In this note, we derive an attenuation function for computing magnitude values equivalent to Mw using strong‐motion data. We analyze 106 earthquakes of the 1 April 2014 Mw 8.1 Pisagua sequence, which occurred along the 1877 seismic gap in northern Chile. We considered both foreshocks and aftershocks with moment magnitude available from moment tensor inversion in the GEOFON bulletin and recorded by the Integrated Plate boundary Observatory Chile strong‐motion network. The maximum peak displacement measured over the double integrated traces is used to construct the magnitude scale, following a nonparametric approach. A bootstrap analysis is performed to assess the uncertainty of the model parameters, and cross‐validation tests are performed to proof the suitability of the derived model in predicting the Mw in the analyzed area, with an uncertainty of 0.2 magnitude units. The derived scale is applied to an early aftershock, which occurred about 155 s after the mainshock, initially missed in bulletins published by rapid global earthquake monitoring agencies (e.g., National Earthquake Information Center and GEOFON), because its phase arrivals at regional/teleseismic distances mix with those of the mainshock and its later arrivals. The estimated magnitude equivalent to Mw is 6.6±0.3, which rank this event as the second largest aftershock of the sequence, after the Mw 7.6 earthquake that occurred on 3 April 2014.