High-Frequency Directivity Effect for an Mw 4.1 Earthquake, Widely Felt by the Population in Southeastern France,

Abstract We document a moderate earthquake in the French Alps (26 February 2012 M w  4.1) that has been much more distinctly felt south of the event than north of it. This discrepancy was especially clear in the two large cities of Nice and Grenoble, both situated at 100 km from the epicenter. This observation was confirmed by ground‐motion measurements that were eight times larger in one city than in the other one, for the same site conditions. Using a time‐domain deconvolution between the broadband recordings of the mainshock and an aftershock used as empirical Green’s functions, we show that the rupture process of this event had a strong directivity effect toward a direction of N155°±5 on an ∼2  km long fault, detectable only at frequencies higher than 1 Hz. The fault size and direction are in good accordance with the location of the aftershocks. Despite the various possible contributions leading to high‐frequency amplification, we show here that this simple directivity effect controlled the intensity felt by the population and the acceleration measured in the cities.