slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling

modeling the diurnal variation of tracer transit velocity through a subglacial channel 1 tracer injections into a moulin connecting to a subglacial channel at unteraargletscher were repeated at intervals of about two hours over two diurnal discharge cycles in august and september 2000 the pronounced hysteresis in the relationships between tracer transit velocity and proglacial discharge may be explained by adjustments of the channel cross section in response to discharge variations or alternatively by modulation of inflow from the tributary moulin into a main subglacial channel to test these hypotheses we employ a physically based time dependent model of channelized subglacial drainage in a series of numerical experiments we explore the general characteristics of each mechanism before the model is applied to diurnal discharge variations measured during the tracer tests at unteraargletscher we found that the ability of a rothlisberger channel to adjust its size to the prevailing hydraulic conditions can contribute to the hysteresis in the velocity discharge relationships however using plausible parameter values r channel hydraulics alone is not satisfactory in reproducing the magnitude of observed transit velocities the model further confirms that retardation of the tracer in the tributary moulin due to inflow modulation has major effects on transit velocities both in magnitude and timing with appropriate assumptions on channel sinuosity and moulin size a combination of both mechanisms can be found that is capable of reproducing the observed transit velocities

diurnal variability of subglacial drainage conditions as revealed by tracer experiments 1 the morphology of the drainage system of unteraargletscher switzerland and the diurnal variability of drainage conditions were investigated by conducting a series of tracer tests over a number of discharge cycles during the ablation season 2000 dye injections into a moulin were repeated at intervals of a few hours and were accompanied by simultaneous measurements of discharge of supraglacial meltwater draining into the moulin and bulk runoff in the proglacial stream fast transit velocities in conjunction with low dispersion values suggest that the tracer was routed through a hydraulically efficient channelized drainage system however detailed analyses reveal a large diurnal variability in terms of transit velocity and dispersion coefficient this finding underlines the difficulty of detecting a possible drainage system evolution based on single tracer tests conducted at coarse temporal intervals furthermore the obtained velocity discharge relationships display pronounced hysteresis we suggest that the evolution of the cross sectional area of an ice walled conduit and the modulation of inflow at the junction of a tributary moulin to a main subglacial channel are responsible for the observed behavior

elucidating changes in the degree of tracer dispersion in a subglacial channel tracer injections into a subglacial channel at unteraargletscher switzer land were repeated at intervals of about 2 hours over two diurnal discharge cycles in august and september 2000 records of dye concentration reveal a pronounced hysteresis in the velocity dispersion relationship thereby indicating alterations in the drainage system theoretical considerations for ro thlisberger channels suggest an evolution of the conduit cross section in response to a diurnally varying discharge we studied the relation between conduit cross section and tracer dispersion with numerical tracerexperiments thevelocity fieldforsteady flow throughagivenconduitgeometry is calculated using a commercial flow solver tracer transport is represented by a scalar volume which is advected by the velocity field experiments were conducted for several scenarios by varying flow velocity conduit geometry and conduit roughness results show only a weak dependence of dispersion on conduit size in contrast changes in roughness of the conduit walls reveal a strong effect on tracer dispersion therefore to explain the observed hysteresis in the velocity dispersion relationship we suggest that the evolution of a subglacial flow path might involve changes in roughness

localized fault slip to the trench in the 2010 maule chile mw 8 8 earthquake from joint inversion of high rate gps teleseismic body waves insar campaign gps and tsunami observations the 27 february 2010 mw 8 8 maule earthquake ruptured 500 km along the plate boundary offshore central chile between 34 s and 38 5 s establishing whether coseismic fault offset extended to the trench is important for interpreting both shallow frictional behavior and potential for tsunami earthquakes in the region joint inversion of high rate gps teleseismic body waves interferometric synthetic aperture radar insar campaign gps and tsunami observations yields a kinematic rupture model with improved resolution of slip near the trench bilateral rupture expansion is resolved in our model with relatively uniform slip of 5 10 m downdip beneath the coast and two near trench high slip patches with 12 m displacements the peak slip is 17 m at a depth of 15 km on the central megathrust located 200 km north from the hypocenter and overlapping the rupture zone of the 1928 m 8 event the updip slip is 16 m near the trench another shallow near trench patch is located 150 km southwest of the hypocenter with a peak slip of 12 m checkerboard resolution tests demonstrate that correctly modeled tsunami data are critical to resolution of slip near the trench with other data sets allowing but not requiring slip far offshore large interplate aftershocks have a complementary distribution to the coseismic slip pattern filling in gaps or outlining edges of large slip zones two clusters of normal faulting events locate seaward along the plate motion direction from the localized regions of large near trench slip suggesting that proximity of slip to the trench enhanced extensional faulting in the underthrusting plate

geometry and flow conditions of subglacial water at south cascade glacier washington state u s a an analysis of tracer injections tracers were injected into south cascade glacier to determine the flow condition and geometry of the subglacial water system results indicate that two distincts drainage basins on the glacier feed the two main streams flowing from the glacier in the largest basin two parallel drainage networks exist one englacial and the other subglacial

velocity discharge relationships derived from dye tracer experiments in glacial meltwaters implications for subglacial flow conditions repeated dye tracer tests were undertaken from individual moulins at haut glacier d arolla switzerland over a number of diurnal discharge cycles during the summers of 1989 1991 it was hoped to use the concepts of at a station hydraulic geometry to infer flow conditions in subglacial channels from the form of the velocity discharge relationships derived from these tests the results however displayed both clockwise and anticlockwise velocity discharge hysteresis in addition to the simple power function relationship assumed in the hydraulic geometry approach clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial channel and that discharges in the two channels vary out of phase with each other anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthrough curve and is best explained by hydraulic damming of moulins or sub englacial passageways despite the complex velocity discharge relationships observed some indication of subglacial flow conditions may be obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity discharge relationships are derived from dye injections conducted during periods when the supraglacial discharge entering the moulin and the bulk discharge vary in phase analyses based on this premise suggest that both open and closed channel flow occur beneath haut glacier d arolla and that flow conditions are highly variable at and between sites

changes in geometry and subglacial drainage derived from digital elevation models unteraargletscher switzerland 1927 97 digital elevation models of the bed and surface of unteraargletscher switzerland are used to reconstruct the theoretical pattern of basal water drainage for the years 1927 1947 1961 and 1997 during which period the glacier was thinning and receding the theoretical drainage pattern for 1997 compares well in a broad sense with the locations of active moulins and the hydraulic connection status of boreholes drilled to the glacier bed changes in the basal water flow pattern over the period 1927 97 that are revealed by the theoretical reconstructions of the subglacial drainage system structure are likely to have resulted from changes in glacier geometry concurrent with the retreat and thinning of the glacier the height of medial moraines increased probably due to the insulating effect of the debris cover reducing the melt of the underlying ice this increase of moraine heights has led to the formation of hydraulic barriers at the glacier bed such that water flow has become channelized beneath the ice along drainage axes that parallel the course of the medial moraines on the glacier surface

dye tracing a jokulhlaup i subglacial water transit speed and water storage mechanism we present results of an investigation of two j okulhlaups glacial lake outburst floods at gornergletscher switzerland using dye tracer experiments and complementary hydrological measurements repeated dye injections into moulins showed that tracer transit speeds were larger after the lake had emptied but when proglacial discharge was still high than during the main phase of the j okulhlaup this counter intuitive finding was modelled by tracer retardation inside the injection moulin this model together with an estimate of the maximum time the tracer takes to transit the injection moulin allows us to calculate bounds on the transit speed in the main drainage channel where the lake water flows these results indicate that the main drainage channel transit speeds are indeed highest during the peak of the flood moreover it is known from a previous study that water amounting to half of the lake volume is temporarily stored within the glacier during a gornergletscher j okulhlaup our observations suggest that this process occurred via lateral spreading of water at the glacier bed

evidence for excess pore water pressure generated in subglacial sediment implications for clast ploughing 1 pore water pressure in front of an object pulled through subglacial sediment and water pressure at the base of a borehole were simultaneously recorded at unteraargletscher switzerland to investigate the generation of excess pore pressures downglacier from ploughing clasts analysis of the strongly correlated pressure records revealed that the amplitude of the pore pressure fluctuations was larger than that of the corresponding borehole pressure fluctuations guided by high temporal resolution measurements of glacier surface motion from a different year this finding suggests that temporal changes in glacier speed lead to changes in the degree of excess pore pressure in front of the ploughing object the generation of excess pore water pressure has the potential to weaken the sediment downglacier from ploughing clasts instead of these clasts providing the roughness at the ice bed interface to resist glacier motion the ploughing process may in fact help to decouple the ice from the bed reducing basal resistance

character of the englacial and subglacial drainage system in the lower part of the ablation area of storglaciaren sweden as revealed by dye trace studies

evolution of the internal drainage system in the lower part of the ablation area of storglaci aren sweden during the 1989 melt season ten tracer experiments were conducted to investigate seasonal diurnal and spatial variations in the internal drainage system in the ablation area of storglaciaren sweden dye was injected into moulins and dye concentration and discharge were monitored in two streams at the glacier terminus sydjakk and nordjakk dye emerged only in sydjakk transit velocities ranged from 0 07 to 0 29 m s implying that drainage was through a well defined conduit system velocity and discharge relations suggest that these conduits are part of a multi branched arborescent network of individually braided wide low passages a progressive decline in dispersivity during the early part of the melt season reflects structural changes in these conduits presumably including a decrease in the degree of braiding and an increase in size the system reached its late season configuration in early august by which time the energy dissipated by the flowing water would have sufficed to melt between 0 7 and 21 m 3 of ice per meter of conduit length depending on conduit size potential closure rates exceed melt rates on conduit walls throughout most of the drainage system and so conduits are probably full of water during periods of higher daily discharge with water pressures exceeding atmospheric pressure variations in tracer return curves and dispersivity on a diurnal time scale also appear to reflect changes in the degree of braiding in this case resulting from drowning of low divides between channels as discharge increases

determining the extent of pressurized flow beneath storglaciaren sweden using results of tracer experiments and measurements of input and output discharge two experiments were conducted on the drainage system beneath the lower part of the ablation zone of storglaciaren a small valley glacier in northern sweden in the first experiment over 70 tracer tests were performed in a cluster of moulins during a 1 month period at sub daily intervals in the second experiment input and output discharge signals were measured on the supraglacial melt stream emptying into a moulin and on the proglacial stream to which the moulin drains the data from these two experiments are used in an idealized model of the subglacial drainage system to calculate the percentage of the system flowing as an open channel results from the tracer experiment suggest that the system is pressurized to within 60 340 m of the snout while analysis of the discharge data indicates pressurized conduits to within 0 415 m of the snout

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

seismic evidence for thermal runaway during intermediate depth earthquake rupture 1 intermediate depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected there are two leading candidates for the physical mechanism behind these earthquakes dehydration embrittlement and self localizing thermal shear runaway a complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate depth earthquake rupture the combination of high stress drop and low radiation efficiency that we observe for mw 4 5 earthquakes in the bucaramanga nest implies a temperature increase of 600 1000 c for a centimeter scale layer during earthquake failure this suggests that substantial shear heating and possibly partial melting occurs during intermediate depth earthquake failure our observations support thermal shear runaway as the mechanism for intermediate depth earthquakes which would help explain differences in their behavior compared to shallow earthquakes

the crust and uppermost mantle structure of southern peru from ambient noise and earthquake surface wave analysis abstract southern peru is located in the northern central andes which is the highest plateau along an active subduction zone in this region the nazca slab changes from normal to flat subduction with the associated holocene volcanism ceasing above the flat subduction regime we use 6 s to 67 s period surface wave signals from ambient noise cross correlations and earthquake data to image the shear wave velocity v s v structure to a depth of 140 km a mid crust low velocity zone is revealed and is interpreted as partially molten rocks that are part of the andean low velocity zone it is oblique to the present trench and possibly indicates the location of the volcanic arcs formed during the steepening of the oligocene flat slab beneath the altiplano plateau the recently subducted slab beneath the forearc shows a decrease in velocity from the normal to flat subduction regime that might be related to hydration during the formation of the nazca ridge which in turn may contribute to the buoyancy of the flat slab the mantle above the flat slab has a comparatively high velocity which indicates the lack of melting and thus explains the cessation of the volcanism above a velocity contrast from crust to uppermost mantle is imaged across the cusco vilcanota fault system and is interpreted as the boundary between two lithospheric blocks

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

geochemistry and mineralogy of sediments and authigenic carbonates from the malta plateau strait of sicily central mediterranean relationships with mud fluid release from a mud volcano system a mud volcano field was recently discovered within the malta plateau in the strait of sicily central mediterranean sea box core sediments and associated authigenic carbonates have been collected in water depths of 140 170 m from two distinctive sectors of the area and analyzed for major trace and rare earth elements stable isotopes and mineralogy relative homogeneity in the mineralogy and geochemistry of bulk sediments and 210pb activity distributions argue against an active mud ejection activity in the malta plateau western sector the sediments show high concentrations of fe as sb and mo exceeding the background values estimated for the strait of sicily active fluid seepage in this area is thought to be responsible for these enrichments and for the formation of authigenic carbonate crusts evidence of bacterial involvement in carbonate mainly aragonite formation is recorded in the authigenic crusts which contain flat and spiralled filaments resembling beggiatoa like sulphide oxidising bacteria and biofilm like remains which occur inside the interspaces carbonate crusts consistently show extremely negative 13c values down to 49 that suggest contribution of carbon originating from anaerobic methane oxidation heavy oxygen isotopic compositions of the authigenic carbonates 2 3 higher than those in box core sediments are interpreted as due to precipitation from fluids enriched in 18o due to seepage of deep isotopically heavy relict messinian brines

subevent location and rupture imaging using iterative backprojection for the 2011 tohoku mw 9 0 earthquake summary knowledge of the rupture speed and spatial temporal distribution of energy radiation of earthquakes is important for earthquake physics backprojection of teleseismic waves is commonly used to image the rupture process of large events the conventional backprojection method typically performs temporal and spatial averaging to obtain reliable rupture features we present an iterative backprojection method with subevent signal stripping to determine the distribution of subevents large energy bursts during the earthquake rupture we also relocate the subevents initially determined by iterative backprojection using the traveltime shifts from subevent waveform cross correlation which provides more accurate subevent locations and source times a bootstrap approach is used to assess the reliability of the identified subevents we apply this method to the mw 9 0 tohoku earthquake in japan using array data in the united states we identify 16 reliable subevents in the frequency band 0 2 1 hz which mostly occurred around or west of the hypocentre in the downdip region analysis of tohoku aftershocks shows that depth phases can often produce artefacts in the backprojection image but the position and timing of our main shock subevents are inconsistent with depth phase artefacts our results suggest a complicated rupture with a component of bilateral rupture along strike the dominant energy radiation between 0 2 and 1 hz is confined to a region close to the hypocentre during the first 90 s a number of subevents occurred around the hypocentre in the first 90 s suggesting a low initial rupture speed and repeated rupture or slip near thehypocentre therupturereachedthecoastalregionabout106kmnorthwestofthehypocentreat43sandtheregionabout110kmnorthofthehypocentreat105swithanorthwardrupture speed 2 0 kms 1 at 60 110 s after 110 s a series of subevents occurred about 120 220 km southwest of the hypocentre consistent wit ha3k ms 1 along strike rupture speed the abundant high frequency radiation in the downdip region close to the coast suggests intermittent rupture probably in the brittle ductile transition zone the lack of high frequency radiation in the updip region suggests the rupture near the trench was more continuous probably due to more homogeneous frictional properties of the shallow slab interface the lack of early aftershocks in the updip region indicates that most of the accumulated slip in the updip region during the interseismic period was probably released during the main shock