04.06. Seismology

The 2002 earthquake sequence of October 31 and November 1 (main shocks Mw=5.7) struck an area of the Molise region in Southern Italy. In this paper we analyzed the co-seismic deformation related to the Molise seismic sequence, inferred from GPS data collected before and after the earthquake, that ruptured a rather deep portion of crust releasing a moderate amount of seismic energy with no surface rupture. The GPS data have been reduced using two different processing strategies and softwares (Bernese and GIPSY) to have an increased control over the result accuracy, since the expected surface displacements induced by the Molise earthquake are in the order of the GPS reliability. The surface deformations obtained from the two approaches are statistically equivalent and show a displacement field consistent with the expected deformation mechanism and with no rupture at the surface. In order to relate this observation with the seismic source, an elastic modeling of fault dislocation rupture has been performed using seismological parameters as constraints to the model input and comparing calculated surface displacements with the observed ones. The sum of the seismic moments (8.9×1017 Nm) of the two main events have been used as a constraint for the size and amount of slip on the model fault while its geometry has been constrained using the focal mechanisms and aftershocks locations. Since the two main shocks exhibit the same fault parameters (strike of the plane, dip and co-seismic slip), we modelled a single square fault, size of 15 km×15 km, assumed to accommodate the whole rupture of both events of the seismic sequence. A vertical E–W trending fault (strike=266°) has been modeled, with a horizontal slip of 120 mm. Sensitivity tests have been performed to infer the slip distribution at depth. The comparison between GPS observations and displacement vectors predicted by the dislocation model is consistent with a source fault placed between 5 and 20 km of depth with a constant pure right-lateral strike-slip in agreement with fault slip distribution analyses using seismological information. The GPS strain field obtained doesn't require a geodetic moment release larger than the one inferred from the seismological information ruling out significant post-seismic deformation or geodetic deformation released at frequencies not detectable by seismic instruments. The Molise sequence has a critical seismotectonic significance because it occurred in an area where no historical seismicity or seismogenic faults are reported. The focal location of the sequence and the strike-slip kinematics of main shocks allow to distinguish it from the shallower and extensional seismicity of the southern Adriatic block from the northern one.

The present-day stress field has been investigated by the analysis of the directions of maximum horizontal stress (ó 1 ) inferred from earthquake focal mechanisms and borehole breakouts in Egypt. The results indicate that strike-slip and normal faulting movements characterize the majority of the earthquake focal mechanisms; only a few events are of reverse faulting type. The analysis of 35 mechanisms suggests that the present-day stress field in Southern Egypt is dominated by a strike-slip stress regime (SS) and it is mainly transtensional (NS: normal faulting with strike-slip component) in Northern Egypt. The orientation of P-axes reflects that the maximum horizontal stress (ó 1) in Southern Egypt is uniform and aligned to nearly E-W direction while in Northern Egypt it is aligned with an even mix of NW-SE and nearly E-W compression. Along the Gulf of Aqaba, the southern part of the Dead Sea Fault (DSF), the focal mechanism solutions indicate that the maximum horizontal stress is presently oriented NW-SE, corresponding to a strike-slip mechanism in concert with geological evidence. More detailed investigations have been performed for the Gulf of Suez. We compare our results to the near-surface stress measurements from borehole breakouts to see if there is a change in orientations with depth. Shallow stress directions derived from borehole breakouts are not consistent with the deep stress directions derived from earthquakes focal mechanisms. About 73% of 30 borehole breakouts measurements indicate NW-SE alignment of the maximum horizontal stress and 27% are ENE-WSW. The direction of ó 1 inferred from the focal mechanism solutions is changing from NE-SW to ENE-WSW. Therefore, at least in this area, the stress direction is not constant throughout the crust.

In this paper we analyse the records of the two Potenza seismic sequences (Southern Apennines, Italy) which occurred in 1990 and 1991, in order to obtain information on the upper crustal structure in this area. The hypocentral depths are mainly concentrated below 10 km which is the supposed upper limit of the crystalline basement. The seismograms recorded at temporary arrays deployed during the two sequences clearly show on the vertical component, an intermediary phase between the P and S waves. For the investigated epicentral distances (less than 30 km) the delay between this secondary phase and the direct S wave arrival is almost constant at each station, suggesting that the observed intermediary phase might be an S to P conversion at a discontinuity shallower than the hypocentral depth. This interpretation has been supported by polarisation analysis and numerical modelling results. Considering the regional geological structure, these latter have also shown that the interface generating strong converted waves could be the top of the Apulian carbonate platform overlaid with recent clay deposits and flysch sediments. 1D inversion of the travel-time data was performed in order to evaluate a local vertical upper crustal profile.

After the 2002 Molise earthquake, a wide survey of the damage and the seismic vulnerability of the churches was carried out, using specific forms extensively tested and recently acknowledged at a national scale (G.U. 07/03/2006). This approach has allowed one to compare the observed and the expected medium damage, calculated through vulnerability curves correlated to the specific form used. This comparison has enabled to evaluate inhomogeneous values in some cases, pointing out how the surveyed damage could not be put down just to a structural lack of the building. Studying the sites morphology for the churches located on the ridges, it was observed a damage level always greater than expected. To study the influence of the local amplification phenomena due to the topography, it has been analyzed some churches damaged by the Appennino Abruzzese (1984) and Molise (2002) earthquakes. These churches are located on the ridges top, with slope in-clination alfa≥15°, height H≥30m and characterized by homogeneous lithological subsoil, almost to consider it having an elastic behavior under dynamical stress. Taking into account this hazard increase inside a vulnerability analyses (based on a macroseismic or a mechanical approach), it has been proposed a double approach, validated through local seismic response (RSL) modeling of the ground (estimation of amplification factors and of elastic response spectra) and through the seismic response of the macroelements, damaged by the reference earth-quake (linear and not-linear kinematic analysis). Through the vulnerability analysis based of the macroseismic approach, the influence of the site morphology on the building behavior has been preliminary defined by a vulnerability modifier (deltaVml), that represents an additional parameter to reach the equivalent value between the surveyed and the expected damage. The study of the amplification effects has been performed using the BEM numerical analysis; the results are represented by the amplification factors (Fa), defined as the ratio between the spectral intensity of output and input, considering the range period of 0.1s-0.5s, and the corresponding elas-tic response spectra: they confirm the results of the macroseismic approach. Considering the mechanical approach, two churches have been examined in detail (S Pietro in Vincoli at Castellino del Biferno and S. Michele Arcangelo at Campolieto): the kinematic analyses have been performed applied the calculated amplified and not-amplified response spectra (considering the earthquake 2002) and the design response spectrum. The analyses show that the results obtained applying the calculated amplified response spectrum are better correlated to the surveyed damage of the churches than the results obtained applying the not-amplified response spectrum.

After the 2002 earthquakes, that struck the southern Italy provinces of Campobasso (Molise) and Foggia (Apulia), the safety assessment of buildings has allowed to evaluate, for different building typologies, the damage level caused by the seismic event; in particular, the survey of the damaged cultural heritage was carried out by the Task Cultural Heritage of the Mixed Operating Centre (COM) in Larino (a village in the epicentral area), coordinated by the “Working Group for the Safeguard of the Cultural Heritage from Natural Risks” (Italy’s Gazzetta Ufficiale n. 116, 21 May 2001 - PCM-DPC Decree, 3 May 2001)”. For this survey activity, the churches were investigated with a specific form proposed by the same Working Group (Ministerial Decree n. 133, 23th January 2001), adopting different forms for other kinds of buildings. The surveyors were organized in teams, called NOPSA (Nuclei Operativi Patrimonio Storico Artistico), composed by officials of the Regional board of the Ministry of Cultural Heritage and Environmental Conservation (architects and art historians), an engineer and a Fire Brigade technician. The first phase of the safety assessment has been developed on the basis of the warnings received by the Larino COM, and most of the surveyed buildings were churches. The damage assessment of public or private buildings began, initially, near the earthquake epicentre (area 1 - IMCS between 6 and 8.5) and subsequently was carried out for all the villages from which inspection requests arrived. At the same time a vulnerability survey was carried out for the churches, using the II level form (Lagomarsino et al., 2004) proposed in the SAVE Project (“Updated Tools for the Seismic Vulnerability Evaluation of the Italian Real Estate and of Urban Systems” – http://gndt.ingv.it). The damage and vulnerability survey has allowed to verify and optimize the procedures for damage assessment immediately after a seismic event (emergency phase) and to obtain useful indications for the reconstruction phase. On the whole 379 monumental buildings in 101 Molise villages and 207 monumental buildings in 44 Apulia villages were investigated. Figure 1 shows the number of monumental buildings surveyed after the earthquakes for each Molise village, subdividing the region according to the seismic classification proposed by the recent Italian Seismic Code (OPCM n. 3274, 2003 – http://zonesismiche.mi.ingv.it).

A "local earthquake tomography" of a large area encompassing the South of Iberia, the sea of Alboran, the Gibraltar arc, and Northern Morrocco, has been performed using first arrival times recorded at various Spanish and Morroccan seismic networks. A total of 52 stations and 639 earthquakes provided over 6300 first P arrivals and 4400 S arrivals. Three features of interest appear in the results: i) a continuous low velocity structure which correlates with the Betics, the Gibraltar arc and the Rif; ii) a high velocity feature which persists to a depth of approximately 30 km, positioned near the coast of Malaga on the northern margin of the Alboran sea; iii) a low velocity feature, extending to a minimum depth of approximately 40 km, which coincides with the Granada basin and a strong negative Bouguer gravity anomaly.

We describe a new catalogue of strong ltalian earthquakes that the Istituto Nazionale di Geofisica in collaboration with SGA, has recently made available to the international scientific community and to the general public. The new catalogue differs from previous efforts in that for each event the usual seismic parameters are complemented by a list of intensity rated localities, a complete list of relevant references, a series of synoptic comments describing different aspects of the earthquake phenomenology. and in most cases even the text of the original written sources. The printed part of the catalogue has been published as a special monograph which contains also a computer version of the full database in the form of a CD-ROM. The software package includes a computer program for retrieving, selecting and displaying the catalogue data.

L’area del terremoto dell’Irpinia del 1930 corrisponde in profondità all'avampaese Apulo, inflesso al di sotto delle unità alloctone accavallate nella porzione frontale del cuneo orogenico, ma non coinvolto nelle strutture dei cosiddetti duplex dell'Apula sensu Patacca and Scandone (2004). In particolare, le profondità ipocentrali del terremoto del 1930 corrispondono al basamento sottostante la successione della Piattaforma Carbonatica Apula. Il campo macrosismico e i dati strumentali disponibili (si veda la sorgente in DISS, 2007, con relativa bibliografia, e Pino et al., sottomesso) suggeriscono una sorgente sismogenetica con un’orientazione e una cinematica che rappresentano una sorta di transizione tra la direzione W-E a cinematica trascorrente destra, che caratterizza la sismicità propria delle aree di avampaese sia affiorante che sepolto, e la direzione NW-SE a cinematica normale, che caratterizza la sismicità connessa all'estensione lungo l'asse della catena Appenninica (si veda il terremoto del 1980). In questo quadro, l’obiettivo dello studio magnetotellurico è stato quello di investigare i volumi di crosta al di sotto della successione Apula per valutare l'eventuale presenza di direzioni preferenziali dell'anisotropia di resistività che fossero confrontabili con la direzione della sorgente del terremoto del 1930. Il verificarsi di tale evenienza avrebbe potuto essere infatti interpretato come indizio di una zona di debolezza regionale, che avrebbe condizionato le caratteristiche geometriche e cinematiche della sorgente del terremoto stesso. Partendo dall’area sismogenetica segnalata nel DISS per questo terremoto, sono stati effettuati in un’area di circa 1000 km2 sondaggi magnetotellurici in 15 siti, nell’intervallo di 0.009- 4000 s. Per ciascun sito si è proceduto alla misura delle tre componenti ortogonali del campo magnetico e di tre componenti del campo elettrico, di cui due lungo la stessa linea e ortogonali alla terza. Ciò ha consentito la stima dei parametri magnetotellurici per due sondaggi adiacenti, al fine di meglio controllare possibili problemi di rumore antropico o strumentale. Le stazioni, fino ad un massimo di tre, hanno operato in contemporanea fungendo l’una per l’altra da remote reference (Gamble et al., 1979). Va sottolineata la buona qualità dei dati acquisiti sia in termini di stime stabili con diverse tecniche di analisi, che per basso scattering delle curve di resistività apparente e fase. Le risposte sperimentali sono state poi comparate con i dati di pozzo disponibili, verificando un ottimo accordo. È stata inoltre eseguita un’analisi sulle proprietà fisiche e geometriche del tensore impedenza, adottando lo schema di decomposizione di Weaver et al. (2000) dal quale è derivato poi lo studio degli invarianti magnetotellurici per la definizione della dimensionalità delle strutture elettriche investigate ai vari periodi (ovvero alle varie profondità). Circa il 75% dei dati analizzati implica strutture assimilabili necessariamente a modelli tridimensionali e le quattro componenti del tensore impedenza sono significativamente diverse da zero. Per questo tipo di strutture, seguendo Weaver et al. (2000), è comunque possibile definire una direzione di eterogeneità elettrica. Ciò è stato fatto per ciascun sondaggio e per ciascun periodo di stima. Mediante la trasformazione di Niblett–Bostick è stato poi ottenuto lo strike elettrico in funzione della profondità stimata. Viene riportata la direzione di strike per i vari sondaggi alla profondità stimata nell’intervallo 8 - 16 km, riferibile quindi a una porzione di crosta al di sotto del resistivo che identifica le successioni della Piattaforma Apula.

Nei primi anni del Settecento il giornalismo italiano ed europeo raggiunge il suo pieno sviluppo. La grande diffusione delle gazzette a stampa (consolidatasi da un paio di decenni con particolare vivacità nell'Italia Centrale) il successo commerciale delle relazioni monografiche a stampa, l’ininterrotto flusso sotterraneo di informazioni riservate che alimenta la produzione di avvisi manoscritti sono alcuni degli aspetti di una realtà vivace e competitiva, che influenza profondamente l'intero sistema della circolazione delle informazioni in età moderna, in una costante interazione con la memorialistica e la diaristica privata e con la stessa documentazione amministrativa. L'analisi sistematica dei resoconti giornalistici sui terremoti del 1703, e in particolare la verifica del loro contenuto informativo, ci consente di evidenziarne il contributo originale rispetto a quello di altre tipologie di fonti e di verificarne l'influenza sul processo di costituzione della tradizione storiografica e sismologica relativa a questi eventi.