focal mechanisms and locations of earthquakes in the vicinity of the 1975 kalapana earthquake aftershock zone 1970 1979 implications for tectonics of the south flank of kilauea volcano island of hawaii the november 29 1975 kalapana earthquake of magnitude 7 1 on the south flank of kilauea volcano hawaii represents a major rift tectonic event it is generally understood that the earthquake resulted from movement on a nearly horizontal fault plane with the crustal block south of kilauea s east rift zone moving south southeastward up to several meters well recorded earthquakes on the south flank of kilauea from 1970 to 1979 were analyzed and focal mechanisms determined to obtain a better understanding of the details of south flank tectonics included are a large number of earthquakes from the aftershock sequence of the 1975 earthquake hypocenters are distributed in a subplanar zone that dips 2 to 3 to the west at a depth of about 8 9 km depth we interpret this pattern to coincide with a portion of the slip plane of the kalapana earthquake and to be consistent with the notion of the slip plane occurring in a weak layer at the top of the old oceanic crust focal mechanisms are remarkably constant throughout this time period both before and after the 1975 event and closely agree with the mainshock mechanism p axis azimuths and slip vectors for the low angle fault planes are normal to the middle and lower east rift zones of kilauea there is suggestion of a change in the orientation of p axes from shallower to steeper plunge at the time of the kalapana earthquake such a change is consistent with a general reduction in the horizontal compressional stress normal to the rift zone at the time of the large earthquake no other evidence of precursory reorientation of the stress field was found and the precise predictability of this event remains in doubt however a general model of the tectonics of the volcanic rift system is now available that satisfies a number of geological and geophysical observations

geochemical precursors to seismic activity studies of earthquake precursory phenomena during the last several decades have found that significant geophysical and geochemical changes can occur prior to intermediate and large earthquakes among the more intensely investigated geochemical phenomena have been 1 changes in the concentrations of dissolved ions and gases in groundwaters and 2 variations in the concentrations of crustal and mantle volatiles in ground gases the concentration changes have typically showed no conanomalies trend either increasing or decreasing and the spatial and temporal distribution of the observed anomalies have been highly variable as a result there is little agreement on the physical or chemical processes responsible for the observed anomalies mechanisms proposed to account for precursory groundwater anomalies include ultrasonic vibration pressure sensitive solubility pore volume collapse fracture induced increases in reactive surfaces and aquifer breaching fluid mixing precursory changes in soil gas composition have been suggested to result from pore volume collapse micro fracture induced exposure of fresh reactive silicate surfaces and breaching of buried gas rich horizons an analysis of the available field and laboratory data suggests that the aquifer breaching fluid mixing ab fm model can best account for many of the reported changes in temperature dissolved ion and dissolved gas concentrations in groundwater ultrasonic vibration and pressure sensitive solubility models cannot reasonably account for the geochemical variations observed and although the pore collapse model could explain some of the observed chemical changes in groundwater and ground gas uncertainties remain regarding its ability to generate anomalies of the magnitude observed other geochemical anomalies in particular those associated with hydrogen and radon seem best accounted for by increases in reactive surface areas irsa model that may accompany precursory deformation around the epicenter of an impending earthquake analysis of the probable response of these models to the earthquake preparation process as well as to other environmental factors suggests that geochemical monitoring programs can provide information that may be valuable in forecasting the probability of an earthquake however because of the complexity of the earthquake preparation process the absolute prediction of seismic events using geochemical methods alone does not presently appear to be feasible

crustal structure of mauna loa volcano hawaii from seismic refraction and gravity data abstract in october 1978 the u s geological survey established a 100 km long seismic refraction profile normal to the kona coast of hawaii island analysis of these data along with available gravity data suggest that the oceanic crust dips about 3 landward under the submerged flank of the island increasing to 8 5 under the kona coast maximum vertical deflection of the base of crust from beneath the deep ocean to a point beneath the summit of mauna loa is about 9 km high velocity high density rocks vp about 7 1 km sec density about 2 9 gm cm 3 comprise the bulk of the volcanic edifice and reach to within a few kilometers of the surface beneath the north flank of mauna loa the data also suggest that an elongate high velocity high density body lies parallel to the kona coast just below the surface this body probably represents an extinct buried rift zone

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

volcano tectonic implications of 3 d velocity structures derived from joint active and passive source tomography of the island of hawaii 1 we present a velocity model of the onshore and offshore regions around the southern part of the island of hawaii including southern mauna kea southeastern hualalai and the active volcanoes of mauna loa and kilauea and loihi seamount the velocity model was inverted from about 200 000 first arrival traveltime picks of earthquakes and air gun shots recorded at the hawaiian volcano observatory hvo reconstructed volcanic structures of the island provide us with an improved understanding of the volcano tectonic evolution of hawaiian volcanoes and their interactions the summits and upper rift zones of the active volcanoes are characterized by high velocity materials correlated with intrusive magma cumulates these high velocity materials often do not extend the full lengths of the rift zones suggesting that rift zone intrusions may be spatially limited seismicity tends to be localized seaward of the most active intrusive bodies low velocity materials beneath parts of the active rift zones of kilauea and mauna loa suggest discontinuous rift zone intrusives possibly due to the presence of a preexisting volcanic edifice e g along mauna loa beneath kilauea s southwest rift zone or alternatively removal of high velocity materials by large scale landsliding e g along mauna loa s western flank both locations also show increased seismicity that may result from edifice interactions or reactivation of buried faults new high velocity regions are recognized and suggest the presence of buried and in some cases previously unknown rift zones within the northwest flank of mauna loa and the south flanks of mauna loa hualalai and mauna kea

multichannel seismic evidence for variations in crustal thickness across the molokai fracture zone in the mid pacific coincident multichannel seismic reflection and refraction data from a n s transect near oahu hawaii provide evidence for thickening of the pacific crust by 1 2 1 km south of the large offset 16 m y molokai fracture zone fz tau p stacks tau sum inversions and forward modeling of the refraction data indicate that the crustal thickening occurs primarily within the lower portion of seismic layer 2 assuming isostatic balance the differences in crustal thickness predict that seafloor having the same age will have different elevations across the fz observations of sea floor depths across the fz east of the hawaiian islands are consistent with this prediction implying that the processes which have generated the crustal differences have been stable for over 50 m y previous correlations between the chemical composition of ridge crest basalts crustal thickness and ridge crest elevation have been attributed to variations in the thermal regime of the upper mantle under mid ocean spreading centers in accord with this hypothesis we propose that the observed differences in crustal structure across the molokai fz may have been produced by small 25 c differences in the thermal regime of the upper mantle beneath the ancestral east pacific rise discontinuous intracrustal reflections located about 1 6 s below the sediment basement interface are observed in migrated reflection data south of oahu these reflections are similar in character to the lower crustal horizon r event observed in the western north atlantic shallower intracrustal reflections possibly from within seismic layer 2 are also observed the observation of these intracrustal reflections in both the atlantic and pacific oceans suggests that they are a fundamental signature of the crustal accretion process at a variety of spreading rates and that they are mappable using modern seismic reflection refraction methods

flexure and seismicity beneath the south flank of kilauea volcano and tectonic implications we present a refined model for the tectonic behavior of kilauea volcano s south flank including flexure calculations and microearthquake data supportive of the model in our model the south flank moves seaward over the downwardly flexed ocean crust in a manner analogous to an accretionary prism in a subduction zone the flank is driven seaward by the gravitational stresses inherent in its shape this driving force is augmented by high density material in kilauea s rift zones elastic flexure calculations predict a configuration for the downwardly flexed crust which agrees with previous seismic refraction and gravity modeling this configuration in turn is consistent with the hypothesis that many earthquakes occur along the volcanic pile ocean crust interface focal mechanisms for these events predominantly indicate southeastward directed overthrusting we also propose a relationship between rift zone intrusion and south flank displacement including a suggestion that the decollement configuration controls the relative activity of kilauea s east and southwest rift zones

the stratigraphy correlation provenance and palaeogeography of the skiddaw group ordovician in the english lake district a new lithostratigraphy is presented for the skiddaw group lower ordovician of the english lake district two stratigraphical belts are described five formations are defined in the northern fells belt ranging in age from tremadoc to early llanvirn they are all mudstone or sandstone dominated of turbidite origin in ascending order they are named the bitter beck watch hill hope beck loweswater and kirk stile formations two formations are defined in the central fells belt ranging in age from late arenig to llanvirn these are the buttermere formation a major olistostrome deposit overlain by the tarn moor formation consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds a revised graptolite and new acritarch biostratigraphy for the skiddaw group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub assemblages the provenance of the group is assessed from detailed petrographical and geochemical work this suggests derivation in the early ordovician largely from an old inactive continental arc terrane lying to the south east with the appearance of juvenile volcanic material in the llanvirn comparisons and correlations of the skiddaw group are made with the isle of man and eastern ireland

seismic detection of a low velocity layer beneath the southeast flank of mauna loa hawaii the authors have identified seismic phases reflected off the top and bottom of a low velocity layer lvl by analysis of seismograms from six small earthquakes in hawaii these events occurred almost directly beneath station ain of the hawaiian volcano observatory hvo seismic network located within the kaoiki seismic zone on the southeast flank of mauna loa the polarity reversals of the first reflected phases provide clear evidence for the existence of a lvl beneath this station the lvl is estimated to be at a depth of about 11 5 km with a thickness of about 800 m the estimated depth to the top of the lvl is consistent with the depth distribution of low angle thrust faulting events from the aftershock sequence of the 1983 kaoiki earthquake presumably this zone is the marine sediment layer buried beneath the volcanic pile along which the volcanic edifice may slip easily to cause large earthquakes mapping the interface between the volcanic pile and oceanic crust and thus determining the depth and thickness of the buried sediment layer is essential for understanding the tectonics of large earthquakes in hawaii

a study of seismic reflection imaging using microearthquake sources abstract we present a method for processing three component digital recordings of microearthquakes to obtain near vertical reflection profiles in regions of shallow seismicity the processing includes magnitude and focal depth normalization and event stacking where stacking is by small localized groups with ray theoretical time and distance corrections applied to compensate for varying focal depths in areas with high seismicity this procedure allows earthquakes to be treated as controlled sources to probe layered structures of the deep crust and upper mantle the validity of our approach is examined using aftershocks of the borah peak idaho earthquake m s 7 3 several thousand events occurred in a nnw trending zone about 10 km wide 75 km long and 15 km deep a small 10 10 km array of nine university of wisconsin three component triggered short period digital seismographs was installed in the region of aftershock activity over a 10 day period about 1000 useable events were recorded of which about 120 have been used for this study hypocenters have been computed using both p and s wave arrivals the latter being essential for stable solutions of events outside the network the borah peak data have been processed to obtain shear wave reflection profiles for the central station station 8 of the digital station array the stacked shear wave transverse record sections reveal coherent reflections from horizons at mid crustal to moho depths the most prominent reflections are from crustal discontinuities in the depth range 18 28 km coherent reflections can be obtained only through stacking which is necessary to improve the signal to noise ratio the major sources of data scatter as manifested by smearing of reflections on the stacked records are crustal heterogeneity and errors in the determination of focal depth and origin time

upper llandovery and wenlock graptolites from prairie creek southern mackenzie mountains northwest territories department of geology the university of western ontario london ont canada n6a 5b7 received january 13 1988 revision accepted april 6 1988 a moderately rich fauna of graptolites comprising species dominated by monograptus sensu stricto retiolites stomato graptus and cyrtograptus is documented from prairie creek southern mackenzie mountains northwest territories grapto lites of the upper llandovery monograptus turriculatus monograptus spiralis and cyrtograptus sakmaricus zones and of the wenlock unnamed zone and cyrtograptus rigidus and monograptus testis cyrtograptus lundgreni zones are recognized the species composition of these zones is significantly different from corresponding zones elsewhere in the canadian cordillera twenty six species are described and illustrated and of these two are new stomatograptus canadensis and cyrtograptus preclarus and two species monograptus aff riccartonensis and cyrtograptus aff sakmaricus might also be new several species particularly monograptus praecedens monograptus veles and monograptus cf mutuliferus strigosus have not been prevously reported in north america une faune moyennement riche en graptolites incluant les especes dominantes monograptus s s retiolites stomato graptus et cyrfograptus a ctc trouvce a prairie creek dans la partie sud des monts mackenzie tenitoires du nod ouest les graptolites du llandovery supcrieur k zones de monograptus turriculatus monograptus spiralis et cyrtograptus sakmaricus et la

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins