Search

This paper both describes and discusses landslides and other ground effects induced by the September–October 1997 seismic sequence, which struck the Umbria and Marche regions (Central Italy). Three main events occurred on 26 September at 00:33 and 09:40 GMT, and 14 October with magnitude M w equal to 5.8, 6.0 and 5.4, respectively; furthermore hundreds of minor but significant events were also recorded. The authors examined an area of some 700 km 2 around the epicentre (Colfiorito). Primary and secondary effects were observed, including surface faulting phenomena, landslides, ground fractures, compaction and various hydrological phenomena. Surface evidence of faulting reactivation was found along the well-known capable faults, to a total length of ca. 30 km. Landslides, which were the most recurrent among the phenomena induced, consisted mainly of rock falls and subordinately of rotational and translational slides, which were generally mobilised by the inertia forces during the seismic motion. The percentage of reactivated old landslides decreased as the distance from the epicentral zone increased; a similar decrease had been observed for the 1980 Irpinia earthquake (Southern Italy). The ground fracture distribution was consistent with the regional structural setting and the general pattern of macroseismic field. Numerous episodes of hydrological changes were observed within the most severely damaged area. All this evidence confirms the relevance of the study of ground surface effects for achieving a more complete evaluation of seismic hazard.

The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved useful for (1) defining the epicentral area and the location of the causative fault; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a better evaluation of the magnitude from intensity data. Of special interest was the observation of surface ruptures generated along segments of a system of normal faults already mapped as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm. Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.

The Campanian province has a rich history of human interaction with volcanic eruptions. In a region currently inhabited by 3 million people, it is crucial to have precise and accurate geochemical characterization of volcanic units within the region so as to identify the spatial distribution of past events. Furthermore, tephrochronology is becoming an important tool in the region for correlating past environmental records. Unfortunately, many of the key units have been geochemically analysed using relatively imprecise methods, making correlation problematic. Although robust correlations have been established in the Campanian province using a range of methods, including stratigraphy and geochronology, more distal correlation requires precise geochemical characterisation of individual glass shards. Here we report major oxide data, geochemically characterising 17 key tephra units within the Campanian province using wavelength dispersive spectrometry (WDS). The new data confirm the trachytic nature of most of the eruptions. To effect more precise correlations between units (especially in distal locations), proximal units must be individually analysed for major oxides using WDS on the vitreous phase, and statistically analysed for robust correlations. In cases where similar geochemistry exists, analysis of trace and rare earth elements may be necessary.

The Campi Flegrei volcanic field, Southern Italy, is composed of a large variety of pyroclastic deposits related to both magmatic and hydromagmatic activity. In spite of similarities of composition, grain size, age and pre-eruptive environment, the various deposits exhibit great differences in terms of degree and type of alteration of the juvenile components. In particular, detailed analyses of pyroclasts collected in the numerous monogenetic volcanic cones show the broad range of alteration, from fresh glass to complete replacement of the glass by the zeolites. The comparative analysis of 17 different deposits has allowed us to conclude that the eruptive and emplacement conditions were the major controls on the zeolitization processes. The relationships between volcanological features of deposits and secondary mineralization are consistent with the glass-fluid phase reaction in a near-closed post-depositional system. Eruptions characterised by separation of water vapour from the pyroclastic particles produce unaltered or poorly altered deposits. In contrast, eruptions characterised by mechanisms that permit the capture and retention of pore water and water vapour produce zeolitized deposits. According to these lines of evidence the zeolitization of pyroclastic products of Campi Flegrei took place immediately after emplacement, during the cooling of wet deposits.

The eruption's first surge instantly killed some people sheltering from the impact. The town of Herculaneum, lying at the foot of Mount Vesuvius on a cliff overlooking the sea, was buried by a succession of pyroclastic surges and flows (currents of volcanic ash and hot gases generated by collapse of the eruptive column) during the plinian eruption of ad 79. The skeletons of 80 of 300 people who had taken refuge in 12 boat chambers along the beach have now been unearthed from the first surge deposit. We have investigated how these people were killed by this surge, despite being sheltered from direct impact, after its abrupt collapse (emplacement) at about 500 °C on the beach. The victims' postures indicate that they died instantly, suggesting that the cause of death was thermally induced fulminant shock1 and not suffocation, which is believed to have killed many of the inhabitants of Pompeii and of Herculaneum itself.