dynamic inversion of the 2015 jujuy earthquake and similarity with other intraslab events fondo nacional de desarrollo cientifico y tecnologico fondecyt of comision nacional de investigacion cientifica y tecnologica conicyt chile 11130230 programa riesgo sismico universidad de chile conicyt pcha magisternacional 2014 22140434

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

modelling of the april 5 2003 stromboli italy paroxysmal eruption from the inversion of broadband seismic data abstract on april 5 2003 one of the largest eruptions in the last decades was observed at stromboli volcano italy the eruption occurred in a period of increased volcanic activity following a first explosion in december 2002 which interrupted the typical moderate strombolian behaviour we present an exhaustive analysis of the available broadband seismic data and relate them to the observed eruption phases prominent features of the seismic signals include an ultra long period signal starting a few tens of seconds prior to the explosive eruption as well as a strong energetic signal a few seconds after the onset of the eruption both signals are not exactly synchronized with the other geophysical observations we present a detailed study of those signals using spectral and particle motion techniques we estimate eruption parameters and seismic source characteristics by different inversion approaches results clearly indicate that the paroxysmal eruption was triggered by a shallow slow thrust faulting dislocation event with a moment magnitude of m w 3 0 and possibly associated with a crack that formed previously by dike extrusion at least one blow out phase during the paroxysmal explosion could be identified from seismic signals with an equivalent moment magnitude of m w 3 7 and is represented by a vertical linear vector dipole and two weaker horizontal linear dipoles in opposite direction plus a vertical force

imaging of subducted lithosphere beneath south america tomographic images are produced for the deep structure of the andean subduction zone beneath western south america the data used in the imaging are the delay times of p pp and pwp phases from relocated teleseismic earthquakes in the region regionally structural features larger than about 150 km are resolved by the data presentations of layer anomaly maps and cross sections reveal 1 the nazca slab is probably continuous laterally and at depth over most regions 2 the offset between the north and south deep earthquake zones containing the 1994 deep bolivia main shock and its aftershocks can be modelled by a northwest striking and steeply northeast dipping slab structure and 3 the nazca slab clearly penetrates the lower mantle beneath central south america but is partly deflected in the southern deep zone

fine details of the wadati benioff zone under indonesia and its geodynamic implications using about 5200 handpicked p s pp sp pcp and scp phases from digitally recorded seismograms together with international seismological centre reported phases we obtain improved hypocentral locations for 1790 earthquakes 983 of them having 90 confidence limits 50 km in the period 1962 to september 1996 along the indonesian subduction zone we use an improved joint hypocenter determination method in which the solution is more stable than in the original one we show that for deep earthquakes use of the core reflected phases is as good as use of depth phases and since such core phases often have larger amplitudes and sharper onsets than the depth phases their arrival times can be read very accurately the positions of the relocated hypocenters show that 1 a portion of the indonesian arc between 110 e and 123 e longitude and deeper than 500 km is dipping southward at an angle of 75 that is in a direction opposite to the upper part of the north dipping slab and 2 east of about 108 e the seismic zone is wider near 670 km than near the 500 km depth the first suggests southward lateral flow in the mantle relative to the plate motion vector here from the contortion of the seismic zone along the eastern portion of the indonesian arc we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10 16s 1 over the last 10 20 myr

fast rise times and the physical mechanism of deep earthquakes earthquakes at depths of 300 km are similar to shallower events in that they are dominantly of double couple character1 implying that shearing motion has taken place at depth but because increased friction at these high pressures inhibits brittle fracture2 3 various other mechanisms related to phase transformations have been invoked to explain the occurrence of deep earthquakes4 10 as yet however no consistent differences have been found between the source characteristics of deep 300 km and intermediate depth 300km earthquakes2 11 13 here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes the rise time is defined as the time from rupture initiation to peak moment release rate when the rise times are scaled to the seismic moment release of the events their average is nearly twice as fast for events deeper than 450 km as for shallower events this difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity

comparisons of shear wave slowness in the santa clara valley california using blind interpretations of data from invasive and noninvasive methods many groups contributed to a blind interpretation exercise for the deter mination of shear wave slowness beneath the santa clara valley the methods in cluded invasive methods in deep boreholes as well as noninvasive methods using active and passive sources at six sites within the valley with most investigations being conducted at a pair of closely spaced sites near the center of the valley although significant variability exists between the models the slownesses from the various methods are similar enough that linear site amplifications estimated in several ways are generally within 20 of one another the methods were able to de rive slownesses that increase systematically with distance from the valley edge cor responding to a tendency for the sites to be underlain by finer grained materials away from the valley edge this variation is in agreement with measurements made in the boreholes at the sites

the 1996 june 17 flores sea and 1994 march 9 fiji tonga earthquakes source processes and deep earthquake mechanisms summary source parameters of the 1996 flores sea and 1994 fiji tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad band seismograph stations both events consisted of several subevents models to approximate the spatial and temporal extent of the source process include point sources propagating point sources and a combination of these for the flores sea event rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of p wave pulses in agreement with the findings reported by other investigators our preferred model suggests bilateral rupture propagation it consists of four point sources that have variable double couple radiation patterns and source time histories and explains well the large compensated linear vector dipole clvd component inferred from the harvard centroid moment tensor cmt solution the main moment release in the fiji tonga event lasted only about 15 s our best model consists of two point sources with a total moment release of 2 8 1020 n m rupture propagated subhorizontally from the nucleation point to the north the termination of rupture was located about 40 km to the north of rupture initiation the inferred velocity of moment release in the flores sea and fiji tonga events was 3 5 km s 1 a value which is higher than that inferred for the great bolivian earthquake of june 1994 other derived source parameters static stress drop radiated seismic energy and maximum seismic efficiency are also significantly different from those inferred for the 1994 bolivian event suggesting that deep earthquake processes do not follow an easily detectable common mechanism

observation and prediction of dynamic ground strains tilts and torsions caused by the mw 6 0 2004 parkfield california earthquake and aftershocks derived from upsar array observationsdynamic ground strains tilts and torsions caused by the 2004 parkfield california earthquake

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

the depth dependence of earthquake duration and implications for rupture mechanisms the duration of rupture is a fundamental characteristic of earthquakes and is important for understanding the mechanics of faulting1 2 the complexity of the seismic source and the incoherence of the high frequency seismic wavefield often inhibit the identification location and timing of features in the later part of earthquake rupture here we sum many teleseismic records from regional seismic arrays producing an unusually clear depiction of the earthquake source at short periods by suppressing background noise and coda generated near the receivers the ending as well as the beginning of rupture is clearly identifiable for most earthquakes examined measurements of 130 large earthquakes show that near 100 km depth rupture duration averages 11 s when scaled to a moment of 1026 dyn cm this decreases to 5 5 s at 650 km depth models of faulting suggest that duration should be inversely proportional to the shear wave velocity and the cube root of stress drop thus to explain the observed twofold decrease in duration with depth stress drops would have to increase by a factor of four as shear velocity increases with depth by only about 20 however observed stress drops show no strong trend with depth3 4 suggesting that the faulting process changes with depth

comparison of the rupture and radiation characteristics of intermediate and deep earthquakes we study the depth dependence of the rupture and radiation characteristics of earthquakes below 70 km depth by investigating the source properties of intermediate 70 300 km depth and deep 300 km depth earthquakes along the fiji and japan subducting slabs the source properties are explored by retrieval of the moment rate functions and by spectral analysis using p waves thirty two earthquakes in the period 1986 1997 with seismic moment 1 1018 n m are analyzed the main conclusions are 1 the dependence of the source duration on the scalar seismic moment m0 follows the usual one third trend on average 2 this behavior is still visible when the source durations are adjusted for the shear wave velocity and the modulus of rigidity at the hypocentral depth 3 the effective stress drop and the effective strain drop for subcrustal earthquakes in fiji and japan tend to be larger than those for the crustal ones 4 the duration and shape of the moment rate functions do not show any systematic difference between intermediate and deep events and 5 no obvious difference in the spectral properties of intermediate and deep events is seen

attenuation of radiated ground motion and stresses from three dimensional supershear ruptures 1 radiating shear and rayleigh waves from supershear ruptures form mach waves that transmit large amplitude ground motion and stresses to locations far from the fault we simulate bilateral ruptures on a finite width vertical strike slip fault of width w and half length l with l w breaking the surface of an elastic half space and focus on the wavefield out to distances comparable to l at distances much smaller than w two dimensional plane strain slip pulse models i e models in which the lateral extent of the slip zone is unbounded accurately predict the subsurface wavefield amplitudes in the shear mach wedge of those models are undiminished with distance from the fault when viewed from distances far greater than w rupture is accurately modeled as a moving point source that produces a shear mach cone and on the free surface rayleigh wave mach fronts geometrical spreading of the shear mach cone occurs radially and amplitudes there decrease with the inverse square root of distance the transition between these two asymptotic limits occurs at distances comparable to w similar considerations suggest that rayleigh mach waves suffer no attenuation in the ideally elastic medium studied here the rate at which fault strength weakens at the rupture front exerts a strong influence on the off fault fields only in the immediate vicinity of the fault for both sub rayleigh and supershear ruptures and at the mach fronts of supershear ruptures more rapid weakening generates larger amplitudes at the mach fronts

rupture velocity inferred from near field shear strain analysis s u m m a r y we propose a new technique to determine the rupture velocity of large strike slip earthquakes by means of simple numerical ground motion simulations we show that when the rupture penetrates a shallow layer of sediment or fractured rock shock waves propagate along the surface fault trace in the forward rupture direction such shock waves which are insensitive to the complexity of slip over the fault plane propagate at a phase velocity equal to the rupture speed we show that those shock waves can be easily isolated in the frequency domain and that phase velocity can then be simply obtained from shear strain

deep earthquake rupture histories determined by global stacking of broadband p waveforms 1 we conduct a systematic investigation of time histories of moment release in deep earthquakes whose physical mechanism remains unknown we have constructed source time functions stfs of 111 large deep earthquakes depth 100 km and mw 6 4 by stacking p waveforms from broadband seismograms recorded by the global seismographic network aligning and stacking waveforms cancels noise and allows us to identify the rupture initiation and termination times accurately each stf is scaled to a reference moment of 1019 n m using the duration moment relation m01 3 in order to investigate depth dependence of the stfs we bin them into three depth ranges 100 350 km 350 550 km and deeper than 550 km based on physical properties of the mantle and the distribution of the data we infer a change in character near 550 km earthquakes deeper than about 550 km are shorter have simpler time functions and are more uniform in their characteristics further we find longer than expected scaled durations and greater complexity in the 350 550 km depth range the scaled durations of the 350 550 km earthquakes do not decrease with depth as 1 vs instead possessing a slightly longer average scaled duration than those 100 350 km deep we further categorize the earthquakes by subduction zone and find a similar depth dependence within individual zones finally the events that have achieved highest moment rate in the first 1 2 s after initiation tend to be deeper and or larger events the latter suggesting that dynamic rupture propagation affects the final size of the earthquake the depth dependence of duration and complexity suggests more complicated rupture histories between 350 and 550 km and a possible change in the physical mechanism of rupture at about 550 km

earthquake source parameters and stress distribution in the adak island region of the central aleutian islands alaska source parameters have been systematically determined for all earthquakes with mw 5 that occurred between 172 w and 179 w longitude in the adak island region of the central aleutian islands during 1977 march 1987 we relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination the first method uses arrival times of direct and prominent reflected phases primarily pwp the second method uses broadband p wave displacement seismograms in an inversion for source depth the analysis of these events provides a well constrained data set for the study of stress release along an active subduction zone three earthquakes which occurred seaward of the trench axis are located just below the crust mantle interface and show extension nearly perpendicular to the trench axis seven events occurred in the wadati benioff zone sixty four events are located in the main thrust zone and except for five unusual events are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle in cross section the thrust zone appears as a thin 10 15 km thickness interplate region which extends from 15 to 50 km depth a 10 15 average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the pacific and north american plates exists for these events the observed slip vectors are oriented more normal to the trench than is predicted by plate motions five earthquakes which occurred as aftershocks to the may 7 1986 earthquake mw 8 0 in the crust of the overriding plate have strike slip mechanisms consistent with right lateral motion on arc parallel fault planes observations of slip vectors along the whole aleutian arc show a similar trend to that observed in the adak island region the largest differences 30 between observed and predicted slip azimuths occur around 175 e we propose a model of plate interaction in which a portion of the along arc motion occurs along a weak strike slip shear zone in the upper plate near the volcanic line the slip azimuths in the main thrust zone fit this model well if the amount of transcurrent slip occurring in the upper plate is 60 of the arc parallel relative plate motion a consequence of the model is along arc extension of the overriding plate between the accretionary wedge and the volcanic line especially in the western part of the aleutian arc calculations based on a tectonic model by geist et al 1988 for the formation of arc summit basins through block rotation and translation suggest that along arc extension has been significant since late miocene or early pliocene

spatial gradient analysis for linear seismic arrays i incorporate the spatial gradient of the wave field recorded from one dimensional arrays into a processing method that yields the horizontal wave slowness and the change of geometrical spreading with distance in general the model for seismic wave propagation is enough to be appropriate for body and surface waves propagating from nearby seismic sources but can be simplified into a plane wave model although computation of the spatial gradient requires that array elements be closer than 10 of the horizontal wavelength seismic array apertures in the usual sense may extend over many horizontal wavelengths and illuminate changes within the wave field array images of horizontal slowness and the relative geometrical spreading changes of seismic waves are derived using filter theory and used to interpret observed array wave fields errors in computing finite difference spatial gradients from array nodes are explicitly considered to avoid spatial aliasing in the estimates i apply the method to interpret waves in strong ground motion and small scale refraction data sets use of the wave spatial gradient accentuates spatial differences in the wave field that can be theoretically exploited in fine scale tomographic studies of structure and is complementary to frequency wavenumber or beam forming array processing techniques

strain tilt and rotation associated with strong ground motion in the vicinity of earthquake faults abstract in the absence of near field records of differential ground motion induced by earthquakes we simulate the time histories of strain tilt and rotation in the vicinity of earthquake faults embedded in layered media we consider the case of both strike slip and dip slip fault models and study the effect of different crustal structures the maximum rotational motion produced by a buried 30 km long strike slip fault with slip of 1 m is of the order of 3 10 4 rad while the corresponding rotational velocity is about 1 5 10 3 rad sec a simulation of the san fernando earthquake yields maximum longitudinal strain and tilt a few kilometers from the fault of the order of 8 10 4 and 7 10 4 rad these values being small compared to the amplitude of ground displacement the results suggest that most of the damage occurring in earthquakes is caused by translation motions we also show that strain and tilt are closely related to ground velocity and that the phase velocities associated with strong ground motions are controlled by the rupture velocity and the basement rock shearwave velocity

duration of deep earthquakes determined by stacking of global seismograph network seismograms the duration of each subevent of 48 earthquakes with magnitude larger than 5 5 and depth greater than 100 km was determined from stacked traces of broadband records of global seismograph network stations we fitted the source time function by one or more triangles convolved with attenuation we found that global stacks of displacement seismograms yield reliable estimates of the rupture duration the durations scaled to a moment of 1019 n m of both the subevents and the entire earthquake show a slight decrease with depth from 9 s for events at 100 km depth to about 7 s for events at 600 km depth assuming that the rupture velocity is a constant fraction of the shear wave speed this decrease can be completely explained by the increase in shear velocity of 20 in this sense deep earthquakes are comparable to intermediate ones for some intermediate depth events vidale and houston 1993 found durations up to twice as long we find that almost all of their slow events have been recorded at large epicentral distances at these distances we conjecture that the end of the p wave train may be extended by the arrival of reflections from the d layer

anomalously low aftershock productivity of the 2019 mw 8 0 energetic intermediate depth faulting beneath peru abstract the 26 may 2019 peru m w 8 0 earthquake struck within the nearly horizontal underthrust nazca plate at depths from 110 to 150 km below the upper amazon near a steep bend in the plate where it plunges down to a deep earthquake zone little prior seismicity occurred in this region but large intraslab events with similar normal faulting mechanisms have occurred to the west the event is situated in a similar slab position to the 2017 puebla morelos mexico earthquake but the remote location resulted in limited loss of life and damage back projection imaging and finite fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170 km long zone at a rupture speed v r 3 km s with three normal faulting patches of up to 4 5 m slip despite the mainshock size it produced only three m4 0 aftershocks within 300 km one nearby the aftershock productivity of the 2019 peru earthquake is very low even among all large intermediate depth earthquakes but similar to that for other large peruvian intraslab events comparison of aftershock productivity of the peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat peru slab

rupture history of the great bolivian earthquake slab interaction with the 660 km discontinuity teleseismic body waves of the great bolivian earthquake of june 9 1994 are analyzed to determine the focal parameters and rupture history broadband seismograms reveal a complex rupture process a small initial event mw 7 2 was followed about 10 s later by a large moment release pulse of about 40 s duration focal mechanisms determined for the mainshock indicate normal faulting with one very shallow ne dipping plane azimuthal variation in body wave displacement pulse widths suggest northward rupture from master event and body wave inversion the main moment release is located 25 50 km ne of the initiation point at about 650 km depth with only small depth variations between the initial and main event this suggests that rupture was n directed on the near horizontal plane because the slab along other parts of the andean arc at about 600 km depth dips steeply a sub horizontal plane may imply shearing perpendicular to slab dip downdip compression on a sub horizontal plane would then imply that the slab does not penetrate the 660 km seismic discontinuity but rather is being sheared out to the ne this interpretation is not unique as other scenarios are also possible such an event immediately above the 660 km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle

the deep peru 2015 doublet earthquakes abstract on 24 november 2015 two events of magnitude mw 7 5 and mw 7 6 occurred at 600 km depth under the peru brazil boundary these two events were separated in time by 300 s deep event doublets occur often under south america the characteristics that control these events and the dynamic interaction between them are an unresolved problem we used teleseismic and regional data situated above the doublet to perform source inversion in order to characterize their ruptures the overall resemblance between these two events suggests that they share similar rupture process they are not identical but occur on the same fault surface dipping westward using a p wave stripping and stretching method we determine rupture speed of 2 25 km s from regional body wave inversion we find that stress drop is similar for both events they differ by a factor of two the similarity in geometry rupture velocity stress drop and radiated energy suggests that these two events looked like simple elliptical ruptures that propagated like classical sub shear brittle cracks

tsunami earthquakes possibly widespread manifestations of frictional conditional stability 1 tsunami earthquakes shallow events that produce larger tsunamis than expected given their surface wave magnitudes ms typically have long durations and a source spectrum depleted in short period energy seven cases of underthrusting tsunami earthquakes provide information on the rupture processes but little constraint on geographic distribution or frequency we compare their rupture characteristics with smaller magnitude earthquakes on circum pacific interplate thrust faults comparable moment release time histories are found for large tsunami earthquakes and for many smaller shallow subduction zone earthquakes with significantly longer durations and additional source complexity than for events deeper than 15 km thus very shallow interplate earthquake ruptures are scale invariant with variable frictional properties on the plate interface controlling the depth dependent rupture process widespread occurrence of small shallow interplate earthquakes with long durations suggests that many subduction faults have frictional properties that may enable large tsunami generating earthquakes to occur fortunately large shallow ruptures are infrequent

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

rupture process of large earthquakes in the northern mexico subduction zone the cocos plate subducts beneath north america at the mexico trench the northernmost segment of this trench between the orozco and rivera fracture zones has ruptured in a sequence of five large earthquakes from 1973 to 1985 the jan 30 1973 colima event m s 7 5 at the northern end of the segment near rivera fracture zone the mar 14 1979 petatlan event m s 7 6 at the southern end of the segment on the orozco fracture zone the oct 25 1981 playa azul event m s 7 3 in the middle of the michoacan gap the sept 19 1985 michoacan mainshock m s 8 1 and the sept 21 1985 michoacan aftershock m s 7 6 that reruptured part of the petatlan zone body wave inversion for the rupture process of these earthquakes finds the best earthquake depth focal mechanism overall source time function and seismic moment for each earthquake in addition we have determined spatial concentrations of seismic moment release for the colima earthquake and the michoacan mainshock and aftershock these spatial concentrations of slip are interpreted as asperities and the resultant asperity distribution for mexico is compared to other subduction zones the body wave inversion technique also determines the moment tensor rate functions but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes an appendix describes the moment tensor rate functions methodology in detail

centroid moment tensor solutions for january march 1997 abstract centroid moment tensor solutions are presented for 162 earthquakes that occurred during the first quarter of 1997 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward dziewonski a m woodward j h 1992 acoustic imaging at the planetary scale in h emert h p harjes eds acoustical imaging plenum new york 19 785 797 a new model of anelastic attenuation of durek and ekstrom durek j j ekstrom g 1996 a radial model of anelasticity consistent with long period surface wave attenuation bull seismol soc am 86 144 158 is used to predict the decay of the waveforms

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

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 temporal distribution of seismic radiation during deep earthquake rupture the time history of energy release during earthquakes illuminates the process of failure which remains enigmatic for events deeper than about 100 kilometers stacks of teleseismic records from regional arrays for 122 intermediate depths of 100 to 350 kilometers and deep depths of 350 to 700 kilometers earthquakes show that the temporal pattern of short period seismic radiation has a systematic variation with depth on average for intermediate depth events more radiation is released toward the beginning of the rupture than near the end whereas for deep events radiation is released symmetrically over the duration of the event with an abrupt beginning and end of rupture these findings suggest a variation in the style of rupture related to decreasing fault heterogeneity with depth

radiated seismic energy and earthquake source duration variations from teleseismic source time functions for shallow subduction zone thrust earthquakes 1 earthquake source time functions deconvolved from teleseismic broadband p wave recordings are used to examine rupture variations for 417 underthrusting earthquakes located on the interplate interface in circum pacific subduction zones moment scaled duration of significant moment release varies with depth with longer duration events occurring in the shallowest 20 km of the megathrusts the source time functions are also used to estimate radiated seismic energy two estimates are obtained a simple scaled triangle substitution for the moment release history provides a minimum estimate while integration of the time function shape provides an estimate limited only by the bandwidth of the teleseismic deconvolutions while these energy calculations underestimate total energy they enable systematic comparisons of rupture process within and between subduction zones we do not find significant depth dependence for radiated energy overall but some regions do show mild trends of increasing energy seismic moment ratios e mo with increasing source depth that correspond to rupture duration variations in those regions the observations of longer rupture duration at shallow depth with moderate e mo may be due to heterogeneous friction and structural features on the shallow plate interface index terms 7215 seismology earthquake parameters 7230 seismology seismicity and seismotectonics 8164 tectonophysics stresses crust and lithosphere keywords radiated energy shallow subduction source parameters

global and along strike variations of source duration and scaling for intermediate depth and deep focus earthquakes the systematic behavior of earthquake rupture as a function of earthquake magnitude and or tectonic setting is a key in our understanding of the physical mechanisms involved during earthquake rupture geophysical evidence suggests that although deep earthquakes including intermediate depth and deep are similar to shallow ones the mechanism involved during deep earthquakes is different from that of shallow ones in particular the magnitude and depth dependence of scaled duration a measure of earthquake rupture duration has led to controversy of what controls deep earthquake behavior here we estimate scaled source durations for 600 intermediate depth and deep focus earthquakes recorded at teleseismic distances and show deviation from self similar scaling no depth dependence is observed which we interpret as due to little differences between intermediate depth and deep focus earthquake mechanisms the data show no correlation between durations and plate age or thermal parameters suggesting that the thermal properties of the plate have little effect on source durations we nevertheless report differences in average source duration and scaling between subduction zones and along strike variations of source durations that more closely resemble the geometry of subduction flat or steep subduction rather than plate age

rupture processes of large deep focus earthquakes from inversion of moment rate functions we determine the source time histories of five recent 1994 1996 large deep focus earthquakes using a method that inverts for fault slip from far field moment rate functions the moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from global seismic network gsn and geoscope stations tests of this method on synthetic data indicate that it is successful in determining the low frequency rupture characteristics of deep earthquakes under a variety of complicating conditions we find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the june 9 1994 great bolivian earthquake are also found for other events comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep focus earthquakes is probably occurring within the active slab interior this provides further evidence that temperature controlled mechanisms such as transformational faulting play a large role in deep earthquake faulting most of the events studied also show a tendency for horizontal rupture propagation suggesting that isobaric processes may be an important factor in controlling progression of the rupture large gaps in the slip distributions point to possible occurrence on multiple fault planes

observation of far field mach waves generated by the 2001 kokoxili supershear earthquake regional surface wave observations offer a powerful tool for determining source properties of large earthquakes especially rupture velocity supershear ruptures being faster than surface wave phase velocities create far field surface wave mach cones along which waves from all sections of the fault arrive simultaneously and over a sufficiently narrow frequency band in phase we present the first observation of far field mach waves from the major kokoxili earthquake tibet 2001 11 14 mw7 9 and confirm that ground motion amplitudes are indeed enhanced on the mach cone theory predicts that on the mach cone bandpassed surface wave seismograms from a large supershear rupture will be identical to those from much smaller events with similar focal mechanisms with an amplitude ratio equal to the ratio of the seismic moments of the two events cross correlation of 15 25 s love waves from the kokoxili event with those from a much smaller mw5 foreshock indicates a high degree of similarity correlation coefficients ranging from 0 8 to 0 95 in waveforms recorded at stations near the far field mach cone this similarity vanishes away from the mach cone these observations provide further evidence for supershear propagation of the kokoxili rupture and demonstrate how this simple waveform correlation procedure can be used to identify supershear ruptures

insights into the mechanism of intermediate depth earthquakes from source properties as imaged by back projection of multiple seismic phases 1 this study investigates the spatial and temporal distribution of energy release of large intermediate depth earthquakes using a modified back projection technique first used to study the 2004 sumatra andaman megathrust event multiple seismic phases are included in the back projection analysis which provides the capability to determine the energy distribution with respect to depth and time a total of 22 intermediate depth earthquakes with moment magnitudes greater than or equal to 6 5 are investigated with hypocentral depths between 100 and 300 km for most of these events the vertical extent of energy release is either below the resolution of this study 5 km or slightly above 15 km this observation agrees with previous studies that find large intermediate depth earthquakes have subhorizontal rupture planes the results also show a significant portion of the events have multiple rupture planes that are well separated in depth the closeness in time of the ruptures on separate planes and the distance between the planes suggest dynamic triggering where the p waves from the first rupture initiate rupture on the second plane we propose that a dehydration embrittlement mechanism combined with preferentially hydrated subhorizontal faults can explain the observations of dominant subhorizontal rupture planes and the frequent occurrence of rupture complexity involving multiple subevents

global rupture parameters for deep and intermediate depth earthquakes we investigate the global rupture parameters for deep and intermediate depth earthquakes from measurements of rupture duration and radiated seismic energy we estimate stress drop apparent stress and radiation efficiency and obtain a detailed earthquake energy budget from scaling of the source parameters we highlight differences between crustal and deep seismicity with the latter showing larger fracture energies the observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate depth events differs in agreement with previous studies we observe along strike variability of rupture properties for deep and intermediate depth earthquakes correlating with slab morphology plate age or presence of volcanic structures

toward understanding slip inversion uncertainty and artifacts 1 the main concept of seismic source tomography the projection lines along which the observed signals are back projected to the fault is extended to complete wavefields the so called dynamic projection strips dpss are defined and a method to construct the strips from individual waveforms is described in this way each individual station role in the inversion can be better understood synthetic models with two asperities two unilateral and one bilateral rupture scenarios are used as examples they are analyzed using two independent slip inversion methods with similar results bias of the rupture speed for all scenarios and a strong false asperity in the middle of the bilateral fault both artifacts can be explained by the dps analysis as inherent nonuniqueness of the inverse problem due to the joint effect of the two true asperities removal of some slip artifacts by imposing various constraints is discussed but most of the constraints are hardly applicable in practice as such it is recommended to look at least for possible indications of the most significant inversion artifacts this seems to be feasible through combining dpss derived from real data and synthetic models of various rupture scenarios for a given fault and stations the ideas are applied to the movri mountain earthquake in greece mw6 3 8 june 2008 it appears that the earthquake was predominantly unilateral however with a nonunique space time slip pattern few equivalent nonsmooth models all fitting the data equally well are illustrated

the 1994 bolivia and tonga events fundamentally different types of deep earthquakes the 1994 bolivia and tonga events show large differences in source and aftershock properties the bolivia earthquake characterized by high stress drop slow rupture propagation and a weak aftershock sequence appears similar to other large deep earthquakes that are isolated from active seismic zones including the 1970 colombia earthquake in contrast the 1994 tonga event showed stress drop rupture propagation velocities and an aftershock sequence similar to that found for shallow earthquakes the aftershock sequence of the 1994 tonga event is many times stronger than has been observed for any other deep earthquake a survey of deep earthquake aftershocks suggests that aftershock occurrence is correlated with the overall subduction zone magnitude frequency relation b value the largest deep events occur in isolated regions lacking smaller earthquakes or in subduction zones characterized by anomalously low b values the 1994 tonga event is an exception and is the largest deep earthquake to occur within an active subduction zone showing a large b value perhaps explaining the strong aftershock sequence the bolivia and tonga events may represent end members of a population of deep earthquakes showing exceptional diversity in source properties or they may represent two fundamentally different types of deep earthquakes perhaps resulting from different physical mechanisms

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

slow but not quite silent regular earthquakes release large amounts of strain accumulated in earth9s crust within a very short time recently a different type of earthquake has been reported from subduction zones around the world deep in earth9s crust slow earthquakes may last weeks or months in their perspective melbourne and webb explain that while slow these earthquakes are not silent as originally assumed the authors highlight the report by rogers and dragert who show that slow earthquakes in the cascadia subduction zone off the western coast of north america are accompanied by a tremor that is absent when slow faulting is not occurring the earthquakes are likely caused by fluid flow a mechanism that may also apply to regular earthquakes

injection induced slip heterogeneity on faults in shale reservoirs abstract managing fluid stimulation protocols is an effective means to mitigate the risk of injection induced earthquakes during shale gas development the success of these protocols is dependent on our understanding of fluid pressure heterogeneity and the associated inhomogeneous slip on critically stressed faults here we show the evolution of velocity weakening zone on a simulated fault derived from fluid injection and velocity stepped experiments and the corresponding non uniform fluid pressure distribution recovered from coupled hydro mechanical simulations our results indicate that the sharp extension of velocity weakening zone occurs before the nucleation of fault rupture which could be an indicator to avoid the reactivation of other fault patches beyond the stimulated zone the dynamic rupture is estimated to extend much faster than the maximum speed of the velocity weakening zone front we infer that the velocity weakening zone may further expand and fully control the fault behavior after multiple slip events

shallow sedimentary rock as a fragile geological feature effects of clay content and hydrology on frictional strength strong long period 3 s seismic waves impose dynamic strains on the shallow subsurface the dynamic strain is the dynamic velocity divided by the phase velocity of the waves the dynamic stress is the strain times the shear modulus a testable hypothesis is that the shear modulus of the rock self organizes so that the rock barely fails in friction with typical imposed dynamic strains the predicted value of stiffness divided by depth is then independent of depth for constant rock density and constant coefficient of friction with the water table at the surface predicted stiffness divided depth deviates from constancy for finite water table depth prior laboratory studies indicate that the coefficient of friction is lower in clay rich rocks than in clay free rocks these effects provide appraisal of the concept in which hydrology and lithology are constrained four boreholes near parkfield california qualitatively exhibit the predicted effects there is some indication of the predicted effect of water table depth within accumulating sediments penetrated by borehole mcglincy mgcy in the santa clara valley of california but the effect of clay is not well resolved due to a dearth of clay rich beds

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1