Marine Geology in the Region of the Messina Straits, and a Puzzling Tale of Faults, Earthquakes and Tsunamis
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Marine Geology in the Region of the Messina Straits, and a Puzzling Tale of Faults, Earthquakes and Tsunamis A. Argnani Institute of Marine Sciences, CNR, Bologna, Italy [email protected] Abstract The Straits of Messina is one of the most tectonically active areas of the Mediter- ranean, and has been the site of the destructive 1908 Messina earthquake. In spite of the hazard potential of this marine area, studies based on modern geophysical data are still lacking. A marine multichannel seismic survey was purposely carried out with the aim to outline the fault pattern in the area of the Messina 1908 earth- quake, and to better understand its significance within the tectonic frame of the re- gion. Within the Messina Straits, faults have been imaged on the Calabrian side, with a 30 km long NW-trending fault, located at the SW tip of Calabria, that is affecting the sea floor, whereas we did not image any extensional fault plane attributable to the Taormina Fault, on the Sicilian side of the straits. The geodynamic implication is that extension in south-eastern Sicily, on the Ionian side of the Hyblean Plateau, and extension in southern Calabria and Messina Straits belong to two different tec- tonic systems and, therefore, cannot be mechanically linked. Finally, the damages produced by the1908 ground shaking were aggravated by the effects of a remarkable tsunami, with up to 11 m of run-up height, that followed the earthquake. The origin of the tsunami associated with the 1908 earthquake is not yet fully understood, but geological and geophysical evidences substantially undermine a recent proposal that the 1908 tsunami originated by a large landslide offshore Giardini Naxos. 1 Introduction and Geo- rine terraces. The flight of emergent ma- logical Setting rine terraces along the coast of Sicily, from Taormina to Briga, were uplifted since 125 ka with rates of 1.07 mm·yr−1 [5]. The oc- The Messina Straits and its surroundings currence of a large and active extensional are one of the most tectonically active areas fault named Taormina Fault, running off- of the Mediterranean, as indicated by sev- shore, has been inferred on the basis of eral lines of geological and geophysical ev- coastal geomorphology (Figure1[6]). Ac- idence. Relatively large earthquakes struck cording to some investigators (e.g., [7]), the area in historical times (Figure1), al- the systems of faults of southern Calabria though the recurrence time for 1908-type and south-eastern Sicily, on the Ionian side events (Mw=7.1) seems to be about 1500 of the Hyblean Plateau (e.g., [8]), both yr [4]. The Sicilian and Calabrian side of associated to large and destructive earth- the straits are characterised by uplifted ma- quakes, can be linked through the Taormina Marine Geology Figure 1: Map of the Messina Straits with location of relevant historical earthquakes (after [1]; [2]). Dashed boxes represent poorly constrained events. Fault. However, along the belt correspond- about 1.0-1.3 mm·yr−1 in the last 300-400 ing to the inferred Taormina Fault a lack ka. GPS velocities [11], indicate a NW-SE- of seismicity is indicated by historical data directed extension at a rate between 1.7 to and recent instrumental records [9] (Figure 3 mm·yr−1 between the Sicilian and Cal- 1). The assumption that the hypothesized abrian sides of the Messina Straits. In par- Taormina Fault is part of a single rift sys- ticular, GPS-derived interseismic strain ap- tem, connecting Calabria to south-eastern pears to fit aseismic slip along a 30°, SE- Sicily, implies that it represents one of the dipping normal fault, locked above about 8 most hazardous seismic gaps in Italy, a po- km (Serpelloni, p.c. 2009). Onshore struc- tential site for large future earthquakes. A tural studies (e.g., [3]) have shown the oc- flight of uplifted marine terraces character- currence of a fault system that runs along izes also the Calabrian coast of the Messina the Sicilian coast, but the faults that have Straits. Twelve to fourteen orders of ter- been most active during the Pleistocene are races have been identified, with the high- those located on the Calabrian side of the est terraces, dated Middle Pleistocene, hav- Messina Straits [3]; these faults belongs to ing an elevation of about 1350 m above sea two main sets trending NW-SE and NE- level [10], with an stimated uplift rate of SW and are not obviously related to the N- 846 Marine research at CNR Figure 2: Map showing the traces of fault planes proposed for the Messina 1908 earth- quake, with the authors annotated. S trend of the faults inferred to be respon- human life in Italy’s history of seismicity, sible for 1908 Messina earthquake (Figure with over 80,000 people dieing in the cities 2). Most authors agree that the extensional of Messina and Reggio Calabria and the faults bounding the Reggio Calabria basin surrounding area. Besides buildings col- have been active through Late Pliocene lapse and fires, the damages produced by and Early Pleistocene. However, subse- ground shaking were aggravated by the ef- quent fault activity is debated, and some fects of a remarkable tsunami, with up to authors suggest that the faults were not ac- 11 m of run-up height, that followed the tive since Middle Pleistocene (e.g., [10]). earthquake [12]. Despite such catastrophic The large 1908 Messina earthquake (Mw = effects the location of the causative fault is 7.1 [2]), for which extensional focal mech- not fully assessed (e.g., [13]). Inverse mod- anisms were obtained, occurred within this elling of seismograms and geodetic lev- tectonic frame. The December 28, 1908 elling, and geological studies have pro- Messina Earthquake has been ranked as duced a variety of results in terms of posi- one of the most destructive events of the tion, direction, length and dip of the fault, last centuries, and costed the highest toll in with the most recent solutions proposing 847 Marine Geology Figure 3: Map with traces of multichannel seismic profiles in the study area. The trace of the supposed Taormina Fault is in red. Onshore faults after Ghisetti [3]. Seismic profiles shown in Figures 4-6 are in thick green lines. long, E-dipping faults trending about N-S a matter of debate [15], and contribution (e.g., [14,2], and references therein Fig- from a so far unidentified submarine slide ure2). At present, the most accepted seis- is called upon. In spite of the hazard poten- mogenic source for the 1908 earthquake tial of this marine region, geophysical sur- is a 40 km-long, blind fault dipping 30° veys purposely devised to investigate the to the ESE which is thought to accounts neotectonic features are lacking, with the for the topography of the Messina Straits notable exception of the early work of Selli (DISS, http://diss.rm.ingv.it). This fault [16] which, however, did not employ mod- plane, which trends NNE-SSW, has a min- ern geophysical techniques. In order to imum depth of 3 km, and would crop out bridge this gap a multichannel seismic sur- along the coast of Sicily (Figure2). As vey, aimed at defining the structural pat- for the seismogenic fault, the source of the tern of the Messina Straits, was carried out tsunami related to the earthquake is still within the frame of INGV-DPC seismolog- 848 Marine research at CNR Figure 4: Seismic profile TAO 09 across the central part of the supposed Taormina fault. Arrows mark the South Calabrian fault. See Figure3 for location. ical projects (Figure3[13]). rine strata along the offshore slope (Fig- ure4), but such deformation cannot be re- lated to footwall uplift of a normal fault. 2 Discussion The geodynamic implication is that exten- sion in south-eastern Sicily, on the Ionian Three key aspects related to the tectonics side of the Hyblean Plateau, and extension of the Messina Straits will be briefly dis- in southern Calabria and Messina Straits, cussed below, based on the results obtained belong to two different tectonic systems from multichannel seismic data. A more and cannot be mechanically linked. The detailed presentation of the data and in- seismological implication is that the lack terpretation procedures is given elsewhere of earthquakes is not indicating the occur- [13], with references therein), together rence of a seismicity gap. with the correlation of seismic units with onshore outcrops and short offshore wells 2.2 The faults of the Messina that allows to date the seismic units. Straits and the 1908 Earth- quake 2.1 The elusive Taormina Fault Within the northern part of the Messina The hypothesized occurrence of a large ex- Straits, the imaged faults are located on the tensional fault parallel to the coastline and Calabrian side and dip to the west. These located offshore, between Taormina and faults appear connected to the fault sys- Briga (Taormina Fault) can be rejected by tem reported onshore near Reggio Calabria seismic data [13]. That stretch of coast- (Figure3[3]). A fault trending NW-SE line is actively deforming, as suggested by has been mapped between the towns of uplifted marine terraces and tilting of ma- Reggio Calabria and Messina. The fault 849 Marine Geology Figure 5: Seismic profile TAO 08 within the Messina Straits. A listric fault (arrows) is flattening at shallow depth. Location in Figure5. plane dips to the west and displays a listric within the narrower part of the Messina shape, with growth strata in the lower part Straits might support the interpretation of of the half graben sedimentary fill, that a seismogenic fault located to the south of can be dated as Late Pliocene-Early Pleis- this area or, alternatively, of a blind fault lo- tocene (Figure5).