earthquake multiplets in the southeastern solomon islands the 350 km stretch of the solomon islands trench that lies adjacent to the islands of guadalcanal and san cristobal has been the site of 10 large shallow earthquakes since 1966 june 15 1966 doublet m s 7 7 7 3 may 20 21 1977 quadruplet m s 6 7 7 5 7 5 7 5 november 4 5 1978 doublet m s 6 9 7 1 and two single events on october 23 1979 m s 7 1 and february 7 1984 m s 7 7 analyses of p wave first motions aftershock distributions p wave forms recorded at epicentral distances of 30 to 90 on the wwssn and 256 s period rayleigh wave spectra obtained from the ida and gdsn networks are used to examine the source characteristics of the events the close temporal association and large size of earthquakes precluded collection of useful seismograms for the latter of the two 1966 events and the second and third events of the 1977 sequence p waveforms analyzed show smooth traces that can be modeled with simple trapezoidal source time functions located at shallow 15 45 km depths focal mechanisms are consistent with oblique subduction of the indian plate beneath the pacific plate at about n73 e estimates of seismic moment for individual events range from 0 2 to 2 3 10 27 dyne cm average stress drops of the sequences are estimated to range between 10 and 40 bars this section of island arc may have ruptured in a similar sequence of earthquakes during the 1930 s conversion of the cumulative seismic moment to displacement and averaging over 40 years suggests a convergence rate of about 4 5 cm year 1 about one half of the rates that have been assessed on the basis of seafloor magnetic lineations

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

paleomagnetism of large igneous provinces case study from west greenland north atlantic igneous province abstract we present new paleomagnetic and multi model stereo photogrammetry data from lava sequences in the west greenland part of the north atlantic igneous province naip the joint analyses of paleomagnetic and photogrammetric data yield a well defined paleomagnetic pole located at lat 73 6 n long 160 5 e n 44 95 6 2 k 13 1 age 61 55 ma which is statistically indistinguishable from a pole recently obtained for the eurasian part of the naip on faroe islands riisager et al earth planet sci lett 201 2002 261 276 combining the two datasets we obtain a joint naip paleomagnetic pole in greenland coordinates lat 71 1 n long 161 1 e n 87 95 4 3 k 13 6 age 61 54 ma the results presented here represent the first study in which photogrammetry profiles were photographed at the exact same locations where paleomagnetic fieldwork was carried out and a direct flow to flow comparison of the two datasets is possible photogrammetry is shown to be particularly useful because of i highly precise dip strike measurements and ii detailed field observations that can be made in the laboratory highly precise determination of the structural attitude of well exposed kanisut mb lava sequences demonstrates that their apparently reliable in field dip strike measurements typically are up to 6 wrong erroneous dip strike readings are particularly problematic as they offset paleomagnetic poles without affecting their confidence limits perhaps more important for large igneous provinces is the recognition of a variable temporal relationship between consecutive lava flows we demonstrate how correct interpretation of paleosecular variation facilitated by the detailed photogrammetry analysis is crucial for the rapidly emplaced vaigat formation lavas inaccurate tectonic correction non averaged paleosecular variation and unrecognized excursional directions may perhaps explain why coeval paleomagnetic poles from large igneous provinces are often discordant the difference between the joint naip paleomagnetitc pole and apparent polar wander path poles suggests that they may be less reliable than suggested by their confidence limits

basin seismic stratigraphy and aspects of prospectivity in the ne baffin bay northwest greenland abstract subsurface mapping in the northeastern part of baffin bay offshore northwest greenland indicates the presence of large basement structures and sedimentary basins we identify eight seismic stratigraphic units that can be genetically related to the regional geology and tectonic development the seismic interpretation includes the top of paleocene to eocene volcanic rocks which cover most parts of the southern and western study area using this horizon as a time marker the seismic units are divided into pre cenozoic and palaeogene to neogene strata based on comparison to equivalent units exposed onshore west greenland and canada and sampled by exploration wells farther south in the disko west offshore nuuk region our interpretation of the faults and tectonic elements suggest that the main extensional faults and basin formation took place mainly during the cretaceous and into the paleocene subsequent compressional tectonics and inversion were accompanied by strike slip faulting and local transtensional faults and flexures this last phase of deformation corresponds to the well documented eurekan orogeny in ellesmere island and northern greenland that developed as a result of northward motion and possible counterclockwise rotation of greenland relative to the north american plate we surmise that these tectonic phases exerted a strong control on basin infill by activation of hinterland sediment sources during cretaceous and palaeogene times the deep basins may include cretaceous and palaeogene organic rich sediments with hydrocarbon source potential we estimate that maturation could have started during the eocene

apatite u th sm he thermochronology on the wrong side of the tracks abstract despite the increasing application of apatite u th sm he thermochronology in a range of settings the technique suffers from two major unresolved problems which preclude reliable thermal history interpretations on a routine basis one problem is the common but unexplained over dispersion of single grain ages compared to predictions based on accepted models of diffusion systematics including enhanced he retentivity due to the buildup of radiation damage in the apatite crystal lattice a second related problem is that the widely adopted rdaam model does not provide an accurate representation of the degree to which helium retentivity is enhanced as a result of the accumulation of radiation damage until these problems are resolved and eliminated thermal history interpretations derived from this method cannot be regarded as reliable routine application of the technique requires a more rigorous quantitative understanding of these factors further experimental studies are required to identify all significant sources of variation in measured ages and to develop more accurate models of the thermal response of the system combined with rigorous calibration and validation against independent methods in well constrained natural settings only once this is achieved will the technique be capable of producing robust and reliable interpretations

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

a simple discrete element model for large multiplet earthquakes adjacent segments of the plate boundary may fail separately as earthquakes that occur very close in time when the time between events is a small fraction of the recurrence interval then the sequence is referred to as doublets or multiplets in the first part of this paper we determine the frequency of occurrence of large multiplet earthquakes we count the number of multiplet earthquakes present in the abe catalogue with aftershocks removed ms 7 1 using a series of space time and magnitude filters we compare this number to the number of multiplet earthquakes present in the abe catalogue where origin times of earthquakes are randomized and epicentral locations and magnitudes are kept constant the difference between the observed and random number of multiplets gives us the percentage of multiplets that do not occur randomly we find that this percentage ranges from 2 to 7 of all large earthquakes based on the best choice of space time and magnitude filters in the second part of this paper we model multiplet earthquakes using three simple frictional slider models the models consist of slider blocks resting on a frictional conveyer belt and interconnected by springs the seismogenic portion of the plate interface is represented by blocks that exhibit stick slip behavior asperities aseismic creep within the seismogenic zone is represented by creepers the creepers are blocks that obey a simple linear creep law the three models differ in that the positions of the creeping blocks within the system of asperities and creepers are different we find that model 3 produces a complex sequence of events consisting of single double and multiplet earthquakes these synthetic sequences of events are comparable to the earthquake cycle observed along nankai trough model 3 produces a high percentage of multiplet earthquakes when the creeper is allowed to relax within a small fraction of the maximum recurrence time of the event sequence this implies that the details of the frictional law invoked for the creeper blocks does not influence the model results significantly the time evolution of model 3 is compared to the postseismic phase of the loading cycle at muroto point japan we suggest that the rapid postseismic deformation that occurs immediately after the nankaido earthquake is due to a rapid creeping zone that is in this region

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

exhuming the top end of north america episodic evolution of the eurekan belt and its potential relationships to north atlantic plate tectonics and arctic climate change

comment on a new model for the paleogene motion of greenland relative to north america plate reconstructions of the davis strait and nares strait regions between canada and greenland by g n oakey and j a chalmers

uplift and denudation of the central alaska range a case study in the use of apatite fission track thermochronology to determine absolute uplift parameters apatite fission track thermochronology aftt on granitic samples collected in the central alaska range in conjunction with geologic constraints from basins to the north nenana basin and south cook inlet of the range is used to constrain the timing amount rate and pattern of surface uplift rock uplift and denudation since the late miocene the conversion from a thermal frame of reference apatite fission track data to an absolute frame of reference with respect to mean sea level which requires constraining the paleoland surface elevation the paleomean annual temperature and the paleogeothermal gradient is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone paz apatite ages at denali mount mckinley range from 16 ma near the summit 6 km elevation to 4 ma at 2 km elevation a distinctive break in slope in the apatite age profile at an elevation of 4 5 km also marked by a change in confined track length distributions marks the base of an exhumed apatite paz rock uplift and denudation are greatest at denali decreasing southward away from the mckinley strand of the denali fault system as shown by progressively older apatite ages 7 35 ma from a suite of samples along the kahiltna glacier a correlative decrease in topography occurs southward from the fault the central alaska range lies within an arc defined by the denali fault with the highest peaks including denali concentrated at the arc apex patterns of rock uplift and denudation within the central alaska range mimic topography between early and late miocene and possibly earlier the central alaska range was most likely an area of relative tectonic and thermal stability rock uplift denudation and mean surface uplift of the denali region began by the late miocene 5 6 ma being 8 5 km 5 7 km and 2 8 km respectively at average rates of 1 5 km m y 1 km m y and 0 5 km m y the amount of rock uplift denudation and surface uplift decreases to 3 km 2 km and 1 km at little switzerland some 45 km south of the denali fault we conclude that the topographic and rock uplift patterns of the central alaska range the shape and proximity of the mckinley strand of the denali fault to these patterns the timing of the onset of rock uplift and denudation at 5 6 ma and a significant change in relative plate motion between north america and the pacific plates circa 5 6 ma are all inherently related

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

the first evidence for morb like lavas from the outer mariana forearc geochemistry petrography and tectonic implications abstract the mariana forearc the region between the active mariana volcanic arc and the trench axis is considered to be composed of rocks entirely of arc origin this conclusion was based upon the lack of an accretionary wedge and recovery of arc volcanics from sites throughout the forearc including the inner trench wall new chemical data from dredged samples demonstrate that exposures of altered basalts similar to mid ocean ridge basalt morb and of ocean island basalt oib exist in the mariana forearc several forearc sites including a 2000 m scarp bounding a large graben at 19 30 n two small scarps in the inner trench wall and a seamount were dredged morb like and oib lavas in addition to island arc tholeiite iat and boninite were recovered all samples exhibit low temperature seafloor alteration and some are metamorphosed to lower greenschist facies iat samples have low tio 2 2 of 1 2 and zr of 60 93 ppm oib samples typically have high tio 2 3 and zr 180 ppm several oib samples contain modal nepheline and all have high alkali contents the presence of oib and morb like lavas in the outer forearc may suggest either that entrapment of philippine sea or kula plate or that obduction of pacific plate has occurred in the evolution of the mariana forearc however the intimate association of morb like lavas with arc lavas in the fault scarp requires that either iat magma was injected into entrapped or obducted oceanic crust or that morb like magma was injected into an arc crust during forearc rifting or deformation

centroid moment tensor solutions for 113 large earthquakes in 1977 1980 abstract the centroid moment tensor technique is applied to 113 large earthquakes of the period 1977 1980 including all the shallow events with magnitude mb ms or ml 6 5 we use digital data recorded by the global digital seismograph network gdsn and international deployment of accelerometers ida networks solutions are determined for three sequences of events close in time by inverting mantle wave data simultaneously for multiple sources

the thermal history of the nares strait kane basin and smith sound region in canada and greenland constraints from apatite fission track and u th sm he dating apatite fission track and u th sm he data for samples with proterozoic neogene lithologic ages from eastern ellesmere devon and baffin islands in the canadian arctic archipelago and northwest greenland provide insights into the complex thermal history of this region most data reflect exhumational cooling of the region as a sediment source area for the developing sverdrup basin to the west during late paleozoic mesozoic time samples proximal to the basin margin record late paleozoic cooling consistent with erosion of uplifted rift flanks and widespread clastic sedimentation during early rifting and thermal subsidence data from distal samples are consistent with early mesozoic epeirogenic denudational cooling during periods of high sediment supply from well developed river systems around northern baffin bay the data record erosional cooling of uplifted rift flanks in response to late cretaceous rifting rapid denudation rates are indicated for southeastern devon island compatible with thick upper cretaceous strata offshore in lancaster sound slower denudation rates are indicated for northwest greenland along nares strait data reflect mafic volcanism or erosional exhumation associated with strike slip and or thrust displacements during the paleogene eurekan orogeny and offer limited support for the existence of a wegener fault fission track ages older than those obtained in central ellesmere island however suggest that active tectonics occurred earlier near the strait and strike slip tectonism and deformation were likely partitioned across a relatively wide belt apatites obtained from archean gneisses and franklinian dykes have up to 500 ppm sm and up to 15 of the radiogenic he is sm derived

recent large earthquakes along the middle american trench and their implications for the subduction process several large shallow earthquakes ms 7 0 have occurred along the middle american trench since the installation of the wwssn network included are the 1965 1968 and 1978 oaxaca events the 1970 chiapas event the 1973 colima event and the 1979 petatlan event these earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes the events have seismic moments ranging from 1 0 1027 dyne cm for the 1968 event to 3 2 1027 dyne cm for the 1978 event all events are of predominantly thrust type consistent with subduction to the northeast of the cocos plate body waves for the 1965 1968 1978 and 1979 events along the trench indicate rather simple faulting processes these events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars the 1970 and 1973 events the easternmost and westernmost respectively of the events studied here are located close to triple junctions for the cocos north america caribbean plates 1970 and the cocos north america rivera plates 1973 these two events generated more complex body waves than the oaxaca and petatlan quakes this source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters this may offer a new perspective to viewing source complexity in relation to subduction zones

body wave waveform constraints on the source parameters of the yangjiang china earthquake of july 25 1969 a devastating earthquake in a stable continental region on july 25 1969 anm s 5 9 earthquake shook the kwangtung province near yangjiang china casualties and extensive damage were reported in the epicentral area the yangjiang earthquake occurred within the passive margin located along china s southeast coast this stable continental setting is seismically one of the most quiet regions of china historic records indicate this earthquake to have been the first devastating one in the area a remarkable feature of this earthquake sequence is that its foreshocks and aftershocks are relatively small in terms of number and magnitude despite the relatively large main shock waveforms of long and short period teleseismicp and long period teleseismicsh waves have been modeled to estimate the source parameters for this earthquake the focal mechanism determined is predominately strike slip with a small normal component strike 263 dip 79 rake 194 and is in good agreement with observations in the epicentral area regarding dislocation direction of fissures distribution of aftershocks and isoseismals the average seismic moment is 5 15 1024 dyne cm and the focal depth is estimated to be 9 km the earthquake is characterized by a rather short source time function and a high stress drop of approximately 380 bars the observed pattern of the foreshock main shock aftershock sequence is interpreted in terms of the large stress drop associated with the main shock and the material properties at and near the source thep axis orientation lies roughly nw se and is consistent with the maximum compressive stress observed along the coast of southeast china this stress orientation is consistent with the nw subducting philippine sea plate and the se ward push from central china due to the convergence between indian and eurasian subcontinents

a bimodal hybrid model for time dependent probabilistic seismic hazard analysis the evaluation of evidence provided by geological studies and historical catalogs indicates that in some seismic regions and faults multiple large earthquakes occur in cluster then the occurrences of large earthquakes confront with quiescence and only the small to moderate earthquakes take place clustering of large earthquakes is the most distinguishable departure from the assumption of constant hazard of random occurrence of earthquakes in conventional seismic hazard analysis in the present study a time dependent recurrence model is proposed to consider a series of large earthquakes that occurs in clusters the model is flexible enough to better reflect the quasi periodic behavior of large earthquakes with long term clustering which can be used in time dependent probabilistic seismic hazard analysis with engineering purposes in this model the time dependent hazard results are estimated by a hazard function which comprises three parts a decreasing hazard of last large earthquake cluster and an increasing hazard of the next large earthquake cluster along with a constant hazard of random occurrence of small to moderate earthquakes in the final part of the paper the time dependent seismic hazard of the new madrid seismic zone at different time intervals has been calculated for illustrative purpose

geological setting and fluid inclusion characteristics of a lead copper barium occurrence hosted in a neoproterozoic mafic sill at kiatak northumberland island northwestern greenland abstract we describe a pb cu ba occurrence in northumberland island northwestern greenland mineralization occupies the brecciated upper contact domain of a neoproterozoic diabase sill belonging to the franklin thule dike swarm 720 716 ma with pyrite bearing black shales of the dundas group upper thule supergroup the host tholeiitic diabase sill is of different composition low tio 2 and p 2 o 5 than the locally crosscutting dikes high tio 2 and p 2 o 5 chloritization carbonatization and silicification are intense in proximity to sulfides coarse grained open space filling galena and minor chalcopyrite are accompanied by 2 generations of calcite and 2 of barite galena contains significant amounts of ag av 400 ppm sb av 700 ppm se av 20 ppm traces of bi cd and sn fluid inclusions in the gangue of the kiatak occurrence indicate two fluid types prior to galena precipitation a cacl 2 nacl rich aqueous brine 20 wt eq nacl cooled from temperatures 300 c and was trapped first in early calcite and with further cooling in barite together with solid bitumen inclusions following galena crystallization secondary inclusions containing a similar brine but of lower salinity higher ca na ratio and lower temperature were trapped in calcite corrosion of galena was followed by precipitation of lower temperature 100 c barite from a second fluid comprising immiscible water and ch 4 despite its location in the contact between shale and large mafic sill the low temperature mineralization postdates the cooling of the sill and may be related to basinal fluid circulation controlled by regional extensional faults parallel to diabase dikes although uneconomic the kiatak occurrence may be witness to a larger metallogenic process that could have formed significant sedex type metal concentrations in strata within the thule supergroup

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

the gilbert islands republic of kiribati earthquake swarm of 1981 1983 abstract a major swarm of intraplate earthquakes at the southeastern end of the gilbert islands chain 3 5 s 177 5 e commenced in december 1981 and lasted through march 1983 no seismicity had been reported in the historical record in this region prior to 1981 but during the swarm 217 events with mb 4 0 were located by the neis with 86 events having mb 5 0 the source region is quite remote and the uniform detection level for the neis is for mb 4 8 a b value of 1 35 is found for the swarm using the maximum likelihood method four events in the sequence were large enough mb 5 6 5 9 to determine focal mechanisms teleseismically using body and surface wave analysis these events are found to have a range of mechanisms from predominantly thrust with a significant oblique component to purely strike slip the compression axes are consistent for all four events with horizontal orientation trending nne ssw this orientation is perpendicular to the direction of plate motion the events are located at depths between 15 and 20 km placing them deep in the oceanic crust or in the upper mantle no obvious bathymetric feature can be related to the fault plane orientations though there is an offset in the island chain near the epicenters while some characteristics of the swarm suggest a magmatic origin the nature of the focal mechanisms the location of the swarm and the large accumulated moment release of the sequence favor a tectonic origin

worldwide doublets of large shallow earthquakes abstract we investigated all the pairs of mw 7 5 shallow earthquakes in the harvard catalog that occurred at a centroid distance of less than 100 km we showed that most of these pairs have similar focal mechanisms because these earthquakes generally should have focal regions in excess of 100 km diameter their rupture zones apparently intersect for all these pairs the time interval is significantly less than the time span needed for plate motion to accumulate the strain released by the first event these observations conflict strongly with quasi periodic recurrence models on which the seismic gap hypothesis is based power law recurrence fits these earthquake ob servations much better introduction many models used to forecast long term seismic hazard are based on the quasi periodic or seismic cycle hypothesis of large earthquake occurrence this hypothesis assumes that the probability of a large earthquake is low after signifi cant release of seismic energy and that danger increases only when tectonic plate motion accumulates strain sufficient for a new strong event several models have been formulated to apply this idea to long term earthquake prediction mccann

a structural fabric defined by topographic lineaments correlation with tertiary deformation of ellesmere and axel heiberg islands canadian arctic digital topographic contours from four 1 250000 scale maps have been gridded to produce a digital elevation model for part of ellesmere and axel heiberg islands in the canadian arctic islands gradient calculations were used to define both east and west dipping slopes defining a pattern of lineaments that have been compared with mapped geological structures in ice covered areas where geological mapping was not possible well defined topographic lineaments have been identified and are correlated to extensions of major structural features the northeast southwest patterns of both topographic lineaments and mapped structures are strongly unimodal and support a single compressive event oriented at 67 west of north this orientation is coincidental with the convergence direction calculated from the kinematic poles of rotation for greenland relative to north america between 56 and 35 ma a minor secondary peak at 70 east of north is observed for thrust and normal fault solutions and is not directly related to the predicted convergence direction whether this represents a unique phase of deformation or is a subcomponent of a single event is not known the agreement of structural components lineament orientations and convergence direction suggests an overwhelming overprint of eurekan deformation on any preexisting structural fabric this study confirms for the first time an excellent compatibility between geological and geophysical constraints for the timing and geometry of the eurekan orogeny

helium diffusion and u th he thermochronometry of titanite we have explored the diffusivity characteristics of radiogenic he in titanite sphene and have developed analytical techniques for u th he dating of this mineral results of incremental he outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44 6 3 4 2 kcal mol and a frequency factor of 60 cm 2 s diffusivity is highly linearly correlated with the inverse square of the grain size indicating that the he diffusion domain in titanite is the crystal itself for typical titanite grains of 200 to 800 m minimum dimension the he closure temperature is in the range 191 to 218 c for a cooling rate of 10 c myr there is no indication in the titanites we studied that radiation damage plays a major role in he diffusion u th he ages of titanites from quickly cooled rocks yield ages with 5 8 1 reproducibility that are concordant with known ages and u th he ages of titanites from slowly cooled rocks are consistent with independently established cooling paths supporting the closure temperature estimates these experiments suggest that titanite u th he ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40 ar 39 ar dating but higher than apatite fission track or u th he dating

paleomagnetism and u pb geochronology of the clarence head dykes arctic canada orthogonal emplacement of mafic dykes in a large igneous province the north south trending clarence head dyke swarm located on devon and ellesmere islands in the canadian high arctic has a trend orthogonal to that of the neoproterozoic franklin swarm that surrounds it the clarence head dykes are dated by the u pb method on baddeleyite to between 716 1 and 713 1 ma ages apparently younger than but within the published age range of the franklin dykes alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages but a comparison of the u pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the clarence head dykes are indeed a distinctly younger event within the period of franklin magmatism the clarence head dykes represent a large swarm tangential to and cogenetic with a giant radiating dyke swarm 800 km from the indicated source the preferred mechanism for the emplacement of the clarence head dykes is the exploitation of concentric zones of e

a fission track transect across nares strait canada greenland further evidence that the wegener fault is a myth many workers continue to model nares strait between greenland and ellesmere island canada as a plate boundary and locus of major cenozoic compression and strike slip marked by the postulated wegener transform fault this despite continuity of precambrian paleozoic provinces across the seaway including an undeformed neoproterozoic mafic dyke swarm that crosses at a high angle as a test of the speculative plate kinematic modelling apatite fission track aft dating was undertaken along a transect across the paleoproterozoic shield of smith sound at the south end of nares strait running from sea level to ca 600 m elevation and as much as 70 km inland on both sides the shield is overlain by three sedimentary basins separated by major hiatuses the aft ages fall into two groups 282 34 and 650 51 ma they indicate maximum depth was 4 km in the lower paleozoic from both coasts to 50 km inland and at 50 km inland temperatures were below the annealing interval throughout phanerozoic time durin

the existence of an inner core super rotation questioned by teleseismic doublets abstract the first evidence for a differential rotation of the earth s inner core with respect to the mantle comes from the detection of a variation over 30 years of the differential travel time anomalies of seismic core phases song x richards p g 1996 seismological evidence for differential rotation of the earth s inner core nature 382 221 224 the comparison of teleseismic doublets provides a powerful means to measure such differential travel times and to test their temporal variation contrary to direct measurements of differential travel times this technique does not require to have an accurate location of the seismic events this method is first checked on examples of doublets for which no differential travel time is expected these tests allow us to point out the difficulties such as polarity reversals which may be encountered in the use of the doublet method it is then applied to deep tonga events recorded in france and to the south sandwich islands events recorded at station college col alaska which led song and richards to propose an inner core rotation for the first path we find no time variation in agreement with previous studies for the path from south sandwich island to col the doublet analysis shows that no significant temporal change in pkp travel time can be detected within the resolution of the method which is about 0 05 s the variation of the residuals observed by song and richards about 0 3 s over 30 years is in a large extent ascribable to hypocenter mislocations consequently present seismological observations do not detect the inner core differential rotation because if it exists it is smaller than about 0 2 year the present detection capacity of seismology

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

large earthquake doublets and fault plane heterogeneity in the northern solomon islands subduction zone in the solomon islands and new britain subduction zones the largest earthquakes commonly occur as pairs with small separation in time space and magnitude this doublet behavior has been attributed to a pattern of fault plane heterogeneity consisting of closely spaced asperities such that the failure of one asperity triggers slip in adjacent asperities we analyzed body waves of the january 31 1974 m w 7 3 february 1 1974 m w 7 4 july 20 1975 14 37 m w 7 6 and july 20 1975 19 45 m w 7 3 doublet events using an iterative multiple station inversion technique to determine the spatio temporal distribution of seismic moment release associated with these events although the 1974 doublet has smaller body wave moments than the 1975 events their source histories are more complicated lasting over 40 seconds and consisting of several subevents located near the epicentral regions the second 1975 event is well modeled by a simple point source initiating at a depth of 15 km and rupturing an approximate 20 km region about the epicenter the source history of the first 1975 event reveals a westerly propagating rupture extending about 50 km from its hypocenter at a depth of 25 km the asperities of the 1975 events are of comparable size and do not overlap one another consistent with the asperity triggering hypothesis the relatively large source areas and small seismic moments of the 1974 doublet events indicate failure of weaker portions of the fault plane in their epicentral regions variations in the roughness of the bathymetry of the subducting plate accompanying subduction of the woodlark rise may be responsible for changes in the mechanical properties of the plate interface

an earthquake doublet in ometepec guerrero mexico on june 7 1982 an earthquake doublet occurred in a gap near ometepec guerrero mexico which had been given a high seismic potential the two earthquakes first event m s 6 9 m b 6 0 16 3 n 98 4 w d 25 km second event m s 7 0 m b 6 3 16 4 n 98 5 w d 8 km of the doublet occurred within five hours of each other we determine the source parameters of these events by inverting surface wave data at a period of 256 s the results are for the first event strike 116 dip 77 slip 88 and seismic moment of 2 8 10 26 dyne cm and for the second event strike 116 dip 78 slip 89 and seismic moment of 2 8 10 26 dyne cm modeling of long period p waves suggests that the first event has a depth of 20 km and is represented by a single trapezoidal source time function with an effective duration of 6 s the second event is best modeled by two sources at depths of 15 and 10 km the combined effective source duration time for the two sources is about 10 s the ratio of the seismic moment obtained from body waves to that from surface waves is 0 5 for the first event and 1 for the second adding the seismic moment of the two events and considering the first week aftershock area 3200 km 2 the stress drop is 4 bars these results suggest that the first event that involved a deeper asperity caused an incremental stress change large enough to trigger the second event if the two events of the doublet broke distinct areas of the subduction zone the coseismic slip is 0 58 m and accounts for about 25 of the total plate motion between the cocos plate and the north america plate accumulated since the last large earthquake in the region other doublets similar to the ometepec doublet have occurred along the middle america trench during the past 70 years a regional distribution of comparable size asperities may be responsible for this relatively frequent occurrence of doublets and for the simplicity of earthquakes in the region the high convergence rate which produces rapid strain accumulation and short recurrence intervals for large earthquakes and the smooth sea floor subducted along the middle america trench may contribute to the homogeneous distribution of comparable size asperities we found a relation log t 1 3 log m 0 t is the average recurrence time and m 0 is the average seismic moment for large earthquakes along the subduction zone in the guerrero oaxaca region where the convergence rate and the properties of the subducted plate are considered relatively uniform a simple asperity model predicts this relation