subseasonal forecasting with an icosahedral vertically quasi lagrangian coupled model part i model overview and evaluation of systematic errors abstractthe atmospheric hydrostatic flow following icosahedral model fim developed for medium range weather prediction provides a unique three dimensional grid structure a quasi uniform icosahe

locked and creeping patches along the hayward fault california the results of mapping locked and unlocked patches on the hayward fault using the local recurrence time method are compared with models derived from surface creep and geodetic measurements the data used were the m 1 0 earthquakes in the usgs catalog since 1981 0 located within 5 km of the fault trace along the central and southern segments agreement with the other methods is satisfactory the 1868 rupture except for its two ends is classified as a locked segment by our method in the northern part of the hayward fault the data are poor and the results of the three methods differ short local recurrence times in the northern part suggest that an asperity exists there which is capable of an m6 5 class event

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

paleoseismic investigations in the santa cruz mountains california implications for recurrence of large magnitude earthquakes on the san andreas fault trenching microgeomorphic mapping and tree ring analysis provide information on timing of paleoearthquakes and behavior of the san andreas fault in the santa cruz mountains at the grizzly flat site alluvial units dated at 1640 1659 a d 1679 1894 a d 1668 1893 a d and the present ground surface are displaced by a single event this was the 1906 surface rupture combined trench dates and tree ring analysis suggest that the penultimate event occurred in the mid 1600 s possibly in an interval as narrow as 1632 1659 a d there is no direct evidence in the trenches for the 1838 or 1865 earthquakes which have been proposed as occurring on this part of the fault zone in a minimum time of about 340 years only one large surface faulting event 1906 occurred at grizzly flat in contrast to previous recurrence estimates of 95 110 years for the santa cruz mountains segment comparison with dates of the penultimate san andreas earthquake at sites north of san francisco suggests that the san andreas fault between point arena and the santa cruz mountains may have failed either as a sequence of closely timed earthquakes on adjacent segments or as a single long rupture similar in length to the 1906 rupture around the mid 1600 s the 1906 coseismic geodetic slip and the late holocene geologic slip rate on the san francisco peninsula and southward are about 50 70 and 70 of their values north of san francisco respectively the slip gradient along the 1906 rupture section of the san andreas reflects partitioning of plate boundary slip onto the san gregorio sargent and other faults south of the golden gate if a mid 1600 s event ruptured the same section of the fault that failed in 1906 it supports the concept that long strike slip faults can contain master rupture segments that repeat in both length and slip distribution recognition of a persistent slip rate gradient along the northern san andreas fault and the concept of a master segment remove the requirement that lower slip sections of large events such as 1906 must fill in on a periodic basis with smaller and more frequent earthquakes

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

dating san andreas fault earthquakes with lichenometry regionalrockfalleventsinthesierranevadaofcaliforniaare caused by distant earthquakes on the northern and southern san andreasfaultaswellasbylocalearthquakes lichenometricdating of synchronous pulses of rockfalls that are presumed to be caused by five historical earthquakes suggests an accuracy of 2 2 3 5 yr 95 confidence level for dating young events comparison of lichenometric ages for prehistorical rockfalls with precise radiocarbon ages of surface rupture times at the pallett creek and wrightwood paleoseismology sites indicates that earthquakes on the southern san andreas fault caused rockfalls 200 to 400 km to the north a major rockfall event at about a d 1690 supports the fumal et al 1993 model for a san andreas fault earthquake that was not detected at pallett creek the a d 1690 event in the chronology indicates that the earthquake recurrence interval was shorter and that the degree of clustering of times of earthquakes was less on the mojave segment of the san andreas fault than previously thought

interseismic coupling and refined earthquake potential on the hayward calaveras fault zone interseismic strain accumulation and fault creep is usually estimated from gps and alignment arrays data which provide precise but spatially sparse measurements here we use interferometric synthetic aperture radar to resolve the interseismic deformation associated with the hayward and calaveras faults hf and cf in the east san francisco bay area the large 1992 2011 sar data set permits evaluation of short and long wavelength deformation larger than 2 mm yr without alignment of the velocity field to a gps based model our time series approach in which the interferogram selection is based on the spatial coherence enables deformation mapping in vegetated areas and leads to refined estimates of along fault surface creep rates creep rates vary from 0 2 mm yr on the northern cf to 14 2 mm yr on the central cf south of the hf surface junction we estimate the long term slip rates by inverting the long wavelength deformation and the distribution of shallow slip due to creep by inverting the remaining velocity field this distribution of slip reveals the locations of locked and slowly creeping patches with potential for a m6 8 0 3 on the hf near san leandro a m6 6 0 2 on the northern cf near dublin a m6 5 0 1 on the hf south of fremont and a m6 2 0 2 on the central cf near morgan hill with cascading multisegment ruptures the hf rupturing from berkeley to the cf junction could produce a m6 9 0 1 the northern cf a m6 6 0 1 the central cf a m6 9 0 2 from the junction to gilroy and a joint rupture of the hf and central cf could produce a m7 1 0 1

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

resolution of fault slip along the 470 km long rupture of the great 1906 san francisco earthquake and its implications data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture because of varying network aperture and station density slip resolution is very uneven although slip is determined within uncertainties of 1 0 m along 60 of the fault constraints are poor on the remaining mostly offshore portions of the rupture slip decreases from maxima of 8 6 and 7 5 m at shelter cove and tomales bay to 4 5 m near mount tamalpais and 2 7 m at loma prieta the geodetically derived slip distribution is in poor agreement with estimates based on analysis of s wave seismograms probably because these waves register only 20 30 of the total seismic moment obtained from longer period surface waves consideration of a range of fault geometries for 1906 slip near loma prieta indicates right lateral motions lie between 2 3 and 3 1 m these values are considerably greater than the 1 5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based this factor along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts casts doubt on some claims of success in predicting the 1989 m 6 9 loma prieta earthquake

re evaluation of 1906 surface faulting geomorphic expression and seismic hazard along the san andreas fault in the southern santa cruz mountains the lack of through going right lateral surface rupture accompanying the loma prieta earthquake has raised questions about the behavior of the san andreas fault in the southern santa cruz mountains we re evaluated previously published reports of ground deformation due to the 1906 earthquake in this region to determine if san andreas fault surface rupture accompanied this event and whether any of the reported values of offset are reliable in addition our search for historical data has brought to light new information about the 1906 earthquake in the southern santa cruz mountains none of the reports provides indisputable evidence for 1906 san andreas fault surface rupture however taken together with a study of the geomorphology of the fault zone and relationships exposed in excavations across the fault they do provide a basis for believing that surface rupture extended through the region in 1906 none of the reported values for 1906 offset is a reliable measurement however calling into question current and pre 1989 seismic hazard assessments for the loma prieta area the geologic and geomorphic relationships indicate that shallow right lateral crustal strain is typically expressed as surface rupture across a narrow san andreas fault zone in contrast to the 1989 loma prieta earthquake

re evaluation of the 1836 hayward fault and the 1838 san andreas fault earthquakes abstract current seismic hazard models include two major earthquakes m 7 in the san francisco bay area that are close in space and time an 1836 event on the northern hayward fault and an 1838 event on the peninsula section of the san andreas fault analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of monterey bay far from the hayward fault and was of m 6 also the 1838 event was not confined to the 60 km peninsula san andreas as current models indicate instead faulting probably extended from san francisco to san juan bautista 140 km indicating a significantly larger earthquake m 7 than previously thought damaging effects of the 1836 earthquake were reported only from santa clara to carmel and no contemporary effects were reported to the north of santa clara or near the hayward fault the illusion of an 1836 hayward earthquake evolved from a newspaper reminiscence published following the 1868 hayward earthquake stating that the 1868 effects in the east bay were similar to those of an 1836 event the article describes various strong effects in the east bay that differ completely from the effects recorded for the 1836 earthquake but are very similar to those documented for the major 1838 san andreas earthquake that caused extensive damage on both sides of san francisco bay based on this and other evidence we conclude that the reminiscence describes the destructive june 1838 effects but it erroneously indicates the date as june 1836 there is no evidence for any major historical earthquakes in the san francisco bay area before the 1838 earthquake back to the founding of mission san francisco dolores in 1776 during the 1838 san andreas fault earthquake the shaking intensity in monterey was as strong as or stronger than during the great 1906 san andreas fault earthquake this suggests that the 1838 san andreas fault rupture may have extended to san juan bautista as it did in 1906 numerous probable aftershocks were felt in the area south of san juan bautista these damaged carmel and santa cruz in 1840 and alisal 16 km west of the san andreas fault in 1841 the northern end of the 1838 faulting was previously assumed to be 25 km south of san francisco however mission san francisco dolores was damaged in 1838 but not in 1906 suggesting that the 1838 faulting extended to san francisco also the 1838 aftershocks were felt in oakland as frequently and violently as those following the major 1868 hayward earthquake suggesting that the 1838 faulting on the san andreas extended to the latitude of oakland the 1838 fault segment ruptured again 68 years later as part of the overlapping 1906 san andreas fault rupture this and similar evidence from southern california indicates that m 7 san andreas fault earthquakes can recur at intervals of 68 years or less when they are followed by m 8 earthquakes on overlapping segments of the fault

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

time dependent model of creep on the hayward fault from joint inversion of 18 years of insar and surface creep data 1 spatial and temporal variations of aseismic fault creep influence the size and timing of large earthquakes along partially coupled faults to solve for a time dependent model of creep on the hayward fault we invert 18 years of surface deformation data 1992 2010 obtained by interferometric processing of 52 and 50 sar images acquired by the ers1 2 and envisat satellites respectively and surface creep data obtained at 19 alinement and 4 creepmeter stations for multi temporal analysis of the sar data we developed a method for identifying stable pixels using wavelet multi resolution analysis we also implement a variety of wavelet based filters for reducing the effects of environmental artifacts using a reweighted least squares approach we inverted the interferometric data to generate a time series of surface deformation over the san francisco bay area with a precision of better than a few millimeters to jointly invert the insar displacement time series and the surface creep data for a time dependent model of fault creep we use a robust inversion approach combined with a kalman filter the time dependent model constrains a zone of high slip deficit that may represent the locked rupture asperity of past and future m 7 earthquakes we identify several additional temporal variations in creep rate along the hayward fault the most important one being a zone of accelerating slip just northwest of the major locked zone we estimate that a slip rate deficit equivalent to mw 6 3 6 8 has accumulated on the fault since the last event in 1868

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

probabilistic seismic hazard analysis considering nonlinear site effect a modified method is proposed for the probabilistic seismic hazard analysis considering nonlinear site effect a case study is presented to show the hazard curves that consider the nonlinearity of the soil layers at gilroy 2 source parameters and an earthquake occurrence model are assigned to the caleveras fault which is assumed the dominant seismic source to the site a relationship between ground surface and bedrock accelerations at the site is established by recordings from three earthquake events using the program nonli3 which calculates the nonlinear responses of the soil layers to strong motions various values of standard errors of the estimated surface pga given the bedrock motions are assumed to perform a sensitivity analysis an alternative nonlinear relation is included in the case study in addition to the nonlinear models a linear response model is also performed using the same soil profile at gilroy 2 the results demonstrate that the hazard curves are sensitive to the standard error of the estimated surface pga values at the site the hazard curve may be severely distorted if the soil condition of a nonlinear site is treated inadequately the alternative nonlinear model obtained empirically from regression analysis and the linear response model are both unsuitable for the hazard assessment considering nonlinear site effect

evidence for surface rupture in 1868 on the hayward fault in north oakland and major rupturing in prehistoric earthquakes wgcep90 estimated the hayward fault to have a high probability 0 45 in 30 yr of producing a future m7 bay area earthquake this was based on a generic recurrence time and an unverified segmentation model because there were few direct observations for the southern fault and none for the northern hayward fault to better constrain recurrence and segmentation of the northern hayward fault we trenched in north oakland unexpectedly we observed evidence of surface rupture probably from the m7 1868 earthquake this extends the limit of that surface rupture 13 km north of the segmentation boundary used in the wgcep90 model and forces serious re evaluation of the current two segment paradigm although we found that major prehistoric ruptures have occurred here we could not radiocarbon date them however the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated ad 1640 1776

uncharacteristic earthquakes on the san andreas fault one model of earthquake recurrence holds that large quakes exhibit regular or characteristic rupture patterns combined with a simple picture of how elastic strain accumulates in the faults this implies that characteristic earthquakes occur at regular intervals in her perspective grant proposes that recent data from study sites along the san andreas fault are difficult to understand with this model the earthquake activity is more irregular and more clustered than the model would suggest

source study of the 1906 san francisco earthquake all quality teleseismic recordings of the great 1906 san francisco earthquake archived in the 1908 carnegie report by the state earthquake investigation commission were scanned and digitized first order results were obtained by comparing complexity and amplitudes of teleseismic waveforms from the 1906 earthquake with well calibrated similarly located more recent earthquakes 1979 coyote lake 1984 morgan hill and 1989 loma prieta earthquakes at nearly co located modern stations peak amplitude ratios for calibration events indicated that a localized moment release of about 1 to 1 5 10 27 dyne cm was responsible for producing the peak the teleseismic body wave arrivals at longer periods 50 to 80 sec we found spectral amplitude ratios of the surface waves require a total moment release between 4 and 6 10 27 dyne cm for the 1906 earthquake comparable to previous geodetic and surface wave estimates thatcher 1975 we then made a more detailed source analysis using morgan hill s body waves as empirical green s functions in a finite fault subevent summation the morgan hill earthquake was deemed most appropriate for this purpose as its mechanism is that of the 1906 earthquake in the central portion of the rupture from forward and inverse empirical summations of morgan hill green s functions we obtained a good fit to the best quality teleseismic waveforms with a relatively simple source model having two regions of localized strong radiation separated spatially by about 110 km assuming the 1906 epicenter determined by bolt 1968 this corresponds with a large asperity on the order of the loma prieta earthquake in the golden gate san francisco region and one about three times larger located northwest along strike between point reyes and fort ross this model implies that much of the 1906 rupture zone may have occurred with relatively little 10 to 20 sec radiation consideration of the amplitude and frequency content of the 1906 teleseismic data allowed us to estimate the scale length of the largest asperity to be less than about 40 km with rough constraints on the largest asperity size and magnitude we produced a suite of estimated synthetic ground velocities assuming a slip distribution similar to that of the loma prieta earthquake but with three times as much slip for purposes of comparison with the recent abundant loma prieta strong motion data set we moved the largest 1906 asperity into loma prieta region peak ground velocity amplitudes are substantially greater than those recorded during the loma prieta earthquake and are comparable to those predicted by the attenuation relationship of joyner and boore 1988 for a magnitude m w 7 7 earthquake

slip rate earthquake recurrence and seismogenic potential of the rodgers creek fault zone northern california initial results instrumental seismicity defines a seismic gap along the rodgers creek fault zone rcfz between santa rose and san pablo bay results of a paleoseismicity study within the gap using offset channels in late holocene alluvial deposits as piercing points indicate a minimum slip rate of 2 1 to 5 8 mm yr for the past 1 300 years a preferred range for the maximum recurrence interval of 248 to 679 years and a surface offset of 2 0 3 minus 0 2 m during the most recent event the rcfz has produced past m7 earthquakes and historical seismicity data indicate a minimum elapsed time of 182 years since the most recent earthquake of this size

slip in the 1868 hayward earthquake from the analysis of historical triangulation data despite the importance of the hayward fault for earthquake hazards in the san francisco bay area neither the rupture length nor the average coseismic slip in the most recent major earthquake which occurred in 1868 is well known we estimate slip in the 1868 earthquake by analyzing triangulation data collected between 1853 1860 and 1876 1891 we apply a denuisancing procedure that projects out the dependence of that data on station coordinates allowing their displacements to be estimated directly the best fit to the data is obtained with a 52 km long by 10 km deep rupture that extends from warm springs to near berkeley considerably longer than the 30 km of reported surface breakage the estimated slip for an earthquake of this length is 1 9 0 4 m and the seismic moment is m0 3 0 0 7 1019 n m corresponding to mw 7 0 the data are inconsistent with regionally high strain rate prior to the 1906 earthquake as had been previously suggested the triangulation data record northeastward displacement of station loma prieta presumably as a result of the 1865 earthquake the fault normal displacement is not easily explained by slip on either the san andreas fault or the 1989 loma prieta rupture surface this observation is however consistent with an mw 6 3 4 thrust event northeast of the san andreas fault in the southern santa cruz mountains thrust belt

long period seismicity at redoubt volcano alaska 1989 1990 related to magma degassing abstract the mass of exsolved magmatic h 2 o is estimated and compared to the mass of superheated steam 25 50 mtons released through the resonating crack producing the december 13 14 1989 swarm of long period seismic events at redoubt volcano results indicate degassing of a h 2 o co 2 so 2 saturated magma upon ascending from at least 12 km to 3 4 km beneath the crater as the source of the superheated steam the mass of exsolved h 2 o 3 2 250 mtons is estimated from solubility diagrams of h 2 o co 2 saturated silicate melts for the ascent history of the redoubt magmas crystal size distribution seismological petrological and geochemical data are used to constrain the ascent history of the two andesitic magmas prior to the eruption two stages of crystallization are inferred from crystal size distributions of plagioclase crystals in andesites erupted in december 1989 the first stage occurred 30 150 years before the eruption in both magmas and the second stage occurred at least 8 years and 15 years before the eruption in the dacitic andesite and rhyolitic andesite respectively the depths of crystallization are constrained from the spatial and temporal variations of volcano tectonic earthquakes locations lahr et al 1994 and from the p wave and s wave velocity structures benz et al 1996 these data suggest that the rhyolitic andesite magma ascended to a depth of 7 8 km within at least 15 years of the eruption within at least 8 years of the eruption the dacitic andesite magma migrated to a depth just below the other magma body where it resided until hours to days of the eruption at this time the dacitic andesite magma mixed with the rhyolitic andesite magma and established the reservoir for the eruption near the top of the reservoir some of the mixed magma was displaced into fractures which extended 4 5 km toward the surface this displaced magma created the eruption conduit and released the fluids related to the resonating crack this scenario is consistent with the trends in major and trace element chemistry and the stability of hornblende in the pre eruption redoubt magmas it also provides a source for the so 2 and co 2 emissions measured during the eruption

timing of late holocene paleoearthquakes on the northern san andreas fault at the fort ross orchard site sonoma county california paleoseismic trenching within fort ross state historic park provides data on the late holocene rupture history of the north coast segment of the northern san andreas fault the 1906 earthquake ruptured through the fort ross orchard site which is characterized by a narrow shutter ridge and associated linear trough containing latest holocene sediments trenches across the northeast facing fault scarp exposed sediments interpreted as scarp derived colluvium and possible fissure fill deposits and tentative upward fault truncations that provide evidence of three possible surface ruptures prior to 1906 coarse grained scarp derived colluvial sediments were deposited after individual surface rupturing earthquakes that predate the 1906 rupture radiocarbon analyses of 31 detrital radiocarbon samples collected from the colluvial deposits constrain the timing of earthquakes over the past approximately 1000 years based on stratigraphic ordering and a statistical comparison of radiocarbon dates using the oxcal program we estimate at a 95 confidence level that three pre 1906 surface ruptures at the orchard site occurred at a d 1660 1812 a d 1220 1380 and a d 1040 1190 previous trenches at the nearby fort ross archae camp site are consistent with these dates and further suggest the occurrence of an earlier event between a d 555 and 950 collectively the fort ross orchard and archae camp sites suggest pre 1906 ruptures at a d 1660 1812 a d 1220 1380 a d 1040 1190 and a d 555 950 the time windows for these ruptures are consistent with results from other sites on the north coast segment of the san andreas fault however additional information on the late holocene history of rupture events on adjacent fault segments is needed to evaluate whether the long term behavior of the san andreas fault involves a mix of large 1906 type ruptures and shorter segment specific ruptures

gravity magnetic and high precision relocated seismicity profiles suggest a connection between the hayward and calaveras faults northern california 1 gravity magnetic and seismicity data profiled across the hayward fault zone were generated as part of ongoing studies to help determine the geologic and tectonic setting of the san francisco bay region these data combined with previous geophysical studies that indicate that the hayward fault zone dips 75 ne near san leandro and follows a preexisting structure reveal a possible direct connection between the seismogenic portion of the hayward and calaveras faults at depth although the relocated seismicity data are regional in nature they suggest that the dip of the hayward fault zone may vary from near vertical in the northwestern part of the fault to about 75 ne at san leandro in the central part of the fault to about 50 ne in the southeastern part of the fault gravity and magnetic data profiled across the hayward fault zone were processed using standard geophysical techniques cross sections of high precision relocated hypocenters were constructed along each profile from the northwestern to the southeastern end of the hayward fault zone profiles and cross sections are referenced to pinole point where the hayward fault enters san pablo bay and are spaced 2 5 km apart topographic profiles shown on the seismicity cross sections were generated using u s geological survey usgs 7 5 min 30 m digital elevation models relocation of seismicity data was accomplished using a regional double difference method the double difference method incorporates ordinary travel time measurements and cross correlation of p and s wave differential travel time measurements relative locations between earthquakes have hypocentral errors of about 100 m horizontally and 250 m vertically absolute location uncertainties were not determined but are probably dramatically improved compared to the usgs s northern california seismic network catalog data

re evaluation of several large historic earthquakes in the vicinity of the loma prieta and peninsular segments of the san andreas fault california abstract the size and location of several of the largest 19th century earthquakes in the san francisco bay area including the june 1838 8 october 1865 21 october 1868 and 24 april 1890 events are re evaluated by comparing these historic events to several modern events in particular the 1989 loma prieta earthquake similar high intensities in the monterey bay area during both the 1989 and 1838 events indicate that the 1838 shock probably ruptured the loma prieta segment in addition to the peninsular segment of the san andreas fault if so the resulting earthquake would have been of moment magnitude m w 7 2 thus like segments of many other plate boundaries the loma prieta segment exhibits heterogeneous rupture in that sometimes it ruptures with the peninsular segment as in 1906 and probably in 1838 and at other times it ruptures alone as in 1989 in contrast the area of high intensities generated by the 1865 earthquake was centered in the santa clara valley and the eastern foothills of the santa cruz mountains indicating that the 1865 shock probably was located northeast of the loma prieta rupture and not on the san andreas fault in addition the felt areas taken to be one of the best measures of relative source size of modified mercalli intensities mmi v and vi for the 1865 shock are considerably smaller than that of the 1989 event relationships between felt areas of intensities v and vi and moment magnitude are developed for the san francisco bay area based on these relationships the average computed moment magnitudes for the 1865 1868 and 1890 earthquakes are 6 51 0 18 6 84 0 20 and 6 32 0 16 respectively if these results are correct then the loma prieta segment taken to include all faults within the san andreas fault zone i e within a zone up to a few kilometers wide appears to rupture with or without the peninsular segment every 76 11 years these data on repeat times may be the most accurate information now available for ascertaining the date of future rupture of that segment and for testing recurrence models the smaller size of the 1865 earthquake would be consistent with it being part of the long term seismic build up to the 1868 event rather than being paired with it as events of similar size if the 1865 earthquake did occur northeast of the loma prieta segment then a future source of that type located closer to san jose and the santa clara valley could pose a considerable hazard to the southern san francisco bay area on going searches of letters and diaries of missionaries and early settlers could yield new information that should help to further constrain the locations and magnitudes of these and other historic earthquakes