control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

the 1930 santa monica and the 1979 malibu california earthquakes the malibu earthquake m l 5 0 that occurred 1 january 1979 in santa monica bay was located at 33 56 9 n and 118 41 3 w and at a depth of 10 km the focal mechanism of the main shock as determined from first motions recorded by short period high gain seismic stations in southern california shows reverse faulting with one plane dipping 55 to 60 to the north striking 275 to 290 and the second plane dipping 30 to 38 to the south and striking 100 to 120 more than 400 aftershocks were recorded during 1979 focal mechanisms for 20 aftershocks of m 3 0 show similar reverse mechanisms as the main shock the main shock and aftershocks m 2 5 were relocated using a local velocity model and a set of corresponding station delays the aftershocks define a volumetric hypocentral zone 4 km 6 km west trending with depths ranging from 6 to 11 km this 1979 malibu main shock aftershock sequence probably occurred on the eastern end of the anacapa dume fault the santa monica earthquake m l 5 2 that occurred 31 august 1930 was relocated using station delays calculated from the 1979 date set the relocated 1930 epicenter is located near the western end of the santa monica fault the 1979 and 1930 main shocks can be interpreted to define a 8 to 10 km north south sidestep between the anacapa dume and santa monica faults possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake

low frequency shelf slope responses induced by localized offshore forcings using a two layer plane model with bottom friction we investigated the shelf slope responses induced by localized offshore forcing sources which 1 propagate in the along slope direction and 2 have oscillating amplitudes the two cases represent typical forcing patterns of cutoff eddy propagations and unstable meanders in western boundary currents when the localized forcing source propagates in the along slope direction it tends to induce components of bottom trapped topographic rossby waves trws with a similar cross slope structure and the shelf slope response depends strongly upon the magnitude of the propagating speed c in the in viscid limit maximum response occurs when c is such that the induced bottom trapped trw components may freely cross the slope region when bottom friction is considered however the shelf slope response due to the bottom trapped trws is important only in large c value cases in small c value cases we found the shelf slope response is determined by surface intensified wave components whose existence is due to the planetary effect for observed warm eddies with c 3 5 cm s 1 the result of the present study suggests that both bottom trapped trws and surface intensified baroclinic waves are significant in determining the shelf slope response in cases when the localized forcing source oscillates in amplitude we found that a localized response peak exists in the shelf slope region along a fixed cross slope section this response peak in the frictional case tends to shift offshore as the oscillating period of the forcing source increases this result is opposed to the inviscid result which shows that the response peak is independent of the forcing s oscillating period the result for the frictional case is qualitatively consistent with that observed across the continental rise south of cape cod thus suggesting the importance of bottom friction in determining the shelf slope response

diffusion controlled spherulite growth in obsidian inferred from h2o concentration profiles spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian the growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass and rejection of non crystal forming components from the crystallizing region water concentration profiles measured by synchrotron source fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth and that it diffuses outward ahead of the advancing crystalline front we compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth using a diffusion controlled growth law we find that spherulites can grow on the order of days to months at temperatures above the glass transition the diffusion controlled growth law also accounts for spherulite size distribution spherulite growth below the glass transition and why spherulitic glasses are not completely devitrified

mineralogical analyses and in vitro screening tests for the rapid evaluation of the health hazard of volcanic ash at rabaul volcano papua new guinea the continuous ash and gas emissions from the tavurvur cone in rabaul caldera papua new guinea during 2007 08 raised concerns regarding how exposure would affect the respiratory health of nearby populations and impact on the environment as part of a formal evaluation of the effects of volcanic emissions on public health we investigated the potential health hazard of the ash using a suite of selected mineralogical analyses and in vitro toxicity screening tests the trachy andesitic ash comprised 2 1 6 7 vol respirable sub 4 m diameter particles the crystalline silica content was 1 9 5 0 wt cristobalite in the bulk sample with trace amounts of quartz and or tridymite scanning electron microscopy showed that the ash particles were angular with sparse fibre like particles 3 60 m max diameter observed in some samples which we confirmed to be caso4 gypsum at 6 wt in the bulk samples and not asbestiform fibres the ash specific surface area was low 0 1 2 7 m2 g 1 the leached solution from one of the ash samples was slightly acidic ph 5 6 but did not contain high levels of toxic metals such as f cu zn mn as ni and cd when compared to previously tested volcanic ash leachates ash samples generated potentially harmful hydroxyl radicals through an iron mediated catalytic reaction in the range of 0 15 2 47 mol m 2 after 30 min of reaction however measurement of particle oxidative capacity potential oxidative stress reaction using ascorbic acid and silica like injury to red blood cells erythrolysis assay i e measurement of cell death nevertheless revealed low biological reactivity the findings suggest that acute exposure to the ash would have a limited potential to exacerbate pre existing conditions such as asthma or chronic bronchitis and the potential for chronic exposure leading to silicosis was low

a simple treatment of very low frequency wind driven sea level variations it is shown that very low frequency sea level variations can be calculated from the sum of the steric or density effect and the wind driven setup derived from barotropic dynamics provided that a weak slope criterion is met this result is shown to be consistent with a recent two layer formulation for wind driven shelf motions

crustal earthquake bursts in california and japan their patterns and relation to volcanoes 1 we analyze 153 bursts of earthquakes in southern california and japan the burst patterns are similar in southern california and japan they fill a spectrum between swarm like sequences without obvious mainshocks and mainshocks with omori law abiding aftershocks in agreement with our previous work the swarm like sequences in japan have more events are more voluminous and tend to expand with time when compared to mainshock aftershock type sequences in both regions we find that the sequences starting with their largest events tend to be much shorter in duration bursts within 50 km of volcanoes are similar in character to those elsewhere except they tend to have longer duration we hypothesize that swarminess is a proxy for fluid pressure redistribution and or aseismic slip driving the seismicity bursts and conversely the mainshock aftershock style sequences have end member behavior that results solely from a cascade of elastic failures the complexity of the spatial seismicity distribution does not correlate with the style of swarm observed indicating that fluid conditions and composition are likely more influential than geometry in determining the patterns we observe

the santa barbara california earthquake of 13 august 1978 the 5 1 m l santa barbara earthquake of 13 august 1978 occurred at 22h54m 52 8s utc the epicenter was located 3 km southeast of santa barbara at 34 23 9 n latitude and 119 40 9 w longitude with a focal depth of 12 7 km the main shock was followed between 13 august and 30 september by 373 aftershocks that were located with the caltech usgs array the aftershock zone extended 12 km wnw from the epicenter and was 6 km wide in the n s direction and it had a very clear temporal development during the first 20 min of activity all the aftershocks were located in a cluster 7 km wnw of the main shock epicenter during the next 24 hr the aftershock zone grew to 11 km in the wnw direction and 4 km in the n s direction during succeeding weeks the zone extended to 12 by 6 km this temporal spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the wnw and the initial rupture plane may have been considerably smaller than the eventual aftershock zone this smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone in cross section the aftershock hypocenters outline a nearly horizontal plane dipping 15 or less at 13 km depth the main shock focal mechanism indicates nne ssw compression and vertical extension the preferred fault plane strikes n80 w and dips 26 nne indicating north over south thrusting with a component of left lateral movement focal mechanisms for 40 aftershocks also indicate compression in the general n s direction for most of these events the north dipping nodal plane dips between 7 and 45 with most dipping 25 or more which is significantly steeper than the plane delineated by the hypocenters themselves these observations are consistent with a tectonic model in which much of the slip during the santa barbara earthquake occurred on a nearly horizontal plane the after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip hence the santa barbara earthquake may be taken as evidence for mid crustal horizontal shearing in the western transverse ranges

aftershocks and temporal clustering of deep earthquakes this paper evaluates how the focal depth the geographic location and magnitude affect the incidence of aftershocks and the occurrence of temporal clustering of nearby events for selected deep earthquakes h 70 km in the catalog of the international seismological centre for comparison we also evaluate shallow h 70 km and deep earthquakes for which locations and moment tensors are reported by the harvard group the primary conclusions are as follows 1 the incidence of aftershocks depends strongly on focal depth with aftershocks being less common for events between 100 and 450 km depth than for shallower or deeper events events with focal depth shallower than 40 km have a higher incidence of aftershocks and more pronounced temporal clustering than do deeper events 2 for the data available at present the incidence of aftershocks and amount of temporal clustering for deep events are independent of geographic region except for dependence caused by regional differences in the focal depths of events 3 deep earthquakes with large magnitudes are more likely to have aftershocks than are smaller magnitude events in accordance with models where the number of aftershocks is a poisson random variable that is a function of magnitude however several large events are anomalous in that they have many more aftershocks or many fewer aftershocks in the catalog than predicted by the model

a source extent analysis of the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 a new method for estimating source extent parameters has been applied to the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 from the stations of the world wide standard seismograph network and canadian seismic network estimates of the duration of far field long period sh waves were made by computing the variance or second central moment 2 of instrument corrected attenuation corrected pulses seventeen measurements for the imperial valley event and 25 for the victoria event were made we inverted for the source duration t the fault length l and the directivity parameter d by least squares we find that for the imperial valley t 9 0 0 5 s d 130 43 km s and l 46 13 km and for the victoria event t 10 9 0 6 s and d 251 71 km s l is not well constrained for this second event the inversion gives 23 km the lower bound obtained by constraining 2 to be positive everywhere on the focal sphere although a value as high as 40 km would still be within the uncertainties the positive values of d for both events indicate that rupture propagation was predominantly to the northwest in order to estimate the degree of unilateralness we have computed the rupture mode index defined by r d lt which is 1 for a unilateral rupture and zero for a bilateral rupture we find a value of r 0 31 0 14 for the imperial valley event significantly less than unity which indicates that a southern component of rupture is also present assuming uniform slip we estimate the northern and southern fault segments to be 26 and 20 km respectively the presence of a bilateral component is supported by near field studies which have made use of stations along the southern half of the imperial fault for the victoria event due primarily to the uncertainty in l the value of r can vary from 0 6 to 1 with the value of 1 being most probable assuming that the rupture is unilateral we can obtain an estimate of fault length from the relation l d t which yields l 23 6 km geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5 8 0 4 for imperial valley and 4 8 0 4 for victoria in units of 1018 n m the presence of a southern component of rupture for the imperial valley event would suggest a closer and perhaps causal relationship to the victoria event which occurred 8 months later and about 60 km to the south in a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of cerro prieto

estimates of velocity structure and source depth using multiple p waves from aftershocks of the 1987 elmore ranch and superstition hills california earthquakes abstract event record sections which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations show multiple free surface reflections pp ppp pppp of the p wave in the imperial valley california the relative timing of these arrivals is used to estimate the strength of the p wave velocity gradient within the upper 5 km of the sediment layer consistent with previous studies a velocity model with a value of 1 8 km sec at the surface increasing linearly to 5 8 km sec at a depth of 5 5 km fits the data well the relative amplitudes of the p and pp arrivals are used to estimate the source depth for the aftershock distributions of the elmore ranch and superstition hills main shocks although the depth determination has large uncertainties both the elmore ranch and superstition hills aftershock sequences appear to have similar depth distribution in the range of 4 to 10 km

faulting style controls for the space time aftershock patterns abstract to understand the complexity and thus to predict earthquake occurrence in size time and space seismicity patterns are characterized by two robust empirical laws the exponential distribution of magnitude and the power law decrease of aftershock rate over time these laws are known as the gutenberg richter law and the omori law respectively using global earthquake catalogs we resolve that on average the k aftershock productivity and the p value exponent of the power law decrease of aftershock rate over time are dependent on the mainshock faulting style strike slip events have a lower aftershock rate n and k values and a larger p value than thrust and normal events respectively within the epidemic type aftershock sequence model strong k n values are driven by a high branching ratio value n within the same framework a relatively higher n value for the thrust events also predicts the lower p value we observe for thrust events as compared to strike slip and normal faulting events respectively furthermore we observed that earthquake interactions through time and space are a function of the faulting style when measured by t the exponent of the power law decrease of earthquake density over space the t values of thrust events for different time windows always remain smaller than those of the strike slip events when changes in faulting styles are driven by stress pattern the anderson faulting theory predicts thrust faulting that requires somewhat larger stresses in absolute magnitude than do normal and strike slip faulting within the framework of rate and state friction law changes in the stress heterogeneity patterns reproduce the p value changes we observe our results suggest that only stress perturbations associated with mainshock rupture affect the productivity and decay rate over time of aftershocks

earthquake depths and the relation to strain accumulation and stress near strike slip faults in southern california earthquakes in the major fault zones are predominantly deep earthquakes in the crustal blocks bounding the fault zones are predominantly shallow in the san jacinto fault zone maximum earthquake depths correlate with surface heat flow these relations together with focal mechanisms geodetic strain measurements and fault zone models are consistent with the following ideas 1 interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes 2 the aseismic slip in a narrow fault zone in the brittle plastic transition region concentrates strain at the base of the brittle fault zone 3 deep earthquakes occur in thelower part of the brittle fault zone due to stick slip failure of highly stressed patches 4 background earhtquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding deep and stick slip shallow regions of the fault zone furthermore the difference in seismicity between the san jacinto and southern san andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases 5 shear stress may be lessmore in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks 6 in the stress field produced by plate motion and slip in the deep fault zone the upper brittle fault zone is not oriented favorably for shear failure lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation less

detecting aseismic strain transients from seismicity data 1 aseismic deformation transients such as fluid flow magma migration and slow slip can trigger changes in seismicity rate we present a method that can detect these seismicity rate variations and utilize these anomalies to constrain the underlying variations in stressing rate because ordinary aftershock sequences often obscure changes in the background seismicity caused by aseismic processes we combine the stochastic epidemic type aftershock sequence model that describes aftershock sequences well and the physically based rate and state dependent friction seismicity model into a single seismicity rate model that models both aftershock activity and changes in background seismicity rate we implement this model into a data assimilation algorithm that inverts seismicity catalogs to estimate space time variations in stressing rate we evaluate the method using a synthetic catalog and then apply it to a catalog of m 1 5 events that occurred in the salton trough from 1990 to 2009 we validate our stressing rate estimates by comparing them to estimates from a geodetically derived slip model for a large creep event on the obsidian buttes fault the results demonstrate that our approach can identify large aseismic deformation transients in a multidecade long earthquake catalog and roughly constrain the absolute magnitude of the stressing rate transients our method can therefore provide a way to detect aseismic transients in regions where geodetic resolution in space or time is poor

vapor phase cristobalite as a durable indicator of magmatic pore structure and halogen degassing an example from white island volcano new zealand vesicles in volcanic rocks are physical records of magmatic degassing however the interpretation of their textures is complicated by resorption coalescence and collapse we discuss the textural significance of vesicle hosted vapor phase cristobalite high t low p sio2 polymorph and its utility as a complement to textural assessments of magmatic degassing using a representative dacite bomb erupted from white island volcano new zealand in 1999 imaging in 2d sem and 3d ct shows the bomb to have 56 bulk porosity almost all of which is connected 99 and devoid of sio2 phases the remaining 1 of porosity is in isolated sub spherical vesicles that have corroded walls and contain small 30 m across prismatic vapor phase cristobalite crystals 98 4 0 4 wt sio2 with diagnostic laser raman spectra halogen degassing models show vapor phase cristobalite to be indicative of closed system chlorine and fluorine partitioning into h2o rich fluid in isolated pores at white island this occurred during shallow 100s of meters ascent and extensive 50 groundmass crystallization associated with slow cooling in a volcanic plug pristine textures in this white island bomb demonstrate the link between pore isolation and vapor phase cristobalite deposition we suggest that because these crystals have higher preservation potential than the bubbles in which they form they can serve as durable qualitative textural indicators of halogen degassing and pre quench bubble morphologies in slowly cooled volcanic rocks e g lava flows and domes even where emplacement mechanisms have overprinted original bubble textures

the nature and formation of cristobalite at the soufriere hills volcano montserrat implications for the petrology and stability of silicic lava domes cristobalite is commonly found in the dome lava of silicic volcanoes but is not a primary magmatic phase its presence indicates that the composition and micro structure of dome lavas evolve during and after emplacement nine temporally and mineralogically diverse dome samples from the soufriere hills volcano shv montserrat are analysed to provide the first detailed assessment of the nature and mode of cristobalite formation in a volcanic dome the dome rocks contain up to 11 wt cristobalite as defined by x ray dif fraction prismatic and platy forms of cristobalite identified by scanning electron microscopy sem are commonly found in pores and fractures suggesting that they have precipitated from a vapour phase feathery crystallites and micro crystals of cristobalite and quartz associated with vol canic glass identified using sem raman are interpreted to have formed by varying amounts of devitrification we dis cussmechanismsof silicatransport and cristobaliteformation and their implications for petrological interpretations and domestability we conclude 1 that silica maybetransported in the vapour phase locally or from one part of the magmatic system to another 2 that the potential for transport of silica into the dome should not be neglected in petrological and geochemical studies because the addition of non magmatic phases may affect whole rock composition and 3 that the extent of cristobalite mineralisation in the dome at shv is sufficient to reduce porosity hence permeability and may impact on the mechanical strength of the dome rock thereby potentially affecting dome stability

properties of aftershock sequences in southern california the temporal behavior of 39 aftershock sequences in southern california 1933 1988 was modeled by the modified omori relation minimum magnitudes for completeness of each sequence catalog were determined and the maximum likelihood estimates of the parameters k p and c with the standard errors on each were determined by the ogata algorithm the b value of each sequence was also calculated many of the active faults in the region both strike slip and thrust were sampled the p values were graded in terms of the size of the standard error relative to the p value itself most of the sequences were modeled well by the omori relation many of the sequences had p values close to the mean of the whole data set 1 11 0 25 but values significantly different from the mean as low as 0 7 and as high as 1 8 exist no correlation of p with either the b value of the sequence or the mainshock magnitude was found the results suggest a direct correlation of p values is with surface heat flow with high values in the salton trough high heat flow and one low value in the san bernardino mountains and on the edge of the ventura basin both low heat flow the large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean 74 mw m2 if the hypothesis that aftershock decay rate is controlled by temperature at depth is valid the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature surface heat flow is taken as an indicator of crustal temperature at hypocentral depths but the effects on heat flow of convective heat transport and variations in near surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution the interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials leading to a faster decay rate higher p value in 1967 mogi proposed such a relation on the basis of japanese data seismic velocity distributions published for southern california generally support the hypothesis with low velocities higher temperatures corresponding to high p values and vice versa

long term exposure to respirable volcanic ash on montserrat a time series simulation frequent ash fallout from long lived eruptions with active phases greater than 5 years may lead to local populations experiencing unacceptably high cumulative exposures to respirable particulate matter ash from montserrat has been shown to contain significant levels of cristobalite and other reactive agents that are associated with an increased risk of developing pneumoconiosis including silicosis and other long term health problems there are a number of difficulties associated with estimating risks in populations due to uncertain and wide ranging individual exposures change in behaviour with time and the natural variation in individual response present estimates of risk in workers and other population groups are simplifications based on a limited number of exposure measurements taken on montserrat 1996 1999 and exposure response curves from epidemiological studies of coal workers exposed to siliceous dust in this paper we present a method for calculating the long term cumulative exposure to cristobalite from volcanic ash by monte carlo simulation code has been written to generate synthetic time series for volcanic activity rainfall ash deposition and erosion to give daily ash deposit values and cristobalite fraction at a range of locations the daily mean personal exposure for pm10 and cristobalite is obtained by sampling from a probability distribution with distribution parameters dependent on occupation ground deposit depth and daily weather conditions output from multiple runs is processed to calculate the exceedance probability for cumulative exposure over a range of occupation types locations and exposure periods results are interpreted in terms of current occupational standards and epidemiological exposure response functions for silicosis are applied to quantify the long term health risk assuming continuing volcanic activity median risk of silicosis profusion 1 0 or higher for an average adult after 20 years continuous exposure is estimated to be approximately 0 5 in northern montserrat to 1 6 in cork hill the occupational group with the highest exposure to ash are gardeners with a corresponding 2 to 4 risk of silicosis in situations where opportunities for in depth exposure studies are limited computer simulations provide a good indication of risk based on current expert knowledge by running the code for a range of input scenarios the cost benefit of mitigation measures such as a programme of active ash clearance can be estimated results also may be used to identify situations where full exposure studies or fieldwork would be beneficial

a tomographic image of mantle structure beneath southern california we determined the variations in seismic structure beneath southern california by using a tomographic method of inversion on teleseismic p delays recorded with the southern california array the algorithm employed was a modified form of an algebraic reconstruction technique art used in medical x ray imaging deconvolution with an empirically estimated point spread function was also used to help in focusing the image the inversion reveals two prominent features beneath the region the first is a thin vertical wedge directly beneath the transverse ranges that is 2 3 faster than the surrounding region this feature deepens to the east attaining a maximum depth of about 250 km beneath the san bernardino mountains the second feature is a major zone of low velocity material that is 2 4 slow under the salton trough rift valley extending to a depth of about 125 km two possible explanations for the spatial association of the transverse ranges with the velocity anomaly below are lithospheric subduction or small scale sublithospheric convection in the region of the big bend of the san andreas fault the low velocity anomaly beneath the salton trough is consistent with convective upwelling there

post seismic motion following the 1997 manyi tibet earthquake insar observations and modelling summary on november 8 1997 a mw 7 6 earthquake occurred in the manyi region of northern tibet near the western end of the kunlun fault over 7 m of left lateral slip occurred on a 200 kmlong fault here we use insar observations of post seismic surface deformation following the manyi earthquake to investigate possible causal mechanisms time series of deformation are constructed from 26 interferograms covering the entire length of the fault for nearly 4 yr after the earthquake three different modelling approaches are used to try and understand the observed variations in surface displacement first order poroelastic models predict displacement fields which do not match those observed modelling of viscoelastic stress relaxation in a half space with standard linear solid rheology beneath an elastic lid provides a reasonable fit to the observations and demonstrates that two relaxation times are needed to characterize the post seismic transient the best fitting viscosity is 4 10 18 pa s and the fully relaxed shear modulus is 32 per cent of the purely elastic value a model with a maxwell viscoelastic halfspace underlying the lid cannot explain the observations at later times larger viscosities are required than at earlier times this increase in effective viscosity with time may be consistent with stress relaxation occurring in a power law rheology the time series are also inverted for distributed afterslip on an extension of the coseismic rupture along strike correlation of coseismic and post seismic slip maxima suggests that afterslip is a plausible mechanism the maximum slip in the afterslip model is 0 72 m after 3 yr and the equivalent moment release is approximately 20 per cent of the coseismic moment this study shows that it is possible to rule out certain mechanisms as the dominant post seismic process but with the current insar data set it is difficult to distinguish between other plausible options

aftershock relocation and frequency size distribution stress inversion and seismotectonic setting of the 7 august 2013 m 5 4 earthquake in kallidromon mountain central greece abstract on august 7 2013 a moderate earthquake noa ml 5 1 noa mw 5 4 occurred in central kallidromon mountain in the pthiotis region of central greece 2270 aftershocks were relocated using a modified 1 d velocity model for this area the b value of the aftershock sequence was b 0 85 for a completeness magnitude of mc 1 7 the rate of aftershock decay was determined at p 0 63 the spatial distribution of the aftershock sequence points towards the reactivation of a n70 10 e striking normal fault at crustal depths between 8 and 13 km a nnw sse cross section imaged the activation of a steep south dipping normal fault a stress inversion analysis of 12 focal mechanisms showed that the minimum horizontal stress is extensional at n173 e no primary surface ruptures were observed in the field however the earthquake caused severe damage in the villages of the kallidromon area the imaged fault strike and the orientation of the long axis of the aftershock sequence distribution are both at a high angle to the strike of known active faults in this area of central greece we interpret the kallidromon seismic sequence as release of extensional seismic strain on secondary steep faults inside the fokida viotia crustal block

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

source parameters of the 1933 long beach earthquake regional seismographic network and teleseismic data for the 1933 m l 6 3 long beach earthquake sequence have been analyzed both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the newport inglewood fault the focal mechanism had a strike of 315 dip of 80 to the northeast and rake of 170 relocation of the foreshock main shock aftershock sequence using modern events as fixed reference events shows that the rupture initiated near the huntington beach newport beach city boundary and extended unilaterally to the northwest to a distance of 13 to 16 km the centroidal depth was 10 2 km the total source duration was 5 sec and the seismic moment was 5 10 25 dyne cm which corresponds to an energy magnitude of m w 6 4 the source radius is estimated to have been 6 6 to 7 9 km which corresponds to a brune stress drop of 44 to 76 bars both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents when the slip estimate from the seismic moment of 85 to 120 cm is compared with the long term geological slip rate of 0 1 to 1 0 mm yr along the newport inglewood fault the 1933 earthquake has a repeat time on the order of a few thousand years

the 3 december 1988 pasadena california earthquake evidence for strike slip motion on the raymond fault the pasadena earthquake m l 4 9 occurred on 3 december 1988 at a depth of 16 km the hypocenters of the earthquake and its aftershocks define a east northeast striking steeply northwest dipping surface that projects up to the active surficial trace of the raymond fault one of the nodal planes of the focal mechanism of the earthquake parallels the raymond fault with left lateral strike slip movement on that plane and is consistent with geomorphic and paleoseismic evidence that the raymond fault is dominantly a left lateral strike slip fault the existence of a component of sinistral slip along the raymond fault had been suspected prior to the earthquake but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse

the respiratory health hazard of tephra from the 2010 centennial eruption of merapi with implications for occupational mining of deposits ashfall into heavily populated areas during the october november 2010 eruption of merapi volcano indonesia created anxiety regarding the growing impacts to health as the eruption escalated and the hazard zone widened we made a preliminary assessment of the respiratory hazards to human health of the tephra deposits ashfall lahar and pdc surge from the eruption using a laboratory protocol specifically developed to study the toxic potential of volcanic ash particles twenty samples collected from a range of locations were analysed for health pertinent mineralogical parameters grain size crystalline silica content morphology surface area bulk chemistry and leachable elements and bio reactivity hydroxyl radical generation haemolytic potential oxidative capacity pro inflammatory response the grain size pertinent to respiratory health was variable ranging from 1 4 15 6 vol sub 4 m and 3 0 28 9 vol sub 10 m diameter material no fibre like particles were observed cristobalite was present in all samples ranging from 1 9 9 5 wt but surface reactivity and in vitro toxicity assays showed low reactivity for all samples tested the risk of direct exposure to ash from fallout was in any case low due to seasonal rains limiting its re suspension and the immediate and effective clean up of communities by local people who supplied the ash to the indonesian construction industry for use as aggregate however mining of the lahar and thick pdc deposits in the valleys draining the volcano is performed on a vast industrial scale which could result in high occupational exposure to thousands of sand miners at merapi during the dry seasons further study of the health hazard of the mined merapi deposits is warranted

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

tephra stratigraphy and eruptive volume of the may 2008 chaiten eruption chile on may 1st 2008 mount chaiten southern chile interrupted a long period of quiescence generating a sequence of explosive eruptions and causing the evacuation of chaiten town located a few kilometers south of the volcano the activity was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level the products were dispersed across a wide area with the finest ash reaching the atlantic coast of argentina our field observations in the proximal medial area 3 25 km from the vent indicate that the may 2008 tephra deposit consists of numerous layers most of which can be correlated with individual eruptive events these layers vary from extremely fine grained ash to layers of lapilli and blocks composed of both juvenile and lithic material here we describe the stratigraphy and physical characteristics of the may 2008 deposits and propose a reconstruction of the timing of the may 2008 events the deposits are mainly associated with the three main explosive phases which occurred on 1st 2nd may 3rd 5th may and 6th may with an estimated bulk tephra volume of 0 5 1 0 km3 integration of both exponential and power law fitting for the 6th may event represented by a layer composed mainly of lithic lapilli and blocks 2 mm an isopleth map was compiled from which a 19 km plume height was determined which is in good agreement with satellite observations

surface cooling winds and eddies over the continental shelf abstractmodels show that surface cooling over a sloping continental shelf should give rise to baroclinic instability and thus tend toward gravitationally stable density stratification less is known about how alongshore winds affect this process so the role of surface momentum input is treated here by means of a sequence of idealized primitive equation numerical model calculations the effects of cooling rate wind amplitude and direction bottom slope bottom friction and rotation rate are all considered all model runs lead to instability and an eddy field while instability is not strongly affected by upwelling favorable alongshore winds wind driven downwelling substantially reduces eddy kinetic energy largely because the downwelling circulation plays a similar role to baroclinic instability by flattening isotherms and so reducing available potential energy not surprisingly cross shelf winds appear to have little effect analysis of the model runs leads to quantitative relations for the wind eff

omori s law applied to mining induced seismicity and re entry protocol development this paper describes a detailed study of the modified omori s law n t k c t p applied to 163 mining induced aftershock sequences from four different mine environments in ontario canada we demonstrate using a rigorous statistical analysis that this equation can be adequately used to describe the decay rate of mining induced aftershock sequences the parameters k p and c are estimated using a uniform method that employs the maximum likelihood procedure and the anderson darling statistic to estimate consistent decay parameters the method considers only the time interval that satisfies power law behavior the p value differs from sequence to sequence with most 98 ranging from 0 4 to 1 6 the parameter k can be satisfactorily expressed by k n1 where is an activity ratio and n1 is the measured number of events occurring during the first hour after the principal event the average values are in a well defined range theoretically 0 8 and empirically 0 3 0 5 these two findings enable us to develop a real time event rate re entry protocol 1 h after the principal event despite the fact that the omori formula is temporally self similar we found a characteristic time tmc at the maximum curvature point which is a function of omori s law parameters for a time sequence obeying an omori process tmc marks the transition from highest to lowest event rate change using solely the aftershock decay rate therefore we recommend tmc as a preliminary estimate of the time at which it may be considered appropriate to re enter an area affected by a blast or large event we found that tmc can be estimated without specifying a p value by the expression tmc an1b where a and b are two parameters dependent on local conditions both parameters presented well constrained empirical ranges for the sites analyzed a 0 3 0 5 and b 0 5 0 7 these findings provide concise and well justified guidelines for event rate re entry protocol development

the july 1986 oceanside ml 5 3 earthquake sequence in the continental borderland southern california an earthquake of m l 5 3 occurred at 32 58 7 n 117 51 5 w southwest of oceanside in san diego county at 13 47 13 july 1986 ut this main shock was followed by an extensive aftershock sequence with 55 events of m l 3 0 during july 1986 the epicenters of the main shock and aftershocks are located at the northern end of the san diego trough bahia soledad fault zone sdt bs where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault the northwest striking sdt bs is one of three strike slip fault systems that constitute the offshore agua blanca fault system the spatial distribution of the aftershocks indicates a unilateral 7 to 9 km long rupture to the east southeast away from the epicenter of the main shock the focal mechanism of the main shock also has an east southeast striking and south dipping plane with mostly reverse movement on it focal mechanisms of the m l 3 0 aftershocks show both reverse and strike slip movement the reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the san diego trough fault as movement is transferred to the west along the santa cruz catalina island escarpment some of the aftershocks that are located to the southeast of the main shock and have strike slip focal mechanisms suggest activation of the northwest trending san diego trough fault a stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of s30 w plunging 18 the maximum principal stress determined from 30 aftershocks that occurred from 15 july to 2 october 1986 has an azimuth of s20 w plunging 18 the values the measure of the relative sizes of the principal stresses are approximately 0 07 and 0 1 respectively indicating that the intermediate and minimum principal stress are of similar magnitude the results of the stress inversion and the focal mechanisms that showed reverse faulting suggest that the inner continental borderland offshore from oceanside is not currently a pure strike slip tectonic regime but rather a strike slip mixed with reverse faulting regime when the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated the change in stress can be described as a progressive counter clockwise rotation of 14 of the orientation of the maximum principal stress to a more southerly direction with the greatest change in stress orientation observed shortly after the main shock the abundance of aftershocks may be related to the large temporal variation in stress orientation that in turn may have resulted from the small stress drop of the main shock

cristobalite in a rhyolitic lava dome evolution of ash hazard prolonged and heavy exposure to particles of respirable crystalline silica rich volcanic ash could potentially cause chronic fibrotic disease such as silicosis in individuals living in areas of frequent ash fall here we show that the rhyolitic ash erupted from chaiten volcano chile in its dome forming phase contains increased levels of the silica polymorph cristobalite compared to its initial plinian eruption ash erupted during the initial explosive phase 2 5 may 2008 contained approximately 2 wt cristobalite whereas ash generated after dome growth began from 21 may 2008 contains 13 19 wt the work suggests that active obsidian domes crystallise substantial quantities of cristobalite on time scales of days to months probably through vapour phase crystallisation on the walls of degassing pathways rather than through spherulitic growth in glassy obsidian the ash is fine grained 9 7 17 7 vol 4 m in diameter the respirable range and the particles are mostly angular sparse fibre like particles were confirmed to be feldspar or glass

cristobalite in volcanic ash of the soufriere hills volcano montserrat british west indies crystalline silica mostly cristobalite was produced by vapor phase crystallization and devitrification in the andesite lava dome of the soufriere hills volcano montserrat the sub 10 micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass the sub 10 micrometer fraction of ash generated by explosive eruptions has much lower contents 3 to 6 percent of crystalline silica high levels of cristobalite in respirable ash raise concerns about adverse health effects of long term human exposure to ash from lava dome eruptions

heat flow and thermotectonic problems of the central ventura basin southern california the ventura basin southern california is located near the big bend area of the san andreas fault system within the transverse ranges physiographic province continuous equilibrium temperature logs were measured in 12 idle oil wells located within the onshore ventura avenue san miguelito filmore oxnard and west montalvo fields to an average depth of about 3100 m 10 200 feet thermal conductivities were measured on all available samples heat flows were calculated with the aid of a thermostratigraphic scheme based on correlative gradient intervals and average thermal conductivity for the appropriate units negative curvature of the ventura avenue temperature profiles may be explained by an increase in thermal conductivity associated with tectonic compaction of the underlying pliocene clastic sequence temperature profiles at fillmore are enigmatic but suggest highly unusual geotectonic conditions basinwide heat flow averages about 48 mw m2 a value which is low relative to most of southern california as heat flow does not vary systematically to the maximum measured depth of about 4 km this anomaly is not easily explained in terms of hydrologic effects or recent uplift and erosion however a diminution of heat flow is an expectable consequence of the accumulation of cold sediments up to 12 km since eocene time if 70 mw m2 is accepted as the background heat flow then the sedimentation effect is probably sufficient to explain the anomaly

aftershocks of the 13 may 1993 shumagin alaska earthquake the 13 may 1993 ms 6 9 shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1 5 by 1 june 1993 of these aftershocks 79 were located by using s p travel times at the only two stations within 570 km of the mainshock epicenter the rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1 0 m and a 28 bar stress drop the magnitude frequency plots give a b value for the aftershock sequence of about 0 4 which is low compared to the background value of approximately 0 8 the decay of the aftershock sequence followed the modified omori law with a p value of 0 79 which is also lower than the typical values of about 1 1 observed in alaska both of these facts can be interpreted as indicating relatively high ambient stress in the shumagin seismic gap and the possibility that the 13 may earthquake was a foreshock to a larger gap filling event to occur within the next few years

characterization of respirable volcanic ash from the soufriere hills volcano montserrat with implications for human health hazards volcanic ash generated in the long lived eruption of the soufriere hills volcano montserrat is shown to contain respirable sub 4 m particles and cristobalite a crystalline silica polymorph respirable particles of cristobalite can cause silicosis raising the possibility that volcanic ash is a respiratory health hazard this study considers some of the main factors which affect human exposure to respirable volcanic ash namely the composition and proportions of respirable ash and the composition and concentrations of airborne suspended particulates the composition size distribution and proportion by weight of respirable particles in representative samples of the soufriere hills tephra dome collapse ash fall deposits dome collapse pyroclastic flow matrix vulcanian explosion ash and mixed ash have been characterized dome collapse ash fall deposits are significantly richer in respirable particles 12 wt than the other tephra samples in particular the matrices of dome collapse pyroclastic flow deposits 3 wt within the respirable fraction dome collapse ash contains the highest proportion of crystalline silica particles 20 27 number of which 97 wt is cristobalite compared with other primary tephra types 0 4 5 6 number this enrichment of crystalline silica in the dome collapse ash is most pronounced in the very fine particle fraction sub 2 m the results are explained as being due to significant size fractionation during fragmentation of pyroclastic flows resulting in a fines depleted dome collapse matrix and a fines rich dome collapse ash deposit for all sample types the sub 4 m fraction comprises 45 55 wt of the sub 10 m fraction aeolian deposit lahar deposit and airborne samples of suspended ash collected on filters were characterized these samples show enrichment of crystalline silica in the respirable fraction 10 18 number the results are consistent with ash in the environment having a mixed origin but originating predominantly from dome collapse eruptions the reworked ash however contains low proportions of respirable ash 3 wt compared to primary ash samples the concentration of ash particles re suspended by road vehicles on montserrat is found to decrease exponentially with height above the ground indicating higher exposure for children compared with adults pm4 concentration at 0 9 m height of two year old child is 3 times that at 1 8 m adult height the composition of the re suspended road particles is similar to that re suspended by the wind

mount st augustine volcano fumarole wall rock alteration mineralogy zoning composition and numerical models of its formation process abstract intensely altered wall rock was collected from high temperature 640 c and low temperature 375 c vents at augustine volcano in july 1989 the high temperature altered rock exhibits distinct mineral zoning differentiated by color bands in order of decreasing temperature the color bands and their mineral assemblages are a white to grey tridymite anhydrite b pink to red tridymite hematite fe hydroxide molysite fecl 3 with minor amounts of anhydrite and halite and c dark green to green anhydrite halite sylvite tridymite with minor amounts of molysite soda and potash alum and other sodium and potassium sulfates the alteration products around the low temperature vents are dominantly cristobalite and amorphous silica with minor potash and soda alum aphthitalite alunogen and anhydrite compared to fresh 1986 augustine lava the altered rocks exhibit enrichments in silica base metals halogens and sulfur and show very strong depletions in al in all alteration zones and in iron alkali and alkaline earth elements in some of the alteration zones to help understand the origins of the mineral assemblages in altered augustine rocks we applied the thermochemical modeling program gasworks in calculations of a reaction of the 1987 and 1989 gases with wall rock at 640 and 375 c b cooling of the 1987 gas from 870 to 100 c with and without mineral fractionation c cooling of the 1989 gas from 757 to 100 c with and without mineral fractionation and d mixing of the 1987 and 1989 gases with air the 640 c gas rock reaction produces an assemblage consisting of silicates tridymite albite diopside sanidine and andalusite oxides magnetite and hercynite and sulfides bornite chalcocite molybdenite and sphalerite the 375 c gas rock reaction produces dominantly silicates quartz albite andalusite microcline cordierite anorthite and tremolite and subordinate amounts of sulfides pyrite chalcocite and wurtzite oxides magnetite sulfates anhydrite and halides halite the cooling calculations produce a anhydrite halite sylvite b cu mo fe and zn sulfides c mg fluoride at high temperature 370 c d chlorides fluorides and sulfates of mn fe zn cu and al at intermediate temperature 170 370 c and e hydrated sulfates liquid sulfur crystalline sulfur hydrated sulfuric acid and water at low temperature

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

seismotectonics of intermediate depth earthquakes from properties of aftershock sequences abstract aftershock sequences following two intermediate depth earthquakes have been analyzed to determine their spatial and temporal distributions the kanaga pass earthquake m b 6 0 occurred at 105 km depth in the central aleutian islands data were provided by the central aleutians seismic network the iwaizumi town earthquake m b 6 4 occurred at 75 km depth under northern honshu japan data were provided by the network of tohoku university for both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others with down dip compression for both both events were followed by numerous aftershocks that decayed with time initially according to the modified omori relation with p value close to 1 in both cases activity at a higher rate than that expected from the omori relation began after several months without a strong aftershock to trigger it the kanaga pass activity is modeled by two omori sequences the second beginning 64 days after the mainshock the p values are 0 92 and 1 04 respectively the corresponding b values are 1 01 and 1 03 the iwaizumi town sequence is modeled by a single omori sequence p 0 96 plus a constant rate of activity 0 25 events day for 63 days beginning 120 5 days after the mainshock the b value of the main sequence was 0 91 0 14 up to the start of the linear surge for which the b value was 1 23 0 47 no strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity in either case the cause of the secondary activity is not known the two sequences occurred in subduction zones with similar properties and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes a strong creep event or silent earthquake in a weakly coupled interplate contact zone is a plausible explanation for the secondary sequences the unusual setting of the iwaizumi event at the bottom of the upper plane of the double planed wadati benioff zone under honshu may play a role in that case

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

groundmass crystallisation and cooling rates of lava like ignimbrites the grey s landing ignimbrite southern idaho usa constraining magmatic and eruptive processes is key to understanding how volcanoes operate however reconstructing eruptive and pre eruptive processes requires the ability to see through any post eruptive modification of the deposit the well preserved grey s landing ignimbrite from the central snake river plain provides an opportunity to systematically investigate the post eruptive processes occurring through a single deposit sheet despite overall compositional homogeneity in both bulk and glass compositions the grey s landing ignimbrite does preserve differences in the abundance of li in plagioclase crystals which are strongly associated with the host lithology li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low average 5 ppm n 262 while plagioclase from the microcrystalline interior of the deposit has higher li contents average 33 ppm n 773 given that no other trace elemental parameter in plagioclase varies we interpret the variability in li to reflect a post depositional process groundmass crystallisation of a rhyolite like grey s landing requires 50 crystallisation of sanidine and variable amounts of a silica rich phase quartz tridymite cristobalite and plagioclase to satisfy mass balance we suggest the low affinity of li for sanidine causes migration of groundmass li into plagioclase during crystallisation even within the microcrystalline interior of the deposit the morphology of the groundmass varies the more marginal finer grained regions are dominated by cristobalite as the sio2 rich phase while tridymite and quartz are additionally found in the more slowly cooled coarser grained portions of thick sections of the ignimbrite numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement a maximum of a few years where the ignimbrite is thickest these numerical models also illustrate that the time at which the potential for rheomorphism ceases either the ignimbrite is a solid glass or 50 crystalline is diachronous across the deposit

formation of cristobalite nanofibers during explosive volcanic eruptions high resolution transmission electron microscopy hrtem observations of unaltered volcanic air fall deposits from the ongoing lava dome explosive eruption at chaiten volcano chilean patagonia revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash 240 c beta form of cristobalite with average lengths of hundreds of nanometers and widths on the order of tens of nanometers we propose that the beta cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide sio which is later oxidized to form one dimensional crystalline silica nanostructures nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer to nanometer sized fragments of silica glass in the volcanic column the formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short and long term health hazards associated with the respirable nanofibrous components of volcanic ash

rossby waves with continuous stratification and bottom friction abstractpublished observations of subinertial ocean current variability show that the vertical structure is often well described by a vertical mode that has a node of horizontal velocity at the bot

spatiotemporal evolution of the 2011 prague oklahoma aftershock sequence revealed using subspace detection and relocation the 6 november 2011 mw 5 7 earthquake near prague oklahoma is the second largest earthquake ever recorded in the state a mw 4 8 foreshock and the mw 5 7 mainshock triggered a prolific aftershock sequence utilizing a subspace detection method we increase by fivefold the number of precisely located events between 4 november and 5 december 2011 we find that while most aftershock energy is released in the crystalline basement a significant number of the events occur in the overlying arbuckle group indicating that active meeker prague faulting extends into the sedimentary zone of wastewater disposal although the number of aftershocks in the arbuckle group is large comprising 40 of the aftershock catalog the moment contribution of arbuckle group earthquakes is much less than 1 of the total aftershock moment budget aftershock locations are sparse in patches that experienced large slip during the mainshock

cristobalite in the 2011 2012 cordon caulle eruption chile cristobalite is a low pressure high temperature polymorph of sio2 found in many volcanic rocks its volcanogenic formation has received attention because 1 pure particulate cristobalite can be toxic when inhaled and its dispersal in volcanic ash is therefore a potential hazard and 2 its nominal stability field is at temperatures higher than those of magmatic systems making it an interesting example of metastable crystallization we present analyses by xrd sem epma laser raman and synchrotron ct of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011 2012 eruption of cordon caulle chile cristobalite was not detected in pyroclasts negating any concern for respiratory hazards but it makes up 0 23 wt of lava samples occurring as prismatic vapour deposited crystals in vesicles and or as a groundmass phase in microcrystalline samples textures of lava collected near the vent which best represent those generated in the conduit indicate that pore isolation promotes vapour deposition of cristobalite mass balance shows that the sio2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass textures of lava collected down flow were modified during transport in the insulated interior of the flow where protracted cooling additional vesiculation events and shearing overprint original textures in the most slowly cooled and intensely sheared samples from the core of the flow nearly all original pore space is lost and vapour deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite vapour deposition and devitrification act concurrently but semi independently both are promoted by slow cooling and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions with insight afforded by the fundamentally different emplacement geometries of flows and domes

shallow subduction tectonics and fault interaction the weber new zealand earthquake sequence of 1990 1992 a series of four large earthquakes with epicenters very close to one another occurred in 1990 1992 in the weber region of the southeast coast of the north island new zealand the region is one of oblique plate convergence and subduction the plate interface being at about 20 km depth the first event february 19 1990 ml 6 1 mw 6 2 occurred within the upper part of the subducting pacific plate on a steeply northwest dipping normal fault as defined by relocated aftershocks the mechanism of this event reflects downdip tension within the subducting plate due to slab pull forces as is common in new zealand the second event may 13 1990 ml 6 2 mw 6 4 occurred on an imbricate fault in the overlying australian plate dipping shallowly northwest with components of both thrusting and dextral strike slip motion the slip vector is similar to the direction of plate convergence showing no arc normal rotation as observed for subduction thrust events in other areas of oblique convergence it is unlikely that the spatial and temporal closeness of these events is coincidence a similar pair occurred to the south in 1942 but the coupling mechanism is not clear perhaps being related to aseismic slip downdip on the subduction interface two subsequent events both of ml 5 5 occurred on august 15 1990 and march 2 1992 and extended the aftershock zone of the deeper first main shock to the northeast matching the extent of the shallower second main shock mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to the relevant mainshock the time history of the aftershocks of the first deeper event shows a period of quiescence beginning 35 days before the second shallow event the mean magnitude of those events that did occur was high this observation cannot be explained by station biases weather or changes in analysis methods and may be due to stress changes induced by precursory aseismic slip which would discourage aftershock occurrence

a simple model for stratified shelf flow fields with bottom friction abstract the effect of bottom friction on the subinertial frequency motion of stratified shelf flow fields is studied in a two layer f plane model with idealized shelf and slope bottom topography coastal trapped fire waves and motion forced by the alongshore component of the wind stress at the coast are considered vertical turbulent diffusion effects are assumed to be present in thin surface and bottom boundary layers but not at the density interface simplifications are achieved by assuming that typical alongshore scales are larger than the offshore scales given by the internal rossby radius of deformation r and the shelf slope width that the upper layer depth is small compared with the lower layer depth and that the topography of the continental margin may be represented by a linear bottom slope of small magnitude some results are not dependent on the presence of variable bottom topography these are obtained first with a flat bottom ocean adjacent to a vertical coast a characteristic feature o

the respiratory health hazards of volcanic ash a review for volcanic risk mitigation studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years and mostly since the eruption of mt st helens in 1980 this review of all published clinical epidemiological and toxicological studies and other work known to the authors up to and including 2005 highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long term health risk silicosis non specific pneumoconiosis and chronic obstructive pulmonary disease in populations from prolonged exposure to ash due to persistent eruptive activity the acute and chronic health effects of volcanic ash depend upon particle size particularly the proportion of respirable sized material mineralogical composition including the crystalline silica content and the physico chemical properties of the surfaces of the ash particles all of which vary between volcanoes and even eruptions of the same volcano but adequate information on these key characteristics is not reported for most eruptions the incidence of acute respiratory symptoms e g asthma bronchitis varies greatly after ashfalls from very few if any reported cases to population outbreaks of asthma the studies are inadequate for excluding increases in acute respiratory mortality after eruptions individuals with pre existing lung disease including asthma can be at increased risk of their symptoms being exacerbated after falls of fine ash a comprehensive risk assessment including toxicological studies to determine the long term risk of silicosis from chronic exposure to volcanic ash has been undertaken only in the eruptions of mt st helens 1980 usa and soufriere hills montserrat 1995 onwards in the soufriere hills eruption a long term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population a more systematic approach to multi disciplinary studies in future eruptions is recommended including establishing an archive of ash samples and a website containing health advice for the public together with scientific and medical study guidelines for volcanologists and health care workers

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths

analysis of relaxation temporal patterns in greece through the retas model approach the temporal decay of eight aftershock sequences in the area of greece after 1975 was examined with main shocks magnitudes of mw 6 6 the analysis was done through the restricted epidemic type aftershock sequence retas stochastic model which enables the possibility to recognize the prevailing clustering pattern of the relaxation process in the examined areas in four of the cases the analysis selected the epidemic type aftershock sequence etas model to offer the most appropriate depiction of the aftershock temporal distribution which presumes that all shocks to the smallest ones in the sample can cause secondary aftershocks while for the rest of the sequences triggering potential seems to have aftershocks above a certain magnitude threshold retas model and they are expected to induce secondary activity the models developed on aftershock data were also applied to forecast real seismicity after the conclusion of the aftershock sequences for four out of eight cases we obtained promising estimations of ensuing seismicity after the end of the sequences with models based only on aftershock data some features of the retas model simulation were also studied like simulating magnitudes revealing that it is reasonable to consider in the model the temporal behavior of the aftershocks magnitudes as well stochastic modeling was also applied to estimate the duration of the relaxation process assuming that the end of each sequence is marked by the divergence of real seismicity from the modified omori formula mof model the latter known to represent pure aftershock activity the obtained results give an indication that possibly low stressing rate results in longer duration of the relaxation process in a region 2007 elsevier b v all rights reserved

sakurajima volcano a physico chemical study of the health consequences of long term exposure to volcanic ash regular eruptions from sakurajima volcano japan repeatedly cover local urban areas with volcanic ash the frequency of exposure of local populations to the ash led to substantial concerns about possible respiratory health hazards resulting in many epidemiological and toxicological studies being carried out in the 1980s however very few mineralogical data were available for determination of whether the ash was sufficiently fine to present a respiratory hazard in this study we review the existing studies and carry out mineralogical geochemical and toxicological analyses to address whether the ash from sakurajima has the potential to cause respiratory health problems the results show that the amount of respirable 4 m material produced by the volcano is highly variable in different eruptions 1 1 18 8 vol the finest samples derive from historical plinian eruptions but considerable amounts of respirable material were also produced from the most recent vulcanian eruptive phase since 1955 the amount of cristobalite a crystalline silica polymorph which has the potential to cause chronic respiratory diseases is 3 5 wt in the bulk ash scanning electron microscope and transmission electron microscope imaging showed no fibrous particles similar to asbestos particles surface reactivity tests showed that the ash did not produce significant amounts of highly reactive hydroxyl radicals 0 09 1 35 mol m 2 at 30 min in comparison to other volcanic ash types a basic toxicology assay to assess the ability of ash to rupture the membrane of red blood cells showed low propensity for haemolysis the findings suggest that the potential health hazard of the ash is low but exposure and respiratory conditions should still be monitored given the high frequency and durations of exposure