physical property measurements icdp boreholes lb 07a and lb 08a lake bosumtwi impact structure ghana physical rock property measurements provide the primary constraints for any geological models hypothesized from geophysical observations previous geophysical models of the bosumtwi impact structure hypothesized that a highly magnetic and dense impact melt sheet might be the source of the observed magnetic anomalies however magnetic susceptibility and density measurements made on international continental scientific drilling program icdp cores lb 07a and lb 08a from the interior of the bosumtwi meteorite impact structure contain no evidence for that both density and magnetic susceptibility logs on both boreholes exhibit low amplitude contrasts between the uppermost polymict lithic breccia and suevite the intermediate monomict lithic breccia and the lowermost bedrock the depth extent of fracture related density reduction is much greater at lb 08a than at lb 07a a total magnetic intensity log from borehole lb 08a supports the suggestion that magnetic anomalies over lake bosumtwi are mainly sourced in undetected and or covered bedrock intrusions like the ones outcropping at the northeast and to the southwest of the lake

estimates of stress drop of the chi chi taiwan earthquake of 20 september 1999 from near field seismograms the apparent stress and stress drop of the chi chi taiwan earthquake are estimated from near field seismograms the estimated apparent stress and stress drop for the southern part of the fault are about 100 bars lower than those for the northern part the estimated ratio e s m also suggests that there is a higher dynamic stress drop in the northern part than in the southern one this indicates the transformation of a higher percentage of strain energy into the seismic wave energy in the northern part than in the southern part based on a parameter proposed by ramon zuniga 1993 we propose that the stress model of frictional overshoot can interpret the rupture of the chelungpu fault on which the chi chi taiwan earthquake occurred manuscript received 31 july 2000

a climatology of midlatitude mesoscale convective vortices in the rapid update cycle abstract climatological characteristics of mesoscale convective vortices mcvs occurring in the state of oklahoma during the late spring and summer of four years are investigated the mcv cases are selected based on vortex detection by an objective algorithm operating on analyses from the rapid update cycle ruc model consistent with a previous study true mcvs represent only about 20 of the mesoscale relative vorticity maxima detected by the algorithm the mcvs have a broad range of radii and intensities and their longevities range between 1 and 54 h their median radius is about 200 km and their median midlevel relative vorticity is 1 2 10 4 s 1 there appears to be no significant relationship between mcv longevity and intensity similar to past estimates approximately 40 of the mcvs generate secondary convection within their circulations the mean synoptic scale mcv environment is determined by the use of a ruc based composite analysis at four different stages in the mcv life cycle defined b

influence of dip and velocity heterogeneity on reverse and normal faulting rupture dynamics and near fault ground motions we investigate the influence of fault dip 35 60 and crustal velocity heterogeneity on rupture dynamics and near fault ground motions from normal and reverse faulting the same initial conditions were used except for the direction of initial shear stress in each dynamic rupture calculation we used two 3d elastic finite element approaches that employ split nodes for the computations in homogeneous and weakly heterogeneous half spaces with faults dipping 50 maximum fault normal peak velocities occurred on the hanging wall however for fault dips 50 maximum fault normal peak velocities occurred on the footwall bilateral and unilateral rupture simulations in weakly heterogeneous media found that reverse faulting slip velocities frequency band 1 3 5 hz were on average 39 larger than those during normal faulting however on average reverse faulting slip velocities were only 16 larger than normal faulting slip velocities for frequencies 50

an observation of rupture pulses of the 20 september 1999 chi chi taiwan earthquake from near field seismograms the ground velocity recordings of the 20 september 1999 chi chi taiwan earthquake recorded at stations near the ruptured fault trace show a simple large amplitude and long period pulse following the s wave which is closely associated with the surface faulting and the rupture process of thrust faulting the conspicuous pulse on the ground velocity seismogram following the s wave arrival called the s 1 phase is interpreted as the superposition of the rupture pulses that nucleate at an asperity near and underneath the station and propagate up dip and laterally along the fault plane toward the surface stations the arrival times of the s 1 phase and the onsets of the permanent displacement at stations near and along the ruptured fault trace increase with hypocentral distance suggesting that the rupture of the chi chi earthquake might have initiated at the hypocenter of the mainshock and propagated both upward and laterally from south to north on the basis of the travel time differences between the s 1 phase and the direct s wave at the stations near and along the ruptured fault trace the rupture velocities varied from 2 28 to 2 69 km sec with an average rupture velocity of about 2 49 km sec the rupture velocities decreased from south to north

the three dimensional dynamics of dipping faults recent two dimensional dynamic simulations of dip slip faulting niel sen 1998 oglesby et al 1998 2000 shi et al 1998 have shown that the asym metric geometry of dip slip faults that intersect the free surface can have large effects on the dynamics of earthquake rupture the nonvertical dip angle of such faults leads to larger motion on the footwall than the hanging wall as well as much larger motion from thrust reverse faults than from normal faults with the same geometry and stress magnitudes in the present work we perform full three dimensional simulations of thrust reverse normal and strike slip faults and show that the same effects exist in three dimensions strike slip fault motion is either in between or lower than the motion of both dip slip faults additional three dimensional effects include strong rake rotation at the free surface the results confirm the findings of the previous studies and further elucidate the dynamic effects of the free surface on fault rupture slip and ground motion they are also borne out by early analyses of the 1999 chi chi taiwan thrust earthquake which displayed higher motion on the hanging wall than on the footwall and a strong oblique component of motion at the surface

teleseismic body waves from dynamically rupturing shallow thrust faults are they opaque for surface reflected phases we investigate whether a shallow dipping thrust fault is prone to wave slip interactions via surface reflected waves affecting the dynamic slip if so can these interactions create faults that are opaque to radiated energy furthermore in this case of a shallow dipping thrust fault can incorrectly assuming a transparent fault while using dislocation theory lead to underestimates of seismic moment slip time histories are generated in three dimensional dynamic rupture simulations while allowing for varying degrees of wave slip interaction controlled by fault friction models based on the slip time histories p and sh seismograms are calculated for stations at teleseismic distances the overburdening pressure caused by gravity eliminates mode i opening except at the tip of the fault near the surface hence mode i opening has no effect on the teleseismic signal normalizing by a haskell like traditional kinematic rupture we find teleseismic peak to peak displacement amplitudes are approximately 1 0 for both p and sh waves except for the unrealistic case of zero sliding friction zero sliding friction has peak to peak amplitudes of 1 6 for p and 2 0 for sh waves the fault slip oscillates about its equilibrium value resulting in a large nonzero 0 08 hz spectral peak not seen in other ruptures these results indicate wave slip interactions associated with surface reflected phases in real earthquakes should have little to no effect on teleseismic motions thus haskell like kinematic dislocation theory transparent fault conditions can be safely used to simulate teleseismic waveforms in the earth

dynamic earthquake ruptures in the presence of lithostatic normal stresses implications for friction models and heat production we simulate dynamic ruptures on a strike slip fault in homogeneous and layered half spaces and on a thrust fault in a layered half space with traditional friction models sliding friction exceeds 50 of the fault normal compressive stress and unless the pore pressures approach the lithostatic stress the rupture characteristics depend strongly on the depth and sliding generates large amounts of heat under application of reasonable stress distributions with depth variation of the effective coefficient of friction with the square root of the shear modulus and the inverse of the depth creates distributions of stress drop and fracture energy that produce realistic rupture behavior this ad hoc friction model results in 1 low sliding friction at all depths and 2 fracture energy that is relatively independent of depth additionally friction models with rate weakening behavior which form pulselike ruptures appear to generate heterogeneity in the distributions of final slip and shear stress more effectively than those without such behavior which form cracklike ruptures for surface rupture on a thrust fault the simple slip weakening friction model which lacks rate weakening behavior accentuates the dynamic interactions between the seismic waves and the rupture and leads to excessively large ground motions on the hanging wall waveforms below the center of the fault which are associated with waves radiated to teleseismic distances indicate that source inversions of thrust events may slightly underestimate the slip at shallow depths

shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting recent thrust faulting models both physical and numerical have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults in this article recent field evidence supporting this strong asymmetry is described earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1 g and 100 cm sec new shattered rock evidence on the hanging wall of thrust faults in southern california supports the occurrence of intense ground motions on the other hand the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in southern california indicate relatively low ground motions thus indicating strong asymmetry in ground motions near the fault trace for thrust faults some current ground motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall this could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the los angeles basin and elsewhere in the world the new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models if such detachment occurs it could also have important implications for understanding thrust fault rupture mechanics for example the paradox of large overthrusts and the heat flow paradox for thrusts

earthquake induced chains of geologic hazards patterns mechanisms and impacts large earthquakes initiate chains of surface processes that last much longer than the brief moments of strong shaking most moderate and large magnitude earthquakes trigger landslides ranging from small failures in the soil cover to massive devastating rock avalanches some landslides dam rivers and impound lakes which can collapse days to centuries later and flood mountain valleys for hundreds of kilometers downstream landslide deposits on slopes can remobilize during heavy rainfall and evolve into debris flows cracks and fractures can form and widen on mountain crests and flanks promoting increased frequency of landslides that lasts for decades more gradual impacts involve the flushing of excess debris downstream by rivers which can generate bank erosion and floodplain accretion as well as channel avulsions that affect flooding frequency settlements ecosystems and infrastructure ultimately earthquake sequences and their geomorphic consequences alter mountain landscapes over both human and geologic time scales two recent events have attracted intense research into earthquake induced landslides and their consequences the magnitude m 7 6 chi chi taiwan earthquake of 1999 and the m 7 9 wenchuan china earthquake of 2008 using data and insights from these and several other earthquakes we analyze how such events initiate processes that change mountain landscapes highlight research gaps and suggest pathways toward a more complete understanding of the seismic effects on the earth s surface

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

the importance of the dynamic source effects on strong ground motion during the 1999 chi chi taiwan earthquake brief interpretation of the damage distribution on buildings the 1999 chi chi taiwan earthquake that originated on a low angle reverse fault showed complexity and uncommon characteristics the records show that the hanging wall side is characterized by larger particle motions than the foot wall and the ground motion is stronger in the northern part than in the southern part of the causative fault although the strongest ground motion occurred near the north ern part of the trace structural damage was heavier in the southern part in order to get a better understanding of the complex damage distribution caused by this earth quake the dynamic rupture process was numerically simulated because of the dif ferences between the observed features of the rupture process in the northern and southern parts of the fault each part was modeled independently by using a 2d discrete element model dem the principal results of the simulation show that the velocity ground motions in the northern part in the frequency range of 0 5 2 hz natural frequency range of standard structures are small near the surface break thus light structural damage might be predicted near the surface rupture moreover in the northern part the fault rupture propagation reaches the surface with a very slow velocity about 1 2 km sec however in the southern part the rupture propa gation reaches the surface with higher velocity about 3 0 km sec these differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part

dynamics of dip slip faulting explorations in two dimensions dynamic models of earthquake rupture and slip are a powerful method by which to investigate the physics of earthquakes owing to both conceptual and computational constraints dynamic earthquake models have largely been limited to cases with geometrical symmetry such as faults in unbounded media or vertical faults however there are both observational and theoretical reasons to believe that nonvertical dip slip faults behave differently from faults with more symmetrical geometries previous observations have shown greater ground motion from thrust reverse faults than normal faults and higher ground motion on hanging walls than on footwalls in the present work two dimensional dynamic simulations of thrust reverse and normal earthquakes show precisely these effects and also elucidate their causes for typical nonvertical dip slip faults the breakdown of symmetry with respect to the free surface allows radiated seismic waves to reflect off the free surface and to hit the fault again altering the stress field on the fault this process can lead to time dependent normal stress and a feedback between the friction rupture processes and seismic radiation this interaction leads to thrust reverse faults producing much higher fault and ground motion than normal faults with the same geometry and stress magnitudes the asymmetric geometry also directly leads to higher motion on the hanging walls of such faults than on the footwalls simulations show that these effects occur for a variety of dip angles but only for faults that either intersect or closely approach the free surface the results emphasize the strong effect that the free surface can have on the dynamics of fault rupture and slip

the lake bosumtwi meteorite impact structure ghana a magnetic image from a third observational level the bosumtwi impact structure in ghana is the youngest and best preserved medium sized impact structure on earth and because of the vast amount of prior geophysical and geological data gathered in the area it constitutes a great natural laboratory to try to develop new geophysical interpretation and modeling techniques during the 2004 international continental scientific drilling program icdp drilling campaign at highlight of this study is that we demonstrate that is possible to acquire at least partial vector data with readily available instrumentation

dynamic rupture processes on a dipping fault and estimates of stress drop and strength excess from the results of waveform inversion summary the dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3 d spontaneous shear crack models the wave equations for the 3 d space are solved numerically by a finite difference method under appropriate boundary conditions the displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault it was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall side and that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the medium particularly of the existence of low velocity surface layers on the basis of the above model the dynamic rupture process of the 1961 kita mino earthquake in central japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis for this purpose a locking fracture criterion was introduced for rupture propagation from which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated the dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips the results revealed quite heterogeneous distribution of strength excess indicating large values near the sw section of the fault at shallow depths and in the central bottom section in contrast to small values at mid depths in the ne section around the rupture nucleus zone the distribution of stress drop was also found to be heterogeneous with the existence of high stress drop zones in the ne shallow and bottom sections and a low stress drop zone at most of mid depths except in the sw section these results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low strength excess and small stress drop then broke the shallower and deeper fault sections with moderate strength excess and high stress drop and finally ruptured barrier zones with high strength excess these zones may be interpreted as weak and strong asperities and barriers respectively

detection and prediction of warm season midtropospheric vortices by the rapid update cycle the authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the rapid update cycle version 2 ruc 2 during the period 1 may to 31 august 1999 a total of 203 vortices meeting conditions of an automated algorithm were found of these 86 were observed to form within organized convection and were termed mesoscale convective vortices mcvs mcvs were favored over a broad area from eastern colorado and western nebraska to the mississippi river valley essentially collocated with the loci of organized convection the remaining 117 vortices termed dry vortices clustered in the immediate lee of the rocky mountains and over the southeastern united states vortices arising from convection had considerably greater intensity and longevity than dry vortices they were roughly five times more likely to be involved with the triggering of new convection a relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices consistent with many dry vortices having a topographic origin and mcvs arising from organized convection parameters such as horizontal scale background vertical wind shear and horizontal deformation were not systematically related to intensity or longevity prediction of mesoscale vortices by the ruc 2 was examined for a subsample of all cases in general the ruc 2 was able to predict the evolution of vortices once analyzed but had virtually no skill at predicting 12 h in advance the formation of the vortices however forecasts of organized convection should still benefit from accurate predictions of long lived vortex tracks

geochemistry and age of ivory coast tektites and microtektites abstract ivory coast tektites were first reported in 1934 from a geographically restricted area at ivory coast west africa although some additional specimens have been found later the total number remains small a few hundred the bosumtwi impact crater in ghana is most likely the source crater for the ivory coast tektites based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions in addition to tektites on land microtektites were found in so far eleven deep sea cores off the west african coast between about 9 n and 11 s and 0 and 23 w defining the extent of the ivory coast tektite strewn field in this study we analyzed eleven ivory coast tektites for their major and trace element composition studied their petrographical characteristics provided major element data for 111 microtektites and major and trace element data for four microtektites we determined the 40ar 39ar step heating age of five ivory coast tektites and four microtektites and obtained fission track dates for ten tektites and one bosumtwi impact glass the tektites have very small intersample and intrasample variations of their major and trace element composition 111 ivory coast microtektites from eleven cores were analyzed for their major element compositional range their compositional range is significantly wider than that of the ivory coast tektites but the majority of all microtektites have compositions very similar to those of the tektites within a factor of 1 2 trace element compositions of the tektites also show little variation between samples the samples do not show any distinct eu anomaly in the ree patterns this characteristic as well as the high absolute ree abundances and la nybn ratios of about 8 indicate that archean rocks are plausible source rocks the major and trace element contents of four individually analyzed ivory coast microtektites show compositions that are very similar to those of the ivory coast tektites however the microtektites contain 20 rel higher abundances of some of the lithophile and siderophile trace elements such as sc cr co ni sr zr ba hf ta th and the rees these differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite forming melt the ivory coast microtektites also have a uniform internal composition duplicate 40ar 39ar step heating age analyses were performed on five tektites the best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs resulting in an age of 1 1 0 05 ma we also tried to date four microtektites by 40ar 39ar age analyses but their young age and small sample size makes it impossible to assign a reliable age to the microtektites one run yielded satisfactory results that were similar to the tektite age in addition we determined the fission track ages for ten individual ivory coast tektite samples and for one impact glass sample from the bosumtwi crater the track size corrected ages for the ivory coast tektites ranged from 0 91 to 1 18 ma resulting in an average fission track age of 1 05 0 11 ma this age is within errors identical to that of the bosumtwi impact glass at 1 03 0 11 ma and to the 40ar 39ar age of 1 1 0 05 ma the preferred age of the ivory coast tektite event is 1 07 ma

dynamic faulting under rate dependent friction we discuss the effects of rate dependent friction on the propagation of seismic rupture on active faults several physicists using burridge and knopoff s box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self similar stress distribution on the fault plane if this model proves to be correct it has important consequences for the origin of the complexity of seismic sources in order to test these ideas on a more realistic earthquake model we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate dependent friction we study rupture dynamics of models with single and twin asperities in our models asperities are places on the fault with a higher value of prestress othewise all fault parameters are homogeneous we show that for models with such asperities a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough for models with asperities we can also observe narrow slip velocity pulses qualitatively similar to the so called heaton pulses observed in some earthquake accelerograms we also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law

dynamic wedge failure reveals anomalous energy radiation of shallow subduction earthquakes abstract dynamically induced coulomb failure in the overriding wedge significantly affects energy radiation of shallow subduction earthquakes for a wedge on the verge of failure extensive fluid assisted coseismic failure due to updip rupture causes significant seafloor uplift above a shallow dipping basal fault the large inelastic uplift greatly enhanced by the presence of free surface significantly dilates the fault behind the rupture front during the rupture propagation which reduces the effective normal stress and sliding friction on the fault and increases the dynamic stress drop and slip velocity as a result slip velocity time histories in the shallow section of the fault tend to have a snail like shape leading to depletion of high frequencies in the slip velocity field and the resultant source time function we also show that the failure in the wedge acts as a large energy sink while contributing to seismic moment giving rise to distributed heat generation i e small heat flow anomaly across the fault low moment scaled radiated energy slow rupture velocity and small directivity which provides a unifying interpretation for nearly all anomalous observations documented for shallow subduction earthquakes

joint inversion of surface and three component borehole magnetic data the inversion of magnetic data is inherently nonunique with respect to the distance between the source and observation locations this manifests itself as an ambiguity in the source depth when surface data are inverted and as an ambiguity in the distance between the source and boreholes if borehole data are inverted joint inversion of surface and borehole data can help to reduce this nonuniqueness to achieve this we develop an algorithm for inverting data sets that have arbitrary observation locations in boreholes and above the surface the algorithm depends upon weighting functions that counteract the geometric decay of magnetic kernels with distance from the observer we apply these weighting functions to the inversion of three component magnetic data collected in boreholes and then to the joint inversion of surface and borehole data both synthetic and field data sets are used to illustrate the new inversion algorithm when borehole data are inverted directly three component data are far more usefu

results of pre drilling potential field measurements at the bosumtwi crater gravity and magnetic measurements were carried out at the bosumtwi crater to determine the geophysical signature of the crater land gravity data was acquired at 163 locations around the structure and on the shore of the lake the separation between the gravity stations was 500 m for radial profiles but 700 1000 m along roads and footpaths that ran parallel to the lakes shore additionally a marine gravity survey was carried out along 14 north south and 15 east west profiles on the lake magnetic data was also acquired along 14 north south profiles on the lake in all marine surveys the line spacing was 800 m and navigation was provided by a garmin 235 echo sounder gps the gravity signature of the crater is characterized by a negative bouguer anomaly with an amplitude of about 18 mgal using the seismic results as constraints the gravity model obtained indicates the central uplift at a depth of 250 m the negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor the impact breccias within the crater and the sedimentary and water infilling of the lake magnetic modeling yielded a model for the causative body which is located north of the central uplift the model has a magnetic susceptibility of 0 03 s i and extends from a depth of 250 to 610 m the causative bodies have been interpreted as impactites

the dynamics of thrust and normal faults over multiple earthquake cycles effects of dipping fault geometry we perform dynamic simulations of thrust and normal faults over multiple earthquake cycles our goal is to explore effects of asymmetric fault geometry on the long term seismicity and dynamics of dipping faults a dynamic finite element method is used to model the interseismic and coseismic processes with a dynamic relaxation technique for the former the faults are loaded by stable sliding along the downward continuation of the faults the asymmetric fault geometry of dipping faults with respect to the free surface cause changes in normal stress during the interseismic and coseismic periods these changes are of opposite sign in the two periods resulting in a stabilization of the normal and shear stresses over many earthquake cycles both faults develop relatively stable event patterns in which a large event that ruptures the entire fault is preceded by a number of small events with various rupture lengths a strong asymmetry in fault and ground motion exists between thrust and normal faults and between the hanging wall and the footwall of both faults on faults dipping less than 70 in both normal and thrust faults the horizontal component of ground motion dominates on the footwall while the vertical component dominates on the hanging wall the above results may have implications in seismic hazard analysis and building design in regions where dip slip faults predominate

a long lived mesoscale convective complex part ii evolution and structure of the mature complex abstract an eight day episode in august 1977 is described wherein 14 mesoscale convective complexes mccs developed in the central united states including one to the immediate ice of the rocky mountains on each day of the episode in part i of this article the daytime genesis of one of these systems was traced from its pre convective roots in the mountains of central colorado to its incipient mcc stage on the plains of eastern colorado in this paper its continued nocturnal development into a large mcc over kansas is followed satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western atlantic ocean analysis is focused on the mature stage of this and a second mcc in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature and also to previous studies of tropical mesoscale convective systems many of the important characteristics of midlatitude mccs fo

petrography and shock related remagnetization of pyrrhotite in drill cores from the bosumtwi impact crater drilling project ghana rock magnetic and magnetic mineralogy data are presented from the international continental scientific drilling program icdp drill cores lb 07a and lb 08a of the bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact related remagnetization mechanism basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided magnetic susceptibility 150 500 10 6 si and natural remanent magnetization 10 3 10 1 a m are generally weak but locally higher values up to 10 6 10 3 si and 43 a m occur sixty three percent of the investigated rock specimens show q values above 1 indicating that remanence clearly dominates over induced magnetization which is a typical feature of impact structures ferrimagnetic pyrrhotite is the main magnetite phase which occurs besides minor magnetite and a magnetic phase with a curie temperature between 330 and 350 c interpreted as anomalous pyrrhotite coercive forces are between 20 and 40 mt brecciation and fracturing of pyrrhotite is a common feature confirming its pre impact origin grain sizes of pyrrhotite show a large variation but the numerous stress induced nanostructures observable by transmission electron microscopy tem are assumed to behave as single domain grains we suggest that the drilled rocks lost their pre shock remanence memory during the shock event and acquired a new stable remanence during shock induced grain size reduction the observed brittle microstructures indicate temperatures not higher than 250 c which is below the curie temperature of ferrimagnetic pyrrhotite 310 c

precarious rock constraints on ground motion from historic and recent earthquakes in southern california new examples of precarious rocks are presented these rocks provide constraints on ground motion for historical and recent large earthquakes an approximate field calibration of the precarious rock methodology was provided by the m 7 1 hector mine earthquake of 16 october 1999 previously documented rocks at granite pass near kelso california brune 1996 were overturned by the earthquake and a nearby strong motion station indicated ground motions of about 0 2 g consistent with the toppling accelerations estimated in the published article the time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka consistent with previously estimated ages for precarious rocks this first example of an actual earthquake field test lends strong support to the precarious rock methodology the other rocks discussed here give constraints on ground motions for historic earthquakes such as the 1812 and 1857 san andreas fault earthquakes the 1899 and 1912 san jacinto fault earthquakes the 1952 kern county earthquake and possibly recent but prehistoric earthquakes on the banning and garlock faults the ground motion constraints for these earthquakes are lower than predicted by some recent ground motion attenuation curves which are extrapolations to near fault distances from a data set dominated by data from larger distances but are generally consistent with peak ground accelerations observed from the recent large turkey and taiwan earthquakes and provide important additional information for seismic hazard analysis on the other hand there are areas where precarious rocks would be expected on the basis of previous studies but are apparently not found this suggests possible earthquakes on previously unrecognized or only recently recognized faults one such area is in northwestern san diego and southwestern orange county between the elsinore and newport inglewood faults the lack of precarious rocks in this area might be attributed to recent earthquakes on the blind thrust faults proposed in the area by grant et al 1999 2002 and rivero et al 2000

the lake bosumtwi meteorite impact structure ghana where is the magnetic source the bosumtwi impact structure ghana is a young and well preserved structure where a vast amount of information is available to constrain any geophysical model previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact melt body underneath the lake recent drilling through the structure did not penetrate such an expected impact melt rock magnetic source a new 3 d magnetic model for the structure was constructed based on a newly acquired higher resolution marine magnetic data set with consideration of the observed gravity data on the lake previous seismic models and the magnetic properties and lithology identified in the two international continental scientific drilling program icdp deep boreholes the new model contains highly magnetic bodies located in the northeast sector of the structure not centered onto the drilling sites as in previous models higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the kumasi batholith which outcrops to the northeast of the structure the possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact melt sheet predicted by previous models of the structure is not supported detailed magnetic scanning of the kumasi batholith to the north and the bansu intrusion to the south would provide a test for this interpretation

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

the lake bosumtwi impact crater ghana the 1 m y old bosumtwi crater ghana has a nearly circular shape with a rim diameter of 11 km north south and 10 km east west it is surrounded by a circular depression and an outer ridge of diameter 20 km polymict breccias averaging at least 20 m thick with clasts as much as 5 m long occur on the outer ridge and the crater rim shows in situ shattered rock patches of suevite have been found in the circular depression north and south of the crater analogy with better known craters suggests that bosumtwi has a central uplift rising to 200 m beneath the lake floor an aeromagnetic anomaly of amplitude 50 nanotesla nt over the northern half of the lake is interpreted as due to a layer of magnetized fallback breccia beneath the lake sediments the normal polarity of the breccia shows that the crater was formed during the normal jaramillo event of 0 97 to 0 85 m y ago which agrees with the magnetic stratigraphy of the related ivory coast microtektites a regional gravity survey indicates a negative bouguer anomaly over the crater there is some geochemical evidence that the meteorite was an iron and its mass and energy are suggested as about 108 tons and 3 1019 joules or 7 3 103 megatons

short slip duration in dynamic rupture in the presence of heterogeneous fault properties recent studies of strong motion data consistently show that the risetime duration of slip at particular locations on the fault is significantly shorter than the overall rupture duration the physical explanation for this observation and its implications have become central issues in earthquake source studies two classes of mechanisms have been proposed to explain short risetimes one explanation is that velocity weakening frictional behavior on the fault surface causes the fault to self heal this possibility is suggested by rate dependent friction observed in laboratory experiments and by some two dimensional dynamic numerical simulations of earthquake rupture it has recently been demonstrated however that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self heal an alternative explanation for short risetimes is that spatially heterogeneous fault strength e g barriers limit the slip duration in this paper we investigate this second explanation for short risetimes by constructing a three dimensional dynamic rupture model for the 1984 morgan hill california earthquake mw 6 2 using a kinematic model previously obtained from waveform inversion of strong motion data we assume velocity independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model the slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data by alternating between dynamic and kinematic modeling we obtain a dynamic model that provides an acceptable fit to the recorded waveforms in this dynamic model the risetime is short over most of the fault which is attributable entirely to the short scale length slip stress drop heterogeneity required by the strong motion data a self healing mechanism such as strongly velocity dependent friction is not required to explain the short risetimes observed in this earthquake

the dynamics of strike slip step overs with linking dip slip faults fault step overs with linking dip slip faults are common features on long strike slip fault systems worldwide it has been noted by various researchers that under some circumstances earthquakes can jump across fault step overs to cascade into large events while under other circumstances rupture is arrested at step overs there is also evidence that fault step overs may be preferential locations for earthquake nucleation the present work uses the 3d finite element method to model the dynamics of strike slip fault systems with step overs and linking dip slip faults i find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step over leading to a larger event additionally dilational step overs with linking normal faults are more prone to through going rupture than compressional step overs with linking thrust faults this difference is due to the sign of the normal stress increment on the dip slip fault caused by slip on the strike slip segments slip on the strike slip segments causes a negative unclamping normal stress increment on the linking normal fault in a dilational step over whereas the opposite effect occurs on the linking thrust fault in a compressional step over even in cases for which both dilational and compressional step overs can experience through going rupture dilational step overs typically experience higher slip particularly on the linking normal fault in the compressional case rupture nucleation on the linking thrust fault may increase the likelihood of through going rupture compared to nucleation on one of the strike slip segments near the intersections between the fault segments the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field the results help to emphasize the importance of two way interactions between nearby fault segments during the earthquake rupture process the results also may have implications for the probability of large earthquakes along geometrically complex strike slip fault systems and may help explain why step overs sometimes act as barriers and other times as nucleation locations for large earthquakes

thrust sheet motion and earthquake mechanisms abstract the mechanism of thrust sheet motion and that of earthquake generation are two aspects of the same phenomenon studies of earthquake mechanisms show that only a part of the thrust fault generally less than 10km in length is in motion at any given instant this part sweeps the length of the fault moving at approximately 3 km s since shear fracture propagation is inherently a dynamic phenomenon it cannot be understood on the basis of the equations of static equilibrium the previously proposed mechanisms all rest on such an equilibrium which necessarily implies that the motion of a thrust sheet occurs simultaneously over its whole length since the latter may exceed 100 km the motion becomes a mechanical enigma this enigma vanishes once the dynamic nature of the problem is recognized the proposed mechanism also suggests that solution features on the fault result from the dissolution of barriers thus explaining the motion of only part of a thrust sheet

dynamic stress changes during earthquake rupture we assess two competing dynamic interpretations that have been pro posed for the short slip durations characteristic of kinematic earthquake models de rived by inversion of earthquake waveform and geodetic data the first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthen ing of the fault surface occurs after passage of the rupture front a hypothesized mechanical behavior that has been referred to as self healing the second inter pretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level a condition we refer to as geometrical constraint these interpretations imply contrasting pre dictions for the time dependence of the fault plane shear stresses we compare these predictions with dynamic shear stress changes for the 1992 landers m 7 3 1994 northridge m 6 7 and 1995 kobe m 6 9 earthquakes stress changes are com puted from kinematic slip models of these earthquakes using a finite difference method for each event static stress drop is highly variable spatially with high stress drop patches embedded in a background of low and largely negative stress drop the time histories of stress change show predominantly monotonic stress change after passage of the rupture front settling to a residual level without significant evidence for dynamic restrengthening the stress change at the rupture front is usu ally gradual rather than abrupt probably reflecting the limited resolution inherent in the underlying kinematic inversions on the basis of this analysis as well as recent similar results obtained independently for the kobe and morgan hill earthquakes we conclude that at the present time the self healing hypothesis is unnecessary to explain earthquake kinematics

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

earthquake nucleation on dip slip faults with depth dependent frictional properties 1 previous research has indicated that for dip slip faults embedded in a half space the nonvertical asymmetric fault geometry can have a significant effect on the seismic nucleation process using quasi static simulation methods and a slip weakening friction law we investigate the coupled effects of asymmetric fault geometry and depth dependent frictional properties on seismic nucleation on dip slip faults as a first order approximation to the depth dependent parameters in the earth we assume that the normal stress tapers to a small value near the free surface and we assume that the drop in frictional coefficient decreases with depth we find that these depth dependent properties have an important effect on the nucleation process of these faults compared to models with homogeneous frictional properties the nucleation point moves down dip the time to unstable slip nucleation decreases and the amount of preseismic surface slip increases significantly however these effects do not dominate the effects of the asymmetric dipping geometry of these faults on the nucleation process thrust faults nucleate farther up dip than normal faults and nucleate earlier as well normal faults have a larger amount of preseismic surface slip than thrust faults the results indicate that both geometrical and frictional heterogeneity can have important effects on nucleation and that both sources of complexity must be taken into account to produce accurate models of the seismic nucleation process

applying wedge theory to dynamic rupture modeling of fault junctions wedges such as accretionary prisms and thin skinned fold and thrust belts occur frequently in nature and can be the site of devastating earthquakes critical wedge theory can be applied to these settings but this steady state description of wedge deformation is at odds with the periodic occurrence of earthquakes we discuss how critical wedge theory applies to the seismic cycle and we use elastic wedge theory to constrain realistic stress states our goal is to determine the rupture behavior of an earthquake in a wedge if rupture initiates on the basal sliding surface will it stay confined to the basal surface or will it propagate onto a fault branch interior to the wedge this information can significantly alter the seismic hazard in areas where fault intersections occur we answer this question using numerical models of dynamic rupture propagation through branched geometries for which the stress state is a pivotal input parameter we apply wedge theory to constrain the stress state but inherent to this theory is the assumption of a weak basal fault we investigate the role of this assumption and determine that rupture is unlikely to propagate from a weak basal fault onto a strong branch fault without the aid of a physical process such as pore fluid migration along the branch this framework is applied to the rupture of the 2008 wenchuan earthquake we find that we are able to reproduce the behavior at some fault intersections but our 2d model is not able to reproduce all the behaviors possibly due to the oblique nature of this event

metalliferous mining geophysics state of the art in the last decade of the 20th century and the beginning of the new millennium the downturn in mining activity experienced during the 1990s did not preclude significant new developments in various areas of mining geophysics the methodology for acquiring and compiling data has kept pace with the latest technological developments from global positioning system navigation to raster displays and parallel computing wavelet transforms principal component analysis and fractals have begun to find successful applications in both processing and interpretation of geophysical data methods of quantitatively interpreting inverting anomalies in terms of 2 d and 3 d models of causative bodies are becoming common for the first time it is possible to make airborne gravity gradient measurements suitable for use in mineral exploration lower transmitter frequencies for airborne time domain electromagnetic em systems have enabled surveys in areas where conductive cover previously screened basement conductors the use of approximation algorithms has allowed the transformation of either time domai

fault geometry and the dynamics of the 1999 chi chi taiwan earthquake the 1999 m 7 6 chi chi taiwan earthquake produced a data set of unparalleled size and quality particularly in the near source region where data have been previously quite scarce the large amount of near source data allows the verification of many predictions of thrust fault behavior for faults that intersect the surface of the earth through rigorous three dimensional dynamic models of the chi chi earthquake it can be shown that many aspects of the observed near source ground motion in this event are direct effects of the asymmetrical dipping fault geometry these effects include the hanging wall moving more than the footwall with strongly peaked velocities right at the fault trace and a transition from predominantly thrust motion in the south of the fault to largely left lateral motion in the north building on the work of oglesby and day 2001 the current work helps to delineate the effects of fault geometry nonuniform prestress and dynamic waves on the physics of the chi chi earthquake and dip slip faults in general in particular we find that a completely homogeneous prestress pattern still fits the gross features of the near source ground motion quite well additionally the strike slip component of motion near the fault trace is seen to be a combination of dynamic and static effects finally dynamic overshoot is seen to be much larger for dip slip faults than for otherwise identical vertical faults the results emphasize the necessity of rigorous models that correctly account for both the effects of fault geometry and dynamic waves in the rupture and slip processes manuscript received 31 july 2000

global patterns of radiated seismic energy and apparent stress radiated energies from shallow earthquakes with magnitudes 5 8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress in contrast to traditional methods which have relied upon empirical formulas these energies are computed through direct spectral analysis of broadband seismic waveforms velocity squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases frequency dependent attenuation and focal mechanism the least squares regression fit of energy es to surface wave magnitude ms for a global set of 397 earthquakes yields log es 4 4 1 5ms which implies that the gutenberg richter relationship overestimates the energies of earthquakes the least squares fit between es and seismic moment m0 is given by the relationship es 1 6 10 5 m0 which yields 0 47 mpa as the average global value of apparent stress however the regression lines of both es ms and es m0 yield poor empirical predictors for the actual energy radiated by any given earthquake the scatter of data is more than an order of magnitude about each of the regression lines on the other hand global variations between es and mo while large are not random when subsets of es m0 are plotted as a function of seismic region and faulting type the scatter in data is substantially reduced the es m0 fits for many seismic regions and tectonic environments are very distinctive and a characteristic apparent stress c can be derived the lowest apparent stresses 3 0 mpa are associated with strike slip earthquakes that occur at oceanic ridge ridge transforms and in intraplate environments seaward of island arcs intermediate values of apparent stress 1 5 a 3 0 mpa are associated with strike slip earthquakes at incipient or transitional plate boundaries in general the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress an energy magnitude me can complement moment magnitude mw in describing the size of an earthquake me being derived from velocity power spectra is a measure of seismic potential for damage mw being derived from the low frequency asymptote of displacement spectra is more physically related to the final static displacement of an earthquake when earthquake size is ranked by moment a list of the largest events is dominated by earthquakes with thrust mechanisms when earthquake size is ranked by energy the list of the largest events is dominated by strike slip earthquakes

experimental investigation of strong ground motion due to thrust fault earthquakes thrust fault earthquakes are studied in a laboratory earthquake setup previously used to investigate analog strike slip seismic events dynamic mode ii ruptures are generated along preexisting faults in an analog material homalite h 100 and their interaction with the free surface is studied for both sub rayleigh and supershear rupture speeds high speed digital photography and laser velocimeter diagnostics are used synergistically to identify and study the ground velocity signatures caused by the various features of the generated ruptures the obtained surface normal motions of both sub rayleigh and supershear ruptures show substantial asymmetry between the hanging and footwall with the hanging wall experiencing much larger velocity amplitudes the main features of the surface velocity traces at various stations can be explained by the calculated arrivals of various waves and fronts mach cones pand s waves and sub rayleigh features in both the sub rayleigh and supershear cases the arrival of the rupture tip generates a prominent rayleigh wave traveling along the simulated earth s surface supershear events feature larger amplitudes of ground shaking profiles all signatures in the surface motion records attenuate and broaden with increasing distance from the fault trace the signatures corresponding to the arrival of the mach fronts attenuate with distance at a slower rate than those from sub rayleigh ruptures the arrival of the updip supershear rupture at the free surface creates a downdip propagating slip feature with its own mach cone these additional mach fronts further amplify ground shaking on the hanging and footwalls

dynamics of earthquake normal faulting two dimensional lattice particle model a 2d lattice particle model is used to simulate the dynamic rupture process of a normal fault the system equations for the particle motions are solved numerically by the finite difference method under a given block boundary condition the flexibility of the implementation of a 2d lattice particle model to simulate an earthquake dynamic process was demonstrated in previous modeling of a shallow angle thrust fault shi et al 1998 numerical results indicate that the particle motions displacement velocity and acceleration along the fault are discontinuous both in the fault parallel and fault normal directions with a localized slip rupture and localized fault separation in the vicinity of the fault outcrop the position at which the fault intersects with the free surface the particle velocity and acceleration increase rapidly both on the hanging wall and the footwall the particle motions on the hanging wall are larger than those on the footwall these motions are amplified as the fault scarp develops rupture breaks out at the surface with strong asymmetry between the hanging wall and the footwall along the free surface as the distance from the fault outcrop increases the particle velocity and acceleration decrease rapidly on the footwall and less rapidly on the hanging wall the asymmetrical particle motion results from the geometrical effect of the dip of the fault the free surface and the dynamic source rupture the combination of all of these effects causes a strong asymmetry in stress when the rupture pulse approaches the free surface the dynamically propagating rupture is characterized by a ramp slip time function accompanied with fault opening the slip pulse becomes sharper when the rupture approaches the free surface consequentially the hanging wall in the vicinity of the fault exhibits a large vibration which generates a strong surface wave propagating along the free surface away from the fault scarp on the hanging wall side this result is similar but significantly different from the numerical simulation of a normal fault with a moving double couple dislocation source benz and smith 1988 in addition the numerical result is qualitatively in agreement with recent foam rubber experiments brune and anooshehpoor 1999 and similar to results from a finite element simulation oglesby et al 1998 oglesby 1999 comparing with a 2d strike slip fault and thrust fault the particle motions in the vicinity of the normal fault on the free surface are smaller for a same size rupture pulse

simulation of the frictional stick slip instability a lattice solid model capable of simulating rock friction fracture and the associated seismic wave radiation is developed in order to study the origin of the stick slip instability that is responsible for earthquakes the model consists of a lattice of interacting particles in order to study the effect of surface roughness on the frictional behavior of elastic blocks being rubbed past one another the simplest possible particle interactions were specified corresponding to radially dependent elastic brittle bonds the model material can therefore be considered as round elastic grains with negligible friction between their surfaces although breaking of the bonds can occur fracturing energy is not considered stick slip behavior is observed in a numerical experiment involving 2d blocks with rough surfaces being rubbed past one another at a constant rate slip is initiated when two interlocking asperities push past one another exciting a slip pulse the pulse fronts propagate with speeds ranging from the rayleigh wave speed up to a value between the shear and compressional wave speeds in agreement with field observations and theoretical analyses of mode ii rupture slip rates are comparable to seismic rates in the initial part of one slip pulse whose front propagates at the rayleigh wave speed however the slip rate is an order of magnitude higher in the main part of pulses possibly because of the simplified model description that neglected intrinsic friction and the high rates at which the blocks were driven or alternatively uncertainty in slip rates obtained through the inversion of seismograms particle trajectories during slip have motions normal to the fault indicating that the fault surfaces jump apart during the passage of the slip pulse normal motion is expected as the asperities on the two surfaces ride over one another the form of the particle trajectories is similar to those observed in stick slip experiments involving foam rubber blocks bruneet al 1993 additional work is required to determine whether the slip pulses relate to the interface waves proposed by brune and co workers to explain the heat flow paradox and whether they are capable of inducing a significant local reduction in the normal stress it is hoped that the progressive development of the lattice solid model will lead to realistic simulations of earthquake dynamics and ultimately provide clues as to whether or not earthquakes are predictable

constraints on ground accelerations inferred from unfractured hoodoos near the garlock fault california abstract in the absence of long term instrumental data the presence of fragile geologic features near active faults can provide physical limits on the level of ground shaking that could potentially have significant implications for seismic hazards this paper introduces a multidisciplinary investigation that uses unfractured hoodoos in seismically active regions to constrain the level of ground accelerations at those locations although there is a large uncertainty associated with the age of the hoodoos because of their rapidly eroding nature they can still be useful in providing physical limits on ground motions associated with recent large events here we consider the fragilities of two hoodoos in the red rock canyon region within a few kilometers of the garlock fault which is an active strike slip fault in a transtensional region with at least a few large earthquakes in the holocene the hoodoos at these sites could be evidence of median or relatively low ground motions associated with large transtensional strike slip earthquakes results of our field and laboratory tests on two hoodoos provide constraints on peak ground accelerations pgas of and using the u s geological survey s usgs probabilistic seismic hazard psh deaggregation the dominant earthquake contributing to the hazard at the site of the hoodoos for the recurrence intervals of 475 975 and 2475 years is located at a distance of 4 8 km and has a magnitude of 7 63 consistent with the observed paleoearthquake evidence on the garlock fault the pgas corresponding to these three return periods are 0 26 0 40 and respectively therefore the survival of the more fragile hoodoo during a presumably large event on the garlock fault in the past 550 years would be consistent with the 2008 seismic hazard level if the ground motions during that event were below the median value

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

the mechanical paradox of large overthrusts the mechanical paradox of large overthrusts originates with a fallacious assumption in the conceptual model upon which mechanical analyses of overthrust faulting have been based the notion that the maximum width of a sheet of rock that may be displaced along an overthrust fault can be determined by using the mohr coulomb failure criterion to compare the strength of the rocks with the stress required to overcome the total frictional resistance to slip and the total cohesion or adhesion over the entire fault surface tacitly assumes that slip is initiated and occurs simultaneously over the entire fault surface this unstated and untenable assumption which is incompatible with what is known about real faults is the real explanation for the mechanical paradox a realistic model for overthrust faulting must be compatible with the nature of real overthrust faults and particularly with the way in which displacements including earthquake displacements occur on real faults most large overthrusts are discrete shear surfaces within a coherent mass of rock that is physically continuous around their ends total displacement clearly varies from place to place over these discrete shear surfaces and moreover must be the cumulative result of many relatively small incremental displacements that also varied from place to place over the finite part of the fault surface on which they occurred each increment began as a local shear failure and propagated along the fault as a smeared out somigliana dislocation at a velocity that was small considering the time required for propagation over the total area of the fault during any individual displacement event whether a megathrust earthquake or a creep event only a small part of the fault surface was in relative motion at any one time fault displacements may appear to occur simultaneously across laboratory specimens that are very small relative to the velocity of propagation of displacement because the transit time is so short when the linear scale is expanded over six orders of magnitude to encompass an entire surface of a large overthrust fault however the transit time is much longer because the velocity of propagation is very small relative to the area of the fault surface and the displacement clearly involves only a very small part of the fault surface at any one time the shape and size of overthrust faults result from the propagation of dislocations and this process is controlled by the strength heterogeneity and anisotropy of the rock mass as well as by variations in the regional stress field but not by the frictional resistance to sliding integrated over the entire fault surface

earthquakes on dipping faults the effects of broken symmetry dynamic simulations of earthquakes on dipping faults show asymmetric near source ground motion caused by the asymmetric geometry of such faults the ground motion from a thrust or reverse fault is larger than that of a normal fault by a factor of 2 or more given identical initial stress magnitudes the motion of the hanging wall is larger than that of the footwall in both thrust reverse and normal earthquakes the asymmetry between normal and thrust reverse faults results from time dependent normal stress caused by the interaction of the earthquake generated stress field with earth s free surface the asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault

simple crack models explain deformation induced by subduction zone megathrust earthquakes following the 2010 maule and 2011 tohoku earthquakes many studies have examined the relation between megathrust earthquakes and subsequent deformation here we apply simple models based on mode ii shear cracks including approximated effects of the free surface to study induced deformation during coseismic and early postseismic stages we distinguish between buried and surface ruptures represented by a full crack and a half crack model respectively we adopt an analogy based approach to interpret the half crack model from well known results of the full crack model which is also validated by our numerical simulations with transferable knowledge between the two models we provide easy ways to understand 1 the contrasting deformation patterns in the frontal wedge of the overriding plate between buried ruptures and surface ruptures 2 the correlation between triggered outer rise normal faulting and surface ruptures and 3 the similar deformation patterns for both buried and surface ruptures toward the down dip end with a preference for normal faulting in the overriding plate and for reverse faulting in the subducting plate these model outcomes are consistent with several recent observations on aftershocks and veins in a paleoaccretionary wedge we further investigate some important transient features during rupture propagation which show that a transition from compressional to extensional deformation in the frontal wedge of the overriding plate is possible even during a single rupture event our work provides alternative views for understanding various aspects of subduction zone megathrust earthquakes and raises the issue of important transient features that were typically ignored in previous studies

effects induced by an earthquake on its fault plane a boundary element study summary mechanical eiects left by a model earthquake on its fault plane in the post seismic phase are investigated employing the displacement discontinuity method simple crack models characterized by the release of a constant unidirectional shear traction are investigated rst both slip componentsparallel and normal to the traction directionare found to be non vanishing and to depend on fault depth dip aspect ratio and fault plane geometry the rake of the slip vector is similarly found to depend on depth and dip the fault plane is found to suier some small rotation and bending which may be responsible for the indentation of a transform tectonic margin particularly if cumulative eiects are considered very signi cant normal stress components are left over the shallow portion of the fault surface after an earthquake these are tensile for thrust faults compressive for normal faults and are typically comparable in size to the stress drop these normal stresses can easily be computed for more realistic seismic source models in which a variable slip is assigned normal stresses are induced in these cases too and positive shear stresses may even be induced on the fault plane in regions of high slip gradient several observations can be explained from the present model low dip thrust faults and high dip normal faults are found to be facilitated according to the coulomb failure criterion in repetitive earthquake cycles the shape of dip slip faults near the surface is predicted to be upward concave and the shallower aftershock activity generally found in the hanging block of a thrust event can be explained by unclamping mechanisms

a method for mapping apparent stress and energy radiation applied to the 1994 northridge earthquake fault zone using the northridge earthquake as an example we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane from the model of wald et al 1996 we estimated apparent stress for each subfault using where g is the modulus of rigidity is the shear wave speed and is the average slip rate the image of apparent stress mapped over the northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous but limited by the strength of the crust indeed over the depth range 5 to 17 km maximum values of apparent stress for a given depth interval agree with a max 0 06s z where s is the laboratory estimate of crustal strength as a function of depth z the seismic energy from each subfault was estimated from the product ada where a is subfault area and d its slip over the fault zone we found that the radiated energy is quite variable spatially with more than 50 of the total coming from just 15 of the subfaults

rupture dynamics on a bimaterial interface for dipping faults dip slip faulting may juxtapose different geologic materials with differ ent properties such that a strong material contrast will naturally tend to form both the material contrast across a fault and dip slip motion on a nonvertical fault lead to normal stress variations during earthquake rupture this normal stress variation will significantly affect dynamic rupture propagation to demonstrate this we model dynamic rupture propagation on two dimensional 2d reverse and normal faults 30 45 and 60 dipping with 20 material contrasts for predominantly up dip rupture propagation we find that normal stress variations due to the free surface and material contrast can either reinforce or counteract each other depending on the configuration for reverse faults we find a larger strength drop for a more com pliant hanging wall and a lower strength drop for a more compliant footwall for normal faults we find a larger strength drop for a more compliant footwall and a lower strength drop for a more compliant hanging wall for both reverse and normal faults ground motion will be more symmetric between the hanging wall and footwall with compliant material on the footwall and more asymmetric if more compliant materials are on the hanging wall our results have important implications for the dynamics of crustal and perhaps subduction zone earthquake faulting where strong bimaterial contrasts across dipping faults are possible in continental settings reverse faulting will tend to advect rigid materials from greater depth onto the hanging wall such that the effects of fault dip and material contrast will counteract one another in subduction zones the hanging wall is likely to be more compliant and hence the material and geometric effects may reinforce one another

precarious rock and overturned transformer evidence for ground shaking in the ms 7 7 kern county earthquake an analog for disastrous shaking from a major thrust fault in the los angeles basin precariously balanced rocks and overturned transformers in the vicinity of the white wolf fault provide constraints on ground motion during the 1952 m s 7 7 kern county earthquake a possible analog for an anticipated large earthquake in the los angeles basin shaw et al 2002 dolan et al 2003 on the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves on the other hand on the hanging wall there is evidence of intense ground shattering and lack of precarious rocks consistent with the intense hanging wall accelerations suggested by foam rubber modeling numerical modeling and observations from previous thrust fault earthquakes there is clear evidence of the effects of rupture directivity in ground motions on the hanging wall side of the fault from both precarious rocks and numerical simulations on the southwest part of the fault which is covered by sediments the thrust fault did not reach the surface blind thrust overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall an effect that may not be as effective when the rupture reaches the surface is not blind transformers near the up dip projection of the fault tip have been damaged or overturned on both the hanging wall and footwall sides of the fault the transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in los angeles we suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the los angeles basin specially given the fact that there is so little instrumental data from large thrust fault earthquakes

earthquake nucleation on dip slip faults 1 the nucleation of unstable slip on a fault is of key importance in our understanding of the seismic cycle we investigate how the asymmetric geometry of dip slip faults affects the nucleation of unstable slip on such faults previous researchers have devoted much effort to understanding this nucleation process on geometrically simple faults using a variety of frictional parameterizations however there are many reasons to believe that earthquake nucleation may be affected by fault complexity the breakdown of symmetry on a nonvertical dip slip fault means that normal stress is not constant during the slip process and that the hanging wall and footwall may not necessarily move equally using a slip strengthening and weakening friction law in a two dimensional quasi static model based on a variational boundary integral method we show that nucleation on dip slip faults is affected by both the dip angle and the direction of slip normal versus thrust reverse under otherwise identical conditions in a homogeneous half space i e neglecting depth dependent frictional or material properties thrust faults nucleate closer to the earth s surface than normal faults and take less time to do so these differences decrease as the dip angle of the fault increases to 90 the amount of preseismic surface slip is a much more complicated function of dip angle the results show that fault geometry may have an important role in the nucleation process as well as the process of dynamic rupture further research is needed to combine rigorous nucleation models with full rupture dynamics

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

dynamics of thrust faulting 2d lattice model particle motions in a foam rubber model of shallow angle thrust faulting show many features different from those commonly assumed in dislocation models of subduction thrusts brune 1996 as a complement to a physical foam rubber experiment we have carried out dynamic simulation using a 2d lattice numerical model the model is constructed as a triangular block sliding over a rectangular elastic block it consists of a 2d set of particles interacting with each other by nonlinear hooke s forces and obeying newton s equations of motion rough surfaces are introduced on the contact plane of the two blocks to simulate realistic friction the numerical simulations demonstrate several key features of thrust faulting including stick slip fault opening and strong breakout and shaking at the hanging wall toe the wedge shaped tip of the outcropping hanging wall block of the thrust fault all consistent with the foam rubber experiments the stick slip motion shows an approximate time and slip repeatable behavior each slip event is characterized by a self healing pulse associated with fault opening and frictional locking the rupture pulse propagates steadily along the fault when rupture reaches the toe of the fault outcrop the hanging wall breaks away from the foot wall and creates a large opening vibration of the hanging wall the peak acceleration observed at the hanging wall toe of the fault outcrop is about 3 to 4 times the peak acceleration in the center part of the fault and about 2 to 3 times the motion of the foot wall such a large increase in peak acceleration at the toe is caused mainly by multiply reflecting stress waves trapped in the wedge shaped hanging wall of the fault the strong asymmetry of the particle velocity between the hanging wall and foot wall is an important feature of the results consistent with the foam rubber model but different from standard dislocation models this dynamic result illustrates the important effects of thrust faulting geometry on fault slip and ground motion

fault dynamic rupture simulation of the hypocenter area of the thrust fault of the 1999 chi chi taiwan earthquake a 2d discrete element model was employed to simulate the rupture propagation and near source ground motion in the epicentral area of the 1999 chi chi taiwan earthquake the observations show that the hanging wall side is characterized by larger particle motions than the footwall side the simulation results reproduce the main features of the recorded ground motion and show that the particle velocity on the hanging wall side and on the footwall side are symmetric in the deeper fault region however as the crack approaches the free surface the hanging wall side is characterized by larger particle motions than the footwall side these results suggest that the difference in the particle motion on the hanging wall and on the footwall is due mainly to the asymmetric geometry of hanging wall and footwall thus the model used leads to good approximations in the vicinity of the epicenter

characteristics of near fault ground motions by dynamic thrust faulting two dimensional lattice particle approaches two dimensional lattice particle models have been used to simulate dynamic thrust faulting and its effects on near fault ground motions the lattice particle modeling approach has been demonstrated as an efficient way to model the dynamic rupture phenomena observed from a foam rubber experiment on dipping faults we constructed a 42 dipping fault model to simulate near fault ground motions under different site conditions in which different combinations of fault geometry blind fault underlying a sedimentary layer and outcropping thrust models and near surface sedimentary layers were investigated in particular dynamic behaviors between a blind fault with an overlying sedimentary layer and an outcropping fault without a sedimentary layer were compared in this simulation a dynamic slip pulse accompanied by fault separation was initiated at the deepest part of the fault and propagated updip along the fault under a subshear rupture velocity a strong asymmetrical ground motion pattern on the hanging wall and footwall caused by near source rupture effects was observed rupture directivity played an important role in determining the size and distribution of peak ground velocities and accelerations on the hanging wall and footwall in the case of an outcropping thrust without a sedimentary layer the hanging wall underwent a stronger ground motion caused by the near surface breakout phase as the rupture reached the free surface in the case of a blind thrust overlying a sedimentary layer the peak ground particle velocity and acceleration could be much larger on the footwall this is a result of the amplification effect of trapped long period seismic energy in the sedimentary layer the seismic energy emitted from the rupture stopping phase was incident to the sedimentary layer and radiated under the rupture directivity effect the radiated long period seismic energy was trapped in the sedimentary layer propagating away from the fault trace toward the footwall side the numerical results show that for an initial slip of 5 to 5 5 m the horizontal peak ground velocity and acceleration could reach about 1 5 m sec and 2 g respectively on the footwall for a blind thrust with a sedimentary layer with the same initial slip level the peak ground velocity and acceleration were about 1 1 m sec and 1 5 g respectively on the hanging wall for an outcropping thrust without a sedimentary layer these results can partially explain the field observations of precarious rock distributions and overturned transformers in the vicinity of the white wolf fault during the 1952 m s 7 6 kern county earthquake furthermore the current simulation can be used for near fault strong motion prediction for large thrust faults in the los angeles basin or similar tectonic settings around the world

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events

the three dimensional dynamics of a nonplanar thrust fault advances in computational methodology have made it possible to ex plore the dynamics of earthquake rupture on nonplanar faults using a method that allows geometrical flexibility we simulate in three dimensions the dynamics of a fault that has an abrupt change in dip with depth using a homogeneous prestress on both fault segments we find that while the resultant final stress field is strongly influenced by the fault bend the fault slip and low frequency ground motion are relatively insensitive to the pure dynamic effects of the nonplanar fault the ground velocity from the nonplanar fault is qualitatively quite similar to that of a planar fault with the same dip angle as the nonplanar fault s shallow segment as the effects of multiple earthquakes accumulate on this fault stress concentrations at the fault bend are compounded but the effect of the free surface on the stress appears to approach a steady state the results of this study imply that for the prediction of peak ground motion from faults that intersect the surface of the earth a bend in the fault at depth may not be a significant factor the very long term effects of the fault bend are not fully determined but could lead to complexity in the rupture and slip process in future events

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the