S41D-01
From Analyses of Fallen Monuments to Tsunami Waveform Modeling: a Century of Seismological Developments Seen Through the Landmark 1908 Messina Earthquake
The 28 December 1908, Messina Straits earthquake (M 7.1) and tsunami was one of the most damaging events in Europe during the last century. There are several factors that make this earthquake truly a scientific landmark: (1) It was the first European earthquake to be well recorded by modern seismological, geodetic and mareographic instrumentation, much like the 1906 earthquake in California. Over 100 seismograms and several mareograms were recorded, many tens of elevation changes were measured and made available to the scientific community, although with limitations in reproducing the original time histories and data sheets. (2) Contemporary workers also strived to gather post-earthquake and post-tsunami data that were as objective and quantitative as possible. Examples include the large set of available tsunami run-up heights that were measured using geodetic instrumentation, and the determination of the epicenter initially derived from a large set of measurements of the directions of fallen monuments and later from travel times of seismic waves. (3) Observations from the 1908 earthquake have been analyzed repeatedly during different scientific eras, each time with the best tools available. Examples of recent efforts include a variable slip analysis based on the coseismic elevation changes, a source-time function derived from digitized seismograms and sophisticated techniques used to model the tsunami. (4) This earthquake spurred the adoption of the first quantitative seismic code in history, about 15 years before similar provisions were adopted in Japan and in the US.
S41D-02 INVITED
The Invaluable and Irreplaceable Role of the Historical Seismograms' Analysis
Based on the general evidence that strongest events repeat on the major geological faults, often with similar features, the reconstruction of the pattern of energy release for the 1908 earthquake source is crucial for the assessment of the seismic risk in the region. At the beginning of the twentieth century, many seismic stations were operating in the world and at least 110 recorded the Messina Straits event. Since 1908, several seismologists analyzed the available seismograms aiming at the determination of earthquake source parameters, but their studies were strongly affected by the state of the seismological knowledge and the available technology. Today, after 100 years, taking advantage of the scientific and technological progress, historical seismograms can be reanalyzed with techniques analogue to those used in modern earthquake studies. Although most original seismograms of the 1908 earthquake are unfortunately lost, the analysis of the available waveforms allowed the reconstruction of the fault kinematics, putting firm and unique constraints not achievable with other investigations.
S41D-03
Reassessment of the interseismic and coseismic deformation in the Messina Straits
The December 28th, 1908 M 7.1 Messina earthquake is one of the largest instrumentally recorded earthquake in the Central Mediterranean and among the first to be recorded by seismological and geodetic instruments. Starting from the evaluation of the present-day geodetically-observed active deformation in the Messina Straits region, we revised the available data on the interseismic and coseismic deformation in the Messina Straits region.GPS observations and terrestrial geodetic measurements show that the main active deformation is given by a 110-160 nstrain/yr right-lateral N-S simple shear which is scarcely consistent with the faulting geometry generally associated with the 1908 M_w=7.1 Messina earthquake. Then we use the levelling data measuring the coseismic deformation of the 1908 earthquake and historical seismograms to evaluate the whole range of faulting mechanisms allowed by the data. This analysis suggests that the available data for the 1908 earthquake allows a larger component of North-South strike slip of what hitherto considered, resolving the discrepancy with the present-day deformation pattern. Finally we discuss these new findings in the light of the present-day regional kinematics between the Calabrian and Sicilian crustal blocks.
S41D-04
Active Extension of the Messina Straits and Kinematics of Sicily and Calabria From Elastic Block Modeling of GPS Data
Eastern Sicily, the Messina Straits and Calabria (Southern Italy) are among the most seismically active areas of the Mediterranean region, and the locus of several destructive earthquakes in the historical record. December 2008 will be the centenary of the most recent major seismic event, the 1908 (M7.1) Messina- Reggio Calabria earthquake. Despite the significant advances in our knowledge of the present-day crustal deformation of the area, thanks to recent developments of new Continuous GPS stations in Italy, the kinematics and geodynamics of the Sicily-Calabria subduction system are still debated. We use GPS measured velocities from across the region to better constrain the motion of Sicily, Calabria and the Ionian Sea in the framework of the slowly NW-ward Nubia-Eurasia plate convergence. GPS data indicates that about 2 mm/yr of NW-SE extension are taken up across the Messina Straits-Peloritani area, and simple dislocation models suggest that the observed velocity gradient is compatible with a SE-ward dipping shallowly locked normal fault. However, any effort of studying the kinematics of crustal deformation of this area, including rates of interseismic loading across the Straits, is challenged by the fact that several active tectonic structures (including the Vulcano and Tindari-Giardini faults and the Calabrian subduction system) likely contribute to the observed GPS velocity gradients in the study area. In order to investigate these effects, we develop an elastic block model of the Sicily-Calabria region that accounts for tectonic block rotations and elastic strain accumulation on regional fault systems. The model reveals that there is a substantial trade-off between interseismic slip-rate estimates of crustal faults in the area and the locking width of the Calabrian subduction interface. The regional kinematics suggest the presence of a complex set of crustal blocks and microplates, including a Sicily micro-plate, a North-Eastern Sicily block and a Calabrian subduction micro-plate moving independently from Nubia and Eurasia.
S41D-05
The 28 December 1908 Earthquake in the Straits of Messina (Southern Italy) and Contemporary Italian Seismology: Theories, Instruments, Observations
Italian seismology has a long and prestigious tradition which has seen the contributions of many scholars ever since the earliest decades of the 18th century. In the last 25 years of the 19th century Italian seismology underwent a very strong and original development, especially from the point of view of the honing of a method of earthquake observation, which significantly influenced European seismology. Born outside the institutions in the second half of the 1870s, the first network of instrumental earthquake observations in Italy was developed and consolidated within the institutions only between the second half of the 1880s and the mid-1890s. A striking series of large earthquakes that affected southern Italy between 1894 and 1908 (1894 Calabria, Me = 6.1, 1905 Calabria Me = 6.5, 1907 Calabria, Me = 6.0 and 1908 Straits of Messina, Me = 7.1) triggered a lively debate within the seismological community and also among scholars and institutions. The early years of the 20th century are the time when, thanks also to Italian experiences, Emil Wiechert and Boris Galitzin designed and set up the first modern seismographs. The 1908 earthquake, one of the strongest earthquakes of Italian history, marked a turning point in the history of Italian seismology and in the relationships bewteen the national government and the scientific community. This work aims at presenting the scientific context of theories, instruments and observations at the time of the 1908 earthquake and discussing their impact on the Italian seismology of that period.
S41D-06
A Seismically-Triggered Underwater Landslide Caused the 1908 Messina Tsunami
On 28 December 1908, at 5.21 a.m., local time, a catastrophic earthquake (MCS maximum intensity = XI, estimated magnitude = 7.1) struck the region of the Messina Straits, Ionian Sea, Southern Italy. Within minutes after the passage of the seismic waves, a tsunami with maximum observed run-up of about 10 m hit the coasts of Calabria and Sicily. The earthquake and tsunami caused major destruction and at least 60,000 deaths. A century after the catastrophic event, the sources of the 1908 Messina earthquake and tsunami remain uncertain. Our hypothesis is that the tsunami was mostly caused by an underwater landslide. To test this hypothesis, we analyzed the available tsunami, bathymetric, seismic, and seismological data to revue previous interpretations and present our new conclusions. Through a simple backward ray-tracing method, we converted the tsunami travel-time data reported in a 100-years-old paper written by Mario Baratta into distances and found that the sources of the earthquake and tsunami are different. Overturning a long-held assumption, reconsideration of the available tsunami, bathymetric, seismic, and seismological data support our hypothesis of a seismically-triggered underwater landslide as the main cause of the Messina tsunami.
S41D-07
The 1908 Messina tsunami. Some comments on the source: earthquake, submarine landslide or a combination of both?
It is almost 100 years to the day that the Messina earthquake and tsunami took place, yet numerous attempts to model the tsunami have failed to effectively simulate the measured runups (e.g. Tinti and Armigliato, 2003). Only by southward extension of the earthquake rupture can the measured tsunami runups be approximated, but there is little evidence to support this interpretation. Additionally, the rupture requires a complex heterogeneous slip mechanism. Billi et al (2008), recognising the discrepancy between the available earthquake simulations and measured runups, attempted to identify the tsunami source location by simple backward ray tracing based on runup height and tsunami arrival times. Their work confirmed a tsunami source to the south of the earthquake rupture. On 2-D bathymetry a submarine landslide was identified at this location, and these authors proposed that, rather than an earthquake, it was this landslide that was the tsunami source. However, there has been no modelling of a landslide generated tsunami to support this hypothesis. Here we revisit the tsunami source of 1908 using a regridded data set of multibeam bathymetry acquired in the Ionian Sea, east of Sicily. This data provides an improved visualisation of seabed morphology that allows a better understanding of the features present. The offshore area is heavily gullied and a number of seabed failures identified. There is the landslide identified by Billi et al (2008) together with another seabed failure, located to the south of the landslide, on the flank of Mount Etna. This second failure is a rotational slump or flank collapse. Preliminary modelling based on the architecture of the failures indicates that the landslide is an unlikely tsunami source. The slump, however, is a more likely candidate; either on its own or in combination with the earthquake. These new interpretations are presented and discussed.
S41D-08 INVITED
The Messina Straits 1908 Tsunami: Observations and Causative Source
After a whole century, the causative source of the tsunami remains one of the most discussed issues about the 1908 earthquake. Recently, two papers reached substantially opposite conclusions and have raised the debate again. The authors of the first paper hypothesize that the origin of the tsunami should be related to a large submarine landslide; conversely, the second paper demonstrates that the run-up distribution along the Sicilian and Calabrian coast is much more compatible with a tsunami generated by a seismic fault than by a submarine landslide. In the first part of the present paper, we will review the observations of the 1908 tsunami, mainly basing on the 1909 account by Giovanni Platania. The second part of the paper will be dedicated to the tsunami causative source. We will discuss several scenarios, by qualitative and quantitative comparison between experimental data and simulated tsunami generated with different kind of sources.