Reappraisal of the 1887 Ligurian earthquake (western Mediterranean) from macroseismicity, active tectonics and tsunami modelling

Abstract

International audienceEarly in the morning of 1887 February 23, a damaging earthquake hit the towns along the Italian and French Riviera. The earthquake was followed by a tsunami with a maximum run-up of 2 m near Imperia, Italy. At least 600 people died, mainly due to collapsing buildings. This 'Ligurian earthquake' occurred at the junction between the southern French-Italian Alps and the Ligurian Basin. For such a historical event, the epicentre and the equivalent magnitude are difficult to characterize with any degree of precision, and the tectonic fault responsible for the earthquake is still under debate today. The recent MALISAR marine geophysical survey allowed the identification of a large system of active faults. We propose that the rupture of some of the segments belonging to this 80-km-long northern Ligurian Faults system connected to a shallow-dipping major thrust plane at depth was the source of the 1887 Ligurian earthquake. We investigated the macroseismic data from the SISFRANCE-08 and DBMI-04 historical databases using several models of intensity attenuation with distance and focal depth. The modelling results are consistent with the off-shore location, with an epicentre around 43.70°-43.78°N and 7.81°-8.07°E, and with a magnitude Mw in the range of 6.3-7.5. Numerous earthquake source scenarios have been tested on the tide gauge record at Genoa harbour. As a result, we present seven characteristic source earthquake scenarios for a shallow strong earthquake occurring below the northern Ligurian margin. The modelled tide gauge records were analysed with the help of basic statistical tools and a simple harmonic analysis, to extract the wave spectrum characteristics. This analysis indicates that scenarios of a magnitude Mw of 6.8-6.9 along a reverse N55°E striking fault are the best candidates to explain the known characteristics of the tsunami that followed. The best-fitting scenarios comprise a 70°-dipping southward fault plane with Mw 6.8 and a 16°-dipping northward fault plane with Mw 6.9, both with reverse kinematics. Taking into account the geometry of the active faults, the location of the macroseismic epicentre and the morphotectonic evolution of the continental slope, we propose that the 1887 Ligurian earthquake corresponded to the reverse faulting of a N55°E striking fault plane dipping to the north with a coseismic slip of 1.5

Frequency and triggering of small-scale submarine landslides on decadal timescales: Analysis of 4D bathymetric data from the continental slope offshore Nice (France)

“…It is characterized by a constant background microseismic activity as well as the occurrence of moderate-size earthquakes (4<Mw<5) every 4 to 6 years that generally cause no damage. Nevertheless, in the past, stronger and destructive events were reported, the more recent one being the 1887 Ligurian earthquake (Mw ~ 6.8-6.9; Larroque et al, 2012) that occurred offshore, close to the city of Imperia (North-Western-Italy) and caused extensive damage and the death of about 600 persons.…”

Seismic Exploration of the Deep Structure and Seismogenic Faults in the Ligurian Sea by Joint Multi Channel and Ocean Bottom Seismic Acquisitions: Preliminary Results of the SEFASILS Cruise

“…This feature is likely responsible for the margin uplift, which is fairly pronounced offshore Imperia, vertically offsetting by more than 1 km the sedimentary sequence, including Messinian markers whose deposition postdates the rifting [6] (Figure 2c). It is also likely responsible for the 1887 Ligurian earthquake that affected the same area, with an assessed magnitude up to Mw6.9, some important damage from Menton to Genova, a 2-m high tsunami, and more than 600 fatalities [3] (yellow star on Figure 1). The tsunami polarity as well as the depth (10-12 km) and focal mechanisms of more recent local earthquakes all point to an active landward verging thrust fault beneath the margin, compatible with the tectonic uplift [2,3].…”

Response to Maarten Van Daele's comments

“…Moreover, the south-western European Alps are in a seismogenic region that experienced strong earthquakes with macroseismic Medvedev-Sponheuer-Kárník (MSK) intensities up to IX and estimated magnitudes higher than 6, e.g. the Ligurian earthquake in 1887 (M w = 6.8; Larroque et al, 2012) and the Visp earthquake in 1855 (M w = 6.2; Fäh et al, 2011;Fig. 1),…”