roller bearing tectonic evolution of the juan fernandez microplate a combined gloria sidescan and hydrosweep multi narrow beam sonar survey shows striking evidence for fan shaped spreading systems bounding pseudofaults and compression ridges associated with 90 of clockwise rotation of the juan fernandez microplate in the past 4 myr a quantitative analysis of this evolution suggests that the microplate has rotated as a rigid roller bearing for most of its history driven at its edges by shear between the surrounding major plates the remarkable similarity between the juan fernandez and easter microplates despite their different tectonic settings suggests that this is a general model for microplate tectonics

coseismic stress changes and crustal deformation on the reykjanes peninsula due to triggered earthquakes on 17 june 2000 1 a large mw 6 5 earthquake struck the south iceland seismic zone sisz on 17 june 2000 the 17 june main shock triggered increased seismicity over a large area and significant slip on at least three distinct faults on the reykjanes peninsula up to 87 km to the west of the event a second large mw 6 4 earthquake in the sisz occurred on 21 june 2000 about 17 km west of the 17 june main shock this event does not appear to have triggered as much activity on the reykjanes peninsula as the 17 june main shock although the epicenter was closer crustal deformation signals due to the june 2000 earthquakes on the reykjanes peninsula were observed with campaign and continuous gps and synthetic aperture radar interferometry with the largest coseismic deformation signal near lake kleifarvatn we model the faults using three uniform slip rectangular dislocations in an elastic half space best fit uniform slip models consistent with seismic and geodetic data indicate that all three faults trend n s and the motion on them was primarily right lateral strike slip our study suggests that the event near kleifarvatn had a significantly larger moment than seismic estimates indicating a component of aseismic slip on the fault lasting no more than several hours static coulomb failure stress change calculations indicate that the event at kleifarvatn increased the coulomb stress at the hypocenter of the nupshlidarhals event by 0 1 0 2 mpa as well as loading the hvalhnukur fault

using microearthquakes to track repeated magma intrusions beneath the eyjafjallajokull stratovolcano iceland 1 we have mapped microearthquakes caused by magma migration preceding and during the flank and summit eruptions in march may 2010 of eyjafjallajokull stratovolcano in iceland using a coalescence microseismic mapping technique spatial and temporal clustering of 5 000 microearthquakes under the eastern flank of the volcano illuminates several northeast southwest striking sub vertical dikes at 2 6 km b s l emplaced before the fimmvor uhals flank eruption in march this intense precursory seismicity had a lateral extent of 6 km east west and 3 km north south a sequence of 386 microearthquakes during the summit eruption refined by double difference relative relocation defines a sub linear trend inclined 5 10 from vertical extending from the upper mantle at 30 km depth to the summit crater this sequence includes two major clusters at 19 km and 24 km b s l each containing 100 earthquakes all microearthquakes display characteristics of brittle fracture with several subsets of events exhibiting closely similar waveforms within clusters this suggests similar repetitive source processes the deeper clusters may be caused by fracturing solidified magma plugs that form constrictions in an otherwise aseismic melt conduit or they may occur at exit points from melt pockets in which case they indicate positions of magma storage at depth the seismicity deeper than 10 km only starts three weeks after the onset of the summit eruption after which the largest clusters occur at progressively greater depths this temporal pattern may result from pressure release at shallow levels in the magmatic plumbing system progressively feeding down to mobilize deeper melt pockets

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

inter book normal fault related shear heating in brittle bookshelf faults abstract this work models shear heat related temperature rise due to inter book dip slip normal faults by a bookshelf mechanism such faults are characterized by temporally changing the dip of the inter book fault planes considerable temperature can be produced by such faulting although the effects of high temperature have not yet been reported around such fault planes this may mean that fluids flowing through such planes do not allow shear heating to accumulate

multiple melt injection along a spreading segment at askja iceland lower crustal earthquakes 12 25 km depth have been detected since august 2005 in the askja volcanic system along the north iceland rift in the normally ductile part of the crust the earthquakes occur in three clusters which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust seismic velocity vp vs ratios are consistent with the presence of partial melt in the lower crust at askja the spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active this pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath iceland compared to the rest of the mid atlantic ridge however the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano askja i e segment center during the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano therefore on long time scales melt supply is probably greater at the segment center with melt redistribution in the upper crust even though there are multiple points of lower crustal injection along the segment

the fissure swarm of the askja volcanic system along the divergent plate boundary of n iceland divergent plate boundaries such as the one crossing iceland are characterized by a high density of subparallel volcanic fissures and tectonic fractures collectively termed rift zones or fissure swarms when extending from a specific volcano volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events when magma intrudes fractures to form dikes and even feeds fissure eruptions we mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern iceland the study area is 1 800 km2 located within and north of the askja central volcano the style of fractures changes with distance from askja close to askja the swarm is dominated by eruptive fissures the proportion of tectonic fractures gets larger with distance from askja this may indicate that magma pressure is generally higher in dikes close to askja than farther away from it volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3 4 identified calderas in askja the average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity as this deviation decreases gradually northward we suggest that the effect of the triple junction of the north american eurasian and the hreppar microplate is a likely cause for this deviation shallow tectonic earthquakes in the vicinity of askja are often located in a relatively unfractured area between the fissure swarms of askja and kverkfjoll these earthquakes are associated with strike slip faulting according to fault plane solutions we suggest that the latest magma intrusions into either the askja or the kverkfjoll fissure swarms rotated the maximum stress axis from being vertical to horizontal causing the formation of strike slip faults instead of the dilatational fractures related to the fissure swarms the activity in different parts of the askja fissure swarm is uneven in time and switches between subswarms as shown by a fissure swarm that is exposed in an early holocene lava nw of her ubrei but disappears under a younger 3500 4500 bp lava flow we suggest that the location of inflation centres in askja central volcano controls into which part of the askja fissure swarm a dike propagates the size and amount of fractures in the kollottadyngja lava shield decrease with increasing elevation we suggest that this occurred as the depth to the propagating dike s was greater under central kollottadyngja than under its flanks due to topography

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

source scaling relations of interface subduction earthquakes for strong ground motion and tsunami simulation the recording on high resolution broadband seismic networks of several great interface subduction earthquakes during the last decade provide an excellent opportunity to extend source scaling relations to very large magnitudes and to place constraints on the potential range of source parameters for these events at present there is a wide range of uncertainty in the median rupture areas predicted for any given seismic moment by current relationships between magnitude and rupture area for subduction interface earthquakes our goal is to develop an updated set of earthquake source scaling relations that will reduce this current large degree of epistemic uncertainty and improve the accuracy of seismic hazard analysis and the prediction of the strong motion characteristics and tsunamis of future subduction earthquakes to achieve this goal we compiled a database including slip models of interface earthquakes that occurred worldwide with moment magnitudes ranging from m 6 75 9 1 we characterized the seismic sources based on well established criteria to estimate the asperity areas as well as the average slip on the faults and we used these parameters to compute an updated set of magnitude scaling relations of the various characteristics of the fault additionally we followed an alternative approach to quantifying slip models for use in developing characteristic slip models of future earthquakes this involved analyzing the 2d fourier transforms of the slip functions in the compiled database and deriving a wavenumber spectral model of the slip distribution

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

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

the kverkfjoll fissure swarm and the eastern boundary of the northern volcanic rift zone iceland rift zones at the divergent plate boundary in iceland consist of central volcanoes with swarms of fractures and fissures extending away from them fissure swarms can display different characteristics in accordance with their locations within the 50 km wide rift zones to better discern the characteristics of fissure swarms we mapped tectonic fractures and volcanic fissures within the kverkfjoll volcanic system which is located in the easternmost part of the northern volcanic rift zone nvz to do this we used aerial photographs and satellite images we find that rifting structures such as tectonic fractures holocene volcanic fissures and hyaloclastite ridges are unevenly distributed in the easternmost part of the nvz the kverkfjoll fissure swarm extends 60 km north of the kverkfjoll central volcano holocene volcanic fissures are only found within 20 km from the volcano the fjallgar ar area extending north of the kverkfjoll fissure swarm is characterized by narrow hyaloclastite ridges indicating subglacial volcanism we suggest that the lack of fractures and holocene volcanic fissures there indicates decreasing activity towards the north in the easternmost part of the nvz due to increasing distance from the long term spreading axis we argue that arcuate hyaloclastite ridges at the eastern boundary of the northern volcanic rift zone are mainly formed during deglaciations when three conditions may occur firstly eruption rate increases due to decompression of the mantle secondly the high tensile stresses accumulated during glaciations due to lack of magma supply may be relieved as magma supply increases during deglaciations thirdly faulting may occur during unloading due to differential movements between the thinner and younger northern volcanic rift zone crust and the thicker and older crust to the east of it

coalescence microseismic mapping earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations if mis picks are made these will contribute to the location therefore causing potential bias for data recorded by a dense seismic array direct imaging methods can be applied instead we describe the coalescence microseismic mapping method which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array by continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio temporal coordinates of the origins of seismic events noisy data are migrated away from the correct origin so do not contribute to errors in location the method is rooted in a bayesian formulation of event location traveltime inversion allows imaging of source locations and has the capacity to handle errors in modelled traveltimes it has the advantage of working with any 3 d velocity model which therefore may include anisotropy it also automatically incorporates both p and s wave data a multiresolution grid search leads to an efficient implementation with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality we discuss the theory and implementation of this method and illustrate it with real data from microseismic events in iceland caused by melt intrusion in the crust

strike slip faulting during the 2014 barðarbunga holuhraun dike intrusion central iceland over a 13 day period magma propagated laterally from the subglacial bar arbunga volcano in the northern rift zone iceland it created 30 000 earthquakes at 5 7 km depth along a 48 km path before erupting on 29 august 2014 the seismicity which tracked the dike propagation advanced in short bursts at 0 3 4 7 km h separated by pauses of up to 81 h during each surge forward seismicity behind the dike tip dropped moment tensor solutions from the leading edge show exclusively left lateral strike slip faulting subparallel to the advancing dike tip releasing accumulated strain deficit in the brittle layer of the rift zone behind the leading edge both left and right lateral strike slip earthquakes are observed the lack of non double couple earthquakes implies that the dike opening was aseismic

multifaulting in a tectonic syntaxis revealed by insar the case of the ziarat earthquake sequence pakistan on 28 29 october 2008 within 12 h two similar mw 6 4 strike slip earthquakes struck baluchistan pakistan as part of a complex seismic sequence interferometric synthetic aperture radar insar data reveal that the peak of surface displacement is near the ziarat anticline a large active fold affected by quaternary strike slip faulting all coseismic interferograms integrate the deformation due to both earthquakes as their causative faults ruptured close to each other the individual signals cannot be separated according to their focal mechanisms each earthquake may have activated a ne sw sinistral or a nw se dextral fault segment which leads to four possible scenarios of fault orientations a nonlinear inversion of the insar data set allows rejecting two scenarios the best slip distributions on the two fault segments for the two remaining scenarios are determined by linear inversion stress change modeling favors a scenario involving two abutting conjugate strike slip faults two other fault segments accommodated left lateral strike slip during the seismic sequence the activated fault system includes multiple fault segments with different orientations and little surface expression this may highlight at a smaller scale the distributed possibly transient character of deformation within a broader right lateral shear zone it suggests that the activated faults delineate a small tectonic block extruding and subtly rotating within the shear zone it occurs in the vicinity of the local tectonic syntaxis where orogenic structures sharply turn around a vertical axis these mechanisms could participate in the long term migration of active tectonic structures within this kinematically unstable tectonic syntaxis

active tectonics of the turkish iranian plateau 1 this paper examines how active faulting in the turkey iran caucasus region accommodates the arabia eurasia collision and the velocity field observed by gps the overall shortening across the zone is in general spatially separated partitioned into right lateral strike slip in the turkish iranian plateau and thrusting in the greater caucasus a band of counterclockwise rotating nw se right lateral strike slip faults accommodates a nw se gradient in ne directed velocity relative to eurasia between the black and caspian seas a nnw sse band of previously unrecognized oblique normal faults is present on the turkey iran border we estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 2 ma at present rates this implies a reorganization of deformation in the collision zone at that time after the initial collision at 12 ma probably in response to mantle induced dynamic uplift

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

space imaging geodesy reveals near circular coseismic block rotation during the 2016 mw 7 8 kaikōura earthquake new zealand

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

field examples of strike slip fault terminations in mongolia and their tectonic significance deformation at the ends of large intracontinental strike slip faults that do not simply link other major structures often involves rotations about a vertical axis we use earthquake slip vectors surface rupture in earthquakes and geomorphology to examine the ends of three major strike slip faults in mongolia in these places a simple pattern is seen consisting of a thrust fault on one side with a displacement that decreases away from the strike slip fault consistent with local rotational deformation strike slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area for example from dominantly strike slip to dominantly dip slip while still accommodating the overall deformation required by larger scale regional motions such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis which may be detected paleomagnetically the kind of strike slip fault termination described here may have consequences for how large strike slip fxaults evolve and grow and for the variation in displacement along their length