Submarine Earthquake Geology: Potential, Limits, Techniques and Results L

GNGTS 2007 SESSIONE 1.1

SUBMARINE EARTHQUAKE GEOLOGY: POTENTIAL, LIMITS, TECHNIQUES AND RESULTS L. Gasperini, A. Polonia, Gi. Bortoluzzi, L.G. Bellucci, V. Ferrante Istituto di Scienze Marine, Geologia Marina, CNR Bologna Paleoseismology is the investigation of individual earthquakes from their geological signatures such as those produced directly along the rupture plane, and those produced indirectly(landslides and, in general, mass wasting processes in the vicinity of faults). If dateable material is recovered from stratigraphic horizons that experienced successive ruptures, slip rate and time separation between large earthquakes can be reconstructed. Earthquake geology has been widely applied to major continental faults over the past decades. While paleoseismology has become a primary tool for seismic hazard evaluation on land, only few paleoseismological studies have been attempted on submarine fault systems, mainly because of the limited resolution of the available geophysical techniques used at sea. Rapid developments in imaging and sampling techniques have now made such studies possible. Submarine paleoseismological studies have the following advantages compared to those undertaken on land: (i) marine sedimentation is generally more continuous in time and space, allowing for regional stratigraphic correlations; (ii) offshore data can be acquired more quickly and at a lower cost. Innovative techniques have been recently developed which makes submarine paleoseismology feasible (see bibliography below). They include (i) high-resolution morphobathymet- ric images of the seafloor, (ii) 3-D and pseudo-3D high resolution seismic reflection imaging, and (iii) detailed stratigraphic reconstruction of the sedimentary record. Although submarine geophysical data have lower resolution than trenching, they provide complete spatial coverage of fault struc- tures, both horizontally and vertically. This will allow accurate long time-scale, high-resolution reconstruction of fault-zone evolution. The North Anatolian Fault in the Marmara Sea, the southern Italian coasts, such as the Calabrian Arc, and other seismogenetic features in the south of Italy, as well as many locations in Greece (e.g. the Gulf of Corinth) represent ideal areas for conducting submarine earthquake geology studies, since long-term geological studies and a unique record of large historical earthquakes/tsunamis dat- ing back to 2000 years BP are available. Moreover, these areas are located along major plate bound- aries, and are characterized by the presence of large scale tectonic features and relative high strain rates. Finally, these areas, are highly populated and prone to seismogenic/tsunamigenic hazards. Results from the marine geological studies carried out in the Sea of Marmara after the 1999 destruc- tive earthquakes are promising, and suggest the feasibility of the paleoseismological approach also underwater. High-resolution geophysical systems detected tiny (few cm) escarpments, features as subtle as anchor drags, cables, narrow fissures and other fault-related features such as mud or gas volcanoes and degassing craters. Geophysical high-resolution images, coupled with accurate pale- ooceanographic/stratigraphic reconstructions allowed for unprecedented quantitative estimate of seismogenic fault behaviour over geological times. We focused on Holocene sediment in several Marmara Sea basins of different sizes. The approach was to test whether (1) the depocenters of the larger basins contain a record of all historic Ms>7 earthquakes within the Marmara Sea region; (2) vertical and strike-slip Holocene deformation can be quantified; and (3) the effects of an earthquake includes both primary structural features due to rupture of the sea floor - strata offset, scarps and tilting - as well as secondary effects due to shaking, such as mass-wasting and gravitational flows. We found evidence of earthquakes that we correlate with historic events in 181 AD, 740 AD, 1063 AD, 1343 AD, 1509 AD, 1766 AD, 1894 AD and 1912 AD. The geologic evidence is primarily from those basins adjacent to the rupture as inferred from historic data. This suggests that coseismic deformation of the sea floor along the rupture is a critical factor in the sedimentary record. A qual- itative sedimentation model has been proposed that relates this coseismic deformation to mass-wasting of the slope, scour of the basin floor, seiche motions and homogenite deposition.

67 GNGTS 2007 SESSIONE 1.1

Moreover, findings of reliable piercing points on both sides of the Sea of Marmara basin enabled us to obtain an accurate estimate of the slip-rate over a geological time (~10000 years) on different fault strands of the NAF system. We discovered displaced sedimentary features (submarine canyons and a river channel) in the Gulf of Izmit and Gulf of Saros that are presently inactive and locally filled by a thick Holocene sedimentary cover. Since we can accurately date the abandoning of these sedimentary features, because they are related to the same episode of sea-level rise that followed the Last Glacial Maximum, we obtained accurate estimates of the slip-rate along the northern strand of the NAF system on both sides of the Marmara basin. The rates we obtained are in the order of 10 mm/yr that represents about one half of what expected from geodetic measurements and accepted plate-tectonic models. This has important implications both for reconstructing the neotectonic set- ting of the submerged portion of the NAF system and to our understanding of fault interactions and seismic hazard in the Marmara region as it suggests that the total Anatolia/Eurasia plate motion is more distributed than previously reported. The surveying techniques and approaches used have therefore the potential of documenting earthquake ruptures of fault segments and to extend the earthquake record far before the known his- tory, thus improving hazard evaluations and the fundamental understanding of earthquake process.

LA SISMICITÀ DI FONDO PUÒ MASCHERARE L’EVIDENZA DI UN DECADIMENTO ESPONENZIALE DEL TASSO DI AFTERSHOCKS B. Lolli e P. Gasperini Dipartimento di Fisica, Università di Bologna I modelli Stretched Exponential modificato (MSE) e Band Limited Power Law (LPL) assumo- no che un decadimento esponenziale negativo domini il tasso di aftershocks a lungo termine. Tuttavia, precedenti studi hanno evidenziato che tali modelli riproducono le sequenze reali meglio del Modello di Omori Modificato (MOM) solamente in pochi casi. Attraverso l’analisi di 29 sequenze di aftershocks reali avvenute in California meridionale ed in Italia mostriamo che, in base ai criteri di informazione di Akaike e Baiesiano, i modelli MSE e LPL sono preferibili rispetto al MOM se il tasso di background è correttamente considerato e la durata della finestra temporale è di almeno un anno. Si può dedurre che la presenza di un decadimento esponenziale negativo sia una caratteristica generale delle sequenze reali e che il modello LPL, che normalmente risulta preferi- bile rispetto al MSE, sia il più adatto a descriverne le proprietà.

LA VERSIONE 3.0 DEL DATABASE EMMA (EARTHQUAKE MECHANISMS OF MEDITERRANEAN AREA) G. Vannucci(1), P. Gasperini(2) e P. Imprescia(3) (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna (2) Dipartimento di Fisica, Università degli Studi di Bologna (3) Dipartimento di Scienze Geologiche, Università degli Studi di Catania Viene presentata la nuova versione del database, sviluppata nell’ambito del progetto sismologi- co S2 della convenzione INGV/DPC. Rispetto alle precedenti versioni (Vannucci e Gasperini, 2003, 2004, disponibili all’indirizzo http://ibogfs.df.unibo.it/user2/paolo/www/emma/) è stato incremen- tato il numero di meccanismi focali nell’area compresa fra le coordinate 32°W-72°E di longitudine e 22°N-52°N di Latitudine. Oltre 10700 soluzioni sono ad oggi presenti nel dataset con un incre- mento di circa il 60% rispetto alla versione 2.2. I dati aggiunti provengono da circa un centinaio di