Source characteristics of the 28 December 1908, Messina Straits earthquake (Mw 7.1): a review
by
Gianluca Valensise
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy

The 1908 earthquake occurred just at the beginning of the instrumental era. Nevertheless, it was recorded by several observatories worldwide, and the permanent elevation changes it produced were precisely measured by promptly resurveying first order leveling lines on both sides of the Straits.

Systematic efforts for quantifying the 1908 earthquake and identifying its causative fault began in the early ‘70s at the time of the first project for a single span bridge that would permanently connect the two shores of the Straits. In the early ‘80s various investigators proposed seismotectonic models that would explain the occurrence of the earthquake as due to the activity of a rather complex set of faults arranged in a Graben fashion. Then in the second half of the ‘80s the fault responsible for the 1908 earthquake started being represented as a single large fault underlying the Straits. Analyses conducted by independent groups of workers on the elevation changes associated with the event converged in suggesting that this fault strikes nearly N-S, is 40-60 km-long and 15-20 km-wide, and dips to the East at a very shallow angle (30º). As a result of this configuration, the fault was seen as being essentially blind and hence unable to produce significant ground dislocation, consistently with the lack of surface faulting reports following the earthquake.

In the early ‘90s this rather established source was related to the overall recent evolution of the Messina Straits. The 1908 was seen as a characteristic earthquake of the Messina Straits that would recur every 1,000 years or so and would prevent the occurrence of smaller but still damaging earthquakes (e.g. in the magnitude range 5.5 to 6.5). Later this finding received further support from historical and archeoseismological analyses, which suggested that an earthquake comparable in size with 1908 may have occurred in the IV century AD, and that no comparably large earthquake has occurred after the IV century and prior to 1908.

Finally, recent seismological analyses suggest that the earthquake nucleated beneath the Calabrian coast about 10 km SE of Reggio Calabria, and that from there the rupture propagated almost unilaterally for about 40 km up to the northern tip of the fault, located near Capo Peloro. These findings are well consistent with the asimmetries of the pattern of damage caused by the earthquake.

In summary, after 30 years of intense research there is substantial consensus on the main characteristics of the 1908 earthquake. The consensus is even stronger for the properties of the fault that have the strongest engineering and seismic hazard implications, such as (1) the fact the fault did not produce surface breaks, (2) its orientation, (3) the unilateral nature and the direction of rupture propagation, and (4) a minimum expected recurrence interval of several centuries. The main open questions concern the exact extent of the coseismic rupture, that some investigators believe to have stretched beyond the Straits both to the North and to the South, and the residual potential for moderate size earthquakes (~5) that may occur on the main or on a subsidiary fault plane (e.g. the Reggio Calabria earthquake of 16 January 1975, M 5.1). These two issues will hopefully be addressed by the results of the GPS campaigns that have been conducted in the Messina Straits during the past 15 years. GPS results might also contribute to a better identification of the Straits’ main fault and to detect any region of anomalous strain accumulation.

Date received: September 18, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-58.