ridge offsets normal faulting and gravity anomalies of slow spreading ridges we develop a model relating mid ocean ridge normal faulting and crustal structure by examining recently available high resolution gravity and multibeam bathymetric data of the mid atlantic ridge and other spreading centers results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major atlantic offsets studied especially along the inside corner side of the offsets individual residual gravity anomaly spikes local peaks which have amplitudes of up to 20 mgal and typical across axis spacing of 10 30 km often coincide with individual major fault scarps suggesting crust thinned by normal faulting theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive ridge parallel low angle faults that originate episodically at inside corners of ridge offset intersections fault scarp heights and the amounts of crustal thinning as inferred from gravity anomalies are consistently larger at inside corners than at outside corners supporting a model in which tectonic extension near ridge offsets is asymmetric with low angle faults occurring preferentially at inside corners these results on spatial variations in seafloor morphology and gravity anomalies further support a three dimensional tectonic faulting model at oceanic spreading centers with three major characteristics 1 low angle faults form preferentially at inside corners where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets 2 low angle faults decrease in throw toward midpoints of long ridge segments where large low angle faults may not be sustained by a weak lithosphere and 3 the residual gravity peaks are statistically larger at transform than near nontransform offsets indicating that the length of a ridge offset and its tectonic style control the development of low angle faults

t phase spectra seismic moments and tsunamigenesis t phase spectrograms and spectra derived from deep ocean hydrophones for tsunamigenic and non tsunamigenic earthquakes in the pacific reveal the following 1 substantial amounts of t phase energy at high frequencies i e 1 hz to at least 35 hz 2 strong correlations of spectral strengths in the 10 hz to 35 hz frequency range to seismic moments and 3 large spectral strengths at high frequencies for tsunamigenic earthquakes these findings are consistent with the low attenuation of the so und fixing and ranging sofar channel which permits efficient propagation of high frequencies to great distances and with the perception that large earthquakes may generate high frequency signals at the water sediment interface which can best be observed by hydroacoustic sensors in the low noise environment of the deep oceans in addition these studies confirm that 1 source bathymetry can strongly influence the character of t phases 2 islands or seamounts near the source area can act as secondary radiators of energy which add to the complexity and duration of the coda 3 there appears to be no significant dependence of t phase strength on earthquake source mechanism and 4 the strength of the t phase is dependent on the purity of its oceanic travel path the strong correlation of t phase spectral strengths to seismic moments suggests that substantial improvements in the reliability of tsunami warnings may be possible by using t phases as an additional independent single station method for estimating seismic moments finally the high frequencies observed in the t phase may be associated with important alternate mechanisms for tsunamigenesis other than faulting i e slumpage compaction or liquefaction

reverse propagation of t waves from the emperor seamount chain 1 analysis of seismo acoustic t waves associated with an intermediate depth earthquake beneath northern japan reveals that the detected t waves are secondary waves scattered by distant seamounts the timing of t wave onset 3000 s after occurrence of the earthquake indicates that the detected t waves did not follow a direct path from the solid liquid conversion point near the japan trench to the observation stations in northeast japan it is hypothesized that the original t waves were reverse scattered back toward japan by peaks of the emperor seamount chain after propagation in the northern pacific ocean the synthesized envelope patterns for acoustics scattered at the seamounts considering this multi path effect reproduce the travel time and amplitude pattern of the observed t waves well with each seamount group producing a discrete amplitude peak of the scattered wave these observations suggest that topographical obstructions act as a secondary source for t wave propagation in the ocean

hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

the january 1998 earthquake swarm at axial volcano juan de fuca ridge hydroacoustic evidence of seafloor volcanic activity on january 25 1998 at 11 33z an intense swarm of earthquakes began within the summit caldera of axial volcano central juan de fuca ridge jdfr the earthquake swarm was detected using the u s navy hydrophones in the northeast pacific ocean the earthquake swarm lasted 11 days and produced 8247 detected 1037 located earthquakes during the first two days of the swarm earthquake activity migrated from the summit caldera 50 km along the south rift zone of axial volcano at rates of 0 92 0 13 and 0 23 0 09 m s 1 earthquake epicenter migration is characteristic of a lateral magma dike injection the largest three swarm earthquakes occurred within the caldera their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit earthquakes with short t wave rise times shallow focal depths cluster at two spots along the summit and south rift zone and are considered possible seafloor eruption sites in situ ground deformation and hydrothermal plume monitors and later shipboard observations confirmed the occurrence of a seafloor volcanic eruption at the volcano s summit

mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

synthesis of earthquake generated t waves t phases excited by earthquakes propagate with low transmission loss along the ocean sound channel and are characterized by long duration wavetrains often having multiple peaks associated with bathymetric highs in the source region although t waves were first identified almost fifty years ago their activation has not been adequately explained in this paper we show that for continental margin earthquakes realistic t phase coda can be computed by expressing the acoustic energy in terms of several low order modes excited by point sources distributed uniformly over the seafloor the conversion of seismic energy to acoustic energy is thus consistent with seafloor scattering

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

ultra long range hydroacoustic observations of submarine volcanic activity at monowai kermadec arc monowai is an active submarine volcanic center in the kermadec arc southwest pacific ocean during may 2011 it erupted over a period of 5 days with explosive activity directly linked to the generation of seismoacoustic t phases we show using cross correlation and time difference of arrival techniques that the eruption is detected as far as ascension island equatorial south atlantic ocean where a bottom moored hydrophone array is operated as part of the international monitoring system of the comprehensive nuclear test ban treaty organization hydroacoustic phases from the volcanic center must therefore have propagated through the sound fixing and ranging channel in the south pacific and south atlantic oceans a source receiver distance of 15 800 km we believe this to be the furthest documented range of a naturally occurring underwater signal above 1 hz our findings which are consistent with observations at regional broadband stations and long range acoustic parabolic equation modeling have implications for submarine volcano monitoring

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

acoustics variability of air gun signals recorded at intermediate ranges within the lau basin 1 during january february 2009 an active source seismic survey was performed over the eastern lau spreading center in the lau back arc basin 21 s 176 s acoustic signals generated by ther v langseth s 36 gun pneumatic source array were recorded within the deep sound channel at offsets of 29 416 km the local ocean acoustic environment is everywhere bottom limited with seafloor depths within the study domain ranging from 1700 2800 m low frequency 4 125 hz sound levels are monitored using root mean square energy flux density and zero to peak measurement techniques from these field data transmission loss is found to exceed the predictions of a geometric spherical spreading model at similar ranges arrival amplitudes vary by up to 20 db and durations vary by a factor of three to six the depth of the seafloor beneath the air gun source exhibits a positive correlation with arrival duration and a negative correlation with range corrected amplitude explaining up to 30 of the observed variation in both parameters the strength of this correlation however varies for stations lying at different azimuths highlighting the importance of seafloor aspect and slope in the coupling of bottom interacting acoustic energy into the sound channel range dependent ray tracing shows that shots deployed over shallower seafloor are more likely to produce sound channel trapped signals that propagate with limited bottom interaction this results in arrivals that are more impulsive with shorter durations and higher amplitudes shots deployed in deeper water typically undergo a larger number of bounces and are characterized by more emergent longer duration and smaller amplitude arrivals

long term seismicity and ground deformation at axial volcano juan de fuca ridge seismicity along the juan de fuca ridge jdfr has been monitored since august 1991 using u s navy hydrophones in the northeast pacific ocean during this time the focus of jdfr seismicity has been axial volcano where a total of 9055 earthquakes have been detected 1669 located this activity occurred within 48 discrete swarms along the summit caldera and flank rift zones from august 1991 to may 1996 axial volcano exhibited only one swarm with 50 earthquakes from may 1996 to november 1997 swarm activity increased significantly with five swarms of 50 earthquakes activity culminated at the volcano in january 1998 with an 11 day 8247 event swarm that was the result of a major eruption along the summit and southern flank seafloor deformation sensors indicated pre eruption swarms in october 1995 may 1996 and june 1996 coincided with subsidence of the caldera floor the 1996 1997 swarm activity increase also coincided with horizontal extension of the north rift zone consistent with pre eruption deformation of the volcano following the 1998 eruption swarm activity dramatically decreased and no earthquakes have been detected since april 1998 this is the longest aseismic period 1 yr observed at the volcano since acoustic monitoring began suggesting the january 1998 swarm marked the end of an eruptive cycle at axial volcano

hydroacoustic monitoring of seismicity at the slow spreading mid atlantic ridge 1 in february 1999 long term hydroacoustic monitoring of the northern mid atlantic ridge mar was initiated six autonomous hydrophones were moored between 15 n and 35 n on the flanks of the mar results from the first year of data reveal that there is significant variability in along axis event rate groups of neighboring segments behave similarly producing an along axis pattern with high and low levels of seismic activity at a wavelength of 500 km this broad scale pattern is likely influenced by the axial thermal regime several earthquake sequences with variable temporal characteristics were detected suggesting fundamental differences in the cause of their seismicity off axis most seismic faulting occurs within a zone 15 km from the axis center

empirical relationship of t wave energy and fault parameters of northeast pacific ocean earthquakes a total of 179 northeast pacific ocean earthquakes that occurred between 1992 1998 are used to investigate the relationship of t wave acoustic energy and earthquake fault parameters the moderate sized 3 7 mw 6 9 earthquakes were recorded by ocean hydrophones and had their source parameters estimated by land based seismic networks the acoustic energy release at the seafloor water interface of each earthquake was estimated by removing the signal attenuation along the propagation path and instrument gain from the hydrophone signal there appears to be a relationship where for a given magnitude earthquake t wave energy is lower for normal and reverse fault earthquakes than strike slip events a more detailed comparison indicates acoustic energy of the earthquake will often decrease as the component of dip slip motion increases these observations suggest fault parameter information is contained within the t wave signal packet

a comparison between the generalized digital environmental model and levitus climatologies two ocean climatologies of temperature and salinity the generalized digital environmental model gdem and the climatological atlas of the world ocean are compared dynamic height fields are computed by season from each climatology for the north atlantic north pacific and indian oceans and are compared on a 1 latitude longitude grid large scale oceanographic features are generally found to be similarly represented in both climatologies gdem appears to render better representations of seasonal variability and regions of high current shear such as the gulf stream because of a different smoothing method and a finer grid spacing maps of dynamic heights from both climatologies are presented and their similarities and differences are discussed the methodology for the construction of gdem is also described in detail

spatial and temporal distribution of seismicity along the northern mid atlantic ridge 15 35 n 1 a detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern mid atlantic ridge between 15 and 35 n is presented two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones the seismically active zone extends 20 km to either side of the ridge axis consistent with earlier results from studies of fault morphology along the axis hydrophone recorded activity shows important variations areas with intense and persistent seismic activity stripes stand in sharp contrast to areas that lack seismicity gaps the regions of persistent activity are a new observation at mid ocean ridges in general the patterns of seismically active inactive regions are also recognized in the 28 year teleseismic record implying that these patterns are maintained at timescales between a few years and a few decades we find no simple relationship between individual segment variables e g length or trend of the segment maximum offset of discontinuities or along axis change in mantle bouguer anomaly mba and water depths and number of hydrophone recorded events there does appear to be a correlation between axial thermal structure and seismicity regions of low and high numbers of events would thus correspond to thinner hotter and thicker colder lithosphere respectively seismicity may reflect thermal structure at short timescales decadal or longer while relief and inferred crustal thickness may integrate this structure over longer periods of time order of 1 myr

on the mechanism of conversion of seismic waves to and from t waves in the vicinity of island shores high frequency seismic records from the polynesian seismic network are used to investigate in detail the processes of conversion of acoustic t wave energy from and to seismic waves at island shores on the source side we study the seismic to acoustic conversion based on t phases from hawaiian events recorded at polynesian stations located on the coral platter on the receiver side we study the acoustic to seismic conversion based on t phases from marine sources recorded across the polynesian islands in both instances our results underline the importance of steep slopes typically 50 in allowing an efficient conversion between p waves in the island structure and t waves in the water column these slopes can be coral reefs or the heads of young presumably unconsolidated basalt flows under this geometry modeling based on raytracing indicates that the seismic record of the t phase consists of a p wave at distances from the conversion point greater than 9 km at shorter distances s waves and surface waves are generated in the absence of a steep slope only surface waves are present these models can be used to compute precise and predictable travel time station corrections allowing the use of seismometers located inland to exploit the superb detection and location capabilities of t waves

evidence of a recent magma dike intrusion at the slow spreading lucky strike segment mid atlantic ridge 1 mid ocean ridge volcanic activity is the fundamental process for creation of ocean crust yet the dynamics of magma emplacement along the slow spreading mid atlantic ridge mar are largely unknown we present acoustical seismological and biological evidence of a magmatic dike intrusion at the lucky strike segment the first detected from the deeper sections 1500 m of the mar the dike caused the largest teleseismic earthquake swarm recorded at lucky strike in 20 years of seismic monitoring and one of the largest ever recorded on the northern mar hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband 3 hz intrusion tremor suggesting a volcanic origin two submersible investigations of high temperature vents located at the summit of lucky strike seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting this magmatic episode may represent one form of volcanism along the mar where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long lived discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems index terms 3035 marine geology and geophysics midocean ridge processes 7280 seismology volcano seismology 8419 8149 tectonophysics planetary tectonics 5475 4259 oceanography general ocean acoustics 9325 information related to geographic region atlantic ocean keywords mid atlantic ridge earthquake hydroacoustic

localization of t wave energy on land revealed by a dense seismic network in japan summary we have investigated the wave propagation characteristics of a seismic t wave on land in northeastern japan using the seismograms recorded by a dense seismic network the analysed t wave is a unique one that was generated by a large intermediate depth earthquake beneath the network that backpropagated from the emperor seamount chain in the northern pacific the t wave has a predominant frequency of 1 5 hz which is lower than the previously reported values spatiotemporal variation of an rms envelope bandpass filtered with a passband of 1 2 hz demonstrates multiple arrivals of the t wave and inhomogeneous distribution of amplitude judging from the spatial pattern of first arrival times and the polarization characteristics such as the relatively large dip angle and high planarity we regard the seismic t wave as a rayleigh wave a possible conversion area estimated by a method similar to the source scanning algorithm is located to the northeast of honshu island and may have a dimension of 100 km the most prominent feature of the spatiotemporal variation of the rms amplitude is the localization of t wave energy in the northeastern part of honshu island this localization can be attributed either to the close proximity of the conversion area to land or to the large extent of the area however these factors are only supplementary because they cannot explain a sharp contrast in amplitude between the two areas with comparable distance but different azimuth from the conversion point an additional and essential factor in explaining the observation is the non isotropic radiation from the conversion points since the conversion occurred on the continental slope the maximum amplitude of converted energy is expected to appear either in the propagation direction of incoming acoustic waves or in the steepest direction of the local topography these factors in particular a convex shape of bathymetry probably caused the focusing of converted energy into the northeastern part of honshu island

microseismicity of the mid atlantic ridge at 7 s 8 15 s and at the logatchev massif oceanic core complex at 14 40 n 14 50 n lithospheric formation at slow spreading rates is heterogeneous with multiple modalities favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate we report microseismicity from three deployments of seismic networks at the mid atlantic ridge mar two networks surveyed the mar near 7 degrees s in the vicinity of the ascension transform fault three inside corner high settings were investigated however they remained seismically largely inactive and major seismic activity occurred along the center of the median valley in contrast at the logatchev massif core complex at 14 degrees 45n seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted logatchev hydrothermal vent field to the north and south of the massif however seismic activity occurred along the ridge axis emphasizing the asymmetry of seismicity at the logatchev segment focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3 5 km depth which we interpret to reflect volume expansion accompanying serpentinization at shallower depth of 2 4 km some earthquakes in the vicinity of the vent field showed normal faulting behavior suggesting that normal faults facilitates hydrothermal circulation feeding the vent field further a second set of cross cutting faults occurred indicating that the surface location of the field is controlled by local fault systems

comparison of mid oceanic earthquake epicentral differences of travel time centroid locations and those determined by autonomous underwater hydrophone arrays 1 mid oceanic interplate earthquakes are difficult to locate accurately because they normally occur far away from land based seismic stations use of water borne t waves recorded by autonomous underwater hydrophone auh arrays records an order of magnitude more highly accurate regional low seismicity along the north mid atlantic ridge than the international seismic centre isc even though the physical meaning of an auh locations is still not well known auh s small location errors are important for better constraining mid oceanic earthquakes comparison of such auh locations with those in isc and harvard centroid moment tensor cmt location catalog and relocated ones based on the high resolution bathymetry and teleseismic p phases is made in this study auh locations are used as a reference to compare the teleseismically determined locations for large earthquakes with known focal mechanisms we find that relocated locations agree with auh ones better than with isc we also note that the centroid vectors from relocated epicenters are usually larger than auh centroid vectors the relocated epicenters and auh locations lie in similar azimuthal directions to the associated cmt epicenters the larger relocated and auh centroid vectors than the error ellipses of auh cmt and relocated ones combined might be explained by the fault rupture process for smaller events relocated location confidence ellipses are usually large enough to cover auh locations and their error ellipses overall the highly accurate auh locations can be used to confirm the mid oceanic earthquake hypocenters and seismicity characteristics and for detail studies of the low level seismicity associated with the plate motions

excitation of t phases by seafloor scattering t phases excited by suboceanic earthquakes are classified into two types abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the sofar velocity channel and slope t phases which are excited at continental or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter in this article it is demonstrated that approximate time frequency characteristics of both classes of t phase can be synthesized under the assumption that t phases are excited by scattering from a rough seafloor seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum at greater depths scattering excites higher order modes which interact weakly with the seafloor along much of the propagation path using known variations in near source bathymetry t phase envelopes are synthesized at several frequencies for several events south of the fox islands that excited both

t wave generation and propagation a comparison between data and spectral element modeling t waves are underwater acoustic waves generated by earthquakes modeling of their generation and propagation is a challenging problem using a spectral element code specfem2d this paper presents the first realistic simulations of t waves taking into account major aspects of this phenomenon the radiation pattern of the source the propagation of seismic waves in the crust the seismic to acoustic conversion on a non planar seafloor and the propagation of acoustic waves in the water column the simulated signals are compared with data from the mid atlantic ridge recorded by an array of hydrophones the crust water interface is defined by the seafloor bathymetry different combinations of water sound speed profiles and sub seafloor seismic velocities and frequency content of the source are tested the relative amplitudes main arrival times and durations of simulated t phases are in good agreement with the observed data differences in the spectrograms and early arrivals are likely due to too simplistic source signals and environmental model these examples demonstrate the abilities of the specfem2d code for modeling earthquake generated t waves

deciphering the ocean bottom pressure variation in the logatchev hydrothermal field at the eastern flank of the mid atlantic ridge ocean bottom pressure data from the logatchev hydrothermal field lhf are presented and analyzed the data were collected with two ocean bottom pressure meters obps constructed at the university of bremen that are capable of recording signals with frequencies up to 0 25 hz over the long term a nearly 2 5 kpa 25 cm water column equivalent pressure variation over 3 7 years is observed which is consistent with uplift followed by subsidence but cannot unequivocally be discerned from instrumental drift medium term pressure variations are compared with satellite surface topography satellite gravity ocean modeling and in situ data from an obp 700 km away it is shown that fluctuations in the oceanic mass distribution dominate the variations in this frequency range and that oceanic modeling and data from a 700 km distant obp are positively correlated with the lhf bottom pressure time series the short term variations are dominated by microseisms originating from sea surface waves and pressure waves from earthquakes as can be shown by comparison with weather buoy and teleseismic data

time clustering behavior of spreading center seismicity between 15 and 35 n on the mid atlantic ridge observations from hydroacoustic monitoring abstract an earthquake catalog derived from the detection of seismically generated t waves is used to study the time clustering behavior of moderate size 3 0 m earthquakes between 15 and 35 n along the mid atlantic ridge mar within this region the distribution of inter event times is consistent with a non periodic non random clustered process the highest degrees of clustering are associated temporally with large mainshock aftershock sequences however some swarm like activity also is evident temporal fluctuations characterized by a power spectral density p f that decays as 1 f are present within the time sequence with ranging from 0 12 to 0 55 for different regions of the spreading axis this behavior is negligible at time scales less than 5 103 s and earthquake occurrence becomes less clustered smaller as increasing size thresholds are applied to the catalog a power law size frequency scaling for mid atlantic ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes or source levels although fractal seismic behavior has been linked to the structure of the underlying fault population in other environments power law fault size distributions have not been observed widely in the mid ocean ridge setting

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

improving epicentral and magnitude estimation of earthquakes from t phases by considering the excitation function a standard technique for locating events with t phases is to pick the peak energy of t phases as the arrival time then proceed as if it was an unscattered phase originating at the epicenter the peak energy arrival time however can shift to different parts of the wave train due to incoherent scattering we show that a 50 reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the log of the entire envelope of the t phase we test the stability of this approach by comparing relative event locations based on this method with those determined by cross correlating waveforms of rayleigh and love surface waves recorded teleseismically using a swarm of earthquakes at the northern end of the easter microplate as an example relative event locations show that there is no systematic bias in t phase locations the t phase location and detection can be extended to much smaller events than are detectable by surface waves we estimate t phase maximum amplitudes of about 50 events within the swarm from the amplitude of the fitted functions rather than directly from the seismograms twenty four of these events are large enough to estimate their magnitudes by surface wave analysis the empirical analysis shows the log of t phase maximum amplitude and surface wave magnitude ms exhibit a relatively uniform linear relationship with much less scatter than in previously published studies of t phase amplitude versus magnitude the reduction in scatter is due to both the stability of the functional estimate of amplitude and the use of a swarm of earthquakes with similar depth mechanism and source receiver geometry the observed t phase coda for shallow earthquakes can be synthesized using a simple model of multiple reverberation seafloor scattering the results show that scattering of energy at a rough seafloor from multiple reverberations in the water column is significant in t phase generation even where the water depths are relatively uniform

the temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow spreading ridge high precision u pb zircon dating of odp holes 735b and 1105a atlantis bank southwest indian ridge abstract ocean drilling program hole 735b at atlantis bank on the southwest indian ridge sampled 1508 m of plutonic oceanic crust hosted in the footwall of an oceanic detachment fault we present new high precision isotope dilution thermal ionization mass spectrometry id tims u pb zircon dates from samples spanning the length of hole 735b and from the shallower adjacent hole 1105a 158 m the new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date whole rock and mineral geochemistry from hole 735b define three main igneous series weighted mean 206pb 238u dates suggest each igneous series intruded beneath the preceding series weighted mean 206pb 238u dates range from 12 175 to 11 986 ma in series 1 11 974 to 11 926 ma in series 2 and 11 936 to 11 902 ma in series 3 0 015 to 0 069 ma weighted mean 206pb 238u dates from hole 1105a range from 11 9745 to 11 9573 ma 0 0082 to 0 0086 ma the hole 1105a dates are coeval with series 2 in hole 735b consistent with previous correlations of fe ti oxide rich layers between the two holes suggesting individual magmatic series formed sheet like bodies that were 250 m thick and extended 1 1 km parallel to the ridge axis e w and 0 48 km in the spreading direction n s the data suggest a total duration of magmatism in hole 735b of 0 214 0 032 ma corresponding to accretion over a horizontal distance of 2 6 0 4 km the crust at atlantis bank was formed during active detachment faulting and the successive underplating of each magmatic unit may have been favored in this environment the combined u pb dates and reported ti in zircon temperatures are consistent with magmatic cooling rates of 10 3 10 4 c ma over the temperature interval of 900 700 c

microearthquake seismicity of the mid atlantic ridge at 5 s a view of tectonic extension we report measurements made with an ocean bottom array which was operated for 10 days on the mid atlantic ridge just south of the 5 s transform fault fracture zone a total of 148 locatable earthquakes with magnitudes 0 5 2 8 were recorded seismic activity appears to be concentrated within the western half of the median valley the median valley seismic zone is bounded in along axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south a few scattered events occurred within the inside corner high on the transform fault and in the western sidewall close to the segment center earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle although a few crustal earthquakes also occurred the presence of earthquakes in the mantle indicates that it is not strongly serpentinized we infer the median valley seismic activity to primarily arise from normal faulting

collision of the izu arc with honshu and the effects of oblique subduction in the miura boso peninsulas abstract collision of the izu arc in central japan is discussed with a focus on its tectonic effects to the east of the arc in the miura boso peninsulas of honshu the tectonics are the combination of the following events philippine sea plate spreading in the late oligocene to early miocene opening of the sea of japan in the middle miocene obduction of ophiolitic rocks in the northeasternmost corner of the philippine sea plate and forearc sedimentation between the honshu and izu arcs oblique subduction has shifted the plate boundary from northeast to southwest from the present mineoka tectonic belt through the miura fold belt to the sagami trough since the miocene remarkable right lateral transpressional deformation occurred throughout this period of the oblique collision and subduction

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

a model for predicting the volumes of vesicles in submarine basalts the total volume percentage of vesicles in submarine pillow basalt rinds can be modelled by the exsolution of h o and co vapor using available solubility determinations for these components in basalt in accordance with observations on natural samples co is predicted to be the major vapor component in vesicles at eruption depths 1 2 km only small initial contents of co 0 1 0 2 wt are consistent with the vesicle volumes observed in most deep sea pillow lavas h o is important only at shallow depths crystallization of pillow interiors can lead to more extensive vesiculation inside of pillow lavas than in the vesicle poor outer rinds

seismicity at intersections of spreading centers and transform faults ocean bottom seismographs were used in a microearthquake monitoring experiment at the eastern junction of the oceanographer transform with the mid atlantic ridge at 35 n microearthquake activity at the junction occurred over a broad area 7 km these microearthquakes cut across the corner between the transform and median valley and are associated with fault scarps that form the inner walls on the west and north sides of the median and transform valleys at intersections of other major fracture zones 100 km offset and slow to moderate spreading centers microearthquake activity is also diffuse and cuts across the corner between the spreading center and transform fault the narrow zone of decoupling 1 km observed between spreading center and transform boundaries by detailed geological studies at the tamayo east pacific rise and vema mid atlantic ridge intersections suggest that the diffuse seismicity 20 to 30 km in width does not reflect a diffuse plate boundary at the transition from rift to transform valley instead the faulting probably reflects internal deformation of the corner by secondary faults off of the plate boundary

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

monitoring pacific ocean seismicity from an autonomous hydrophone array since may 1996 an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial pacific to provide long term monitoring of seismic activity including low level volcanic signals along the east pacific rise between 20 n and 20 s and the galapagos ridge the instruments and moorings were designed to continuously record low frequency acoustic energy in the sofar channel for extended periods and produce results comparable to those previously derived by using the u s navy sound surveillance system sosus in the northeast pacific the technology and methodology developed for this experiment including instrument design mooring configuration analysis software location algorithms with an analysis of errors and a predicted error field are described in detail volcanic activity is observed throughout the pacific along with seismicity along transform faults subduction zones and intraplate regions comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to or better than sosus volcanic seismicity along the fast spreading east pacific rise appears similar to documented examples in the northeast pacific but with much shorter durations one example from the intermediate spreading galapagos ridge is comparable to northeast pacific examples and several episodes of activity were observed in the wilkes transform fault zone a site of continuing off axis seismicity is located near 18 s and 116 w isolated intraplate earthquakes are observed throughout the study area earthquake information from this experiment and future observations will be provided through the world wide web and earthquake data centers

modal scattering a key to understanding oceanic t waves the excitation mechanism of oceanic t waves has been a puzzle for almost fifty years with refraction from a sloping seafloor and seafloor scattering as two of the most commonly invoked mechanisms by representing the earthquake source field as a normal mode sum it can be seen that both mechanisms are very closely related strict modal orthogonality prohibits the existence of t waves in a laterally homogeneous semi infinite half space or radially symmetric sphere as energy cannot be transferred from one mode to another in an homogeneous medium deterministic non planar bathymetry random boundary roughness upper crustal heterogeneity or a combination of these provides a physical mechanism to break the strict orthogonality we show that modal scattering from the rough seabottom in the epicentral region converts energy from the directly excited ocean crustal water column modes to the propagating acoustic modes comprising the oceanic t wave submarine earthquake fault orientation also appears to be reflected in the t wave excitation

new age data for mid atlantic ridge hydrothermal sites tag and snakepit chronology revisited the chronologies of tag and snakepit hydrothermal fields have been established using 210pb pb 230th 234u and 14c dating at the tag field a mn oxide record indicative of low temperature events began at least 125 000 years and possibly 140 000 years ago with maximum intensities at 15 000 7000 and 4000 years before present high temperature events giving rise to sulfide deposits began about 100 000 years ago and have been intermittent to the present day a presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20 000 years decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000 7000 and 9000 years ago after a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago the snakepit field is much younger and no sulfides older than 4000 years have been recovered relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at tag reactivation of snakepit took place about 80 years ago and is presently concurrent with that of tag comparison with hydrothermal sites on the east pacific rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits

seismic reflection imaging of an oceanic detachment fault atlantis megamullion mid atlantic ridge 30 10 n abstract we present multichannel seismic reflection data collected over the atlantis megamullion at the eastern ridge transform intersection of atlantis fracture zone on the northern mid atlantic ridge and over its conjugate crust these data image for the first time the internal structure of a young well developed megamullion dome formed by tectonic extension across a long lived oceanic detachment fault the exposed corrugated detachment fault surface exhibits a sharp coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor at the termination of the megamullion the fault is imaged 13 km along strike beneath a volcanic hanging wall block at a sub seafloor depth of 0 2 0 5 s two way travel time reaching north as far as 30 19 n the eastward dipping of the fault beneath the hanging wall block is estimated to be 6 14 the corrugated fault surface is underlain by a continuous strong and relatively smooth reflection d at 0 2 0 25 s sub bottom below the central axis of the dome this reflection deepens up to 0 6 s sub bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope we suggest that atlantis massif formed by sequential slip on two different detachment faults that merged at depth with breakaways as little as 2 km apart the initial detachment is represented by reflection d and the second corresponds to the presently exposed fault surface in this interpretation much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity it lies in contact with less altered higher velocity mantle below the first detachment resulting in the strong smooth character of reflection d mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism in conjugate crust corresponding to termination of the megamullion observed sub bottom reflections are interpreted as base of seismic layer 2a this layer is as thick as or thicker 570 900 m than layer 2a in normal atlantic crust and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming

deep seafloor arrivals in long range ocean acoustic propagation ocean bottom seismometer observations at 5000 m depth during the long range ocean acoustic propagation experiment in the north pacific in 2004 show robust coherent late arrivals that are not readily explained by ocean acoustic propagation models these deep seafloor arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km the travel times for six of 16 observed deep seafloor arrivals correspond to the sea surface reflection of an out of plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers this out of plane bottom diffracted surface reflected energy is observed on the deep vertical line array about 35 db below the peak amplitude arrivals and was previously misinterpreted as in plane bottom reflected surface reflected energy the structure of these arrivals from 500 to 3200 km range is remarkably robust the bottom diffracted surface reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor

regional seismicity of the mid atlantic ridge observations from autonomous hydrophone arrays summary autonomous hydrophones arrays are an excellent tool for monitoring mid ocean ridge seismic activity the major advantage of using arrays of autonomous hydrophones for recording deep ocean ridge earthquakes is its low magnitude detection thresholds achievable using hydroacoustic techniques regional analysis of the detection thresholds of the different autonomous hydrophones arrays deployed along the mid atlantic ridge reveals the strong influence of the detection threshold in the number of recorded events and it must be taken into account in any further analysis in this study the analysis of both autonomous hydrophones and teleseismically detected mid atlantic ridge seismicity reveals that the background seismicity from the relatively short recording periods of the autonomous hydrophones mimic the results of the much longer teleseismic recording it also reveals that seismicity generally cluster at both the segment scale and on mantle bouguer anomaly maxima the big majority of these clusters seem to be related to dyke intrusions and propagation along the mid atlantic ridge these dyke intrusions interact with the mainshock aftershock sequences the seismic sequences mainshock aftershock analysis reveals that the strength of the faults is highly influenced by the mode or style of faulting detachment faults which are ubiquitous along the mid atlantic ridge can produce more prolific shorter duration seismic sequences revealing faster and reduced strain releases in comparison to higher angle normal faults this reduced strain release is most likely to occur due to the presence of higher levels of serpentinization on detachment faults higher levels of serpentenisation can also promote an aseismic transient slip on the mainshock aftershock sequences

temporal and spatial history of the 1999 2000 endeavour segment seismic series juan de fuca ridge 1 two large clusters of earthquake activity in june of 1999 and january of 2000 have dominated recent seismicity along the endeavour segment of the juan de fuca ridge the impacts of the june 1999 sequence on the hydrologic system which include changes in vent temperature and chemistry within the main endeavour vent field have been well documented previously analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm like behavior characterized by the absence of a dominant main shock event the epicentral locations of events within the two swarms overlap spatially with centroid positions near 47 49 and 47 46 n latitude during the june 1999 swarm the initial activity spans the along axis region where a shallow axial magma chamber reflector was later imaged the epicenters then migrate 12 km to the south at a rate of 0 3 m s consistent with lateral dike propagation a distinct subcluster of events also occurred in the vicinity of surveyor volcano on the overlapping portion of the cobb segment given its distance from the main swarm this activity may represent a triggered response to dynamic shaking the january 2000 swarm has a more limited along axis extent relative to the june 1999 swarm with no indication of lateral migration much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999 although it contains fewer total events and is of shorter duration relative to the june 1999 swarm the january 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude as in situ temperature monitoring was not in place during january 2000 and vent fluids were not sampled until june 2000 the impacts of this swarm on the hydrothermal system are unknown the southernmost tip of the endeavour segment also is found to be a region of repeating swarm activity although morphologic evidence indicates the cobb segment has been propagating northward recently this seismic activity suggests that the western limb of the endeavour cobb overlap zone remains active

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

coupled seismoacoustic modes on the seafloor 1 wave to wave coupling arises when an acoustic pulse selects a rayleigh mode of the same speed and both travel together swapping energy across an interface ewing et al 1957 a distinctive signal is observed at the hawaii 2 observatory for purely oceanic paths from earthquakes on the blanco and mendocino fracture zones off the coast of north america the signal appears to be a composite of undispersed higher rayleigh modes propagating along the ocean floor both in the sediments and in the water the new coupled modes are identified by their frequency composition and their phase and group velocities seismoacoustic coupling at the seafloor is conditioned on a the presence of a low velocity interface at the ocean floor and b the wavelength of the rayleigh component being shorter than the depth of the water layer

estimating the horizontal and vertical direction of arrival of water borne seismic signals in the northern philippine sea conventional and adaptive plane wave beamforming with simultaneous recordings by large aperture horizontal and vertical line arrays during the 2009 philippine sea engineering test philsea09 reveal the rate of occurrence and the two dimensional arrival structure of seismic phases that couple into the deep ocean a ship deployed controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels ninety t phases from unique azimuths were recorded between yeardays 107 to 119 t phase azimuth and s minus p phase time of arrival range estimates were validated using united states geological survey seismic monitoring network data analysis of phases from a seismic event that occurred on yearday 112 near the east coast of taiwan approximately 450 km from the arrays revealed a 22 clockwise evolution of t phase azimuth over 90 s two hypotheses to explain such evolution body wave excitation of multiple sources or in water scattering are presented bas

hydroacoustic ray theory based modeling of t wave propagation in the deep ocean basin offshore eastern taiwan t waves are conventionally defined as seismically generated acoustic energy propagating horizontally over long distances within the minimum sound speed layer in the ocean sofar axis minimum however t waves have also been observed by ocean bottom seismometers in ocean basins at depths greater than the sofar axis minimum previously non geometrical processes such as local scattering at rough seafloor and water sediment interface coupling have been proposed as possible mechanisms for deep seafloor detection of t waves here we employ a new t wave modeling approach based on hydroacoustic ray theory to demonstrate that seismoacoustic energy can propagate to reach deep seafloor previously considered as shadow zone of acoustic propagation our new hydroacoustic simulations explain well the observations of t waves on ocean bottom seismometers at deep ocean basins east of taiwan and shed new light on the mechanism for deep ocean t wave propagation

tracking three meddies with sofar floats abstract three meddies were tracked for up to two years in the canary basin using neutrally buoyant sofar floats these meddies have cores of warm salty mediterranean water and are approximately 100 km in diameter 800 m thick and are centered at a depth of 1100 m meddy 1 was tracked for two years 1984 86 with five floats as it drifted 1090 km southward with a mean velocity of 1 8 cm s 1 four shipboard surveys made during these two years revealed the nearly total decay of meddy 1 by gradual mixing processes meddy 2 drifted 530 km southwestward over 8 5 months with a mean velocity of 2 3 cm s 1 until it collided with hyeres seamount near 31 n 29 w the floats trapped in this meddy then stopped looping abruptly implying a major disruption of this meddy meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1 1 cm s 1 a comparison of the velocity of meddies to the velocity of nearby floats at 1100 m depth outside of the meddies shows clearly that all three m

on the influence of slopes source seabed and water column properties on t waves generation at shore the term t waves is generally associated with acoustic waves generated by seismic events that subsequently travel horizontally in the ocean at the speed of sound in this paper we use a time domain spectral element method to perform a parametric study of the influence of seafloor slope source position and media properties for a typical downslope t wave generation scenario we find that the energy and duration of these waves are particularly sensitive to the environment in particular the slopes and physical characteristics of the seabed play a crucial role for both the generation and the conversion of these waves likewise the depth and position of the earthquake relative to the slope is of great importance with the presence of privileged areas for the generation of t waves which we map

controls on the t phase energy fluxes recorded on juan fernandez island by continental seismic wave paths and nazca bathymetry conicyt pcha doctorado nacional 2017 21171480 prs programa de riesgo sismico universidad de chile fondecyt 1170430

palaeoceanography and tectonic setting of the jurassic coast range ophiolite central california evidence from the extrusive rocks and the volcaniclastic sediment cover abstract the primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis late jurassic c 169 161 ma coast range ophiolite remnants are exposed e and w of the san andreas fault in central california ophiolite volcanic rocks are mainly sub alkaline tholeiitic basalts but include subordinate intermediate and silicic flows and breccias available immobile major and trace element data indicate that the coast range ophiolite formed above a subduction zone possibly in a fore arc setting as suggested by the presence of high magnesian andesites and boninites the ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault controlled ocean floor interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water local interbeds of arc derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting depositionally overlying deep water successions up to 600 m thick comprise radiolarian rich tuffaceous and volcaniclastic sediments of callovian mid tithonian age 165 148 ma depositional processes included airfall turbidity currents mass flow and current reworking small si fe mn lenses along the lava sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation at one locality llanada the ophiolite is overlain by up to 1 8 km of younger tithonian basic intermed and acid lava flows submarine lahars and mainly silicic pyroclastic sediments of inferred volcanic arc origin the volcaniclastic cover passes conformably upwards into late tithonian and younger terrigenous deep water clastic sediments of the great valley group deposited in a fore arc setting the coast range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc possibly developed above an eastward continent ward dipping subduction zone within the equatorial pacific ocean in this scenario short lived 5 8 ma supra subduction zone spreading to form the ophiolite was followed by callovian tithonian arc volcanism shedding tuffaceous debris onto the adjacent fore arc subduction was active separately along the western american continental margin with the result that the coast range arc ophiolite complex eventually collided with north america triggering the late jurassic nevadan orogeny and initiating franciscan subduction accretion the coast range ophiolite remnants at least those west of the san andreas fault were apparently translated northwards to their present positions after late cretaceous

p wave detection thresholds pn velocity estimates and t wave location uncertainty from oceanic hydrophones p wave arrivals recorded by the u s navy s sound surveillance system sosus hydrophone arrays were used to estimate earthquake detection thresholds and pn velocities in the northeast pacific ocean the navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary t waves however use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment the p wave detection threshold of the sosus hydrophones was quantified using the epicentral distance and magnitude of 250 northeast pacific ocean earthquakes earthquakes with body wave magnitudes as low as 2 have detectable p wave arrivals at epicentral distances of 500 km earthquakes with mb between 3 5 and 5 were detected 50 of the time at distances of 100 1500 km while events with mb 5 were all detected even out to distances of 1000 1500 km both p and t wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes the peak amplitude of the t wave coda and the onset of the p wave were used as the earthquake arrival times to estimate event locations t wave arrival time residuals have a gaussian distribution with zero mean which implies that using t wave peak amplitude is consistent with using the p wave onset as the arrival time there are typically 6 stations used to derive a t wave based location hence location error ellipses are not well constrained a monte carlo technique was employed to estimate t wave event location uncertainty t wave locations have error bars of 1 km in latitude and longitude when 3 hydrophones are used for a location estimate the detected p wave arrivals and earthquake locations were used to measure pn velocities pn velocity values of 7 9 0 1 and 8 0 0 1 km s were found for the pacific and juan de fuca plates respectively a pn velocity of 7 5 0 1 km s was measured for rays traveling northward from the mendocino triple junction along the cascadia subduction zone a pn velocity of 7 7 0 3 km s was estimated for ray paths originating onshore western north america and traveling to the offshore hydrophones

generation of hydroacoustic signals by oceanic subseafloor earthquakes a mechanical model summary for more than 15 yr the recording of hydroacoustic signals with hydrophones moored in a minimum sound velocity channel called the sofar sound fixing and ranging channel has allowed for detection and localization of many small magnitude earthquakes in oceanic areas however the interpretation of these hydroacoustic signals fails to provide direct information on the magnitudes focal mechanisms or focal depths of the causative earthquakes these limitations result in part from an incomplete understanding of the physics of the conversion across the seafloor interface from seismic waves generated by subseafloor earthquakes to hydroacoustic t waves to try and overcome some of these limitations we have developed a 2 d finite element mechanical model of the conversion process by computing an exact solution of the velocity field of the waterborne t waves our model shows that a double couple source mechanism of a subseafloor earthquake generates t waves whose take off angles are adequate to allow penetration into the sofar channel and efficient trapping by this waveguide furthermore our model confirms that a double couple source with a high s wave content produces higher amplitude t waves than a simple explosive source which only generates p waves

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

fracture alignments in marine sediments off vancouver island from ps splitting analysis alignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction and extension and basement faulting the orientation of these alignments can be estimated through analyses of s wave splitting if sensors in ocean bottom observations are deployed through free fall sensor orientation needs to be determined in order for the recorded data to be used for such analyses here we estimate the sensor orientation from the linear particle motions of p to s ps phases converted at the sediment basement interface and also from t waves that are excited by earthquakes and propagate in the seawater we examine waveforms of local earthquakes recorded by 32 ocean bottom seismometers obss that were deployed through free fall for three months in 2010 off vancouver island where the strike slip nootka fault zone nfz intersects the deformation front of the cascadia subduction zone because the particle motion of the ps wave was corrected by estimating splitting parameters the fast polarization direction which reflects s wave anisotropic structure within the sediment can also be evaluated consequently we could estimate the fast polarization direction at obss deployed near the nfz and west of the deformation front the obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left lateral motion of the fault in the basement along the nfz margin normal cracks due to horizontal compression west of and slightly away from the deformation front and frontal thrust faults within the accretionary prism near the deformation front

recent faulting and microearthquakes at the intersection of the vema fracture zone and the mid atlantic ridge new air gun reflection profiles 3 5 khz reflection profiles and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault disturbances of the thick normally flat lying turbidite deposits provide indications of recent vertical motions at the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley the depression is terminated abruptly on the south by the active transform fault which acts as a locus for vertical as well as horizontal displacement flat lying undisturbed sediments terminate abruptly at the fault the western boundary of the depression is much broader and is characterized by a series of slumplike steps to the west there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression this suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary to the east there is clear evidence of uplift and tilting of sedimentary layers a basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief the transition between the spreading center and the transform fault appears to take place within 1 2 km the width of the transform fault just east of the depression is less than a kilometer microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley at the basement ridge in the eastern portion of the depression and in the crestal mountains very little activity is associated with the western edge of the transform depression and the trace of the transform fault

geochemical evidence for the tectonic setting of the coast range ophiolite a composite island arc oceanic crust terrane in western california the middle to late jurassic age coast range ophiolite cro of california contains two geochemically distinct volcanic rock associations that formed in different tectonic settings volcanic rocks from the southern cro point sal cuesta ridge stanley mountain llanada quinto creek and del puerto and parts of the northern cro healdsburg elder creek are similar to low k tholeiites and calc alkaline rocks of the island arc suite the thin volcanic sections of these ophiolite remnants suggest formation by intra arc rifting in contrast volcanic rocks from stonyford seamount and paskenta in the northern cro are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid ocean ridge basalts or ocean island tholeiites these rocks are associated with tithonian radiolarian cherts and may be part of the franciscan complex alternatively they may represent a change in tectonic setting within the cro during the late jurassic regardless the cro as currently conceived cannot be considered a single terrane with one mode of origin

normal mode composition of earthquake t phases understanding the nature of the coupling between the underwater acoustic field and the land seismic field is important for evaluating the performance of the t phase stations in the international monitoring system for the comprehensive nuclear test ban treaty for upslope propagation in an ocean environment the places where underwater acoustic field energy couples into the land seismic field are determined to first approximation by the local water depth and the normal mode composition of the acoustic energy therefore the use of earthquake generated t phases as natural probes of water to land coupling characteristics is aided by knowledge of their modal composition data collected by a 200 element 3000 m aperture vertical hydrophone array during a 1989 experiment in the deep northeast pacific ocean are used to determine the mode composition of t phase arrivals from two mb 4 1 earthquakes near the w est coast of the u s one occurring offshore and the other on land results from an eigenanalysis approach and conventional mode decomposition for the two events are consistent and show that at 5 hz the offshore event s arrivals have higher order mode content compared to those from the event on land single hydrophone recordings at pt sur of two mb 4 4 hawaiian events in 1996 and 1997 one occurring offshore and the second on land display time frequency arrival structures that are explainable by the dispersion characteristics over the oceanic path although other effects due to complex source time functions and shear wave and dispersive propagation effects along the initial land path cannot be separated with these single element data differences in these two events arrival structures suggest differences in normal mode content consistent with those seen in the pair of 1989 events ocean path dispersion also appears to play a significant role in determining the i n water arrival structure from a 1995 french nuclear test at mururoa recordings of two hawaiian events in 1997 by the t phase station vib and the seismic station at berkeley illustrate that the water land coupling confuses the relative timing between normal modes resulting in apparent loss of information about the source

seismicity of the atlantis massif detachment fault 30 n at the mid atlantic ridge 1 at the oceanic core complex that forms the atlantis massif at 30 n on the mid atlantic ridge slip along the detachment fault for the last 1 5 2 ma has brought lower crust and mantle rocks to the seafloor hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the massif mostly on the southeastern section while detected seismicity along the adjacent ridge axis was sparse in 2005 five short period ocean bottom seismographs obs were deployed on and around the massif as a pilot experiment to help constrain the distribution of seismicity in this region analysis of six months of obs data indicates that in contrast to the results of the earlier hydroacoustic study the vast majority of the seismicity is located within the axial valley during the obs deployment and within the array seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4 0 and 4 5 the aftershock sequences were located on the western side of the axial valley adjacent to the atlantis massif and close to the ridge transform intersection they follow omori s law and constitute more than half of the detected earthquakes the obs data also indicate a low but persistent level of seismicity associated with active faulting within the atlantis massif in the same region as the hydroacoustically detected seismicity within the massif the data indicate a north south striking normal fault and a left lateral strike slip fault near a prominent transform parallel north facing scarp both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the atlantis transform alternatively the normal faulting within the massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60 beneath the axis and the strike slip events may indicate transform parallel movement on adjacent detachment surfaces

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

spectra and magnitudes of t waves from the 1993 earthquake swarm on the juan de fuca ridge a swarm of earthquakes on the crest of the juan de fuca ridge was detected in june and july 1993 by a network of hydrophones the activity migrated 60 km along the crest suggesting a lateral dike injection and the possibility of a volcanic eruption subsequent geologic and oceanographic investigations confirmed that an eruption had taken place examination of the individual acoustic arrivals shows changes in the character of the signal that are consistent with an injection of magma a reduction in the rise time of the wave packet and a proportional increase in high frequency energy was observed and is interpreted to result from a shoaling of the earthquake source region second the source magnitudes were largest at the onset of the swarm and became smaller over time also consistent with shoaling of the dike the appearance of the t wave arrivals changed significantly 5 days after the beginning of the swarm potentially indicating the onset of a surface eruption

arctic abyssal t phases coupling seismic energy to the ocean sound channel via under ice scattering earthquakes along the mid arctic ridge radiate earthborne compressional and shear waves that couple into the arctic acoustic channel and propagate as t phases the t phase energy is transmitted above the epicenter through the seafloor at near vertical angles scattering from the ice surface couples some of this energy into waterborne angles at 5 hz the coupled energy is about 40 db down from the incident levels and at 15 hz it is about 30 db down scattering accounts for the characteristic spectral shape of the abyssal t phase that is more energetic at 15 hz than 5 hz several t phase arrivals were recorded on hydrophones during the fram ii experiment from the measured acoustic levels a transmission loss prediction between source and receiver and the scattering conversion losses the microearthquake acoustic source level in the water column is estimated to be about 250 db re 1 pa hz at 5 hz and 235 db re 1 pa hz at 15 hz these levels lead to a crustal attenuation estimate of 0 52 db km at 5 hz that is consistent with fram ii refraction measurements work supported by onr

evidence for weak oceanic transform faults 1 we present the results of a series of 3 d boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid ocean ridge spreading centers we find that the time averaged strength of transform faults is low and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness specifically mechanical coupling of only 5 best explains the observed patterns of strike slip and oblique normal faulting near a ridge transform intersection on time scales shorter than a typical earthquake cycle transient locked periods can produce anomalous reverse faulting similar to that observed at the inside corner ic of several slow spreading ridge segments furthermore we predict that extensional stresses will be suppressed at the ic due to the shear along the transform resisting ridge normal extension this implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many ics index terms 3035 marine geology and geophysics midocean ridge processes citation behn m d j lin and m t zuber evidence for weak oceanic transform faults geophys res lett 29 24 2207 doi 10 1029 2002gl015612 2002

modelling sound propagation in the southern ocean to estimate the acoustic impact of seismic research surveys on marine mammals summary modelling sound propagation in the ocean is an essential tool to assess the potential risk of air gun shots on marine mammals based on a 2 5 d finite difference code a full waveform modelling approach is presented which determines both sound exposure levels of single shots and cumulative sound exposure levels of multiple shots fired along a seismic line band limited point source approximations of compact air gun clusters deployed by r v polarstern in polar regions are used as sound sources marine mammals are simulated as static receivers applications to deep and shallow water models including constant and depth dependent sound velocity profiles of the southern ocean show dipole like directivities in case of single shots and tubular cumulative sound exposure level fields beneath the seismic line in case of multiple shots compared to a semi infinite model an incorporation of seafloor reflections enhances the seismically induced noise levels close to the sea surface refraction due to sound velocity gradients and sound channelling in near surface ducts are evident but affect only low to moderate levels hence exposure zone radii derived for different hearing thresholds are almost independent of the sound velocity structure with decreasing thresholds radii increase according to a spherical 20 log10 r law in case of single shots and according to a cylindrical 10 log10 r law in case of multiple shots a doubling of the shot interval diminishes the cumulative sound exposure levels by 3 db and halves the radii the ocean bottom properties only slightly affect the radii in shallow waters if the normal incidence reflection coefficient exceeds 0 2