Environmental effects following a seismic sequence: the 2019 Cotabato - Davao del Sur (Philippines) earthquakes

Abstract

In the period of October – December 2019, the Cotabato – Davao del Sur region (Philippines) was hit by a seismic sequence comprising four earthquakes with magnitude MW > 6.0 (EQ1-4; max magnitude MW 6.8). The earthquakes triggered widespread environmental effects, including landslides and liquefaction features. We documented such effects by means of field surveys, which we supplemented with landslide mapping from satellite images. Field surveys allowed us to gather information on 43 points after EQ1, 202 points after EQs2-3 and 87 points after EQ4. Additionally, we built a multi-temporal inventory of landslides from remote sensing, comprising 190 slope movements triggered by EQ1, 4737 after EQs2-3, and 5666 at the end of the sequence. We assigned an intensity value to each environmental effect using the Environmental Seismic Intensity (ESI-07) scale. Our preferred estimates of ESI-07 epicentral intensity are VIII for the first earthquake and IX at the end of the sequence, which is in broad agreement with other events of similar magnitude globally. This study, which is the first case of the application of the ESI-07 scale to a seismic sequence in the Philippines, shows that repeated documentation of environmental damage and the evaluation of the progression through time may be useful for time-dependent susceptibility assessment and for investigating the role played by ground shaking and by other mechanisms (e.g., rainfall).