F. Courboulex

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

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°) 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.

Complex Normal Faulting in the Apennines Thrust-and-Fold Belt: The 1997 Seismic sequence in Central Italy

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.

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.

Source study and tectonic implications of the 1995 Ventimiglia (border of Italy and France) earthquake (ML=4.7)

We analyse the source process and the aftershock distribution of the April 21, 1995, Ventimiglia, ML=4.7 earthquake using the records of permanent high dynamic broad-band seismic stations and a temporary network deployed on land and at sea few hours after the earthquake. This event occurred on the western Mediterranean coast, near the border between Italy and France, at a depth of 9 km, at a point where Alpine tectonic units and Late Oligocene extensional structure overlap and are currently undergoing compressional stress. The focal solutions of the mainshock and three aftershocks depict a dominant reverse faulting with an important strike-slip component, which underlines two nodal planes: a NW–SE-dipping north fault and a NE–SW-dipping south fault. We operate a careful re-location of the aftershocks using a master-event technique and data from the temporal network and obtain a predominant NW–SE alignment. Then, we analyse the rupture process using an empirical Green function approach. We find that the mainshock broke a 0.5 to 1 km fault length and that the rupture propagated during 0.1–0.2 s probably in a SE direction. Those two arguments, together with the recent fault trace that exists close to the epicentre, leads us to propose that this event expresses the reactivation of an old transverse NW–SE structure with a dextral movement. This study thus emphasizes the role of inherited, deep-rooted, transcurrent features in the tectonic reactivation of this passive margin. It also underlines the importance of combining short-period and broad-band seismology to better resolve and understand regional tectonic processes in areas of moderate seismic activity and complex geology.

Observations of vertical ground accelerations exceeding gravity during the 1997 Umbria-Marche (central Italy) earthquakes

We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.

Sismos à l’Ecole: A Worldwide Network of Real‐Time Seismometers in Schools

After each large destructive earthquake in the world, shocking images are presented to the adults and children on the TV and through the Internet. All sorts of questions arise: Some of them are very rational, whereas others are driven by fear of a global catastrophe or even a curse. For example, in March 2011, after the giant Tohoku, Japan, earthquake, seismologists everywhere were asked to provide explanations and often to give their opinion on what had happened. How could there be such a disaster? Can it happen to us? Many took the time to talk to students in classrooms to make clear what we know and what we are still trying to understand and also tried to limit irrational fears. Teachers, as well as researchers, were on the front line, particularly those who actively participate in networks of educational seismology in the United States (e.g., Levy and Taber, 2005), England (e.g., Denton, 2009), Italy (e.g., Cantore et al. , 2003; Solarino and Eva, 2009), Switzerland (e.g., Sornette and Haslinger, 2009), and other countries in the world. Thanks to Sismos a l’Ecole (SaE) network stations, accessible in real time in various parts of the world, students saw the waves of the many aftershocks passing through their school and through other schools in the world. Thus, they immediately understood that the waves of Japanese earthquakes first arrived at the high school stations in Taipei, then in Canberra, and then in Istanbul before arriving under their feet at their school. Let us see how this network is structured and how it allows …

Influence of Source, Path, and Site Effects on the Magnitude Dependence of Ground‐Motion Decay with Distance

We study the physical causes of the magnitude dependence of ground‐motion decay with distance that is observed on real data. Using stochastic simulations, we analyze the role played by anelastic attenuation Q (f), path duration, site effect (κ0), finite‐fault effect, and stress drop for a magnitude range between 4 and 7. We systematically look at peak ground acceleration, peak ground velocity, and pseudoabsolute spectral acceleration at different frequencies. We find that path duration, κ0, and stress‐drop variations have a minor effect on this dependence. At close distances (typically distances smaller than the largest fault length), the main effect is the finite‐fault effect that tends to decrease the ground‐motion values of larger events. At larger distances (>30  km), it is the effect of Q (f) that is preponderant and tends to lower the ground‐motion values of smaller events.

Une expérience multi-antennes à Annot pour l'analyse des effets de site en sismologie

Abstract For two months, ground motions induced by natural seismicity were recorded by 4 dense arrays, each of them including 9 seismological receivers. The target zone is a 400 km2 area and displays a characteristic topographic organization. The sites correspond to flat valleys filled with surficial soft sediments. The data recorded during this experiment will be used to characterize the wavefields through the different valleys (main energetic azimuthal contributions, apparent velocities) in the aim to quantify site effects.

Quantified sensitivity of small lake sediments to record historic earthquakes: Implications for paleoseismology: LAKE SENSITIVITY TO RECORD EARTHQUAKES

Seismic hazard assessment is a critical but challenging issue for modern societies. A key parameter to be estimated is the recurrence interval of damaging earthquakes. This requires the establishment of earthquake records long enough to be relevant, i.e., far longer than historical observations. We study how lake sediments can be used for this purpose and explore conditions that enable lake sediments to record earthquakes. This was achieved (i) through the compilation of eight lake-sediment sequences from the European Alps to reconstruct chronicles of mass movement deposits and (ii) through the comparison of these chronicles with the well-documented earthquake history. This allowed 24 occurrences of mass movements to be identified, of which 21 were most probably triggered by an earthquake. However, the number of earthquake-induced deposits varies between lakes of a same region, suggesting variable thresholds of the lake sequences to record earthquake shaking. These thresholds have been quantified by linking the mass movement occurrences in a single lake to both intensity and distance of the triggering earthquakes. This method offers a quantitative approach to estimate locations and intensities of past earthquake epicenters. Finally, we explored which lake characteristics could explain the various sensitivities. Our results suggest that sedimentation rate should be larger than 0.5 mm yr−1 so that a given lake records earthquakes in moderately active seismotectonic regions. We also postulate that an increasing sedimentation rate may imply an increasing sensitivity to earthquake shaking. Hence, further paleoseismological studies should control carefully that no significant change in sedimentation rates occurs within a record, which could falsify the assessment of earthquake recurrence intervals.