dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia received 17 july 2012 revised 29 august 2012 accepted 4 september 2012 published 12 october 2012 1 during continental rifting the interaction between faulting and magmatic intrusions is not well understood using insar and seismicity we show that a 0 06 km 3 dike was intruded along the dallol segment ethiopia and was accompanied by a mw 5 5 earthquake and associated fault slip along the western flank of the rift the intrusion was fed by a previously unidentified magma chamber under dallol the total seismic moment release was 2 3 10 17 nm 10 of the geodetic moment this is a higher proportion than during the 2005 2009 dabbahu rifting episode which ranged between 1 4 of the geodetic moment a larger component of faulting occurs at dallol than at dabbahu segment a feature we interpret to be related to the proximity 10 km of the dallol segment to the rift margin where welldeveloped faults facilitate slip citation nobile a c pagli d keir t j wright a ayele j ruch and v acocella 2012 dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia geophys res lett 39 l19305 doi 10 1029 2012gl053152

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

contrasted styles of rifting in the eastern gulf of aden a combined wide angle multichannel seismic and heat flow survey continental rifts and passive continental margins show fundamental along axis segmentation patterns that have been attributed to one or a number of different processes extensional fault geometry variable stretching along strike preexisting lithospheric compositional and structural heterogeneities oblique rifting and the presence or absence of eruptive volcanic centers the length and width scales of the rift stage fault bounded basin systems change during the late evolution of the new plate boundary and the role of magmatism may increase as rifting progresses to continental rupture along obliquely spreading ridges first order mid ocean ridge geometries originate during the synrift stage indicating an intimate relationship between magma production and transform fault spacing and location the gulf of aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore this obliquely spreading rift is considered magma poor and has several large offset transforms that originated during late stage rifting and control the first order axial segmentation of the spreading ridge widely spaced geophysical transects of passive margins that produce only isolated 2 d images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models using closely spaced new geophysical and geological observations from the gulf of aden we show that rift sectors between transforms have a large internal variability over short distances 10 km the ocean continent transition oct evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed we suggest that this small scale variability may be explained 1 by the distribution of volcanism and 2 by the along strike differences in time averaged extension rate of the oblique rift system the volcanism may be associated with 1 the long offset alula fartak fracture zone which may enhance magma production on its younger side or 2 channeled flow from the afar plume material along the newly formed oct and the spreading ridge oblique extension and or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma poor margins

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

the volcano seismic crisis in afar ethiopia starting september 2005 we describe the seismo volcanic crisis that occurred in northern afar in late 2005 which involved 15 earthquakes greater than m5 and a small explosive silicic eruption from a vent called da ure at 12 651 n 40 519 n close to dabbahu volcano the purpose is to pull together the different sources of information into a coherent preliminary interpretation of what happened the main geophysical data are the locations of the largest earthquakes and a radar interferogram of unusually high quality that reveals injection of a 60 km long dyke with surface deformation expressed as normal faulting subsidence occurred around the dabbahu volcanic edifice most of the dyke is likely to have been basaltic rather than silicic although the eruption was silicic the volume of the subsidence represents at most 25 of the magma injected into the dyke the silicic eruption was possibly triggered by interaction of basaltic magma with a shallow silicic reservoir at about the same time as the eruption and dyking episode some activity appears also to have taken place at the lava lake at erta ale volcano some 150 km to the north of the eruption site we evaluate the possibility that there may be some link by calculating stresses associated with opening of the fissure and looking at the activity of the lava lake as revealed by the thermal anomaly seen by weather satellites

detection of changes in ridge crest morphology using repeated multibeam sonar surveys a quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area the technique involves mathematical gridding of bathymetric surveys from different years co registration of grids based on prominent features covered in both surveys calculation of slope weighted depth difference grids and determination of the boundaries of statistically significant differences the resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor the method is applied to data from the southern juan de fuca ridge where a seafloor volcanic eruption of 0 05 km3 is inferred to have occurred between 1983 and 1987 camera tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 105 m2 in area or about 200 300 m in diameter and 5 15 m in thickness to be detected through repeated sea beam bathymetric surveys

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

effects of three dimensional earth structure on cmt earthquake parameters abstract we investigate errors in centroid earthquake parameters due to unmodeled structural heterogeneity we generate a simulated dataset consisting of synthetic seismograms for 50 earthquakes and 150 stations distributed globally to generate the synthetic seismograms we use a spectral element wave propagation package specfem3d globe that accounts for the earth s three dimensional structure an established centroid moment tensor inversion algorithm from the global cmt project is used to invert the synthetic dataset with and without added noise for earthquake source parameters this algorithm uses a one dimensional earth structure together with approximate corrections for three dimensional structure to model the seismograms we interpret the differences between the estimated source parameters and the parameters used to compute the synthetic dataset as errors due to unmodeled structural heterogeneity and the presence of noise we expect that the errors obtained in this study are representative of the errors in the global cmt catalogue we find that the errors in scalar moment moment tensor elements and location are small on average the depth and centroid time are however biased by a small amount we find that the error in depth can be reduced significantly by applying corrections for the difference in the velocity structure at the source and receiver locations from the earth s average structure in the cmt inversions this modification has a minimal effect on the errors in centroid time we do not find large errors in scalar moments even where the crustal thickness at the source is very different from the earth s average crustal thickness

structure and evolution of the eastern gulf of aden insights from magnetic and gravity data encens sheba md117 cruise summary magnetic and gravity data gathered during the encens sheba cruise 2000 june in the eastern gulf of aden provide insights on the structural evolution of segmentation from rifted margins to incipient seafloor spreading in this study we document the conjugate margins asymmetry confirm the location of the ocean continent transition oct previously proposed by seismic data and describe its deep structure and segmentation in the oct gravity models indicate highly thinned crust while magnetic data indicate presence of non oceanic high amplitude magnetic anomalies where syn rift sediments are not observed thus the oct could be made of ultra stretched continental crust intruded by magmatic bodies however locally in the north the nature of the oct could be either an area of ultra slow spreading oceanic crust or exhumed serpentinized mantle between the alula fartak and socotra fracture zones the non volcanic margins and the oct are segmented by two n027 e trending transfer fault zones these transfer zones define three n110 e trending segments that evolve through time the first evidence of oceanic spreading corresponds to the magnetic anomaly a5d and is thus dated back to 17 6 ma at least reconstruction of the spreading process suggests a complex non uniform opening by an arc like initiation of seafloor spreading in the oct the early segmentation appears to be directly related to the continental margin segmentation the spreading axis segmentation evolved from three segments 17 6 to 10 95 ma to two segments 10 95 ma to present at the onset of the spreading process the western segment propagated eastwards thus reducing the size of the central segment the presence of a propagator could explain the observed spreading asymmetry with the northern flank of the sheba ridge being wider than the southern one

elastic thickness control of lateral dyke intrusion at mid ocean ridges abstract magmatic accretion at slow spreading mid ocean ridges exhibits specific features although magma supply is focused at the centre of second order segments melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions it has been previously suggested that an along axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation however this cannot account for the poor correlation between dyke opening and surface elevation in the 2005 2010 series of 14 dyke intrusions of afar ethiopia using mechanical arguments constrained by both geodetic and seismological observations we propose that the large dykes that initiate near the mid segment magma source are attracted toward segment ends as a result of a thickening of the elastic brittle lithosphere in the along rift direction this attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long term focusing of melts and comparatively colder hence stronger segment ends the axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along axis elastic brittle thickness rather than the primary cause of lateral dyke injections

seafloor spreading event in western gulf of aden during the november 2010 march 2011 period captured by regional seismic networks evidence for diking events and interactions with a nascent transform zone in november 2010 intense seismic activity including 29 events with a magnitude above 5 0 started in the western part of the gulf of aden where the structure of the oceanic spreading ridge is characterized by a series of n115 trending slow spreading segments set within an ew trending rift using signals recorded by permanent and temporary networks in djibouti and yemen we located 1122 earthquakes with a magnitude ranging from 2 1 to 5 6 from 01 november 2010 to 31 march 2011 by looking in detail at the space time distribution of the overall seismicity and both the frequency and the moment tensor of large earthquakes we reexamine the chronology of this episode in addition we also interpret the origin of the activity using high resolution bathymetric data as well as from observations of sea floor cable damage caused by high temperatures and lava flows the analysis allows us to identify distinct active areas firstly we interpret that this episode is mainly related to a diking event along a specific ridge segment located at e044 in light of previous diking episodes in nearby subaerial rift segments for which field constraints and both seismic and geodetic data exist we interpret the space time evolution of the seismicity of the first few days migration of earthquakes suggests initial magma ascent below the segment center this is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall the cumulative seismic moment associated with this sequence reaches 9 1 1017 nm and taking into account a very low seismic versus geodetic moment we estimate an horizontal opening of 0 58 to 2 9 m the seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes over the whole period the opening is estimated to 1 76 8 8 m across the segment a striking feature of this episode is that the seismicity remained confined within one individual segment whereas the adjacent en echelon segments were totally quiescent suggesting that the magma supply system of one segment is disconnected from those of the neighboring segments second we identify activity induced by the first intrusion with epicenters aligned along a n035 e trending 30 km long at the northwestern end of the active opening segment this group encompasses more than 7 earthquakes with magnitude larger than 5 0 and with strike slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area we propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation at the locus of the trend change of the spreading ridge which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust

local magnitude scale and seismicity rate for tanzania east africa abstract a local magnitude scale is developed for tanzania east africa using data collected by the 1994 1995 tanzania broadband seismic experiment the waveform data from 1189 local and regional earthquakes located within east africa were corrected for instrument response and convolved with the nominal wood anderson torsion seismograph response appropriate for the original definition of local magnitude a total of 24 710 maximum s wave amplitudes were incorporated into an iterative regression for individual event local magnitudes 38 horizontal component station factors and 2 linear distance dependent factors in the log a 0 term of the equation for m l the resulting distance correction log a 0 is given by log a 0 0 776 log r 17 0 000902 r 17 2 0 where r is hypocentral distance in kilometers this distance correction yields much less ground motion attenuation than observed for southern california and is similar to that observed for local s waves and regional lg waves propagating in eastern north america normalizing the distance correction using the standard constraint of 1 mm ground motion at 100 km for a magnitude 3 0 earthquake results in a bias of nearly half of a magnitude unit between m b and m l with m l being larger normalizing using the constraint of 10 mm of motion at 17 km for m l 3 0 removes the bias in magnitude measures and indicates that structure within tanzania is relatively high q the seismicity rate of tanzania for 1994 1995 was examined using the gutenberg richter seismicity distribution and is seen to follow the relation log n 4 63 0 84 m l where n is the number of earthquakes per year of local magnitude m l or greater the catalog of events used in this study is seen to be complete to magnitude 2 to 2 5

the ml scale in southern california measurements 9 941 of peak amplitudes on wood anderson instruments or simulated wood anderson instruments in the southern california seismographic network for 972 earthquakes primarily located in southern california were studied with the aim of determining a new distance correction curve for use in determining the local magnitude m l events in the mammoth lakes area were found to give an unusual attenuation pattern and were excluded from the analysis as were readings from any one earthquake at distances beyond the first occurrence of amplitudes less than 0 3 mm the remaining 7 355 amplitudes from 814 earthquakes yielded the following equation for m l distance correction log a 0 log a 0 1 110log r 100 0 00189 r 100 3 0 where r is hypocentral distance in kilometers a new set of station corrections was also determined from the analysis the standard deviation of the m l residuals obtained by using this curve and the station corrections was 0 21 the data used to derive the equation came from earthquakes with hypocentral distances ranging from about 10 to 700 km and focal depths down to 20 km with most depths less than 10 km the log a 0 values from this equation are similar to the standard values listed in richter 1958 for 50 200 km the effect at close distances is consistent with that found in several other studies and is simply due to a difference in the observed 1 r geometrical spreading for body waves and the 1 r 2 spreading assumed by gutenberg and richter in the construction of the log a 0 table m l s computed from our curve and those reported in the caltech catalog show a systematic dependence on magnitude small earthquakes have larger magnitudes than in the catalog and large earthquakes have smaller magnitudes by as much as 0 6 units to a large extent these systematic differences are due to the nonuniform distribution of data in magnitude distance space small earthquakes are preferentially recorded at close distances relative to large earthquakes for large earthquakes however the difference in the two magnitudes is not solely due to the new correction for attenuation magnitudes computed using richter s log a 0 curve are also low relative to the catalog values the differences in that case may be due to subjective judgment on the part of those determining the catalog magnitudes the use of data other than the caltech wood anderson seismographs the use of different station corrections or the use of teleseismic magnitude determinations whatever their cause the departures at large magnitude may explain a 1 0 0 7 proportionality found by luco 1982 between m l s determined from real wood anderson records and those from records synthesized from strong motion instruments if it were not for the biases in reported magnitudes luco s finding would imply a magnitude dependent shape in the attenuation curves we studied residuals in three magnitude classes 2 0 m l 3 5 3 5 m l 5 5 and 5 5 m l 7 0 and found no support for such a magnitude dependence based on our results we propose that local magnitude scales be defined such that m l 3 correspond to 10 mm of motion on a wood anderson instrument at 17 km hypocentral distance rather than 1 mm of motion at 100 km this is consistent with the original definition of magnitude in southern california and will allow more meaningful comparison of earthquakes in regions having very different attenuation of waves within the first 100 km

capturing magma intrusion and faulting processes during continental rupture seismicity of the dabbahu afar rift continental rupture models emphasize the role of faults in extensional strain accommodation extension by dyke intrusion is commonly overlooked a major rifting episode that began in 2005 september in the afar depression of ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture earthquakes recorded on a temporary seismic array 2005 october to 2006 april direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the 60 km long dabbahu rift segment defined in earlier remote sensing studies epicentral locations lie along a 3 km wide 50 km long swath that curves into the se flank of dabbahu volcano a second strand continues to the north toward gab ho volcano considering the 8 m of opening in the september crisis we interpret the depth distribution of microseismicity as the dyke intrusion zone the dykes rise from 10 km to the near surface along the 60 km long length of the tectono magmatic segment focal mechanisms indicate slip along nnw striking normal faults perpendicular to the arabia nubia plate opening vector the seismicity insar continuous gps and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode persistent earthquake swarms at two sites on dabbahu volcano coincide with areas of deformation identified in the insar data 1 an elliptical northwestward dipping zone of seismicity and subsidence interpreted as a magma conduit and 2 a more diffuse 8 km radius zone of shallow seismicity 2 km above a shadow zone interpreted as a magma chamber between 2 5 and 6 km subsurface insar and continuous gps data show uplift above a shallow source in zone 2 and uplift above the largely aseismic gab ho volcano the patterns of seismicity provide a 3 d perspective of magma feeding systems maintaining the along axis segmentation of this incipient seafloor spreading segment

seismicity and subsidence following the 2011 nabro eruption eritrea insights into the plumbing system of an off rift volcano nabro volcano situated to the east of the afar rift zone erupted on 12 june 2011 eruptions at such off rift volcanoes are infrequent and consequently the plumbing systems are poorly understood we present posteruption synthetic aperture radar sar images from the terrasar x satellite and posteruption continuous seismic activity from a local seismic array interferometric analysis of sar data reveals a circular 12 km wide signal subsiding at 200 mm yr we inverted for the best fit mogi source finding a 4 1 107 m3 yr volume decrease at 7 1 km depth between 31 august and 7 october 2011 we located 658 and relocated 456 earthquakes with local magnitudes between 0 4 and 4 5 seismicity beneath the se edge of nabro at 11 km depth is likely associated with high strain rates from deep magma flow into the modeled reservoir this suggests that magma is supplied through a narrow conduit and then stored at 7 km depth we interpret seismicity at 4 6 km depth as brittle fracturing above the inferred magma reservoir focal mechanisms delineate a thrust fault striking ne sw and dipping 45 to the se across the caldera floor we propose that the crustal response is to slip on this fault which crosscuts the caldera rather than to deform on ring faults the ne sw fault plane is not associated with measurable surface deformation indicating that it does not contribute much to the caldera deformation we show that subsidence of the caldera is controlled by magma chamber processes rather than fault slip

seismic character of volcanic activity at the ultraslow spreading gakkel ridge never before has a volcanic eruption on a slow or ultraslow spreading mid ocean ridge been both observed seismically and confirmed on the seafloor during the first half of 1999 a long lived volcanic spreading event occurred on the ultraslow spreading gakkel ridge in the arctic ocean the seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption sonar images from the u s s hawkbill which passed over the area within four months of the start of activity are consistent with the presence of a large recently erupted flow and a volcanic peak directly in the area of seismic activity seismic activity began in mid january and continued vigorously for three months a reduced rate of activity persisted for an additional four months or more in total 252 events were large enough to be recorded on global seismic networks although a limited number of volcanic spreading events have been observed globally the duration and magnitude of the gakkel ridge swarm when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates suggest that a negative power law relationship may exist between these parameters and spreading rate fault activation in response to magmatic emplacement appears to have occurred over a broad region suggesting that magma may have been tapped from mantle depths the slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth in combination with bathymetric setting and sidescan sonar confirmation the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow spreading rates

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

thermomechanics of mid ocean ridge segmentation the mechanics responsible for the initiation of the orthogonal pattern characterizing mid ocean ridges and transform faults are studied using numerical models the driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions thermal stress can exert ridge parallel tension comparable in magnitude to spreading induced tension when selectively released by ridges and ridge parallel structure two modes of ridge segment growth have been identified in plan view an overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform like fault as the ratio of thermal stress to spreading induced stress increases the patterns of localized plastic strain change from the overlapping to connecting mode the orthogonal pattern marks the transition from one mode to the other besides the amount of stress from each driving force the rate of stress accumulation is crucial in determining the emergent pattern this rate dependence is characterized by the spreading rate normalized by a reference cooling rate pe when pe is paired with the ratio of thermal stress to the reference spreading induced stress they unambiguously define stability fields of the two modes the obliquely connecting the orthogonally connecting and the overlapping mode are similar to ridge transform fault intersections observed in ultra slow slow to intermediate and fast spreading centers respectively the patterns are also sensitive to the strain weakening rate fracture zones were created in part as a response to thermal stress

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

september 2005 manda hararo dabbahu rifting event afar ethiopia constraints provided by geodetic data faults the volume of the 2005 dike 1 5 2 0 km 3 is not balanced by sufficient volume loss at dabbahu and gabho volcanoes 0 42 and 0 12 km 3 respectively taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment above the dike faults slipped by an average 3 m yielding an estimated geodetic moment of 3 5 10 19 nm one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm between dabbahu and ado ale volcanic complexes significant opening occurred on the western side of the dike the anomalous location of the dike at this latitude offset to the east of the axial depression may explain this phenomenon a two stage intrusion scenario is proposed whereby rifting in the northern manda hararo rift was triggered by magma upwelling in the dabbahu area at the northern extremity of the magmatic segment although vigorous dike injection occurred during the september 2005 event the tectonic stress deficit since the previous rifting episode was not fully released leading to further intrusions in 2006 2009

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

january 2006 seafloor spreading event at 9 50 n east pacific rise ridge dike intrusion and transform fault interactions from regional hydroacoustic data 1 an array of autonomous underwater hydrophones is used to investigate regional seismicity associated with the 22 january 2006 seafloor spreading event on the northern east pacific rise near 9 50 n significant earthquake activity was observed beginning 3 weeks prior to the eruption where a total of 255 earthquakes were detected within the vicinity of the 9 50 n area this was followed by a series of 252 events on 22 january and a rapid decline to background seismicity levels during the subsequent 3 days because of their small magnitudes accurate locations could be derived for only 20 of these events 18 of which occurred during a 1 h period on 22 january these earthquakes cluster near 9 45 n and 9 55 n at the distal ends of the young lava flows identified posteruption where the activity displays a distinct spatial temporal pattern alternating from the north to the south and then back to the north this implies either rapid bilateral propagation along the rift or the near simultaneous injection of melt vertically from the axial magma lens short duration t wave risetimes are consistent with the eruption of lavas in the vicinity of 9 50 n on 22 january 2006 eruptions on 12 and 15 16 january also may be inferred from the risetime data however the locations of these smaller magnitude events cannot be determined accurately roughly 15 h after the last earthquakes were located adjacent to the eruption site a sequence of 16 earthquakes began to the north northeast at a distance of 25 40 km from the 9 50 n site these events are located in vicinity of the clipperton transform and its western inside corner an area from which the regional hydrophone network routinely detects seismicity coulomb stress modeling indicates that a dike intrusion spanning the known eruptive zone to the south 9 46 9 56 n would act to promote normal faulting or a combination of normal faulting and transform slip within this region with stress changes on the order of 1 10 kpa

automated near realtime cmt inversion the earthquake research institute eri of the university of tokyo has completely automated the process of determining source mechanisms of earthquakes using waveform data the solutions are automatically distributed using the internet without being checked the process is initiated by receipt of e mail giving the origin time and location of an earthquake to determine the source mechanism we apply the harvard centroid moment tensor cmt inversion method to long period body wave data the portion of the seismogram from the first arrival to the initial surface wave the method has been successfully applied since october 1993 to all of the major earthquakes in the world for which we received event e mail 108 of the 242 cmt solutions in 1994 were determined within 6 hours of the event with an average delay of 3 8 hours for local events e g events around japan it is also possible to determine cmt solutions within 30 minutes from the occurrence of the earthquakes using an efficient dialup data retrieval system operated by eri

crustal structure of the gulf of aden southern margin evidence from receiver functions on socotra island yemen breakup of continents in magma poor setting occurs primarily by faulting and plate thinning spatial and temporal variations in these processes can be influenced by the pre rift basement structure as well as by early syn rift segmentation of the rift in order to better understand crustal deformation and influence of pre rift architecture on breakup we use receiver functions from teleseismic recordings from socotra which is part of the subaerial oligo miocene age southern margin of the gulf of aden we determine variations in crustal thickness and elastic properties from which we interpret the degree of extension related thinning and crustal composition our computed receiver functions show an average crustal thickness of 28 km for central socotra which decreases westward along the margin to an average of 21 km in addition the crust thins with proximity to the continent ocean transition to 16 km in the northwest assuming an initial pre rift crustal thickness of 35 km undeformed arabian plate we estimate a stretching factor in the range of 2 1 2 4 beneath socotra our results show considerable differences between the crustal structure of socotra s eastern and western sides on either side of the hadibo transfer zone the east displays a clear intracrustal conversion phase and thick crust when compared with the western part the majority of measurements across socotra show vp vs ratios of between 1 70 and 1 77 and are broadly consistent with the vp vs values expected from the granitic and carbonate rock type exposed at the surface our results strongly suggest that intrusion of mafic rock is absent or minimal providing evidence that mechanical thinning accommodated the majority of crustal extension from our observations we interpret that the western part of socotra corresponds to the necking zone of a classic magma poor continental margin while the eastern part corresponds to the proximal domain

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

lithospheric structure of a nascent spreading ridge inferred from gravity data the western gulf of aden the aden spreading ridge somalia arabia plate boundary does not connect directly to the red sea spreading ridge it propagates toward the east african rift through the afar depression where the presence of a hot spot has been postulated from seismological and geochemical evidence the spreading direction n37 e is highly oblique to the overall trend n90 e of the ridge we present and interpret new geophysical data gathered during the tadjouraden cruise r v l atalante 1995 in the gulf of aden west of 46 e these data allow us to study the propagation of the ridge toward the afar and to discuss the processes of the seafloor spreading initiation we determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data a striking bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust the inversion of the bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere which should depend not only on the seafloor spreading but also on the hot spot beneath east africa our results allow us to define three distinct lithospheric domains in the western gulf of aden east of 44 45 e the lithosphere displays an oceanic character thermal subsidence recorded for the last 10 ma and constant crustal thickness between 43 30 e and 44 10 e the lithosphere is of continental type but locally thinned beneath the axial valley the central domain defined between 44 10 e and 44 45 e is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust it displays en echelon basins that are better interpreted as extension cells rather than accretion cells

lunar and solar tidal variabilities in mesospheric winds and eej strength over tirunelveli 8 7 n 77 8 e during the 2009 major stratospheric warming 1 mesospheric wind observations by the medium frequency radar and geomagnetic field observations at tirunelveli 8 7 n 77 8 e 1 75 n dip angle are used to study the relative importance of solar and lunar influences in the variabilities of mesospheric tides and equatorial electrojet eej strength during the unprecedented major stratospheric sudden warming ssw of 2009 it is observed that the afternoon reversal in the eej popularly known as counter electrojet occurs consecutively for several days during the ssw event when there is an enhancement of solar semidiurnal tide in both zonal wind at 90 km and eej strength over tirunelveli although the amplitude of lunar tides also shows enhancement it is much less than that of solar the diurnal tidal amplitude in zonal wind and eej strength also shows large enhancement just before the onset of ssw however solar semidiurnal tide dominates diurnal tide during the ssw the diurnal tidal phase in zonal wind shifts to a few hours earlier during the ssw the lunar semidiurnal tidal phase shifts to later hours in both zonal wind and eej strength the main observation of the present study is that the large semidiurnal tide observed during the ssw 2009 is mostly solar driven and only partly lunar driven although tidal planetary wave interaction also may play a vital role although a similar behavior is noticed during the ssw 2006 also the large lunar semidiurnal tide observed in the eej strength without having large lunar semidiurnal tide in the underlying mesospheric winds needs further investigation

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

magma transport and storage at piton de la fournaise la reunion between 1972 and 2007 a review of geophysical and geochemical data abstract since the middle of the 20th century improvement in volcano monitoring techniques has provided large data sets that can be used to evaluate the evolution of magma plumbing systems at piton de la fournaise volcano abundant eruptive activity and a dense monitoring network are especially conducive to such research analysis of the extensive set of geophysical and geochemical data associated with the last 35 years of activity provides new insights into magma transport and storage at piton de la fournaise two periods of frequent eruptions separated by 6 years of inactivity can be distinguished 1972 1992 and 1998 2007 considering these two periods we show evidence of major changes in the shallow plumbing system in 2000 during 1972 1992 and 1998 2000 lava compositions and weak long term eruptive precursors lasting few days to three weeks characterized by no significant summit inflation and about ten volcano tectonic earthquakes per day suggest that eruptions were fed by the progressive drainage of an occasionally recharged shallow magma reservoir geophysical evidence of shallow magma reservoir recharge was only recorded in 1986 and 1998 in contrast from 2000 to 2007 geophysical data highlight the appearance of long term precursors 1 to 5 months of summit inflation and strong seismicity up to one hundred events per day preceding cycles of successive eruptions each cycle of eruptions was characterized by a sequence 3 to 10 months in duration of summit and near summit proximal eruptions and ended with a distal low altitude oceanite olivine abundances 20 vol eruption on the eastern flank of the volcano within each of these eruptive cycles lavas became progressively more magnesian over time the preferential motion of the eastern flank caused by continuous recharging of the shallow reservoir since 2000 would favour the occurrence of distal eruptions towards this flank at the end of a cycle

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

volcanic hybrid earthquakes that are brittle failure events 1 volcanoes generate a variety of pre eruptive low frequency seismic signals hybrid earthquakes comprise a class of these signals having high frequency onsets followed by low frequency ringing they are used empirically to predict eruptions but their ambiguous physical origin limits their diagnostic use the short duration near field hybrid seismograms associated with the 2004 mount st helens eruption indicate that much of the prolonged signal is due to path rather than resonating fluids we show using seismic source spectra that the hybrids have a corner frequency seismic moment relationship that scales consistently with brittle failure the unusually low frequency of these earthquakes can result from low rupture velocities combined with strong path effects due to their shallow sources this new application of near field instrumentation provides the first seismological evidence for brittle failure as a major process in dome building and suggests that hybrids should not be used as direct indicators of fluids

propagation of rifting along the arabia somalia plate boundary the gulfs of aden and tadjoura the localization and propagation of rifting between arabia and somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern gulf of aden and the gulf of tadjoura using bathymetric magnetic seismological and structural evidence large scale southwestward propagation of the aden ridge markedly oblique to the arabia somalia relative motion vector began about 30 myr ago between the error and sharbithat ridges it was an episodic process with stages of rapid propagation mostly at rates 10 cm yr interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the arabia somalia plate the longest pause was at the shukra el sheik discontinuity 45 e where the ridge tip stalled for 13 myr between 17 and 4 ma west of that discontinuity rifting and spreading took place at an azimuth n25 10 e and rate 1 2 0 3 cm yr different from those of the global arabia somalia motion vector n39 1 73 cm yr implying an additional component of movement n65 10 e 0 7 0 2 cm yr due to rotation of the danakil microplate at shukra el sheik the typical oceanic ridge gives way to a narrow wsw trending axial trough resembling a large fissure across a shallow shelf this trough is composed of about eight rift segments which result from normal faulting and fissuring along n110 n130 e trends all the segments step to the left southwestward mostly through oblique transfer zones with en echelon normal faults only two segments show clear significant overlap there is one clear transform the maskali fault between the obock and tadjoura segments the latter segment which encroaches onland is composed of two parallel subrifts iboli ambabbo that propagated northwestward and formed in succession the most recent southwestern subrift ambabbo represents the current tip of the aden ridge we propose a mechanical model in which the large scale propagation of the ridge followed a wsw trending zone of maximum tensile stress while the small scale propagation of its nw trending segments was dictated by the orientation of that stress oblique propagation was a consequence of passive lithospheric necking of the arabia somalia plate along its narrow section in map view between socotra and the kink of the red sea ethiopian rift above the afar plume individual ridge segments oriented roughly perpendicular to plate motion like lithospheric cracks were forced to jump southward because of confinement within the necking zone self sustaining plate scale necking may explain why the aden ridge did not connect with the red sea through bab el mandeb but continued straight into afar

comparison of dike intrusions in an incipient seafloor spreading segment in afar ethiopia seismicity perspectives 1 oceanic crust is accreted through the emplacement of dikes at spreading ridges but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood between 2005 and 2009 the 70 km long dabbahu manda hararo rift segment in ethiopia has experienced 14 large volume dike intrusions 9 of which were recorded on temporary seismic arrays a detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems all of the migrating swarms of earthquakes started from a 80 of energy is released during the propagation phase with minimal seismic energy release after the dike propagation ceases we interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip coincident with the onset of low frequency earthquakes dike lengths show no systematic reduction in length with time suggesting that topographic loading and stress barriers influence dike length as well as changes in tectonic stress the propagation velocities of all the dikes follow a decaying exponential northward propagating dikes had faster average velocities than those that propagated southward suggesting preconditioning by the 2005 megadike or ongoing heating from a subcrustal magma source north of the midsegment

manifold methods for assimilating geophysical and meteorological data in earth system models and their components abstract a novel manifold method of reconstructing dynamically evolving spatial fields is presented for assimilating data from sensor networks in integrated land surface subsurface oceanic lake models the method was developed based on the assumption that data can be mapped onto an underlying differential manifold in this study the proposed method was used to reconstruct meteorological forcing over lake michigan the bathymetry of an inland lake gull lake and precipitation over the grand river watershed in michigan in the first case study hourly meteorological forcing data were reconstructed and used to run a three dimensional hydrodynamic model of lake michigan and to quantify the improvement that results from the use of the new method in the second example the bathymetry of gull lake was reconstructed from measured scatter point data using the manifold technique a hydrodynamic model of gull lake was developed and refined using the improved bathymetry in the last case study improved daily participation data for a six year period over the grand river watershed were used as input to an integrated distributed hydrologic model all three examples illustrate the superior performance of the manifold method over standard methods in terms of accuracy and computational efficiency our results also indicate that using the cross validation technique to evaluate the performance of data reconstruction methods can lead to misleading conclusions about their relative performance

rift zone development around a plume related magma centre on the isle of skye scotland a model for stress inversions using the example of the isle of skye in scotland we investigate the influence of pressure variations in upper crustal magma reservoirs on the development of rift type normal faulting around central volcanoes the regional synmagmatic stress regime is of strike slip type in scotland during the lower tertiary during a prolonged period of overall high pressure in the skye magma reservoir gabbro intrusion stage crustal extension results from the injection of basaltic dykes parallel to the trend of the far field maximum stress during a subsequent period of pressure decrease in the reservoir granites intrusion stage normal faults trending parallel to the dykes are initiated these faults tilt the upper crustal blocks along with the former dyke swarm and associated lava pile finite element modelling shows that a decrease of magma pressure in a circular cavity may lead as in skye to a change from a regional strike slip to a local rift type normal stress regime

seismoacoustic recordings of a spreading episode on the mohns ridge a period of very active seismicity near 72 7 n 4 e marks an episode of seafloor spreading on the mohns ridge the earthquakes were recorded from november 1995 to january 1996 by onshore seismic stations and by u s navy hydrophone arrays in the north atlantic both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events the hydrophone arrays recorded 2 3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed during the most intense phase of activity a level of 50 200 events per day was sustained throughout the episode initial locations of the events were obtained from the seismic bulletin further refinement of the epicenters was possible using p s converted to an acoustic phase at the seafloor and t waves in the hydrophone data analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm a few events occurred at a more distant location the activity tends to concentrate in one area or another for short periods a few days but at times it is clear that events occur simultaneously at more than one location we have not found evidence of steady migration of activity such as might accompany propagation of a magma filled dike we thus infer that despite the 50 70 km length of ridge involved in the spreading episode rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas

strain accommodation by slow slip and dyking in a youthful continental rift east africa continental rifts begin and develop through repeated episodes of faulting and magmatism but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented in highly evolved rifts tensile stresses from far field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1 2 3 these rifting crises are rarely observed in thick lithosphere during the initial stages of rifting here we show that most of the strain during the july august 2007 seismic crisis in the weakly extended natron rift tanzania was released aseismically deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping this event provides compelling evidence for strain accommodation by magma intrusion in addition to slip along normal faults during the initial stages of continental rifting and before significant crustal thinning

geometry of ionic arrangements and magnetic interactions in ordered ferri ilmenite solid solutions and its effect on low t magnetic behavior 1 the magnetic properties of metastable chemically ordered ilmenite rich fe2o3 fetio3 solid solutions in the temperature range 500 0 k represent a complex amalgam of the incompatible properties of the end members while hematite is controlled by adjacent layer antiferromagnetic interactions ilmenite is controlled by double layer antiferromagnetic interactions the complex geometry of the exchange coupled ions is visualized and tentative explanations of property changes with temperature are provided by schematic 2 d ionic images along a common 1 1 2 0 plane showing the exchange interactions in the end members and in ilm 90 80 and 70 solid solutions accompanied by parallel temperature and frequency dependent susceptibility diagrams to provide a larger scale visualization of the extent of exchange clusters and networks 3 d images of ilm 95 90 85 80 75 and 70 containing 2592 atomic positions were also constructed both types of images together provide a conceptual overview of the physical mechanisms governing this complex solid solution system and its low temperature magnetic behavior even though the geometric approach does not in itself provide a quantitative model of physical properties it is an indispensable prerequisite for designing realistic quantitative models judging their validity and interpreting model results it also supplies information on the distributions numbers and orientations of magnetic exchange interactions which can become guidelines for more sophisticated calculations

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

shallow axial magma chamber at the slow spreading erta ale ridge thermal models predict that spreading velocity at mid ocean rifts should influence the geometry of the underlying magma chamber insar data from the slow spreading ethiopian rift identify a shallow elongated magma chamber a feature usually associated with fast spreading rifts beneath the erta ale segment implying that spreading velocity may not be so important after all

reappraisal of the arabia india somalia triple junction kinematics abstract we propose alternative kinematics for the arabia india somalia triple junction based on a re interpretation of seismological and magnetic data the new triple junction of the ridge ridge ridge type is located at the bend of the sheba ridge in the eastern gulf of aden at 14 5 n and 56 4 e the owen fracture zone arabia india boundary is connected to the sheba ridge by an ultra slow divergent boundary trending n80 e 10 marked by diffuse seismicity the location of the arabia india rotation pole is constrained at 14 1 n and 71 2 e by fitting the active part of the owen fracture zone with a small circle the finite kinematics of the triple junction is inferred from the present day kinematics since the inception of the accretion 15 18 ma ago the sheba ridge has probably receded 300 km at the expense of the carlsberg ridge which propagated northwestward in the gulf of aden while an ultra slow divergent plate boundary developed between the arabian and indian plates the overall geometry of the new triple junction is very similar to that of the azores triple junction

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

propagation of an oblique spreading centre the western gulf of aden abstract the east west trending gulf of aden rift between arabia and somalia is spreading along a n35 e direction it propagates westward inside the african continent toward the afar triple junction oblique to the spreading direction these combined propagation and oblique rifting generate a special faulting pattern associated with an irregular distribution of volcanism we analyse this pattern using a structural map derived from the analysis of multibeam bathymetric data and backscattering images from the 1995 tadjouraden cruise in the western gulf of aden the rift valley is bounded by faults trending from n110 e to n130 e in the eastern part of the surveyed area where the valley trends n90 e and by large faults in the central western part where the valley is oriented n70 e the direction of extension deduced from the fault histogram is n37 e 10 over the whole area whatever the rift trend therefore this bend of the plate boundary does not result from a change in the stretching direction this bending increases the rifting obliquity westward and therefore modifies the faulting pattern faults display two distinct patterns either en echelon on the walls indicating a right lateral component of shear or purely normal and parallel to the rift trend inside the valley volcanoes are mostly located in the eastern part of the rift where they are aligned into swarms trending roughly perpendicular to the extension direction deduced from fault directions backscattering images show that recent lava flows fill the axial valley of the eastern part whereas in the central and western parts volcanoes are scattered in the middle part of the valley this distribution is interpreted as a westward propagation of the volcanism and as the direction of deformation migration the thickness of the upper brittle layer of the lithosphere is estimated from the width if the inner graben is inside the rift it indicates a deepening of the brittle ductile transition from east 6 km to west 16 km these values are typical of oceanic and continental systems respectively and consistent with the depth of earthquakes in the western part this points out the existence of a transitional area between 44 e and 45 20 e characterised by the lack of recent volcanism the nature of this mechanical transition is discussed in terms of propagation of an spreading centre into a continental or transitional afar type lithosphere

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

crustal structure of the rifted volcanic margins and uplifted plateau of western yemen from receiver function analysis we analyse p wave receiver functions across the western gulf of aden and southern red sea continental margins in western yemen to constrain crustal thickness internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism faulting and mechanical crustal thinning during continental breakup we analyse teleseismic data from 21 stations forming the temporary young conjugate margins laboratory yocmal network together with gfz and yemeni permanent stations analysis of computed receiver functions shows that 1 the thickness of unextended crust on the yemen plateau is 35 km 2 this thins to 22 km in coastal areas and reaches less than 14 km on the red sea coast where presence of a high velocity lower crust is evident the average vp vs ratio for the western yemen plateau is 1 79 increasing to 1 92 near the red sea coast and decreasing to 1 68 for those stations located on or near the granitic rocks thinning of the crust and by inference extension occurs over a 130 km wide transition zone from the red sea and gulf of aden coasts to the edges of the yemen plateau thinning of continental crust is particularly localized in a 30 km wide zone near the coastline spatially co incident with addition of magmatic underplate to the lower crust above which on the surface we observe the presence of seaward dipping reflectors sdrs and thickened oligo miocene syn rift basaltic flows our results strongly suggest the presence of high velocity mafic intrusions in the lower crust which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the sdrs our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western gulf of aden and southern red sea volcanic margins followed by a second phase of extension along the red sea margin

volcanism jump and propagation on the sheba ridge eastern gulf of aden segmentation evolution and implications for oceanic accretion processes the rifting between arabia and somalia which started around 35 ma was followed by oceanic accretion from at least 17 6 ma leading to the formation of the present day gulf of aden bathymetric gravity and magnetic data from the encens sheba cruise are used to constrain the structure and segmentation of the oceanic basin separating the conjugate continental margins in the eastern part of the gulf of aden between 51 degrees e and 55 5 degrees e data analysis reveals that the oceanic domain along this ridge section is divided into two distinct areas the eastern area is characterized by a shorter wavelength variation of the axial segmentation and an extremely thin oceanic crust in the western segment a thicker oceanic crust suggests a high melt supply this supply is probably due to an off axis melting anomaly located below the southern flank of the sheba ridge 75 km east of the major alula fartak transform fault this suggests that the axial morphology is produced by a combination of factors including spreading rate melt supply and the edge effect of the alula fartak transform fault as well as the proximity of the continental margin the oceanic domains have undergone two distinct phases of accretion since the onset of seafloor spreading with a shift around 11 ma at that time the ridge jumped southwards in response to the melting anomaly propagating ridges were triggered by the melting activity and propagated both eastward and westward the influence of the melting anomaly on the ridges decreased stopping their propagation since less than 9 ma from that time up to the present the n025 degrees e trending socotra transform fault developed in association with the formation of the n115 degrees e trending segment 2 in recent times a counter clockwise rotation of the stress field associated with kinematic changes could explain the structural morphology of the alula fartak and socotra hadbeen fracture zones

form and growth of an embryonic continental rift insar observations and modelling of the 2009 western arabia rifting episode summary a magma driven rifting episode occurred at the harrat lunayyir harrat al shaqa volcanic field western arabia between 2009 april and july it was accompanied by a swarm of more than 4000 m 2 earthquakes the largest ever documented in that region with a peak mw 5 7 shock on may 19 we combine interferometric synthetic aperture radar insar measurements and elastic modelling with seismic moment calculations to resolve the evolution of surface deformation associated with this event phase discontinuities and low coherence lineaments are used to infer the location of the main active structures during the various deformation stages and descending track interferograms that span the entire period are inverted to resolve the slip and opening distributions along two graben bounding normal faults and a dyke respectively assuming negligible rift parallel displacements we combine ascending and descending track interferograms to derive the vertical and rift perpendicular deformation which add up to a maximum surface extension of 1 5 m across the rift and subsidence of 0 8 m the far field deformation is dominated by the dyke opening whereas the near field displacements are mostly associated with movements along the faults the cumulative seismic moment released during the entire swarm period accounts for about 14 per cent of the total geodetic moment compared to about 55 per cent at the 2007 gelai tanzania and about 8 per cent at the 2005 manda hararo dabbahu afar events we propose that the differences in moment partitioning ratios are due to the different crustal and seismogenic layer thicknesses in the three regions and represent different stages in the evolution of a volcanic rift the gelai event represents the most juvenile stage of rifting the dabbahu event represents the most evolved and the harrat lunayyir event represents a rift that is intermediate between the two in its degree of maturity

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

origin and models of oceanic transform faults abstract mid ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics a fundamental problem of plate tectonics is how this pattern has formed and why it is maintained gross scale geometry of mid ocean ridges is often inherited from respective rifted margins indeed transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge both analog and numerical models of transform faults were investigated since the 1970s two main groups of analog models were developed thermomechanical freezing wax models with accreting and cooling plates and mechanical models with non accreting lithosphere freezing wax models reproduced ridge ridge transform faults inactive fracture zones rotating microplates overlapping spreading centers and other features of oceanic ridges however these models often produced open spreading centers that are dissimilar to nature mechanical models on the other hand do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading three main types of numerical models were investigated models of stress and displacement distribution around transforms models of their thermal structure and crustal growth and models of nucleation and evolution of ridge transform fault patterns it was shown that a limited number of spreading modes can form transform faults microplates overlapping spreading centers zigzag ridges and oblique connecting spreading centers however the controversy exists whether these patterns always result from pre existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion therefore two types of transform fault interpretation exist plate fragmentation structures vs plate accretion structures models of transform faults are yet relatively scarce and partly controversial consequently a number of first order questions remain standing and significant cross disciplinary efforts are needed in the future by combining natural observations analog experiments and numerical modeling

excitation of a buried magmatic pipe a seismic source model for volcanic tremor recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so called long period or low frequency events accordingly we can view the long period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor generating system we present a seismic model in which the source of tremor is the acoustic resonance of a fluid filled volcanic pipe triggered by excess gas pressure the model consists of three elements namely a triggering mechanism a resonator and a radiator for simplicity we assume a hemispherical trigger cylindrial resonator and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk shaped radiator shutting off its bottom considering the simple case of a source buried in a homogeneous half space we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances the results demonstrate that the displacement attributed to the pipe dominates the near field motion while that due to the disk is representative of the intermediate and far fields the trigger itself has a smaller contribution mainly limited to the field in the proximity of the source the characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well developed harmonic wave train dominated by rayleigh waves at larger distances no clear shear arrival can be detected in the synthetic seismograms the displacement spectrum reflects the organ pipe modes of the conduit and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid in the case of the cylindrical magma column considered the radiation loss is proportional to the square of the pipe radius while the loss related to viscous damping is inversely proportional to the same factor indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir the relative importance of the pipe and disk elements likewise is a function of the conduit cross section this suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near and far field observations

broadband calibration of the r v marcus g langseth four string seismic sources 1 the r v marcus g langseth is the first 3 d seismic vessel operated by the u s academic community with up to a four string 36 element source and four 6 km long solid state hydrophone arrays this vessel promises significant new insights into earth science processes the potential impact of anthropogenic sound sources on marine life is an important topic to the marine seismic community to ensure that operations fully comply with existing and future marine mammal permitting requirements a calibration experiment was conducted in the gulf of mexico in 2007 2008 results are presented from deep 1 6 km and shallow 50 m water sites obtained using the full 36 element 6600 cubic inches seismic source this array configuration will require the largest safety radii and the deep and shallow sites provide two contrasting operational environments results show that safety radii and the offset between root mean square and sound exposure level measurements were highly dependent on water depth

the seismic moment budget of slowly spreading ridges mid ocean ridges with well developed median valleys are commonly the sites of large normal faulting earthquakes1 2 a recent global study3 6 of the centroid depths and source mechanisms of such earthquakes has yielded two important conclusions first that all the largest mid ocean ridge earthquakes occur beneath the inner floor of the median valley and second that the maximum thickness of the seismogenic layer increases with decreasing spreading rate these results lend support to the hypothesis that the median valley arises from the necking of a strong brittle lithosphere under horizontal extension7 9 here we compare the global rate of seismic moment release along slowly spreading mid ocean ridges with the rate predicted if widening of the median valley occurs entirely by seismogenic extension of the brittle lithosphere beneath the inner floor we find that earthquakes account for no more than about 10 20 of the rate of plate separation across the central median valley

seismic precursors to the mount st helens eruptions in 1981 and 1982 six categories of seismic events are recognized on the seismograms from stations in the vicinity of mount st helens two types of high frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers medium and low frequency earthquakes occur at shallow depths less than 3 kilometers within the volcano and increase in number and size before eruptions temporal changes in the energy release of the low frequency earthquakes have been used in predicting all the eruptions since october 1980 during and after eruptions two types of low frequency emergent surface events occur including rockfalls and steam or gas bursts from the lava dome

sequence of rifting in afar manda hararo rift ethiopia 2005 2009 time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling thirteen dike intrusions in the manda hararo rift afar ethiopia from september 2005 to june 2009 studied using an extensive interferometric synthetic aperture radar insar data set provide insight into the mechanics of a major active rift kinematic inversions of insar data reveal that dikes opened by 0 8 3 5 m at an average 5 km depth with volumes of 0 04 0 2 km 3 with up to 12 m opening and a volume greater than 1 km 3 for the september 2005 megadike dikes have their source in a midsegment magma reservoir which induces a local shallowing of the brittle ductile boundary presumably due to thermal weakening of the lithosphere the smaller dikes in 2006 2009 were emplaced in regions of minimum opening of the september 2005 megadike above the central magma reservoir in contrast the most voluminous dike intrusions in 2006 2009 occurred near the locus of the peak of maximum opening of the september 2005 megadike 10 km north of the magma source this may suggest that tension on the plate boundary was highest there both prior to 2005 and possibly also after 2005 evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes presumably because dikes in the manda hararo rift are intruded at low magma pressure and high tectonic stress time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

contrasting structural styles of the eastern and western margins of the southern red sea the 1988 sonne experiment abstract seismic investigations in the sudanese and yemeni sections of the red sea showed contrasting structural styles across the two surveyed areas of the opposing flanks off sudan seaward of an about 50 km broad zone of stretched continental crust different domains of oceanic crust were identified the oldest of these domains was found under an up to 6 km thick cover of sediments which include a 1 to 2 km thick pre evaporite sequence these sediments are interpreted to be part of a pull apart basin generated during the initial stages of red sea evolution towards the red sea axial trough a structural boundary separates this from younger oceanic crust which is covered only by post evaporite sediments this sequence thins progressively from about 800 m at the boundary to zero at the axial trough off yemen in contrast oceanic crust is confined to the axial trough zone which terminates southward of 16 n further south to 14 n no oceanic crust has yet been formed stretched continental crust was found across the entire flank of the southern red sea and at the tihama coastal plain further inland beginning at the escarpment the crust thickens in several major steps to a thickness of 35 km in central yemen

low frequency hybrid earthquakes near a magma chamber in afar quantifying path effects areas of active volcanism contain elaborate velocity structures that complicate interpretations of earthquake source mechanisms we examine the spectral characteristics of 805 earthquakes that immediately followed a large volume basaltic dike intrusion and associated silicic flank eruption of dabbahu volcano in the afar depression as recorded on near source seismometers we use these results to quantify the contribution of scattering and attenuation to the observed spectra of low frequency hybrid and volcano tectonic earthquake clusters from beneath dabbahu volcano and around the dike zone we find strong variations in the signal amplitude and frequency content of earthquakes recorded at stations separated by as little as 2 km caused by preferential attenuation of high frequencies depending on the vantage point these observations imply that there are large impedance contrasts near the cooling solidifying and recently intruded dike we estimate the intrinsic absorption attenuation coefficient qi and inverse scattering length g0 averaged over a 300 sq km area beneath dabbahu our results are consistent with the highest attenuation coefficients from studies of volcanic provinces in italy qi 1 0 02 g0 0 1 km 1 for a signal at 2 hz the magnitude of these two parameters indicates there are large impedance contrasts present in the area due to the recent intrusion of magma and associated fracturing

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

effects of topography and crustal heterogeneities on the source estimation of lp event at kilauea volcano summary the main goal of this study is to improve the modelling of the source mechanism associated with the generation of long period lp signals in volcanic areas our intent is to evaluate the effects that detailed structural features of the volcanic models play in the generation of lp signal and the consequent retrieval of lp source characteristics in particular effects associated with the presence of topography and crustal heterogeneities are here studied in detail we focus our study on a lp event observed at kilauea volcano hawaii in 2001 may a detailed analysis of this event and its source modelling is accompanied by a set of synthetic tests which aim to evaluate the effects of topography and the presence of low velocity shallow layers in the source region the forward problem of green s function generation is solved numerically following a pseudo spectral approach assuming different 3 d models the inversion is done in the frequency domain and the resulting source mechanism is represented by the sum of two time dependent terms a full moment tensor and a single force synthetic tests show how characteristic velocity structures associated with shallow sources may be partially responsible for the generation of the observed long lasting ringing waveforms when applying the inversion technique to kilauea lp data set inversions carried out for different crustal models led to very similar source geometries indicating a subhorizontal cracks on the other hand the source time function and its duration are significantly different for different models these results support the indication of a strong influence of crustal layering on the generation of the lp signal while the assumption of homogeneous velocity model may bring to misleading results

post rift volcanism and high heat flow at the ocean continent transition of the eastern gulf of aden a high heat flow 900 mw m 2 has been observed over a volcanic structure at the ocean continent transition in the eastern gulf of aden oman margin the anomaly is superposed to a progressive increase of heat flow across the margin and can be interpreted either by 1 heat refraction 2 fluid discharge or 3 cooling magma the two first explanations cannot be ruled out definitely by modelling analysis but require unlikely thermal conductivity or permeability values the third one implies that the latest activity of the volcano was about 100 000 years old and therefore continued c 18 ma after the break up of africa and arabia this potential mechanism is consistent with other lines of evidence of post rifting activity in the gulf of aden and could invalidate the conventional assumption that rifted margins become passive after the break up of continents

decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano caldera forming volcanic eruptions are low frequency highimpact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days with devastating effects on local and global scales1 because no such eruption has been monitored during its long build up phase the precursor phenomena are not well understood geophysical signals obtained during recent episodes of unrest at calderas such as yellowstone usa and campi flegrei italy are difficult to interpret and the conditions necessary for large eruptions are poorly constrained2 3 here we present a study of pre eruptive magmatic processes and their timescales using chemically zoned crystals from the minoan caldera formingeruption of santorini volcano greece4 which occurred in the late 1600s bc the results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption5 6 despite the large volume of erupted magma4 40 60 cubic kilometres and the 18 000 year gestation period between the minoan eruption and the previous major eruption most crystals in the minoan magma record processes that occurred less than about 100 years before the eruption recharge of the magma reservoir by large volumes of silicic magma and some mafic magma occurred during the century before eruption and mixing between different silicicmagmabatches was still taking place during the final months final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period with major growth phases immediately before eruption these observations have implications for the monitoring of long dormant but potentially active caldera systems

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data