Eliana Esposito

Surface Faulting of the 6 April 2009 Mw 6.3 L’Aquila Earthquake in Central Italy

This paper documents evidence of surface faulting associated with the 6 April 2009 moderate-sized earthquake (ML 5.8, Mw 6.3) in the central Apennines of Italy, which caused major damage to the town of L’Aquila and its surroundings. Coseismic surface ruptures were mapped for a minimum distance of 2.6 km along the Paganica fault, a fault still poorly investigated relative to the other active faults nearby, which bound much wider range fronts. Surface rupture length (SRL) and maximum displacement parameters (2.6 km minimum and 10–15 cm, respectively) are in agreement with what is expected for an Mw 6.3 event in the Italian Apennines tectonic environment. Different viewpoints exist on the amount of SRL and the number of activated faults. We propose a pattern of sympathetic and secondary slip on an array of faults around the master seismogenic structure. Past seismicity and evidence for larger Holocene offsets on this and other capable faults nearby prove that the 2009 event is not a good reference event for assessing the seismic hazard of the region. Nevertheless, the 2009 L’Aquila earthquake once more confirmed the importance of detailed geological studies for a proper seismic hazard assessment, and it clearly illustrates the need to pay attention to moderate events and supposedly minor active faults. Indeed, this type of earthquake is rather frequent in the whole Mediterranean region and is potentially much more destructive than in the past, due to the expanding urban centers and infrastructures inside their epicentral regions and even right above the traces of capable faults.

AREAL DISTRIBUTION OF GROUND EFFECTS INDUCED BY STRONG EARTHQUAKES IN THE SOUTHERN APENNINES (ITALY)

Moderate to strong crustal earthquakes are generally accompanied by a distinctivepattern of coseismic geological phenomena, ranging from surface faulting to groundcracks, landslides, liquefaction/compaction, which leave a permanent mark in thelandscape. Therefore, the repetition of surface faulting earthquakes over a geologictime interval determines a characteristic morphology closely related to seismic potential. To support this statement, the areal distribution and dimensions of effects of recent historical earthquakes in the Southern Apennines are being investigated in detail. This paper presents results concerning the 26 July 1805 earthquake in the Molise region, (I = X MCS, M = 6.8), and the 23 November 1980 earthquake in the Campania and Basilicata regions (I = X MSK, Ms = 6.9). Landslide data are also compared with two other historical earthquakes in the same region with similar macroseismic intensity. The number of significant effects (either ground deformation or hydrological anomalies) versus their minimum distance from the causative fault have been statistically analyzed, finding characteristic relationships. In particular, the decay of the number of landslides with distance from fault follows an exponential law, whereas it shows almost a rectilinear trend for liquefaction and hydrological anomalies. Most effects fall within the macroseismic area, landslides within intensity V to VI, liquefaction effects within VI and hydrologicalanomalies within IV MCS/MSK, hence at much larger distances. A possible correlation between maximum distance of effects and length of the reactivated fault zone is also noted. Maximum distances fit the envelope curves for Intensity and Magnitude based on worldwide data. These results suggest that a careful examination of coseismic geological effects can be important for a proper estimation of earthquake parameters and vulnerability of the natural environment for seismic hazard evaluation purposes.

Ground effects and surface faulting in the September-October 1997 Umbria-Marche (Central Italy) seismic sequence

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.

Detailed study of the Potentino seismic zone in the Southern Apennines, Italy

Abstract Most relevant macroseismic and instrumental data concerning past and current seismic activity of the Potentino, southern Italy, have been collected in order to study the seismic behaviour of this area. Ten among the most important recent and historical earthquakes occurred in the area from 1826 to 1990 have been studied by means of macroseismic methodologies; macroseismic fields for all the events indicate a general isoseismal lengthening in the direction of the Apennines chain (northwest-southeast), a rapid attenuation of intensity westward of the studied area and a strong propagation eastwards; this configuration is strongly dependent on the source mechanisms of the seismic events and the geological dishomogeneities of the concerned areas. As regards earthquakes which have been instrumentally recorded in the Campania-Basilicata area from 1980 to 1991, more than 1300 accurate hypocentral locations have been computed for events with MD ⩾ 2.5, using data from Osservatorio Vesuviano (O.V.) and Istituto Nazionale di Geofisica (I.N.G.) seismic networks. Special emphasis is put on the Potenza May 5, 1990 earthquake (MS = 5.4 NEIC—National Earthquake Information Center; I = VII MSK) which was the strongest recent event in the area and was felt in a large sector of southern Italy. As regards the seismic sequence following this event, about 110 aftershocks with MD ⩾ 1.9 have been located very close to the town of Potenza, northeast of it, with a clustering-type space distribution. The occurrence of the events is strongly time concentrated, in so defining a main shock-aftershocks typical sequence. Moreover, the macroseismic investigation of the main event has allowed to map out isoseismal lines bounding VII, VI, V and IV MSK areas; the isoseismals pattern shows that attenuation and amplification of intensity are observed in the same areas as for the previous events. Beginning from May 26, 1991 an increase of seismicity, initiated by an earthquake of ML = 4.7 NEIC and I = VI-VII MSK was observed nearly in the same area as the 1990 sequence. Accurate locations of 84 events with MD ⩾ 1.9 have been computed. However, this sequence seems to have a slightly different character from the 1990 one, in that events are scattered in time and seismic energy release is comparable to a swarm-type seismic sequence. As regards the previous sequences, fault plane solutions for the highest-magnitude events and for appropriate groups of events have been worked out; the mechanisms indicate normal faulting motion with T axes oriented orthogonal to the Apenninic chain. In conclusion, detailed analyses on the historical and recent seismicity of the Potentino area and adjacent ones have allowed to identify at least four seismogenic zones which show activity with different features. Such zoning represents a contribution to the seismic hazard assessment in the Southern Apennines on a regional scale.

Insights into flood-dominated fan-deltas: very high-resolution seismic examples off the Amalfi cliffed coasts, eastern Tyrrhenian Sea

Abstract A high-resolution (IKB-Seistec) seismic survey calibrated with gravity-core data, off the Amalfi coast, a rocky coastal area on the southern side of the Sorrento Peninsula (Italy), documents the internal stratigraphic architecture of a series of small fan-deltas that develop at the mouth of major bedrock streams. The fan-delta system mostly postdates the Plinian eruption of Vesuvius of ad 79 and displays various phases of development associated with periods of high sediment supply from the adjacent river basins. During these periods landscape-mantling loose pyroclastic deposits (mostly air-fall tephra from Vesuvius) were quickly eroded and delivered to the continental shelf by sheet wash and flash flood events. Depositional processes on the foresets were dominated by sediment gravity flows originating from hyperpycnal river flow and pyroclastic fall deposits. This in turn created favourable conditions for sea-floor instability, soft sediment failure, slumping and sliding, which characterize the deltaic stratigraphic architecture. The intermittently increased sediment yield during the various phases of the evolution of the fan-delta system was probably influenced also by the morphoclimatic regime. This may have resulted in varying rates of progradation of the delta foresets, tentatively correlated with the main climatic oscillations of the last 2000 years. The Amalfi fan-delta system represents a small-scale analogue for larger flood-dominated fan-deltas of the world and may be regarded as a useful example for a better understanding of inner-shelf, mixed siliciclastic–volcaniclastic fan-delta systems in the stratigraphic record.

Earthquake Hazard and the Environmental Seismic Intensity (ESI) Scale

The main objective of this paper was to introduce the Environmental Seismic Intensity scale (ESI), a new scale developed and tested by an interdisciplinary group of scientists (geologists, geophysicists and seismologists) in the frame of the International Union for Quaternary Research (INQUA) activities, to the widest community of earth scientists and engineers dealing with seismic hazard assessment. This scale defines earthquake intensity by taking into consideration the occurrence, size and areal distribution of earthquake environmental effects (EEE), including surface faulting, tectonic uplift and subsidence, landslides, rock falls, liquefaction, ground collapse and tsunami waves. Indeed, EEEs can significantly improve the evaluation of seismic intensity, which still remains a critical parameter for a realistic seismic hazard assessment, allowing to compare historical and modern earthquakes. Moreover, as shown by recent moderate to large earthquakes, geological effects often cause severe damage”; therefore, their consideration in the earthquake risk scenario is crucial for all stakeholders, especially urban planners, geotechnical and structural engineers, hazard analysts, civil protection agencies and insurance companies. The paper describes background and construction principles of the scale and presents some case studies in different continents and tectonic settings to illustrate its relevant benefits. ESI is normally used together with traditional intensity scales, which, unfortunately, tend to saturate in the highest degrees. In this case and in unpopulated areas, ESI offers a unique way for assessing a reliable earthquake intensity. Finally, yet importantly, the ESI scale also provides a very convenient guideline for the survey of EEEs in earthquake-stricken areas, ensuring they are catalogued in a complete and homogeneous manner.

The use of documentary sources for reconstructing flood chronologies on the Amalfi rocky coast (southern Italy)

Abstract Documentary source materials are essential for retrospective reconstruction of flood events occurring in past centuries. This paper presents methods of research and archiving of historical data from the 16th century to the present. The quality and completeness of the various original sources were evaluated and carefully analysed in their historical context, to avoid serious mistakes. Systematic investigation of about 3000 documents, mainly found in national State Archives and libraries, allows us to identify and localize at least 106 flood events occurring along the Amalfi coast (southern Italy) for five centuries between the years 1500 and 2000. The collected data provide useful details on flood dynamics, size of flooded areas, flood duration, damage level, number of victims and induced geological effects. When available in sufficient quantity, the flood data allow determination of very useful parameters such as the severity class, to identify large floods and their recurrence interval.

Impact of Ground Effects for an Appropriate Mitigation Strategy in Seismic Area: The Example of Guatemala 1976 Earthquake

Guatemala is one of the most seismically active countries of Central America. It has suffered several dramatic earthquakes during the last century, the most destructive of which was the event associated with the Motagua fault, occurred on 4, February 1976, that caused 23,000 deaths, and 77,200 injuries. Noteworthy were also the coseismic geological effects, either directly linked to the earthquake source, such as a remarkable displacement along a 230 km long strike-slip fault, or caused by the ground shaking, such as 50,000 landslides, numerous liquefaction phenomena, ground cracks and ground deformation over an area of about 18,000 km2. In spite of this, the estimated intensity was relatively low (Imax = IX MM, only in few localities). For this reason, in order to achieve a better definition of seismic hazard, we have conducted a re-evaluation of the 1976 earthquake taking into account the environmental effects that have contributed substantially to the dramatic level of recorded impact, both in terms of loss of human life and damage affecting housing and infrastructures. The analysis of primary and secondary effects has permitted a better evaluation of seismic intensity in 24 localities that were hit by the earthquake and allowed for establishing a new epicentral Intensity, characterized by XI ESI. The recalculated seismic intensity is consequently two degrees higher than previous estimates and definitely more consistent with the scenario of the actual damage caused by the earthquake.

Landslides Induced by Historical and Recent Earthquakes in Central-Southern Apennines (Italy): A Tool for Intensity Assessment and Seismic Hazard

Analysis of distribution of landslides (rock falls and coherent slides), induced by 12 moderate to strong earthquakes occurred in the last three centuries in Central–Southern Apennines, has permitted to investigate the relationship of their maximum distance versus magnitude and ESI epicentral intensity.

Flood Historical Data for Flood Risk Estimation in Coastal Areas, Eastern Tyrrhenian Sea, Italy

A reconstruction of historical floods occurred along the Amalfi coast, during the last five centuries is presented. The analysis of historical sources allowed to achieve a chronological reconstruction of more than 100 floods, four of which classified as catastrophic events. In this task, the level of information was decisive to carry out space–time identification, estimate the affected area and define the type of damage to the structures, and the environment (e.g. mud flow, debris flow, rock falls, shoreline progradation, fan deltas), which may be relevant for the recognition of similar events within the geologic record. The magnitude of the events was finally estimated, taking into account the size of the areas affected by flooding as well as the type of effects induced on the urban and physical environment and the recurrence intervals.