evidence for transform faulting in the ionian sea the cephalonia island earthquake sequence of 1983 accurate locations of aftershocks of the january 17 1983 ms 7 0 main shock in the ionian islands have been determined as well as fault plane solutions for this main shock and its largest aftershock which are interpreted as a right lateral strike slip motion with a thrust component on a fault striking in about a ne sw direction

identifying and removing noise from the monterey ocean bottom broadband seismic station mobb data when compared to quiet land stations the very broadband monterey ocean bottom seismic station mobb shows increased long period background as well as signal generated noise both sources of noise are unavoidable in shallow ocean bottom installations and postprocessing is required to remove them from seismic observations the long period background noise observed for periods longer than 20 s is mainly due to infragravity waves and ocean currents the shorter period signal generated noise on the other hand is due to reverberations of the seismic waves in the shallow sedimentary layers as well as in the water layer we first present the steps that were taken prior to and during the instrument deployment to minimize instrument generated noise as well as to avoid noise due to water flow around the instrument we then present results from two postprocessing methods that can be used to remove the long period background noise which both utilize the ocean bottom pressure signal locally recorded on a differential pressure gauge dpg one consists of subtracting the locally recorded ocean bottom pressure from the vertical seismic acceleration signal in this case the frequency independent scale factor is linearly estimated from the data the other one makes use of the transfer function between the vertical seismic and pressure signal to predict the vertical component deformation signal the predicted signal is then removed from the vertical seismic data in either frequency or time domain we also present two methods that can be used to remove the signal generated noise one employs the empirical transfer function constructed from mobb data and nearby land station data that do not show the signal generated noise the other one uses a synthetic transfer function computed by modeling the response of shallow layers at the mobb location using either of the two transfer functions most of the signal generated noise can be removed from the mobb data by deconvolution

seismic source parameter relations for earthquakes in greece a data set of nineteen mainly shallow moderate to large earthquakes which occurred in the aegean and the surrounding area has been used to derive empirical relations for kinematic fault parameters thus the relations between seismic momentm 0 and magnitudem s andm b and betweenm 0 andm s and fault dimensionss andl have been determined from these relations and theoretical ones it was deduced that earthquakes in the aegean and the surrounding events chiefly interplate are characterized by low average stress drop values values of ranging from 1 to 30 bar are consistent with the data it was also found that in general terms most of the data obey the geometrical similarity conditionl 2w wherel is the fault length measured along the strike andw is its width measured along the dip for strike slip faults however the conditionl 4w seems to hold

removing infragravity wave induced noise from ocean bottom seismographs obs data deployed offshore of taiwan abstract vertical ocean bottom seismograph obs data at frequencies below 0 05 hz are contaminated by noise induced by infragravity waves we constructed the transfer function between pressure and velocity data from obss deployed in taiwan waters to remove the wave pressure induced noise from seismic recordings data were analyzed from five portable broadband obss deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled shallow buried seismograph eos1 installed on the seafloor at 300 m depth removing long period noise from these obs data improves the identification of teleseismic phases such as p s ss p diff and pkikp that are otherwise ambiguous or unidentifiable for eos1 infragravity wave signals completely mask the p and s waveforms in the 10 50 s period band suitable for centroid moment tensor cmt solutions for most of the local events application of the transfer functions to predict and remove wave deformation yielded clean prominent p and s waveforms at these periods and aided in the cmt determination for small events jointly with land stations the relative amplitudes of the wavenumber normalized transfer function for some of the obss are mostly determined by the thickness of the sediment at the obs site

seismicity in the hellenic volcanic arc relation between seismic parameters and the geophysical fields in the region summary a study of the spatial distribution of seismicity and seismic hazard and geophysical parameters is undertaken along the hellenic volcanic arc hva seismicity of the area is divided in a cellular manner allowing analysis of the localized seismicity parameters and representation of their regional variation as contour maps extreme value analysis locates a zone of highest seismicity just west of centre of the hva around the cyclades islands non probabilistic modelling of the strain energy release process in terms of magnitudes equivalent to the average annual energy release rate m and the maximum possible energy release in a seismic cycle m3 enhances this picture maximum values of m and m3 are more tightly localized in the central cyclades west of amorgos island and also in the east of the hva in the sporadhes north west of rhodes isocyclic contours of the waiting times for m3 show values in excess of 70 years in the cyclades and eastern sporadhes i e the zones of largest earthquakes show the longest strain accumulation periods the waiting time for the hva as a whole is about 54 years and at present it appears to be about 75 per cent through to accumulation of the notional maximum strain energy the b value of the magnitude frequency distribution shows a consistent spatial variation along the hva which is not sensitive to subdivision into different focal depth ranges regional contour maps of the b values and maps of local values of heat flow magnetic and electrical anomalies compared with the bouguer gravity anomaly map indicate correlations which reflect the deformation of the aegean shallow crust in relation to the deep intrusive magmatic processes the correlations between the different geophysical fields are displaced commensurate with an aegean extension of approximately 45 km

the weibull log weibull transition of interoccurrence time for synthetic and natural earthquakes abstract we study the interoccurrence time distributions of events by analyzing synthetic catalogues and three natural catalogues of the japan meteorological agency jma the southern california earthquake data center scedc and the taiwan central weather bureau tcwb we find a universal feature i e the weibull log weibull transition in the interoccurrence time statistics this transition demonstrates that the interoccurrence time statistics of earthquakes possess the hybrid weibull and log weibull statistics we further find that the crossover magnitude m c from the superposition regime to the pure weibull regime is averagely proportional to the plate velocity in the end of this paper we summarize a region independent relation i e m c m max 0 54 0 06 which represents a novel empirical relation related to the weibull log weibull transition for earthquake processes

integration of geological and seismological data for the analysis of seismic hazard a case study of japan seismic hazard analyses are associated with large uncertainties when historical data are insufficient to define secular rates of seismicity such uncertainties may be decreased with geological data in areas where seismicity is shallow and produced by quaternary faulting to illustrate we examine intraplate japan large intraplate earthquakes in japan characteristically produce surface ruptures along mappable quaternary faults and show a systematic relation between seismic moment m 0 and rupture length i log m 0 23 5 1 94 log i it is observed that within the bounds placed by geologically assessed slip rates the mean regional moment release rate m 0 resulting from slip on mapped quaternary faults is in accord with estimates of m 0 determined with the 400 yr record of seismicity recent work also shows that when the repeat time t of earthquakes on quaternary faults in southwest japan is assumed to equal m 0 m 0 g where m 0 is estimated for rupture extended over the entire fault length and m 0 g is the geologically assessed moment release rate of each fault the moment frequency distribution of earthquakes predicted from the geologic record is virtually identical to that seen with the 400 yr record of seismicity these observations indicate that the geologic record of quaternary fault offsets contains sufficient information to predict both the spatial and size distribution of intraplate earthquakes in japan a contour map of the average recurrence time of ground shaking of jma intensity v is thus computed using an empirical relation between seismic moment and the areal distribution of seismic intensity and assuming that the repeat time t of earthquakes on each quaternary fault equals m 0 m 0 g the map demonstrates how quaternary fault data may be used to assess long term seismic hazard in areas of active faulting where historical records of seismicity are relatively short or absent another shortcoming of conventional seismic hazard analysis is that hazard is not considered a function of the time since each fault in a region last ruptured a simple procedure is used to demonstrate how the time dependent nature of the earthquake cycle affects the evaluation of seismic hazard the distribution of seismic shaking characteristic of large interplate earthquakes offshore of japan is estimated from published isoseismal maps the observed average repeat times of ruptures along specific segments of the plate boundaries then provide the basis to make probabilistic estimates of the next expected time of seismic shaking due to plate boundary earthquakes when data are too few to document the average repeat times of rupture the estimates of probability are calculated with data relating to the relative coseismic slip during past earthquakes and the rate of interseismic strain accumulation interpreted within the framework of the time predictable model of earthquake occurrence results are displayed as maps of instantaneous seismic hazard the probability that seismic shaking will occur conditional to knowledge of where in time each fault in a region presently resides with respect to the earthquake cycle

the seismic parameter b of the frequency magnitude relation and its association with the geological zones in the area of greece abstract the parameter b of the frequency magnitude relation has been accurately calculated for each of the 21 seismic zones into which the aegean and surrounding area 34 n 43 n 18 e 30 e has been divided on the basis of several seismotectonic criteria these 21 seismic zones have been geographically separated into three a b c groups regions according to the values of the parameter b the outer region a is characterized by a relatively high value 1 03 the middle region b by an intermediate value 0 84 and the innermost region c by a low value 0 60 the boundaries of these three groups of zones almost coincide with the boundaries of well known geological zones this observation is interpreted in terms of mechanical heterogeneity of the material and of structural and stress conditions in the area

coastal cliff ground motions from local ocean swell and infragravity waves in southern california 1 ground motions atop a southern california usa coastal cliff are compared with water level fluctuations observed at the cliff base and with ground motions observed 10 km inland at high tide cliff top ground motions in three frequency bands were generated locally by ocean waves at the cliff base 1 high frequency 0 3 hz shaking caused by waves impacting the cliff and 2 gravitational loading induced swaying at the frequency of the incident sea swell waves 0 05 0 1 hz and 3 slow swaying at infragravity frequencies 0 006 0 05 hz at high tide at infragravity and incident sea swell wave frequencies cliff top vertical ground displacement and cliff base water level fluctuations are coherent and oscillate in phase with occasional deviation at sea swell frequencies and spectral levels at the cliff top are much higher than at the inland seismometer in contrast at double frequencies 0 1 0 3 hz spectral levels of vertical motions are nearly identical inland and at the cliff top consistent with a common distant or spatially distributed source at low tide when ocean waves did not reach the cliff base power levels of vertical ground motions at the cliff top decreased to inland levels at incident wave frequencies and higher and only infragravity band motions were noticeably forced by local ocean waves

surface fault traces fault plane solution and spatial distribution of the aftershocks of the september 13 1986 earthquake of kalamata southern greece a shallow earthquake ofms 6 2 occurred in the southern part of the peloponnesus 12 km north of the port of the city of kalamata which caused considerable damage the fault plane solution of the main shock geological data and field observations as well as the distribution of foci of aftershocks indicate that the seismic fault is a listric normal one trending nne ssw and dipping to wnw the surface ruptures caused by the earthquake coincide with the trace of a neotectonic fault which is located 2 3 km east of the city of kalamata and which is related to the formation of messiniakos gulf during the pliocene quaternary tectonics field observations indicate that the earthquake is due to the reactivation of the same fault

a crustal velocity model for locating earthquakes in monterey bay california the seismicity of the monterey bay displays a sparse distribution of events with a majority in northern monterey bay on the san gregorio fault the paucity of near shore and offshore seismic recording instruments and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events on major faults that crosscut the bay new three component seismic data were acquired during 1997 1999 by the monterey bay aquarium research institute mbari margin seismology project using ocean bottom digital corehole and standard seismometers and coastal digital reftek instruments obtained from iris passcal by the university of california santa cruz we have analyzed phases from earthquakes in the monterey bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent northern california seismic network a new one dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the salinian block in the center of monterey bay velocities in the 10 16 km layer are consistent with continental crustal velocities a sharp increase in velocity at 16 km suggests a boundary that results from underplating of oceanic crust this underplated zone appears to extend to a depth of 27 30 km where we observe normal upper mantle velocities near 8 0 km sec new details from the ocean bottom corehole and coastal reftek instruments for events along the sgf and monterey bay fault zones hint at local fine scale structures and have implications for tectonic history and plate reconstruction interpretation

seismic faults in the aegean area abstract reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the aegean and surrounding area thrust faults having an about nw se strike occur in the outer seismic zone along western albania westernmost part of mainland of greece ionian sea south of crete south of rhodes the inner part of the area is dominated by strike slip and normal faulting strike slip with an about ne sw slip direction occurs in the inner part of the hellenic arc along the line peloponnesus cyclades dodecanese southwest turkey as well as along a zone which is associated with the northern aegean trough and the northwesternmost part of anatolia all other regions in the inner part of the area are characterized by normal faulting the slip direction of the normal faults has an about sw ne direction in crete n38 e and an about e w direction n81 e in a zone which trends n s in eastern albania and its extension to western mainland of greece in all other regions central greece southern yugoslavia and bulgaria western turkey the slip of the normal faults has an about n s direction

nearfield interseismic vertical strain on the central garlock fault southern california 1984 1991 repeated precise leveling of fault crossing arrays at two sites on the central and western parts of the garlock fault discloses minor height changes of a few centimeters across the fault in seven years during that time the seismicity of the fault has been very low relative to other parts of california although other investigators have measured a millimeter or two of horizontal displacement per year near each of the leveling arrays the vertical changes measured in this study probably result from nontectonic differential subsidence across the fault due to the effects of the 1985 1991 drought in southern california the vertical displacement at cameron could turn out to be tectonic but at the koehn lake site the subsidence may be related to nearby pumping of groundwater the changes and rates of heights reported here should not be extrapolated in time or space because of the relatively short period of observations and because these sites measure only minuscule parts of the 250 km long fault

occurrence patterns of large earthquakes in the seismic zones along the hellenic arc occurrence patterns of large shallow and intermediate depth earthquakes in the seismic zones along the hellenic arc have been investigated it is shown that throughout this active region the earthquakes tend to occur in a rather systematic manner at each time period earthquakes occur within a discrete activated segment of the arc these occurrence patterns are considered to offer an insight into identifying where large earthquakes are expected to occur in the future

seismicity of the hellenic subduction zone in the area of western and central crete observed by temporary local seismic networks temporary local seismic networks were installed in western crete in central crete and on the island gavdos south of western crete respectively in order to image shallow seismically active zones of the hellenic subduction zone more than 4000 events in the magnitude range between 0 5 and 4 8 were detected and localized the resulting three dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central crete from the mediterranean ridge to the cretan sea furthermore a three dimensional structural model of the studied region was compiled based on results of wide angle seismics surface wave analysis and receiver function studies the comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished high interplate seismicity along the interface between the subducting african lithosphere and the aegean lithosphere was found south of western crete where the interface is located at about 20 to 40 km depth an offset between the southern border of the aegean lithosphere and the southern border of active interplate seismicity is observed in the area of crete the offset varies laterally along the hellenic arc between about 50 and 70 km a southwards dipping zone of high seismicity within the aegean lithosphere is found south of central crete in the region of the ptolemy trench it reaches from the interface between the plates at about 30 km depth towards the surface in comparison the aegean lithosphere south of western crete is seismically much less active including the region of the ionian trench intraplate seismicity within the aegean plate beneath crete and north of crete is confined to the upper about 20 km between 20 and 40 km depth beneath crete the aegean lithosphere appears to be seismically inactive in western crete the southern and western borders of this aseismic zone correlate strongly with the coastline of crete

properties of the 1979 monte negro southwest yugoslavia seismic sequence focal properties of the monte negro earthquake 15 april 1979 m 7 1 and its seismic sequence foreshocks and aftershocks which occurred near the southwestern coast of yugoslavia are investigated fault plane solutions of the main shock and its largest aftershock 24 may 1979 m 6 3 and the spatial distribution of the shocks of this sequence show that the seismic fault strikes about southeast northwest parallel to the coast and dips northeast towards the continent it is a strike slip left lateral fault with a considerable thrust component its length is 95 km and its width 12 km an aseismic belt which separated the aftershock foci into two groups the northwestern and southeastern is interpreted as a section of the fault that slipped smoothly during the main shock the aftershock foci were barriers where stress had been induced one of these barriers broke later and produced the largest aftershock of 24 may

historical seismicity and rates of crustal deformation along the margins of the ordos block north china earthquakes in china show an empirical relation between seismic moment m 0 and the areal distribution of modified mercalli intensities vi and viii log m 0 16 66 0 91 log a viii and log m 0 14 35 1 16 log a vi where a and m 0 are measured in squared kilometers and newton meter respectively the empirical relations may be used to estimate m 0 for historical earthquakes in china to within a factor of three on average when sufficient isoseismal data exist this observation and an extensive collection of isoseismal maps are used to estimate m 0 for large earthquakes that occurred along the margins of the ordos block during the last 700 yr focal parameters of the historical events are inferred from the orientation and displacements across quaternary faults average rates of crustal deformation are then estimated from the 700 yr historical record with formulas that relate the occurrence rate of seismic moment in a region to rates of crustal strain the shanxi and northern ningxia graben systems are situated along the eastern and northwestern edges of the ordos block respectively normal faults in the two systems trend northeasterly and are characterized by a component of right lateral slip the deformation resulting from slip during earthquakes in each of the respective fault systems is estimated at about 0 5 to 1 0 mm yr of both right lateral shear and north by northwest extension the weihe graben system bounds the southern edge of the ordos block strikes easterly and conjugate to the shanxi and northern ningxia fault systems and exhibits left lateral normal fault displacements the average rate of deformation across weihe is described by about 1 0 mm yr of north by northwest extension and 1 5 mm yr left lateral east west shear the hetao fault system delineates the northern edge of the ordos block and displays quaternary faults similar in orientation and mechanism to that observed in weihe although mapped faults in hetao exhibit evidence of quaternary displacement crustal deformation rates are not estimated because there exists no historical record of large earthquakes in the area in southern ningxia at the southwest boundary of the ordos block deformation occurs by slip on left lateral strike slip faults oriented in an easterly azimuth and thrust faults with strikes ranging from southeast to south the average deformation rate in southern ningxia is found to be about 4 0 mm yr of east by northeast contraction and 10 0 mm yr of left lateral shear deformation of each of the fault systems is consistent with a regional compressive stress that trends northeast and results in an average of about 3 0 mm yr each of contraction at n70 e and extension of n160 e across the entire region inasmuch as uncertainties in estimates of m0 for historical earthquakes are about a factor of three a similar uncertainty is attached to rates of crustal strain determined in this manner

probabilities of occurrence of large earthquakes in the aegean and surrounding area during the period 1986 2006 repeat times of large shocks are obtained for 17 seismic fracture zones of the aegean and surrounding area from times of historic and present century earthquakes the mean standard deviation of the repeat times is approximately 50 of any one observation

probabilities of occurrence of large plate rupturing earthquakes for the san andreas san jacinto and imperial faults california 1983 2003 the san andreas san jacinto and imperial faults in california are divided into 19 segments conditional probabilities are calculated that a particular segment will be the site of a large plate rupturing earthquake i e an event that breaks the entire down dip extent of the seismogenic zone during the next 20 years the sizes of such events which account for most of the slip that occurs seismically appear to vary greatly for different segments of these faults repeat time of large shocks coseismic displacement moment release rupture length and seismic magnitude appear to correlate with one another and to be a function of the tectonic style of different parts of those fault zones tectonic inhomogeneities on a scale of about 1 to 100 km are much larger than displacement in any single seismic event and may be regarded as being invariant in their effects upon earthquake generation over many cycles of large shocks it is this invariance that appears to lead to a given segment of a fault rupturing repeatedly in events of nearly the same size since repeat time varies however for a given segment of a fault a simple probabilistic approach is used to forecast the likelihood of large future earthquakes for each segment using as input the time of the last large shock the average recurrence time and the standard deviation of time intervals between events dates of the last large shocks are available for most of the segments investigated repeat times are estimated from times of historic and prehistoric events tectonic similarity and times calculated from coseismic displacement in the last large shock divided by a rate of fault motion or strain buildup the standard deviation of time intervals divided by the average repeat time is close to 1 3 for three fault segments in california for which data are available on historic and prehistoric earthquakes that normalized value is adopted for all calculations of conditional probability magnitudes and seismic moments of future plate rupturing events are estimated for each fault segment several of the segments have well constrained probabilities for rupture during the next 20 years whereas others have large uncertainties eight of the segments constituting about 40 of the total lengths of the three faults have moderate to high probability and thus should be instrumented and studied extensively so as to provide shorter time warnings in some cases two adjacent segments may be nearly in phase in their loading history and may break in a single event the segment of the san andreas fault from opposite san jose to san juan bautista which ruptured less than 1 5 m in 1906 and which probably also broke in 1838 is calculated to have a moderate to high probability of an earthquake of magnitude 6 to during the next 20 years a 325 km section of the southern san andreas between tejon pass and the salton sea has about a 25 chance of rupturing in a shock of magnitude near 8 during the next 20 years all the other segments have only a small chance i e less than 5 to 10 of rupturing in an event of magnitude near 8 during that interval

cross shore decay of cliff top ground motions driven by local ocean swell and infragravity waves 1 ground motions at the frequencies between 0 01 and 0 1 hz of ocean infragravity and swell waves were observed on a cross shore transect extending landward from the edge of a southern california coastal cliff cliff top ground motions are coherent and in phase with water level fluctuations at the cliff base vertical ground motions at infragravity and single frequencies decay rapidly with inland distance from the cliff edge e folding scale is about 12 m and at the edge decrease by several orders of magnitude between high tide when waves reach the cliff base and low tide when the waterline is about 50 m from the cliff base the observed cross shore decay scales are qualitatively consistent with gravitational loading and attraction of water waves at tidally modulated distances from the cliff base at approximately constant distance from the waterline ground motions vary roughly linearly with nearshore swell wave energy in contrast to these locally forced ground motions double frequency band 0 1 0 2 hz cliff top vertical ground motions are remotely generated with spatially uniform magnitudes approximately equal to those observed 14 km inland near the cliff edge ground tilt dominates the observed large relative to vertical cross shore acceleration at infragravity frequencies contributes significantly to cross shore acceleration at swell frequencies and is a small fraction of cross shore acceleration at higher frequencies

estimation of earthquake source parameters of the may 4 1972 event of the hellenic arc by the inversion of waveform data abstract average source parameters of the may 4 1972 m s 6 5 earthquake southwest of crete are obtained by the inversion of body waves a source depth of 40 km is determined which places the source in the mantle the source parameters of the major double couple are strike 309 dip 18 rake 89 and seismic moment m 0 2 62 10 25 dyn cm the source time function is of 5 s duration with a long rise time the mechanism obtained corresponds to a pure thrust with the p axis almost perpendicular to the hellenic arc although a point source is assumed the resulting synthetics match satisfactorily the observed waveforms

long term earthquake prediction in the hellenic trench arc system abstract data concerning seismic sequences which have occurred in the hellenic trench sedimentary arc tectonic system have been used to investigate the time variation of the difference d 1 between the magnitude of the main shock and that of the largest aftershock the data are well fitted by an harmonic function with period of 29 years for both the western seismic zone ionian islands s peloponnesus w crete and the south southeastern zone crete karpathos rhodos it has been observed that the largest earthquakes in both zones are associated with low values of the d 1 t function based on these observations we suggest that a strong seismic activity with earthquakes of m 7 0 will break out soon in the hellenic trench arc system the largest of these earthquakes are expected between 1983 and 1987 a quiescence in seismicity which started fifteen years ago in the southern part of the western zone also indicates the generation of a very strong earthquake in this area during the next five years or so

identifying and removing tilt noise from low frequency 0 1 hz seafloor vertical seismic data low frequency 0 1 hz vertical component seismic noise can be reduced by 25 db or more at seafloor seismic stations by subtracting the coherent signals derived from 1 horizontal seismic observations associated with tilt noise and 2 pressure measurements related to infragravity waves the reduction in effective noise levels is largest for the poorest stations sites with soft sediments high currents shallow water or a poorly leveled seismometer the importance of precise leveling is evident in our measurements low frequency background vertical seismic spectra measured on a seafloor seismometer leveled to within 1 10 4 radians 0 006 degrees are up to 20 db quieter than on a nearby seismometer leveled to within 3 10 3 radians 0 2 degrees the noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides indicating that it is caused by tilting due to seafloor currents flowing across the instrument at low frequencies this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect to the earth s gravitational field the effect is much stronger on the horizontal components than on the vertical allowing significant reduction in vertical component noise by subtracting the coherent horizontal component noise this technique reduces the low frequency vertical noise on the less precisely leveled seismometer to below the noise level on the precisely leveled seismometer the same technique can also be used to remove background noise due to the seafloor pressure field up to 25 db noise reduction near 0 02 hz and possibly due to other parameters such as temperature variations

seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america a quantitative reappraisal the seismic potential of the chilean and southern peruvian margins of south america is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years 1984 2004 long term estimates of seismic potential or the conditional probability of recurrence within a specified period of time are based on 1 statistical analysis of historic repeat time data using weibull distributions and 2 deterministic estimates of recurrence times based on the time predictable model of earthquake recurrence both methods emphasize the periodic nature of large and great earthquake recurrence and are compared with estimates of probability based on the assumption of poisson type behavior the estimates of seismic potential presented in this study are long term forecasts only as the temporal resolution or standard deviation of both methods is taken to range from 15 to 25 of the average or estimated repeat time at present the valparaiso region of central chile 32 35 s has a high potential or probability of recurrence in the next 20 years coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971 1973 average repeat times also suggest this area is due for a great shock within the next 20 years flanking segments of the chilean margin coquimbo illapel 30 32 s and talca concepcion 35 38 s presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years in contrast the rupture zone of the great 1960 earthquake 37 46 s has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years in the north the seismic potentials of the mollendo arica 17 18 s and arica antofagasta 18 24 s segments which last ruptured during great earthquakes in 1868 and 1877 are also high but poorly constrained

improvement of seismic observation in the ocean by use of seafloor boreholes we developed a long term high quality seismic ocean floor borehole observatory system the neath seafloor equipment for recording earth9s internal deformation nereid four nereid borehole observatories were installed in the japan trench off sanriku area jt1 jt2 in the northwestern pacific basin wp2 and in the philippine sea wp1 the borehole sensors are cemented in the borehole to assure good coupling of sensors to the ground as well as to avoid effects of water flow around the sensors which may have been a problem in previous borehole installations the nereid seismic records from two of the observatories jt1 wp2 were free from long period noise due to turbulence in the seafloor boundary current or to water flowing around the sensor that is significant on the seafloor the infragravity wave noise clearly observed around 0 01 hz on the horizontal components was significantly higher in the jt1 seismometer in the sediment because of the low shear modulus of the sediment ocean waves of long wavelength cause the infragravity wave noise it is thus necessary to install seismometers in boreholes below the sediments to reduce the infragravity wave noise

on noise reduction in vertical seismic records below 2 mhz using local barometric pressure we propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mhz the method consists in subtracting the locally recorded scaled atmospheric pressure signal from the gravity record the scale factor is frequency independent and can be linearly estimated from the data while such a pressure correction was previously only used in the analysis of tidal records it is equally efficient at the low frequency end of the normal mode band and we use gravity recordings of the big 1994 shikotan and bolivian quakes to demonstrate this point our examples also show that this correction permits to achieve noise levels well below the new low noise model nlnm peterson 1993 the success of the correction also sheds some light on the physical processes responsible for the generation of low frequency seismic noise at quiet observatories