the undrained response of sands with additions of particles of various shapes and sizes as a natural deposit sand is rarely found clean that is without fractions of silt or clay the engineering behaviour of two different host sand gradations mixed with additions of particles of various shapes and sizes is compared with that of clean sand anisotropically consolidated specimens are sheared in triaxial compression and extension at their loosest initial state resulting from the same depositional method namely air pluviation additive materials at contents less than 2 5 by weight can dramatically change the undrained behaviour of the host sand the addition of rotund particles e g silt should be distinguished from that of flat or platy particles of different size e g kaolin silt or sand size mica the importance of shape and location of additives in modifying the sand structure is not reflected in measures such as void ratio mixtures of both sands with silt size mica exhibit the most unstable response although these had the lowest values of void or granular void ratio the undrai

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

tectonic setting and source parameters of the september 19 1985 michoacan mexico earthquake analysis of body waves and long period surface waves from the september 1985 earthquake in coastal michoacan mexico shows that the event was an interplate subduction event with a low dip angle fault plane 9 striking parallel to the mid america trench 288 and a small component of left lateral motion 72 with a point source depth of 17 km and a seismic moment in excess of 1 10 28 dyn cm the earthquake was a multiple event with a second source of identical moment fault geometry and depth occurring approximately 26 s after the first directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first this suggests that the earthquake first broke the northern portion of the michoacan gap propagated with low moment release through the rupture zone of the 1981 playa azul earthquake and then broke the remaining asperity in the southern section of the gap the seismic moment determined from rayleigh and love waves is between 1 0 1 7 10 28 dyn cm m w 7 9 8 1 the largest moment determined to date for a mexico subduction earthquake comparison of seismograms at pasadena with records of other large mexico events shows that the michoacan earthquake is basically the same size as the 1932 jalisco mexico earthquake and clearly larger than other significant events in mexico since 1932 the seismic moment and the time since the last large earth quake in michoacan in 1911 fit an empirical relation between moment and recurrence time found for the guerrero oaxaca region of the subduction zone the large aftershock on september 21 m s 7 5 has the same geometry as the mainshock a somewhat larger source depth 22 km a simple time function and a seismic moment between 2 9 4 7 10 27 dyn cm m w 7 6 7 7

characterization of coarse soils derived from igneous rocks for rammed earth abstract rammed earth refers to a conventional construction method as well as a material comprising gravel sand and fine soil there has been an increasing interest due to it being a sustainable and environmentally friendly material the strength of rammed earth depends on the material and environment e g temperature and relative humidity while rammed earth is an established and accepted construction material in some countries e g australia and new zealand its use in south china is mostly related to historical structures of which the unesco tulou buildings in fujian province is an example in addition south china has a sub tropical climate with a distinctive rainy humid and hot season followed by a dry and cool season and soils are granular due to the underlying igneous geology with a coarse fraction sand and gravel in excess of 80 their appropriateness as earthen materials is at first sight debatable this paper aims to assess the pertinence of three representative soils from hong kong residual soil alluvium and completely decomposed granite for unstabilised rammed earth unconfined compressive strength tests and shrinkage measurements at different relative humidity as well as the organics content are determined to aid the interpretation of the strength and shrinkage data a fundamental characterization is conducted namely of the wettability and particle attributes size and shape by comparing the results to established guidelines for earthen construction the three soils are shown to achieve strengths in the range 0 2 1 4 mpa and shrinkage after equilibration for one week up to 4 with the residual soil outperforming the others moreover it was found the strength of the soils was influenced by the relative humidity and was strongly dependent on changes of dry density the wettability results confirmed the higher organics for the alluvium while the more rounded particle shape and greater content of potassium feldspar of the completely decomposed granite corroborated with its lower strength the results highlight the importance of conducting a fundamental characterization of natural soils for rammed earth via the particle attributes wettability and mineralogy

effects of nonplastic fines on static liquefaction of sands des essais triaxiaux en compression non draines ont ete effectues sur des echantillons reconstitues de sable du nevada et d ottawa avec variation systematique de la teneur en fines particules inferieures a 0 074 mm afin d evaluer l effet de la fraction fine sur le potentiel de liquefaction statique lors d un chargement monotonique les fines etaient non plastiques les resultats indiquent clairement que la presence de fines peut augmenter considerablement le potentiel de liquefaction statique des quatre sables propres etudies lorsqu on utilise la technique du montage a sec avec un entonnoir les fines semblent creer dans le sol une structure particulaire tres compressible ce qui accroit ainsi le potentiel de liquefaction alors meme que la densite augmente accroitre la teneur en fines d un sable propre peut aboutir a l augmentation du potentiel de liquefaction jusqu a un seuil maximum pour lequel les fines dominent le comportement non draine du sol a ce point la le potentiel de liquefaction tend a devenir constant vis a vis de l indice de densite on a trouve que la liquefaction statique complete pouvait intervenir a un indice de densite superieur a 60 pour des teneurs en fines elevees

discussion of cyclic strength of sand under sustained shear stress by j yang and h y sze in their technical paper the authors study the effect of initial shear stress on the cyclic strength and liquefaction potential of sand they introduce the concept designated as thresholda inwhicha represents the sustained shear stress level and they suggest that this parameter can be related well to a state parameter c a linear relationship between cyclic strength andc is proposed and the line representing this relationship rotates as a changes the authors present extensive results fromexperiments performed on themodel fujian sand onwhich they base their generalizations they also mention a lack of sufficient data from other sources that would enable a better understanding of the problemconsidered especially because someofficial geotechnical organizations for example the national center for earthquake engineering research nceer committee still do not recommend existing approaches as geotechnical standards in order to meet these needs we would like to add information about the similar behavior of another model sand called skarpa the discussers hope that our limited results will be included in respective databases concerning the effect of sustained shear stress on the cyclic behavior of sands we have also found some inconsistencies in the paper by yang and sze and have addressed these in additional comments

seismic rupture patterns in oaxaca mexico the spatio temporal patterns of seismic activity for events with body wave magnitude mb 4 0 are investigated for the three most recent large earthquakes in the oaxaca region of southern mexico august 23 1965 ms 7 6 mw 7 5 august 2 1968 ms 7 1 mw 7 3 november 29 1978 ms 7 8 mw 7 6 a master catalogue of earthquakes is compiled for the analyses using the international seismological center isc catalogue supplemented by the national earthquake information service neis catalogue the events are then relocated with the joint hypocenter determination jhd method at this magnitude threshold we find that the aftershock zone of the 1965 earthquake was seismically quiescent for at least 20 months prior to the main event the 1968 earthquake was preceded by 1 year of foreshock type activity clustered in the aftershock zone the 1978 earthquake was preceded by 43 months of quiescence and one event mb 4 7 within the aftershock zone 4 months prior to the mainshock we also find that a segment of the subduction zone in oaxaca remains unbroken by these earthquakes in addition some catalogue problems are pointed out which are important to interpretations of seismicity patterns

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

coseismic and postseismic stress changes in a subducting plate possible stress interactions between large interplate thrust and intraplate normal faulting earthquakes 1 a large intraplate normal faulting earthquake mw 7 5 occurred in 1999 in the subducting cocos plate below the downdip edge of the ruptured thrust fault of the 1978 oaxaca mexico earthquake mw 7 8 this situation is similar to the previous case of the 1997 normal faulting event mw 7 1 that occurred beneath the rupture area of the 1985 michoacan mexico earthquake mw 8 1 we investigate the possibility of any stress interactions between the preceding 1978 thrust and the following 1999 normal faulting earthquakes for this purpose we estimate the temporal change of the stress state in the subducting cocos plate by calculating the slip distribution during the 1978 earthquake through teleseismic waveform inversion the dynamic rupture process and the resultant coseismic stress change together with the postseismic stress variations due to plate convergence and the viscoelastic relaxation process to do this we calculate the coseismic and postseismic changes of all stress components in a three dimensional space incorporating the subducting slab the overlying crust and uppermost mantle and the asthenosphere for the coseismic stress change we solve elastodynamic equations incorporating the kinematic fault slip as an observational constraint under appropriate boundary conditions to estimate postseismic stress accumulations due to plate convergence a virtual backward slip is imposed to lock the main thrust zone the effects of viscoelastic stress relaxations of the coseismic change and the back slip are also included the maximum coseismic increase in the shear stress and the coulomb failure stress below the downdip edge of the 1978 thrust fault is estimated to be in the range between 0 5 and 1 5 mpa and the 1999 normal faulting earthquake was found to take place in this zone of stress increase the postseismic variations during the 21 years after the 1978 event modify the magnitude and patterns of the coseismic stress change to some extent but are not large enough to overcome the coseismic change these results suggest that the coseismic stress increase due to the 1978 thrust earthquake may have enhanced the chance of occurrence of the 1999 normal faulting event in the subducting plate if this is the case one of the possible mechanisms could be static fatigue of rock materials around preexisting weak planes involved in the subducting plate and it is speculated that that one of these planes might have been reactivated and fractured because of stress corrosion cracking under the applied stress there for 21 years

forewarning of static liquefaction landslides abstractstatic liquefaction failure and the ensuing flow slide of slopes have produced significant damages to structures and even loss of lives based on a very large number 1 632 of laboratory s

teleseismic time functions for large shallow subduction zone earthquakes broadband vertical p wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum pacific in the last 45 yr most of the records studied come from a common instrument the pasadena california benioff 1 90 seismometer propagation and instrument effects are deconvolved from the p wave records using a damped least squares inversion to obtain the teleseismic source time function the inversion has the additional constraint that the time function be positive everywhere the period band over which the time functions are considered reliable is from 2 5 to 50 sec fourier displacement amplitude spectra computed for each of the 1 90 p wave trains indicate spectral slopes measured between 2 and 50 sec of 1 0 to 2 25 with an average value of 1 5 these values assume an average attenuation of t 1 0 the seismic moments derived from the p wave time functions compare well with other published values for earthquakes having moments smaller than 2 5 10 28 dyne cm m w 8 2 because the 1 90 seismometer has little response at very long periods this technique underestimates the moments of the very largest events the time functions are characterized using five parameters 1 spectral slope between 2 and 50 sec 2 roughness of the time function 3 multiplicity of sources 4 pulse widths of individual sources and 5 overall signal duration the 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere convergence rates and maximum size of earthquakes comparing the time function parameters with age rate and m w of the subduction zone does not yield obvious global trends however most of the subduction zones do behave characteristically and can be grouped accordingly

a unified framework for evaluating in situ state of sand with varying fines content in the design and construction of large scale earth structures such as hydraulic fills for artificial islands or tailings dams a major concern is the susceptibility of sand fills to flow failure o

undrained monotonic response of clean and silty sands a series of undrained monotonic triaxial compression tests were performed on ottawa sand containing 0 5 10 and 15 of non plastic silt the paper focuses on distinctive states of the monotonic undrained response of sands namely the critical state the phase transformation state the quasi steady state and the state of undrained instability onset of flow liquefaction specimens were prepared using slurry deposition and moist tamping to investigate the effect of the initial sand fabric on these states it was observed that an increase in the non plastic fines content leads to a downward shift of the critical state line in the void ratio mean effective stress space and to an increase of the critical state friction angle the present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behaviour in the early stages of shearing with its influence vanishing at large strains moist tamped specimens demonstrate considerably larger undrained instability s

the november 29 1978 oaxaca mexico earthquake a large simple event the recent oaxaca mexico earthquake november 29 1978 mw 7 6 ms 7 8 seismic moment m0 3 2 1027 dyne cm is of special interest because of its location within a predetermined seismic gap the event excited long period 100 200 s multiple rayleigh and love waves which were well recorded by the wwssn these data along with p wave first motion data and p waveforms were used to constrain the source mechanism the results indicate an oblique thrust mechanism consistent with subduction of the cocos plate to the northeast beneath mexico dip 14 strike n90 w rake 54 hence this event is indeed of the type anticipated by ohtake et al 1977 a local network of stations installed in a joint university of mexico california institute of technology program began operation 20 days prior to the mainshock gonzalez 1979 mcnally et al 1979 mcnally 1981 ponce et al 1980 the 43 foreshocks of magnitude m 2 8 were recorded by the network in a period of 20 days prior to the mainshock these events show an interesting spatial and temporal pattern which culminates in the last 1 8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake this pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity with supplemental stations the network continued operation until december 12 1978 singh et al 1980 and provided good hypocentral control for the more than 169 aftershocks of magnitude m 2 8 the area of the aftershock zone determined from these events is 5525 km2 85 km by 65 km in spite of the large size of the event p waves for the oaxaca event indicate an extremely simple source at the period range of the wwssn long period seismographs this simplicity suggests that the p waves were generated by a limited portion of the rupture surface perhaps by the breaking of a fault zone asperity this result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves such simplicity also appears to be the case for the 1965 and 1968 mexican earthquakes on the adjacent eastern and western ends of the oaxaca aftershock zone respectively chael et al 1980 this type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries

influence of fines on the collapse of loose sands a study has been carried out to investigate the influence of fines and gradation on the skeletal collapse behaviour of loosely prepared sand samples loose sand samples formed by moist tamping and consolidated to the same effective stress level were prepared with varying percentages of both plastic and non plastic fines 74 m samples were isotropically consolidated and subjected to monotonic undrained triaxial compression results indicate that increased percentages of fines 0 5 this change is represented by a decreased strain softening behaviour and a shifting of the steady state line variation of the gradation of the host sand appears to have little effect on the monotonic undrained behaviour a supplemental study involving use of the scanning electron microscope was carried out to evaluate the findings of the test program in terms of sand fabric key words colla

the undrained behaviour of clayey sands in triaxial compression and extension the paper describes an experimental investigation of the stress strain behaviour of anisotropically consolidated clayey sands carried out using instrumented and computer controlled triaxial cells the specimens were prepared by sedimenting ham river sand into a kaolin suspension for a given granular void ratio as the clay content of the sedi mented clayey sands increases from 4 6 to 10 the undrained brittleness and strains to phase transformation also increase when the specimens are sheared in compression when sheared in extension liquefaction takes place for clay contents up to 7 5 increases in granular void ratio give rise to increased brittleness and potential to liquefaction the effects of overconsolidation on strength and stiffness characteristics have been examined the effective stress paths for undrained shear of the normally consolidated soil form a boundary for the effective stress paths for the lightly overconsolidated soil l article decrit une etude experimentale effectuee a l aid

collapse of loose sand with the addition of fines the role of particle shape this paper describes a fundamental study to explore the role of particle shape in the shear behaviour of mixtures of sand and fine particles fines through macro scale and grain scale laboratory experiments together with interpretations in the framework of critical state soil mechanics and conceptual micromechanics models two non plastic fines of distinct shape angular crushed silica fines and rounded glass beads were added to two uniform quartz sands toyoura sand and fujian sand to produce four binary mixtures laboratory quantification of particle shape and size was conducted for both fines and sands and a series of undrained triaxial tests was performed to investigate the overall response of the mixed soils at different percentages of fines one of the significant findings is that the critical state friction angle of a mixed soil is affected not only by the shape of coarse particles but also by the shape of fine particles and this shape effect is coupled with fines content when a small amount

ground motions from subduction zone earthquakes a total of 258 horizontal components of accelerogram data recorded at soil sites during thrust normal and strike slip earthquakes occurring in seven subduction zones around the pacific ocean were analyzed the results of statistical analyses of 5 per cent damped pseudo velocities psv computed from these accelerograms at 10 periods t between 0 1 and 4 sec indicated no significant differences in the average psv levels of the thrust normal and strike slip data from the northern honshu zone each of these groups of northern honshu data which together comprised one half of the total data base were fit equally well by the same attenuation model the analyses also revealed that at intermediate periods 0 8 t 3 sec the psv observed at stiff soil sites within the northern honshu nankai kuril mexico and alaska zones were on the average significantly greater than the psv observed at similar sites in the peru northern chile and new britain bougainville zones this observation indicates that differences in the source and or travel path characteristics between the two groups accounted for the differences in psv independent evidence supporting source differences is the correlation noted between psv in this period band and the characteristic source complexities for these zones which hartzell and heaton inferred from the teleseismic data of the larger magnitude earthquakes certain anomalous tectonic characteristics of the new britain bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff soil sites within this zone some of the differences in the psv among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics geology greatly influenced the mexican psv data from soft soil sites to a lesser extent geology probably affected the alaskan data which were recorded mostly on deep stiff soils and the new britain bougainville data recorded at yonki which is underlain by softer soil deposits

diatom response to climatic warming over the last 200 years a record from gonghai lake north china abstract annual monitoring data and centennial and millennial scale paleolimnological records have revealed an increase in small celled planktonic diatom species relative to heavier tycho planktonic and small benthic diatoms in many high altitude lakes presumably in response to recent climatic warming in addition to changes in the composition and structure of lacustrine diatom assemblages there have also been changes in diatom biodiversity however these changes are not regionally consistent in this study we chose a temperate mid latitude lake to explore the response of changes in diatom assemblages and biodiversity to recent climatic warming the diatom record spans the past ca 200 years and reveals that since 1966 ce the small celled cyclotella ocellata stelligera has replaced the previously dominant large celled phytoplankton cyclotella schroeteri these changes are in accordance with the increasing trend in global mean surface temperature biodiversity increased remarkably from 1966 ce but began to decline after 1990 ce presumably in response to rapid climatic warming our findings indicate that the response of diatom biodiversity to climate change was a nonlinear process

attenuation of modified mercalli intensity with distance in mexico abstract two relations are proposed to predict the attenuation of modified mercalli intensity i with distance d for mexican earthquakes i e ln i b 0 b 1 ln d d 9 b 2 d d 9 b 3 ln m s ln i b 0 b 1 d d 9 b 2 ln d d 9 b 3 ln m s m s is the earthquake surface wave magnitude d is a distance related to the maximum i mapped for an earthquake i or to m s the coefficients b i i 0 1 2 3 were obtained by fitting in a least square sense the information contained in the intensity maps of 32 events to the relations those events were classified in three groups according to their epicentral location focal mechanism and depth i e events related to the subduction zone intermediate depth earthquakes in south central mexico and to shallow crustal events along the trans mexican volcanic belt the i predicted by the proposed relations compare well with the i observed for historical earthquakes not included in the fitting results obtained from a parametrical study showed that the attenuation of i with d is different for each of the three types of earthquakes for distances of less than about 200 km the earthquakes associated with the subduction zone have a larger attenuation than the ones originating in the south central region of mexico for greater distances d 200 km the opposite behavior is observed the events located in the trans mexican volcanic belt have a larger attenuation with distance than that of events in the other two regions from these results it seems advisable in mexico to use several attenuation relations to estimate the seismic hazard at a site depending on the particular tectonic setting and the path of the events under consideration

nonuniform seismic slip rates along the middle america trench revised estimates of seismic slip rates along the middle america trench are lower on the average than plate convergence rates but match them locally for example oaxaca along the cocos north american plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction for at least 81 yr and possibly several hundred years no major m sub s or 7 5 shallow earthquake is known to have occurred near the orozco fracture zone and tehuantepec ridge areas compared with the average recurrence periods for large earthquakes 33 8 yr since 1898 and 35 24 yr between 1542 and 1979 this suggests that either a large m or 8 4 event may be anticipated at such locations or that these are points of aseismic subduction large coastal terraces and evidence suggesting tectonic uplift are found onshore near the orozco fracture zone the larger discrepancy between plate convergence and seismic slip rates along the cocos carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate we used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip the observations appear consistent with a possible forthcoming episodemore of more intense seismic activity based on a series of comparisons with carefully delineated aftershock zones we conclude that the zones of anomalous seismic activity can be indentified by a systematic automated analysis of the worldwide earthquake catalog m sub b or 4 less

crustal structure of oaxaca mexico from seismic refraction measurements seismic refraction and gravity data have been analyzed to obtain a model of the compressional wave structure of the ocean to continent transition in the state of oaxaca in southwestern mexico crustal thickness on the continent at the latitude 18 n is 45 4 km based on reflected phases from the moho discontinuity the crust has been modeled with three layers with velocities of 4 3 to 4 6 5 0 to 5 7 and 6 85 to 7 0 km sec each with positive velocity gradient the crust thins to 10 km at the coast near pinotepa nacional where precambrian metamorphic rocks are exposed 45 km from the mid america trench offshore the oceanic structure consists of an 8 km thick crust with a normal crustal velocity structure spudich and orcutt 1980 the apparent dip of the subducting plate beneath western mexico is 10 on the oceanic side strong reflections suggest a minimum depth of 35 km for the lithosphere asthenosphere boundary the asthenosphere has a seismic velocity of 7 6 km sec and a thin lid in which the velocity is 8 6 km sec

strength and dilatancy of silty sand abstractfines content can have a great influence on the mechanical behavior of sand the effect of fines content has not been taken into account in most of the strength dilatancy relationships for sand an attempt was made in this paper to account for the influence of fines content on the strength dilatancy relationship of silty sands the test data on ottawa sands with nonplastic fines were reanalyzed to examine the strength and dilatancy relationship for silty sand with different fines contents the ratio of excess angle which is the difference between the peak friction angle and the critical state friction angle to the relative dilatancy index was found to be a function of fines content furthermore a strength dilatancy relationship was also modified to account for the influence of confining pressure on the strength dilatancy relationship of silt sands

patterns of local seismicity preceding the petatlan earthquake of 14 march 1979 local seismic activity m 2 3 during the 4 1 2 day period preceding the petatlan earthquake of 14 march 1979 ms 7 6 was monitored by a seismic network deployed by the hawaii institute of geophysics these data allow us to revise the location of that event and to study its pattern of foreshocks the foreshock hypocenters are concentrated above the ocean continent plate boundary and landward of the hypocenter of the main shock the spatial distribution of foreshocks suggests that the foreshocks occurred along existing zones of weakness within the continental plate during the 1 1 2 day period preceding the petatlan event foreshocks were concentrated within 100 km of the epicenter of that earthquake the pattern of precursory seismic activity we observed during that period is similar to that observed during the same time period prior to the oaxaca earthquake of 29 november 1978 however the 17 hr period of seismic quiescence for events with m 2 8 preceding the oaxaca earthquake is not found in our data where a magnitude 4 foreshock occurs within 28 min of and 2 km from the petatlan event this suggests that the spatially larger set of foreshocks may have greater diagnostic value than does the sequence of precursory events within 30 km of the main shock

seismic potential of acapulco san marcos region along the mexican subduction zone recently seismic quiescence based on p d e and i s c data has been reported in the acapulco san marcos region 16 17 2 n 98 5 99 7 w mcnally 1981 in order to assess the seismic potential of this region we have estimated seismic moments and slips associated with 1957 ms 7 5 and 1907 ms 8 0 events which occurred in this area historical data suggest that earthquakes occurred in this region in 1845 ms 7 9 and probably also in 1820 ms 7 6 we find preference for the time predictable model in the data set which if valid forecasts the next large earthquake between now and 1993 with 1985 as the most likely year although this forecast can not be taken as definitive since it is based on several assumptions each of which are reasonable but uncertain the implications of reported quiescence and the results of this paper suggest a careful monitoring of the area

source characteristics of earthquakes in the michoacan seismic gap in mexico we investigated the source characteristics of large earthquakes which occurred in the michoacan mexico seismic gap during the period from 1981 to 1986 in relation to historical seismicity in the region the rupture pattern of the michoacan gap during this period can be characterized by a sequential failure of five distinct asperities before 1981 the michoacan gap had not experienced a large earthquake since 1911 when an m s 7 8 earthquake occurred the recent sequence started in october 1981 with the playa azul earthquake which broke the central part of the gap body wave modeling indicates that the playa azul earthquake is 27 km deep with a seismic moment of 7 2 10 27 dyne cm it is slightly deeper than the recent michoacan earthquakes and its stress drop is higher suggesting a higher stress level at depths in the michoacan gap the seismic moment of the 19 september 1985 m w 8 1 earthquake was released in two distinct events with the rupture starting in the northern portion of the seismic gap and propagating to the southeast with low moment release through the area already broken by the 1981 playa azul earthquake the rupture propagated further southeast with an m w 7 5 event on 21 september 1985 another aftershock occurred on 30 april 1986 to the northwest of the 19 september main shock body wave modeling indicates that this event has a simple source 10 sec long at 21 km depth and fault parameters consistent with subduction of the cocos plate 280 12 and 70 and m 0 2 0 to 3 1 10 26 dyne cm m w 6 8 to 6 9 although this distribution of asperities is considered characteristic of the michoacan gap whether the temporal sequence exhibited by the 1981 to 1986 sequence is also characteristic of this gap or not is unclear it is probable that depending on the state of stress in each asperity the entire gap may fail in either a single large event with a complex time history or a sequence of moderate to large events spread over a few years the seismic moment and the time since the last earthquake in michoacan in 1911 fit an empirical relation between moment and recurrence time found for the guerrero oaxaca region of the mexico subduction zone

analysis of the petatlan aftershocks numbers energy release and asperities the petatlan earthquake of march 14 1979 ms 7 6 occurred between the middle america trench and the mexican coast 15 km southwest of petatlan guerrero mexico from seismograms recorded on smoked paper fm and digital tapes we have identified 255 aftershocks with coda lengths greater than 60 s that occurred 11 hours to 36 days after the main shock based on these events the aftershock epicentral area defined during the period between 11 and 60 hours was about 2000 km2 between 11 hours and 6 days it was about 2400 km2 although the area grew to 6060 km2 in 36 days most of the activity was still confined within the area defined after 6 days this suggests that the smaller aftershock area might represent an asperity the distribution of events and energy release per unit area confirm the existence of heterogeneity in the aftershock area thus our data support the concept of an inhomogeneous rupture area that includes an asperity as suggested by chael and stewart 1982 to account for the differences they computed for the body and surface wave moments from wwssn data however the combination of the moments reichle et al 1982 report for body and surface waves from ida data and the rupture areas reported in this paper results in a solution that is most physically realizable in terms of stress drop and slip we calculate stress drops of 5 and 15 bars the former for the average over the entire area the latter for the asperity and an average slip of 60 cm for the entire area and 120 cm for the asperity these values for slip are 30 and 60 respectively of the convergence of the cocos plate relative to the north america plate during the 36 year period between the last two major earthquakes in the petatlan area hypocenters of the aftershocks define a zone about 25 km thick dipping 15 with an azimuth of n20 e which is perpendicular to the middle america trench most aftershocks are below the main shock the b value estimated for aftershocks in the range 3 0 mcl 4 5 is 60 higher than one determined similarly by hsu 1981 for the foreshocks

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

thermal regime from bottom simulating reflectors along the north ecuador south colombia margin relation to margin segmentation and great subduction earthquakes 1 the north ecuador south colombia nesc margin has three transverse morphotectonic segments the manglares tumaco and patia segments each with different tectonic and structural patterns following the 1906 subduction earthquake the nesc margin has been the site of three megathrust events in 1942 1958 and 1979 for which the rupture zones abut one another we first investigated variations in heat flow derived from bottom simulating reflectors bsr observed along multichannel seismic lines sampling the three morphotectonic segments strong along strike variations of the bsr derived heat flow just landward of the deformation front suggest that each morphotectonic segment has a specific thermal regime finite element thermal models show that these variations are mainly produced by changes in the age and the dip of the oceanic plate and local hydrothermal cooling we then examined the relationship between seismogenesis and thermal structure along the plate boundary the updip limits of the 1942 and 1979 seismogenic zones estimated from the aftershock area and rupture zone reach the trench where the temperature at the top of the subducting plate appears to be 50 60 c between these events the seismogenic zone of the 1958 earthquake is restricted to a region landward of a prominent outer basement high where the temperature appears to be 100 120 c we propose that on the nesc margin the updip limit of the seismogenic zone is primarily controlled by low temperature processes except for the 1958 event for which the seismogenic updip limit appears to be related to a structural feature in the upper plate

cyclic strength of sand under sustained shear stress the existence of initial shear stress can have a significant effect on the cyclic strength or liquefaction potential of sand this effect is not yet fully understood because of a lack of convergence and consistency in the existing data and interpretations leading to great uncertainty in quantifying the effect for practical applications this paper presents new experimental results on a silica sand to validate the concept known as threshold below which the cyclic strength of sand increases with and above which the cyclic strength decreases with with representing the sustained shear stress level on the basis of a series of monotonic loading tests on the same sand and in the framework of critical state soil mechanics it is further confirmed that threshold can be well related to a state parameter in the void ratio mean effective stress plane and thereby a unified and consistent interpretation can be established a new platform is proposed on which the relationship between cyclic strength and

on the role of grain shape in static liquefaction of sand fines mixtures this technical note presents new experimental evidence for the role of grain shape in the static liquefaction behaviour of sand fines mixtures the laboratory experiments comprise a series of strain controlled triaxial tests on several types of sand fines mixtures that are intended to clarify a concern remaining in a previous study by the same authors whether the observed effect of grain shape was due to the differences in hardness of the added fines the comparison of new test results with the previous ones confirms that it is grain shape rather than grain hardness that is responsible for the observed differences in overall behaviour adding fines of rounded shape into a clean sand indeed results in a marked increase of liquefaction potential compared with adding fines of more angular shape into the same sand it is also shown that the concept of the combined roundness is effective and useful for quantifying in an integrated manner the influence of grain shape and fines content on the overall friction a

source spectra and rms acceleration of mexican subduction zone earthquakes strong motion sm data of six mexican subduction zone earthquakes 6 4 ms 8 1 recorded near the epicentral zone are analyzed to estimate their far field source acceleration spectra at higher frequencies f 0 3 hz apart from the usual corrections such as geometrical spreading 1 r average radiation pattern 0 6 free surface amplification a factor of 2 and equal partitioning of the energy into two orthogonal horizontal components a factor of 1 sqrt 2 the observed spectra are corrected for a frequency dependentq q 100f a site dependent filter e kf and amplification ofs waves near the surface a factor of about 2 atf 2hz we taker as the average distance from the rupture area to the site if we model the high frequency plateau f 1 hz of the source spectra by a point source 2 model and interpret them in terms of brune s model we obtain between 50 and 100 bars for all earthquakes the low frequency broadband teleseismicp wave spectra corrected witht 1 0 s agrees within a factor of two with sm source spectra near 1 hz the 2 model is inadequate to explain the observed source spectra in a broad frequency range these resemble spectra given bygusev 1983 with some differences

correlation of foreshocks and aftershocks and asperities a close correlation in spatial distribution of local seismic activity and energy release patterns before and after the 1979 petatlan mexico earthquake suggests heterogeneity within the fault plane of this major low angle thrust event associated with subduction along the middle america trench a simple two asperity model is proposed to account for the complexity foreshocks and aftershocks of the neighboring 1981 playa azul earthquake showed a similar pattern as both events occurred at the junction of the orozco fracture zone and the middle america trench we speculate that the observed complex fault plane is caused by subduction of the rugged ocean floor of the orozco fracture zone short term precursory seismicity prior to the petatlan earthquake can be explained by using the asperity model and migration of a slip front from the south east to the north west across the main shock source region

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

source mechanism of two 1994 intermediate depth focus earthquakes in guerrero mexico abstract in may and december 1994 two medium size intermediate depth focus earthquakes occurred in guerrero mexico eastward of the rupture area of the great michoacan earthquake of september 19 1985 even though these are not major earthquakes 6 4 m w they were widely felt through central and southern mexico with minor damage at zihuatanejo and acapulco located along the pacific coast and mexico city both earthquakes separated by 100 km have similar focal depths and magnitudes however their focal mechanisms based upon the polarities of first arrivals show some differences the may earthquake shows a clear normal faulting mechanism 307 55 108 whereas the december earthquake mechanism solution suggests an initial thrust faulting 313 62 98 process although previous analysis including local and teleseismic stations reported a normal faulting for the december earthquake we find that modeling using the cmt focal mechanism solution fails to reproduce the first 5 sec of the observed p wave signal at the nearest broadband station 168 km and the s wave polarity at two strong ground motion local stations 32 53 km in fact the best fit for these stations is obtained using the thrust focal mechanism calculated from the first motion method seismic moment value and rupture duration time deduced from the teleseismic spectral analysis are 2 0 10 18 n m and 6 9 sec for the may event 2 8 10 18 n m and 7 1 sec for the december earthquake from the inferred seismic moment an average of 15 bars for both earthquakes is obtained inversion of teleseismic p wave data indicates a better fit using the cmt focal mechanism solution normal faulting than the first motion mechanism for both earthquakes although the adjustment9s differences are small for the may event for this earthquake the rupture consisted of two sources separated by 7 sec starting at a depth of 40 km and then propagating downdip reaching a depth of 60 km the december earthquake however released all its energy at a depth of 50 km in two main sources separated by 10 sec the non double couple components values are 0 004 and 0 01 for the may and december events respectively indicating that the december shock has a small contribution of non double couple radiation that could be the result of a changing mechanism this result agrees with the hypothesis that a slab subducting at a shallower angle our case is associated with the existence of random subfaults with different fault orientations from a tectonic point of view the complexity of the december earthquake could be the result of the observed complexity of the stress distribution around 101 w and the existence of compressional events beneath the normal faulting earthquakes near the coastline this feature permits the flexural stresses associated to the slab bending upward to become subhorizontal at the guerrero region we conclude that the may earthquake corresponds to a pure normal faulting whereas the december shock is a complex event with a variable fault geometry

a source study of the thessaloniki northern greece 1978 earthquake sequence summary the thessaloniki northern greece earthquake sequence appears to have occurred along faults forming a graben structure this graben situated in the border region between the serbomacedonian massif and the vardar zone is bounded to the south west by clearly exposed north west striking north east dipping normal faults relative hypocentre determinations fault plane solutions surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes the source parameters of the mainshock mb 6 1 ms 6 4 were determined by computing body wave synthetic seismograms in the time domain and comparing them with the observed this modelling constrained the orientation of faulting determined by the p wave fault plane solution it also constrained the source depth to 6 mp 2 km similar depths were calculated by a relative relocation method for the other three large events of the sequence the dislocation time function required for the far field had a total duration of 9 mp 1 5 s the body wave moment was 5 2 mp 1 8 times 1025 dyne cm for a fault length of 35 km and a fault width of 17 km both estimated from the aftershock distribution the static stress drop was found to be 4 bar an accelerograph record of the mainshock shows two distinct events 3 4 s apart these two events were unresolved by the long period data in the far field but the unusually long duration of the mainshock time function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time seismic energy released in this fashion can account for waveform complexities observed at some stations

stress state dependency of small strain shear modulus in silty sand and sandy silt of ganga this study explores the effect of stress state on small strain modulus gmax in the naturally occurring ganga sand and silt using bender elements during a series of carefully performed undrained t

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

effect of soil contraction tendency on static liquefaction triggering analysis static liquefaction failure occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield peak shear strength of the saturated liquefiable cohesionless soil in this study the effect of soil contraction tendency on static liquefaction failure of cohesionless soils is characterised by an empirical relationship between soil brittleness index and undrained yield strength from a database of 702 laboratory shear tests performed by the author and collected from past literature the application of this relationship for estimating the static liquefaction triggering strength of cohesionless soils is validated by comparison with several cases of liquefaction failures

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are