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

SUMMARY Early 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 2m 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 of6.8–6.9alongareverseN55 ◦ Estrikingfaultarethebestcandidatestoexplaintheknown 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 1.5m.