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[1] We study a seismic swarm that occurred in 1989–1990 at the Alban Hills volcano and interpret the seismicity pattern in terms of Coulomb stress changes caused by magma intrusion in a local volcanic source and the extensional tectonic stress field. We first image the three-dimensional (3-D) structure of the volcano through a tomographic inversion of P waves and S-P arrival times recorded by a temporary local network. A high Vp and Vp/Vs body exists beneath the area of most recent volcanic activity, which we interpret as a solidified magma body delimiting the position of the volcanic source. We have relocated 661 events (M ≤ 4.0) using this 3-D velocity model and we have computed 64 fault plane solutions. Elevation changes, measured between 1951 and 1994 along a 33-km-long line crossing the western part of the volcano, reveal an uplift of 0.3 m. We model these data to constrain the position and geometry of the volcanic source. We compute the vertical deformation in a homogeneous half-space, testing different volcanic sources (spherical magma chamber, sill and dike). We model the Coulomb stress changes caused by the local volcanic source and the regional tectonic stress field. The inflation of magma generates an increase of Coulomb stress larger than 0.5 MPa in the area where the seismicity is located. More than 85% of fault plane solutions are consistent with the stress perturbations induced by the volcanic source. We conclude that microearthquakes at the Alban Hills are promoted by elastic stress changes caused by volcanic unrest episodes.

[1] We model the dynamic propagation of a 2-D in-plane crack obeying to either slip weakening (SW) or rate- and state-dependent friction laws (R&S). We compare the value of slip weakening distance (Dc), adopted or estimated from the traction versus slip curves, with the critical slip distance measured as the slip at the time of peak slip velocity (D′c). The adopted friction law and the constitutive parameters control the slip acceleration as well as the timing and the amplitude of peak slip velocity. Our simulations with R&S show that the direct effect of friction and the friction behavior at high slip rates affect the timing of peak slip velocity and thus control the ratio D′c/Dc. The difference observed in this study between the Dc values and the inferred D′c can range between few percent up to 50%.

[1] We compute the temporal evolution of traction by solving the elasto-dynamic equation and by using the slip velocity history as a boundary condition on the fault plane. We use different source time functions to derive a suite of kinematic source models to image the spatial distribution of dynamic and breakdown stress drop, strength excess and critical slip weakening distance (D c ). Our results show that the source time functions, adopted in kinematic source models, affect the inferred dynamic parameters. The critical slip weakening distance, characterizing the constitutive relation, ranges between 30% and 80% of the total slip. The ratio between D c and total slip depends on the adopted source time functions and, in these applications, is nearly constant over the fault. We propose that source time functions compatible with earthquake dynamics should be used to inter the traction time history.

[1] In this study we aim to understand the effect of static and dynamic stress changes in promoting earthquake failures on secondary faults. Toward this goal we solve the equation of motion of a spring-slider dynamic system including inertia and using rate- and state-dependent constitutive laws. We separately investigate the dynamic response of this fault analog system to a sudden stress change represented either as a stress step or as a stress pulse, which are used to model permanent (static) and transient (dynamic) stress perturbations. The induced earthquake failure does not occur immediately at the application of the coseismic stress change, but it is delayed in time: we define this time interval as the triggering delay. For a given stress perturbation, we analyze the dependence of triggering delays on different system conditions and constitutive parameters. Our results clearly show that the effects of static and dynamic stress changes are quite different. While a static stress change is able to advance as well as to delay an induced instability depending on its sign, a dynamic stress pulse is only able to promote a nearly instantaneous failure, provided its amplitude is positive and large enough with respect to the direct effect of friction. In other words, dynamic stress changes can only cause nearly instantaneous failures, without any relevant triggering delay. These results should be considered in interpreting the seismicity rate changes caused by large earthquakes at least as long as seismic events are interpreted as sliding instabilities obeying rate- and state-dependent friction laws.

We investigate the coseismic stress redistribution during the 25 March 1998 great Antarctic plate (MW 8.1) earthquake, in which the mainshock consisted of two distinct subevents separated in time by several tens of seconds. We compute the dynamic stress time histories for the fault geometry and the rupture and slip history determined by Henry et al. (2000), using the discrete wavenumber and re- flectivity method of Cotton and Coutant (1997), both for a homogeneous and a strat- ified half-space. We first image the coseismic stress evolution caused by the first subevent on the fault plane of the second one for both the velocity models. We compute both shear and normal stress changes and a time-dependent Coulomb failure function (CFF). Our results show that the shear stress changes have larger amplitudes than the other stress components and hence are the primary control on the evolution of the CFF. The dynamic stress amplitudes are larger than the static stress pertur- bations, with the largest positive dynamic stress peak on the second subevent fault plane reaching slightly less than 0.2 MPa at 60 sec and 65 sec after the nucleation, for the layered and the homogeneous crustal models, respectively. We suggest that the dynamic stress changes caused by the first subevent promoted a nearly instan- taneous failure on the second subevent fault.

[1] We study the dynamic traction behavior within the cohesive zone during the propagation of earthquake ruptures adopting rate- and state-dependent constitutive relations. The resulting slip-weakening curve displays an equivalent slip-weakening distance (D0eq), which is different from the parameter L controlling the state variable evolution. The adopted constitutive parameters (a, b, L) control the slip-weakening behavior and the absorbed fracture energy. The dimension of the nucleation patch scales with L and not with D0eq. We propose a scaling relation between these two lengthscale parameters which prescribes that D0eq/L ∼ 15.

Broadband seismograms recorded at a borehole three-component (high dynamic range) seismic station in the Po Valley (Northern Italy) were analyzed to study the velocity structure of the shallow sedimentary layers as well as the local site effects in soft sediments. The broadband borehole seismometer was installed at a depth of 135 m just below the quaternary basement, while a second digital broadband seismometer was installed in the same site at the Earth surface. The velocity structure in the shallower layers was determined both by means of cross-hole and up-hole measurements and by inverting seismic data recorded during a seismic exploration experiment.Velocity discontinuities are quite well related to the stratigraphy of the site. We are interested to record local earthquakes as well as regional and teleseismic events. The analyzed data set includes local, regional and teleseismic events, most of which were recorded during the seismic sequence that started on October 15, 1996, near Reggio Emilia 80 km away from the borehole site. The orientation of the borehole sensor is determined using the recordings of a teleseismic event and of some local earthquakes. The noise reduction for the borehole sensor is 2 decades in power spectral density at frequencies larger than 1.0 Hz. We studied the site amplification of the shallow alluvial layers by applying the spectral ratio method. We analyzed the spectral ratios of noise recorded by the surface and borehole seismometers as well as those from local earthquakes. We compared these observations with a theoretical model for the site response computed by the Haskell-Thomson method.

Publisher Summary This chapter discusses the recent developments in understanding how earthquakes interact with each other. The new theoretical framework for earthquake cycles is based on calculating the stress changes caused by one event and assessing where and what mechanism of earthquakes these changes may promote. For studying such stress interaction, the computation of the stress field outside a rupturing fault is analyzed. This is different from investigating the dynamic rupture growth requiring the reconstruction of the spatiotemporal evolution of the stress on the fault plane. The chapter discusses the theoretical background of earthquake sequences and reviews some of the simple examples that allowed stress coupling concepts to be accepted. The success of simple stress modeling led to the introduction of several modifications, adaptations, and refinements of the ideas.

We investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault. The dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods. After a transient phase, the stress time history evolves to the final static stress value. We compare the static stress changes resulting from this model with those computed from a static dislocation model. We have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 (MS 6.9) Irpinia earthquake. These two subevents are separated in time by nearly 20 s, while the third (and last) subevent occurred 40 s after the rupture onset. We compute the dynamic stress changes caused by the rupture of the first subevent. Our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault. On the average the static stress level on the second subevent (20 s) fault plane is reached nearly after 14 s. The dynamic rupture did not jump from a rupturing segment to the adjacent one immediately, but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event. The dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3.4 km/s. The delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults. In particular, rate- and state-dependent frictional law can explain a delayed instability after a sudden change in stress. Using the estimated values of the subevent triggering delay and the shear stress change, we attempt to constrain the parameter Aσ on the 20 s fault. The values here inferred agree well with those resulting from previous studies.

During the past 1000 years, eruptions of Vesuvius have often been accompanied by large earthquakes in the Apennines 50-60 km to the northeast. Statistical investigations had shown that earthquakes often preceded eruptions, typically by less than a decade, but did not provide a physical explanation for the correlation. Here, we explore elastic stress interaction between earthquakes and eruptions under the hypothesis that small stress changes can promote events when the Apennine normal faults and the Vesuvius magma body are close to failure. We show that earthquakes can promote eruptions by compressing the magma body at depth and opening suitably oriented near-surface conduits. Voiding the magma body in turns brings these same normal faults closer to Coulomb failure, promoting earthquakes. Such a coupling is strongest if the magma reservoir is a dike oriented normal to the regional extension axis, parallel to the Apennines, and the near-surface conduits and fissures are oriented normal to the Apennines. This preferred orientation suggests that the eruptions issuing from such fissures should be most closely linked in time to Apennine earthquakes. Large Apennine earthquakes since 1400 are calculated to have transferred more stress to Vesuvius than all but the largest eruptions have transferred to Apennine faults, which may explain why earthquakes more commonly lead than follow eruptions. A two-way coupling may thus link earthquakes and Vesuvius eruptions along a 100-km-long set of faults. We test the statistical significance of the earthquake-eruption correlation in the two-way coupling zone, and find a correlation significant at the 95% confidence level.

Geophysical Research Letters, vol. 25, pp. 2861-2864, 1998; International audience; A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (•40 ø) to the southwest, and confined in the upper •8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

Anisotropy is a common property of the Earth's crust and the upper mantle; it is related to the strain field of the medium and therefore to geodynamics. In this paper we describe the different possible origins of anisotropic behavior of the seismic waves and the seismological techniques used to define anisotropic bodies. In general it is found that the fast polarization direction is parallel to the absolute plate motion in cratonic areas, to the spreading direction near rifts or extensional zones, and to the main structural features in transpressive regimes. The delay times between fast and slow waves reflect the relative strength and penetration at depth of the deformation field. The correspondence between surface structural trends and anisotropy in the upper mantle, found in many regions of the world, strongly suggest that orogenic processes involve not only the shallow crust but the entire lithosphere. Recently in Italy both shear wave splitting analysis and Pn inversion were applied to define the trend of seismic anisotropy. Along the Northern Appeninic arc fast directions follow the strike of the arc (i.e., parallel to the strike of the Miocene-Pleistocene compressional features), whereas in the Tyrrhenian zone fast directions are about E-W SW-NE; parallel to the post-Miocene extension that is thought to have reoriented the mantle minerals fabric in the astenosphere.

In the opinion of the undersigned, AGU's position statement regarding the conviction of Italian seismologists, issued following the 22 October 2012 conviction of six Italian scientists and one government official related to the 2009 L'Aquila earthquake (see Eos, 93(44), 444, 10.1029/2012EO440013), is absolutely right and correct. We believe that Franco Marenco's opinion disagreeing with AGU's position (Eos, 94(6), 63, doi:10.1002/2013EO060006) is misleading because it is based only on biased information gathered from the media. We invite Marenco and anyone who is interested in better understanding the L'Aquila trial and related issues to retrieve and read original documents and information from http://processoaquila.wordpress.com/.

The earthquake of June 12, 1995 has been located using local and regional data (41°48.8'N, 12°30.8°6E at a depth of about 11.5 km) a few kilometers inside the city limit of Rome, in its southernmost part. This is the first event that occurred in Rome for which instrumental data are available. The local magnitude estimated from digital recordings is ML 3.6 and it was largely felt reaching intensity VI MCS. We constrained the focal mechanism by analyzing the S-wave polarization and it agrees well with the distribution of P-wave polarities. The fault plane solution shows a dominant strike slip mechanism (strike 275°, dip 70°, rake - 140°). Seismic moment, M0 = 2.3 ± 0.6 1021dyne × , was computed from S-wave displacement spectra of horizontal components of ground motion digital waveforms. The corresponding source radius ranges between 200 and 500 m, depending on the assumed stress drop (100 bars or 10 bars, respectively). The earthquake was preceded by a ML 2.6 foreshock. The seismic sequence lasted a few days during which 38 aftershocks were recorded. The seismicity pattern shows the characteristics of a mainshock-aftershock sequence, rather than swarm behavior which seems to characterize the activity of the neighboring seismogenic areas of the Alban Hills. We used a master event algorithm to locate some of the aftershocks. Results show that the relocated aftershocks are clustered in a small volume in proximity of the mainshock hypocenter.

The strong motion accelerograms recorded during the 1990 Eastern Sicily earthquake have been analyzed to investigate source and attenuation parameters. Peak ground motions (peak acceleration, velocity and displacement) overestimate the values predicted by the empirical scaling law proposed for other Italian earthquakes, suggesting that local site response and propagation path effects play an important role in interpreting the observed time histories. The local magnitude, computed from the strong motion accelerograms by synthesizing the Wood-Anderson response, is ML = 5.9, that is sensibly larger than the local magnitude estimated at regional distances from broad-band seismograms (ML = 5.4). The standard omega-square source spectral model seems to be inadequate to describe the observed spectra over the entire frequency band from 0.2 to 20 Hz. The seismic moment estimated from the strong motion accelerogram recorded at the closest rock site (Sortino) is Mo = 0.8 x 1024 dyne.cm, that is roughly 4.5 times lower than the value estimated at regional distances (Mo = 3.7 x 1024 dyne.cm) from broad-band seismograms. The corner frequency estimated from the accelera- tion spectra i.5 J; = 1.3 Hz, that is close to the inverse of the dUl.ation of displacement pulses at the two closest recording sites. This value of corner tì.equency and the two values of seismic moment yield a Brune stress drop larger than 500 bars. However, a corner frequency value off; = 0.6 Hz and the seismic moment resulting from regional data allows the acceleration spectra to be reproduced on the entire available frequency band yielding to a Brune stress drop of 210 bars. The ambiguity on the corner frequency value associated to this earthquake is due to the limited frequency bandwidth available on the strong motion recordil1gs. Assuming the seismic moment estimated at regional distances from broad-band data, the moment magnitude for this earthquake is 5.7. The higher local magnitude (5.9) compared with the moment magnitude (5.7) is due to the weak regional attenuation. Beside this, site amplifications due to surface geology have produced the highest peak ground motions among those observed at the strong motion sites.

n this paper we describe the location and the fault plane solution of the December 13, 1990, Eastern Sicily earthquake (ML = 5.4), and of its aftershock sequence. Because the main shock location is not well constrained due to the geometry of the permanent National Seismic Network in this area, we used a "master event" algorithm to locate it in relation to a well located aftershock. The revised location is slightly offshore Eastern Sicily, 4.8 km north of the largest aftershock (ML = 4.6) that occurred on December 16, 1990. The main shock has a strike-slip mechanism, indicating SE-NW compression with either left lateral motion on a NS plane, or right lateral on an EW plane. Two days after the main event we deployed a local network of eight digital stations, that provided accurate locations of the aftershocks, and the estimate of source parameters for the strongest earthquake. We observed an unusual quiescence after the ML = 5.4 event, that lasted until December 16, when a ML = 4.6 earthquake occurred. The fault plane solution of this aftershock shows normal faulting on E-W trending planes. Between December 16 and January 6, 1991, a sequence of at least 300 aftershock" was recorded by the local network. The well located earthquakes define a small source region of approximately 5 x 2 x 5 km3, with hypocentral depths ranging between 15 and 20 km. The paucity of large aftershocks, the time gap between the main shock occurrence and the beginning of the aftershock sequence (3.5 days), their different focal mechanisms (strike-slip vs. normal), and the different stress drop between main shock and after- shock suggest that the ML = 5.4 earthquake is an isolated event. The sequence of aftershocks began with the ML = 4.6 event, which is probably linked to the main shock with a complex mechanism of stress redistribution after the main faulting episode.

In this paper we analyze the seismicity along the Apennines in order to relate the seismicity pattern to the seismogenic environment and to the active tectonic processes. The analysis of crustal and subcrustal seismicity show that the northern and southern Apennines are characterized by two distinct patterns of seismicity. These two domains are separated by two important lithological discontinuities (the Ancona — Anzio and the Ortona — Roccamonfina lines). The seismicity along the Apennines is mainly concentrated in a narrow belt running along the chain, with an evident geometrical offset which corresponds to the Ancona-Anzio line. We discuss these observations considering both the seismological evidence and the present tectonic regime in which the earthquakes occur. We focus on the southern Apennines, where the largest earthquakes (6 ≤ M ≤ 7.0) have occurred. We analyze the main features of some recent seismic sequences in order to discuss the seismicity patterns in terms of the hypothesis of segmentation of the southern Apennine seismogenic belt. This work aims to be a preliminary contribution to the important goals of understanding the spatial and temporal patterns of seismicity and of identifying active faults.

[1] A 2D finite element approach is used to model the mechanical coupling between faults and the interaction between large earthquakes. The regional tectonic stress, the fault geometry and the Coulomb friction law control the resulting slip patterns. To test the potential of this approach we investigate the Landers and Hector Mine earthquakes which occurred in southern California. The resulting coseismic displacement satisfactorily fits the available GPS data. We show that the stress changes induced by the 1992 MW = 7.3 Landers earthquake modulate the applied regional tectonic stress reducing the slip on the faults of the subsequent 1999 MW = 7.1 Hector Mine rupture. We emphasize that fault interaction due to stress transfer can alter the slip distribution of impending earthquakes.

The strong-motion accelerograms written by moderate and strong earthquakes occurring in Central Italy in recent years have been used to estimate physical quantities characterizing the spectrum of motion in the band of engineering interest (0.1 Hz≲f≲20 Hz). The source, propagation and site response parameters obtained in these analyses have therefore been employed to check if theoretical models may be used to correctly predict the trend of the peak values of seismic excitationvs. earthquake size in the areas investigated. For the selected data set, representative of a seismotectonic environment generating earthquakes with prevalent normal faulting (up to a magnitudeMl∼7, at depths varying roughly from 5 to 15 km), the assumed spectral model allows successful statistical predictions showing that specific site response is the main factor responsible for the large fluctuations often observed in the data behaviour, at similar magnitudes and distances from focus.

A long sequence of moderate-magnitude earthquakes (5 M 6) struck central Italy in September and October 1997. At the end of the sequence a year later, the seismogenic area extends for about 60 km along the Apennines. The analysis of historical seismicity suggests that this seismic sequence filled a 700-year gap in this portion of the chain. Other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments, probably due to the in- volvement of shallow and complex structures inherited by the compressive tectonics. The distribution of seismicity and the fault-plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles (40) to the southwest. The focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the Apenninic chain (northeast-southwest), consistently with the Quaternary tectonics of the internal sector of the northern Apen- nine belt and with previous earthquakes in adjacent regions. Three mainshocks oc- curred on distinct 5- to 10-km-long fault segments, adjacent and slightly offset be- tween each other. High-quality aftershock locations show that seismicity is confined within the sedimentary Mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks, which nucleated at 6-km depth. Faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface. Most of the aftershock focal mechanisms are dominated by normal faulting. Several strike-slip events occurred at shallow depths, reactivating portions of pre-existing thrust planes that segment the normal fault system. The spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes.

The horizontal components of 45 strong motion accelerograms recorded in Friuli, Italy, from May to September 1976, have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes. Two different stress parameters, the Brune stress drop Δσ and the apparent stress σa, have been considered. The Friuli main shock has a strong stress drop (Δσ ≈ 800 bars), while the aftershocks have stress drops which range from 250 to 400 bars. Five accelerograms written by the Montenegro, Yugoslavia, earthquake of April 1979 have also been analyzed. This thrust faulting earthquake had a stress drop of Δσ ≈ 900 bars. The stress drops of these thrust faulting earthquakes are compared to the stress drops determined by Rovelli et al. (1988a) for nine normal faulting earthquakes which occurred in the Apennines region from 1979 to 1984. The radiated energies and apparent stresses have also been calculated for the set of Apennine earthquakes. The thrust faulting earthquakes in the Alpine-Dynaride region exhibit higher values for both the Brune and the apparent stress than the normal faulting earthquakes in the Apennines region. Excluding the Friuli and Montenegro main shocks, the Brune stress drops of the thrust faults are approximately 3 times those of the normal faults, while the apparent stresses are twice as large. This difference is statistically significant at a confidence level of 95%. The same difference also emerges from the scaling of the peak ground motions recorded in these two regions, suggesting an enhanced severity of the seismic input in the Alpine-Dynaride environment compared with the Apennines.

The analysis of static stress changes due to co-seismic dislocations has been widely applied in recent years to study the variation in failure stress on well-known segmented seismogenic faults. These studies assert that earthquakes induce changes in static stress on neighboring faults that may delay, advance, or trigger impending earthquakes. So far, most of the investigations have focused on California strike-slip faults and, more recently, on thrust faults. In this work, we study the static stress changes in an extensional tectonic setting. Following Okada (1992), we calculate static stress changes caused by normal-faulting earthquakes in the southern Apennines (Italy) using a 3D dislocation model in an elastic half-space. We first studied the static stress changes produced by the 1980 Irpinia earthquake. This normal-faulting event consisted of four fault segments that ruptured together within 40 sec. The last two subevents occurred 20 and 40 sec after the nucleation on two normal faults parallel to the main one. Because the rupture of these subvents are separated by a sufficiently long time interval, we assume that, before each subevent, the dynamic stress has reached its static configuration. Therefore, we analyze the static stress changes caused by each subevent in order to determine the variations of the initial conditions for subsequent ruptures. Moreover, we applied our procedure to study the static stress changes after the 1980 Irpinia earthquake on the fault zone where the 1990 and 1991 Potenza earthquakes (M ≈ 5) occurred. Our calculations indicate that, even if the amount of stress change depends on the fault geometry assumed for the 20-sec subevent of the Irpinia fault, the static stress increased on the Potenza fault after the 1980 Irpinia earthquake. This event also increased the static stress on seismogenic faults that are believed to have ruptured in the 1732 and 1857 earthquakes (M ≈ 6.5). In these calculations, we have considered the effect of a regional extensional stress field whose minimum principal axis is horizontal and oriented 216° N. The results show that the 1980 Irpinia earthquake increased the static stress by roughly 1 bar on the 1732 fault zone. Right lateral strike-slip faults oriented nearly E-W, such as the Potenza fault, are favorably oriented with respect to the total stress resulting from the extensional regional stress field and the coseismic stress induced by the Irpinia earthquake. Finally, we investigate the static stress changes caused by several historical earthquakes that occurred along the southern Apennines seismogenic belt. Most of the large historical earthquakes occurred in areas of static stress increase, yielding evidence for fault interaction.

We study coseismic and postseismic stress fields caused by a normal faulting earthquake in a self-gravitating, stratified, viscoelastic spherical Earth over distances from a few to hundreds of kilometers. We investigate the contribution of postseismic relaxation on the induced Coulomb stress for extensional tectonic settings accounting for the effects of the Earth stratification. We use a numerical code based on the spherical self-gravitating Earth model developed by Piersanti et al. [1995, 1997]. We study how postseismic relaxation can modify the state of stress at the base of the seismogenic layer where large earthquakes are believed to nucleate. We compare our results with those obtained by means of a three-dimensional dislocation model in an elastic half-space, which does not account for the time-dependent postseismic stress transfer. The viscoelastic relaxation process modifies the coseismic stress changes during time periods from several decades to centuries. The postseismic stress is generally greater than the coseismic stress change. Postseismic relaxation increases the Coulomb stress near the causative faults and tends to reduce the stress shadow areas. The temporal evolution of Coulomb stress reveals that in addition to the viscosity value, the thickness of the elastic layer controls the time at which the relaxation process is completed. A larger thickness of the elastic layer yields a faster relaxation in the first few decades after the seismic event but smaller postseismic stress amplitudes at longer timescales. One of the most significant results of this study is the extreme sensitivity of the timescales of the viscoelastic relaxation to small changes in the thickness and depth of the shallowest viscoelastic layer as well as in variation of the viscosity. Such a result suggests that the interpretation of the time evolution of the postseismic signals only in terms of viscosity values could lead to misleading conclusions.

This paper describes the results of a seismic refraction profile conducted in October 1992 in the Sannio region, Southern Italy, to obtain a detailed P-wave velocity model of the upper crust. The profile, 75 km long, extended parallel to the Apenninic chain in a region frequently damaged in historical time by strong earthquakes. Six shots were fired at five sites and recorded by a number of seismic stations ranging from 41 to 71 with a spacing of 1-2 km along the recording line. We used a two-dimensional raytracing technique to model travel times and amplitudes of first and second arrivals. The obtained P-wave velocity model has a shallow structure with strong lateral variations in the southern portion of the profile. Near surface sediments of the Tertiary age are characterized by seismic velocities in the 3.0-4.1 km/s range. In the northern part of the profile these deposits overlie a layer with a velocity of 4.8 km/s that has been interpreted as a Mesozoic sedimentary succession. A high velocity body, corresponding to the limestones of the Western Carbonate Platform with a velocity of 6 km/s, characterizes the southernmost part of the profile at shallow depths. At a depth of about 4 km the model becomes laterally homogeneous showing a continuous layer with a thickness in the 3-4 km range and a velocity of 6 km/s corresponding to the Meso-Cenozoic limestone succession of the Apulia Carbonate Platform. This platform appears to be layered, as indicated by an increase in seismic velocity from 6 to 6.7 km/s at depths in the 6-8 km range, that has been interpreted as a lithological transition from limestones to Triassic dolomites and anhydrites of the Burano formation. A lower P-wave velocity of about 5.0-5.5 km/s is hypothesized at the bottom of the Apulia Platform at depths ranging from 10 km down to 12.5 km; these low velocities could be related to Permo-Triassic siliciclastic deposits of the Verrucano sequence drilled at the bottom of the Apulia Platform in the Apulia Foreland.

Statistical seismology has been emerging as a new discipline at the interface between earthquake physics, earthquake statistics, hazard assessment, and society. A workshop in Italy was sponsored by the Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV), the Swiss Federal Institute of Technology Zurich (ETH), and the Japanese Institute of Statistical Mathematics (ISM), to discuss the state of the art and future directions. Conference participants discussed how to use available tools and techniques of statistical seismology to advance earthquake science. Building increasingly accurate timedependent earthquake forecast models at various spatial and temporal scales is widely recognized as an important challenge, and various such models were presented and discussed at the workshop. Exploiting time dependence for hazard assessment requires us to develop a detailed understanding of the behavior of regional fault systems and corresponding earthquake catalogs spanning decades. We can no longer analyze individual faults or earthquake sequences in isolation.

We examined shear wave splitting in SKS and S phases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split - shear wave ( ?) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ? in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW-SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ? oriented about E-W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E-W post-orogenic extension observed at the surface.

International audience; We investigate the rupture history of the three largest magnitude earthquakes of the 1997 Umbria-Marche sequence by inverting GPS, DInSAR and near-source strong motion waveforms. We use the frequency domain inversion procedure proposed by Cotton and Campillo (1995) and calculate the Green's functions for a layered half-space using the discrete wavenumber and reflectivity methods. We first invert GPS measurements and DInSAR interferograms to image the coseismic slip distribution on the fault planes in a layered half space for the two earthquakes that occurred on September 26, 1997 at 00:33 UTC (Mw = 5.7) and 09:40 UTC (Mw = 6.0) near Colfiori-to. We also invert DInSAR interferograms to infer the slip distribution during the subsequent earthquake that occurred on October 14, 1997 at 15:23 UTC (Mw = 5.6) in the SE section of the seismogenic zone near Sellano. We also explore the set of acceptable solutions using a genetic algorithm to have information on the available resolution of geodetic data. The slip models obtained by geodetic data inversion are used to perform a forward mod-eling of strong motion waveforms for all three events. We adopt a constant rupture velocity of 2.6 km/s and a constant rise time of 1 s. Our results show that these rupture models provide an acceptable fit to recorded waveforms. Finally, we invert the recorded ground displacements, collected during the September 26th 09:40 main shock and the October 14th Sellano earthquake, to constrain the rupture history. We use the geodetic slip distribution as starting model for the iterative inversion procedure. The retrieved rupture models are consistent with those inferred from geodetic data and yield a good fit to recorded seismograms. These rupture models are characterized by a heterogeneous slip distribution and an evident rupture directivity in agreement with previous observations.

[1] We model the traction evolution and shear stress degradation near the tip of a propagating dynamic rupture by solving the elastodynamic equation for a 2-D in-plane fault obeying rate- and state-dependent friction laws and adopting a finite difference numerical method. Modeling results clearly show that our dynamic solution implies a slip dependence of fault friction, as previously observed either in laboratory experiments or in theoretical models. However, the resulting equivalent slip-weakening distance (d0eq) is different from the length scale parameter (L) characteristic of the rate and state formulation. We demonstrate that the state variable evolution controls the slip acceleration and the absorbed fracture energy. The adopted constitutive parameters a, b, and L affect the traction dependence on slip. We present the results of several numerical simulations, performed after a careful control of the available resolution of the cohesive zone, to unravel the dependence of the equivalent slip-weakening distance on the constitutive parameters. We also propose analytical relations to interpret our numerical results, which point out that the traction evolution within the cohesive zone cannot be prescribed a priori in the framework of rate-and-state constitutive laws. In particular, the yield stress and the kinetic friction level depend on particular slip velocity values characteristic of specific stages of the breakdown process. Finally, we discuss how the adopted evolution law affects the slip-weakening curve by comparing the simulations performed with a slip and a slowness law. The former yields smaller equivalent slip-weakening distances than the latter.

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate- and state-dependent friction law, where we differentiate velocity-weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity-strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

[1] Pore pressure changes are rigorously included in Coulomb stress calculations for fault interaction studies. These are considered changes under undrained conditions for analyzing very short term postseismic response. The assumption that pore pressure is proportional to fault-normal stress leads to the widely used concept of an effective friction coefficient. We provide an exact expression for undrained fault zone pore pressure changes to evaluate the validity of that concept. A narrow fault zone is considered whose poroelastic parameters are different from those in the surrounding medium, which is assumed to be elastically isotropic. We use conditions for mechanical equilibrium of stress and geometric compatibility of strain to express the effective normal stress change within the fault as a weighted linear combination of mean stress and fault-normal stress changes in the surroundings. Pore pressure changes are determined by fault-normal stress changes when the shear modulus within the fault zone is significantly smaller than in the surroundings but by mean stress changes when the elastic mismatch is small. We also consider an anisotropic fault zone, introducing a Skempton tensor for pore pressure changes. If the anisotropy is extreme, such that fluid pressurization under constant stress would cause expansion only in the fault-normal direction, then the effective friction coefficient concept applies exactly. We finally consider moderately longer timescales than those for undrained response. A sufficiently permeable fault may come to local pressure equilibrium with its surroundings even while that surrounding region may still be undrained, leading to pore pressure change determined by mean stress changes in those surroundings.

The modeling of earthquake initiation and subsequent rupture propagation requires the use of a fault constitutive law controlling the traction evolution and the slip acceleration that yields a finite energy flux at the crack tip. The determination of the temporal evolution of dynamic traction during the propagation of an earthquake rupture has been the major task of many recent investigations. Ohnaka (2004) presented a detailed discussion in favor of a “rational” governing law for earthquake ruptures which is consistent with laboratory experiments and, according to the author, is based on the “physics of rock friction and fracture”. He concluded that this constitutive law must be slip-dependent because the slip dependency is a “more fundamental property of the shear rupture than the ratedependency”. Ohnaka (2004) also suggested that this slipdependent law model the whole seismic cycle (therefore including earthquake nucleation and long-term fault restrengthening) and considered the proposed law to be a governing relation unifying the different features of the earthquake failure process. In order to support the proposed slip-dependent law, Ohnaka (2004) presented a questionable comparison between the slip- and the rate- and state-dependent (referred to as RS hereafter) constitutive laws (see section 2 of that paper). As previously mentioned, his discussion focused on demonstrating that the slip dependency is more important than the rate dependency. The goal of the present paper is not to open a debate on the more appropriate constitutive law, neither to support RS friction in favor of slip-dependent laws. Instead, our aim is to provide an alternative interpretation of the slip dependency of shear traction as well as to discuss the reasons why RS friction laws yield a traction evolution consistent with slip-weakening (referred to as SW hereafter). Although we share a common goal with Ohnaka in our search for the best framework within which to define a unified constitutive law that models earthquake initiation, propagation and arrest, we think that the complexity and the diversity of real fault zone structures make this task difficult to accomplish. Our reasoning is explained in detail in the following discussion.

SUMMARY An analysis of the Zihuatanejo, Mexico, earthquake of 1994 December 10 (M = 6.6), based on teleseismic and near-source data, shows that it was a normal-faulting, intermediate-depth (H = 50 k 5 km) event. It was located about 30 km inland, within the subducted Cocos plate. The preferred fault plane has an azimuth of 130", a dip of 79" and a rake of -86". The rupture consisted of two subevents which were separated in time by about 2s, with the second subevent occurring downdip of the first. The measured stress drop was relatively high, requiring a Acr of about a kilobar to explain the high-frequency level of the near-source spectra. A rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below Zihuatanejo, based on the depth and the rupture extent of this event, is 40 km. This event and the Oaxaca earthquake of 1931 January 15 (M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, lowangle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event.