Seismic Microzonation Studies in the City of Ragusa (Italy)
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Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in (2008) - Sixth International Conference on Case Geotechnical Engineering Histories in Geotechnical Engineering 16 Aug 2008, 8:45am - 12:30pm Seismic Microzonation Studies in the City of Ragusa (Italy) Salvatore Grasso University of Catania, Catania, Italy Lauretta Spina University of Catania, Catania, Italy Michele Maugeri University of Catania, Catania, Italy Follow this and additional works at: https://scholarsmine.mst.edu/icchge Part of the Geotechnical Engineering Commons Recommended Citation Grasso, Salvatore; Spina, Lauretta; and Maugeri, Michele, "Seismic Microzonation Studies in the City of Ragusa (Italy)" (2008). International Conference on Case Histories in Geotechnical Engineering. 24. https://scholarsmine.mst.edu/icchge/6icchge/session03/24 This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. SEISMIC MICROZONATION STUDIES IN THE CITY OF RAGUSA (ITALY) Salvatore Grasso Lauretta Spina Michele Maugeri University of Catania University of Catania University of Catania Viale A. Doria, 6 Viale A. Doria, 6 Viale A. Doria, 6 95125 Catania 95125 Catania 95125 Catania ABSTRACT The geotechnical zonation of the subsoil of the city of Ragusa suggests a high vulnerability of the physical environment added to site amplification of the ground motion phenomena. These elements concur on the definition of the Seismic Geotechnical Hazard of the city of Ragusa that should be correctly evaluated, through geo-settled seismic microzoning maps. Based on the seismic history of Ragusa, the following scenario events have been considered: the “Val di Noto” earthquake of January 11, 1693 (M=7.3), the “Etna” earthquake of March 1, 1818 (M=5.9), the 1895 earthquake (IMAX= 7 EMS), the Modica earthquake of January 23, 1980 (MW = 4.63) and the “Sicilian Earthquake” of December 13, 1990 (ML=5.6). Despite of its lower magnitude, a medium size, local earthquake, such as the 1990 “Sicilian” event, has to be accounted for the seismic hazard assessment of Ragusa, since it may cause heavy damage to the most urbanized area. According to historical data, the epicentre of this earthquake was located in the sea. This earthquake is considered a tectonic earthquake and is associated to the strike-slip segment of the Ibleo-Maltese fault system. This fault system is the major seismogenic structure of Eastern Sicily, and it is considered the responsible of the major historical earthquakes which struck this area in the past. According to the response spectra obtained through the application of the 1-D non-linear models, the city of Ragusa has been divided into some zones with different peak ground acceleration at the surface. Shaking maps for the central area of the city of Ragusa were generated via GIS for the scenario earthquakes. The maps represent an important tool for the seismic improvement of the buildings, indispensable for the mitigation of the seismic risk. INTRODUCTION The Val di Noto earthquake of January, 11 1693 is the best destroyed many villages on the south-east side of Mt Etna. A remembered by Sicilians. Telluric movements started at 9 p.m. total amount of 72 people died because of the great number of (Barbano, 1985) of January 9: at the first shake many houses that collapsed. The isoseismal map explained that the buildings collapsed and were badly damaged in all of the earthquake was perceived almost in every part of Sicily from towns in the south-east of Sicily spanning from Catania to Siracusa to Noto and Palermo. Probably at Ragusa major Noto (Barbano, 1985). But the real destructive shock hit two damages were caused by the shock of March 1, 1818 with days later, on January 11 at about 2 p.m. (corresponding to 9 epicentre located near the city (M=5.9, Ioss = 7 EMS). The p.m. local time) (Barbano, 1985); the area had already been Modica earthquake of January 23, 1980 (MW = 4.63) with stricken and so the result was that two earthquakes overlapped epicentre near the city of Ragusa represents the “medium” causing great damages over an area of 14.000 km square. Due event for the ZS 78. to the huge number of people died (nearly 60.000) and the extent of the damage (more than 45 towns and small villages) The earthquake of December 13, 1990 brought to an end a this seismic event may be considered alongside the one that period of seismic dormancy lasted a long time thus reaching a stroke Messina in 1908 the strongest earthquake ever occurred local magnitude ML = 5.4 with a focus depth of about 6-12 in historic times. The shock of January 9 which hit Lentini, km. Even if it was internationally recognized as a “moderate” Augusta and Catania had an intensity measuring VII on MCS earthquake it provoked serious damages to many buildings. while that of January 11 which developed from the epicentre The accelerations recorded are influenced by episodes of local (situated at sea but not far from the coast) measured XI on amplification as it is shown by the response test of the soil. MCS (Barbano, 1985), as it resulted from the buildings The analysis of the available data showed that the greatest destroyed and the persistent damages on the soil) and IX-X on effects of the earthquake were felt in Augusta (near Siracusa) MCS in Ragusa (Barbano, 1985). with an intensity of VIII MCS causing considerable damages even in modern r.c. buildings. Moreover, the analysis The Etna earthquake that took place on February 20, 1818 was revealed the presence of local factors which provoked the one of the feeblest ever occurred but its effects were noticed amplification of the effects of the shake. The damages are also over a vast area. In fact, this quake was sensed in almost every to be attributed to a seismic activity characterized by part of Sicily and in the south of Calabria (Imposa and subsequent shakes of minor entity in comparison with the Lombardo, 1985). The quake occurred at 18:20 (G.M.T.) and main one. In the south-eastern Sicily there are two areas where Paper No. 3.49 1 seismicity is mainly distributed: long the Ionian coast Catalogue is used to estimate the return period (figure 3) and (earthquakes of magnitude M > 7.0) and in the hinterland area probability of occurrence (figure 4) of earthquakes using the (earthquakes of magnitude lower than 5.5). There are approach proposed by Gresta et al., 2004. evidences from the late Quaternary period that the Ibleo- Maltese fault system is the most probable source for the great earthquakes that struck the region (1169, 1693,1818 earthquakes). This fault system is mainly made up of normal faults NNW-SSE oriented, divided into three segments of fault, the most northern of which continues on the ground up to the Etnean area (Timpa), the central segment reaches the Gulf of Catania while the most southern part lies at sea between Augusta and Siracusa (see figure 1). The Hyblean foreland may be considered as part of the northern margin of the African continental crust, which is Figure 2. Seismicity distribution of the city of Ragusa, after bounded to the north by the thrust front of the Apennine Gresta et al., 2004. allocthonous units. ESTIMATION OF THE SEISMIC HAZARD FOR THE CITY OF RAGUSA From figure 2 that shows the seismic history of the city of Ragusa (Gresta et al., 2004), it is possible to observe that there is a low frequency of occurrence for seismic events with MCS intensity I>5-6 and also for those with 3<I<5-6. Figure 3. Return period vs MCS intensity, after Gresta et al., 2004. RAGUSA Figure 4. Probability of occurrence vs time period, after Gresta et al., 2004. Figure 1. Tectonic sketch of SE Sicily. All kinds of faults, more (continuous) or less (dashed) reliable, are indicated with simple lines. EBT78 and IBL-MAL (heavy From figures 3 and 4 it is possible to observe that return segments) are the tentative sources for the 9 period is low for intensity classes I=6 and I=7 (moderate January damage). For higher intensity classes (I=8, I=9, I=10, heavy and 11 January 1693 earthquakes. N. 78 and 79 damage), the return periods are high, so that the probability of indicate seismogenic areas. occurrence is very low. Paper No. 3.49 2 Historical maps of the city of Ragusa allowed to reconstruct Figure 6. Tectonic scheme of Sicily with localization of the the damage frame of the city for the scenario earthquakes. Hyblean Plateau. Damage scenario for the Val di Noto earthquake of January In some part of the urban area, detritus was found. 11, 1693 is represented in the figure 5, reporting the EMS 98 The subaerial part of the Iblean Plateau is constituted by a damage of the typical “forma piscis”. complex sedimentary and volcanic sequence, characterized by shallow water marine sediments and both sub aqueous and sub aerial volcanics. Late Triassic-Early Jurassic volcanic rocks are not exposed, but they were encountered in drill holes (Patacca et al., 1979). The most ancient outcropping volcanic rocks were emplaced in Upper Cretaceous (Barberi et al., 1974; Grasso et al., 1983) and are constituted mainly by submarine lavas and pillow breccias.