a rupture model for the 28 june 1992 landers california earthquake we have modeled displacement time histories to infer the large scale rupture process of the 28 june 1992 m 7 4 landers california earthquake we have used integrated accelerograms from four terrascope stations at distances between 70 and 150 km the earthquake process consists of unilateral rupture propagation 3 km s on two distinct segments with different strikes n10 w and n40 w the two segments are 20 and 30 km long with constant slip of 3 5 m and 5 2 m respectively the excitation of surface waves resulting from a low velocity surficial layer plays a critical role in matching the synthetic waveforms to the observed displacements the displacements due to both body waves and surface waves are strongly affected by directivity matching the synthetics to the data requires a one second delay between the end of rupture on the first segment and the initiation of rupture on the second segment the seismic moment of the subevents are 2 8 1019 n m and 8 4 1019 n m leading to a total moment of 1 12 1020 n m m 7 4

rupture process of the 1987 superstition hills earthquake from the inversion of strong motion data a pair of significant earthquakes occurred on conjugate faults in the western imperial valley involving the through going superstition hills fault and the elmore ranch cross fault the first event was located on the elmore ranch fault m s 6 2 and the larger event on the superstition hills fault m s 6 6 the latter event is seen as a doublet teleseismically with the amplitudes in the ratio of 1 2 and delayed by about 8 sec this 8 sec delay is also seen in about a dozen strong motion records these strong motion records are used in a constrained least squares inversion scheme to determine the distribution of slip on a 2 d fault upon closer examination the first of the doublets was found to be itself complex requiring two episodes of slip thus the rupture model was allowed to have three separate subevents treated as separate ruptures with independent locations and start times the best fits were obtained when all three events initiated at the northwestern end of the fault near the intersection of the cross fault their respective delays are 2 1 and 8 6 sec relative to the first subevent and their moments are 0 4 0 9 and 3 5 10 25 dyne cm which is about half of that seen teleseismically this slip distribution suggests multi rupturing of a single asperity with stress drops of 60 200 and 15 bars respectively the first two subevents were confined to a small area around the epicenter while the third propagated 18 km southwestward compatible with the teleseismic and afterslip observations

southern surface rupture associated with the 1992 m7 4 landers earthquake did it all happen during the mainshock approximately three minutes after the magnitude 7 4 landers mainshock on 28 june 1992 a m5 7 aftershock occurred south of the mainshock epicenter with a location of 34 7 65 n 116 23 82 w this aftershock was recorded on an array of portable digital seismic instruments deployed in morongo valley 21 km southwest of the event although peak accelerations are found to differ by approximately 50 at stations with similar general site conditions within 500 m of each other there is good coherence of arrivals across the array for frequencies 1 hz we use the recordings to determine the apparent phase velocity and azimuth of propagation across the array and show that the event clearly ruptured to the south with a rupture length of 11 km and a rupture velocity of approximately 3 0 km s our results suggest that at least some of the mapped surface rupture south of the town of yucca valley 11 km in extent maximum displacement of 20 cm may have been associated with this aftershock if this is the case then the fault that produced the southern end of the landers mainshock the johnson valley fault need not continue at depth across the active left lateral east west trending pinto mountain fault

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

empirical green s function analysis of recent moderate events in california i use seismic data from portable digital stations and the broadband terrascope network in southern california to investigate radiated earthquake source spectra and discuss the results in light of previous studies on both static stress drop and apparent stress applying the empirical green s function egf method to two sets of m 4 6 1 events i obtain deconvolved source spectra estimates and corner frequencies the results are consistent with an 2 source model and constant brune stress drop however consideration of the raw spectral shapes of the largest events provides evidence for a high frequency decay more shallow than 2 the intermediate f 1 slope cannot be explained plausibly with attenuation or site effects and is qualitatively consistent with a model incorporating directivity effects and a fractional stress drop rupture process as suggested by haddon 1996 however the results obtained in this study are not consistent with the model of haddon 1996 in that the intermediate slope is not revealed with egf analysis this could reflect either bandwidth limitations inherent in egf analysis or perhaps a rupture process that is not self similar i show that a model with an intermediate spectral decay can also reconcile the apparent discrepancy between the scaling of static stress drop and that of apparent stress drop for moderate to large events

analysis of broadband records from the 28 june 1992 big bear earthquake evidence of a multiple event source the 28 june 1992 big bear earthquake occurred at 15 05 21 gmt and is considered to be an aftershock of the earlier m w 7 3 landers earthquake from overall aftershock locations and long period focal studies rupture is generally assumed to have propagated northeast no surface rupture was found however and the mainshock locations determined from both strong motion and terrascope data are mutually consistent and do not lie on the assumed fault plane further directivity analysis of records from the terrascope array suggests significant short and long period energy propagating northwest along the presumed antithetic fault plane this observation is supported by significant early aftershocks distributed along both the presumed rupture plane and the antithetic plane to the northwest an empirical green s function egf approach using both the m w 5 2 28 june 1992 14 43 gmt foreshock and the m w 5 0 17 august 1992 aftershock produces consistent results and suggests that the big bear event comprised at least two substantial subevents from the egf results we infer that the second and possibly a third subevent occurred on the presumed northeast striking mainshock rupture surface but that significant moment release occurred on the antithetic northwest striking surface we present results from line source fault modeling of broadband displacement recordings of the big bear mainshock which indicate that a two fault event is necessary to produce the observed waveforms the limitations imposed by the mainshock location and directivity analysis require that the initial rupture be towards the northwest striking 320 this was followed approximately 4 sec later by bilateral rupture along a northeast southwest fault that strikes 50 east of north

observations of loma prieta aftershocks from a dense array in sunnyvale california a small aperture 300 m four station array was deployed in sunnyvale california for 5 days to record aftershocks of the loma prieta earthquake of october 1989 the purpose of the array was to study the seismic response of the alluvium filled santa clara valley and the role of surface waves in the seismic shaking of sedimentary basins strong motion records of the loma prieta mainshock indicate that surface waves produced the peak velocities and displacements at some sites in the santa clara valley we use the recordings from the dense array to determine the apparent velocity and azimuth of propagation for various arrivals in the seismograms of four aftershocks with magnitudes between 3 6 and 4 4 apparent velocities are generally observed to decrease with increasing time after the s wave in the seismograms phases arriving less than about 8 sec after the s wave have apparent velocities comparable to the s wave and appear to be body waves multiply reflected under the receiver site or reflected by crustal interfaces for times 10 to 30 sec after the direct s wave we observe long period 1 to 6 sec arrivals with apparent velocities decreasing from 2 5 to 0 8 km sec we interpret these arrivals to be surface waves and conclude that these surface waves produce the long duration of shaking observed on the aftershock records much of the energy in the 40 sec after the s wave is coming approximately from the direction of the source although some arrivals have backazimuths as much as 60 different from the backazimuths to the epicenters two of the aftershocks show arrivals coming from 30 to 40 more easterly than the epicenters this energy may have been scattered from outcrops along the southeastern edge of the basin in contrast the deepest aftershock studied d 17 km displays later arrivals with backazimuths 30 to 40 more westerly than the epicenter a distinct arrival for one of the aftershocks propagates from the southwest possibly scattered from the western edge of the basin synthetic seismograms derived from a plane layered crustal model do not produce the long period love waves observed in the waveforms of the ml 4 4 aftershock these love waves may be generated by the conversion of incident s waves or rayleigh waves near the edge of the basin

shallowly driven fluctuations in lava lake outgassing gas pistoning kīlauea volcano abstract lava lakes provide ideal venues for directly observing and understanding the nature of outgassing in basaltic magmatic systems k lauea volcano s summit lava lake has persisted for several years during which seismic and infrasonic tremor amplitudes have exhibited episodic behavior associated with a rise and fall of the lava surface gas pistoning since 2010 the outgassing regime of the lake has been tied to the presence or absence of gas pistoning during normal behavior no gas pistoning the lake is in a spattering regime consisting of higher tremor amplitudes and gas emissions in comparison gas piston events are associated with an abrupt rise in lava level up to 20 m during which the lake enters a non spattering regime with greatly decreased tremor and gas emissions we study this episodic behavior using long term multidisciplinary monitoring of the lake including seismicity infrasound gas emission and geochemistry and time lapse camera observations the non spattering regime i e rise phase of a gas piston cycle reflects gas bubbles accumulating near the top of the lake perhaps as a shallow foam while spattering regimes represent more efficient decoupling of gas from the lake we speculate that the gas pistoning might be controlled by time varying porosity and or permeability in the upper portions of the lava lake which may modulate foam formation and collapse competing models for gas pistoning such as deeply sourced gas slugs or dynamic pressure balances are not consistent with our observations unlike other lava lakes which have cyclic behavior that is thought to be controlled by deeply sourced processes external to the lake itself we show an example of lava lake fluctuations driven by cycles of activity at shallow depth and close to the lake s surface these observations highlight the complex and unsteady nature of outgassing from basaltic magmatic systems

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

high latitude plasma outflow as measured by the dmsp spacecraft 1 we have examined the vertical ion flux in the topside high latitude ionosphere from measurements of the vertical ion drift and ion number density made by the dmsp f13 satellite we found that the average vertical flux over the entire high latitude region near 800 km altitude is quite small this suggests that most of the vertical ion flux is associated with a large scale breathing of the upper ionosphere in the polar cap the vertical ion flux is uniformly downward at all locations however in the auroral zone the ion flux is highly structured and a net upward flux is produced only by spatially and temporally confined events containing upward fluxes in excess of 1010 cm 2 s 1 that have no downward counterparts

determination of earthquake energy release and ml using terrascope we estimated the energy radiated by earthquakes in southern california using on scale very broadband recordings from terrascope the method we used involves time integration of the squared ground motion velocity and empirical determination of the distance attenuation function and the station corrections the time integral is typically taken over a duration of 2 min after the p wave arrival the attenuation curve for the energy integral we obtained is given by q r cr n exp kr r 2 2 h ref 2 with c 0 49710 n 1 0322 k 0 0035 km 1 and h ref 8 km where is the epicentral distance a similar method was used to determine m l using terrascope data the station corrections for m l are determined such that the m l values determined from terrascope agree with those from the traditional optical wood anderson seismographs for 1 5 6 5 m l saturates the ratio e s m 0 m 0 seismic moment a measure of the average stress drop for six earthquakes the 1989 montebello earthquake m l 4 6 the 1989 pasadena earthquake m l 4 9 the 1990 upland earthquake m l 5 2 the 1991 sierra madre earthquake m l 5 8 the 1992 joshua tree earthquake m l 6 1 and the 1992 landers earthquake m w 7 3 are about 10 times larger than those of the others that include the aftershocks of the 1987 whittier narrows earthquake the sierra madre earthquake the joshua tree earthquake and the two earthquakes on the san jacinto fault the difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times the aftershocks which occurred on the fault plane where the mainshock slippage occurred had a very short time to heal hence a low stress drop the repeat time of the major earthquakes on the frontal fault systems in the transverse ranges in southern california is believed to be very long a few thousand years hence the events in the transverse ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times such as the san andreas fault and the san jacinto fault the observation that very high stress drop events occur in the transverse ranges and the los angeles basin has important implications for the regional seismic potential the occurrence of these high stress drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events characterization of earthquakes in terms of the e s m 0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the los angeles basin and the transverse ranges

an analysis of the 1985 nahanni earthquakes abstract two large magnitude earthquakes occurred in 1985 near the north nahanni river in the northeast cordillera northwest territories canada on 5 october m s 6 6 and 23 december m s 6 9 these events were unprecedented for this region of the cordillera and the december event is the largest earthquake ever known in the eastern part of the canadian cordillera this report presents detailed studies of the nahanni earthquakes their source parameters macroseismic effects aftershock sequences triggered rock avalanches their relationship to the geology of the epicentral area and their seismic hazard implications parameters calculated here for these two major events are parentheses indicate other studies the earthquakes occurred within a small relatively undeformed plateau the mackenzie plain in the foreland fold belt created along the northeastern cordillera during the columbian or laramide orogeny that fold belt is now subjected to northeast southwest horizontal compressive stresses possibly very similar to the original laramide stress regime and the fault plane implied by the seismic mechanisms is very similar to the laramide faults in the area however no surface fault break has been found and none of the mapped laramide faults in the epicentral area appears to be involved in the aftershock activity it is thus believed that the nahanni earthquakes occurred on a blind laramide thrust fault under the mackenzie plain the large magnitudes of the nahanni earthquakes show that the present seismic zoning for building code purposes in the eastern cordillera north of 60 n needs reassessment and suggest that the zoning in the eastern cordillera in british columbia and alberta may also need revision the felt areas of the earthquakes show that regional anelastic attenuation in the eastern canadian cordillera is significantly anisotropic and much lower along strike than across strike the felt areas also appear to scale with seismic moment rather than m b magnitude the thrust mechanisms regional stress regime shallow focal depth and high velocity sedimentary rocks in the focal region suggest that the nahanni earthquakes and particularly the near field strong ground motion recorded for the second event are ideal engineering design data for critical facilities in eastern north america

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

probability of multifault rupture abstract earthquake hazard modelers often assume that earthquake ruptures will be confined to individual faults and that earthquakes magnitudes will not exceed values related to the mapped fault length however earthquakes have been observed to jump from one fault to another producing a rupture longer than the originating fault the magnitudes of these earthquakes can exceed the value based on the originating fault length we estimated the probability of jumping as a function of interfault distance based on retrospective observations from past earthquakes a simple model consistent with the few available observations suggests that an earthquake on one strike slip fault may jump to another with a probability that decreases exponentially with separation the scale distance is about 3 km the model implies a nonnegligible probability of jumping as much as 10 km many california faults are within 10 km of one another we identified those and wrote a simple program to calculate the interfault distance and thus the jump probabilities we also formulated a simple way to estimate the magnitude frequency distribution for a pair of nearby faults as a function of their independent magnitude distributions and their separation distance allowing for multifault rupture increases the estimated upper magnitude limits for faults relative to a model in which faults are independent if the magnitude frequency distribution is constrained to be consistent with a fixed moment rate from fault slip rates for example then increasing the upper magnitude limits will reduce the rate of intermediate and small events

a three dimensional simulation of seismic waves in the santa clara valley california from a loma prieta aftershock abstract the finite difference method is used to propagate elastic waves through a 3 d model of the santa clara valley an alluvium filled basin that underlies the city of san jose california the model was based on depth to bedrock information from water wells in the area the simulation corresponded to a region 30 east west by 22 north south by 6 depth km and contained about 4 million grid points synthetic seismograms from the simulation are accurate at frequencies up to 1 hz motions from a magnitude 4 4 aftershock of the loma prieta earthquake were modeled snapshots of ground motion and synthetic seismograms from the simulation are presented the simulation illustrates s to surface wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics because of the slow group velocity of the love waves sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges sites near the center of the basin also show larger peak amplitudes on the transverse component array analysis of the synthetic seismograms indicates that love waves tend to propagate parallel to the eastern and western edges of the valley rayleigh waves are produced along the southern margin of the basin from incident s waves large radial motions occur where a rayleigh wave impinges on the northeast margin of the valley some rayleigh waves travel westward across the basin after being scattered from the eastern edge of the valley synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the sunnyvale dense array for this aftershock although the duration of the tranverse component is not matched by the synthetic seismogram using this basin model the simulation indicates that the love waves observed on the actual seismograms were produced by conversion of incident s waves at the southern margin of the santa clara valley 2 d simulations show how the s to love wave conversion is affected by the angle of incidence of the s wave and the sharpness of the velocity transition between the alluvium and bedrock as more accurate basin models are developed 3 d simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes

southern surface rupture associated with the m 7 3 1992 landers california earthquake although most evidence suggests that the 28 june 1992 m 7 3 landers earthquake ruptured unilaterally north significant surface rupture was mapped on the eureka peak and burnt mountain faults to the south of the landers epicenter an eyewitness account reports that surface rupture occurred on the northern eureka peak fault within approximately 35 sec of the mainshock initiation array analysis of the landers mainshock provides evidence in support of this report a significant southern subevent in the early mainshock coda i also analyze dense array recordings of a m 5 6 aftershock that occurred 3 min after the mainshock at 34 7 65 n 116 23 82 w and show that there is strong evidence that this event was also associated with significant rupture on the eureka peak fault this analysis thus suggests that the eureka peak fault rupture was not caused by direct bilateral mainshock rupture but instead was initially triggered less than a minute after the mainshock and reruptured by the m 5 6 aftershock results for the evolution of the landers sequence suggest that mainshock subevents may in some cases be accurately described as aftershocks i e disjoint triggered events that occur within the duration of mainshock strong ground motion

teleseismic body wave analysis of the 1988 armenian earthquake long period and broadband body waves from 14 digital seismic stations are used to investigate the rupture process of the december 7 1988 earthquake near spitak armenia ussr the inversion of these data gives the following centroidal source parameters strike 299 degree dip 64 degree rake 151 degree depth 6 3 km and seismic moment 1 5 times 10 sup 19 nm indicating that on average the earthquake had a strike slip mechanism with a substantial reverse component the mechanism with a substantial reverse component the broadband waveforms however show significant complexity they are best fit with a source model that includes three sub events very similar in size but with distinct focal mechanisms and locations rupture apparently initiated as a shallow reverse fault at a point of maximum bending on a right lateral strike slip fault and then extended bilaterally first towards the southeast and then towards the west this interpretation agrees with the aftershock distribution and fault lineations observed on landsat images

slip distribution of the 1992 landers earthquake and its implications for earthquake source mechanics we use near source 10 to 164 km displacement seismograms to image the slip distribution and rupture history of the 28 june 1992 mw 7 3 landers california earthquake aftershock seismograms from similar distances are modeled to find the velocity model and frequency range 0 05 to 0 25 hz over which theoretical green s functions are most accurate and the measure of fit is used as an upper bound on theoretical error in the mainshock inversion we represent the rupture surface with three planar segments divided into 3 by 3 km elements extending from the surface to 18 km depth and solve for the slip distribution and the rupture time model that minimizes misfit to both recorded seismograms and mapped surface displacement in a least squares sense we investigate two faulting models one with a uniformly short 3 sec rise time everywhere on the fault surface and the other with a variable rise time 2 to 6 sec we perform sensitivity tests using synthetic data from a hypothetical earthquake to evaluate model resolution and solution stability in the sensitivity tests both fault models recover similar slip and rupture features but neither is capable of imaging details of the rise time for the landers earthquake we find an average rupture velocity of 2 5 km sec and use this average for a starting model in a linearized inversion for rupture time in our solutions of slip amplitude distribution the southernmost johnson valley fault segment has 20 of the total seismic moment 6 to 8 1019 n m with small displacements near the hypocenter the homestead valley segment contributes half of the moment with the largest slip amplitudes 25 to 35 km northwest of the hypocenter at 4 to 12 km depth and the camp rock emerson segment contributes the remaining moment with the largest slip amplitude 35 to 50 km northwest of the hypocenter in the shallow crust 9 km there is some evidence that the rupture front is delayed as it encounters high slip regions suggesting that prior to the mainshock these areas were further from failure owing either to greater strength or lower prestress

long dormancy low slip rate and similar slip per event for the emerson fault eastern california shear zone excavations in a playa along the 1992 rupture of the emerson fault reveal evidence of two paleoseismic events with only one large prehistoric rupture in the past 15 millennia accelerator mass spectrometer radiocarbon ages of charcoal from playa sediments and from fault scarp colluvium directly beneath the playa beds indicate that the last large prehistoric slip event occurred about 9000 ka trench wall exposures revealed clear evidence of at least one pre 9 ka rupture at the playa site the event horizon of this earthquake is between two pedogenic carbonate layers that have radiocarbon ages of 14 8 ka and 24 1 ka implying that the earthquake occurred about 20 000 years b p the actual bracketing ages for this rupture are likely a few thousand years older because of the mean residence time for the pedogenic carbonate and calibration of the 14c ages by 230th dating despite the large uncertainties a dormant period of at least 6 kyr to as much as 13 kyr separates the older event from the 9 ka event because the scarp formed by the penultimate event is similar in height to the scarp formed by the 1992 mw 7 3 landers earthquake the penultimate rupture was at least locally similar in size to the most recent rupture this similarity supports the concept of characteristic slip for the emerson fault preliminary results from paleoseismic studies at other sites on the 1992 rupture suggest that large ruptures occurred on other nearby faults within a few hundred years of this penultimate event on the emerson fault the interseismic period that preceded the 1992 earthquake on the emerson fault was about 40 times longer than the average interval between large events on the nearby san andreas fault therefore in comparison to events on the san andreas fault the 1992 landers earthquake was an exceedingly rare event

seismic stereometry reveals preparatory behavior and source kinematics of intermediate size earthquakes although moderate size earthquakes are poorly studied by lack of near fault observations they can provide key information about larger damaging earthquakes here we propose a new approach inspired by double difference relocation that uses high coherency waveforms recorded at neighboring sensors to study the preparation phase and dynamics of moderate size earthquakes we validate this technique by analyzing the 2016 m w 5 2 borrego springs earthquake in southern california and find consistent rupture velocities of 2 km s highlighting two main rupture asperities the analysis of the 2019 ml5 2 le teil earthquake in france reveals slow nucleation at depth that migrates to the surface and propagates northward with a velocity of 2 8 km s highlighting two main rupture events also imaged by insar by providing unprecedented resolution in our observation of the rupture dynamics this approach will be useful in better understanding the preparation phase and rupture of both tectonic and induced earthquakes plain language summary small and moderate size earthquakes are much more numerous yet studied in much less detail than large and damaging earthquakes this is because to study small earthquakes precisely one needs seismometers very close to the rupture which is rarely the case it is still unclear if small and large earthquakes start the same way thus studying more systematically small earthquakes could help to answer this question here we show that we can use two close by seismometers as we would do with 3 d glasses to observe with a stereoscopic effect the evolution of intermediate size earthquakes when they rupture along a fault we can retrieve the direction of rupture the speed at which the fault breaks and the length of the rupture important parameters that help to characterize the earthquakes using this new method we can sometimes also observe the behavior of the fault before the main rupture which will be useful to better understand how earthquakes start

the temporal and spatial evolution of the 19 september 1985 michoacan earthquake as inferred from near source ground motion records abstract a frequency domain inversion technique is employed to image the rupture process of the 19 september 1985 michoacan mexico earthquake a linear system is derived from the elastodynamic representation theorem and is solved for the spatial slip velocity distribution in the frequency bandwidth 0 0 to 0 32 hz the temporal behavior of slip velocity on the fault plane is then recovered by fourier synthesis model complexity is penalized by seeking a solution that propagates with the most nearly constant rupture velocity and that produces the most nearly constant spatial slip distribution at 0 hz the inversion of surface static displacements as inferred from strong motion recordings of the guerrero accelerograph array gives an estimate of the static offset on the fault plane coastal uplift data inferred from the mortality of intertidal organisms bodin and klinger 1986 is also used to further constrain the spatial slip distribution a low frequency image of the rupture process of the earthquake has been obtained using full space green9s functions two areas of large slip amplitude are discernible one located downdip of the reported hypocenter and the other updip of the projection on the fault plane of station la union their separation is approximately 100 km assuming that full space green9s functions underestimate static offset values at the surface of a half space by a factor of 0 5 the inferred seismic moment is 10 6 10 27 dyne cm 90 per cent of which is released in these two areas sustaining the largest slip the total inferred rupture duration is approximately 56 sec from the hypocenter the rupture front proceeds with an average velocity estimated at 2 8 km sec there are indications of bilateral propagation both from the hypocentral area and from an area surrounding the projection on the fault plane of the strong motion station la union an upper bound on rupture duration at the two principal subevents is 8 sec

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults