reverse slip structures at oceanic diverging plate boundaries and their kinematic origin data from tertiary crust of west and south iceland abstract normal faulting predominates within rift zones but reverse fault plane solutions are occasionally found for earthquakes along the mid atlantic ridge furthermore field observations of striae and in situ measurements in places show off rift compression and reverse faulting we analyse eight cases of reverse slip motion observed in the eroded tertiary crust of the borgarfjor ur and hreppar rift jump blocks respectively in west and south iceland these structures were the only ones observed where both striae and marker horizons could be used to deduce the sense of reverse slip motion the vertical displacements are less than 10 m and the motions are of local origin we believe these structures are formed independently from regional compression as they have variable kinematic origins some occur in association with dykes sills or cone sheet without evidence of horizontal shortening across intrusions others are due to local bends of regional steeply dipping normal faults or secondary fractures riedel or secondary fractures reverse slip striations are found at the irregularities of steeply dipping normal faults or at the tips of steeply dipping normal faults uplifted by an underlying propagating dyke striations even conspicuous ones do not necessarily reflect significant amounts of displacement if horizontal compression acts in iceland its magnitude is not sufficient to shorten the rocks and to close the fractures of various tectonic settings the observed examples are explained as local kinematic adjustments to companion structures without regional compressional stress field or shortening and can lead to misinterpretation if not put into the correct context

on strike slip faulting in layered media summary we study the effects of structural inhomogeneities on the stress and displacement fields induced by strike slip faults in layered media an elastic medium is considered made up of an upper layer bounded by a free surface and welded to a lower half space characterized by different elastic parameters shear cracks with assigned stress drop are employed as mathematical models of strike slip faults which are assumed to be vertical and planar if the crack is entirely embedded within the lower medium case a a cauchy kernel integral equation is obtained which is solved by employing an expansion of the dislocation density in chebyshev polynomials if the crack is within the lower medium but it terminates at the interface case b a generalized cauchy singularity appears in the integral kernel this singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface finally the case of a crack crossing the interface is considered case c the crack is split into two interacting sections each placed in a homogeneous medium and both open at the interface two coupled generalized cauchy equations are obtained and solved for the dislocation density distribution of each crack section an asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface where a jump discontinuity is present as a corollary the stress drop must be discontinuous at the interface with a jump proportional to the rigidity contrast between the adjoining media this finding is shown to have important implications for the development of geometrical complexities within transform fault zones planar strike slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack interface junction planar strike slip faulting may take place only in mature transform zones where a repetitive earthquake cycle has already developed if the rheology is perfectly elastic otherwise the fault cannot be planar we infer that strike slip faulting at depth is plausibly accompanied by en echelon surface breaks in a shallow sedimentary layer where the stress drop is lower than prescribed by the discontinuity condition while ductile deformation or steady sliding at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer endowed with lower rigidity but higher stress

geologic investigations of a slip gap in the surficial ruptures of the 1992 landers earthquake southern california a 3 km long gap in the dextral surficial rupture of the 1992 mw 7 3 landers earthquake occurs at the north end of a major fault stepover between the johnson valley and homestead valley faults this gap is situated along a segment of the landers rupture that has been modeled geophysically as having a deficit in average slip at depth to better evaluate the nature of the slip gap we document in detail the character and distribution of surficial rupture within it along the gap is a northwest trending thrust fault rupture with an average of less than 1 rn of northeast directed reverse slip and nearly no oblique right slip we interpret this rupture to be limited to the shallow crust of the northern end of the stepover and to have been the secondary result of dextral shear rather than a mechanism of rigid block slip transfer from the landers kickapoo fault a zone of en echelon extensional ruptures also occurs along the slip gap which we interpret as the secondary result of diffuse dextral shear that accommodated less than 0 5 rn of west northwest extension these secondary ruptures represent a discontinuity in the surficial dextral rupture of the landers earthquake which we propose resulted from the lack of a mature fault connection between the johnson valley and homestead valley faults the rupture pattern of the slip gap implies a significant deficit in net surficial slip which compares favorably with some geophysical models aspects of this rupture pattern also suggest a temporal sequence of rupture that compares favorably with geophysical interpretations of the dynamic rupture propagation

fault kinematics at the hengill triple junction sw iceland derived from surface fracture pattern abstract the hengill triple junction in iceland is formed by the intersection of the western volcanic zone wvz reykjanes peninsula oblique rift rpor and a transform zone the south iceland seismic zone sisz the 19 13 km2 study area thus contains extensional structures related to volcanic systems of the two rift zones hengill and hromundartindur as well as strike slip structures characteristic of the transform zones high definition aerial photographs satellite images and digital elevation models are used supported by field studies to accurately map and classify tectonic features in an effort to separate n s trending strike slip faults from the fissure swarms of the volcanic systems a portion of the southern hengill fissure swarm lies within the area it trends ne sw and exhibits extensional features characteristic of spreading ridges i e normal faults open fractures and eruptive fissures it is clearly delineated by surface fractures and is traced a length of 28 km southwest from the central volcano terminating before reaching the coastline the fissure swarm is 2 3 5 km wide in the study area widening southwards elevation profiles across the fissure swarm suggest a spreading rate comparable to the southern wvz which spreads at 4 5 mm year a portion of the nesjahraun eruptive fissure is anomalously smooth and may represent secondary rifting east west shearing in the rpor and sisz is accomplished by many smaller and less conspicuous northerly oriented strike slip faults in a bookshelf faulting manner these parallel faults are overprinted on the volcano tectonic structures of the rpor evidence of 11 northerly striking strike slip faults is found within the study area the zone of bookshelf faulting is thus shown to be continuous across the triple junction area from the sisz to the rpor finally several structures eruptive fissures exposed dikes and faults strike near perpendicular to the fissure swarm and may be the result of changes in the direction of maximum stress caused by rifting events however their tectonic significance remains uncertain

the structure of seismogenic strike slip faults in the eastern part of the reykjanes peninsula oblique rift sw iceland abstract a considerable part of the seismic risk to the capital area of iceland is provided by a series of strike slip faults at the mid atlantic plate boundary within a distance range of 15 35 km about two thirds of the icelandic population lives within this distance range from the source areas of m 6 6 5 earthquakes the structure of the plate boundaries in sw iceland is relatively complex three branches meet in the hengill triple junction the oblique rift of the reykjanes peninsula the south iceland seismic zone a transform zone and the western volcanic zone a rift zone the reykjanes peninsula oblique rift has an over all trend of 70 highly oblique with respect to the spreading direction 101 in this region it contains both volcanic systems and seismogenic strike slip faults the fissure swarms of individual volcanic systems contain normal faults and fissures with a ne trend also quite oblique to the plate boundary the fissure swarms fade out towards the ne and sw as they extend into the plates on either side overprinting this pattern of volcano tectonic structures are sets of parallel northerly striking transcurrent faults that generate the largest earthquakes in this zone their surface expressions are en echelon fracture arrays and push up structures the sense of displacement is right lateral in the 15 km long section of the rift studied here we find evidence for at least six faults of this type the length of individual faults may exceed 12 km the distance between them varies from 1 to 5 km and together they form a bookshelf type fault system taking up the left lateral component of plate movements across the oblique rift zone

source mechanism of the 1987 vatnafjoll earthquake in south iceland the may 25 1987 vatnafjoll earthquake occurred on the mid atlantic plate boundary at a junction between a transform the south iceland seismic zone and a spreading segment the eastern volcanic zone the south iceland seismic zone has had the most destructive earthquakes in the history of iceland with average intervals of 80 100 years and estimated maximum magnitudes around 7 the 1987 vatnafjoll earthquake was the largest earthquake mb 5 8 ms 5 8 mw 5 9 in south iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region foreshocks and aftershocks of the vatnafjoll earthquake were located with relatively good accuracy they define an elongated n s epicentral area 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km we determined centroid source parameters of the vatnafjoll earthquake by inverting long period teleseismic p and sh waves the centroid source mechanism and foreshock and aftershock distribution indicate n s right lateral strike slip faulting on a near vertical plane the rupture initiated near the base of the crust and propagated upward along an approximately 10 km long fault but did not reach the surface the centroid depth is well constrained and is between 5 and 8 km depth at the 95 confidence level the seismic moment is 9 1 1024dyn cm the source has relatively short duration of 3 s which may indicate higher stress drop than global average of earthquakes of this size the south iceland seismic zone is a 70 km long e w striking transform zone between two overlapping rift zones the eastern and western volcanic zones it is characterized by bookshelf type of tectonics that is there is no surface faulting in the direction of the transform instead n s right lateral strike slip faults are distributed along and perpendicular to the transform zone the vatnafjoll earthquake has characteristics of earthquakes in the south iceland seismic zone and is probably not related directly with magmatic activity in the eastern volcanic zone the transform zone thus extends farther into the rift zone than was previously recognized

stress changes modelled for the sequence of strong earthquakes in the south iceland seismic zone since 1706 the south iceland seismic zone is roughly speaking situated between two sections of the mid atlantic ridge i e the reykjanes ridge southwest of iceland and the eastern volcanic zone on the island it is a transform zone where earthquakes are expected to occur on e w trending left lateral shear faults equivalent to conjugate n s oriented right lateral rupture planes in fact earthquakes take place on en echelon n s oriented faults which is indicated by the distribution of main shock intensities aftershocks as well as by surface fault traces the stress field continuously generated in the fault zone by opening of the adjacent ridges is computed and superimposed on the stress field changes induced by a series of 13 earthquakes m 6 between 1706 and 2000 the level of the pre seismic stress field is analysed as well as the size of the area under high stress finally the post seismic stress field of june 2000 is analysed to see where high stresses might have accumulated the modelling indicates that the rupture planes located on separated parallel n s striking zones are dense enough to lead to an area wide stress release by the series of events the obtained pre seismic stress level for most events is high and stable with the exception of situations when several strong shocks occur over a time span of several days i e display typical main shock aftershock patterns the size of areas under high stress aside from of the rupture plane i e where no event occurs at the specific time is of medium to small size

push ups fracture patterns and palaeoseismology of the leirubakki fault south iceland abstract field measurements and gps mapping were conducted along the leirubakki fault a 010 trending right lateral strike slip fault located in the holocene lava flows of the south iceland seismic zone because of poor exposures south and north of the holocene lava flow dissected by the fault its length can only be traced for 7 5 km the exposed surface rupture contains fractures that range in size over four orders of a magnitude from the main fault through fault segments 1000 2000 m long fracture arrays 100 250 m long to individual fractures 8 125 m long detailed measurements were made of 63 push ups located at junctions between individual fractures and between fracture arrays and ranging in maximum height from 0 35 to 4 35 m using the push ups the maximum right lateral displacement on the leirubakki fault is estimated at 2 67 m using empirical relationships established globally it is concluded that for a maximum displacement of 2 67 m the true rupture length of the leirubakki fault is around 50 km and that it gave rise to an earthquake of a moment magnitude 7 1

tectonic significance of the south iceland seismic transform zone the subaerial expression of the mid atlantic ridge on iceland comprises two overlapping spreading axes referred to as the west volcanic zone wvz and the east volcanic zone evz respectively the way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes we found four different strike slip stress phases an early n s compression phase a preceded a ne sw compression phase b this phase was followed by a se nw compression phase c however we cannot date an e w compression phase d relative to the other phases numerical modeling based on the assumption that the wvz has been permanently active during the last 3 myr and that the evz is propagating southward confirms that stress directions have rotated clockwise by more than 140 these results fit perfectly with our field analysis and we propose that phase a corresponds to initial evz ridge tip propagation phase b to emplacement of the evz southern tip near torfajokull and phase c to an extreme southern location near the surtsey islands of the evz southern tip phase d could correspond to an intermediate stage between phases b and c we suggest that this sequence of tectonics recorded in a regional overlapping ridge tip setting is directly analogous to smaller scale and more common phenomena at second order ridge discontinuities throughout the global mid ocean ridge system

rift transform kinematics in south iceland deformation from global positioning system measurements 1986 to 1992 crustal deformation in the plate boundary regions in south iceland is estimated from repeated global positioning system gps geodetic measurements in the period 1986 1992 we compare coordinate solutions for the 1986 and 1989 surveys with the results from the most recent survey in 1992 horizontal position uncertainty is about 2 cm for the 1986 and 1989 coordinates and about 4 mm for the 1992 coordinates little internal deformation is observed in the area west of the western volcanic zone within the north american plate and at the southern tip of the eastern volcanic zone within the eurasian plate the observed relative velocity of these two areas is 2 1 0 4 cm yr in direction n117 11 e 1 uncertainties compatible with the 1 94 cm yr widening in direction n 104 e across south iceland predicted from the nuvel 1 global plate motion model left lateral shear strain is developing across the intervening transform zone the e w trending south iceland seismic zone sisz strain is concentrated within a 20 to 30 km wide zone that correlates with seismic activity in the sisz about 85 15 of the relative plate motion is accommodated by this zone in such a way that the area south of the sisz is moving toward the east with the eurasian plate and the area north of the sisz is moving toward the west with the north american plate accordingly north of the sisz the western rift zone can accommodate a maximum of 15 15 of the relative plate motion within the sisz the shear strain results in an anticlockwise rotation of lineaments oriented north south at a rate of 0 5 1 rad yr the shear strain accumulation can be accommodated by bookshelf faulting on mapped recent north south faults in the sisz if the deformation is accommodated by an array of n s faults spaced 1 5 km apart an average slip rate of about 0 5 5 mm yr is required on each fault the rate of geometric moment release due to earthquakes averaged over centuries m 0 expected from such an array is about the same as expected from a simple transform fault 2vad where 2v is the relative plate motion a is the total length of the seismic zone and d is the thickness of the brittle crust if 2v 0 85 0 15 1 94 cm yr d 10 15 km ana a 75 85 km then m 0 1 0 2 5 107 m3 yr comparable to the rate of geometric moment release in large historical earthquakes in the sisz for the last several centuries we conclude that historical seismicity in the sisz can be attributed to left lateral shear accumulation across the seismic zone at a similar rate throughout historical times as during the years 1986 1992

revision of magnitudes of large shallow earthquakes 1897 1912 abstract in previous research trace amplitudes of surface wave maxima recorded by undamped milne seismographs were used to determine the surface wave magnitudes ms of large shallow earthquakes which occurred prior to 1912 for this purpose the effective gain of these instruments was calibrated by using the surface wave magnitudes ms gr which were calculated from the unpublished worksheets for seismicity of the earth of gutenberg and richter in this paper the real quality of ms gr is critically re evaluated by using independent sets of data it is found that ms gr for the period 1904 1909 is considerably overestimated the average excess from the real ms is 0 5 units for 1904 1906 0 4 for 1907 0 3 for 1908 1909 and 0 0 for 1910 1912 this overestimation is so systematic and large that the previous results are all redetermined the average effective gain of milne instruments is revised to be 21 9 previously the gain depended on ms this revision results in systematic reduction in the previously assigned magnitudes the revised values of ms for 264 shallow earthquakes with ms 6 8 and over in the period 1897 1912 inclusive are listed the present revision is large enough to preclude the possibility of the high activity of large shallow earthquakes around the turn of the century the present results have a direct effect on all the magnitude catalogues of shallow earthquakes which occurred prior to 1909

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

the 2008 may 29 earthquake doublet in sw iceland summary on 2008 may 29 an earthquake doublet shook the southwestern part of iceland the first main shock originated beneath mt ingolfsfjall located near the western margin of the south iceland seismic zone sisz approximately 40 km east of the capital reykjavik immediate aftershock activity was recorded by the sil seismic network operated by the icelandic meteorological office imo with both n s and e w structures illuminated over a broad area a continuous gps cgps network also operated by the imo recorded coseismic offsets with up to 200 mm of horizontal motion at the closest stations we estimate the coseismic surface deformation observed by campaign and continuous gps and satellite radar data insar we invert the geodetic data to find the optimal geometry location and slip on the main faults accounting for variation in the elastic parameters of the crust with depth our models indicate that most of the slip occurred on two n s structures spaced 5 km apart from a joint inversion of gps and insar data for variable slip models we find that most of the slip for the first ingolfsfjall event was concentrated at 2 4 km depth with a maximum of 1 9 m whereas the slip on the second kross fault was located deeper at 3 6 km depth with up to 1 4 m of motion the models give similar geodetic moments for the two main events equivalent to a moment magnitude of mw5 8 and mw5 9 for the first and second event respectively our estimated composite moment therefore equals a mw6 1 for the doublet smaller than the mw6 3 estimated from teleseismic data e g neic and harvard the geodetic data support rupture on two main faults and analysis of high rate 1 hz cgps data suggests that slip on the second fault initiated within 3 s of the first main shock static coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity i e at the location of the largest slip patch on the second fault however we cannot rule out dynamic stress triggering due to the short time between the two main events the 2008 may 29 earthquake doublet appears to be a continuation of the earthquake sequence that started in 2000 june when two mw6 5 events struck the eastern and central part of the south iceland seismic zone in the span of 81 hr the 2000 june 2008 may sequence has released about half of the moment accumulated by plate motion since the previous earthquake sequence in 1896 1912 therefore continued earthquake activity with moderate size events rupturing n s faults in the sisz in the coming decades is likely

ocean ridge discontinuities in iceland abstract the tjornes fracture zone in north iceland and the south iceland seismic zone are ocean ridge discontinuities that are partly exposed on land these are zones of high seismicity but contrasting style of crustal deformation detailed field studies and numerical modelling suggest that the tjornes fracture zone has all the characteristics of an oceanic transform fault is 7 9 ma old and developed along the zone of maximum shear stress between the nearby tips of the rift zones of the kolbeinsey ridge and north iceland conversely the results indicate that the south iceland seismic zone developed within an area of shear stress concentration between the overlapping rift zones in south iceland the development of the south iceland seismic zone is a consequence of the propagation of the eastern rift zone to the southwest during the past 3 ma

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

strain changes for the 1987 vatnafjoll earthquake in south iceland and possible magmatic triggering on may 25 1987 at 11 31 a m 5 8 earthquake occurred at the southern end of vatnafjoll volcanic ridge in south iceland this is the largest event in the south iceland lowland since a m 7 0 earthquake in 1912 which was located approximately 15 km to the west of the vatnafjoll earthquake vatnafjoll is located at the junction of the south iceland seismic zone a left lateral transform zone and the eastern volcanic zone which is a zone of rifling and volcanism in may and june 1987 several foreshocks and aftershocks were recorded on the local seismic network as well as the mainshock a clear coseismic step associated with the mainshock was observed at all operating stations of a volumetric strainmeter network in southern iceland steps associated with some foreshocks and aftershocks were also observed at the closest strain stations slow strain changes before and after the mainshock lasting a few days were also observed forward modeling of the coseismic strainmeter signals of the mainshock suggests a double couple solution where the slip is mostly right lateral strike slip on a subvertical plane with a northerly strike the solution has a good fit to observations and is in good agreement with interpretation of seismometer data this solution indicates a stress field similar to that in the south iceland seismic zone the slow strain changes which start about 10 min after the first foreshock may indicate magma involvement in the process changes associated with an intrusion and pressure release may affect the strain field and possibly trigger the mainshock the strainmeter records open up a new view of the seismic strain event as a combination of seismic strain release and a slower process of magma intrusion

the mechanics of the south iceland seismic zone the south iceland seismic zone appears to act as a transform fault between offset sections of the mid atlantic ridge however there is no morphological expression of a throughgoing transform between the ridges the anticipated throughgoing east west transform is represented by a series of north south strike slip faults we study the mechanics of the south iceland seismic zone using boundary element methods first we examine the implications of the differences between the geometry of an orthogonal ridge transform ridge system and the geometry of the structures observed in iceland then we consider the role of the north south faults in accommodating transform motion in an east west direction we conclude that the north south faults can accommodate deformation if they are longer or more spatially frequent than mapped the south iceland seismic zone is subject to sequences of large earthquakes every 45 112 years in comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models we confirm that the system of north south faults can act as a transform fault the seismic moment release derived from the earthquake magnitudes is 1 4 times greater than that predicted by our models if our application of the moment magnitude relation is correct the above could imply that the brittle layer extends to 14 km rather than the 10 km assumed in our models other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and very long baseline interferometry vlbi

a new precipitation weighted method for determining the meteoric water line for hydrological applications demonstrated using australian and global gnip data summary the relationship between 2h and 18o in precipitation at a site known as the local meteoric water line lmwl is normally defined using an ordinary least squares regression olsr which gives equal weighting to all data points regardless of the precipitation amount they represent however smaller precipitation amounts are more likely to have a lower d excess due to re evaporation of raindrops below the cloud base or biases in the sampling method in this paper we present an equation for a precipitation amount weighted least squares regression pwlsr that will correct these biases for use in groundwater and surface hydrology applications new lmwl equations are presented for australian sites in the global network of isotopes in precipitation gnip where the pwlsr consistently produces a lmwl with a larger gradient than the olsr perth and alice springs exhibit the largest change in slope this is consistent with the higher frequency of small monthly precipitation amounts with low d excess values occurring at these sites in summer for perth and throughout the year for alice springs the pwlsr method was also applied to 288 stations in the gnip data base n 36 and the difference between the slopes of the lmwls a slopepwlsr slopeolsr calculated for these stations the mean change in slope a was 0 12 with 56 of sites showing an increase in slope or positive a value and 44 having a decrease in slope or negative a sites with mediterranean climates showed the greatest increase in slope the magnitude of the change in slope followed some general trends showing a positive correlation with average 2h and 18o composition and rainfall variability and negative correlation with period of record n

near bottom cross shelf heat flux along central florida s atlantic shelf break winter months near bottom current and temperature data from a 154 day period in winter 1984 1985 are combined to investigate cross shelf heat fluxes along central florida s atlantic coast between 27 and 30 n temperature and current time series recorded at the shelf break exhibit considerable variability over time scales of the order of 1 2 weeks low frequency temperature fluctuations superimposed onto a seasonal cooling of approximately 0 4 c week decrease from south to north cross shelf motion results in little net displacement at latitudes 27 and 30 n but results in considerable net shoreward displacement at latitude 28 n cross shelf heat flux is shoreward at all study sites but values decrease approximately linearly from south to north the study shows that a shoreward flux of relatively cool water can continue through winter months in the vicinity of cape canaveral compared with summer up welling in the same area the characteristic time scale is considerably shorter because the heat flux appears to be related to frontal eddies embedded in the cyclonic shear zone of the florida current rather than to seasonal variations in volume transport combined with a westward shift of the axis of the florida current

behaviour of a rupture of the 21 june 2000 earthquake in south iceland as revealed in an asphalted car park abstract a major earthquake m 6 6 occurred on 21 june 2000 in south iceland this paper presents an unusual example of left lateral strike slip displacement recorded in a newly asphalted car park surface through a mechanically consistent pattern of open fissures and pressure ridges resulting from simple shear and rotation measurement of these features allows accurate reconstruction of the local deformation the behaviour of the asphalt layer resembles that of analogue physical models especially in terms of rotations induced by shear deformation it is finally shown that through a wide range of scales some basic patterns associating rotation and opposite senses of strike slip exist in the south iceland seismic zone

the 1912 iceland earthquake rupture growth and development of a nascent transform system we have mapped in detail surface ruptures of the 1912 magnitude 7 0 strike slip earthquake in south iceland this earthquake ruptured fresh basalt flows that had covered the pre existing fault the observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic scale observations from laboratory experiments the shear offset distributed across the zone of surface fractures produced by this earthquake is right lateral and is in the range of 1 to 3 m total mapped rupture length is 9 km but total rupture length is probably at least 20 km this interplate earthquake had an exceptionally high ratio of slip to fault length and by inference stress drop the north south trending rupture of the 1912 earthquake is part of the bookshelf faulting in the east west trending south iceland seismic zone we ascribe the bookshelf faulting in the south iceland seismic zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust

structural analysis of the brigham city weber segment boundary zone wasatch normal fault utah implications for fault growth and structure we investigate the history kinematics principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the wasatch fault zone utah three fault types developed in archean crystalline rocks reflect progressive uplift of fault related footwall rocks chlorite breccias and phyllonites reflect deep level reaction assisted plastic deformation along the north striking part of the segment planar fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment youngest faults are hematitecoated extremely narrow polished surfaces slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north striking portion of the segment slip vectors and fault orientation along the northwest striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace yet slip is confined to a broad fault parallel zone small faults at the southernmost tip of the segment indicate a strong influence of the north striking weber segment to the south inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that 3 may have reoriented approximately 60 over the life of the segment subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest plunging bedrock high which is reflected by a sharp bend in the brigham city segment the southern end of the brigham city segment may have started as a straight north striking fault which has bent due to changes in stress orientations and or interaction with the adjacent weber segment

ground ruptures of the 1974 and 1983 kaoiki earthquakes mauna loa volcano hawaii the november 30 1974 ml 5 5 and november 16 1983 ml 6 6 earthquakes generated left stepping en echelon ground cracks within the kaoiki seismic zone on the southeast flank of mauna loa volcano hawaii the general trend of the ruptures n48 55 e parallels a nodal plane of the main shocks focal mechanisms the ruptures themselves consist of short predominantly extension cracks which are up to 20 m long and strike roughly e w 30 50 clockwise from the overall trend of the zones some of the cracks are linked by secondary fractures and rubble breccia to form left stepping crack arrays which are themselves linked to form longer en echelon systems of ground rupture geologic maps and field observations indicate that these features emerge from an underlying strike slip fault and they form a fracture process zone above its tip the maximum displacement measured across cracks in the 1983 rupture zone is 0 5 m trilateration data however suggests that the overall shear displacement was about 1 5 m at depth elastic solutions indicate that a region of significant tensile stress can exist above buried strike slip faults we suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the earth s surface by subsequent growth and linkage of these cracks into the observed arrays we infer that the crack arrays accommodate increased displacement with depth and they merge downward into the parent strike slip fault at an estimated 1 2 km depth where strike slip displacement was probably more or less continuous along the 7 km length of the rupture in the kaoiki region only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years this suggests that the recent 10 year periodicity of moderate magnitude kaoiki strike slip events may not have extended far into the past the tectonic significance of strike slip faulting on mauna loa volcano remains enigmatic

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

postseismic deformation following the june 2000 earthquake sequence in the south iceland seismic zone 1 we observe postseismic deformation on two spatiotemporal scales following mw 6 5 earthquakes in the south iceland seismic zone on 17 and 21 june 2000 we see a rapidly decaying deformation transient lasting no more than 2 months and extending about 5 km away from the two main shock ruptures this local month scale transient is captured by several radar interferograms and is also observed at a few campaign gps sites located near the faults a slower transient with a characteristic timescale of about a year is detected only by gps measurements the month scale deformation pattern has been explained by poroelastic rebound due to postearthquake pore pressure changes in contrast the year scale deformation can be explained by either afterslip at 8 14 km depth or viscoelastic relaxation of the lower crust and upper mantle in response to the coseismic stress changes the optimal viscoelastic models have lower crustal viscosities of 0 5 1 1019 pa s and upper mantle viscosity of 3 1018 pa s because of the limitations of our gps campaign data we consider both afterslip and viscoelastic relaxation as plausible mechanisms explaining the deformation field both types of postseismic deformation models suggest that the areas of large coseismic stress increase east of the 17 june and west of the 21 june ruptures continue to be loaded by the postseismic deformation

scaling differences between large interplate and intraplate earthquakes a study of large intraplate earthquakes with well determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes in which m 0 l 2 or u l where l is rupture length and u is slip in contrast to interplate earthquakes for which 1 10 5 for for the intraplate events 6 10 5 which implies that these earthquakes have stress drops about 6 times higher than interplate events this result is independent of focal mechanism type this implies that intraplate faults have a higher frictional strength than do plate boundaries and hence that faults are velocity or slip weakening in their behavior this factor may be important in producing the concentrated deformation that creates and maintains plate boundaries

seismic anisotropy in the crust at the mid atlantic plate boundary in south west iceland summary seismograms of microearthquakes recorded on stations of the sil network in south western iceland exhibit strong shear wave splitting and are consistent with being caused by aligned parallel cracks in the uppermost crust splitting times of 0 1 0 3 s are observed with the larger values 0 2 0 3 s occurring beneath stations in the highly heated western volcanic zone and the smaller values 0 1 s in the younger less fractured south iceland seismic zone five of the stations have a fast shear wave polarization azimuth of n30 e n40 e7 parallel to the axis of the western volcanic zone i e the mid atlantic ridge in iceland and in the direction of maximum horizontal compression defined by fault dyke and fissure strikes and microearthquake focal mechanisms station gyg in the north western part of the network has the significantly different azimuth of n70 e this is parallel to one of the local strike slip fault trends and is probably due to a heated rock fabric caused by those faults

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory