inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

earthquake focal mechanisms and the intraplate setting of the bermuda rise a number of intraplate earthquakes occurring in the western north atlantic ocean are located near the perimeter of the bermuda rise focal mechanisms and depths of two earthquakes november 24 1976 mb 5 1 m0 2 96 1023 dyne cm and march 24 1978 mb 6 1 m0 3 58 1025 dyne cm were determined using rayleigh wave amplitude data in the period range 20 50 s the 1978 earthquake occurred approximately 380 km southwest of bermuda near magnetic anomaly m4 118 m y b p the focal mechanism for the 1978 event is of thrust type and has nodal planes striking 340 the depth of this event is 6 km below the seafloor near the local depth to mono the strike of the fault planes does not parallel the trends of either fracture zones 300 or magnetic lineations 035 in the area the fault planes do however parallel the strike of a magnetic gradient in the epicentral area the 1976 earthquake occurred approximately 300 km northeast of bermuda near muir seamount the depth of this event is 10 km below the seafloor the available data are suggestive of one nodal plane striking between 320 and 340 and nearly parallel to the trend of muir seamount and other volcanic features in the region in contrast to the 1978 event the 1976 earthquake appears to exhibit a significant component of strike slip motion p axes of both mechanisms are subparallel to the direction of absolute plate motion for north america we suggest however that strain release in the bermuda rise area is not occurring along major fracture zones or topography parallel to seafloor spreading anomalies but rather on smaller scale structures the stresses induced by variations of crustal thickness may be responsible for triggering intraplate seismicity in this region

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

the 1973 hawaii earthquake a double earthquake beneath the volcano mauna kea summary the 1973 hawaii earthquake occurred north of hilo at a depth of 40 to 50km the location was beneath the east flank of mauna kea a volcano dormant historically but active within the last 4000 yr aftershocks were restricted to a depth of 55 35km the event and its aftershock sequence are located in an area not normally associated with the seismicity of the mauna loa and kilauea calderas the earthquake was a double event the epicentres trending ne sw the events were of similar size and faulting mechanism the fault plane solutions obtained by seismic waveform analysis are a strike slip fault striking ew and dipping 55 s the auxiliary plane a ns vertical plane with a faulting plunge of 35 the axis of maximum compressive stress is aligned with the direction of the gravity gradient associated with the island of hawaii the fault plane striking ew parallels a surface feature the mauna kea east rift zone the earthquakes were clearly not associated with volcanic activity normally associated with mauna loa and kilauea and may indicate a deep seated prelude to a resumption of activity at mauna kea

the geoid small scale convection and differential travel time anomalies of shear waves in the central indian ocean we have measured 37 differential travel times for the phase pairs ss s sss ss scs2 scs scs2 s sscs2 sscs and sscs2 ss each pair included a surface bounce point in the central indian ocean between 4 n and 21 s and 69 and 85 e residuals were calculated relative to both the j b travel time tables and the pem c earth model and were corrected for the elevation of the bounce points the mean j b residual associated with bounce points in the central indian basin is 2 8 0 7 s fast this contrasts with 4 2 s slow reported for the western pacific by sipkin and jordan and is similar to the 4 0 s found for old continental nucleii after correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique we find that travel times in the mantle beneath the central indian basin are midway in character between those beneath pre cambrian shields and those beneath the western pacific ocean basin the fastest differential travel times occur along a line passing close to the center of the large indian ocean geoidal low there is a clear increase in travel time with distance from the center of the geoid low suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the central indian and pacific basins the chagos laccadive ridge seems to coincide with a transition from the fast differential travel times of the central indian basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers there are also variations within the central indian basin that are not related to surface geological features or to the age of the seafloor these variations are periodic with a wavelength of about 640 km and with a nne trend which agrees approximately with the trend that would be expected for linear convective rolls in the upper mantle

change of crustal potential energy by earthquakes an indicator for extensional and compressional tectonics earthquakes cause a potential energy change in earth s interior because they generate permanent vertical deformation in the gravitational field the cumulative change of this potential energy in the crust computed for the period between 1977 and 1998 shows systematic patterns in various tectonic provinces regions of extensional tectonics show systematic potential energy loss and regions of compressional tectonics typically show energy gain therefore we claim that the crustal potential energy change can serve as a good indicator of compressional and extentional tectonics

simulating the formation of hurricane katrina 2005 1 the formation of hurricane katrina 2005 is simulated using coamps with high resolution 27 km 9 km and 3 km grid spacing whereas most atlantic hurricanes form in conjunction with easterly waves no clear synoptic scale tropical disturbance was evident as a precursor for katrina the triply nested simulation initialized 66 h before katrina was identified as a tropical depression successfully predicts the location and timing of katrina s formation in the 3 km domain the organization of the cyclone is associated with concentrated precipitation resolved with microphysics in the southeast part of the circulation the storm does not form in the 9 km domain when the kain fritsch cumulus scheme and the microphysics are used however a separate simulation using microphysics alone in the 9 km domain captures katrina s formation though with less organized structure this study suggests that explicitly resolving clouds in high resolution models holds promise for predicting tropical cyclone tc formation

original mineralogy of carboniferous worm tubes evidence for changing marine chemistry and biomineralization petrographic and geochemical evidence indicates that the original mineralogy of calcareous worm tubes in carboniferous limestones changed from calcitic in the mississippian to aragonitic in the pennsylvanian this change in mineralogy of caco 3 produced by polychaete worms which are relatively simple biomineralizers parallels changes observed in nonbiological carboniferous marine carbonates suggesting that skeletal secretion of caco 3 by polychaetes responded to a global change in seawater chemistry this is the first documentation of long term change in the mineralogy of biologically induced biomineralization it raises the possibility that other bioinduced biomineralization changed in response to changing marine chemistry in contrast to biocontrolled biomineralization by more sophisticated organisms that persisted with no mineralogical change

intraplate seismicity of the southern part of the pacific plate we present a compilation of all the known seismicity of the part of the pacific plate located south of the equator this includes historical earthquakes dating back to 1937 in addition to the gilbert islands swarm of 1981 1983 we identify 60 events at 32 epicenters in the magnitude range 4 3 6 9 our results indicate that the level of seismicity of the southern pacific plate is comparable to that of other plates including continent bearing ones most of the seismic activity is concentrated in the youngest parts of the plate but a few large events are present in cretaceous lithosphere with the exception of events from the gilbert islands swarm earthquakes located in lithosphere 35 ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push in younger areas the situation is much less clear cut the final geometry of the focal mechanisms is controlled by local tectonics in particular due to the miocene reorientation of the spreading at the east pacific rise the bulk of the pacific plate is particularly vulnerable to ridge push and this explains the numerous strike slip events and the absence of magnitude 6 events in most of the plate correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal finally a number of areas with high seismic release are identified as potential targets for future exploration

slow earthquakes along oceanic fracture zones evidence for asthenospheric flow away from hotspots abstract the regime of strain release along transform faults of the mid oceanic ridge system is studied it is shown that earthquakes along certain fracture zones exhibit systematic discrepancies between values of their magnitudes measured at short and long periods implying a regime of slower strain release also observed in a pattern of complex body waveshapes these slow fracture zones do not correlate with simple geographic or kinematic properties but usually occur in the neighborhood of hotspot volcanism frequently also characterized by gravity and bathymetry anomalies we propose that regimes of slow strain release may be due to a partial lubrication of the fault along these fracture zones which may itself be due to asthenospheric flow from the nearby hotspots along the pattern of pipelines described by vogt and johnson and by morgan

subduction seismicity and tectonics in the lesser antilles arc we have studied the mechanisms of 17 earthquakes along the lesser antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate no large thrust earthquakes occurred during the 1950 1978 study period the three large magnitude seven events are all normal faults one is a normal faulting event seaward of the trench its aftershock sequence includes strike slip events on differently oriented faults probably due to lateral block motion in response to the main shock a second indicates extension within the slab at depth these observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative the third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench this unusual geometry may represent a flexural response to the subduction of the barracuda ridge a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some mid atlantic ridge fracture zones thus the barracuda ridge is not buoyant and does not affect benioff zone dip strike slip faulting occurs at depth in the subduction zone along a concentration normal to the arc and maymore indicate a fossil fracture zone there is no direct evidence in the shallow seismicity for the hypothetical north america south america caribbean triple junction through some of the oceanic intraplate seismicity is consistent with such a boundary less

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

heat transfer and intraplate deformation in the central indian ocean nineteen new heat flow measurements made across deformed oceanic lithosphere in the central indian ocean and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region many of the temperature depth profiles are nonlinear upward convection of water is the most likely explanation for the curvature of the temperature profiles since other possible causes including variations in bottom water temperatures conductivity changes with depth in the sediments and experimental error can be eliminated this interpretation requires water velocities of the order of 7 x 10 sup 8 m s which is unusual because the lithosphere is relatively old 72 82 m y and a thick sedimentary cover 1 2 5 km is present these observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment water interface we infer that extra heat is being generated at shallow depths perhaps less than 35 km in the plate although the specific mechanism by which deformational energy is converted into heat is difficult to determine

average seismic source parameter relations for mid plate earthquakes mid plate earthquakes which are defined as having their epicenters at least 500 km from plate margins represent a small portion of the total global seismicity however because those occurring within continental interiors are characterized by anomalously large damage areas they are of particular interest empirical mb ms and m o data are used to develop scaling relations for mid plate earthquakes from them it is found that the corner period t o2 of their far field displacement spectra varies as the one fourth power of the seismic moment which indicates that average stress drop increases as seismic moment increases by use of savage s 1972 rectangular bilateral dislocation model the derived spectra of mid plate earthquakes yield estimates of fault rupture length and width average fault displacement and source rise time as a function of seismic moment these relations show that large mid plate earthquakes such as the 1886 south carolina and 1811 and 1812 new madrid events do not require large fault rupture length

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

the bermuda earthquake of march 24 1978 a significant oceanic intraplate event the bermuda earthquake m 6 occurred near the westerly extension of the kane fracture zone roughly 370 km southwest of the island of bermuda it is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of north america because of its size and location it has provided an excellent set of wwssn body waves they can be used to infer its depth and faulting parameters by waveform modeling techniques the results indicate a north northwest striking thrust mechanism strike n20 w dip 42 ne rake 90 with the hypocenter located at a depth of 11 km which for an oceanic crust places it predominantly in the mantle the event had a seismic moment of 3 4 1025 dyne cm and its time history was modeled with a symmetric trapezoidal time function 3 s in duration the north northwest strike of the event is in good agreement with the bathymetry of the area the epicenter being close to the southwestern edge of the bermuda rise the strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region the presence of such fracture zones is indicative of local weak zones in the lithosphere the bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the north american plate in the direction of its absolute motion this is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of north america

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans