Leonello Serva

Geological evidence for strong historical earthquakes in an “aseismic” region: The Pollino case (Southern Italy)

Abstract The Pollino Range is the southernmost segment of the Southern Apennines at the boundary with the Calabrian Arc. While several strong earthquakes (magnitude 6.5–7.0) have occurred in nearby regions, the Pollino area has no known historical record of seismic events of magnitude > 5. We carried out an aerial photograph interpretation and a field survey of the Pollino fault (the major Quaternary normal fault of the area) in order to characterize geologically the seismic potential of this structure. We dug two sets of trenches across fault scarps within the apecies of latest Pleistocene to Holocene alluvial fans at the Masseria Quercia Marina (MQM) and Grotta Carbone (GC) sites, in the central segment of the southern Pollino Range front. At both sites we identified two surface faulting events affecting the alluvial fan deposits and two overlying colluvial units of historical age. The penultimate event produced a vertical offset of 80–90 cm at GC and 50–60 cm at MQM; while the last event produced a vertical offset of 40–50 cm at GC and few centimeters of offset at MQM. Detailed geomorphological field observations suggest that the two historical earthquakes reactivated the entire length of the Masseria Marzano-Civita segment of the Pollino fault (rupture length about 18 km). For events in this range of rupture length and vertical displacement, comparison with surface faulting earthquakes in the Apennines (and abroad) indicates a magnitude of 6.5–7.0. Therefore, the maximum potential earthquake and the seismic hazard of the Pollino area are significantly larger than that suggested by the available historical seismic catalogue.

Stress Drop, Slip Type, Earthquake Magnitude, and Seismic Hazard

Care must be taken to provide reliable antiseismic protection in earthquake-prone areas where the impact of a large earthquake in megacities with industrial facilities as well as in ordinary buildings is liable to cause massive loss of human life and to cripple the nations economy. This protection needs to take into account not only vibratory ground motion but also permanent ground failure, and notably surface-faulting hazard, a fact tragically illustrated during the recent events of Turkey and Taiwan. The purpose of this contribution is to conduct a survey of our current state of knowledge concerning theoretical relationships between earthquake source parameters, such as moment magnitude (M, M w), surface wave magnitude ( M S), seismic moment ( M O), stress drop (Δσ), rupture length ( L ), and displacement on the fault ( D ). A relationship is proposed that links M S to L and Δσ: M S = 2 log L + 1.33 log Δσ + 1.66, using a simple rupture model. Earthquake data from all over the world for which the parameters of rupture length, fault width ( W ), fault displacement, and surface-wave magnitude are available have enabled stress drop values to be computed for each event using both geological observation (Δσ1) and the previously proposed equation (Δσ2). The results obtained tend to indicate that stress drop values increase versus fault width (depth) up to approximately 15 km (corresponding perhaps to the brittle-ductile boundary). This increase is more pronounced in the case of reverse faults than it is for those with strike-slip or normal mechanisms. An equation of the type Δσ = kW n has been used to fit the data, and preliminary values for k and n have been supplied for the three slip types. For depths in excess of 15 km the data do not display significant variation (Δσ < 100 bars). These results are in agreement with certain laboratory models of the continental lithosphere. Although more data, particularly in the large-magnitude range, are needed to ascertain whether it is the W or the L model that better describes earthquake scaling laws, stress drop does not appear to be fault-length dependent, thus being supportive of the L model. The risk of surface faulting is dependent on the dynamic environment of the fault, that is, the stress drop, the rupture length, and the fault width. Although statistics show that surface faulting appears in most instances at magnitudes of at least 6.1, data from certain regions indicate that seismicity at superficial depths is under certain conditions accompanied by significant surface faulting even for magnitudes as small as 5.5, suggesting a change in scaling law. The threshold magnitude for surface faulting is accordingly seen to depend on the rheology of materials in the fault area and on the stress environment. Manuscript received 31 May 2000.

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.

Geological constraints for earthquake faulting studies in the Colfiorito area (central Italy)

On September 26, 1997, at 00.33 h(GMT), a Mw 5.7 earthquake occurred in the axial zone of theUmbria-Marche Apennines of central Italy, in the Colfiorito basin area. At09.40 h (GMT), a Mw 6.0 earthquake again struck the area withinthe Colfiorito basin, a major intramontane basin filled with Quaternarycontinental deposits. The two main shocks, and the associated aftershockswere within a roughly NNW-SSE trending zone of largest damage (Imax10), in which ground deformation has been observed. Along this trend,Cello et al. (1997a) had mapped a few capable faults, showingtranstensional to pure extensional kinematics. Field inspection of themapped faults, carried out after the main shocks, revealed that some ofthem were locally reactivated (for lengths of several hundreds metres andsurface slip in the range of 2–8 cm) during the September 26, 1997earthquakes.

First study of fault trench stratigraphy at Mt. Etna volcano, Southern Italy: understanding Holocene surface faulting along the Moscarello fault

Paleoseismology, the study of past earthquakes based on their geological record in the stratigraphy and landscape, is a successful newly developing field of research. The application of fault trench studies in volcanic environments is one of the youngest branches of paleoseismology. In this paper, we present the results of the first exploratory trenches excavated at Mt. Etna in Sicily, the largest European volcano. Modern surface faulting at Etna is a very well known feature, which poses significant hazard to the local community, both in terms of ground displacement of essential lifelines and ground shacking from frequent damaging earthquakes. However, while the geomorphology and the seismicity of the active fault in the Etna region consistently show very high rates of tectonic activity, the Holocene cumulative throw and slip-rates, along with the nature (coseismic vs. creeping fault slip), dimension and timing of the displacement events, are still poorly constrained. For this purpose, we selected as a sample area the Moscarello fault, one of the most outstanding segment of the Timpe system of active normal faults in the volcano’s lower eastern flank. Displaced landforms and volcanic units at the Fondo Macchia basin, in the central sector of this fault, indicate some hundreds of meters of vertical offset in the last ca. 80 kyr, with a long-term slip-rate substantially higher than 1.5–2.0 mm/yr. According to the historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150 years during shallow (H < 5 km) and moderate magnitude (M < 4.8) earthquakes. These events were associated with severe damage in a narrow epicentral area (macroseismic intensities up to the IX–X grade of the MSK scale) and extensive surface faulting (end-to-end rupture length up to 6 km, vertical offsets up to 90 cm). This clearly indicates very high modern rates of deformation along this fault. We conducted trench investigations at the Fondo Macchia site, in a point where eyewitnesses observed ca. 20 cm of coseismic vertical displacement after the April 21, 1971, , earthquake. The excavated sections provided direct stratigraphic evidence for a vertical slip-rate of 1.4–2.7 mm/yr in the last ca. 6 kyr. This should be regarded as a minimum slip-rate for the central section of the fault. We explored a single scarp at a single site, while we know from recent historical observations that several parallel scarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rate documented for the modern period might be likely extended back in the past to a time-span of some thousands of years at least. As expected, for such a volcanic environment, the activity rates of the Moscarello fault are also significantly higher than for the Apennines normal faults, typically showing slip-rates lower than 1 mm/yr. The agriculturally reworked trench hangingwall stratigraphy did not allow to recognize individual displacement events. Nevertheless, the sedimentary structures observed in the trench footwall strongly suggest that, as for the last 150–200 years of detailed historical record, fault behavior at Fondo Macchia is governed by coseismic surface displacement rather than fault creep. This research confirms that paleoseismology techniques can be effectively applied also in active volcanic environments, typically characterized by rheology and, consequently, seismicity and fault dynamics very different from those of other tectonic environments in which paleoseismology has been firstly developed and is today extensively applied.

The Muzaffarabad, Pakistan, earthquake of 8 October 2005: surface faulting, environmental effects and macroseismic intensity

Abstract The Mw 7.6 Muzaffarabad earthquake of 8 October 2005, occurred on a lateral equivalent of the main ramp of the Hymalaia frontal thrust, and is the result of the collision tectonics between the Indian and Eurasian plates. The epicentre was located near the town of Basantkot (Muzaffarabad), and the focal depth was about 13 km. The Muzaffarabad earthquake provides unequivocal evidence about the localization of severe damage, intense ground shaking and secondary environmental effects near the surface expression of the source fault. We analyse its nature, and impact on man-made structures and the physical environment, on the basis of a detailed survey and macroseismic study of the affected areas conducted by the Micro Seismic Studies Programme (MSSP) Team (Ishfaq Ahmad Research Laboratories, Pakistan Atomic Energy Commission) immediately after the mainshock, assisted by a careful review of the subsequent data and literature. In the course of the field survey, the displacement and surface expression of the causative fault, and accompanying secondary environmental effects were observed at a number of places along a capable thrust fault structure. We refer to this structure as the Kashmir Thrust (KT) capable fault following the terminology of local research geologists in Pakistan; the seismological evidence of this structure is already known in the literature as the Indus–Kohistan Seismic Zone. A complex, clearly segmented, at least 112-km-long surface rupture was mapped along the KT. The maximum values of vertical displacement (on the order of 4 to 7 m) were observed mainly between Muzaffarabad and Balakot, along the central segment of the rupture (52 km) associated with maximum slip at depth and a major portion of the energy release. Both the NW Alai segment (38 km) and SE Bagh segment (22 km) are characterized by scattered minor surface ruptures with a few centimetres of displacement, accompanied by extensive surface cracking, landslides and severe damage, concentrated in a narrow belt along the fault trace. A maximum intensity of XI on the Modified Mercalli Intensity (MMI) scale and on the Environmental Seismic Intensity scale (ESI 2007) was recorded in the epicentral area between Muzaffarabad and Balakot. Extremely severe damage and very important secondary environmental effects in the hanging wall adjacent to the trace of the causative fault plane are mainly due to near-fault strong motion and rupture directivity effects. To our knowledge, this is the first study to present field observations over the whole near-field of the earthquake, and to include the intensity map of the entire meizoseismal region.

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.

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.

Quaternary geology and paleoseismology in the Fucino and L’Aquila basins

This 2 days-long field trip aims at exploring field evidence of active tectonics, paleoseismology and Quaternary geology in the Fucino and L’Aquila intermountain basins and adjacent areas, within the inner sector of Central Apennines, characterized by extensional tectonics since at least 3 Ma. Each basin is the result of repeated strong earthquakes over a geological time interval, where the 1915 and 2009 earthquakes are only the latest seismic events recorded respectively in the Fucino and L’Aquila areas. Paleoseismic investigations have found clear evidence of several past earthquakes in the Late Quaternary to Holocene period. Active tectonics has strongly imprinted also the long-term landscape evolution, as clearly shown by numerous geomorphic and stratigraphic features. Due to the very rich local historical and seismological database, and to the extensive Quaternary tectonics and earthquake geology research conducted in the last decades by several Italian and international teams, the area visited by this field trip is today one of the best studied paleoseismological field laboratories in the world. The Fucino and L’Aquila basins preserve excellent exposures of earthquake environmental effects (mainly surface faulting), their cumulative effect on the landscape, and their interaction with the urban history and environment. This is therefore a key region for understanding the role played by earthquake environmental effects in the Quaternary evolution of actively deforming regions, also as a major contribution to seismic risk mitigation strategies.