals deep these anomalies suggest but do not prove the presence of considerable amounts of magma in the shallow part of the crust that could have been the source for all magma erupted in the vicinity of mt katmai and mt trident this century

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

seismic velocity structure of the central taupo volcanic zone new zealand from local earthquake tomography abstract the 3 d distribution of p wave velocity vp and the p wave s wave velocity ratio vp vs are derived for the crust in the central taupo volcanic zone tvz new zealand by tomographic inversion of p and s wave arrival time data from local earthquakes resolution in the seismogenic mid crust 4 6 km is good but poorer above and below these depths the 3 d velocity model has several vp anomalies as large as 5 in the mid lower crust 4 10 km and more than 10 in the upper crust 0 4 km the model achieves a 55 reduction in data variance from an initial 1 d model young caldera structures okataina rotorua and reporoa are characterised by low vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies we attribute these anomalies to large volumes of low vp low density volcaniclastic sediments that have filled these caldera collapse structures although there are no vp anomalies which suggest the presence of molten or semi molten magma beneath the tvz a large high vp anomaly of more than 15 and a high vp vs anomaly are observed coincident with a diorite pluton beneath the ngatamariki geothermal field however vp anomalies cannot be seen beneath the largest geothermal fields waimangu waiotapu and reporoa and consequently if such anomalies exist they must be below the resolution of our data a prominent vp contrast of 5 10 occurs at a depth of about 6 km beneath the boundary between the taupo reporoa depression and the taupo fault belt tfb coincident with the eastern limit of the seismic activity beneath the tfb we interpret this velocity contrast as being caused by the presence of extensive non molten intrusives beneath the taupo reporoa depression we suggest that the high velocity material beneath the taupo reporoa depression is isolated from regional extension in the tvz and from the resulting faulting and seismicity which occurs preferentially within the weaker material of the tfb we are unable to determine whether greywacke which forms the basement beneath the eastern most part of the tvz continues further west or is replaced by a volcanic rock such as andesite which has similar vp and density vp vs anomalies are much smaller than vp anomalies and generally have little spatial relationship to the vp pattern there is a widespread decrease in vp vs from 1 76 to 1 70 1 73 between 4 and 6 km depth over much of the study area and vp vs is high southwest of the okataina caldera where vp is low hypocentres calculated using the 3 d velocity model differ little from those obtained using a 1 d model with station terms however some groups of earthquakes are more tightly clustered following relocation there is a slight decrease in the estimated thickness of the seismogenic zone with 73 of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle ductile transition from 6 to 6 5 km

three dimensional p and s wave velocity structure below unzen volcano abstract unzen volcano located in southwestern japan erupted on november 17 1990 after 198 years of dormancy and has been ejecting lava since may 20 1991 in this paper we present three dimensional p and s wave velocity variations below unzen volcano using 22 473 p and 14 349 s wave arrival times from 3457 local earthquakes recorded on a local network of twelve seismic stations the model was parameterized by 24 000 approximately 2 0 km cubic blocks targeting a volume of 120 100 20 km a prominent low velocity anomaly greater than 4 slowness perturbation starting from 2 5 to 5 0 km depth beneath the volcano is observed on both the p and s wave inversions below unzen volcano slightly lower amplitude low velocity anomalies are further observed to a depth of 12 5 km for p and 7 5 km for s waves shallow low velocity anomalies are observed below the chijiwa bay area northwest of unzen and deeper anomalies are seen below shimabara bay southwest of shimabara peninsula this nw se trending feature represents an elongated system of dykes which supply the shallow magma reservoirs

p and s velocity structure of the yellowstone volcanic field from local earthquake and controlled source tomography the three dimensional p and s velocity from vp vs ratios distribution and improved hypocenters for the yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled source explosions the p velocity model has an rms residual of 0 09 s whereas the vp vs ratio model calculated from 511 earthquakes has an rms residual of 0 29 s high p and s velocities outside the yellowstone caldera represent thermally undisturbed basement and sedimentary rocks a caldera wide 15 decrease from regional p velocities at depths of 6 to 12 km is coincident with a 60 mgal gravity anomaly and is interpreted as a hot subsolidus granitic batholith with a quasi plastic rheology localized 30 reductions from regional seismic velocities and higher vp vs ratios 8 km beneath yellowstone s resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful less than 2 ma silicic volcanism additional low seismic velocities and vp vs ratios and a 20 mgal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt hypocenters relocated with the three dimensional p velocity model show subparallel alignment with nw se trending postcaldera volcanic vents and normal faults northwest of the caldera focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera reflecting thinning and heating of the seismogenic crust

multiple seismogenic processes for high frequency earthquakes at katmai national park alaska evidence from stress tensor inversions of fault plane solutions the volcanological significance of seismicity within katmai national park has been debated since the first seismograph was installed in 1963 in part because katmai seismicity consists almost entirely of high frequency earthquakes that can be caused by a wide range of processes i investigate this issue by determining 140 well constrained first motion fault plane solutions for shallow depth 9 km earthquakes occurring between 1995 and 2001 and inverting these solutions for the stress tensor in different regions within the park earthquakes removed by several kilometers from the volcanic axis occur in a stress field characterized by horizontally oriented 1 and 3 axes with 1 rotated slightly 12 relative to the nuvel1a subduction vector indicating that these earthquakes are occurring in response to regional tectonic forces on the other hand stress tensors for earthquake clusters beneath several katmai cluster volcanoes have vertically oriented 1 axes indicating that these events are occurring in response to local not regional processes at martin mageik vertically oriented 1 is most consistent with failure under edifice loading conditions in conjunction with localized pore pressure increases associated with hydrothermal circulation cells at trident novarupta it is consistent with a number of possible models including occurrence along fractures formed during the 1912 eruption that now serve as horizontal conduits for migrating fluids and or volatiles from nearby degassing and cooling magma bodies at mount katmai it is most consistent with continued seismicity along ring fracture systems created in the 1912 eruption perhaps enhanced by circulating hydrothermal fluids and or seepage from the caldera filling lake manuscript received 25 april 2002

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

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

deformation of new trident volcano measured by ers 1 sar interferometry katmai national park alaska using synthetic aperture radar sar interferometry we detect several centimeters of uplift that accumulated during two years 1993 1995 around the vent of the new trident volcano in alaska s katmai national park the areas of image coherence correspond to fresh blocky lavas while coherence is lost in ash covered areas from the uplift gradient we estimate the depth of a pressure source under new trident volcano to be approximately 0 8 2 0 km our results show that in spite of the difficult sub arctic environment of southern alaska strain build up can be monitored over a two year period showing the potential for global monitoring of volcano deformation using sar interferometry

seismic image of the mount spurr magmatic system the three dimensional p wave velocity structure of mount spurr is determined to depths of 10 km by tomographic inversion of 3 754 first arriving p wave times from local earthquakes recorded by a permanent network of 11 seismographs results show a prominent low velocity zone extending from the surface to 3 4 km below sea level beneath the southeastern flank of crater peak spatially coincident with a geothermal system p wave velocities in this low velocity zone are approximately 20 slower than those in the shallow crystalline basement rocks beneath crater peak an approximately 3 km wide zone of relative low velocities correlates with a near vertical band of seismicity suggestive of a magmatic conduit no large low velocity zone indicative of a magma chamber occurs within the upper 10 km of the crust these observations are consistent with petrologic and geochemical studies suggesting that crater peak magmas originate in the lower crust or upper mantle and have a short residence time in the shallow crust earthquakes relocated using the three dimensional velocity structure correlate well with surface geology and other geophysical observations thus they provide additional constraints on the kinematics of the mount spurr magmatic system

seismicity at the volcanoes of katmai national park alaska july 1995 december 1997 abstract upper crustal seismicity located within katmai national park occurs mainly within four distinct clusters originating near martin mageik mm subnet volcanoes trident volcano katmai caldera and snowy volcano analyses of earthquake frequency magnitude distributions reveal high b values beneath mm subnet 1 5 normal b values at trident volcano 1 0 and intermediate b values at katmai caldera 1 3 for all seismicity occurring between july 1995 and december 1997 detailed analyses of subsets of b values and hypocenter locations at mm subnet reveal a temporal increase in b value associated with an increase in the maximum depth of seismicity the changes occurred during a shallow earthquake swarm beneath mageik volcano between october 16 and 25 1996 and again in november december 1997 before the swarm the weighted least squares b value was 1 01 at mm subnet increased to 1 59 during the swarm and remained anomalously high until april 1997 the corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after september 18 1996 but shifted to 10 km during both the october 1996 swarm and the november december 1997 period the november december 1997 event is not associated with an increase in the rate of seismicity or the b value the october 1996 swarm had a cumulative moment release of 5 0 10 20 dyn cm and decayed from a peak rate of 110 events per day with a modified omori law p value of 1 06 0 11 modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20 40 c km 1 in agreement with the p value estimate we infer that a simple pressurizing intrusion was not associated with the october swarm because higher stresses should increase seismicity and lower the b value the opposite of what we observed alternatively an actively degassing intrusion would reduce the effective stress and increase the b value at shallow depths while the increased stress would induce seismicity at depth surface temperature measurements taken at the mageik crater lake in july 1995 and again in july 1997 revealed an increase of about 10 c in the lake water temperature consistent with a degassing intrusive event

three dimensional p wave velocity structure derived from local earthquakes at the katmai group of volcanoes alaska abstract the three dimensional p wave velocity structure beneath the katmai group of volcanoes is determined by inversion of more than 10 000 rays from over 1000 earthquakes recorded on a local 18 station short period network between september 1996 and may 2001 the inversion is well constrained from sea level to about 6 km below sea level and encompasses all of the katmai volcanoes martin mageik trident griggs novarupta snowy and katmai caldera the inversion reduced the average rms travel time error from 0 22 s for locations from the standard one dimensional model to 0 13 s for the best three dimensional model the final model from the 6th inversion step reveals a prominent low velocity zone 3 6 5 0 km s centered at katmai pass and extending from mageik to trident volcanoes the anomaly has values about 20 25 slower than velocities outboard of the region 5 0 6 5 km s moderately low velocities 4 5 6 0 km s are observed along the volcanic axis between martin and katmai caldera griggs volcano located about 10 km behind northwest of the volcanic axis has unremarkable velocities 5 0 5 7 km s compared to non volcanic regions the highest velocities are observed between snowy and griggs volcanoes 5 5 6 5 km s relocated hypocenters for the best 3 d model are shifted significantly relative to the standard model with clusters of seismicity at martin volcano shifting systematically deeper by about 1 km to depths of 0 to 4 km below sea level hypocenters for the katmai caldera are more tightly clustered relocating beneath the 1912 scarp walls the relocated hypocenters allow us to compare spatial frequency size distributions b values using one dimensional and three dimensional models we find that the distribution of b is significantly changed for martin volcano which was characterized by variable values 0 8 b b b b b b values indicating the robustness of the observations the strong high b value region at mageik volcano is mainly associated with an earthquake swarm in october 1996 that possibly indicates a shallow intrusion or influx of gas the new velocity and spatial b value results in conjunction with prior gravity bouguer anomalies up to 40 mgal and interferometry several cm uplift data provide strong evidence in favor of partially molten rock at shallow depths beneath the mageik katmai novarupta region moderately low velocities beneath martin and katmai suggest that old mostly solidified intrusions exist beneath these volcanoes higher relative velocities beneath the griggs and snowy vents suggest that no magma is resident in the shallow crust beneath these volcanoes

orbitally mediated cycles from the turonian of southern england their potential for high resolution stratigraphic correlation detailed sampling of two sections of turonian aged chalk from southeast england for foraminifera has revealed that cyclical abundance changes in many species and genera were mediated by orbitally induced climatic cycles these cycles can be used for high resolution stratigraphic correlation of the two sections and as a possible means of re estimating the duration of the turonian period

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

earthquake classification location and error analysis in a volcanic environment implications for the magmatic system of the 1989 1990 eruptions at redoubt volcano alaska abstract determination of the precise locations of seismic events associated with the 1989 1990 eruptions of redoubt volcano posed a number of problems including poorly known crustal velocities a sparse station distribution and an abundance of events with emergent phase onsets in addition the high relief of the volcano could not be incorporated into the hypoellipse earthquake location algorithm this algorithm was modified to allow hypocenters to be located above the elevation of the seismic stations the velocity model was calibrated on the basis of a posteruptive seismic survey in which four chemical explosions were recorded by eight stations of the permanent network supplemented with 20 temporary seismographs deployed on and around the volcanic edifice the model consists of a stack of homogeneous horizontal layers setting the top of the model at the summit allows events to be located anywhere within the volcanic edifice detailed analysis of hypocentral errors shows that the long period lp events constituting the vigorous 23 hour swarm that preceded the initial eruption on december 14 could have originated from a point 1 4 km below the crater floor a similar analysis of lp events in the swarm preceding the major eruption on january 2 shows they also could have originated from a point the location of which is shifted 0 8 km northwest and 0 7 km deeper than the source of the initial swarm we suggest this shift in lp activity reflects a northward jump in the pathway for magmatic gases caused by the sealing of the initial pathway by magma extrusion during the last half of december volcano tectonic vt earthquakes did not occur until after the initial 23 hour long swarm they began slowly just below the lp source and their rate of occurrence increased after the eruption of 01 52 ast on december 15 when they shifted to depths of 6 to 10 km after january 2 the vt activity migrated gradually northward this migration suggests northward propagating withdrawal of magma from a plexus of dikes and or sills located in the 6 to 10 km depth range precise relocations of selected events prior to january 2 clearly resolve a narrow steeply dipping pencil shaped concentration of activity in the depth range of 1 7 km which illuminates the conduit along which magma was transported to the surface a third event type named hybrid which blends the characteristics of both vt and lp events originates just below the lp source and may reflect brittle failure along a zone intersecting a fluid filled crack the distribution of hybrid events is elongated 0 2 0 4 km in an east west direction this distribution may offer constraints on the orientation and size of the fluid filled crack inferred to be the source of the lp events

new structural limits on magma chamber locations at the valley of ten thousand smokes katmai national park alaska new structural data from the novarupta basin katmai national park alaska site of the largest volcanic eruption of this century 1912 provide limits for the location of magma chambers associated with this eruption to investigate the subsurface structure of the 1912 vents and to support an interdisciplinary study of this young volcanic system a project of geologic mapping of surficial and bedrock structures in the vent region of the 1912 eruption has been undertaken landslide scarps arcuate grabens a monoclinal fold and truncated ridges circumscribe the novarupta basin marking the inferred outer rim of the vent a set of radial fissures crosses the southern margin of the basin striking approximately 140 degree subparallel to the dominant bedrock joint set these fissures and joints along with the local plate motion vector and the inferred regional stress orientation are consistent with a feeder dike propagating from a reservoir beneath trident volcano to the eruptive vent