source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

selection of earthquake scaling relationships for seismic hazard analysis a fundamentally important but typically abbreviated component of seismic hazard analysis is the selection of earthquake scaling relationships these are typically regressions of historical earthquake datasets in which magnitude is estimated from parameters such as fault rupture length and area the mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area we compile a worldwide set of regressions and make a first order shortlisting of regressions according to their relevance to a range of tectonic regimes plate tectonic setting and fault slip type in existence around the world regression relevance is based largely on the geographical distribution age and quantity quality of earthquake data used to develop them our compilation is limited to regressions of magnitude or seismic moment on fault rupture area or length and our shortlisted regressions show a large magnitude range up to a full magnitude unit for a given rupture length or area across the various tectonic regimes these large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic hazard application in different tectonic environments

kilometre scale folding in the teslin zone northern canadian cordillera and its tectonic implications for the accretion of the yukon tanana terrane to north america reply the teslin zone in south central yukon has previously been described as a discrete zone with a steep foliation unique to the zone it includes the anvil assemblage and the narrowest portion of the

dynamic source inversion of the m6 5 intermediate depth zumpango earthquake in central mexico a parallel genetic algorithm we introduce a method for imaging the earthquake source dynamics from the inversion of ground motion records based on a parallel genetic algorithm the source model follows an elliptical patch approach and uses the staggered grid split node method to simulate the earthquake dynamics a statistical analysis is used to estimate errors in both inverted and derived source parameters synthetic inversion tests reveal that the average rupture speed vr the rupture area and the stress drop may be determined with formal errors of 30 12 and 10 respectively in contrast derived parameters such as the radiated energy er the radiation efficiency r and the fracture energy g have larger errors around 70 40 and 25 respectively we applied the method to the mw 6 5 intermediate depth 62 km normal faulting earthquake of 11 december 2011 in guerrero mexico inferred values of 29 2 6 2 mpa and r 0 26 0 1 are significantly higher and lower respectively than those of typical subduction thrust events fracture energy is large so that more than 73 of the available potential energy for the dynamic process of faulting was deposited in the focal region i e g 14 4 3 5 1014j producing a slow rupture process vr vs 0 47 0 09 despite the relatively high energy radiation er 0 54 0 31 1015 j and energy moment ratio er m0 5 7 10 5 it is interesting to point out that such a slow and inefficient rupture along with the large stress drop in a small focal region are features also observed in both the 1994 deep bolivian earthquake and the seismicity of the intermediate depth bucaramanga nest

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

the oaxaca mexico earthquake of 1931 lithospheric normal faulting in the subducted cocos plate a study of the great earthquake in oaxaca mexico on 15 january 1931 shows that it was a normal fault earthquake striking east west the lack of evidence of surface faulting near the source region1 and the focal depth of 40 km inferred from body wave modelling suggest that the earthquake occurred within the subducted slab the large size of the earthquake ms 8 0 strongly suggests that the rupture broke through the entire lithosphere of the subducted cocos plate other large normal fault earthquakes in the japanese2 aleutian3 peruvian4 and sumba5 arcs show lithospheric faulting seawards of the trench or very close to it the 1931 event however occurred farther down dip in the slab 150 km landward of the trench below the bottom of what appears to be the seismogenic plate interface we speculate here that large although infrequent normal fault earthquakes such as the 1931 oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding mexican continental plate the epicentral location of the 1858 michoacan earthquake6 ms 7 5 suggests that it may represent another example of lithospheric normal faulting in the middle american arc such earthquakes for which the periods of recurrence are unknown constitute an extremely high seismic risk to population centres of mexico

an estimate of shear wave q of the mantle wedge in mexico we utilize earthquake recordings at two broadband seismographs located on the coast of the gulf of mexico to estimate q of the mantle wedge from the data at a station situated in the laguna verde nuclear power plant lvig about 100 km northwest of veracruz and at the eastern edge of the mexican volcanic belt we estimate an upper bound of shear wave q of the mantle wedge q f 120 f 0 75 0 1 f 10 hz as compared with q f 251 f 0 58 for the average path through subducted slab and continental lithosphere curiously the estimated q of the mantle wedge at station scig which is located on the yucatan block is about the same as the average q in southern mexico there are several possible explanations for the difference 1 very few recordings at scig 2 a site effect at scig masking the effect of low q of the mantle wedge and 3 relatively high mantle q beneath the yucatan block the third possibility is supported by surface wave tomography that reveals a thick cold mantle lithosphere below the yucatan block and near absence of mantle lithosphere in the backarc region of central mexico a higher density of seismographs along the gulf coast is needed to resolve these issues our study predicts diminished ground motions at the laguna verde nuclear power plant if the seismic waves pass through the mantle wedge or traverse below the popocatepetl or orizaba volcanoes

eclogite from central yukon a record of subduction at the western margin of ancient north america eclogite occurring in central yukon at faro and near last peak as lenses interleaved with muscovite quartz blastomylonite has the chemical and field characteristics of group c rocks from sigmoidal inclusion trails in garnet from geothermometry and geobarometry and from mineral parageneses the eclogite is inferred to have a crustal protolith and to have followed a hysteretic subduction cycle p t trajectory transformation of basic igneous rock into schist was followed by eclogite metamorphism during which pressure was at least 1000 mpa and temperature was between 600 and 700 c uplifting involved passage through the stability field of glaucophane the eclogite and its host rocks were then subjected to greenschist fades metamorphism and deformation with temperature at approximately 400 c the rocks were emplaced as thrust sheets against or onto the western north american cratonal margin the tectonic boundary ranges from nearly vertical where it is outlined by a zone of steeply dipping melange t

an improved set of a x parameters for fe mg ca garnets and refinements of the orthopyroxene garnet thermometer and the orthopyroxene garnet plagioclase quartz barometer

seismic source parameters of normal faulting inslab earthquakes in central mexico we studied 62 normal faulting inslab earthquakes in the mexican subduction zone with magnitudes in the range of 3 6 mw 7 3 and hypocentral depths of 30 z 108 km we used different methods to estimate source parameters to observe differences in stress drop corner frequencies source dimensions source duration energy to moment ratio radiated efficiency and radiated seismic energy the behavior of these parameters is derived we found that normal faulting inslab events have higher radiated seismic energy energy to moment ratio and stress drop than interplate earthquakes as expected this may be explained by the mechanism dependence of radiated seismic energy and apparent stress reported in previous source parameter studies the energy to moment ratio data showed large scatter and no trend with seismic moment the stress drop showed no trend with seismic moment but an increment with depth the radiated seismic efficiencies showed similar values to those obtained from interplate events but higher than near trench events we found that the source duration is independent of the depth we also derived source scaling relationships for the mentioned parameters the low level of uncertainties for the seismic source parameters and scaling relationships showed that the obtained parameters are robust therefore reliable source parameter estimation can be carried out using the obtained scaling relationships we also studied regional stress field of normal faulting inslab events heterogeneity exists in the regional stress field as indicated by individual stress tensor inversions conducted for two different depth intervals z 40 km respectively while the maximum stress axis 1 appears to be consistent and stable the orientations of the intermediate and minimum stresses 2 and 3 vary over the depth intervals the stress inversion results showed that the tensional axes are parallel to the dip direction of the subducted plate at depths greater than 40 km the maximum horizontal stresses are oblique to the dip direction following the general trend of the slab geometry

backflow and margin parallel shear within an ancient subduction complex lineated and foliated tectonites of the teslin suture zone yukon represent the deep seated part of a west dipping subduction complex that formed seaward of western north america in permian triassic time the tectonites were progressively underplated onto the hanging wall plate during b type subduction with continued subduction and underplating the boundary between the hanging wall and subduction channel migrated oceanward resulting in growth of the hanging wail plate at depth these tectonites preserve a progressive record of backflow dextral margin parallel shear and downflow within the subduction channel early jurassic a type subduction of the leading edge of western north america resulted in eastward overthrusting of the tectonites as a coherent block onto north american strata these tectonites provide empirical evidence that backflow margin parallel displacement and downflow can all occur within the deeper parts of subduction zones

percolation threshold and permeability of crystallizing igneous rocks the importance of textural equilibrium the permeability of a crystal mush fundamentally controls the ability of its melt to migrate or segregate with respect to the solid phase and thus controls the extent to which compaction and porous media convection can occur in crystallizing igneous cumulates in particular the existence of a percolation threshold which defines the porosity at which a crystallizing rock becomes impermeable can limit the effectiveness of these processes we present three dimensional numerical models of the topology of porosity in both texturally equilibrated and nonequilibrated crystal melt systems these models enable porosity vs permeability relationships and hence percolation thresholds to be calculated the permeability of the nonequilibrated models was calculated using a network simulation method these models confirm that there is no percolation threshold for a perfectly texturally equilibrated rock with dihedral angles of 20 these results suggest that the volume of trapped melt in an igneous cumulate may be controlled by the ability of the crystallizing crystal melt system to reach textural equilibrium

the great mexican earthquake of 19 june 1858 expected ground motions and damage in mexico city from a similar future event abstract the description of the great earthquake of 19 june 1858 is unusual damage and high intensities were reported both in the state of michoacan and in mexico city although a coastal epicenter for this earthquake cannot be ruled out the reports agree better with an intermediate depth about 50 km normal faulting event in the subducted cocos plate a careful examination of the reports of this event and other normal faulting events below the mexican altiplano suggests that a likely location is 18 0 n 100 8 w near the epicenter of the 6 june 1964 m 7 3 h 55 km event this location is 220 km sw of the city the magnitude of the earthquake is estimated to be about 7 7 we synthesize expected ground motions in cu a hill zone site in the city from an event similar to that of 1858 using records from the 23 may 1994 earthquake 18 0 n 100 6 w h 50 km m 5 7 as an empirical green9s function and stress parameter of 50 160 and 300 bar the expected peak horizontal acceleration in cu of 160 bar is about 30 gals similar acceleration was recorded in cu during the 1985 michoacan earthquake m 8 0 we compute expected ground motions at many sites in mexico city using empirical transfer functions and random vibration theory and compare these motions and the expected damage in the city with those from the 1985 michoacan earthquake results show that the overall expected damage during the postulated earthquake is and 1 of that during the michoacan earthquake for 160 and 300 bar respectively a greater percentage of low rise construction which constitute about 80 of the total in the city will be damaged during the postulated earthquake than during the michoacan earthquake the expected ground motions for 50 bar are smaller at all periods than those from the michoacan earthquake as the present building code for mexico city contemplates coastal earthquakes of magnitude greater than 8 0 the case of 50 bar is not of interest in this article this preliminary study suggests a need for a more careful evaluation of expected ground motion in the valley of mexico from the postulated earthquake and its impact on the current design spectra of the city

eclogitic rocks in the st cyr klippe yukon and their tectonic significance until recently the nisutlin allochthonous assemblage a part of the yukon tanana composite terrane interpreted as trench melange from a late paleozoic mesozoic arc system was the only tectonic

p t evolution of the teslin suture zone and cassiar tectonites yukon canada evidence for a and b type subduction mineral composition and quantitative thermobarometric studies indicate that the teslin taylor mountain and nisutlin terranes within the teslin suture zone tsz yukon record widespread high p t metamorphic conditions consistent with subduction zone dynamothermal metamorphism the highest p t conditions 575 750 c and 9 17 kbar are preserved in tectonites formed during normal dip slip ductile shear dextral strike slip tectonites record lower p t conditions 400 550 c and 5 8 kbar and tectonites which show reverse shear have peak temperatures of c 420 c and a minimum peak pressure of 3 kbar dynamothermal metamorphism took place in a west dipping b type subduction zone outboard of western north america in permo triassic time tsz tectonites were underplated against the hangingwall plate of the subduction zone following subduction of the ocean basin which separated north america from the hangingwall plate tsz tectonites were overthrust eastward as a coherent structural package as a result of a type subduction of cassiar strata in early jurassic time par autochthonous cassiar tectonites which comprised the leading edge of the western north american margin record prograde moderate p high t metamorphism 550 750 c and 7 13 kbar synchronous with top to the east ductile shear metamorphism occurred as a result of subduction of the north american margin into the tsz subduction zone in early jurassic time following metamorphism cassiar tectonites cooled slowly from 500 to 300 c during the period middle jurassic to middle cretaceous tsz and cassiar tectonites were deformed during changing p t conditions data from each of these tectonite packages indicate that grain scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy the composition of the new grains reflects the p t conditions under which that particular grain was deformed

is there truly a hard rock site in the valley of mexico to understand the cause of observed amplification of seismic waves even at hill zone sites in the valley of mexico digital accelerographs have been installed at three especially chosen sites two of these sites madi and texc located on hard pleistocene lava a few meters in thickness overlying oligocene andesites were expected to be free of site effects analysis of the data recorded by these and other accelerographs during three moderate shallow subduction zone events however shows significant amplification at madi and texc between 0 2 and 0 6 hz with respect to established attenuation relations the cause of the amplification at hill zone sites in the valley of mexico including madi and texc may be pervasive low s wave velocity in and complex structure of the upper layers of the volcanic rocks if so then there may not be a truly hard rock site in the valley

plate waves and the 1980 huajuapan de leon mexico earthquake abstract we propose two alternate structural models featuring a dipping oceanic plate in the subduction zone of southwestern mexico based on the relocation by the master event method of 77 aftershocks of the 1980 huajuapan mexico earthquake recorded by the mexican continental aperture seismic network resmac the aftershock locations roughly define the rupture surface of the main event as a normal fault dipping towards the continent the rupture involved a thickness of about 25 km in the top of the subducted plate these results are consistent with the fault plane solution both models feature a waveguide in the top of the subducted plate which may account for the large relative amplitudes of plate waves relative to direct waves at all stations and the large high frequency content observed at a station on the coast compared with stations inland

direct evidence of a subducted plate under southern mexico the hypothesis1 of subduction of oceanic plates under active continental margins has become a fundamental idea in plate tectonics yet the evidence for the existence of oceanic plates beneath continental borders is still largely circumstantial in seismology the presence of a new layer or structural unit is taken as confirmed only after seismic phases corresponding to reflections or refractions from that layer have been identified on seismograms i report here the presence of a mantle phase interpreted as a seismic wave refracted from a dipping interface on records of aftershocks from the intermediate earthquake of 24 october 1980 in the mixtec highlands of south central mexico which may be direct evidence on the structural position of the subducted plate under the mexican active continental margin

subevent structure of large earthquakes a ground motion perspective our ability to predict ground motions from future earthquakes hinges on the accurate modeling of the radiation from the earthquake source a successful approach to modeling earthquake radiation has been to represent faults as a series of discrete independently rupturing subfaults although their physical interpretation has remained largely unclear our simulation of ground motions from twenty six well recorded moderate to large earthquakes substantiates the hypothesis that large ruptures are made up of a sequence of smaller subevents the size of a subevent follows a simple linear relationship with the size of the earthquake fault in an apparently deterministic manner the strength of the high frequency radiation is controlled by the maximum slip velocity which varies stochastically over a small range both the characteristic subevent size and the slip velocity appear to be region independent indicating a remarkable uniformity in earthquake source properties this observation helps to reduce the uncertainty in ground motion prediction by constraining its essential and previously ambiguous parameters

an examination of the cataclastic fabrics and structures of parts of nisutlin anvil and simpson allochthons central yukon test of the arc continent collision model abstract collision of an arc complex against the yukon part of the north american craton during the mesozoic resulted in northeastward transport of arc rocks onto the craton the arc rocks comprise three distinguishable tectonostratigraphic assemblages called nisutlin allochthon quartz muscovite and chlorite schist derived from sedimentary and intermediate volcanic protoliths anvil allochthon amphibolite gabbro ultramafic rocks and sepentinite and simpson allochthon granitic and granodioritic rocks and schist derived from them deformation recovery and recrystallization structures show that parts of the allochthons are blastomylonite formed at temperatures of 350 700 c and at depths between 15 and 40 km the stacking order of the allochthons is inconsistent and complex and locally the cataclastic rocks are overthrust by autochthonous strata the allochthons are truncated by late steep faults that have mainly strike slip displacement and may be contemporaneous with tintina fault the proposed tectonic history of the region is supported by detailed results of the study

evolution of the yukon tanana terrane evidence from southeastern yukon territory the yukon tanana terrane ytt in southeastern yukon consists predominantly of a mid paleozoic volcanic plutonic arc assemblage built on continental crust the ytt had experienced strong deformation and metamorphism by late triassic time by mid cretaceous the metamorphic complex and late triassic clastic rocks derived from it were imbricated with middle and upper paleozoic ophiolitic sheets the ytt differs markedly from adjacent parts of the north american continental margin in both its depositional and tectonic histories the composition and tectonic history of the ytt is comparable to that of the kootenay and barkerville terranes of southern and central british columbia from which the ytt may have been displaced

the seismic alert system for mexico city an evaluation of its performance and a strategy for its improvement the seismic alert system sas for mexico city has now been in operation for about 15 years the sas takes advantage of the fact that the city is located more than 300 km from the foci of many of the potentially damaging earthquakes the system consists of 15 accelerometers located along the coast of the state of guerrero above a segment of subduction plate boundary that is a mature seismic gap an algorithm estimates the magnitudes of earthquakes from the near source accelerograms and issues public and restricted alerts for earthquakes with m 6 and 5 m 6 respectively an evaluation of the sas s performance during 1991 2004 reveals a surprisingly high rate of failure and false alerts this poor performance results from an inadequate detection algorithm and a limited areal coverage by the sas this renders the alert system of limited use in this article we propose an alternative strategy for detecting earthquakes potentially damaging to mexico city that differs substantially from the one presently implemented by the sas although our analysis is based on close to source accelerograms of 45 mexican earthquakes and the corresponding peak accelerations recorded at a reference site in mexico city cu there is no restriction on the distance to the field station except that its location should provide sufficient alert time to mexico city based on these data an attenuation relation is derived to compute expected peak acceleration at cu a red from root mean square acceleration a rms at a field station the relation permits specification of an a red threshold given the peak acceleration at cu a cu for which an alert is desired along with prescribed probabilities of failure and false alert we find that the use of bandpass filtered 0 2 1 0 hz accelerograms leads to an improved performance of the sas the choice of the filter is guided by the frequency band of amplification of seismic waves in the lake bed zone of mexico city we think that a single level of general public alert may be the best option a good choice appears to be an alert for a cu 2 gal for 0 2 1 0 hz bandpass filtered accelerograms with 1 probability of failure to accomplish this we must set a red 0 8 gal the data since 1985 suggest that such an alert would occur about once or twice a year and the event will be felt by most persons in the lake bed zone the proposed algorithm along with an array of sensors located 30 to 40 km apart and distributed in a roughly semicircular arc of 310 km radius centered at mexico city should considerably improve the areal coverage and performance of the sas and potentially save thousands of lives

eclogite facies fluid infiltration constraints from δ 18 o zoning in garnet in situ analysis reveals that eclogite facies garnets are zoned in 18o with lower values in the core and rims that are 1 5 to 2 5 higher this pattern is present in 9 out of 12 garnets analyzed by sims from four orogenic eclogite terranes and correlates with an increase in the mole fraction of pyrope and mg fe ratio from core to rim indicating prograde garnet growth at the maximum temperatures and the time scales experienced by these garnets calculated intragranular diffusion distances for oxygen are small 5 from two kimberlites are homogeneous in 18o possibly due to diffusive exchange which is possible for prolonged periods at higher mantle temperatures homogeneity of 18o in garnets outside the normal mantle range 5 6 may be common in kimberlitic samples

stochastic finite fault modeling of ground motions from the 1999 chi chi taiwan earthquake application to rock and soil sites with implications for nonlinear site response the stochastic method for simulating strong ground motions from finite faults is applied to the records of the 1999 chi chi taiwan earthquake the method involves discretization of the fault plane into smaller subfaults each of which is assigned an x 2 spectrum the contributions from all subfaults are empirically atten uated to the observation site and summed to produce the synthetic acceleration time history the method is initially calibrated against the data recorded at 24 rock sites located within 7 120 km from the mainshock hypocenter and providing a broad azimuthal coverage of the fault plane the accuracy of the simulations is quantified through the model bias defined as the logarithm of the ratio of the observed to simulated spectrum averaged over all stations the calibrated model for the chi chi event has a near zero average bias in reproducing the ground motions at rock sites in the frequency range from 0 1 to 20 hz an unusually low value is found for the radiation strength factor s controlling the high frequency radiation level and directly related to the maximum slip velocity on the fault compared with the mean value found for north american earthquakes this result reflects the observed low peak ground ac celerations of the chi chi mainshock and physically its lower than usual slip ve locities the calibrated model is then used to simulate soil site site class d records using the linear response assumption the simulated soil site input motions are amplified by the weak motion amplification functions estimated by the spectral ratio technique from available aftershock records this analysis reveals an average reduction in strong motion amplification to about 0 5 0 6 of that in weak motions with an ac celeration threshold for detectable nonlinearity near 200 300 cm sec 2 however the derivation of soil site specific weak motion amplification was limited by the amount of aftershock data available further improvement in the quantification of nonlinear soil response during the chi chi earthquake may be possible with the release of additional aftershock datasets

igneous microstructures from kinetic models of crystallization during crystallization a variety of competing kinetic processes determine the evolution of the igneous microstructure yet the relative contribution of each process remains elusive to this end a stochastic algorithm is developed to yield a detailed spatial representation of the igneous microstructure during progressive crystallization this algorithm is used to test a variety of kinetic models for nucleation and crystal growth that yield realistic igneous microstructures the algorithm itself relies on a theoretical model of crystallization the avrami method and can therefore be internally validated in addition to being verified against natural microstructures using crystal size distributions csds the most realistic simulated igneous microstructure in which the csds remain approximately log linear throughout the crystallization interval is produced using a kinetic model involving exponential nucleation rate and constant growth rate an extension of the algorithm is used to simulate multiply saturated mineral growth emulating basalt crystallization through simultaneous nucleation and growth of plagioclase and clinopyroxene a fundamentally important feature of this procedure is the numerical production of detailed 3d representations of the microstructure that can be interrogated to study many physical and chemical processes for example the critical crystallinity necessary for the onset of a finite yield strength and the interstitial melt flow within the igneous microstructure depend on the percolation of the solid and melt phases respectively bounds on the percolation thresholds the critical crystallinity at which the phase of interest is connected or contiguous such that it extends across the full microstructure from one face to the opposite face are determined for the simulated igneous microstructure and are comparable with previously published results for the percolation threshold of both the melt phase and solid network the simulated igneous microstructure can also be used as input into other physical models to ascertain the physical properties of partially molten magmas that are otherwise difficult to estimate by experiment in a real sense these computed microstructures are equivalent to 3d microtomographic images of partially molten basalt within a solidification front

a source and wave propagation study of the copalillo mexico earthquake of 21 july 2000 mw 5 9 implications for seismic hazard in mexico city from inslab earthquakes the copalillo earthquake of 21 july 2000 m w 5 9 is the closest well located inslab event to mexico city ever to be recorded in this study we analyze local and regional broadband and accelerometric recordings to determine the source parameters of the earthquake and the attenuation of ground motion with distance and to obtain a preliminary estimate of the seismic hazard posed to the city by such events our results show that the earthquake occurred at a depth of about 50 km most probably in the subducted oceanic crust the waveform inversion discriminates between the two nodal planes the fault plane defined by the following strike 305 dip 32 and rake 80 the rupture propagated nearly unilaterally along the strike toward northwest with a small downdip component the observed source spectrum can be well explained by an 2 source model with m 0 6 0 10 25 dyne cm and a stress drop of 360 bar we find that high frequency ground motion f 3 hz which is related to a max during inslab earthquakes is not amplified at ciudad universitaria cu a hill zone site in the valley of mexico that is known to suffer amplification at low frequencies 0 1 f m w 7 0 event could give rise to an a max value of 30 40 gal the cu recordings indicate that the a max value of 30 gal could have a return period of about 40 yr about the same as from shallow dipping thrust earthquakes along the mexican subduction zone which have been regarded as posing the highest hazard for the city an inslab earthquake with an a max value of about 40 gal could cause heavy damage to small buildings at certain locations of the city we conclude that seismic hazard from inslab earthquakes to mexico city has so far been underestimated

geochronologic and thermobarometric constraints on the metamorphic history of the fairbanks mining district western yukon tanana terrane alaska this study presents new petrologic and thermochronologic information from the fairbanks district of east central alaska that indicate a complex metamorphic and structural history for the western yukon tanana terrane garnet biotite and garnet pyroxene thermometry and jadeite barometry yield prograde temperatures and pressures for the chatanika eclogite 523 c 14 15 kbar 1 kbar 100 mpa cooling from peak eclogitization is estimated from 40 ar 39 ar single grain geochronology at 210 180 ma secondary white mica ages of 140 115 ma along the fault contact between eclogite and underlying lower amphibolite facies rocks constrain the age of the event that placed the chatanika eclogite over the fairbanks schist based on observations from field mapping and diamond drill samples we interpret this structural contact as a thrust fault garnet biotite mineral pairs are reset by as much as 200 c within this fault zone biotite and white mica ages of 100 110 ma combined with jurassic amphibole ages in fairbanks schist samples indicate the fairbanks schist and chatanika eclogite cooled through biotite and white mica argon closure temperatures in the early cretaceous intrusion of mid cretaceous calc alkalic gold related granitic plutons in the fairbanks district are evidenced by loss of radiogenic argon in many of the 40 ar 39 ar age fractions eocene basalt is visible in six widely separated localities within the eastern part of the fairbanks district however the pervasiveness of a 50 ma resetting event in samples as far as 30 km from present day basalt localities indicates the eocene flows were either deposited throughout the fairbanks area or are associated with large plutons at depth

moment tensor and rupture model for the 1949 olympia washington earthquake and scaling relations for cascadia and global intraslab earthquakes we reanalyzed the 13 april 1949 olympia washington earthquake by using digitized records and first motion polarities from long period seismograms the moment tensor mechanism is normal faulting with a down dip trending t axis similar in style to other cascadia intraslab earthquakes the total seismic moment is 1 3 1026 dyne cm m w 6 7 and the hypocenter depth is 60 km additional inverse modeling for the kinematic rupture process assuming the steeply east dipping fault plane from the moment tensor resulted in a slightly higher total moment of 1 9 1026 dyne cm m w 6 8 the earthquake ruptured to the south with at least two subevents the combined area of asperities and seismic moment for the 1949 earthquake was compiled with those from the 1965 seattle tacoma and the 2001 nisqually earthquakes and with those from japan and mexico to develop a source scaling relation separate from shallow global strike slip earthquakes we infer that deeper intraslab earthquakes have a significantly smaller combined area of asperities than those compiled for shallower strike slip earthquakes with the same seismic moment this difference in rupture area leads to a 3 to 5 fold increase in stress drop for earthquakes with seismic moments between 1024 and 1028 dyne cm

empirical corrections for basin effects in stochastic ground motion prediction based on the los angeles basin analysis amplification corrections are presented for the finite fault stochastic ground motion simulation model these corrections represent the total effect of the los angeles basin on the ground motion spectra spectral amplification ratios were calculated by dividing the observed spectra for the 1994 northridge and 1987 whittier narrows earthquakes including shear and basin generated waves by the simulated spectra created assuming an average rock site condition smoothed amplification data were plotted above 3d images of the basin revealing a general correlation between the estimated basin depth and total basin amplification for both earthquakes over three frequency ranges low 0 2 2 hz intermediate 2 8 hz and high 8 12 5 hz the depth dependent corrections are derived from the regression of the combined data from both earthquakes in order to reduce an uncertainty caused by the azimuth of incoming waves ground motion duration is defined as the time for 95 of the acceleration spectral energy to pass after the s wave arrival due to ambiguity in defining a basin parameter controlling duration it was impossible to develop a generic equation that would relate the duration ratio observed synthetic to some characteristic of the basin users are cautioned though that the durations within the basin may be as much as four times longer than the simulated ones the procedure is outlined for potential users who wish to use the results of this study in synthesizing more accurate earthquake ground motions taking into account complicated basin geometry and near surface effects the results are directly applicable to engineering simulation of strong ground motions in a sedimentary basin environment manuscript received 21 may 2002

permeability and melt flow in simulated and natural partially molten basaltic magmas abstract a lattice boltzmann method is used to calculate grain scale low reynolds number single phase interstitial porous melt flow in two independent igneous microstructures 1 a microstructure simulated using a stochastic algorithm for progressive crystallization and 2 a natural microstructure obtained through x ray computed tomography ct of a partially melted basalt in both cases the error in the calculated flow field due to finite discretization and domain size has been estimated or demonstrated to be insignificant visually the porous melt flow tends to localize into high flux channels especially with increasing crystallinity and this impression is quantitatively confirmed by increasing skewness of the velocity distribution in the direction parallel to the imposed pressure gradient driving the flow a change from uniform to localized melt flow in naturally occurring situations may have a profound effect on the distribution of trace elements and if the melt is reactive the chemical and structural evolution of the igneous microstructure through localized phase change permeabilities of both microstructures are determined from the calculated steady flow field the permeabilities are then fitted to two different correlation models which are based on the rumpf gupte and carman kozeny relations in these models we express permeability as functions of melt fraction and either the mean crystal length or specific surface area extrapolated permeability to higher melt fractions using both correlation models for the partially melted basalt is shown to be within a factor of four of experimentally determined permeability of a similar sample we determine permeability estimates in the relatively unexplored melt fraction range of 30 80 and find consistency with previous work for permeability estimates in the melt fraction range of 20 30

intraslab seismicity and thermal stress in the subducted cocos plate beneath central mexico we present a model of the subducting cocos slab beneath central mexico that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment based on the recently developed thermal models for the central mexico subduction zone the thermal stresses due to non uniform temperature contrast in the subducting slab are calculated using a finite element approach the slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep the calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 mpa on the other hand the top of the slab experiences high compressive thermal stresses up to 110 mpa depending on the elastic constants used and location along the flat part of the subducting plate these compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes and two different sources of stress are proposed the trenchward migration of the mexican volcanic arc for the last 7 ma indicates an increase of the slab dip through time this observation suggests that the gravity torque might exceed the suction torque considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0 5 10 16 1 5 10 16 n m the upper half would exhibit tensional stresses of 40 110 mpa that can actually balance the compressive thermally induced stresses an alternative stress source might come from the slab pull force caused by the slab positive density anomaly based on our density anomaly estimations 75 20 kg m 3 a 350 km slab length dipping at 20 into the asthenosphere induces a slab pull force of 1 7 10 12 4 6 10 12 n m this force produces a tensional stress of 41 114 mpa sufficient to balance the compressive thermal stresses at the top of the flat slab the linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60 180 mpa inside the flat slab core also our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40 80 mpa this study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath central mexico and also it shows that thermal stresses due to non uniform reheating of subducting slabs play a considerable role in the total stress field

the permeability controlled accumulation of primary magma the initial accumulation of primary magma occurs just after the mantle has become permeable the accumulation is caused by the compaction of the residuum which either may be controlled by the rate of creep or by the rate of flow of the interstitial melt experimental results suggest that the rate of compaction is controlled by the permeability and a model for the accumulation process is worked out on this basis the compaction causes the formation of a lower compaction boundary and an upper layer of melt the ascending mantle of plumes and convection currents will form layers of melt situated 20 100 m apart the type of partial melting for this accumulation is critical melting

moment tensor estimation using regional seismograms from a tibetan plateau portable network deployment the authors present the results of moment tensor inversions using regional seismograms recorded during the 1991 1992 passcal passive seismic experiment on the tibetan plateau using data recorded over a one year period they estimate the source mechanisms for 38 earthquakes within and near the tibetan plateau with moment magnitudes m sub w ranging from 3 8 to 5 0 with only one exception all the events within the plateau but away from the plateau boundary indicate either strike slip or extensional faulting a result consistent with previous studies three fourths of all the events occurred at depths less than 10 km and all but two at depths less than 15 km several events located near the southeastern section of the plateau exhibit an unusual m sub b m sub w difference of approximately one magnitude unit 19 refs 3 figs

paleozoic and mesozoic high pressure metamorphism at the margin of ancestral north america in central yukon eclogite and blueschist in the yukon tanana and slide mountain terranes in the yukon and alaska preserve a partial record of middle paleozoic late paleozoic and early mesozoic subduction and subsequent unroofing near faro eclogites are commonly mylonitized and partially retrogressed the analysis of fe mg exchange between garnet and clinopyroxene suggests temperatures t in the 400 500 c range for eclogite and jadeite 40 mol quartz suggest minimum pressures p of 11 13 kilobar at that temperature range clinopyroxene garnet epidote quartz suggest pressure near 15 kilobar p h 2 o p s eclogite near last peak which contains brown hornblende late stage oligoclase and biotite yielded p t estimates up to 625 c at a minimum pressure of 14 kilobar eclogite near ross river yielded temperatures near 525 c and a minimum pressure near 13 kilobar the previously proposed permian triassic age of high pressure tectonism for the entire yukon tanana terrane in the yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities in this study isotopic 40 ar 39 ar analysis of white mica from eclogite in the simpson range yielded a date of 344 1 ma eclogite and blueschist from ross river yielded white mica 40 ar 39 ar dates of 267 3 ma and 273 3 ma respectively white mica in eclogite from faro was dated at 260 3 ma white mica in two eclogites from stewart lake about 2 km apart yielded 40 ar 39 ar dates of 228 1 ma and 346 3 ma white mica in last peak eclogite for which a concordant u pb zircon date is 269 ma yielded an integrated date of 236 1 ma overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others the cooling ages indicate significant plate convergence as early as carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone when this is considered together with the range of ages of other high pressure occurrences the north american cordillera is seen to be characterized by multiepisodic and diachronous high pressure metamorphism

hydrogeochemistry of seasonal flow regimes in the chena river a subarctic watershed draining discontinuous permafrost in interior alaska usa abstract thawing of permafrost and a shift in the timing of major seasonal transitions spring melt and fall freeze up are two anticipated effects of climate warming in northern regions these fundamental changes in terrestrial ecosystems could have major ramifications for the timing and fluxes of carbon and nutrient exports from watersheds and the geochemical signatures of northern rivers discontinuous permafrost which underlies watersheds in interior alaska usa is expected to degrade rapidly in response to climate warming and this will likely alter subsurface flows and flow paths water residence times water soil and water rock interactions this study of the chena river in interior alaska was undertaken to quantify dissolved organic carbon doc total dissolved nitrogen tdn major ion strontium isotope and stable oxygen and hydrogen isotope values during two year long time series sampling periods our goal was to identify the hydrogeochemical signatures associated with the major seasonal flow regimes periods of increased discharge such as spring melt and major summer precipitation events yielded elevated doc and tdn concentrations diluted major ion concentrations and shifted stable oxygen and hydrogen isotope ratios from base flow toward precipitation values during dry summer periods nutrient concentrations decreased and strontium isotope values were indicative of a higher proportion of silicate versus carbonate mineral dissolution winter base flow had a unique geochemical signal with a slight increase in tdn concentrations compared to typical summer conditions and a lower proportion of silicate versus carbonate mineral dissolution since flow paths in subarctic watersheds can change dramatically over the course of a year we interpreted our results within the context of a schematic model for subsurface flow to identify how permafrost degradation might affect nutrient exports and hydrogeochemical patterns in these watersheds

evidence of low q below popocatepetl volcano and its implication to seismic hazard in mexico city seismograms recorded in mexico city reveal that the amplitudes of seismic waves whose wavepaths pass below popocatepetl presently an volcano before reaching the city are diminished by a factor of about one third at frequencies greater than 1 hz as compared to those which do not cross the volcano the high attenuation of seismic waves below the volcano may be attributed to the presence of magma and partial melting of rocks q of shear waves below the volcano is roughly estimated as 60 a consequence of the large attenuation of high frequency seismic waves is a decrease in the seismic hazard to low rise buildings in mexico city from intraplate earthquakes whose wavepaths cross the volcano

modeling finite fault radiation from the ωn spectrum the high frequency seismic field near the epicenter of a large earth quake is modeled by subdividing the fault plane into subelements and summing their contributions at the observation point each element is treated as a point source with an o92 spectral shape where co is the angular frequency ground motion contributions from the subsources are calculated using a stochastic model attenuation is based on simple geometric spreading in a whole space coupled with regional anelastic atten uation q operator the form of the co n spectrum with natural n follows from point shear dislocation theory with an appropriately chosen slip time function the seismic moment and comer frequency are the two parameters defining the point source spectrum and must be linked to the subfanlt size to make the method complete two coefficients aa and k provide this link assigning a moment to a subfault of specified size introduces the stress parameter ao the relationship between comer frequency dislocation growth rate and fault size is established through the coefficient k which is inherently nonunique these two parameters control the number of subsources and the ampli tudes of high frequency radiation respectively derivation of the model from shear dislocation theory reveals the unavoidable uncertainty in assigning ogn spectrum to faults with finite size this uncertainty can only be reduced through empirical vali dation the method is verified by simulating data recorded on rock sites near epicenters of the m8 0 1985 michoacan mexico the m8 0 1985 valparaiso chile and the m5 8 1988 saguenay qurbec earthquakes each of these events is among the largest for which strong motion records are available in their respective tectonic environ ments the simulations for the first two earthquakes are compared to the more de tailed modeling of somerville et al 1991 which employs an empirical source function and represents the effects of crustal structure using the theoretical impulse response both methods predict the observations accurately on average the precision of the methods is also approximately equal the predicted acceleration amplitudes in our model are generally within 15 of observations an unexpected result of this study is that a single value of a parameter k provides a good fit to the data at high frequencies for all three earthquakes despite their different tectonic environments this suggests a simplicity in the modeling of source processes that was unanticipated

rupture history of september 30 1999 intraplate earthquake of oaxaca mexico mw 7 5 from inversion of strong motion data near source strong motions are inverted to estimate the rupture history of intraslab normal faulting september 30 1999 oaxaca mexico earthquake two focal mechanisms harvard and neic cmt solutions are tested for the source geometry the inversion with the ne dipping fault plane of the harvard solution best matches the data strike 295 dip 50 rake 82 we estimated the slip distribution on the fault and the associated rupture front propagation as well as the rise time the inversion results show that the rupture mainly propagated from ese to wnw and slightly downdip with an average rupture velocity of about 3 km s the rise time ranges between 1 and 2 s the slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2 5 m located about 20 km and 40 km wnw of the hypocenter most of the slip is released at an average depth of 45 km a smaller area with a maximum slip of 1 5 m is also observed close to the hypocenter the total co seismic moment released is equal to 1 8 1020 nm

slip distribution of the 1992 landers earthquake and its implications for earthquake source mechanics we use near source 10 to 164 km displacement seismograms to image the slip distribution and rupture history of the 28 june 1992 mw 7 3 landers california earthquake aftershock seismograms from similar distances are modeled to find the velocity model and frequency range 0 05 to 0 25 hz over which theoretical green s functions are most accurate and the measure of fit is used as an upper bound on theoretical error in the mainshock inversion we represent the rupture surface with three planar segments divided into 3 by 3 km elements extending from the surface to 18 km depth and solve for the slip distribution and the rupture time model that minimizes misfit to both recorded seismograms and mapped surface displacement in a least squares sense we investigate two faulting models one with a uniformly short 3 sec rise time everywhere on the fault surface and the other with a variable rise time 2 to 6 sec we perform sensitivity tests using synthetic data from a hypothetical earthquake to evaluate model resolution and solution stability in the sensitivity tests both fault models recover similar slip and rupture features but neither is capable of imaging details of the rise time for the landers earthquake we find an average rupture velocity of 2 5 km sec and use this average for a starting model in a linearized inversion for rupture time in our solutions of slip amplitude distribution the southernmost johnson valley fault segment has 20 of the total seismic moment 6 to 8 1019 n m with small displacements near the hypocenter the homestead valley segment contributes half of the moment with the largest slip amplitudes 25 to 35 km northwest of the hypocenter at 4 to 12 km depth and the camp rock emerson segment contributes the remaining moment with the largest slip amplitude 35 to 50 km northwest of the hypocenter in the shallow crust 9 km there is some evidence that the rupture front is delayed as it encounters high slip regions suggesting that prior to the mainshock these areas were further from failure owing either to greater strength or lower prestress

regional wave propagation from mexican subduction zone earthquakes the attenuation functions for interplate and inslab events the seismic waves from subduction zone earthquakes are significantly affected by the presence of 3d variation in crust and upper mantle structure around the source area these heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances this is illustrated by studying shallow interplate earthquakes along the mexican subduction zone and deeper inslab normal faulting earthquakes in the subducted cocos plate beneath mexican mainland the strong motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and mexico city we compare the wavefield from two large earthquakes of different source type during the shallow h 17 km interplate 1995 guerrero earthquake m w 7 3 the lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source the presence of a lateral velocity gradient in the crust caused by the subduction of the cocos plate enhances the lg wave amplitude which is then amplified further in the mexican volcanic belt by amplification in the low velocity volcanic rocks both effects lead to very large ground motions along the path from the coast to the mexican inland in the frequency band from 0 2 to 4 hz however for the deeper h 40 km inslab normal faulting 1999 oaxaca earthquake m w 7 5 the amplitude of the lg phase is too small to produce the abnormal wave propagation and the direct s wave and its multiple sms reflections between the free surface and moho show a simple attenuation with increasing distance we compare these observations with numerical simulations of seismic wave propagation using the fourier spectral method the results provide a key to the understanding of seismic wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3d heterogeneous structure

frequency domain inversion of strong motions application to the 1992 landers earthquake we present a frequency domain inversion in which the observed earthquake strong ground motions are used to constrain the space time dependence of slip on a fault green s functions are numerically evaluated and the parameters describing the rupture are the local slip rupture time and rise time these parameters are simultaneously evaluated without additional constraints this procedure allows for large variations in the local rupture velocity the june 28 1992 landers earthquake mw 7 3 offers an exceptional opportunity to apply this technique to a major strike slip event we model the rupture evolution including local differences in slip durations and variations in rupture velocity our final results are in good agreement with other inversion studies geodetic and surface observations the main discrepancies occurred at depth and at the end of the johnson valley fault we show that a relatively low resolution could be an explanation for these differences rupture velocity and slip are extremely heterogeneous both along strike and with depth a moment of 0 90 1020 n m was found the slip distribution shows that this event consists of a series of regions of high slip subevents separated by regions of relative low slip approximately 50 of the moment was released on the homestead valley fault in this region of large slip the rupture velocity inferred by our inversion is well constrained and is equal to 3 0 km s at depth and 2 5 km s near the surface our inversion favors the hypothesis that the duration of the slip at each point of the fault is of the order of the duration of rupture of each subevent

inslab earthquakes of central mexico peak ground motion parameters and response spectra we developed equations to predict pseudoacceleration response spectra 5 damping peak ground acceleration and peak ground velocity at free field rock sites for intermediate depth normal faulting inslab earthquakes of central mexico the data set comprises 16 earthquakes 5 2 m w 7 4 35 h 138 km recorded at local and regional distances r 400 km it represents a homogeneous catalog with respect to tectonic regime fault mechanism and soil class our results show larger amplitudes in the epicentral area from inslab events than from interplate events a consequence of higher stress drops during the former type of earthquakes peak ground accelerations from moderate to large m w 6 0 inslab events significantly exceed those from interplate events with similar magnitude reaching almost three times for the largest events the ground motion due to inslab events however decays faster than for the thrust events our results are in reasonable agreement with other studies based on japanese and the worldwide data

some aspects of source characteristics of the 19 september 1985 michoacan earthquake and ground motion amplification in and near mexico city from strong motion data abstract the fourier amplitude ratio of the 19 september m s 8 1 to the 21 september 1985 m s 7 6 michoacan earthquakes obtained from data in and near mexico city increases from about 3 5 at 0 5 hz to about 10 at 0 2 hz this increase is neither seen in the coastal data nor in the teleseismic broadband gdsn p wave spectra it is also not predicted by 2 or gusev9s scaling laws we tentatively attribute the increase at frequencies of less than 0 5 hz to a special path and or a depth of energy release effect for the 19 september earthquake the damage to mexico city is mostly related to 0 5 hz energy since at f 0 5 hz the fourier amplitude ratios in and near mexico city along the coast and at teleseismic distances are nearly equal we attribute this energy to the source characteristics of the 19 september earthquake the evidence suggests that the 19 september was about twice more energetic at f 0 5 hz than expected from scaling laws and from spectral regression of coastal earthquakes recorded in mexico city the analysis of strong motion data recorded in mexico city shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in ciudad universitaria the frequency at which the maximum relative amplification occurs varies from site to site and lies between 0 2 and 0 7 hz the data suggest that f o 4 h where f o frequency at which maximum relative amplitude occurs and and h are the shear wave velocity and the thickness of the clay layer respectively relatively small if any nonlinear behavior of the clay is found at strains of about 0 2 per cent during the great 19 september earthquake for a given lake bed site maximum relative amplitude and f o are roughly constant independent of magnitude depth and azimuth of sources located more than about 200 km ground motion at lake bed sites in mexico city with f o 0 5 hz appear to be amplified 75 times with respect to hard rock coastal sites at equal distances from the source this is also confirmed by m l calculation of the 19 and 21 september earthquakes for the 19 and 21 september earthquakes the average values of m l at hard rock coastal sites 100 distance 250 km are 7 4 and 6 7 whereas they are 8 7 and 8 4 in the lake bed zone distance 300 km respectively

structural and kinematic evolution of the teslin suture zone yukon record of an ancient transpressional margin abstract the teslin suture zone tsz yukon which forms the fundamental boundary between rocks deposited along the ancient margin of north america and allochthonous terranes to the west preserves a complex history of pre mid jurassic convergence and dextral strike slip translation which is overprinted locally by late cretaceous dextral strike slip shear the tsz comprised of a 15 20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata basalt peridotite and granitoids is divisible into distinct elongate structural domains based on the distribution of two well defined but differently oriented stretching lineations lds and lss lds and lss both formed during non coaxial ductile deformation lds trends e w and plunges down dip whereas lss trends nnw sse and plunges shallowly two 1 2 km wide nnw trending anastomosing shear zones of lss cross cut lds structures indicating lss is younger other field relations indicate further that lds began forming earlier than lss followed by a period of coeval lds and lss formation latest motion was dominantly parallel to lss synkinematic mineral assemblages associated with both lds and lss related structures record greenschist to albite epidote amphibolite facies conditions lds tectonites record a complex movement history to the west textural asymmetries indicate west side down or normal shear whereas in eastern domains top to the east thrust style asymmetries dominate lss tectonites consistently record dextral or top to the north shear parallel to lss tectonic motion began as chiefly penetrative dip slip shear lds at a high angle to the present trend of the tsz and evolved to dominantly dextral strike slip shear lss parallel to the trend of the zone the geometry kinematics and sequence of deformation in tsz tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin parallel translation