the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

strain partitioning and stress rotation at the north anatolian fault zone from aftershock focal mechanisms of the 1999 izmit mw 7 4 earthquake summary we investigate aftershock focal mechanisms of the m w 7 4 izmit earthquake of 1999 august 17 on the western north anatolian fault zone nafz spatial clustering and the orientation of 446 fault plane solutions are analysed the izmit mainshock occurred as a right lateral slip on an ew trending near vertical fault plane aftershock clusters define four individual fault segments focal mechanisms surrounding the epicentres of the izmit and subsequent duzce mainshock m w 7 1 1999 november 12 indicate predominantly strike slip but also normal faulting aftershocks in the area between the izmit and duzce segments are mainly related to ew oriented normal faulting delineating a small pull apart structure beneath the easternmost sea of marmara alignments of aftershocks suggest branching of the nafz into three or more active segments that differ significantly in terms of their focal mechanism characteristics the distribution of aftershock focal mechanisms corresponds to fault segmentation of the nafz in the izmit duzce region produced by coseismic slip areas with large amounts of coseismic slip show aftershocks that are predominantly strike slip but low slip barriers show mostly normal faulting aftershocks stress tensor inversions of the aftershock focal mechanisms show rotations of the local stresses following the izmit mainshock in the izmit sapanca area the maximum horizontal compressive stress axis is horizontally rotated counter clockwise by 8 with respect to the coseismic and long term regional stress field towards the eastern end of the rupture karadere duzce area stresses are rotated clockwise we conclude that the izmit earthquake caused significant stress partitioning along the rupture the direction of stress rotation is related to the orientation of the individual fault segments along the nafz

seismotectonics of sierra pie de palo a basement block uplift in the andean foreland of argentina we operated a digitally recording seismic network from september 1987 to may 1988 in the san juan province of northwestern argentina the network was located in the thin skinned precordillera and thick skinned sierras pampeanas tectonic provinces of the andean foreland our data provide a detailed view of the crustal seismicity of sierra pie de palo one of the most seismically active mountain blocks of the sierra pampeanas and the site of the destructive m 7 3 caucete earthquake in 1977 most of the well located shallow earthquakes we recorded occurred directly beneath the range between 9 and 30 km depth with a maximum activity around 25 km depth the morphology of the range seismicity pattern and focal mechanisms strongly suggest sierra pie de palo is cut into two blocks by a northeast southwest striking fracture in the northern block seismicity occurs along a well defined west dipping seismic zone while the southern block is characterized by east dipping seismic layers both dipping zones connect to a subhorizontal mid crustal detachment that we interpret as the lower boundary of the brittle failure domain the complex fault geometry and large variety of focal mechanism orientations we observe suggest that the ongoing seismic activity is a continuation of the aftershock sequence of the 1977 caucete earthquake we therefore interpret the two principal dipping zones to be buried thrusts activated during the complex rupture of the 1977 earthquake inversion of the focal mechanism data for the orientation of the regional deviatoric stress tensor shows the maximum compressive stress u oriented at an azimuth of n107 e this direction is approximately orthogonal to the trend of the sierra but rotated 27 clockwise with respect to the current nazca south america convergence direction this result suggests that local boundary effects within the sierras pampeanas province partially control the shortening direction in the crust it may also indicate that a component of sinistral strike slip motion exits within the precordillera to the west of the sierras pampeanas

homogeneity of small scale earthquake faulting stress and fault strength small scale faulting at seismogenic depths in the crust appears to be more homogeneous than previously thought i study three new high quality focal mechanism datasets of small m 3 earthquakes in southern california the east san francisco bay and the aftershock sequence of the 1989 loma prieta earthquake i quantify the degree of mechanism variability on a range of length scales by comparing the hypocentral distance between every pair of events and the angular difference between their focal mechanisms closely spaced earthquakes interhypocentral distance 2 km tend to have very similar focal mechanisms often identical to within the 1 sigma uncertainty of 25 this observed similarity implies that in small volumes of crust while faults of many orientations may or may not be present only similarly oriented fault planes produce earthquakes contemporaneously on these short length scales the crustal stress orientation and fault strength coefficient of friction are inferred to be homogeneous as well to produce such similar earthquakes over larger length scales 2 50 km focal mechanisms become more diverse with increasing interhypocentral distance differing on average by 40 70 mechanism variability on 2 to 50 km length scales can be explained by relatively small variations 30 in stress or fault strength it is possible that most of this small apparent heterogeneity in stress or strength comes from measurement error in the focal mechanisms as negligible variation in stress or fault strength 10 is needed if each earthquake is assigned the optimally oriented focal mechanism within the 1 sigma confidence region this local homogeneity in stress orientation and fault strength is encouraging implying it may be possible to measure these parameters with enough precision to be useful in studying and modeling large earthquakes

spatially heterogeneous stress field in the source area of the 2011 mw 6 6 fukushima hamadori earthquake ne japan probably caused by static stress change

seismotectonics of the loma prieta california region determined from three dimensional v p v p v s and seismicity three dimensional vp and vp vs velocity models for the loma prieta region were developed from the inversion of local travel time data 21 925 p arrivals and 1 116 s arrivals from earthquakes refraction shots and blasts recorded on 1700 stations from the northern california seismic network and numerous portable seismograph deployments the velocity and density models and microearthquake hypocenters reveal a complex structure that includes a san andreas fault extending to the base of the seismogenic layer a body with high vp extends the length of the rupture and fills the 5 km wide volume between the loma prieta mainshock rupture and the san andreas and sargent faults we suggest that this body controls both the pattern of background seismicity on the san andreas and sargent faults and the extent of rupture during the mainshock thus explaining how the background seismicity outlined the along strike and depth extent of the mainshock rupture on a different fault plane 5 km away new aftershock focal mechanisms based on three dimensional ray tracing through the velocity model support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression the subvertical or steeply dipping san andreas fault and the fault surfaces that ruptured in the 1989 loma prieta earthquake are both parts of the san andreas fault zone and this section of the fault zone does not have a single type of characteristic event

stress field fluctuations along the dead sea rift since the middle miocene the dead sea transform dst is the suture between the african and the arabian plates both of which subduct northward under the eurasian plate the collision of these plates with eurasia and their relative motion are the source for their internal deformation especially in the vicinity of the dead sea transform geological observations indicate that two distinct paleostrain regimes operated adjacent to the dead sea rift and in the sinai israel subplate 1 wnw shortening less than 10 and nne extension beginning in the turonian is associated with the development of the syrian arc fold belt and attributed to the syrian arc stress field sas and 2 middle miocene to recent nnw shortening and ene extension associated with the 105 km sinistral displacement along the dead sea transform and the opening of the red sea is attributed to the dead sea stress field dss it was originally suggested that the sas terminated during the middle miocene with initiation of the dss however trend and age analysis of many structures shows that formation of sas compatible structures continued after the middle miocene up to the recent in some middle miocene and younger rocks structures such as faults dikes volcanic lineaments and tectonic stylolites compatible with both stress fields were reported in a few locations structures compatible with both stress fields crosscut each other it is suggested that the movements which resulted in the development of the syrian arc and other sas compatible structures are continuing to the recent and that dss movements are superimposed upon them the overlapping of sas and dss structures adjacent to the dst results from spatial and temporal fluctuations in the overall stress state we propose that such variations could be caused by the superposition of large earthquake stress drops associated with movements along the dst on a steady background plate scale stress regime associated with the sas thus dss compatible structures should form in preseismic activity periods when dst related elastic strain is high on the other hand the sas compatible structures should form during interseismic activity times subsequent to large local stress release along the dst

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

aftershock slip behavior of the 1989 loma prieta california earthquake an analysis of 745 aftershocks of the m7 1 loma prieta earthquake of 17 october 1989 reveals a wide variety of focal mechanisms at the northwestern end of the aftershock zone earthquakes that apparently occurred off the main rupture plane exhibit mechanisms with predominantly reverse slip on planes nearly parallel to the san andreas fault at the southeastern end the mechanisms exhibit right lateral motion on near vertical planes suggesting that these aftershocks involve slip on the san andreas fault few of the aftershock mechanisms in the central zone resemble the main shock mechanism strike n130 degree e dip 70 degree sw rake 104 degree but instead exhibit reverse right lateral left lateral and normal motion on planes subparallel to the main shock rupture plane the dip of the aftershock zone is parallel to the main shock slip plane and includes the main shock hypocenter however the lack of agreement between the main shock and the aftershock mechanisms suggests that few of the aftershocks occurred on the main shock slip plane this behavior is consistent with observations of aftershock sequences for other dip slip events and also with studies indicating that main shock rupture zones are at all other times mostly aseismic if the stress drop for the main shock relievedmore most of the tectonic stress the mechanisms could reflect the heterogeneity of the near field stress redistribution alternatively the variety of the aftershock mechanisms may reflect deformation by block motion within a narrow zone adjacent to the main shock rupture plane less

seismicity rate immediately before and after main shock rupture from high frequency waveforms in japan 1 we analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the hi net borehole array in japan by scrutinizing high frequency signals we detect 5 times as many aftershocks in the first 200 s as in the japan meteorological agency catalogue after correcting for the changing completeness level immediately after the main shock the aftershock rate shows a crossover from a slower decay with an omori s law exponent p 0 58 0 08 between 20 and 900 s after the main shock to a faster decay with p 0 92 0 04 after 900 s the foreshock seismicity rate follows an inverse omori s law with p 0 73 0 08 from several tens of days up to several hundred seconds before the main shock the seismicity rate in the 200 s immediately before the main shock appears steady with p 0 35 0 50 these observations can be explained by the epidemic type aftershock sequence etas model and the rate and state model for a heterogeneous stress field on the main shock rupture plane alternatively nonseismic stress changes near the source region such as episodic aseismic slip or pore fluid pressure fluctuations may be invoked to explain the observation of small p values immediately before and after the main shock

some characteristics of the stress field of the 1995 hyogo ken nanbu kobe earthquake we investigate the characteristics of the stress field associated with the 1995 hyogo ken nanbu kobe earthquake we use for the study a tomographic model of the fault slip inferred from the numerous near field recordings and we calculate the space and time evolution of shear stress on the fault during the earthquake we show that the spatial distribution of stress drop is very heterogeneous the apparent strength of the fault at the onset of the earthquake is low of the order of 1 mpa or less which indicates that the pre earthquake tectonic shear stress was close to the static friction over most of the fault at many locations the stress drop rotates significantly during sliding as was originally proposed by spudich 1992 we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake the pre earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 mpa average values of the initial and final shear stresses over the fault are 3 3 and 1 6 mpa respectively indicating that about half of the pre earthquake tectonic stress was released during the earthquake there is a relatively strong correlation between the initial and final stress distributions which suggests that intrinsic fault properties not modified by the earthquake control the spatial distribution of tectonic stress over the fault we show that on average over the fault the dynamic friction coefficient is equal to about 40 of the static friction coefficient diagrams depicting the evolution of shear stress as a function of slip are consistent with slip weakening behavior but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion

stress before and after the 2002 denali fault earthquake 1 spatially averaged absolute deviatoric stress tensors along the faults ruptured during the 2002 denali fault earthquake both before and after the event are derived using a new method from estimates of the orientations of the principal stresses and the stress change associated with the earthquake stresses are estimated in three regions along the denali fault one of which also includes the susitna glacier fault and one region along the totschunda fault estimates of the spatially averaged shear stress before the earthquake resolved onto the faults that ruptured during the event range from near 1 mpa to near 4 mpa shear stresses estimated along the faults in all these regions after the event are near zero 0 1 mpa these results suggest that deviatoric stresses averaged over a few tens of km along strike are low and that the stress drop during the earthquake was complete or nearly so

an insight into the bucaramanga nest abstract we used the local seismicity for the period of 1993 to 2001 in the northeast of colombia to show the existence of two slabs in the north and south of the bucaramanga nest the northern slab has a dip angle of about 25 and the southern slab has a 50 dip angle while the dip in the bucaramanga nest is about 29 in order to explain the nature of the bucaramanga nest we proposed the scenario of collision between these two slabs using a 3d finite element model fem we show that collision can concentrate modify and perturb the stress field the active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the bucaramanga nest the perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro earthquakes and the complexity in the source of the moderate size earthquakes in the bucaramanga nest

seismic activity offshore martinique and dominica islands central lesser antilles subduction zone from temporary onshore and offshore seismic networks this work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the central lesser antilles island arc from martinique to guadeloupe islands during the whole recording period extending from january to the end of august 2007 more than 1300 local seismic events were detected in this area a subset of 769 earthquakes was located precisely by using hypoellipse we also computed focal mechanisms using p wave polarities of the best azimuthally constrained earthquakes we detected earthquakes beneath the caribbean forearc and in the atlantic oceanic plate as well at depth seismicity delineates the wadati benioff zone down to 170 km depth the main seismic activity is concentrated in the lower crust and in the mantle wedge close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed we propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense thick and rigid crustal block and the lighter and more flexible outer forearc seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate

seismicity and shear strain in the southern great basin of nevada and california this study examines the relationship between the distribution of small earthquakes ml 4 3 and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events the southern great basin seismic network sgbsn region the great basin is a unique continental extensional province characterized by normal and strike slip faulting high heat flow crust of thin to normal thickness and high elevations the sgbsn is operated to provide data to address suitability issues pertaining to yucca mountain nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository suitability issues include estimation of the probability of occurrence of future damaging earthquakes the characterization of the mechanisms that drive hydrologic flow and the identification of fractures faults that might act as flow conduits or barriers this study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field where strain concentrates there should be a greater number of small earthquakes strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field these strain field models are then interpreted with regard to the regional tectonics and site suitability issues modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults clusters in a wide band along and extending northeast of the northern half of the furnace creek fault may correspond to elevated shear strains along the fault and a broad cluster in the pahranagat shear zone may be associated with shear strain arising from a distribution of smaller localized faults the relatively large number of small earthquakes in the southern and eastern portions of the nevada test site is consistent with the strain field models a minimum in shear strain at yucca mountain is predicted by all models consistent with an almost total lack of earthquakes observed there the region to the west of the death valley furnace creek fault system the portion of the study area with the most active deformation but few small earthquakes is an area of low shear strain a possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep while there is also a relative lack of earthquakes at yucca mountain this may be indicative of a lack of accumulating strain energy and thus a lower potential for a large earthquake

seismotectonic investigation of biga peninsula in sw marmara region using steerable filter technique potential field data and recent seismicity we examine seismotectonic setting of biga peninsula in western anatolia canakkale region using the steerable filter technique and recent seismicity one of the most important issues in geophysics is to observe borders or margins of tectonic geologic discontinues for this purpose we apply this filter technique to gravity anomaly map of biga peninsula we observe undetected buried faults in biga peninsula using the steerable technique where they have never been seen in the geological maps before these buried faults comply with recent seismicity for this region focal mechanisms of past earthquakes m 3 5 are in good agreement with fault orientations this observation shows that we have to take into account these fault locations and consider for preparing future seismic hazard maps the geometry of fault segments reveals mostly strike slip faulting regime with ne sw trending direction of t axis in the entire study region according to high resolution hypocenter relocation of the biga earthquake sequences in the observation period between 5 january 2005 and 14 november 2015 extends from n to s direction the stress tensor inversion results indicate a predominant normal stress regime with a nw se oriented maximum horizontal compressive stress s h according to strong discrepancy of density in the biga peninsula is characterized by numerous small segmented secondary faults these buried or undetected fault locations indicate that these segments are large enough to increase earthquake stress failure towards nw se and n s directions respectively seismotectonic setting of biga peninsula is divided into sub regions by ne sw trending secondary faults with normal and major strike slip components this output is verified by steerable filter and local regional seismotectonic analysis we propose a new seismotectonic model for biga peninsula and update the orientation of active fault segments according to our model north anatolian fault zone cross cuts the southern marmara basins as a narrow fault segment and continues towards aegean sea

empirical green s function analysis of loma prieta aftershocks abstract following the 18 october 1989 loma prieta earthquake an array of 22 three component iris passcal instruments was deployed in the santa cruz mountains we use an empirical green9s function method to study source properties of aftershocks as well as attenuation in the epicentral area using pairs of closely spaced events we show that the largest aftershocks are well modelled by an 2 source spectrum and we obtain estimates of the p wave corner frequency for a total of 15 events brune stress drops are observed to have no systematic variation over the range of moments spanned by the data set down to 10 19 dyne cm there is a suggestion that aftershocks that occurred outside the mainshock rupture have higher stress drops than those that occurred within the mainshock rupture a m l 3 4 foreshock in the subsequence associated with the 18 april 1990 m l 5 4 watsonville earthquake yields a stress drop estimate of 820 bars this value is well outside the stress drops obtained for all other events 6 to 266 bars site specific attenuation parameters suggest large lateral and vertical heterogeneity in p wave q structure station holy in a valley near the surface trace of the san andreas fault is characterized by low q values while stations to the north of the aftershock zone are characterized by the highest q values

conditions for earthquake surface rupture along the san andreas fault system california earthquake ruptures that nucleate along faults at depth can only propagate to the earth s surface if the shear stresses during faulting exceed the yield strength of the uppermost crust where fluid pressures are usually hydrostatic suprahydrostatic fluid pressures may be required at depth for the seismic reactivation of misoriented faults such as the san andreas fault system based on fracture criteria for the wall and fault rock slip events that can cause ground breakage are estimated to occur at seismogenic depth along fault planes that form angles of less than 65 on average with the maximum principal stress direction such slip events require minimum shear stresses of about 30 mpa at hypocentral depth for faults with long interseismic periods and thus inferred high cohesive strength the predicted reactivation angle is 55 suggesting that several segments of the san andreas fault system in southern california including the san bernardino area parts of the elsinore fault zone and the san jacinto fault provide the most probable hypocentral sites for future large earthquakes that disrupt the surface to provide further constraints on locations for such earthquakes we urgently need to investigate frictional properties of fault rocks and the state of stress at depth by employing laboratory experiments focal mechanism studies and by drilling into seismically active faults such as the san andreas fault system

using regional moment tensors to constrain the kinematics and stress evolution of the 2010 2013 canterbury earthquake sequence south island new zealand abstract on september 3 2010 a m w 7 0 u s geological survey moment magnitude earthquake ruptured across the canterbury plains in south island new zealand since then new zealand gns science has recorded over 10 000 aftershocks m l 2 0 and larger including three destructive m w 6 0 earthquakes near christchurch we treat the canterbury earthquake sequence as an intraplate earthquake sequence and compare its kinematics to an andersonian model for fault slip in a uniform stress field we determined moment magnitudes and double couple solutions for 150 earthquakes having m w 3 7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on south island new zealand the majority 126 of these double couple solutions have strike slip focal mechanisms with right lateral slip on ene fault planes or equivalently left lateral slip on sse fault planes the remaining focal mechanisms indicate reverse faulting except for two normal faulting events the strike slip segments have compatible orientations for slip in a stress field with a horizontal 1 oriented n115 e and horizontal 3 the preference for right lateral strike slip earthquakes suggests that these structures are inherited from previous stages of deformation reverse slip is interpreted to have occurred on previously existing structures in regions with an absence of existing structures optimally oriented for strike slip deformation despite the variations in slip direction and faulting style most aftershocks had nearly the same p axis orientation consistent with the regional 1 there is no evidence for significant changes in these stress orientations throughout the canterbury earthquake sequence

an example of triggered earthquakes in western turkey 2000 2015 afyon aksehir graben earthquake sequences abstract we investigate significant earthquake sequences in the observation period between 2000 and 2015 along the afyon aksehir graben system in western anatolia these sequences include a series of strong events with moment magnitudes m w larger than 5 5 e g eber m w 5 8 sultandagi m w 6 5 and cay m w 5 8 earthquakes the mainshocks are shallow focus normal events at depths of 8 15 km focal depths of aftershocks range from 2 to 30 km the focal mechanisms of the aftershocks are mainly normal faulting with variable strike slip components the geometry of focal mechanisms reveals a normal faulting regime with nne ssw trending direction of t axis in the entire activated region a stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the afyon aksehir graben stress field the stress tensor inversion results display a predominant normal stress regime with a nw se oriented maximum horizontal compressive stress s h according to high resolution hypocenter relocations of the afyon aksehir graben seismic sequences six clusters are revealed the aftershock activities in the observation period between 17 january 2000 and 26 october 2015 extend from east to west direction seismic cross sections show that a complex pattern of the hypocenter distribution with the activation of sultandagi fault segments the western clusters are associated with a fault plane trending mainly e w and dipping towards se while the eastern clusters are related to a fault plane trending nw se and dipping towards vertical the best constrained focal depths are mainly confined in the crust depth

the aftershock sequence of the 2011 mineral virginia earthquake temporal and spatial distribution focal mechanisms regional stress and the role of coulomb stress transfer abstract the aftershocks of the 23 august 2011 m w 5 7 mineral virginia earthquake were recorded by 36 temporary stations installed by several institutions we located 3960 aftershocks from 25 august 2011 through 31 december 2011 a subset of 1666 aftershocks resolves details of the hypocenter distribution we determined 393 focal mechanism solutions aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane other aftershocks occurred 10 20 km to the northeast a large percentage of the aftershocks occurred in regions of positive coulomb static stress change and 80 of the focal mechanism nodal planes were brought closer to failure however the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity aftershocks at depths less than 4 km exhibit reverse mechanisms with north northwest trending nodal planes most focal mechanisms at depths greater than 6 km are similar to the mainshock with north northeast trending nodal planes a concentration of aftershocks in the 4 6 km depth range near the mainshock are mostly of reverse type but display a 90 range of nodal plane trend those events appear to outline the periphery of mainshock rupture where positive coulomb stress transfer is largest the focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km along with the mainshock point to the possibility of a depth dependent stress field prior to the occurrence of the mainshock analysis of earthquake occurrence using a new magnitude scale indicates a gutenberg richer law b value of 0 864 and an omori law p value of 1 085 indicative of a typical aftershock sequence online material catalogs of aftershock location magnitude and focal mechanisms

source mechanisms and stress fields of the 15 16 june 2013 crete earthquake sequence along hellenic subduction zone 15 june 2013 mw 6 1 off shore southern crete earthquake and its aftershock sequence along hellenic subduction zone are examined centroid moment tensors cmts for 40 earthquakes with moment magnitudes mw between 3 5 and 6 1 are determined by applying a waveform inversion method the mainshock is shallow focus thrust event with a minor strike slip component at a depth of 20 km the seismic moment mo of the mainshock is estimated as 2 07 1018 nm and rupture duration of the mainshock is 4 s the focal mechanisms of aftershocks are mainly thrust faulting with a strike slip component the geometry of the moment tensors mw 3 5 reveals a thrust faulting regime with n s trending direction of p axis in the entire activated region according to high resolution cmt solutions of the off shore southern crete earthquake sequence one main cluster consisting of 40 events is revealed the aftershock activity in the observation period between 15 june and 15 july 2013 extends from n to s and nw to se directions seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of two segments the subduction interface is clearly revealed with high resolution hypocenter source relocation and moment tensor solution the best constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate depth 30 km and is ranging from about 5 28 km depth a stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the off shore southern crete stress field the stress tensor inversion results indicate a predominant thrust stress regime with a ne sw oriented maximum horizontal compressive stress sh according to variance of the stress tensor inversion to first order the southern crete region is characterized by a homogeneous interplate stress field we also investigate the coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along southern crete and surrounding regions positive lobes with stress more than 0 3 bars are obtained for the mainshock indicating that these values are large enough to increase the coulomb stress failure toward wnw ese direction

seismotectonic setting of the karadere duzce branch of the north anatolian fault zone between the 1999 izmit and duzce ruptures from analysis of izmit aftershock focal mechanisms abstract we investigate aftershock focal mechanisms along the eastern part of the izmit mw 7 4 august 17 1999 rupture zone during the time period august 22 1999 october 17 1999 two spatial clusters of aftershock activity are analyzed representing the karadere fault kf and the duzce area da based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors 10 km events north and east of the easternmost end of the karadere fault we observe a high variance in stress field orientation correlated with lower b values while the karadere fault reflects a predominant dextral strike slip regime with normal faulting components the duzce area further to the east that also hosted the forthcoming mw 7 2 mainshock 87 days after the izmit earthquake can be subdivided into a dominantly ne sw extensional normal faulting regime below the duzce basin db and a first order strike slip regime along the western duzce fault df we conclude that the duzce basin was set under tension by the izmit rupture and partly released the slip deficit by extensional faulting on karadere fault parallel to the coseismic displacement at the same time this area and in particular the duzce fault that bounds the duzce basin to the south reflects mostly strike slip events representing a major asperity along the north anatolian fault zone nafz before initiating the duzce rupture 87 days after the izmit event

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

geodetic estimate of coseismic slip during the 1989 loma prieta california earthquake offsets in the relative positions of geodetic stations resulting from the loma prieta earthquake can be explained with a dislocation model that includes buried oblique slip on a rupture surface extending 37 km along the strike of the san andreas fault dipping 70 degree to the sw and extending from a depth of about 5 to 17 5 km assuming uniform slip on a rectangular surface the mean values for a range of reasonable fault geometries are 1 6 plus minus 0 3 m right lateral strike slip and 1 2 plus minus 0 3 m reverse slip slip on an adjacent extension of the rupture to the southeast recorded in the aftershock sequence is not well constrained by the geodetic data the geodetic data clearly preclude rupture extending near the surface

the 16 april 2015 m w 6 0 offshore eastern crete earthquake and its aftershock sequence implications for local regional seismotectonics we examine the 16 april 2015 m w 6 0 offshore eastern crete earthquake and its aftershock sequence in southern aegean sea centroid moment tensors for 45 earthquakes with moment magnitudes m w between 3 3 and 6 0 are determined by applying a waveform inversion method the mainshock is shallow focus thrust event with a strike slip component at a depth of 30 km the seismic moment m o of the mainshock is estimated as 1 33 1018 nm and rupture duration of the mainshock is 3 5 s the focal mechanisms of aftershocks are mainly thrust faulting with a strike slip component the geometry of the moment tensors m w 3 3 reveals a thrust faulting regime with ne sw trending direction of t axis in the entire activated region according to high resolution hypocenter relocation of the eastern crete earthquake sequence one main cluster consisting of 352 events is revealed the aftershock activity in the observation period between 5 january 2015 and 7 july 2015 extends from n to s direction seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of three segments the subduction interface is clearly revealed with high resolution hypocenter relocation and moment tensor solution the best constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate depth 40 km and are ranging from about 4 5 to 39 km depth a stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the offshore eastern crete stress field the stress tensor inversion results indicate a predominant thrust stress regime with a nw se oriented maximum horizontal compressive stress s h according to variance of the stress tensor inversion to first order the crete region is characterized by a homogeneous interplate stress field we also investigate the coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along crete and surrounding regions positive lobes with stress more than 3 bars are obtained for the mainshock indicating that these values are large enough to increase the coulomb stress failure toward ne sw and nw se directions respectively

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

the 10 june 2012 fethiye mw 6 0 aftershock sequence and its relation to the 24 25 april 1957 ms 6 9 7 1 earthquakes in sw anatolia turkey abstract the 10 june 2012 mw 6 0 aftershock sequence in southwestern anatolia is examined centroid moment tensors for 23 earthquakes with moment magnitudes mw between 3 7 and 6 0 are determined by applying a waveform inversion method the mainshock is a shallow focus strike slip with reverse component event at a depth of 30 km the seismic moment mo of the mainshock is estimated as 1 28 1018 nm and rupture duration of the fethiye mainshock is 38 s the focal mechanisms of the aftershocks are mainly strike slip faulting with a reverse component the geometry of the focal mechanisms reveals a strike slip faulting regime with ne sw trending direction of t axis in the entire activated region a stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the fethiye earthquake stress field the stress tensor inversion results indicate a predominant strike slip stress regime with a nw se oriented maximum horizontal compressive stress sh according to variance of the stress tensor inversion to first order the fethiye earthquake area is characterized by a homogeneous interplate stress field the coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the gulf of fethiye and surrounding region positive lobes with stress more than 0 4 bars are obtained indicating that these values are large enough to increase the coulomb stress failure towards nnw sse and e w directions

structure deformation and strength of the loma prieta fault northern california usa as inferred from the 1989 1990 loma prieta aftershock sequence analysis of aftershock focal mechanisms of the october 1989 m 7 1 loma prieta earthquake reveals details of structure and deformation aftershocks below 4 6 km define an alignment plane oriented 131 65 this alignment includes three planar segments defining a restraining bend in a blind strike slip fault dextral strike slip dominates in the south and reverse dextral slip dominates in the north a distribution similar to slip during the main shock above this fault dominantly reverse faults striking roughly 110 lie en echelon along the main fault trend in the 17 sets of spatially clustered aftershocks a more complex substructure is defined by local planar hypocenter alignments and by preferred orientations of aftershock shear planes the maximum shortening axes make angles with the normal to these planes that average 46 and 53 respectively these angles are incompatible with a weak fault for which 0 20 orientations of preferred aftershock shear planes resemble riedel shears relative to local hypocenter alignment planes the similarity of slip distributions inferred from the aftershock inversions and from the main shock implies that the stress drop during the main shock was incomplete and thus that the loma prieta fault is not unusually weak triaxial brittle deformation is partitioned into a pair of plane strains rotated relative to each other about a common principal axis the shape of the triaxial strain ellipsoid determines the angle of rotation evidence supports our micropolar model of brittle deformation which predicts that the antisymmetric part of the seismic moment tensor records effects of block rotations

oxidative sulfide dissolution on the early earth recent evidence suggests that the biological production of oxygen in earth s surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more however the potential effects of early oxygen production on surface ocean chemistry have remained little explored and questions persist regarding the locus of oxidation of crustal material i e subaerial and or submarine settings here we revisit the notion of spatially restricted oxygen oases in the archean surface ocean by employing a simple steady state box model of the surface ocean in a coastal upwelling system using pyrite as an example we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere we find that it is possible to establish strong air sea gas exchange disequilibrium with respect to o 2 however in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering transport in the subaerial realm despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric po 2 relative to those attainable in productive regions of the surface ocean however broad scale subaerial weathering of sulfides would likely have required either local o 2 production at the site of weathering or transient increase in atmospheric po 2 above the most plausible levels characteristic of a pervasively reducing atmosphere

properties of the aftershock sequence of the 1999 mw 7 1 hector mine earthquake implications for aftershock hazard we investigate the spatial and temporal seismicity parameters and the related probabilistic aftershock hazard for the aftershock sequence of the 1999 m w 7 1 hector mine mainshock and compare it with the neighboring 1992 m w 7 3 landers sequence using a catalog of 11 000 earthquakes we determine the earthquake size distribution b value the aftershock decay rate p value and the seismic activity rate a value the b values are high b 1 2 within the rupture area significantly lower b 0 7 north of the rupture area and increase with time since the mainshock probabilistic aftershock hazard maps computed automatically as early as 4 days after the mainshock identified the northernmost part of the sequence as the highest hazard region these maps show a good agreement between the forecasts and the recorded large aftershocks based on the asymmetrical b value and hazard patterns for both the hector mine and landers sequences we hypothesize that the mainshock rupture directivity and slip distribution influence aftershock hazard current static or dynamic stress triggering models cannot resolve this spatial and temporal evolution of the hazard stress tensor inversions of 1400 relocated first motion focal mechanisms show predominantly a strike slip stress state with a sw ne trend of the greatest principal stress the heterogeneity of the stress field is unusually high near the hector mine and landers mainshock ruptures particularly near patches of large slip

the october 17 1989 loma prieta california earthquake and its aftershocks geometry of the sequence from high resolution locations hypocenters of the loma prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane most aftershocks cluster around the perimeter of the zone surrounding a relatively aseismic center which approximates the region of mainshock rupture at its southeastern end the dipping aftershock zone warps into a vertical surface that corresponds to the san andreas fault in the central and northwestern parts of the zone at depths above 10 km the aftershocks define numerous disjoint fault structures the large component of reverse slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend we do not believe that the loma prieta earthquake occurred on the sargent fault however we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault

seismotectonics of the central denali fault alaska and the 2002 denali fault earthquake sequence in this article we analyze the spatial and temporal variations in the seismicity and stress state within the central denali fault system alaska before and during the 2002 denali fault earthquake sequence seismicity for 30 years prior to the 2002 earthquake sequence along the denali fault was very light with an average of four events with magnitude m l 3 per year we observe a significant increase in the seismicity rate prior to the m w 7 9 event of 3 november 2002 within its epicentral region starting about 8 months before its occurrence the majority of the aftershocks of the m w 7 9 event are located within the upper 11 km of the crust and form several persistent clusters with a few aseismic patches along the ruptured fault the most active aftershock source is associated with the epicentral region of the earthquake the overall b value of the aftershock sequence is 0 96 with the highest b values within the epicentral region we estimate that it will take 14 years for the seismicity rate to drop back to the background level the stress regime across the region varies in space and time the inferred stress regime prior to the 2002 sequence is predominately strike slip along the central part of the rupture zone the orientations of the least and intermediate stress axes are reversed after the 2002 earthquake sequence the maximum compressive stresses along the denali fault rotate clockwise by up to 35 the greatest rotations occur in the area of the rupture step over from the denali to the totschunda fault the inferred stress regime after the 2002 sequence reflects an interchanging thrusting and strike slip faulting along the ruptured fault the thrust faulting is concentrated in the epicentral region of the m w 7 9 event and along the rupture segments showing the largest surface offsets

seismotectonics of the antalya basin and surrounding regions in eastern mediterranean from 8 to 28 december 2013 mw 5 0 5 8 earthquake sequence abstract the 8 28 december 2013 mw 5 0 5 8 antalya basin earthquake sequence in eastern mediterranean is examined centroid moment tensors for 16 earthquakes with moment magnitudes mw between 3 6 and 5 8 are determined by applying a waveform inversion method all earthquakes are shallow focus thrust events at a depth of 40 45 km the seismic moments mo of the earthquakes are estimated as 4 10 1016 5 54 1017 n m and rupture durations of the mainshocks are 20 22 s the focal mechanisms of the aftershocks are mainly thrust faulting with a strike slip component and reveal nw se trending direction of t axis in the entire activated region according to high resolution hypocenter relocation of the antalya earthquake sequence seven main clusters are revealed the aftershock activity in the observation period between 1 december 2013 and 23 january 2015 extends in an n to s direction a seismic cross section indicates that a complex pattern of the hypocenter distribution with the activation of seven segments the westernmost cluster cluster 1 is associated with a fault plane trending mainly wnw ese and dipping vertical while the cluster 5 is related to a fault plane trending nne ssw and dipping towards sse the best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust depth

correlating seismicity parameters and subsidence in the tokai region central japan 1 we demonstrate that transients in subsidence rate in the tokai area of central japan correlate with changes in the rate and size distribution of earthquakes three largely independent seismic catalogs cover this region national research institute for earth science and disaster prevention nied japanese meteorological agency jma and japan university network catalog junec comparing seismicity patterns between these three data sets offers a unique opportunity to verify seismicity anomalies based on independent sources a study of data quality magnitude reporting consistency and consistency across the three data sources reveals that the nied data for the period 1981 2002 are best suited for seismicity analysis we spatially and temporally map seismicity rates finding that a significant decrease in the earthquake rate of m 2 0 events coincides with an accelerated subsidence period however this anomaly disappears when including smaller magnitudes in the analysis this relative quiescence of larger events can readily be explained when interpreting the transient in seismicity in the framework of a change in the earthquake size distribution or b value the background b value of about 0 8 increased in the period 1987 5 1989 5 to a value of b 1 2 a highly significant change that can be confirmed in all three data sets the correlation between subsidence and b value change is significant stress tensor inversions of focal mechanism data for the anomalous period show an increase in thrusting type earthquakes while a unique interpretation of the relationship between subsidence and b value is not possible we propose that an increase in the locking stress a slow stick event is consistent with all observations

source characteristics and b values of the tuz golu fault zone in central anatolia turkey abstract we investigate significant earthquake sequences in the observation period between 2000 and 2018 along the tuz golu fault zone in central anatolia these sequences include a series of strong events with moment magnitudes mw 5 0 e g 30 july 2005 bala ankara mw 5 2 20 december 2007 bala ankara mw 5 7 and 26 december 2007 mw 5 6 afsar ankara earthquakes the mainshocks are shallow focus strike slip events with minor normal component at depths of 12 15 km focal depths of aftershocks range from 2 to 30 km the focal mechanisms of the aftershocks are mainly normal faulting with variable strike slip components the geometry of focal mechanisms reveals a normal faulting regime with e w and also ne sw trending directions of t axis in the entire activated region a stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the tuz golu fault zone stress field the stress tensor inversion results display a predominant normal stress regime with a nne ssw oriented maximum horizontal compressive stress sh according to high resolution hypocenter relocations of the tuz golu fault zone seismic sequences eight clusters are revealed the aftershock activities in the observation period between 5 march 2000 and 2 june 2018 extend from nw to se direction seismic cross sections show that a complex pattern of the hypocenter distribution with the activation of tuz golu fault zone segments the western clusters are associated with a fault plane trending mainly n s and dipping towards almost vertical while the eastern clusters are related to a fault plane trending nw and dipping towards se direction the best constrained focal depths are mainly confined in the crust depth

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

use of fault striations and dislocation models to infer tectonic shear stress during the 1995 hyogo ken nanbu kobe earthquake dislocation models of the 1995 hyogo ken nanbu kobe earthquake derived by yoshida et al 1996 show substantial changes in direction of slip with time at specific points on the nojima and rokko fault systems as do striations we observed on exposures of the nojima fault surface on awaji island spudich 1992 showed that the initial stress that is the shear traction on the fault before the earthquake origin time can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points from yoshida s slip model we calculated dynamic stress changes on the ruptured fault surfaces to estimate errors we compared the slip velocities and dynamic stress changes of several published models of the earthquake the differences between these models had an exponential distribution not gaussian we developed a bayesian method to estimate the probability density function pdf of initial stress from the striations and from yoshida s slip model striations near toshima and hirabayashi give initial stresses of about 13 and 7 mpa respectively we obtained initial stresses of about 7 to 17 mpa at depths of 2 to 10 km on a subset of points on the nojima and rokko fault systems our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near hirabayashi and ogura on awaji island our results indicate that the nojima fault slipped at very low shear stress and fractional stress drop was complete near the surface and about 32 below depths of 2 km our results at depth depend on the accuracy of the rake rotations in yoshida s model which are probably correct on the nojima fault but debatable on the rokko fault our results imply that curved or cross cutting fault striations can be formed in a single earthquake contradicting a common assumption of structural geology

aftershocks properties of the 2013 shonbe m w 6 3 earthquake central zagros iran abstract the spatial and temporal seismicity parameters stress state as well as the partitioning of the seismic energy of the shonbe earthquake sequence have been investigated the geodetic data and the stress tensor inversions of the focal mechanisms before and after the shonbe main shock indicate the same ne sw trend of the greatest principal stress this stress orientation demonstrates a non heterogeneous stress state in that region of the central zagros which can be confirmed by the fractal dimension and the estimated p value the aftershock focal mechanisms show a diversity which may be associated with the approximately uniaxial stress field acting nearly perpendicular to the main shock fault plane in which the co seismic stress release is nearly complete the spatial distributions of the b and p values have been investigated and compared the high p value regions e g far to the southeast south and southwest of the main shock are approximately consistent with the high b value regions and the low p value regions e g the limited areas at the north northeast of the main shock are nearly consistent with the low b value regions the limited areas surrounding the main shock and also at the east southeast of the main shock the p values are not in agreement with the b values which may be attributed to the local tectonic features in those regions the seismic moment ratio and the bath s law have been used to constrain the energy partitioning between the main shock and the aftershocks it is found that a high fraction of the energy is released by the shonbe main shock and nearly one third of the energy released by the aftershocks this may be associated with the high stress imparted by the blind thrust main shock to the overlying crust in which promotes failure in the surrounding area and is often relieved by secondary faults and typically produce a high number of aftershocks

imaging stress and faulting complexity through earthquake waveform similarity while the rupture processes of nearby earthquakes are often highly similar characterizing the differences can provide insight into the complexity of the stress field and fault network in which the earthquakes occur here we perform a comprehensive analysis of earthquake waveform similarity to characterize rupture processes in the vicinity of ridgecrest california we quantify how similar each earthquake is to neighboring events through cross correlation of full waveforms the july 2019 ridgecrest mainshocks impose a step reduction in earthquake similarity which suggests variability in the residual stress field and activated fault structures on length scales of hundreds of meters or less among these aftershocks we observe coherent spatial variations of earthquake similarity along the mainshock rupture trace and document antisimilar aftershock pairs with waveforms that are nearly identical but with reversed polarity these observations provide new high resolution constraints on stress transfer and faulting complexity throughout the ridgecrest earthquake sequence

rotation of the principal stress directions due to earthquake faulting and its seismological implications earthquake faulting results in stress drop over the rupture area because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault the principal stress directions must rotate to adapt such a change of the state of stress using two constraints i e the normal stress on the fault and the vertical stress the overburden pressure which do not change before and after the earthquake we derive simple expressions for the rotation angle in the 1 axis for a dip slip earthquake the rotation angle is only a function of the stress drop ratio defined as the ratio of the stress drop to the initial shear stress and the angle between the 1 axis and the fault plane but for a strike slip earthquake the rotation angle is also a function of the stress ratio depending on the faulting regimes the 1 axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal the rotation of the stress field has several important seismological implications it may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks the rotation angle can be used to estimate the stress drop ratio which has been a long lasting topic of debate in seismology

the 1991 sierra madre earthquake sequence in southern california seismological and tectonic analysis the m l 5 8 sierra madre earthquake of 28 june 1991 occurred at a depth of 12 km under the san gabriel mountains of the central transverse ranges since at least 1932 this region had been quiescent for m 3 the mainshock focal mechanism derived from first motion polarities exhibited almost pure thrust faulting with a rake of 82 on a plane striking n62 e and dipping 50 to the north the event appears to have occurred on the clamshell sawpit fault a splay of the sierra madre fault zone the aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes having a b value of 0 6 twenty nine single event focal mechanisms were determined for aftershocks of m 1 5 the 4 km long segment of the clamshell sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east northeast striking fault plane dipping to the north to the west several thrust aftershocks with east striking nodal planes suggest some complexity in the aftershock faulting such as a curved rupture surface in addition several strike slip and normal faulting events occurred along the edges of the mainshock fault plane indicating secondary tear faulting the tectonic stress field driving the coexisting left lateral strike slip and thrust faults in the northern los angeles basin is north south horizontal compression with vertical intermediate or minimum principal stress axis

two and three dimensional numerical simulations of the stress field at the thrust front of the northern apennines italy 1 two and three dimensional mechanical models simulating the seismic cycle along thrust faults are devoted to the active front of the northern apennines buried under the plio pleistocene sediments of the eastern po plain the models aim at understanding two peculiar characters of the stress field in the region 1 analyses of borehole breakouts and other well data integrated with seismological information and field evidence show along depth variations from extensional to strike slip to compressional for increasing depth of the stress style along the active mirandola anticline and 2 seismic and borehole breakout data suggest strong lateral rotations of the stress axes along the frontal and lateral ramp portions of the mirandola thrust fault modeling results allowed us to understand that the mechanical decoupling along faults strongly affects the stress distribution in their vicinity under the form of strong rotations of the principal stress axes as a consequence the slip either fast or slow along active blind thrust faults could drive to local stress fields very different from that associated with regional tectonics this could explain together with other processes such as differential compaction the along depth variation of the stress field observed in the mirandola region and the strong lateral rotations of the principal stress axes observed along the lateral ramp of the mirandola thrust fault finally the seismic activity along one of the thrust fault ramps along the apennines front should not interfere with the state of stress along adjacent ramps

direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern california in inverting populations of focal mechanisms to estimate stress misfits may arise owing to 1 errors in determining the fault plane solutions and 2 real variations in the state of stress among a number of techniques that invert for stress most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on p wave first motions in a highly nonlinear manner which is difficult to propagate into stress inversions to address these issues we present a new method for constraining stress tensors in which first motion observations are directly inverted for stress without assuming that focal mechanisms are known the technique produces estimates of four stress parameters a suite of focal mechanisms consistent with both the stress tensor and the first motions and estimates of the uncertainty in stress it also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor when tested against artificial data the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks as expected utilizing first motions from the southern california catalog the method is applied to several clusters and aftershock sets from 1981 to 1992 typical uncertainties in stress orientation exceed 20 30 at the 95 confidence level resulting from 5 mispick rates typical of even the cleanest data these uncertainties exceed some previous estimates for similar populations presumably because they account for focal mechanism un certainties and perhaps indicating that some previously reported stress variations are not statistically significant most populations are found to be internally consistent with a homogeneous stress field the most significant exceptions being aftershock sequences of some major earthquakes in at least one case the 1992 landers sequence aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses overall the method provides a useful and robust approach for evaluating stress constraints provided by p wave first motions

rate and state of background stress estimated from the aftershocks of the 1989 loma prieta california earthquake estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of loma prieta aftershocks this technique was previously applied to the landers aftershock sequence gross and kisslinger 1997 and is based upon the seismicity model of dieterich 1994 dieterich s theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area we used two independently derived source models to compute the stress step from the mainshock and to determine how effective that stress step was in triggering aftershocks a background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity the best fitting background stress state has its greatest compressive stress plunging 17 to n13 e and an intermediate stress very close in magnitude to the least principal stress the small shear stresses at depth and low coefficient of friction suggest that high pressure pore fluids may be present inside active faults the estimated stress rate of 70 pa d 0 25 bar yr is comparable to the stress rate found for the southern san andreas fault system we found a best fitting effective coefficient of friction 0 2 for the loma prieta area significantly less than 0 6 estimated for landers in previous work variations in aftershock decay rate within the loma prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by burgmann et al 1997 some of the postseismic slip occurred on structures that did not slip during the mainshock so the postseismic and coseismic stress step fields have different spatial distributions the effectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insignificant in this case

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

type of faulting and orientation of stress and strain as a function of space and time in kilauea s south flank hawaii earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of kilauea volcano hawaii are used to infer the state of stress and strain as a function of time and space we have determined 870 fault plane solutions from p wave first motion polarities for events with magnitudes m l 2 5 and depth ranging between 6 and 12 km faulting is characterized by a mixture of decollement reverse and normal faults most large earthquakes with magnitude m 7 rupture the decollement plane since it is the only surface large enough to generate magnitude 7 or larger earthquakes the percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972 1983 the percentage of decollement type focal mechanisms becomes dominant after 1983 this pattern of faulting activity suggests that pressure was building up within kilauea s rift zone prior to the 1983 puu oo eruption overall a single stress orientation with the maximum compressive stress oriented se perpendicular to the rift and dipping at 45 is compatible with the coeval existence of decollement reverse and normal faults however in a crustal volume east of longitude 155 10 w we find a change of the orientation of 1 from nearly horizontal to plunging 45 se occurring in 1979 this stress rotation suggests magma movements within the aseismic part of kilauea s east rift zone the strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed we observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements while stress stays constant or by a rotation of stress while strain remains constant assuming that the model of a noncohesive coulomb wedge is appropriate for kilauea s south flank we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0 85

aftershocks are well aligned with the background stress field contradicting the hypothesis of highly heterogeneous crustal stress 1 it has been proposed that the crustal stress field contains small length scale heterogeneity of much larger amplitude than the uniform background stress this model predicts that earthquake focal mechanisms should reflect the loading stress rather than the uniform background stress so if the heterogeneous stress hypothesis is correct focal mechanisms before and after a large earthquake should align with the tectonic loading and the earthquake induced static stress perturbation respectively however i show that the off fault triggered aftershocks of the 1992 m7 3 landers california earthquake align with the same stress field as the pre landers mechanisms the aftershocks occurred on faults that were well oriented for failure in the pre landers stress field and then loaded by the landers induced static stress change aftershocks in regions experiencing a 0 05 to 5 mpa coseismic differential stress change align with the modeled landers induced static stress change implying that they were triggered by the stress perturbation contrary to the heterogeneous stress hypothesis these triggered aftershocks are also well aligned with the pre landers stress field obtained from inverting the pre landers focal mechanisms therefore the inverted pre landers stress must represent the persistent background stress field earthquake focal mechanisms provide an unbiased sample of the spatially coherent background stress field which is large relative to any small scale stress heterogeneity the counterexample provided by the landers earthquake is strong evidence that the heterogeneous stress model is not widely applicable

on the origin of diverse aftershock mechanisms following the 1989 loma prieta earthquake summary we test the hypothesis that the origin at the diverse suite of aftershock mechanisms following the 1989 m 7 1 loma prieta california earthquake is related to the post main shock static stress field we use a 3 d boundary element algorithm to calculate static stresses combined with a coulomb failure criterion to calculate conjugate failure planes at aftershock locations the post main shock static stress field is taken as the of a sum of a pre existing stress field and changes in stress due to the heterogeneous slip across the loma prieta rupture plane the background stress field is assumed to be either a simple shear parallel to the regional trend of the san andreas fault or approximately fault normal compression a suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared allowing for uncertainties in both mechanism parameters and earthquake locations to well constrained mechanisms reported in the us geological survey northern california seismic network catalogue we also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes assuming either plane to be a likely rupture plane various permutations of the assumed background stress field fractional coefficients of aftershock fault planer methods of comparisons etc explain between 52 and 92 per cent of the aftershock mechanisms we can explain a similar proportion of mechanisms however by comparing a randomly reordered catalogue with the various suites of synthetic aftershocks the inability to duplicate aftershock mechanisms reliably on a one to one basis is probably a function of the combined uncertainties in models of main shock slip distribution the background stress field and aftershock locations in particular we show theoretically that any specific main shock slip distribution and a reasonable background stress field are able lo generate a highly variable suite of failure planes such that quite different aftershock mechanisms may be expected to occur within a kilometre of less of each other this scale of variability is less than the probable location error of aftershock earthquakes in the loma prieta region we successfully duplicate a measure of the variability in the mechanisms of the entire suite of aftershocks if static stress changes are responsible for the generation of aftershock mechanisms we are able to place quantitative constraints on the level of stress that must have existed in the upper crust prior to the loma prieta rupture this stress level appears to be too low to generate the average dip across the main shock rupture plane possible reasons for this result range from incorrect initial assumptions of homogeneity in the background stress field friction and fault geometry to driving stresses that arise from deeper in the crust or upper mantle alternatively aftershock focal mechanisms may be determined by processes other than or in addition to static stress changes such as pore pressure changes or dynamic stresses

localization patterns in sandbox scale numerical experiments above a normal fault in basement abstract the finite element program elfen is used to study the effect of basement fault dip on the evolution of shear band patterns in unconsolidated sand the material properties and boundary conditions of the model were chosen to correspond to generic sandbox experiments model results reproduce the range of structural styles found in corresponding sandbox experiments with a basement fault dip of 60 and lower a graben structure is formed composed of a synthetic shear band followed by one or more antithetic shear bands with a basement fault dip of 70 and steeper a reverse precursor shear band forms first followed by a synthetic normal shear band that accommodates all further displacement the dip of the synthetic shear band is close to the basement fault dip for basement fault dips between 60 and 70 we observe a transition in localization patterns an analysis of the stress fields and velocity vectors in the model explains the first order aspects of the relationships observed we consider the observed precursor dominated and graben dominated structural domains to be important components of normal fault systems in which the first order structural style and deformation patterns are only weakly dependent on the details of the rheology of the model materials and explore the interesting problem of the change in structural style from precursor dominated to graben dominated structural domains above a normal fault in basement we find similar structural domains in sandbox experiments for the same set of boundary conditions but with slightly different material properties suggesting that the modeled patterns are robust within these two structural domains i e will occur over a range of similar material properties and boundary conditions the results of this study contribute to our ability to validate numerical models against experiments in order to finally better simulate natural systems

earthquake prediction research and the earthquakes of 2000 in the south iceland seismic zone abstract the purpose of this paper is to describe and to model long and short term processes that preceded two large earthquakes that occurred in 2000 in the south iceland seismic zone sisz the results are based on some key findings from multinational earthquake prediction research projects in the sisz since 1988 it involves a fusion of significant old and new results the paper presents a new hypothesis for earthquake build up processes in the region followed by a discussion on how interseismic and preseismic observations can be explained by this model and a discussion about a plausible earthquake cycle in light of the new hypothesis the research described here started in 1988 with the south iceland lowland sil project a significant outcome was the development of a seismic acquisition and evaluation system the sil system retrieving source information from earthquakes down to magnitude zero the research continued with the use of more multidisciplinary and multinational projects revealing new information about crustal processes related to large earthquakes in the area the validity of the work was tested in 2000 when two magnitude 6 6 m s earthquakes occurred in the central part of the sisz the earthquakes had a long term assessment of place a short term warning within short about the location and size of a probable impending earthquake was issued 25 hours before the second earthquake

seismicity of the 24 may 2014 mw 7 0 aegean sea earthquake sequence along the north aegean trough abstract the northern aegean sea was hit by a large size mw 7 0 earthquake on 2014 may 24 centroid moment tensor solutions for 40 events with moment magnitudes mw between 3 3 and 7 0 are computed by applying a waveform inversion method on data from the turkish and greek broadband seismic networks the time span of data covers the period between 2014 may 24 and 2014 june 26 the mainshock is a shallow focus strike slip event at a depth of 15 km focal depths of aftershocks range from 6 to 30 km the seismic moment mo of the mainshock is estimated as 4 60 1019 nm the calculated rupture duration of the north aegean sea mainshock is 40 s the focal mechanisms of the aftershocks are mainly strike slip faulting with a minor normal component the geometry of focal mechanisms reveals a strike slip faulting regime with ne sw trending direction of t axis in the entire activated region a stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the northern aegean sea stress field along the north aegean trough the stress tensor inversion results indicate a predominant strike slip stress regime with a nw se oriented maximum principal compressive stress 1 in the development of the north aegean trough in aegean sea is in good agreement with the resolved stress tensors with respect the newly determined focal mechanisms the effect of the propagating of the north anatolian fault into aegean sea is very clearly pronounced according to high resolution hypocenter relocation of the north aegean sea seismic sequence three main clusters are revealed the aftershock activity in the observation period between 2014 may 24 and 2014 july 31 extends from the mainshock cluster from ne to the sw direction seismic cross sections indicate that a complex pattern of the hypocenter distribution with the activation of seventeen segments the eastern cluster is associated with a fault plane trending mainly ene wsw and dipping vertical while the western is related to a fault plane trending ne sw and dipping towards nnw the best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust depth 30 km and is operating in the approximate depth range from 1 5 to 30 km consequently coulomb stress analysis is performed to calculate the stress transfer and correlate it with the activated region positive lobes with stress more than 3 bars are obtained indicating that these values are large enough to increase the coulomb stress failure towards ene wsw direction