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

modification of a linear bar trough system by a standing edge wave abstract detailed fluid motions and bathymetric changes in the lacustrine nearshore zone at wymbolwood beach georgian bay canada were monitored during storms to test theoretical postulates about the importance of low frequency wave motions to nearshore circulation of water and sediments eighteen continuous resistance wave staffs and two electromagnetic current meters were deployed along one shore normal and two shore parallel instrument arrays over a period of two months during one intense storm a mode 3 standing edge wave at 0 035 hz was the predominant low frequency motion prior to the storm the bathymetry was characterized by linear bar trough systems and during the storm these features migrated onshore and became rhythmic in response to the length scales and relatively fixed locations of nodal and antinodal lines of the standing edge wave however a well defined crescentic bar as predicted by the model under the condition of zero net sediment flux divergence did not evolve because the hydrodynamic forcing ceased before an equilibrium condition could be attained nevertheless the location of scour and accretion features demonstrated the tendency towards such a crescentic form the offshore and alongshore irregularities of the post storm bathymetry are not readily explained by alternative hydrodynamic interactions

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

formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

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

field observations of swash zone infragravity motions and beach cusp evolution 1 fluid flows consistent with low mode edge waves were evident in video observations of swash motions during a field experiment in which beach cusps developed on an initially smooth beach as beach cusps grew energy lying along low mode dispersion curves increased the most energetic edge wave propagation direction changed from upcoast to downcoast as the orientation of the cusp horns rotated these observations suggest a coupling between morphodynamics and hydrodynamics and are evidence that beach cusp evolution might control low mode edge wave dynamics

beach cusp morphodynamics detailed measurements of three dimensional beach cusp morphology were made on a steep gradient low energy microtidal beach in perth western australia during the field campaign a variety of wave conditions and tidal ranges were experienced and these differing hydrodynamic conditions were reflected in a consistent pattern of morphological changes to the beach cusp system a useful parameter to delineate between trends of cusp destruction and re formation appeared to be the surf similarity parameter tan i0 l0 where h0 is offshore wave height l0 is deep water wave length and tan is beach gradient for 1 2 a small storm was experienced at the start of the field campaign period and resulted in considerable erosion of the beach face the cusp morphology across the lower beachface was destroyed but a subtle remnant of the pre storm cusp morphology was preserved on the upper beachface when cusps reformed after the storm under the influence of declining wave conditions they appeared at the same location and with the same dimensions as the pre storm cusp morphology hence it is considered that the cusp re formation was controlled more by the antecedent morphology than the hydrodynamic conditions this indicates that positive feedback between swash hydrodynamics and beachface morphology necessary to form beach cusps does not require a large variation in relief 1997 john wiley sons ltd

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

development of cusp related gravel size and shape facies at malin head ireland samples of beach gravels were obtained from foreshore cusp deposits at portmore beach near malin head republic of ireland axial dimensions a b and c were measured and the maximum projection sphericity was derived for each of 4360 clasts each sample was also categorized according to depositional subenvironment principal components analysis and cluster analysis were used to identify potential facies zonation results indicate that reworking of the foreshore by incident and edge waves resulted in clast size and shape segregation both parallel and perpendicular to the beach patterns of zonation were distinctly different from those described in earlier facies descriptions showing a complex set of foreshore facies the distribution of these facies was controlled primarily by location within beach cusp systems primary and secondary facies demonstrate alongshore variability of a magnitude comparable to across shore differences the results have important implications for palaeoenvironmental reconstruction morphosedimentary description and research sampling design and provide the basis for a conceptual model of gravel beach cusp facies

the origin of swash cusps on beaches abstract genetically there are two types of beach cusps those formed in the surfzone by the nearshore circulation system and those formed on the beachface by the swash and backwash the latter are here called swash cusps and a simple model relating the physical dimensions of swash cusps to the properties of the incident wave field and the mean beach slope is developed as in some previous models the cusp wavelength is controlled by the longshore wavelength of edge waves but the edge waves are now required only to provide small periodic perturbations on an originally uniform beach further cusp growth occurs because of the interactions between the incident waves and the perturbed topography the maximum cusp height apex to valley is hypothesized to be proportional to the significant vertical excursion of swash motions it follows from the above assumptions that the maximum possible cusp steepness cusp height cusp longshore wavelength is proportional to the mean on offshore beach slope this result agrees with field data and the value of the ratio of maximum cusp height to beach slope differs from that obtained theoretically by guza and bowen 1981 using a complicated non linear model with rather different physics the present model implies that edge waves although necessary for initiating the initial bedform perturbation need not persist for the development of mature cusp topography

morphological changes in a cuspate sandy beach under persistent high energy swells renaca beach chile abstract the present work proposes a morphological evolution model for sandy cuspate beaches which depends on two parameters swash excursion s and offshore wave steepness h0 l0 the model was derived from observations of morphology and wave climate at renaca beach in central chile comprising in situ instrumentation topographic surveys wave modeling and optical imagery it was found that wave steepness alone was unable to explain the morphological changes observed on the beach rather results indicate that swash excursion is better to distinguish between erosive and accretionary events with swash excursion exceeding 29 5 m for more than 12 h being indicative of erosive events while morphological changes are led by s wave steepness correlates with the erosive potential of waves especially when h0 l0 is greater than 0 017 although derived for a specific beach under a limited range of conditions this work adds to the understanding of the dynamic behavior of beaches exposed to persistent high energy swells

observations of morphological change and sediment transport on a steep gravel beach abstract observations of morphological change and swash sediment loads concurrent with measurements of water level waves and currents were obtained during a spring to spring tidal cycle on a steep macrotidal gravel beach in southwest england the wave conditions experienced during the field experiment were relatively calm and the significant inshore wave height and period averaged 0 4 m and 6 s respectively incident frequency water motions dominated the hydrodynamics and the phasing between water surface elevation and cross shore and alongshore currents was inconsistent with standing edge wave motion morphological change was directly related to maxima in shoreline elevation and four modes of berm adjustment identified regression and rollover in response to falling and rising water levels respectively and removal and reformation during and after storms respectively swash processes transported substantial amounts of sediment at rates comparable to sand beaches and the net transport under modal wave conditions was estimated to be 1 kg per swash onshore per unit metre beach width steep hydrodynamic gradients induced by the abrupt slope breaks of the step and berm are one of the main causes of the large morphological variability characteristic of gravel beaches

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

morphological evolution of beach cusps and associated swash circulation patterns abstract a numerical model capable of simulating the motion of water particles on beach cusp morphology under the influence of an initial velocity and gravity is presented the model indicates that the typical swash pattern on beach cusps is three dimensional with wave uprush diverging at the cusp horns resulting in concentrated backwash streams in the embayment the degree of horn divergence is an increasing function of the parameter e s 2 where e quantifies the prominence of the beach cusps s is the horizontal swash excursion length and is the cusp spacing the numerical experiments are supplemented with detailed field measurements of beach cusp morphological change the field data include three types of morphological response each characterised by a particular pattern of swash circulation that can be delineated using e s 2 when e s 2 beach cusp morphology is large and or subdued in relation to the swash length swash circulation is essentially two dimensional oscillatory flow and results in steepening of the beachface and infilling of the cusp embayments for e s 2 0 015 to 0 15 wave uprush is deflected from the cusp horns and flows into the embayments where it exits in a concentrated backwash stream horn divergent flow the ensuing swash backwash inequality reinforces cusp development and maintains existing cusp morphology when e s 2 0 15 beach cusps are small and or pronounced in relation to the length of the swash overtopping and ponding of the cusp horn takes place and swash circulation is from the embayment to the horn horn convergent flow as a result cusp horns are eroded and accretion occurs in the embayments

field observations of beach cusps and swash motions abstract we tested the hypothesis of cusp formation by the longshore structure of synchronous or subharmonic mode zero edge waves using detailed field measurements of foreshore topography incident waves and swash motions two distinct intervals of cusp formation were observed alongshore separated swash measurements were analyzed to detect structures at wavelengths and frequencies consistent with the measured cusp spacings for both events no statistical support for cusp generation by the traditional subharmonic or synchronous edge wave mechanisms was found however once cusps became well developed there was evidence of interaction between fluids and topography at a wavelength equal to the cusp spacing but at the period of the incident swash we propose that this interaction could have been responsible for the generation of the second set of cusps with a spacing of half the pre existing cusp wavelength

atmospheric hydrodynamic coupling in the nearshore 1 natural beach shorelines commonly present morphological rhythmic or non rhythmic features of varying geometrical characteristics traditionally their formation is believed to be due to wave induced processes a line thoroughly investigated during the last few decades however these natural beach formations are frequently bounded by coastal cliffs or capes and are affected by intense winds a fact not previously considered this paper presents the results of a field survey demonstrating the existence of atmospheric hydrodynamic coupling in the nearshore region outside of the breaking zone at carchuna beach motril spain where the atmospheric conditions are influenced by a lateral geographic obstacle cape sacratif

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

holocene sea level change and evolution of a mixed coral reef and mangrove system at iriomote island southwest japan abstract exposed fossil microatolls and core samples from a coral reef and a mangrove forest at the yutsun river mouth iriomote island southwest japan reveal the internal structure and temporal changes in the sedimentary processes of a mixed reef mangrove system evidence from the core samples and fossil microatolls suggests sea level reached its present position before 5100 cal yr b p and a relative sea level highstand of 1 1 1 2 m above the present sea level occurred from 5100 to 3600 cal yr b p this was followed by a gradual fall in relative sea level the tectonically corrected sea level curve indicates a stable sea level after 5100 cal yr bp with a sea level highstand of up to 0 4 m between 5100 and 3600 cal yr b p a nearshore reef dominated by massive porites and arborescent acropora initially developed at 6500 3900 cal yr b p reef development was potentially terminated by relative sea level fall and sediment discharge from the yutsun river that affected the backreef environment an offshore coral reef reached present day sea level by 1000 cal yr b p forming a wave break that enabled the foundation of mangrove forest on the fringing reef after 1000 cal yr b p the reef development was significantly delayed compared with other coral reefs in the region with similar medium to high energy conditions but it provided a calm environment in the backreef area that allowed the development of mangroves these features demonstrate the chronology and causal relationship between coral reef and mangrove development under the influence of holocene sea level change and river discharge

video observations of beach cusp morphodynamics beach cusps are a common feature of steep reflective and intermediate beaches although many observations of beach cusp spacing exist there are few observations quantifying the incipient formation evolution and eventual destruction of these features beach cusp morphodynamics were analyzed using a 3 year dataset of video images collected at tairua beach new zealand twenty four beach cusp episodes were selected to monitor the cycle of beach face changes from planar to the appearance of the cusp patterns and back to planar again observations show that beach cusp disappearance can be ascribed not only to the erosive influence of storms but also to the persistence of accretionary conditions leading to the infilling of beach cusp bays and the development of an alongshore continuous berm we also report observations of changes in beach cusp spacing over time which can be attributed to the merging of adjacent cusps within the cusp field with the overall cusp spacing re adjusting to accommodate the disappearance of a horn although the self organization theory provides a better fit to the data and theory we were unable to conclusively refute any of the mechanisms causing beach cusp formation since both existing theories subharmonic standing edge wave and self organization can predict the trend in the observed beach cusp spacing these observations show that initial beach cusp formation primarily occurs around 70 of the episodes analyzed under mildly accretionary conditions and when accretion persists the pattern disappearance is likely to occur as a result of bay infilling to form a featureless berm

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

a mechanism for the generation of wave driven rhythmic patterns in the surf zone the coupling between topographic irregularities and wave driven mean water motion in the surf zone is examined this coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation this circulation in turn creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large scale bed forms to investigate this coupling mechanism the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases first the breaking line is considered to be fixed and second the perturbations in water depth that produce a displacement of the breaker line are accounted for the first case shows that the basic topography can be unstable with respect to two different modes a giant cusp pattern with shore attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect in the second case the varying breaker line may have a strong influence on the circulation this is clear for the giant cusp topography whose growth is totally inhibited in contrast the morphology and the growth of the crescentic pattern remains almost unchanged

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

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

the role of tides in beach cusp development 1 field measurements of morphology and swash flow during three episodes of beach cusp development indicate that tides modulate the height and cross shore position of beach cusps during rising tide beach cusp height decreases as embayments accrete more than horns and the cross shore extent of beach cusps decreases during falling tide beach cusp height increases as embayments erode more than horns and cross shore extent increases a numerical model for beach cusp formation based on self organization extended to include the effects of morphological smoothing seaward of the swash front and infiltration into the beach reproduces the observed spacing position and tidal modulation during rising tide water particles simulating swash infiltrate preferentially in embayments causing enhanced deposition during falling tide exfiltration of water particles combined with diversion of swash from horns causes enhanced erosion in embayments smoothing of beach morphology in the swash zone seaward of the swash front and in the shallow surf zone accounts for most of the observed tidal modulation even in the absence of infiltration and exfiltration despite the qualitative and in some cases quantitative agreement of the model and measurements the model fails to reproduce observed large deviations of horn orientation from shore normal some aspects of beach cusp shape and deviations from the basic tidal modulation possibly because of the simplified parameterization of cross shore sediment transport and the neglect of the effects of sea surface gradients on flow index terms 4546 oceanography physical nearshore processes 4255 oceanography general numerical modeling 3220 mathematical geophysics nonlinear dynamics keywords beach cusp tides infiltration exfiltration erosion

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

rhythmic beach cusp formation a conceptual synthesis abstract observations of a sandy beach at santa barbara california during february 1980 documented the sequence of beach cusp formation replacement by a second set of rhythmic cusps obliterating the first set and finally complete planation of all rhythmic cusps during a major storm although neither cusp building event was observed during its initiation the wave and tide conditions during formation were well documented by in situ instrumentation based on these field observations and those by other investigators a coherent conceptual theory of beach cusp formation is proposed including formative maintenance and destructive phases this theory proposes rhythmic cusp formation during relative stillstands of sea level of sufficient duration to permit initial perturbation of beach geometry regularly spaced cusps acquire their initial spacing from subharmonic edge waves such that spacing derives from the incident wave period growth of the initial perturbation continues until cusps reach a limiting value not necessarily with continuation of edge wave motion the cusp spacing persists until a change in wave conditions either destroys the cusps or establishes a radically different spacing and finally that large storms will destroy the rhythmic cusp field leaving either a planar beach or a beach with arhythmic cusp like features

erosion accretion patterns and multiple beach cusp systems on a meso tidal steeply sloping beach abstract this study examines daily variations in intertidal topography and beach cusps on a meso tidal steeply sloping beach during a five month period the observations are based on coastal imagery and frequent topographic surveys and are extracted from patterns of contours positioned 1 and 3 m above msl lower higher beach face respectively the intertidal topography was very sensitive to variations in the wave regime triggering rapid transitions from erosive to accretive states and vice versa as well as transformations of the cusp systems often involving severe horn erosion and beach face scarping distinct beach cusp systems were observed at different levels of the beach with their evolution being controlled by i wave forcing ii tidal modulation of the wave power at different levels of the beach and iii interactions between the existing beach cusp systems the lower beach face rhythmical patterns changed constantly and merged with the upper beach face cuspate features typically when the significant wave height exceeded h o 1 5 m the upper beach face pattern was persistent during the monitoring period re emerging with similar spacing and alignment despite being eroded during energetic events this persistence is also maintained by the interactions between swash and irregularities e g embayments in the dune backshore morphology the spacing of the observed beach cusps ranged from c min 8 m to c max 67 m these values were underestimated by the edge wave and self organization theories and were better predicted by the sunamura 2004 formula beach cusp growth was linked to breaker angles of near normal incidence and narrow banded frequency wave conditions bay infilling was found to be the most dominant cusp reduction mechanism for larger features c 30 m while smaller features vanished during erosive conditions the most intense cusp growth episodes were linked to accretive conditions while larger features were shown to emerge during more energetic erosion events these findings highlight the need for more longer term observations of diverse beach morphologies to formulate a global theory regarding beach cusp formation and evolution

test of edge wave forcing during formation of rhythmic beach morphology 1 this paper reports on concurrent measurements of evolving rhythmic beach morphology and the associated water motion on a macrotidal coarse sandy beach beach cusps with a spacing of 19 m developed over a 3 day period of low energy swell after which they were destroyed by energetic wind waves the beach cusps were clearly accretionary features and developed mainly during the rising tide detailed hydrodynamic measurements were conducted at three locations in the swash and inner surf zone in the vicinity of a developing cusp horn and revealed that energetic standing mode zero subharmonic edge waves were not present during the initiation of the beach cusps specifically analysis of the hydrodynamic data indicated that 1 the amount of subharmonic energy was limited 10 of the total energy 2 none of the computed energy spectra exhibited a significant peak at the subharmonic frequency 3 the cross shore velocity amplitude at the subharmonic frequency was 2 3 times larger than the longshore velocity amplitude 4 the longshore current motion at the subharmonic frequency was barely coherent between adjacent locations in the surf zone and 5 the time series of pressure and cross shore current were in phase at either side of the developing cusp horn it is concluded that at least during this event of beach cusp formation energetic standing edge waves are not a prerequisite to initiate the development of rhythmic beach morphology

relation between beachface morphology and wave climate at trafalgar beach cadiz spain abstract two years of offshore wave data and daily time exposure images from trafalgar beach a 2 km long sandy beach located on the southwest coast of spain that frequently exhibits rhythmic features were used to 1 explore the variability of the beachface morphology and 2 determine environmental conditions associated with the different morphological states the beachface morphology at three distinct alongshore sectors was analyzed and classified and five different morphological states were found that are related with the presence or absence of beach cusps and a berm 1 large beach cusps 2 small beach cusps 3 low tide terrace 4 plane beach berm and 5 plane beach the predominant beachface morphology is characterized by the presence of large beach cusps and the main wave climate consisted of offshore significant wave heights ranging from 0 5 1 m and wave periods between 4 and 12 s an alongshore variation of the morphology is found which might be related to the nearshore wave variability swan wave model results the morphologies are in some cases well correlated with the daily offshore incident wave climate described by the daily maximum significant wave height and the corresponding period particularly for the moderate to high energy wave conditions small beach cusps appear under short period waves whereas when the wave periods are longer the morphology tends to change to large beach cusps this transition only occurs if the forcing is maintained as constant for a certain duration which depends itself on the wave energy it is concluded that correlations over 90 are only found for the highest wave energy conditions or under long wave periods for the remainder it is not possible to generally correlate the beachface morphology based only on the wave forcing because the previous morphological state cannot be ignored

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

concepts in gravel beach dynamics abstract the dominant processes in gravel beach dynamics are reviewed highlighting some common themes which unify the various components of the gravel beach system the repercussions of which impart on how gravel beach dynamics might be understood conceptually in particular gravel beach dynamics are thought to be highly dependent on the temporal and spatial variation in grain size and the continual adjustments made by an active beach step both of which act not only as the expression of changing morphodynamic conditions but also as a controlling influence morphodynamics the notion that the exchanges on beaches between the hydrodynamics sediment transport and morphological change takes the form of reciprocal relationships which are mediated through feedback mechanisms in such a way that they cannot be thought of or studied independently is not a new one yet it appears that for the gravel beach morphodynamics must be re defined to describe conditions where variations in sediment size are thought to deserve parity rather than as merely a sequent entity or boundary condition morpho sedimentary dynamics is a phrase coined to intuit such cause and effect detailing the co evolution of morphology hydro hydraulics and sediment properties whilst acknowledging causative pluralism feedbacks and multiplier effects this is the recommended conceptual framework within which to crystallise thought and organise further research for the gravel beach essentially it increases the minimum number of parameters needed to describe the state of the gravel beach as a physical system therefore it is advised that simplicity will be most expedient in our future modelling efforts if complexity is to be adequately encapsulated

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

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

on beach cusp formation a system of shallow water equations and a bed evolution equation are used to examine the evolution of perturbations on an erodible initially plane beach subject to normal wave incidence both a permeable under darcy s law and an impermeable beach are considered it is found that alongshore periodic morphological features reminiscent of swash beach cusps form after a number of incident wave periods on both beaches on the permeable impermeable beach these patterns are accretional erosional in both cases flow is horn divergent spacings of the cusps are consistent with observations and are close to those provided by a standing synchronous linear edge wave an analysis of the processes leading to bed change is presented two physical mechanisms are identified concentration gradient and flow divergence which are dominant in the lower and upper swash respectively and their difference over a wave cycle leads to erosion or deposition on an impermeable beach infiltration enters this balance in the upper swash a bed wave of elevation is shown to advance up the beach at the tip of the uprush with a smaller wave of depression on the backwash it is found that cusp horns can grow by a positive feedback mechanism stemming from decreased increased backwash on positive negative bed perturbations

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

flow velocities sediment transport and morphological change in the swash zone of two contrasting beaches abstract as part of a large uk research project focussing on the role of swash on shoreline change two field experiments were conducted on contrasting sandy beaches a flatter dissipative beach and a steeper intermediate beach this paper describes almost 300 h of nearshore velocity data collected during this project supplemented by suspended sediment transport sediment tracer and beach profile data during both field experiments the upper part of the beach experienced progressive accretion indicating that onshore sediment transport prevailed in the swash and inner surf zones suspended sediment transport and tracer dispersal were also onshore the associated cross shore flow field was however dominated by offshore directed mean flows especially on the steeper beach velocity skewness defined as u 3 u 2 1 5 where u is the cross shore velocity and kurtosis defined as u 3 u u 2 2 were mainly directed offshore implying that energetics based sediment transport models are unable to predict the direction of sediment transport in the swash zone possible explanations for the observed accretion include enhanced bed shear stress on the uprush due to in exfiltration effects flow acceleration and or bore turbulence and advection of bore entrained sediment into the swash zone

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

beach cusps as self organized patterns computer simulations of flow and sediment transport in the swash zone on a beach demonstrate that a model that couples local flow acceleration and alongshore surface gradient is sufficient to produce uniformly spaced beach cusps the characteristics of the simulated cusps and the conditions under which they form are in reasonable agreement with observations of natural cusps the self organization mechanism in the model is incompatible with an accepted model in which standing alongshore waves drive the regular pattern of erosion and deposition that gives rise to beach cusps because the models make similar predictions it is concluded that currently available observational data are insufficient for discrimination between them

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

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

morphometric and sediment analysis of beach cusp in correlation to rip currents a case study from tropical coast west coast of india the presence of rhythmic beach cusps in foreshore regions are of persistent research interest to coastal researchers engineers with respect to their formation in beaches morphometric and sediment analysis were used to study beach cusp morphology at vengurla beach a gentle slope beach characterized by medium sand the cusp dimensions such as spacing elevation amplitude and depth were measured for detailed analysis the distinctive sediment characteristics between the horn and the bays of the cusp are different and cusp measurements show a high level of irregularity horns and bays have medium sand but sorting is varying sorting variations clearly demonstrated the difference in energy condition between horns and bay negative skewness of few horn samples has been correlated with erosional trend based on the grain size analysis result it was observed that vengurla beach cusps follow the regular grain size distribution pattern additionally the low intensity rip currents were also noticed in the study area the rate of occurrence and strength of rip currents are decided by the nearshore bathymetry and play an imperative role in rip current formation the present study is based upon the field measurements along vengurla coast and contributes to morphometric analysis of beach cusps regarding beach cusps there are no documented databases along the indian coast understanding the beach cusp formation and its seasonal changes along the beaches will be useful for effective coastal management

alongshore variation in beach cusp morphology in a coastal embayment the variation in beach cusp characteristics was examined along a 1km long embayed beach pearl beach new south wales australia the beach cusp morphology had formed during the previous day and or night and displayed a marked alongshore variation in cusp spacing the edge wave mechanism of beach cusp formation could not account for the observed trend in cusp spacing because no relationship could be established between the spacing of the cusps and the gradient of the beachface on the other hand the cusp spacing was strongly related to the horizontal swash excursion providing some support for the self organization model of beach cusp formation copyright 1999 john wiley sons ltd

pocket beach hydrodynamics the example of four macrotidal beaches brittany france during several field experiments measurements of waves and currents as well as topographic surveys were conducted on four morphologically contrasted macrotidal beaches along the rocky iroise coastline in brittany france these datasets provide new insight on the hydrodynamics of pocket beaches which are rather poorly documented compared to wide and open beaches the results notably highlight a cross shore gradient in the magnitude of tidal currents which are relatively strong offshore of the beaches but are insignificant inshore despite the macrotidal setting the hydrodynamics of these beaches are thus totally wave driven in the intertidal zone the crucial role of wind forcing is emphasized for both moderately and highly protected beaches as this mechanism drives mean currents two to three times stronger than those due to more energetic swells when winds blow nearly parallel to the shoreline moreover the mean alongshore current appears to be essentially wind driven wind waves being superimposed on shore normal oceanic swells during storms and variations in their magnitude being coherent with those of the wind direction we also found evidence for the contribution of infragravity waves to the formation of an intertidal sand bar through prolonged duration of swash processes at the same beach level and to the development of beach cusps but not to their initiation for which the observed standing edge wave pattern may have been enhanced by the enclosed pocket beach setting the paper highlights some of the specificities of pocket beach with regards to their hydrodynamics and morphodynamics more detailed observations would be needed however prior to a more comprehensive review

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

short term morphological change and sediment dynamics in the intertidal zone of a macrotidal beach abstract this paper describes the morphological and sedimentological evolution of amacrotidal beach over a 20 day period under varying hydrodynamicconditions signi cant breaker heights of 0ae3 2 m and tidal ranges of 2 5 m during the eld campaign an intertidal bar developed around the mid tidelevel migrated onshore welded to the upper beach and was then attenedunder energetic wave conditions the bar had a wave breakpoint origin and itsformation was triggered by a reduction in tidal range causing more stationarywater level conditions rather than an increase in wave height most of theonshore bar migration took place while the bar was positioned in the inner tomid surf zone position such that the bar moved away from the breakpoint andexhibited divergent behaviour the depth of disturbance over individual tidalcycles was 10 20 of the breaker height such values are more typical of steepre ective beaches than gently sloping dissipative beaches and are consideredto re ect the maximum height of wave generated ripples the grain sizedistribution of sur cial sediments did not vary consistently across the beachpro le and temporal changes in the sedimentology were mostly unrelated tothe morphological response the lack of clear links between beach morphologyand sedimentology may be in part due to shortcomings in the samplingmethodology which ignored the vertical variability in the sediment sizecharacteristics across the active layer keywords beach sediment depth of disturbance intertidal bar macrotidalbeach morphology introductionbeaches respond rapidly to changing wave andtide conditions through the redistribution of theirsediments by oscillatory and quasi steady cur rents resulting in spatial patterns of erosion sediment divergence and accretion sedimentconvergence given suf cient time these bed level changes modify the beach shape by theformation modi cation destruction and ormigration of secondary morphological features such as berms beach cusps swash bars ripchannels and nearshore bars the dynamics ofthese secondary morphological features and theirrole in affecting beach stability has been a majorfocus of beach research over the last few decades e g komar 1998 short 1999 most short term days to weeks eld investi gations addressing the morphological response ofsandy beaches to wave and tide forcing have beenconducted in microtidal or mesotidal settings beaches in macrotidal settings have on the otherhand only received limited attention despitetheir common occurrence short 1991 a mainreason for this imbalance is that macrotidalbeaches are often planar and lack pronouncedsecondary morphological features indeed theonly type of macrotidal beach that does featureprominently in the coastal literature is the ridgeand runnel beach which is characterized bysedimentology 2007 54 39 53

test of self organization in beach cusp formation 1 field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self organization wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization the self organization hypothesis is tested using measurements from three experiments on a barrier island beach in north carolina beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer swash front motions were recorded on video during daylight hours and morphology was measured by surveying at 3 4 hour intervals three signatures of self organization were observed in all experiments first time lags between swash front motions in beach cusp bays and horns increase with increasing relief representing the effect of morphology on flow second differential erosion between bays and horns initially increases with increasing time lag representing the effect of flow on morphology change because positive feedback causes growth of beach cusps third after initial growth differential erosion decreases with increasing time lag representing the onset of negative feedback that stabilizes beach cusps a numerical model based on self organization initialized with measured morphology and alongshore uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle reproduces the measurements except for parts of one experiment where limited surveys and a significant low frequency component to swash motions might have caused errors in model initialization

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