Earthquake Source and Tsunami Analysis of the Mw 6.0 Event Offshore Bejaia (Algeria) on March 18, 2021

The French National Tsunami Warning Center (CENALT) hosted by the French Alternative Energies and Atomic Energy Commission (CEA) continuously monitors the seismic activity in the Western Mediterranean Sea, and warns the French Civil Security within 15 minutes after a possibly tsunamigenic earthquake. On March 18th, 2021, a magnitude 6.0 earthquake occurred offshore northern Algeria, at about 21 km away from the town of Bejaia. This moderate quake is an event of interest for the region as it also generated small-amplitude tsunami waves travelling across the Mediterranean Sea toward Spain and France that were recorded by tidal gauges.

Here, we present a multi-technology analysis of the event combining seismic source inversions and tsunami simulations. First, seismic moment tensor solutions obtained from the available approaches at CEA (TDMT, FMNEAR, and GRiD MT), using regional broadband stations filtered at long periods, are compared. All methods correctly determine the focal mechanism and magnitude of the event. As opposed to the others, GRiD MT provides the advantage of automatically determining the origin time and the location (latitude, longitude, depth) using a 4D grid search approach. Despite well identified source parameters, the event's depth remains difficult to constrain by the three methods.

Second, tsunami observations are shown on the French coastline, exhibiting amplitudes of a few centimeters in several harbours, without any flooding. These observations confirm that the event can be defined as a threshold regarding tsunami warning and forecast, consistent with the decision matrix in place in the North East Atlantic and Mediterranean Tsunami Warning System (NEAMTWS). They are compared to tsunami simulation carried out with a non linear shallow model accounting for frequency dispersion when necessary (Taitoko code), and based on the seismological models obtained previously.

Both seismological inversion and tsunami modelling capacity are essential to better characterize tsunami hazard in the course of an event, from the very first minutes to a future ability to numerically forecast the amplitudes in detail at the harbour scale.