broadband time domain modeling of earthquakes from friuli italy short period sp and long period lp seismograms written by the main shock and two principal aftershocks of the 1976 friuli italy earthquake sequence are modeled in the time domain using synthetic seismograms the main shock occurred on 6 may 1976 20h 00m m s 6 5 and both aftershocks on 15 september 1976 03h 15m m s 6 0 and 09h 21m m s 5 9 source models were determined initially by trial and error and then refined using a waveform inversion program two point sources of radiation are required to adequately model the aftershock short period records for the 09h 21m aftershock the model derived from short period records also produces good fits to the long period data the seismic moment of this earthquake is found to be 0 8 to 1 0 10 25 dyne cm the sp model for the 03h 15m aftershock on the other hand predicts long period synthetics which do not agree with the observations in particular the sp moment 0 37 10 25 dyne cm is about formula times smaller than the lp moment 1 10 25 dyne cm adding a long period component to the sp model considerably improves lp waveform and moment agreement in the case of the main shock a reasonable fit to the observed sp data is obtained using three point sources of radiation however lp synthetics computed using this model do not agree with the observations and the sp moment 0 65 10 25 dyne cm is a small fraction of the lp moment 3 to 5 10 25 dyne cm time function durations indicate that the individual events inferred from the sp records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars in comparison stress drops estimated from lp data are found to be 12 bars main shock and 24 bars 09h 21m aftershock these observations suggest that the short period instruments are sensitive to the high frequency radiation emitted from small high stress drop areas on the fault plane whereas the long period instruments record the overall motion during the earthquake

effects of earthquake induced landslides on sediment budget and transport over a 50 yr period landsliding during a magnitude 7 7 earthquake in 1929 in northwestern south island new zealand delivered up to 400 000 m 3 km 2 of sediment to stream channels first to third order channels were buried to depths of as much as 10 m with sedimentary rock debris for distances ranging from 2 of the 92 km 2 study area because of the coarse grain size of the sedimentary rock debris and trapping of sediment in numerous landslide dammed lakes sand and pebble sized debris originating from landslides on a 6 6 km 2 area of granite has been transported downstream no more than 10 km at least 50 to 75 of this granitic debris is retained in the fourth order 92 km 2 catchment 50 yr after the earthquake the long retention times of sediment and the volume of debris supplied by landslides during a large earthquake suggest that earthquake induced landsliding is the principal sediment supply mechanism in the region

s wave attenuation and site effects in the region of friuli italy we used strong motion records from the 1976 friuli earthquake m 6 4 and 10 of the biggest aftershocks recorded by the national accelerograph network of the electrical power company of italy to estimate the quality factor q of s waves in this region the wide distance range of the recordings 10 r 190 km permits us to analyze the spectral amplitude decay of the records using a nonparametric approach e g anderson and quaas 1988 castro et al 1990 anderson 1991 we obtained attenuation functions for a set of 18 frequencies ranging between 0 4 and 25 0 hz the values of q retrieved from the attenuation functions obtained follow the frequency dependent relation q 20 4f a test of the method was made using a second data set consisting of digital seismograms from the friuli venezia giulia seismograph network in spite of the different size of the volume sampled by these data 10 r 131 km the frequency dependence of q obtained q 16 1f0 92 is similar to that obtained with the strong motion data set the near surface attenuation was also estimated using the model proposed by anderson and hough 1984 and anderson 1991 we found that 0 is smaller for the strong motion stations located on rock compared to stations located on either shallow or soft sediments to estimate the site response of the strong motion stations we corrected the spectral records for the attenuation effect and then inverted the corrected records to separate source and site effects using the inversion scheme proposed by andrews 1986 to verify the site amplification estimates obtained we also calculated the transfer function of each site using nakamura s 1989 method for s wave e g lermo and chavez garcia 1993 in general the shapes of the site functions obtained with the inversion are consistent with the transfer functions obtained calculating the horizontal to vertical component ratio

seismic moments local magnitudes and coda duration magnitudes for earthquakes in central california onscale seismograms recorded at stations in the u s geological survey s usgs central california seismic network calnet have been used to estimate the seismic moment m and local magnitude ml for earthquakes of 1 ml 4 located on the san juan bautista and parkfield sections of the san andreas fault the coyote lake section of the calaveras fault the sargent fault and near livermore these data together with m and ml estimates for 4 ml 6 earthquakes in these areas cannot be fit with a single linear log m versus ml relation rather the data are consistent with log m 1 5 ml 16 for 3 ml 6 with log m 1 2 ml 17 for 1 1 2 ml 3 1 2 and with a slope of to 1 fro 1 2 ml 1 1 2 whereas usgs coda duration magnitude md is consistent with ml for 1 1 2 ml 3 md is larger than ml at ml 1 1 2 and smaller than ml at ml 3 log m can be estimated to a precision of 0 2 for 1 md 3 1 2 earthquakes in central california by applying log m 1 2 md 17 to the md that have been routinely published by the usgs

source parameter analysis from strong motion records of the friuli italy earthquake sequence 1976 1977 abstract time and frequency domain analyses are applied to strong motion data recorded in friuli italy during 1976 to 1977 an inversion procedure to estimate spectral parameters low frequency level corner frequency and high frequency decay has been applied to displacement spectra using a simple earthquake source model with a single corner frequency the data were digitized accelerograms from enea enel portable and permanent networks instrument corrected sh waves were selected from a set of 138 three component hand digitized records and 28 automatically digitized records thirty eight events with stations having 8 to 32 km epicentral distance were studied different stress drop estimates were performed showing high values 200 to 300 bars on the average with seismic moments ranging from 2 8 10 22 to 8 0 10 24 dyne cm the observation of systematic higher values of brune stress drop obtained from corner frequencies with respect to other time and frequency domain estimates of stress release and the evidence on time series of multiple rupture episodes suggest that the observed corner frequencies are most probably related to subevent ruptures rather than the overall fault size seven events recorded at more than one station show a good correlation between rms brune and dynamic stress drops and a constant scaling of this parameter as a function of the seismic moment when single station events are also considered a slight moment dependence of these three stress drop estimates is observed differently this may be explained by an inadequacy of the 2 high frequency decay of the source model or by high frequency attenuation due to propagation effects the high frequency cutoff of acceleration spectra indicates the presence of an f max in the range of 5 to 14 hz except for the stations where local site effects produce spectral peaks

the seismic spectrum radiated energy and the savage and wood inequality for complex earthquakes abstract we have integrated velocity squared spectra in order to determine the seismic energy radiated during fault rupture the high frequency spectral fall off and the shape of the spectrum at the corner frequency are critical to the energy calculation high frequency spectral fall offs of 2 beyond the corner frequency in a brune 1970 source model return radiated energies approximately equal to that of an orowan 1960 type fault failure where the final stress level is equal to the dynamic frictional stress any spectra with an extended intermediate slope of 1 would therefore result in higher radiated energies savage and wood 1971 proposed a model in which the final stress level was less than the dynamic stress level and that this was the result of overshoot they based their model on the observation that the ratio of twice the apparent stress to the stress drop was typically around 0 3 we show that for such a ratio to exist high frequency spectral fall offs of 3 would be required composite spectra have been constructed for several moderate to large earthquakes these spectra have been compared to that predicted by the haskell 1966 model and velocity squared spectra have been integrated to determine the radiated energy in all cases this ratio twice the apparent stress to the stress drop is greater than or equal to one violating the savage and wood 1971 inequality and provides evidence against overshoot as a source model

evidence for the variation of stress drop between normal and thrust faulting earthquakes in italy the horizontal components of 45 strong motion accelerograms recorded in friuli italy from may to september 1976 have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes two different stress parameters the brune stress drop and the apparent stress a have been considered the friuli main shock has a strong stress drop 800 bars while the aftershocks have stress drops which range from 250 to 400 bars five accelerograms written by the montenegro yugoslavia earthquake of april 1979 have also been analyzed this thrust faulting earthquake had a stress drop of 900 bars the stress drops of these thrust faulting earthquakes are compared to the stress drops determined by rovelli et al 1988a for nine normal faulting earthquakes which occurred in the apennines region from 1979 to 1984 the radiated energies and apparent stresses have also been calculated for the set of apennine earthquakes the thrust faulting earthquakes in the alpine dynaride region exhibit higher values for both the brune and the apparent stress than the normal faulting earthquakes in the apennines region excluding the friuli and montenegro main shocks the brune stress drops of the thrust faults are approximately 3 times those of the normal faults while the apparent stresses are twice as large this difference is statistically significant at a confidence level of 95 the same difference also emerges from the scaling of the peak ground motions recorded in these two regions suggesting an enhanced severity of the seismic input in the alpine dynaride environment compared with the apennines

depth determination of north italian earthquakes using grafenberg data surface reflections such as pp and sp have long been used to determine the source depth of teleseismic events due to difficulties in identifying these phases at single narrow band stations they have rarely been used for regional events correlation over an array of stations and the possibility of applying different filters to broadband data allows such phases to be identified we describe a method to determine the source depth which uses such depth phases at regional distances complete theoretical seismograms are an important aid for the interpretation of these depth phases the method is demonstrated using small events from the lombardia region and large events from friuli northern italy good agreement is achieved with depth determinations by other authors for the friuli events relative source depths are determined very accurately since only time differences need to be compared

the 1997 1998 umbria marche sequence central italy source path and site effects estimated from strong motion data recorded in the epicentral area 1 we apply a nonparametric spectral inversion scheme to calculate source spectra s wave attenuation and site transfer functions from strong motion records in the epicentral region of the 1997 1998 umbria marche seismic sequence central italy we use records from moderate size earthquakes 4 6 ml 5 9 to parameterize the spectral amplitude decay in the distance range from 5 to 40 km we find that the average quality factor q can be approximated by q f 49 f0 9 between 0 5 and 8 hz and the geometrical spreading by r 0 9 at high frequencies f 8 hz the dependence of q on frequency weakens and it takes an approximate constant value of 318 we fit the source spectra to the square model and calculate an average stress drop of 2 1 mpa the average value is consistent with the previous estimates from the weak events 1 4 ml 4 5 of the umbria marche seismic sequence the most remarkable site effects are found in correspondence of large sedimentary basins filled by alternation of sandy clayey deposits the estimated spectral parameters are used to simulate acceleration spectra recorded during several earthquakes of the umbria marche sequence both point source and finite fault effects are considered furthermore attenuation relationships for peak ground velocity and ground acceleration are estimated using synthetic data and compared to existing relationships

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

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

ground motion waveforms and source spectral scaling from close distance accelerograms in a compressional regime area friuli northeastern italy abstract an aftershock sequence was recorded by an sma 1 instrument installed at gemona del friuli on a massive limestone site during the period from may to october 1976 the sequence consisted of small to moderately large 10 21 m 0 10 25 dyne cm dominant thrust earthquakes that occurred within 15 km of the station the instrument was triggered by p waves for 24 events the resulting records are particularly suitable to the investigation of source properties for the friuli earthquakes acceleration velocity and displacement waveforms have been analyzed in order to investigate source properties durations t and areas 0 of the shear wave pulses have been measured from the displacement time histories and compared with the values of corner frequency f c and seismic moment m 0 estimated in the frequency domain from the acceleration spectra of the same events the comparison between these two set of data reveals that attenuation correction is important also for correct estimations of source parameters from ground motions at close focal distance and moderate magnitudes after correcting for attenuation stress drops are approximately constant 300 bars on the average in the investigated seismic moment range for this sequence the behavior of peak ground motions versus earthquakes size is consistent with the trend obtained from stochastic simulations assuming an omega squared spectral model with a constant stress drop of 300 bars the good results obtained by applying to friuli earthquakes an omega squared model with a constant stress drop of 300 bars are further confirmed by the analysis of spectral ratios for different size earthquakes of the sequence this analysis demonstrates that the finding of this spectral scaling model is not biased to first order by whole path or near site propagation properties whose effect can be significant when data from a single station are investigated

sub milliarcsecond determination of pole position using global positioning system data the determination of pole position variations from the analysis of 23 days of global positioning system gps data continuously collected with a globally distributed 21 station network is discussed the comparison of the estimated pole positions from these data with results from 11 very long baseline interferometry vlbi experiments conducted during this same interval yields weighted root mean square differences about the mean differences of 0 5 mas for both x and y pole position estimates from an analysis using single day arcs and 1 0 and 0 8 mas from an analysis using three weekly multiday arcs these results are of sufficient quality that gps could provide an alternative technique to vlbi and satellite laser ranging for regular monitoring of high time resolution variations in the position of the earth s rotation axis

determination of earthquake energy release in the eastern mediterranean region summary seismic energy radiated by earthquakes in the eastern mediterranean region is estimated using the short period 50 samples per second seismic recordings made by the israel seismic network during 1990 1997 our data set is the whole s wave window from sn until lg falls to less than twice the noise level from 133 earthquakes with a high signal to noise ratio we obtained the attenuation function 1 850 0 005 log r 0 00460 0 00005 r log e 0 05 where the distance range is 50 r 1500 km we tested the dependence of the attenuation function on the azimuth and the distance despite the different propagation paths of the waves travelling through the continental crust of the arabian shield to the east or the intermediate crust of the mediterranean sea to the west we show that from a statistical point of view the attenuation functions are similar the energy estimation involves time domain integration of the squared ground motion velocity assuming that the attenuation is known following the method of kanamori et al 1993 for the magnitude range 3 0 mb 6 2 magnitude determined by the national earthquake information service neis we obtained the magnitude energy relationship log e0 2 09 0 10 mb 8 86 0 42 comparison of the seismic energy and the seismic moment suggests that e m00 19 and that orowan s stress drop increases as m00 19 a refinement of the results is expected with the application of the method of mayeda walter 1996 for energy estimation from coda envelopes using a large data set of broad band observations

determination of s wave energy release of earthquakes in the region of friuli italy summary the s wave energy released by the 1976 friuli earthquake ml 6 6 and nine aftershocks with local magnitudes ranging between 4 7 and 6 4 was estimated using strong motion records near the source and at regional distances to determine the seismic energy radiated from the source we corrected the spectral records for attenuation along the path using the q frequency relation q 20 4f1 01 and for the effect of near surface attenuation using specific values of k0 reported by castro et al 1996 for the recording sites of the region the remaining path effects were separated from the source functions using a non parametric inversion the displacement source functions show an intermediate 1 fall off before a high frequency fall off of 2 which provides more energy compared with a single 2 decay when we compare the seismic energy es evaluated by integrating the square of the velocity source spectra with the energy estimated from the local magnitudes via gutenberg richter s 1956 relation we find that es radiated from the source is on average a factor of 6 higher than that obtained from ml in addition comparing the seismic moments mo reported in previous studies cipar 1981 cocco rovelli 1989 with es we observed that es increases proportionally to mo1 24 this rate of increase of the seismic energy with moment indicates that the orowan stress drop also increases with mo the apparent stress calculated with the ratio es mo is also greater than half the brune stress drop a 2 suggesting that for the thrust faulting earthquakes of friuli the final stress was greater than the frictional stress during the 1976 sequence

the influence of landsliding on sediment supply and channel change in a steep mountain catchment abstract rates of sediment supply by landsliding to an alluvial channel in a small catchment in central switzerland were estimated over an 11 month study period fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport landslide derived sediment discharge to the channel was greatest during the winter and spring months while most of the fluvial sediment transport occurred during short intense summer storms approximately 98 m 3 of sediment was delivered directly to the study reach by landsliding 80 m 3 was transported into the reach from upstream and 70 m 3 was transported out of the reach thus there was a net accumulation of 100 m 3 of sediment during the 11 month study decadal scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004 during this 11 year period a total of 1500 m 3 of sediment has accumulated in the study reach aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales

site response attenuation and source spectra of s waves along the guerrero mexico subduction zone acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra the site response and the quality factor q of s waves this method is applied to moderate earthquakes 4 0 0 8 hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions when the source spectra are interpreted in terms of brune s model the stress drops obtained are less than 20 bars for the four larger events analyzed the site functions obtained indicate that although all the stations used are on rock the effect of the near surface geology caused important amplifications at some stations

moment energy stress drop and source spectra of western united states earthquakes from regional coda envelopes we present a new method to estimate stable seismic source parameters such as energy moment and orowan stress drop using regional coda envelopes from as few as one broadband station we use the method to compute path and site corrected seismic moment rate spectra for 117 recent western united states earthquakes empirical green s function corrections were applied to our surface and body wave coda envelope measurements to generate s wave source spectra these source spectra provide stable single station estimates of radiated seismic energy es and seismic moment mo that for common events are in excellent agreement with network averaged estimates obtained using local and regional data teleseismic moment estimates are compatible with our regional results but teleseismic energy estimates appear to be nearly an order of magnitude low we estimated the seismic moment of events ranging between mw 2 2 and 7 3 and energy estimates for which we had measured at least 70 of the total energy generally events above mw 3 3 we use these estimates to examine the behavior of derived parameters such as the orowan stress drop 2 es mo while the earthquakes we studied have a small range in orowan stress drop generally between 0 1 and 20 mpa they show a strong tendency for orowan stress drop to increase with moment approximately as mo0 25 we believe this is a source effect and is not due to inadequate bandwidth or attenuation correction and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern california many of the large high stress drop earthquakes show complexity in their moment rate spectra near the corner frequency and cannot be fit by a simple square model instead above the first corner frequency the spectral decay ranges between f 1 0 and f 1 5 this leads to larger estimates of radiated energy than predicted with a simple square model and has implications for seismic hazard estimation coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy 1 coda amplitudes vary little with geology and source radiation anisotropy and allow accurate single station applications 2 path corrected coda amplitude measurements can be applied to very large regions allowing a comparison of source parameters throughout the western united states using a common methodology and stations 3 because long period coda can last for hours for large local and regional events it allows the analysis of seismograms with clipped early arrivals

effects of coarse woody debris on morphology and sediment storage of a mountain stream system in western oregon effects of coarse woody debris cwd on channel morphology and sediment storage were investigated at five sites representative of first order to fifth order streams in the steep and bedrock confined stream first second order interaction between the channel and cwd was limited except where breakage upon falling produced cwd pieces shorter than channel width channel widening steepening and sediment storage associated with cwd were observed predominantly in third to fifth order streams variation in channel width and gradient was regulated by cwd in the fifth order stream most of the cwd pieces derived from the riparian forest interacted directly with the channel without being suspended by sideslopes in this system cwd promoted lateral channel migration but sediment storage was temporary with annual release and capture

determination of the characteristic frequency of two dams located in the region of calabria italy we estimated the characteristic frequency of vibration of two dams located in southern italy using spectral amplitudes of 13 local earthquakes recorded by three component digital stations installed on top of the structures and on the free field one of the dams arv located in arvo lake is an earth dam and the other trp is an arch dam located in the lake ampollino we separated the source and path effects from the records on the dams by using spectral ratios between the horizontal and the vertical components of motion h v ratios we also calculated simultaneous inversions for source site and path effects using different site constraints in general the characteristic frequencies fn retrived from both approaches are consistent with the expected natural frequency of resonance of the structures for arv we estimated from the spectral records an fn 2 5 hz and a value of 2 0 hz when we calculated the one dimensional response using the shake code schnabel et al 1972 for trp the results of the inversion and the spectral ratio between the free field site give fn 8 0 hz we concluded that the inversion technique gives reliable estimates of fn for the sites analyzed

partial stress drop and frictional overshoot mechanism of seismic events induced by mining the values of uniformly estimated apparent stress a and brune s stress drop taken as a measure of static stress drop from 850 seismic events with moment magnitude ranging from 3 6 to 3 6 induced at the underground research laboratory url in canada western deep levels wdl gold mine in south africa and two coal and two copper mines in poland were collected to study the stress release mode in various mining environments for this the quantity epsilon a 2 where is the static stress drop proposed by zuniga 1993 as an indicator of stress drop mechanism was used the events induced at the url are characterized by low values of epsilon corresponding to a partial stress drop mechanism whereas all the events at wdl display a frictional overshoot mechanism in which final stress reaches a lower value than that of frictional stress the events at polish coal and copper mines on the other hand are in good agreement with the well known orowan s condition such that the final stress is equal to the dynamic frictional stress the brune stress drop however is heavily model dependent through the source radius corner frequency relation the orowan s condition for the events from the url would be met if a constant in the source radius corner frequency relation is equal to 1 82 0 12 and for the events from wdl if it is equal to 3 92 0 40 in contrast to brune s constant of 2 34 the smoothed values of epsilon displayed as a function of time represented by the consecutive event numbers for selected sets of events imply that the largest seismic events in a given set occur when the epsilon is low and a partial stress drop mechanism is dominant the large events are then followed by high epsilon values when a frictional overshoot mechanism begins to dominate