S43C-1893
Stochastic and Full-Wavefield Finite-Fault Ground-Motion Simulations of the M7.1, Messina 1908 Earthquake (Southern Italy)
In the framework of an ongoing Italian national research project we are studying the Messina 1908 earthquake, the first to be recorded adequately by seismological and geodetic instrumentation that allowed subsequent quantitative investigations of its source properties. We use a high-frequency stochastic finite- fault modeling (Motazedian and Atkinson, 2005) to simulate the ground-shaking for a number of different source models, either constrained from past source studies of this event or simulated. Although inherently kinematic, our approach accounts for the physics of the source using a procedure to generate physically consistent earthquake-rupture models (Guatteri et al., 2004). Considering the width of the seismogenic zone and appropriate source-scaling relation, we generate heterogeneous slip models that obey to the source complexity of past earthquakes (Mai and Beroza, 2002). By also constraining the point of rupture initiation based on empirical findings and energy-balance arguments (Mai et al., 2005), we generate a suite of earthquake source models to compute far-field ground-shaking. The Housner parameter from the stochastic high-frequency simulations is than compared with the felt intensity (MCS scale). The developed procedure is a necessary tool to take into account the influence of directivity effects in simulating ground shaking scenarios using realistic slip distribution on the fault. Furthermore, we carry out full-wavefield ground-motion calculations (at frequencies f < 3 Hz) to compare those low-frequency simulations with (a) the stochastic simulations and (b) appropriate ground-motion prediction equations. The combined approach helps to examine the validation range of the two methods (distinguishing the influence of the near-field and far-field motions on the shaking level), and may serve as a basis to develop a hybrid technique which combines the two methods for generating fully broadband synthetic seismograms.
S43C-1894
Broadband Ground Motion Simulations in the Messina Strait Area (Southern Italy): Appraising Strong Motion Variability due to Complexity in Source and Earth Structure.
Strong ground motion variability due to source complexity and heterogeneous Earth structure has been the subject of a large number of studies. As widely recognized, this variability is particularly evident in near-field ground motion complexity, but also present in high-frequency far-field recordings. To study more quantitatively the effects of source and medium complexity on near-field ground motions, we carry out a numerical experiment, considering physics-based earthquake rupture models and different 1D crustal velocity models. Focusing on the Messina Strait area, struck by a devastating large earthquake (M 7.1) one century ago, we calculate broadband (0-10 Hz) seismograms by combining full waveform low-frequency (0- 3.5 Hz) synthetics and physics-based stochastic high-frequency seismograms (Mai and Olsen, 2008). From recently published studies of the area, we select three representative crustal models and address the influence of Green's functions variability on ground-shaking levels. Simultaneously, we also consider physics- based source models using the pseudo-dynamic approximation to dynamic rupture (Guatteri et al., 2004). This approach allows slip, rise-time and rupture velocity to vary over the fault plane consistent with rupture dynamics, incorporating also constraints on the hypocenter location (Mai et al., 2005). We validate our simulations by comparing common ground motion parameters (peak ground displacement, peak ground velocity, peak ground acceleration and spectral acceleration) with the recent "next generation" (NGA) ground motion prediction equations. Our work furnishes realistic strong motion time series for future earthquakes, adoptable for engineering design purposes, but highlights also their variability due to physics-based source characteristics and different crustal models.
S43C-1895
Offshore deformation in the Calabrian accretionary wedge and implications for the 1693 Catania earthquake and tsunami (Eastern Sicily)
Southern Italy has been struck repeatedly by very strong historical earthquakes as in 1169, 1693, 1783, 1905, 1908, often associated with destructive tsunami. While certain events (like 1908 Messina or 1783 Calabria) are associated with known crustal normal faults, which produced observed surface ruptures, the source of the strongest earthquake in the Italian catalog, the Catania earthquake of 1693 is still unknown. It may have been generated by the NW dipping subduction zone beneath Calabria and Eastern Sicily. Identifying its origin will have a major impact on the hazard assessment and our understanding of modern day tectonics in this region. The 1693 earthquake, struck E Sicily with intensities of X - XI and killed up to 60,000 people, destroying Catania, Syracuse and Augusta. It also generated a 5-10 m high tsunami which swept these cities. The 1169 earthquake had similar intensities (X to XI) and a similar isoseismal pattern, suggesting a similar source. Because of the tsunami generated in 1693 and because the isoseismals are open to the sea, the majority of the source region appears to be offshore. The nearby subduction fault plane is thus a strong candidate for both events. However, a lack of instrumentally recorded thrust earthquakes suggests that if subduction is active, the fault plane is locked (like Cascadia or Nankai). Reported GPS motions from the region are ambiguous, and thus the current activity of the Calabrian subduction remains a matter of debate. The offshore accretionary wedge is known from seismic and bathymetric investigations to include compressional anticlines and recently mud volcanoes have been discovered on the upper wedge. We present preliminary results from reprocessed 96-channel seismic reflection data from the 1997 French cruise Archimede (R/V Le Nadir) crossing the deformation front at the toe of the Calabrian prism, the Ionian abyssal plain and the deformation front of the Mediterranean Ridge (Hellenic subduction system). A more recent HR seismic cruise CALAMARE, with the Italian vessel R/V Urania (Apr. 2008) investigated the lateral boundaries of the Calabrian prism. Both data sets also investigated the Malta-Hyblean escarpment, also proposed as a candidate source for great earthquakes offshore Sicily. Additional work is in progress, including planned cruises to complete multi-beam bathymetric mapping of the Calabrian prism and border regions. A cruise proposal for deep seismics (MCS and OBS work) will also be submitted. The goal of this and future work will to seek evidence of continued tectonic activity of the system.
S43C-1896
Strain Accumulation in the Messina Straits (Southern Italy) From Terrestrial Geodetic Measurements and GPS observations
We use geodetic (terrestrial and GPS) measurements at different spatial and temporal scales, to study the present-day style and rate of active crustal deformation in the Messina Straits southern Italy). The first set of observations consists of triangulation and trilateration measurements from a small-aperture (~ 10 km wide) terrestrial network located in the northern part of the Straits, surveyed between 1971 and 2004. The second set of measurements consists of continuous GPS observations from the larger aperture RING network (inter-station distance in the study area ~ 10-30 km).The results show that the main deformation pattern from GPS and triangulation measurements is given by a 110-160 nstrain/yr right-lateral N-S simple shear. Although affected by an unclear systematic bias the trilateration measurements are statistically consistent with this pattern. This deformation appears to correspond to the transition between collision in Sicily and subduction in Calabria and is determined by the differential retrograde motion of the Ionian lithosperic slab relative to Sicily, where the arrival of buoyant continental lithosphere has caused the end of subduction activity. The rate of observed deformation implies a tectonic loading of the order of 1.1-1.6 mm/yr over the 10 km wide Messina Straits network and 6-9 mm/yr over the larger (50-60 km) subarray of the RING network. The understanding of how this deformation is presently accomodated by faulting is presently unclear but very important for the evaluation of the seismic hazard. We find here that the current strain accumulation in the Messina Straits confirms the hypothesis which considers the Messina Straits as an important boundary between distinct crustal domains. On the other hand the style of interseismic deformation appears scarcely consistent with the faulting geometry generally associated with the 1908 M_w=7.1 Messina earthquake.
S43C-1897
The 1908 Messina Earthquake and the Messina Straits Seismicity as Shallow Expression of Wide Depth-Range Regional Geodynamic Processes
New tomographic results in the Calabro-Peloritan Arc region show that the Messina Straits area delineates the transition between the only sector where the Ionian slab is in-depth continuous and therefore still capable to retreat (Southern Calabria) and the south-western boundary of the subduction system now interested by continental collision (central-western Sicily). This geodynamic framework produces the seismogenic stress field detected in the Straits crustal structure, characterised by ESE-oriented extension and by minor strike- slip component. Stress field and seismicity analyses performed in the area reveal that the 1908 earthquake source is characterised by normal faulting, is located in the Straits and it strikes about NNE-SSW with the top below Sicilian shoreline and dip toward east. Also, in spite of the very low seismicity levels recorded since the installation of local seismic network (late '70s), the analysis performed on historical catalogue data (CPTI, http://emidius.mi.ingv.it/CPTI/) for the period 1780-2002 highlights that the Messina Straits is one of the most active areas in Italy, also excluding phases of maximum destructive capability.
S43C-1898
Synthetic Strong Ground Motions for the 28 December 1908 Messina Earthquake: Investigating Different Source Geometries
We apply stochastic strong ground motion simulation for finite faults (Beresnev and Atkinson, 1997) to estimate the spatial distribution of peak ground acceleration from the destructive 1908 Messina earthquake, aiming to shed light on the geometry of the causative fault. We present the results of a comparative study of different source geometries, which mainly show normal faulting, but have different dip, strike and depth. Computed peak ground acceleration values (PGA) based on these sources, converted to macroseismic intensities using empirical formulas, are compared to previously reported macroseismic intensities of the 1908 earthquake (e.g. Baratta, 1910; Bottari et al., 1986; Database of Macroseismic Intensities, Monachesi and Stucchi, 1997). From all models tested and the comparisons we conclude that: a) the 1908 earthquake source is probably located off-shore in the Messina Straits; b) a length of about 15-20 km seems to be more realistic than a length of 30 km proposed by several authors, c) the best fits have been obtained by the NNE- SSW trending sources (Schick, 1977 and DISS 3.0.4, 2007).
S43C-1899
Messina 1908-2008: understanding crust dynamics and subduction in Southern Italy
To mark the centennial anniversary of the 1908 earthquake that shook Messina, Italy, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) has begun the "Messina 1908- 2008" research project. The aim is to clarify the extension deformation processes that occur in the Messina Strait and to understand relationships between subduction and crustal deformation there by merging existing data and studies, and by collecting new and more detailed seismological, geodetic, historical, and satellite observations. More than 20 permanent seismic stations and about 15 temporary stations are located in the study region. A dense permanent geodetic network also operates in the region, several campaign surveys are newly available, and new geodetic campaign measurements were performed in March 2008. In addition, during July 2008, five ocean bottom seismometers (OBS) were deployed to better monitor the area largely covered by the sea. Records of historical earthquakes that struck the Strait of Messina will be analyzed, and synthetic aperture radar images will help define surface deformation of the region. The Messina 1908-2008 project's assemblage of a database and integration of innovative technologies could transform our understanding of the crust and mantle structure of the active tectonics and seismic hazards of the Strait of Messina.
S43C-1900
Estimate of Seismological Parameters for the 1908 Messina Earthquake Through a new Data set Within the SISMOS Project.
The 1908 earthquake is one of the most catastrophic events in Italian history, recorded by most of the historical seismic stations existing at that time. Some of the seismograms recorded by these stations have already been used by many authors for the purpose of studying source characteristics, although only copies of the original recordings were available. Thanks to the Euroseismos project (2002-2007) and to the Sismos project, most of the original data (seismogram recordings and instrument parameter calibrations) for these events are now available in digital formats. Sismos technical facilities now allow us to apply the modern methods of digital-data analysis for the earthquakes recorded by mechanical and electromagnetic seismographs. The Sismos database has recently acquired many original seismograms and related instrumental parameters for the 1908 Messina earthquake, recorded by 14 stations distributed worldwide and never before used in previous works. We have estimated the main event parameters (i.e. location, Ms, Mw and focal mechanism) with the new data set. The aim of our work is to provide the scientific community with a reliable size and source estimation for accurate and consistent seismic hazard evaluation in Sicily, a region characterized by high long-term seismicity.