new insights into the mechanics of sill emplacement provided by field observations of the njardvik sill northeast iceland sills are concordant sheet like bodies of magma their mechanics of emplacement is an important but still not fully understood topic the well exposed basaltic njardvik sill in the extinct tertiary dyrfjoll volcano in northeast iceland offers exceptionally clear insights into the mechanism of sill emplacement the sill is multiple and consists of at least 7 units sills all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows each sill unit was supplied with magma from an inclined sheet the contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets since the njardvik sill consists of separate units it obviously did not evolve into a homogeneous magma body nevertheless the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections the local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills the njardvik sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill using these field observations as a basis a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile along which the sill emplacement takes place the results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties there the orientation of the maximum principal compressive stress 1 is horizontal hence such contacts can represent interfaces along which sill emplacement is encouraged once a sill has been emplaced it extends the stress field with a horizontal orientation of 1 consequently inclined sheets and dykes injected near the sill will be deflected into sills the injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma in case of the njardvik sill the injection frequency was low so subsequently emplaced sill units can be distinguished

shallow seismic source parameter determination using intermediate period surface wave amplitude spectra summary estimating reliable depths for shallow seismic sources is important in both seismo tectonic studies and in seismic discrimination studies surface wave excitation is sensitive to source depth especially at intermediate and short periods owing to the approximate exponential decay of surface wave displacements with depth a new method is presented here to retrieve earthquake source parameters from regional and teleseismic intermediate period 100 15s fundamental mode surface wave recordings this method makes use of advances in mapping globaldispersiontoallowhigherfrequencysurfacewaverecordingsatregionalandteleseismic distances to be used with more confidence than in previous studies and hence improve the resolution of depth estimates synthetic amplitude spectra are generated using surface wave theory combined with a great circle path approximation and a grid of double couple sources are compared with the data source parameters producing the best fitting amplitude spectra are identified by minimizing the least squares misfit in logarithmic amplitude space the f test is used to search the solution space for statistically acceptable parameters and the ranges of these variables are used to place constraints on the best fitting source estimates of focal mechanism depth and scalar seismic moment are determined for 20 small to moderate sized 4 3 mw 6 4 earthquakes these earthquakes are situated across a wide range of geographic and tectonic locations and describe a range of faulting styles over the depth range 4 29km for the larger earthquakes comparisons with other studies are favourable however existing source determination procedures such as the cmt technique cannot be performed for the smaller events by reducing the magnitude threshold at which robust source parameters can be determined the accuracy especially at shallow depths of seismo tectonic studies seismic hazard assessments and seismic discrimination investigations can be improved by the application of this methodology

wide aperture beamforming of depth phases by timescale contraction abstract teleseismic depth phase signals can be combined with constructive interference from stations spaced as much as 1000 km apart each trace is first deconvolved using an operator based on the p phase of that trace then its timescale is stretched or contracted to eliminate the moveout of pp p time with distance traces can then be added with constructive interference of depth phases out to 10 or more angular distance between the stations on the focal sphere depth phases pp or sp representing hypocentral depths ranging from 6 to 204 km were found using canadian national seismograph network cnsn data for 45 events recorded at ranges of 2000 to 10 800 km which had been reported in the reviewed event bulletins reb of the prototype international data center pidc including 23 events for which the reb reported no depth phases for 28 events for which a catalog depth reb or pde from usgs was available for comparison 18 had depths agreeing either within 6 km or within 15 7 can be explained by assuming an sp phase was misassociated as pp 1 is just outside the error bar of the free depth reported in the reb and only 2 disagree significantly thus using only canadian data depth phases were found by beamforming for many events for which no depth phases had been found by the pidc using worldwide data including arrays the depth phase beamforming method has the potential of substantially increasing the number of earthquakes for which teleseismic depth phases can be detected

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

thermal and tectonic consequences of india underthrusting tibet the tibetan plateau is the largest orogenic system on earth and has been influential in our understanding of how the continental lithosphere deforms beneath the plateau are some of the deepest 100 km earthquakes observed within the continental lithosphere which have been pivotal in ongoing debates about the rheology and behaviour of the continents we present new observations of earthquake depths from the region and use thermal models to suggest that all of them occur in material at temperatures of t 600 1c thermal modelling combined with experimentally derived flow laws suggests that if the indian lower crust is anhydrous it will remain strong beneath the entire southern half of the tibetan plateau as is also suggested by dynamic models in northwest tibet the strong underthrust indian lower crust abuts the rigid tarim basin and may be responsible for both the clockwise rotation of tarim relative to stable eurasia and the gradient of shortening along the tien shan

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

emplacement related fabric and multiple sheets in the maiden creek sill henry mountains utah usa the maiden creek sill is a small intrusion w 1k m 2 in map view that is part of the tertiary igneous complex of the henry mountains in southern utah field relationships indicate that 1 this sill has a complex map view geometry consisting of an elliptical main body from which radiate several finger like lobes 2 the intrusion is composed of two igneous sheets presumably representing distinct magma pulses and 3 the current exposure of the intrusion corresponds closely to the original intrusive geometry these relationships allow us to examine in detail the relationships between fabric development intrusive geometry and emplacement processes in this intrusion this sill was emplaced into the flat lying sediments of the colorado plateau during a time of tectonic quiescence fabrics within the intrusion can therefore be interpreted in terms of igneous emplacement processes fabric data are derived from field measurements image analysis x ray computed tomography and anisotropy of magnetic susceptibility in the center of each sheet magmatic fabrics are subhorizontal and poorly defined in the outermost 5 15 cm of each sheet solid state fabrics are developed sub parallel to the contact the two distinct intrusive sheets have similar composition thickness and areal extent we infer that emplacement of the first sheet established the initial map view geometry of the intrusion and the second sheet closely followed the contact between that first sheet and the host rock the example of the maiden creek sill demonstrates that sheeted emplacement can be an important process even in the early stages of construction of small intrusions in addition to other larger intrusions studied elsewhere q 2005 elsevier ltd all rights reserved

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

observation and prediction of dynamic ground strains tilts and torsions caused by the mw 6 0 2004 parkfield california earthquake and aftershocks derived from upsar array observationsdynamic ground strains tilts and torsions caused by the 2004 parkfield california earthquake

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

crustal and upper mantle structure under the tien shan from surface wave dispersion long period seismograms of nuclear explosions and natural earthquakes in central asia recorded at three digital stations of the laboratoire de detection geophysique ldg network in france attest to a significant propagation effect in the tien shan area this allows for an investigation of the deep structures under the range which has been the site of particularly intense intracontinental deformation since about the middle miocene group velocity curves have been obtained by regionalization for pure paths across either the central or eastern tien shan they indicate a crust of approximately 50 55 km in thickness and particularly low shear wave velocities under the central tien shan our study suggests lateral variation of the deep structures under the range which could be related to a westward increase along the range of cumulative amount and rate of shortening

accurate relative earthquake hypocenters reveal structure of the burma subduction zone the burma arc links the himalaya to the andaman sumatra trench and comprehension of the geological features in the region is vital to understand the active tectonic processes in the area subduction on the sumatra trench produced the devastating 2004 sumatra earthquake m w 9 0 but the mechanisms accommodating relative india sundaland motion in burma are still unknown previous seismological studies of the area use only earthquake catalog hypocenters so structural details of subducted material remain poorly understood here accurate relative hypocenters for 81 earthquakes are estimated using first arrivals picked from regional and teleseismic recordings depth determination is improved using measured travel time differences between p onsets and depth phases pp and sp the results clearly illustrate a slab 21 25 n dipping 25 at 40 80 km deep 40 at 80 120 km deep and 60 at 120 160 km deep and its strike follows the indo burman ranges at latitudes 25 n earthquakes become more shallow and a diffuse picture of hypocenters with an east west lateral discontinuity at depth is observed indicating lateral deformation of the slab previous studies suggest there is a transition at 90 km deep from shallower strike slip earthquakes to deeper thrust type events here accurate depth estimates combined with confirmation of two global centroid moment tensor solutions using body wave modeling shows that thrust and strike slip earthquakes both occur deeper than 100 km in the burma arc

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

evidence for a high velocity slab associated with the hindu kush seismic zone we used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the pamir hindu kush region in tadjikistan and afghanistan data from 29 analog seismic stations in tadjikistan and northern afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28 to 87 and covering a broad range of azimuths we inverted for velocity perturbations over a rectangular grid with a block size of 99 99 km the model extended to a depth of 350 km with a 50 km thick first layer and two 150 km thick deeper layers the results show a strong and well resolved zone of high velocities in the upper mantle at depths greater than 200 km coincident with the location of the hindu kush seismic zone no clear velocity perturbations are associated with the pamir seismic zone above 200 km little correlation is observed with the seismic zone but indications of thicker crust under the pamir and thinner crust under the tadjik depression are seen the high velocities are most likely caused by the presence of oceanic lithosphere at depth

emplacement of a xenolith rich sill lajitas texas abstract a 2 15 m thick hawaiite sill in the cretaceous boquillas formation west of the big bend national park texas locally contains a zone 1 12 m thick containing 25 vol xenoliths the inclusions ranging in size from 2 cm to 1 m are dominated by cumulate gabbros and white quartzite the mafic inclusions broke from the sidewall cumulates in a reservoir that fed the hawaiite sill many xenoliths show contacts between quartzite and igneous rock marked by clinopyroxene crystals perpendicular to the contacts the quartzite xenoliths are permeated variably by quenched mafic liquid now devitrified glass with plagioclase clinopyroxene smectite and some quartz grains are fringed by inverted tridymite stratigraphic reconstruction indicates that the sill was emplaced at a depth shallower than 0 5 km but the source of the paleozoic quartzite and sidewall cumulate xenoliths was at least 1 km below the sill the high concentration of xenoliths their lack of imbrication orientation or sorting the preservation of unbroken thin slabs of limestone and the absence of xenoliths at the top and bottom of the sill all support emplacement by plug flow of xenolith charged magma in the center lubricated by less viscous xenolith free magma at the margins crystal size distribution of plagioclase indicates that mean crystal residence time for cumulates was about 75 years and crystallization time at the aphyric upper margin of the sill was about 2 years

mechanisms and duration of non tectonically assisted magma emplacement in the upper crust the black mesa pluton henry mountains utah abstract a new study of black mesa pluton henry mountains colorado plateau utah usa indicates that it is a classic example of a small upper crustal pluton assembled over a few years by incremental amalgamation of discrete magma pulses the results of our petrostructural study of the pluton interior allow us to constrain the geometry kinematics and timing of the processes the symmetric internal fabric is interpreted as an evidence for a feeding by below and not laterally the observed rotation of the lineation from wnw ese on the very top to nne ssw below lead us to propose that the fabric at the base of the pluton is a record of magma infilling process and the fabric at the very top is a record of the strain due to the relative movement between magma and wallrocks a consequence is that except at the contact between pluton and wallrocks top and margins the stretching direction recorded by the lineation is not parallel to the flow direction of the magma i e displacement the black mesa pluton is a sheeted laccolith on its western edge and a bysmalith on its eastern edge this e w asymmetry in pluton geometry construction and the symmetrical internal fabric indicates that the apparently different west and east growth histories could have occurred simultaneously our field data indicate pluton growth through an asymmetric vertical stacking of sill like horizontal magma sheets one dimensional thermal models of the pluton provide maximum limits on the duration of its growth we have constrained the number the thickness and the frequency of magma pulses with our structural observations including 1 the emplacement of the pluton by under accretion of successive magma pulses 2 the absence of solid state deformation textures at internal contacts and 3 the apparent absence of significant recrystallization in the wallrocks our results suggest that the emplacement of the black mesa pluton was an extremely rapid event with a maximum duration on the order of 100 years which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m yr finally our data show that the rates of plutonic and volcanic processes could be similar a significant result for interpretation of magma transfer in arc systems

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

shallow sill and dyke complex in western hungary as a possible feeding system of phreatomagmatic volcanoes in soft rock environment abstract neogene alkaline basaltic rocks in the western pannonian basin are eroded remnants of maars tuff rings tuff cones scoria cones and lava fields the erosion level of these volcanoes is deep enough to expose diatreme zones associated with the phreatomagmatic volcanoes the erosion level is deeper yet in the west exposing shallow dyke and sill swarms related to former intra plate volcanoes the basanitic sills are irregular in shape and their lateral extent is highly variable individual sills reach a thickness of a few tens of metres and they commonly form dome like structures with rosette like radial columnar joint patterns the largest sill system identified in this region is traceable over kilometres and forms a characteristic ridge running north east to south west elevation differences in the position of the basanitic sills within an otherwise undisturbed layer cake like siliciclastic succession indicate emplacement of the basanite magma at multiple levels over kilometre scale distances the margins of sills in the system are irregular at a dm to mm scale undulating contacts of the sills together with gentle thermal alteration in the host sediment over cm to dm distances indicate the soft but not necessarily wet state of the host deposits at the time sills were intruded parts of the sill complex show a complicated relationship with the host sediment in form of peperitic zones and irregularly shaped disrupted peperite textures this is interpreted to reflect inhomogenities in water content and rheology of the siliciclastic deposits during intrusion the current summit of the longest continuous ridge preserves a small diatreme that seems to cut through an otherwise disk like sill indicating of relationship between sill emplacement and phreatomagmatic explosive eruptions

radial dikes laccoliths and gelatin models the northern adel mountains of central montana provide several excellent examples of laccoliths tangentially attached to feeder dikes that are members of radial swarms the radial distribution of the dikes appears to be controlled by stresses imposed by the load of an overlying central volcano model experiments using colored gelatin to simulate magma injected into a tank of uncolored gelatin duplicate the geometry of the radial dikes and field relations between feeder dikes and laccoliths as observed in the field

application of a cepstral f statistic for improved depth estimation the depth of a seismic event can be determined using peaks corresponding to depth phase delay times found in the cepstrum of the seismic signal until now however there has been no method available to determine the significance of various peaks in the cepstrum we have formulated a cepstral f statistic by using a classic approach to detect a signal in a number of stationarily correlated time series the method attaches a statistical significance level to peaks in the cepstrum of seismic data caused by echoes in the signal the method is particularly well suited for detecting depth phase echoes pp and sp recorded on regional seismic arrays detections determined from the peaks in the cepstral f statistic are then stacked as a function of pp p and sp p travel times predicted from the iaspei global 1 d model kennett and engdahl 1991 using a modification of the method of depth phase beamforming woodgold 1998 murphy et al 1999 tests on synthetic data show the method is most successful when the p wave arrival has a signal to noise ratio snr greater than 4 7 and the depth phase exhibits an snr greater than 2 tests on complex regional data suggest that the depth phase snr must be between 4 and 8 for consistent successful application of the method we tested the method by using events from the hindu kush region of afghanistan with well determined depths as recorded on arrays at teleseismic distances we determined the correct depths for 14 of the 19 events in this dataset 74 success rate and we determined incorrect depths for two events for a false alarm rate of 10 5 the difference between the depths calculated by the cepstral method and the prototype international data center pidc decrease with increasing magnitude for events with m b 3 8 the average difference between the cepstral and pidc depths was 7 9 km with no false alarms to test the operational capabilities of this method as a tool for data center use we first analyzed 32 events that occurred on 12 february 2000 as located by the pidc additionally we analyzed 29 events located in central and south america between 05 may and 15 june 2000 with event characteristics published by the international data center idc most of these events were also located by the national earthquake information center our method determined statistically significant depths for 40 of these 61 events with 11 having a low snr at three or more recording arrays while another 10 were either too shallow for analysis or did not exhibit depth phases only seven of the 61 events had depth phases listed in the reviewed event bulletin produced by the pidc and idc for these events the average difference between the reported and cepstral f statistic depths was 4 0 km the pidc idc fixed the depth of 26 of 61 events to 0 0 the cepstral f statistic method determined depths ranging from 16 to 92 km for 15 of the events that had been constrained as surface foci overall we believe the method would be most valuable when used by analysts to detect possible depth phases that could then verify or improve network calculated focal depths manuscript received 25 february 2001

interpretation of p seismograms from shallow undersea earthquakes the time between direct p and the surface reflections pp and or sp on teleseismic p seismograms is one of the most reliable methods of estimating the depth of a seismic source however correctly interpreting the p seismogram in terms of p pp and or sp is critical to the success of the method using observations from the north pacific intraplate earthquake of 7 march 1988 npe we show that the interpretation of p seismograms from shallow undersea earthquakes can be far from straight forward even in the absence of strong bathymetry gradients above the source the effect of an unconsolidated sedimentary layer on the seafloor and interference between the water reverberations pnwp and snwp where n indicates the n th reverberation can give apparently anomalous polarities on seismograms recorded by narrow band seismograph systems for cross correlation type analyses care must be taken when choosing the reference pulse because the pulse shape due to the water reverberations depends on the acoustic impedance contrast at the seafloor as well as on the relative amplitudes of upward radiated p and s

surface reflections and s to p conversions on p seismograms summary theoretical studies using seismic sources that radiate pulses of around 1 s duration or less show that for a double couple source in what are presumed to be realistic crustal structures the most prominent arrivals on short period sp p seismograms should be p pp and sp some seismograms that show p and the surface reflections are observed and can be adequately modelled however the number of such seismograms observed from shallow earthquakes is small usually observed seismograms are either simple that is they show a prominent first arrival and little else or the seismograms are complex showing 30s or more of arrivals with amplitudes greater than or equal to the amplitude at the onset the absence of clear surface reflections on both simple and complex seismograms is difficult to account for it may be of course that the absence of such reflections is the result of scattering and defocussing of the p and s waves by lateral variations in the crust above the source such a hypothesis is difficult to test without very detailed crustal structures for earthquake zones an alternative explanation of simple and complex sp seismograms proposed here is that many earthquakes may be due to unilateral fractures that propagate to increasing depth with time on steeply dipping fault planes for such sources the corner frequencies of pp and sp will be below the corner frequencies of p and s radiated downwards the consequence of this could be that the amplitude of pp and sp in the sp recording band is reduced relative to p if this happens then on paths where p leaves the source near an antinode in its radiation pattern simple seismograms would be observed on paths where p leaves the source near a node in the radiation pattern s to p conversions due to the incidence of downward travelling s on minor discontinuities in wavespeed in the mantle could be of significant amplitude relative to p and pp sp and produce complex seismograms examples are shown of observed complex and simple seismograms that can be explained on the unilateral fracture model

improved focal depth determination through automated identification of the seismic depth phases pp and sp a new fully automated processing algorithm based on network beamforming of raw automatic detection data has been implemented to improve the analyst s capability to identify the seismic depth phases pp and sp and thereby to improve focal depth estimation accuracy in seismic bulletins this algorithm has been tested on data recorded from a sample of 150 subcrustal earthquakes located in the pamir hindu kush hokkaido and central honshu seismic zones and has successfully identified candidate depth phases for over 70 of the events analyzed including some with magnitudes as low as m b 3 75 several supplemental analyst tools for validating such candidate depth phases identified by the network beamforming algorithm have also been implemented and evaluated in a preliminary fashion these include the f detector blandford 1974 which provides a quantitative statistical measure of whether a particular detection has a vector slowness consistent with that expected for a depth phase and the pearce 1977 1980 focal mechanism algorithm which provides a means for determining whether the observed candidate depth phase amplitude patterns across the network of observing stations are consistent with the regional tectonic environments in which the earthquakes occur results of the tests conducted to date indicate that the use of these automated analyst tools should significantly improve the confident identification of pp and sp phases for inclusion in hypocenter inversion analyses

saucer shaped sill with lobate morphology revealed by 3d seismic data implications for resolving a shallow level sill emplacement mechanism we use 3d seismic data to describe the 3d geometry of a large igneous intrusion the solsikke compound sill and address a number of issues related to sill emplacement the solsikke compound sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer shaped depressions and linear discontinuities one of the saucer shaped sub elements of the compound sill the solsikke lobate sill has a previously unrecognized morphology it has a basal feeder and consists of a bifurcating network of interlinked lobe shaped sill segments we propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments the lobate pattern indicates that the solsikke lobate sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation the feeder location is coincident with a fault intersection suggesting that magma transport from the underlying source exploited the zone surrounding the intersection

using regional geophysical databases to facilitate the identification of surface reflections on p seismograms the relative amplitude method allows the focal mechanism of an earthquake to be determined using the relative amplitudes of p and the surface reflections pp and sp observed on p seismograms recorded at long range regional structural information is helpful when interpreting p seismograms in terms of p pp sp and other arrivals such as near source s to p conversions here we re interpret array seismograms from the 1972 january 12 gulf of suez earthquake and demonstrate that many of the regionally dependent parameters required by the method can be obtained not only from conventional published sources but also from geophysical databases that can be easily accessed via the internet for example one interpretation of the p seismogram recorded by the seismometer array at gauribidanur india gba from the gulf of suez earthquake is that the first large amplitude arrival after p is pp followed by sp however modelling shows that this interpretation requires an anomalously high poisson s ratio in the above source structure after careful consideration of plausible arrival times and relative amplitudes of p pp and sp at gba and on seismograms recorded by two other arrays our preferred model has a focal mechanism with s 105 85 165 and a source depth of 7 8 km in a poisson crust 23 km thick in our preferred model the first large amplitude arrival after p at gba is not pp but an s to p conversion at the moho independent studies suggest the moho is about 25 km deep in the epicentral region our example demonstrates that the relative amplitudes of p and surface reflections have the potential to identify seismic disturbances that are too deep to be possible violations of the comprehensive nuclear test ban treaty ctbt the availability of data from seismometer arrays within the international monitoring system and easily accessible geophysical databases containing regional structural parameters will allow the relative amplitude method to be used to help verify the ctbt

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

on the probability of mistaking an earthquake for an explosion using the simplicity of p abstract the observation that p waves from an earthquake are generally more complex than those from an underground explosion was first noted in the 1960s this difference in p wave character may still be the only way of identifying small earthquakes close to the detection threshold of a given station network however there are certain earthquakes that produce simple seismograms the frequency at which such earthquakes are mistaken for explosions using the complexity criterion is known as the false alarm rate a simple seismogram can be defined as having only one large amplitude phase that can be caused by several mechanisms we examine the proportion of simple seismograms produced by a randomly oriented double couple source buried in three different crustal structures observed at a single station at a range of epicentral distances the model predicts that if the hypocenter of a small earthquake near the detection threshold is in a region with thick sediments then the false alarm rate at a single station can be as high as 50 in the teleseismic distance range 30 to 90 the results suggest that the false alarm rate is minimized if 1 array stations are used 2 the station is at a large epicentral distance 3 the source region structure has few layers or 4 the disturbance is recorded at more than one station

geometry and growth of sill complexes insights using 3d seismic from the north rockall trough doleritic sill complexes which are an important component of volcanic continental margins can be imaged using 3d seismic reflection data this allows unprecedented access to the complete 3d geometry of the bodies and an opportunity to test classic sill emplacement models the doleritic sills associated with basaltic volcanism in the north rockall trough occur in two forms radially symmetrical sill complexes consist of a saucer like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined commonly ragged outer rim bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano with an elongate concave upwards trough like geometry bilaterally symmetrical sills climb away from the magma source from which they originate both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge producing hybrid sill complexes radial sill complexes consist of a series of radiating primary flow units with dimensions up to 3 km each primary flow unit rises from the inner saucer and is fed by primary magma tube primary flow units contain secondary flow units with dimensions up to 2 km each being fed by a secondary magma tube branching from the primary magma tube secondary flow units in turn are composed of 100 m scale tertiary flow units a similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex this branching flow pattern as well as the interaction between flow units of varying orders provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger the data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature these features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening the branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex suggesting that the general features of the laccolith emplacement model also apply

magma and sediment i emplacement mechanism of late carboniferous tholeiite sills in northern britain president s anniversary address 1981 like other great tholeiite sheets of the world the midland valley sill and whin sill were emplaced in thick accumulations of sediments which at the time of intrusion were not much older nor had been orogenically deformed however the carboniferous sediments of northern britain had been deposited under the influence of continuous differential subsidence so that they were already disposed in syn sedimentary basins and swells when intrusion began by drawing palinspastic contours of the sills relative to a late westphalian marine band approximating to the surface at the time of emplacement it is established that the dolerites mainly follow bedding planes dipping at angles of as much as 5 down to the bottoms of the basins isopachs of the sills indicate a further relationship for the intrusions are thickest at the bottoms of the basins these facts are not explained by any intrusion mechanism previously postulated though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step and stair transgression of sediment bedding it is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0 5 and 1 0 km below the contemporary surface thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up dip under pressure of head the sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow

hydroacoustic infrasonic and seismic monitoring of the submarine eruptive activity and sub aerial plume generation at south sarigan may 2010 abstract explosive submarine volcanic processes are poorly understood due to the difficulties associated with both direct observation and continuous monitoring in this study hydroacoustic infrasound and seismic signals recorded during the may 2010 submarine eruption of south sarigan seamount marianas arc are used to construct a detailed event chronology the signals were recorded on stations of the international monitoring system which is a component of the verification measures for the comprehensive nuclear test ban treaty numerical hydroacoustic and infrasound propagation modelling confirms that viable propagation paths from the source to receivers exist and provide traveltimes allowing signals recorded on the different technologies to be associated the eruption occurred in three stages separated by three hour periods of quiescence 1 a 46 h period during which broadband impulsive hydroacoustic signals were generated in clusters lasting between 2 and 13 min 95 of the 7602 identified events could be classified into 4 groups based on their waveform similarity the time interval between clusters decreased steadily from 80 to 25 min during this period 2 a five hour period of 10 hz hydroacoustic tremor interspersed with large amplitude broadband signals associated infrasound signals were also recorded at this time 3 an hour long period of transient broadband events culminated in two large amplitude hydroacoustic events and one broadband infrasound signal a speculative interpretation consistent with the data suggests that during phase 1 transitions between endogenous dome growth and phreatomagmatic explosions occurred with the magma ascent rate accelerating throughout the period during phase 2 continuous venting of fragmented magma occurred and was powerful enough to breach the sea surface during the climactic phase 3 discrete powerful explosions occurred and sufficient seawater was vaporised to produce the contemporaneous 12 km altitude steam plume

modification of the regional stress field by magma intrusion and formation of tabular granitic plutons granitic intrusions typically have large aspect ratios with an horizontal major axis about 4 8 times the vertical one combined structural and geophysical gravity data allows distinction between two main types of plutons flat floored plutons the most represented are rather thin 3 4 km and extend in every horizontal direction with a gently dipping floor toward several root zones these contrast with the thick 10 km wedge shaped plutons more elongated in one direction with a few root zones we interpret the shape of flat floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma magma is preferentially emplaced into the plane 1 2 perpendicular to the least principal stress component 3 this plane is initially vertical except for compressional conditions this dilation causes a local re organization of the stress field by increasing the minor and intermediate principal stress components when they overcome the lithostatic load a drastic change in the orientation of the opening plane results switching from vertical to horizontal this constitutes a change from vertically oriented dike shaped intrusions to sub horizontal laccoliths crustal anisotropies vertical faults or horizontal zones of different rheology also contribute to modify the shape of the intrusions the continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings rather we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement

geology of the chinese nuclear test site near lop nor xinjiang uygur autonomous region china abstract the chinese underground nuclear test site in the kuruktag and kyzyltag mountains of the xinjiang uygur autonomous region of northwest china is the location of sixteen underground tests that occurred between 1969 and 1992 the largest test to date conducted on 21 may 1992 had a reported yield of about one megaton geophysical properties of the rocks and a large scale geologic map of part of the test area were published by the chinese in 1986 and 1987 and are the first site specific data available for this test site in areas of low relief underground nuclear testing has occurred below the water table in shafts drilled vertically into dense low porosity paleozoic granitic and metasedimentary rocks additional testing in areas of more rugged terrain has occurred in horizontal tunnels probably abovethe water table at least one of these tunnels was driven into granite the upper 50 m of the rock in the area of the vertical tests is weathered and fractured these conditions have been shown to influence the magnitude of the disturbance of the land surface after a nuclear explosion these descriptions suggest hard rock coupling at depth and a closer resemblance to the former soviet test site in eastern kazakhstan than to the u s test site in nevada

accurate focal depth determination of oceanic earthquakes using water column reverberation and some implications for the shrinking plate hypothesis a b s t r a c t investigation of oceanic earthquakes is useful for constraining the lateral and depth variations of the stress and strain rate fields in oceanic lithosphere and the thickness of the seismogenic layer as a function of lithosphere age thereby providing us with critical insight into thermal and dynamic processes associated with the cooling and evolution of oceanic lithosphere with the goal of estimating hypocentral depths more accurately we observe clear water reverberations after the direct p wave on teleseismic records of oceanic earthquakes and develop a technique to estimate earthquake depths by using these reverberations the z h grid search method allows the simultaneous determination of the sea floor depth h and earthquake depth z with an uncertainty less than 1k m which compares favorably with alternative approaches we apply this method to two closely located earthquakes beneath the eastern pacific these earthquakes occurred in 25 ma old lithosphere and were previously estimated to have similar depths of 10 12 km we find that the two events actually occurred at dissimilar depths of 2 5 km and 16 8 km beneath the seafloor respectively within the oceanic crust and lithospheric mantle the shallow and deep events are determined to be a thrust and normal earthquake respectively indicating that the stress field within the oceanic lithosphere changes from horizontal deviatoric compression to horizontal deviatoric tension as depth increases which is consistent with the prediction of lithospheric cooling models furthermore we show that the p axis of the newly investigated thrust faulting earthquake is perpendicular to that of the previously studied thrust event consistent with the predictions of the shrinking plate hypothesis

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

effect of interarray elevation differences on infrasound beamforming summary the international monitoring system infrasound network will upon completion contain 60 microbarometer arrays with apertures of between 1 and 4 km these arrays are located within a variety of terrains leading to large ratios of interelement elevation differences to array aperture for those arrays situated in areas of significant topography systematic errors in beamforming estimates caused by neglecting the vertical extent of the arrays are quantified for both signal backazimuth and apparent velocity of the 43 arrays certified as of 2011 january i14cl on juan fernandez island has the greatest topography across an array with a least squares fitted plane through the array elements having an 8 1 slope from the horizontal compared to a network mean of 1 6 beamforming errors both backazimuth and apparent velocity are a function of the arrival azimuth and become increasingly large for steeply inclined arrivals such that systematic errors will be significantly larger for signals returned from the thermosphere compared to those from the stratosphere at several arrays azimuthal errors due to array topography are comparable in magnitude to deviations often associated with atmospheric propagation these findings are illustrated using signals recorded in greenland at i18dk where differences between results processed using both full 3 d array geometry and the 2 d topography neglected approximation exhibit good correspondence to theoretical predictions

earthquake depth estimation using the f trace and associated probability determining the depths of seismic disturbances is of great importance when monitoring compliance with the comprehensive nuclear test ban treaty ctbt in plate tectonic studies and in seismic hazard assessments we have developed a semiautomated method that uses the output from the f detector a processing technique that detects correlated signals crossing a seismometer array as a tool for identifying candidate depth phases and hence estimating hypocenter depth the f trace can be interpreted approximately in terms of probability this can be used to help interpret signals and identify candidate depth phases here a method of calculating f probability traces that optimally highlight candidate depth phases is presented the values of the probability trace at the predicted times for pp and sp for a given depth are then weighted and summed over several high quality medium aperture array stations a clear maximum in the trace sum function with depth is interpreted as the probable hypocenter depth results for earthquakes from the lop nor and hindu kush regions indicate that this method of depth estimation is effective for many earthquakes with magnitude greater than about m b 4 0 and depths greater than around 10 km the method is best suited as a tool to be used by analysts to estimate the depths of individual seismic disturbances of special interest or groups of earthquakes in regions where there is a good azimuthal distribution of medium aperture array stations

toward more realistic formulations for the analysis of laccoliths abstract the published laccolith analyses are based on the linear plate bending theory and the a priori assumption that the width of the laccolith is fixed this is not the case in an actual situation the dimension of the laccolith in the horizontal plane has to result from an additional matching condition at the separation lines the published analyses are generalized by dropping the a priori assumption that the width of the laccolith is prescribed by assuming that the magmatic pressure is not constant and by taking into consideration the vertical compressibility of the overburden plate and base in the contact region in order to determine the magnitude of the magmatic pressure a condition is postulated that equates the measured volume of the intruded magma in a laccolith with the corresponding analytical expression for the volume the obtained closed form solution appears to satisfy many of the intuitive expectations it was evaluated numerically and the results are presented as graphs it may be concluded that even very small laccoliths may exist provided the magmatic pressure is sufficiently larger than the overburden weight we also show the dependence of the laccolith size on its stratigraphic position the thicker the overburden h the larger the size of the laccolith for an overburden plate of given thickness the larger the volume v of the intruded magma the larger the laccolith width 2a and its height the paper concludes by discussing a published analysis for laccolith with flexible underburden and overburden it is shown that this analysis is based on a formulation that is of questionable validity

emplacement mechanism of the great whin and midland valley dolerite sills the great whin and midland valley dolerite sill complexes appear to have been emplaced laterally from the walls of feeder dykes the overall thickness of each sill increases with depth as would be expected if the magma finally reached a state of hydrostatic equilibrium variations in the thickness of the sills with estimated intrusion depth imply that the head of magma was about 100 m below the contemporary ground surface in the areas of the present day outcrops at the end of the intrusive episodes before hydrostatic equilibrium was established the magma pressure would probably have been somewhat greater so it is likely that intrusion of the sills was accompanied by the extrusion of flood basalts step and stair transgressions of the bedding are commonly found within the sills mostly stepping downwards in the direction of bedding dip the reason for this directionality is that the weight of sediments floating on an intruding sill has a downdip component that applies a tensile stress to intersecting fractures below the sill when magma is moving downdip and to intersecting fractures above the sill when magma is moving updip

northern chile earthquake of 1980 may 26 evidence of unilateral fracture summary p seismograms recorded in europe from some of the earthquakes that occur at depths of around 100 km in northern chile show three prominent arrivals p bp and pp bp being interpreted as a conversion of downward radiated s to p at a discontinuity in wavespeed beneath the source on broad band seismograms the duration of the p pulse is around 5s that of bp around 2s and that of pp around 12s these differences in duration have been interpreted as doppler effects due to the motion of the source of seismic radiation this being assumed to be a near unilateral fracture propagating downwards on a steeply dipping fault plane for the downward radiated pulses p and bp the source is travelling towards the observer whereas for the upward radiated pulse pp the source is receding from the observer the near unilateral fracture interpretation of the differences in pulse duration is based on data from the earthquake of 1976 november 30 recorded on effectively one azimuth 30 40 which is towards europe to give further support to the interpretation requires data from a wider range of azimuths but this is not available for the 1976 earthquake however data from the northern chile earthquake of 1980 may 26 are available from four stations covering an azimuthal range of 180 eskdalemuir scotland eka azimuth 31 6 blacknest england bna azimuth 35 6 gauribidanur india gba azimuth 95 2 and warramunga australia wra azimuth 212 0 the 1980 earthquake which appears to have a very similar mechanism to that of the 1976 earthquake is investigated here to see if the observations are consistent with the near unilateral fracture mechanism the first motion of p and pp where these can be observed and the general form of the broad band seismograms are consistent with the presence of a nodal plane dipping steeply towards the east p is observed to be small relative to the surface reflection pp at eka bna and gba where p leaves the source close to a node and p is large relative to pp at wra where p leaves the source away from the node and the take off of pp is close to a node taking the steeply dipping nodal plane to be the fault plane the fault dimensions and fracture speeds compatible with the pulse durations are estimated using one of the compatible solutions fault length 40 km maximum width 18 km fracture speed 0 5p wavespeed maximum dimension oriented downdip seismograms are computed and compared to the observed the computed seismograms simulate the pulse durations and relative amplitudes of most of the main pulses the detailed form of the main pulses is not modelled possibly because the fault model used is oversimplified the complexity of the observed pulses presumably indicates that the distribution of slip on the fault plane varies less smoothly than assumed in the model to determine the detailed distribution and time history of slip on the fault requires data from a larger number of well distributed stations than is available here however if it is accepted that the pulse durations of p bp and pp given are reliable and that hp is an s to p conversion at a boundary below the source then the conclusion seems to be inescapable that the source was a near unilateral fracture propagating downwards on a steeply dipping fault plane

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

multi objective analysis of body and surface waves from the market rasen uk earthquake summary the 2008 february 27 market rasen uk earthquake mb 4 5 is used to show how earthquake source parameters can be estimated using a multi objective optimization approach to the joint inversion of teleseismic body wave and regional surface wave observations to estimate the source depth short period teleseismic seismograms are interpreted in terms of p and the depth phases pp and sp using the f statistic and its associated probability theory results of this analysis are inconclusive as two possible source depths are identified to identify our preferred depth a method of estimating source depth and mechanism by modelling surface wave amplitude spectra is applied to data recorded at regional distances a source depth of 22 km is found to be consistent with the observed body and surface waves upper and lower amplitude bounds and polarities are then placed on the phases interpreted as p pp and sp for a source depth of 22 km the gaussian relative amplitude method is used to calculate a likelihood for all possible orientations of the double couple source a multi objective optimization approach can then used to compare these likelihoods with calculated misfits between observed and synthetic surface wave amplitude spectra for all orientations of the double couple source our preferred focal mechanism s 205 50 and 15 is found by normalizing and summing both misfits to find the focal mechanism with the lowest combined misfit synthetic body wave seismograms surface wave amplitude spectra and seismograms are in good agreement with the observed data for our preferred focal mechanism and depth

an enigmatic earthquake in the continental mantle lithosphere of stable north america abstract the existence of earthquakes within continental lithospheric mantle remains a highly controversial topic here we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the wind river range of central wyoming combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically observed depth phases we demonstrate that the 2013 wind river earthquake a m w 4 7 highly oblique thrust faulting event occurred at 75 8 km well beneath the base of the crust the magnitude mechanism and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere as a result of tectonic rather than magmatic processes