The 1877 megathrust earthquake of North Chile two times smaller than thought? A review of ancient articles

Abstract

The 1877 North Chile megathrust earthquake is commonly considered as one of the largest historical earthquakes that occurred on the Chilean subduction zone. The literature generally attributes it a magnitude close to 9, associated with a rupture of about 500 km long. Because it occurred nearly a century and a half ago, the area is often described as a mature seismic gap, that is to say, a place where a great subduction earthquake similar to that of 1877 would be imminent. A careful study of historical articles describing the earthquake and subsequent tsunami shows that the size of the rupture has most likely been greatly overestimated. First of all, it seems that the area of severe shaking, corresponding to intensity VIII on the Mercalli scale, has been artificially increased. Secondly, it appears that once errors and exaggerations are corrected, nowhere was the subsequent tsunami actually higher than 10 m. It also appears that the initial exaggerations have been further amplified in the modern literature to reach today’s estimation of the earthquake size. In reality, all historical observations, i.e. the intensities of destruction on the continent due to the earthquake itself and the characteristics of the subsequent tsunami, concur to attribute to the earthquake a rupture of only ∼225 km long, located between 20.5°S and 22.5°S. Both this reduced length and its localization match very well the patchy coupling revealed by recent GPS measurement in the area and support the hypothesis of a rupture of the Loa segment alone. The estimation of an earthquake magnitude from the subsequent tsunami is always difficult because the amplitude of the tsunami at a given place depends on many more parameters than the seismic moment alone. The source localization which changes the azimuth and path towards a given receiving place, the source depth, the rupture velocity, all play a role. They remain largely unknown for the 1877 event. However, the comparison of the 1877 revised figures with recent tsunamis suggest the magnitude should also be lowered to around 8.5 : slightly larger than Illapel 2015, significantly smaller than Maule 2010. In all cases, the seismic hazard of the region should be revised downwards: the next expected earthquake in the region should be either significantly smaller than feared (magnitude ∼8.5 rather than ∼9), or occur much later than announced.