seismic hazard in central southern africa summary seismic hazard maps of central southern africa where hazard has been expressed in terms of peak ground acceleration for an annual probability in excess of show relatively high values that distinguish the seismic hazard potential of the deka fault zone the mid zambezi basin luangwa rift and western central mozambique in areas such as central southern africa where little is known about the geology of the region and the fault systems have not been fully mapped seismic hazard potential may be estimated from seismicity and broad scale fault features for this region such potential is based on earthquake magnitude m 2 6 events of such magnitude have recently occurred in the mid zambezi basin southern zimbabwe and western central mozambique this paper follows the conventional probabilistic hazard analysis procedure defining seismic source zones from seismicity based on instrumental records from a cataloque that spans a period of 83 years geological and geomorphological features in the region are described on the mesoscale and are correlated with the seismicity as broad fault zones the scarcity of strong motion accelerogram data necessitated the formulation of attenuation values based on random vibration theory

near source attenuation of peak horizontal acceleration strong motion data recorded within 50 km of the rupture zone were used to study near source attenuation characteristics of horizontal peak ground acceleration for worldwide earthquakes of magnitudes 5 0 to 7 7 the data base consisted of 229 horizontal components of peak acceleration recorded from 27 earthquakes including the 15 october 1979 imperial valley earthquake these data were found to be adequately represented by the functional relationship pga a exp b m r c m d where pga represents the mean of the peak values scaled from the two horizontal components of each recording m is richter magnitude and r is distance from the fault rupture zone peak acceleration was found to be lognormally distributed with a standard error representing a 45 per cent increase in the median estimate the regression analysis statistically confirmed the results of earthquake simulation studies that have predicted peak acceleration to become independent of magnitude and distance in the near field an extensive sensitivity study showed that predictions based on our attenuation relationships are stable with respect to reasonable model and parameter variations an analysis of residuals was used to investigate the behavior of peak acceleration with respect to various earthquake site and recording parameters the more significant findings being 1 a similarity in the level of acceleration recorded on soil or rock 2 larger than average accelerations recorded at sites located on shallow soils or in areas of steep topography 3 larger than average accelerations associated with earthquakes having reverse fault mechanisms and 4 lower than average accelerations recorded in large embedded structures

novel monitoring of antarctic ice shelf basal melting using a fiber optic distributed temperature sensing mooring measuring basal melting of ice shelves is challenging and represents a critical component toward understanding ocean ice interactions and climate change in november 2011 moorings containing fiber optic cables for distributed temperature sensing dts were installed through the mcmurdo ice shelf antarctica 200 m and extending 600 m into the ice shelf cavity the high spatial resolution of dts allows for transient monitoring of the thermal gradient within the ice shelf the gradient near the ice ocean interface is extrapolated to the in situ freezing temperature in order to continuously track the ice ocean interface seasonal melt rates are calculated to be 1 0 mm d 1 and 8 6 mm d 1 and maximum melting corresponds to the arrival of seasonal warm surface water in the ice shelf cavity between january and april the development of continuous surface based techniques for measuring basal melting represents a significant advance in monitoring ice shelf stability and ice ocean interactions

stochastic prediction of ground motion and spectral response parameters at hard rock sites in eastern north america empirical predictions of ground motions from large eastern north american earthquakes are hampered by a lack of data for such events for this reason most prediction techniques have been based at least in part on data from the seismically active and well instrumented western north america concentrating on the prediction of response spectra on hard rock sites we have used a relatively new theoretical technique that does not require western data to make ground motion predictions for eastern north america this method often referred to as the stochastic model has its origins in the work of hanks and mcguire who treat high frequency motions as filtered random gaussian noise for which the filter parameters are determined by a seismological model of both the source and the wave propagation the model has been successfully applied to the predictions of ground motions in the western united states and to short period magnitudes from large to great earthquakes worldwide for our application the essential parameters of the model are estimated by using existing data from small to moderate eastern north american earthquakes a crucial part of the model is the relation between seismic moment and corner frequency the relation proposed in 1983 by nuttli for mid plate earthquakes leads to predictions of ground motions that are lower than available data by a factor of about 4 on the other hand a constant stress parameter of 100 bars gives model predictions in good accord with the data to aid in applications the ground motion predictions are given in the form of regression equations for earthquakes of magnitude 4 5 to 7 5 at distances within 100 km of the source the explanatory variables are hypocentral distance and moment magnitude m because predictions are often required in terms of m g rather than m we have used the theoretical model to establish a relation between the two magnitudes the predicted relation agrees with the sparse data available although the large uncertainties in the observed magnitudes for the larger events as well as the sensitivity of the theoretical magnitude to the attenuation model make it difficult to discriminate between various source scaling models

fault plane solutions of the deka fault zone and mid zambezi valley an analysis of twenty one fault plane solutions from the deka fault zone and the mid zambezi valley shows normal dip slip faulting with a strike slip component the fault planes strike in a south

precise measurement of changes in ice shelf thickness by phase sensitive radar to determine basal melt rates 1 we describe a novel technique for measuring the basal melt rate at a point on an ice shelf the method uses a phase sensitive radar to observe the change in thickness of the ice column between an upper reference horizon and the ice shelf base the observed thickness change is a combination of basal melting horizontal ice divergence and firn compaction the latter two components are determined independently by the repeat survey of surface markers and from estimates of surface accumulation rate and density at the depth of the reference horizon the technique has been applied at a site on george vi ice shelf antarctica where a melt rate of 2 78 0 08 m yr 1 was observed over a twelve day period our technique is applicable to almost any ice shelf or floating glacier tongue and its high precision over short time intervals will permit the measurement of seasonal variations in melt rate

determination of lg wave attenuation using single station seismograms a case study for zimbabwe summary in this paper we show how the quality factor q may be calculated using a single seismograph station and a number of events recorded on analogue seismograms we followed nuttli s 1973 method and extended it to one seismograph station using the single station bulawayo bul we determined a mean q value of 650 for zimbabwe furthermore we considered different propagation paths over zimbabwe as the seismic waves travelled to bul and found a low q value of 350 for the deka fault zone the q value of 650 obtained in this study agrees well within error with that of 603 reported by chow et al 1980 using the multistation multi event method and lies within the range of q values 454 759 reported by xie mitchell 1990 using a back project method to image large scale lateral variations of lg coda q the results obtained are important as the q value constitutes part of the input data in seismic hazard calculations the method may be used to determine q in regions where there may be only one station with reliable analogue seismogram data

a back projection method for imaging large scale lateral variations of lg coda q with application to continental africa summary a new method is developed which uses back projection tomography to regionalize large scale lateral variations of coda q for lg waves which have traversed long continental paths successful use of this method requires precise and stable single trace measurements of lg coda q xie nuttli 1988 the method converges rapidly and requires minimal computer storage it also allows quantitative estimation of resolution and error in imaging lateral variations of coda q the spatial resolution of this method is limited by the uneven spatial coverage of the data base by our limited knowledge of lg coda generation and by the trade off between the stability and the spatial resolution of the coda q inversion the method is applied to a large set of digital lg coda data from africa where large scale lateral variations of lg coda q are found to correlate well with major tectonic features most of africa is stable and like other stable regions has relatively high coda q values the lowest values of coda q are associated with the african rift system other regions of low q values include the atlas mountains and cape fold belt regions of mesozoic and younger deformation the lateral variation of frequency dependence of lg coda q correlates in most regions with that of q at 1 hz our analysis of the spatial resolution of this method indicates that the resolving power of lg coda q imaging is comparable to that of velocity tomography using long period surface waves using empirical approaches we estimate that uncertainties in lg coda q and its power law frequency dependence parameter are less than about 60 and about 0 2 respectively for most of africa

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

two years of oceanic observations below the fimbul ice shelf antarctica 1 the mechanisms by which heat is delivered to antarctic ice shelves are a major source of uncertainty when assessing the response of the antarctic ice sheet to climate change direct observations of the ice shelf ocean interaction are extremely scarce and in many regions melt rates from ice shelf ocean models are not constrained by measurements our two years of data 2010 and 2011 from three oceanic moorings below the fimbul ice shelf in the eastern weddell sea show cold cavity waters with average temperatures of less than 0 1 c above the surface freezing point this suggests low basal melt rates consistent with remote sensing based steady state mass balance estimates for this sector of the antarctic coast oceanic heat for basal melting is found to be supplied by two sources of warm water entering below the ice i eddy like bursts of modified warm deep water that access the cavity at depth for eight months of the record and ii fresh surface water that flushes parts of the ice base with temperatures above freezing during late summer and fall this interplay of processes implies that basal melting at the fimbul ice shelf cannot simply be parameterized by coastal deep ocean temperatures but instead appears directly linked to both solar forcing at the surface as well as to the dynamics of the coastal current system

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

spectral estimates of seismic shear waves the scaling of seismic sources according to brune 1970 1971 with a stress drop of 100 bars allows accurate prediction of average fourier spectra of seismic shear waves in the far field for the frequency band 1 to 10 hz random vibration theory under the assumption of stationary response accurately predicts average linear elastic response spectrum amplitudes in this same frequency band these conclusions are reached through comparison of theoretical estimates with strong motion observations during a wide range of california earthquakes local magnitudes 4 0 to 7 2 distances 9 to 130 km thus these models are an accurate physically based method of estimating high frequency strong ground motion for a range of earthquake magnitudes and distances and offer the advantage over purely empirical methods of a physical basis for extrapolating to predict ground motions for magnitudes and distances which are poorly documented with data

effects of temporal variations in seismicity on seismic hazard the accuracy of stationary mathematical models of seismicity for calculating probabilities of damaging shaking is examined using the history of earthquakes in china from 1350 a d to 1949 a d during this time rates of seismic activity varied periodically by a factor of 10 probabilities of damaging shaking are calculated in 62 cities in north china using 50 yr of earthquake data to estimate seismicity parameters the probabilities are compared to statistics of damaging shaking in the same cities for 50 yr following the data window these comparisons indicate that the seismic hazard analysis is accurate if 1 the maximum possible earthquake size in each seismogenic zone is determined from the entire seismic history rather than from a short time window and 2 the future seismic activity can be estimated accurately the first condition emphasizes the importance of realistically estimating the maximum possible size of earthquakes on faults the second indicates the need to understand possible trends in seismic activity where these exist or to develop an earthquake prediction capability with which to estimate future activity without the capability of estimating future seismicity stationary models provide less accurate but generally conservative indications of seismic ground shaking hazard in the united states the available earthquake history is brief but gives no indication of changing rates of activity the rate of seismic strain release in the central and eastern united states has been constant over the last 180 yr and the geological record of earthquakes on the southern san andreas fault indicates no temporal trend for large shocks over the last 15 centuries both observations imply that seismic activity is either stationary or of such a long period that it may be treated as stationary for seismic hazard analyses in the united states

north america pacific plate boundary an elastic plastic megashear evidence from very long baseline interferometry i investigate a number of lithospheric deformation models based upon the concept that the motions of points near the north america pacific plate boundary are a linear combination of north america and pacific velocities this concept entails a shear strain transition zone in the vicinity of the boundary the best of these models fit 95 of the variance in 139 very long baseline interferometry vlbi length and transverse velocity observations taken over 5 5 years and require only 90 of the relative plate motion predicted by rm2 residual misfits in the megashear models are largely explained by a 6 1 mm yr component of basin and range spreading perpendicular to the plate boundary instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the san andreas fault some of this deformation will be recovered through elastic rebound the rest will be permanently set through plastic processes thus depending on the elastic plastic composition of the borderlands the width and pattern of strain accumulation in the megashear over geological time may or may not be significantly different than what is currently observed because vlbi data have been assembled for only a small fraction of the earthquake cycle plastic and elastic strains are indistinguishable other information will have to be brought to bear to discriminate elastic and plastic behaviors and reconcile instantaneous and geologic deformation near the north america pacific margin

attenuation of strong ground motion in canada from a random vibrations approach abstract theoretical relations are developed for the attenuation of horizontal peak ground acceleration pga and velocity pgv in canada with emphasis on eastern canada the theoretical basis is a composite of several approaches at intermediate epicentral distances results from random vibration theory hanks and mcguire 1981 boore 1983 which can be used to describe pga and pgv in terms of shear radiation implied by brune9s 1970 source model are applied to canadian earthquakes motions are extended into the near source region using arguments concerning magnitude scaling joyner and boore 1981 at distances greater than two crustal thicknesses ground motions are attributed largely to the lg phase and are attenuated accordingly nuttli 1982 the resulting attenuation relations for pga and pgv in eastern canada are in excellent agreement with strong motion data from moderate eastern canadian and central united states earthquakes compared to empirical attenuation relationships based on modified mercalli intensity the theoretical model predicts a stronger dependence of peak ground motions on magnitude in other respects amplitude levels and distance decay empirical and theoretical predictions are similar the model predicts that eastern events will have higher peak accelerations but similar peak velocities in comparison to their western counterparts of the same moment magnitude this arises from the relative enrichment of eastern earthquakes in higher frequencies

observations of flow and ice ocean interaction beneath the mcmurdo ice shelf antarctica 1 the first oceanographic data from beneath the mcmurdo ice shelf antarctica are presented here with synchronous observations from southern mcmurdo sound multilevel current profiles at three sites over spring tide revealed primarily k1 diurnal tides with net transport from mcmurdo sound into the basin beneath the mcmurdo ice shelf the time series in southern mcmurdo sound captured the 14 day tidal phase shift and showed that transport into the subice shelf cavity was even greater over neap tide with average transport 1 8 sv mcmurdo sound sea ice cover during this period was the heaviest in 25 years of satellite record and was associated with the proximity of two massive icebergs b 15a and c 19 the warmest waters observed beneath the ice shelf were set at the surface freezing temperature probably through recent interaction with this extensive sea ice cover observed ranges of temperature and salinity were narrowed to the extent that neither antarctic surface water nor high salinity shelf water were observed beneath the ice shelf we anticipate that in years with more typical summer sea ice cover surface waters swept beneath the ice shelf will be significantly warmer having even greater potential for basal melt near the ice shelf front our observations also exhibit some of the interactions particular to the subice shelf ocean environment these include occurrence of platelet ice in ice shelf meltwater boundary layer friction effects from the ice cover on the ocean and tidal currents driving enhanced melting near the ice shelf front

an investigation into earthquake ground motion characteristics in eastern north america abstract a random vibration model of the hanks mcguire type is used to predict peak ground motions at rock sites in eastern north america the assumed geometric decay and distance dependent duration approximate the propagation of direct body waves at short distances and lg waves at regional distances the model predicts peak acceleration and velocities as well as response spectra and magnitude for a given seismic moment and corner frequency to predict ground motions for a given lg magnitude m lg the model is first used to calculate the seismic moment corresponding to that magnitude assuming a source scaling law then knowing the moment and corner frequency the model is used to calculate peak ground motions available data from strong motion recordings and from the ectn and lrsm networks modified to estimate horizontal ground motions on rock where appropriate are used to verify the model9s assumption and predictions modified mercalli intensity data are also used ground motions predicted by the model with a stress drop of 50 to 200 bars agree with the ground motion data but the m lg values computed by the model for given seismic moments do not agree with nuttli9s 1983 moment versus m lg data for large earthquakes resolution of the latter disagreement awaits the collection of instrumental data from large events in eastern north america

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front