Direction Of Rupture Propagation Research Articles

Geological Observations of Damage Asymmetry in the Structure of the San Jacinto, San Andreas and Punchbowl Faults in Southern California: A Possible Indicator for Preferred Rupture Propagation Direction

We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults.

Comment on “The wrinkle-like slip pulse is not important in earthquake dynamics” by D. J. Andrews and R. A. Harris

[1] Andrews and Harris [2005] (hereinafter referred to as AH) found in several numerical simulations that effects associated with prescribed stress heterogeneities and timeweakening friction can suppress those generated dynamically by rupture on a bimaterial interface, and concluded that the wrinkle-like rupture mode is generally not important for earthquake dynamics. Their results are based on selective cases, rather than a systematic parameter-space study, and they do not form the basis for a general conclusion. Moreover, most of their cases are associated with conditions for which the bimaterial effects are weak due to combination of assumed parameter values, mixed rupture modes, relatively small propagation distance, and coarse grid. Considerations of a larger body of theoretical results and observations suggest that the wrinkle-like rupture may be relevant to earthquake dynamics. [2] Weertman [1980] showed analytically that mode II slip pulse on a bimaterial interface governed by Coulomb friction produces dynamic changes of normal stress that depend on the slip function, material properties and direction of rupture propagation. Subsonic propagation in the direction of slip of the compliant solid reduces dynamically the normal stress, whereas propagation in the opposite direction increases the normal stress. Adams [1995] showed analytically that mode II rupture on a bimaterial Coulomb interface has a strong dynamic instability that transfers energy during propagation to shorter wavelengths, leading to pulse sharpening and increasing slip velocity with propagation distance. The discussed effects are pure mode II phenomena on a bimaterial interface, and they do not exist for mode III rupture or in a homogeneous solid. For additional theoretical background, see Ranjith and Rice [2001] and Ben-Zion [2001]. [3] The analytical properties of mode II slip pulse on a bimaterial interface render numerical simulations of such rupture highly challenging. Very fine grid is needed to capture the sharp features associated with the pulse, while large propagation distance is required to reach a dynamic regime that is relatively free of initial transients and reflects the attractor of the evolving dynamic behavior. Andrews and Ben-Zion [1997] simulated mode II ruptures on a bimaterial Coulomb interface with a nucleation consisting of a localized stress drop having a favored propagation direction. They observed wrinkle-like slip pulses with properties compatible to the analytical results of Weertman and Adams. Similar pulses were found in subsequent simulations with increasing refinements and incorporation of additional physical ingredients, including a nucleation mechanism that generalizes the procedure of Andrews and Ben-Zion to a symmetrically expanding source without a preferred direction and slip-weakening friction [Shi and Ben-Zion, 2006]. [4] An assessment of the possible relevance of the wrinkle-like pulse to earthquake dynamics should focus on situations associated with mode II ruptures. Important such cases are large earthquakes on plate-bounding strikeslip faults. These earthquakes (referred to below as the ‘‘target’’ earthquakes) saturate the seismogenic zone, propagate predominately as mode II rupture, and are likely to be affected primarily by the elastic properties of the bounding crustal blocks (rather than the myriad of possible smallscale heterogeneities). As discussed below, the simulations of AH address in a rather limited way the dynamics of the large target earthquakes.

Material contrast does not predict earthquake rupture propagation direction

Earthquakes often occur on faults that juxtapose different rocks. The result is rupture behavior that differs from that of an earthquake occurring on a fault in a homogeneous material. Previous 2D numerical simulations have studied simple cases of earthquake rupture propagation where there is a material contrast across a fault and have come to two different conclusions: 1) earthquake rupture propagation direction can be predicted from the material contrast, and 2) earthquake rupture propagation direction cannot be predicted from the material contrast. In this paper we provide observational evidence from 70 years of earthquakes at Parkfield, CA, and new 3D numerical simulations. Both the observations and the numerical simulations demonstrate that earthquake rupture propagation direction is unlikely to be predictable on the basis of a material contrast.

Hydroacoustic Constraints on the Rupture Duration, Length, and Speed of the Great Sumatra-Andaman Earthquake

On 26 December 2004, one of the largest recorded earthquakes occurred, triggering a devastating tsunami that killed an estimated 300,000 people. The event was initially classified as Mw 9.0 based on the analysis of seismic body and surface waves (Nettles and Ekstrom, 2004; Ji, 2005; Park et al. , 2005). Classical methods of magnitude calculation are hampered by the long duration of the event, however, since late-arriving phases from the earliest portion of the rupture may obscure first-arriving energy sourced from other portions of the rupture. This same phenomenon also limits our ability to constrain the earthquake's duration, length, and speed of propagation. To work around these limitations, Stein and Okal (2005) studied Earth's low-order normal modes to estimate the size of the earthquake. Their results indicated a moment of 1.3 × 1030 dyne-cm ( Mw 9.3), which is ∼2.5 times larger than initial reports, and a centroid position near 7°N. In contrast with previous analyses, which had suggested displacement occurred primarily along the southern third of the aftershock region (Ji, 2005; Yagi, 2005; Yamanaka, 2005), these results argue that significant slip occurred along the entire aftershock zone. For shallow submarine earthquakes, hydroacoustic recordings of tertiary ( T ) waves can provide additional constraints on rupture length, as well as velocity and direction of rupture propagation ( e.g. , Bohnenstiehl et al. , 2004). T waves are formed when seismically generated energy is scattered at the seafloor-ocean interface and becomes trapped in the ocean's low-velocity wave guide, which is known as the SOund Fixing And Ranging (SOFAR) channel (Tolstoy and Ewing, 1950). Locating the source of a T wave, therefore, does not necessarily provide an estimate of the earthquake epicenter, but rather the position of the T -wave radiator. This is particularly true for the subduction zone …

Dynamic rupture on a material interface with spontaneous generation of plastic strain in the bulk

We discuss 2D simulations of in-plane ruptures on a fault between different solids in a model that includes dynamic generation of plastic strain off the fault. The fault is governed by a regularized friction with a gradual response to abrupt changes of normal stress, while plastic strain in the bulk is governed by a Coulomb yielding criterion. The occurrence of off-fault damage stabilizes the slip velocity on the fault. Stable self-sustaining ruptures propagate on the material interface as narrow unidirectional pulses in the direction of slip on the more compliant side of the fault. Plastic strain is generated only on the stiffer side of the fault (the tensional quadrant in the direction of rupture propagation) in a strip of approximately constant width that is correlated with the slip velocity on the fault. Simulations for various values of confining normal stress, material contrast and cohesion in the bulk suggest that significant generation of off-fault damage is limited to conditions that correspond to the top few kilometers of the crust. The results are compatible with field observations of shallow asymmetric damage patterns in the structure of several large faults.

Source fault structure of the 2003 Bam earthquake, southeastern Iran, inferred from the aftershock distribution and its relation to the heavily damaged area: Existence of the Arg-e-Bam fault proposed

[1] We investigate the hypocenter distribution of aftershocks of the December 26, 2003 Bam earthquake Mw 6.5 by using a temporal seismic network. The hypocenters distribute linearly over about 20 km in parallel with a line 3.5 km west of the geological Bam fault and extend from the south of Bam city to the heavily damaged area in the eastern part of the city including the historical mud brick citadel “Arg-e-Bam”. Based on the hypocenter distribution, we propose a schematic 3-D structural model of a new fault that we have named the Arg-e-Bam fault. We suggest that the Bam earthquake occurred not in the Bam fault but in the Arg-e-Bam fault. Bam city is located just above northern part of the Arg-e-Bam fault and also on the rupture propagation direction from the asperity. This may be one of the main reasons why eastern part of Bam city suffered the heaviest damage.

Effects of Fault Dip and Slip Rake Angles on Near-Source Ground Motions: Why Rupture Directivity Was Minimal in the 1999 Chi-Chi, Taiwan, Earthquake

We study how the fault dip and slip rake angles affect near-source ground velocities and displacements as faulting transitions from strike-slip motion on a vertical fault to thrust motion on a shallow-dipping fault. Ground motions are computed for five fault geometries with different combinations of fault dip and rake angles and common values for the fault area and the average slip. The nature of the shear-wave directivity is the key factor in determining the size and distribution of the peak velocities and displacements. Strong shear-wave directivity requires that (1) the observer is located in the direction of rupture propagation and (2) the rupture propagates parallel to the direction of the fault slip vector. We show that predominantly along-strike rupture of a thrust fault (geometry similar in the Chi-Chi earthquake) minimizes the area subjected to large-amplitude velocity pulses associated with rupture directivity, because the rupture propagates perpendicular to the slip vector; that is, the rupture propagates in the direction of a node in the shear-wave radiation pattern. In our simulations with a shallow hypocenter, the maximum peak-to-peak horizontal velocities exceed 1.5 m/sec over an area of only 200 km^2 for the 30°-dipping fault (geometry similar to the Chi-Chi earthquake), whereas for the 60°- and 75°-dipping faults this velocity is exceeded over an area of 2700 km^2. These simulations indicate that the area subjected to large-amplitude long-period ground motions would be larger for events of the same size as Chi-Chi that have different styles of faulting or a deeper hypocenter.

Statistical Modeling of Rock Failure under Biaxial Compression. Part I: Shear Crack Interaction

The shapes and dimensions are determined for the regions of tensile stress concentration under the interaction of two shear cracks in the biaxial compression field. The statistical model for shear microcrack accumulation and main rupture formation is developed. The influence exerted by microcrack orientation and ratio of principal stresses on the direction of main rupture propagation is studied.

Observing Earthquakes Triggered in the Near Field by Dynamic Deformations

We examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments. Our analysis focuses on seismicity at close range (within the aftershock zone), complementing published studies of long-range triggering. Our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions. Long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction. Therefore, we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically, with a majority of triggered earthquakes in the direction of rupture propagation. Bilaterally propagating ruptures also may be directive, and we propose simple criteria for assessing their directivity. We compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes ( M 5.7 to M 8.1) that occurred in California and Idaho in the United States, the Gulf of Aqaba, Syria, Guatemala, China, New Guinea, Turkey, Japan, Mexico, and Antarctica. Nine of these mainshocks had clearly directive, unilateral ruptures. Of these nine, seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction. The two exceptions include an earthquake preceded by a comparable-magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results. Similar (but weaker) correlations were found for the bilaterally rupturing earthquakes we studied. Although the static stress change also may trigger seismicity, it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus. For several of the directive earthquakes, we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change. Manuscript received 1 March 2002.

The 2002 Denali fault earthquake, Alaska: a large magnitude, slip-partitioned event.

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

Spatial and temporal rupture process of the January 26, 2001, Gujarat, India, M S=7.8 earthquake

The source parameters, such as moment tensor, focal mechanism, source time function (STF) and temporal-spatial rupture process, were obtained for the January 26, 2001, India, MS=7.8 earthquake by inverting waveform data of 27 GDSN stations with epicentral distances less than 90°. Firstly, combining the moment tensor inversion, the spatial distribution of intensity, disaster and aftershocks and the orientation of the fault where the earthquake lies, the strike, dip and rake of the seismogenic fault were determined to be 92°, 58° and 62°, respectively. That is, this earthquake was a mainly thrust faulting with the strike of near west-east and the dipping direction to south. The seismic moment released was 3.5×1020 Nm, accordingly, the moment magnitude MW was calculated to be 7.6. And then, 27 P-STFs, 22 S-STFs and the averaged STFs of them were determined respectively using the technique of spectra division in frequency domain and the synthetic seismogram as Green’s functions. The analysis of the STFs suggested that the earthquake was a continuous event with the duration time of 19 s, starting rapidly and ending slowly. Finally, the temporal-spatial distribution of the slip on the fault plane was imaged from the obtained P-STFs and S-STFs using an time domain inversion technique. The maximum slip amplitude on the fault plane was about 7 m. The maximum stress drop was 30 MPa, and the average one over the whole rupture area was 7 MPa. The rupture area was about 85 km long in the strike direction and about 60 km wide in the down-dip direction, which, equally, was 51 km deep in the depth direction. The rupture propagated 50 km eastwards and 35 km westwards. The main portion of the rupture area, which has the slip amplitude greater than 0.5 m, was of the shape of an ellipse, its major axis oriented in the slip direction of the fault, which indicated that the rupture propagation direction was in accordance with the fault slip direction. This phenomenon is popular for strike-slip faulting, but rather rare for thrust faulting. The eastern portion of the rupture area above the initiation point was larger than the western portion below the initiation point, which was indicative of the asymmetrical rupture. In other words, the rupturing was kind of unilateral from west to east and from down to up. From the snapshots of the slip-rate variation with time and space, the slip rate reached the largest at the 4th second, that was 0.2 m/s, and the rupture in this period occurred only around the initiation point. At the 6th second, the rupture around the initiation point nearly stopped, and started moving outwards. The velocity of the westward rupture was smaller than that of the eastward rupture. Such rupture behavior like a circle mostly stopped near the 15th second. After the 16th second, only some patches of rupture distributed in the outer region. From the snapshots of the slip variation with time and space, the rupture started at the initiation point and propagated outwards. The main rupture on the area with the slip amplitude greater than 5 m extended unilaterally from west to east and from down to up between the 6th and the 10th seconds, and the western segment extended a bit westwards and downwards between the 11th and the 13th seconds. The whole process lasted about 19 s. The rupture velocity over the whole rupture process was estimated to be 3.3 km/s.

Source Characteristics and Propagation Effects of the Puebla, Mexico, Earthquake of 15 June 1999

The analysis of arrival times indicates that the focus of the 15 June 1999 (20:42 UTC; 6.3 m b; 6.5 M s; 7.0 M w) Puebla, Mexico, earthquake is located at 18.15° N and 97.52° W and is 68 km deep in the contact region between the continental American and the Cocos subducting plate. First-motion data and regional and teleseismic waveform modeling indicate that the main rupture occurred as normal faulting along a fault striking N64° W and dipping approximately 42° toward the north-northeast. The aftershock activity, though scarce, is concentrated mainly toward the west of the mainshock. Three successive subevents are necessary to fit observed and synthetic waveforms. The location in space and time of subevents indicates that the rupture started as a small release at 76 km deep and then propagated toward the northwest releasing most of the seismic energy at 73 km deep. Teleseismic spectral analysis indicates that the total seismic moment release was 2.0 × 1019 N-m, and the mainshock fault length is of the order of 18 km with a stress drop of 15 bars. The source-time function consisted of three sources with a total duration of 10 sec. Recorded acceleration amplitudes of the mainshock show a clear enhancement that can be up to eight times higher toward the northwest of the epicenter in the frequency range from 0.06 Hz to 6.0 Hz, compared with the opposite direction. The distribution of reported seismic intensity also shows this feature. The directivity effect cannot be fully explained by the orientation of the fault, neither by the rupture propagation direction; therefore, we investigate the possibility that the directivity could be due to a regional propagation effect by analyzing the Lg coda decay of the mainshock. The average frequency-dependent Q Lg quality factor for the vertical component in the frequency range from 0.2 Hz to 2 Hz was Q Lg( f ) = 200 ± 12 f 0.60±0.17 and Q Lg( f ) = 202 ± 7 f 0.53±0.09 for paths toward the northwest and southeast of the epicenter, respectively. The resulting average factor Q o and η do not show significant azimuthal fluctuations. Therefore, variations of crustal attenuation properties cannot explain the notable extent of felt area and observed damage northwest of the epicenter. Probably the presence to the north of the Transmexican Volcanic Belt has played an important role in the enhancement of low-frequency seismic signals.

Three‐dimensional nonplanar simulation of the 1992 Landers earthquake

We have simulated the rupture process of the 1992 Landers, California, earthquake using a realistic nonplanar fault geometry, including fault bends and branches, through a three‐dimensional boundary integral equation method. Considering an external loading system due to local tectonics as well as a nonplanar fault structure, we could reproduce the characteristic features of this earthquake such as spontaneous rupture transfer between faults, pattern of slip distribution, and propagation of rupture front without assuming any laterally heterogeneous distribution of parameters. We could also reproduce seismic waveforms near the faults, especially in the forward direction of rupture propagation, where rupture directivity controlled by rupture transfer between faults significantly affects ground motion.

Source Time Function of Seismic Events at Rudna Copper Mine, Poland

— The empirical Green's function deconvolution technique is used to retrieve the source time functions from the records of P waves of seven seismic events that occurred at the Rudna copper mine in 1996 and were located in the middle of the underground network. Their moment magnitudes ranged from 2.1 to 2.9. The records of smaller events from the same area and with similar source mechanism, with moment magnitudes ranging from 1.5 to 2.0, were accepted as empirical Green's functions. The relative source time functions were successfully retrieved at a number of stations for six events. Directivity effects, implying unilateral rupture propagation, were observed in five cases. The azimuth of rupture propagation direction and the rupture velocity were estimated from the distribution of pulse widths and pulse maximum amplitudes as a function of the cosine of station azimuths. The rupture propagated approximately either from south to north or from north to south. The rupture velocity was low, ranging from 0.25 to 0.54 of the shear-wave velocity. The source dimensions, represented by the fault length, were also small in comparison with those estimated in the frequency domain and ranged from 80 to 250 m.

Evidence for azimuthal and temporal variations of the rupture propagation of the 1999 Chi‐Chi, Taiwan Earthquake from dense seismic array observations

Data recorded by the SMART‐2 array and its nearby accelerometers in eastern Taiwan were used to make direct measurements of the rupture propagation of the 1999, Mw=7.6 Chi‐Chi, Taiwan earthquake. The N‐th root stack method was employed to window and stack seismic waveforms, and it revealed that coherent seismic energy showed clear temporal changes with regard to azimuth. The results of this study elicited unambiguous observations as to rupture propagation direction, rupture duration time and spatial distribution of the major asperities on the fault plane. The rupture propagation of the Chi‐Chi earthquake was reconstructed and demonstrated that the source of the early arrived coherent energy was close to the epicenter and that the subsequent major energy moved southward. Twelve seconds after the initial rupture, clear arrival energy was resolved and then propagated northward. The horizontal apparent rupture velocity is estimated at 2.1 km/sec for its northward propagation. The largest source of radiated energy was identified and its location was near the northern end of the earthquake fault. The strong shaking was continuous for about 24 seconds. The analysed results of this study provide direct observations of the rupture properties of this earthquake. These properties may serve as valuable constraints for other Chi‐Chi earthquake rupture propagation studies based on other indirect measurement methods.

Estimation of rupture propagation direction and strong motion generation area from azimuth and distance dependence of source amplitude spectra

Strong motion generation areas which reproduce ground motions in 0.2 to 10Hz were estimated using the empirical Green's function method. This strong motion generation area was somewhat smaller than the total rupture area, and coincident with the area of asperities derived from heterogeneous slip distributions estimated by waveform inversions using lower frequencies (<1Hz). We confirmed that the azimuth and distance dependence of observed source amplitude spectra in the near‐source area, i.e. rupture directivity effects, were controlled by rupture propagation style and size of the strong motion generation area. We found that the source displacement spectra at stations in forward rupture propagation directions had higher corner frequencies and steeper high‐frequency decays, compared with stations in sideways directions. Stations in backward directions had opposite tendencies. Different relationships between size and average corner frequencies of the strong motion generation area were proposed for unilateral and bilateral ruptures with radial propagation.

Teleseismic inversion for the second-degree moments of earthquake space-time distributions

Determining detailed descriptions of the space–time distribution of an earthquake's rupture using only teleseismic data is often a difficult and non unique process. However, a description of an earthquake that goes beyond a point source is often needed for many seismotectonic problems. We have developed a method that inverts measurements of the shifts in traveltime and amplitude of various seismic phases at global stations for the second degree polynomial moments of an event's space–time distribution. Our values for the second moments of the 1995 Mw 8.0 Jalisco, Mexico, thrust event correspond to a characteristic rupture length of 121±10 km along strike, a characteristic rupture width of 76±11 km downdip, a characteristic duration of 29±2 s, and a direction of rupture propagation along the strike of the subduction zone to the NW. For the 1995 Mw 7.2 Gulf of Aqaba earthquake, our estimates of the second moments correspond to a characteristic rupture length of 53±2 km, a characteristic duration of 12±1 s, and rupture propagation to the NE along the strike of the Dead Sea Transform. Our estimates of the first and second moments agree well with values from local seismic and geodetic networks. For both the Jalisco and Aqaba events, we are able to resolve the fault plane ambiguity associated with the events' moment tensors using our estimates of the second spatial moment.

Dynamic simulation for tempo-spatial distribution of strain energy density in inhomogeneous stratified crust

Through simulating the research on dynamic variations of strain energy density (SED) in seismogenic model with hard inclusion, the authors have gained further knowledge to such problems as the process of earthquake preparation, initial rupture, conditions of the initial rupture and fracture propagation direction, etc. Results of the research show that SED (strain energy density) in soft inclusion is very high during the initial period of earthquake preparation. And the increment of SED in the soft area decreases at the later stage of the process. Meanwhile, the increment increases quickly in hard inclusion and in the intersection zone of the inclusion with an erecting fault, where the increment of SED is maximum. Thus, the intersection zone between hard inclusion with larger elastic modulus and erecting fault becomes the place where the initial rupture or earthquake occurs. The fracture in the end part of the hard inclusion spreads along a direction nearly vertical to the erecting fault, so the theoretical fracture direction is consistent with that calculated by digital simulation.

Fracture characteristics of the 1997 Jiashi, Xinjiang, China, earthquake swarm inferred from source spectra

Broadband P and S waves source spectra of 12 MS≥5.0 earthquakes of the 1997 Jiashi, Xinjiang, China, earthquake swarm recorded at 13 GDSN stations have been analyzed. Rupture size and static stress drop of these earthquakes have been estimated through measuring the corner frequency of the source spectra. Direction of rupture propagation of the earthquake faulting has also been inferred from the azimuthal variation of the corner frequency. The main results are as follows: 1 The rupture size of MS≥6.0 strong earthquakes is in the range of 10–20 km, while that of MS=5.0–5.5 earthquakes is 6–10 km. 2 The static stress drop of the swarm earthquakes is rather low, being of the order of 0.1 MPa. This implies that the deformation release rate in the source region may be low. 3 Stress drop of the earthquakes appears to be proportional to their seismic moment, and also to be dependent on their focal mechanism. The stress drop of normal faulting earthquakes is usually lower than that of strike-slip type earthquakes. 4 For each MS≥6.0 earthquake there exists an apparent azimuthal variation of the corner frequencies. Azimuthally variation pattern of corner frequencies of different earthquakes shows that the source rupture pattern of the Jiashi earthquake swarm is complex and no uniform rupture expanding direction exists.

The Umbria-Marche (central Italy) earthquakes: relation between rupture directivity and sequence evolution for the Mw > 5 shocks

We derive the relative moment rate function for the shocks with Mw > 5 in the 1997-98 central Italy seismic sequence, started with the Mw = 5.7, 09/26/97 00:33 event, by applying an empirical Green functionmethod. By using three conveniently placed broadband MedNet stations we obtain apparent source time functions indicating clear directivity effects for all of the analysed earthquakes, with rupture propagation generally directed outward from the epicentral area of the09/03/97 foreshock sequence. Taking a simple source model, forward modeling is utilized for determining fault parameters such as length, rupture propagation velocity and direction and, for the three major events,slip distribution. The effects of the break propagation directions onthe sequence evolution is discussed. A possible inconsistencyresults for the shallow 04/03/98 shock, which could be avoided if alink with the deep 03/26/98 event is assumed.Our analysis evidences the importance of a quick determination for the fault parameters in order to image the possible induction of fracturing on nearby structures.