Observing sea level and current anomalies driven by a megathrust slope-shelf tsunami: The event on February 27, 2010 in central Chile

Abstract

At 03:34 (local time) on 27 February 2010, the world's sixth largest earthquake on record (8.8Mw) occurred off central Chile, rupturing a segment about 550km long of the Nazca–South America plate boundary fault parallel to the coastline. This earthquake triggered a destructive tsunami that affected the entire Pacific basin, especially the Chilean coast in an extension of ∼580km. Here we analyze observations of currents and sea level anomalies caused by the tsunami that were recorded ∼30km east of the epicenter by an RDI 600kHz ADCP installed facing upward at 35m depth. Although the ADCP sampling rate was not optimal for this kind of phenomenon, we were able to document for the first time near-field ocean hydrodynamics caused by an earthquake-triggered tsunami. We combine our observations with measurements of sea level recorded by tide gauges located along the Chilean coast.The ADCP recorded the passage of five waves of about 1.5–2m amplitude during approximately 4h after the earthquake. The first wave hit the coast in about 20min with slow flows and propagated faster northwards (where the trench is deeper) than it did to the south. The first strong flow arrived cross-shore (after the first wave) with a vertical mean speed of 65cm/s. Unlike this first cross-shore flow, the following peak flows were increasingly aligned with the coastline and had magnitudes that, in at least two cases, exceeded 70cm/s. These flows were consistent with edge waves driven by the contact between the tsunami and the coastline. Current and sea level measurements had more energetic periods at 35, 50 and 72min, displaying a coherent relationship with the dominant oscillation modes of large-scale resonance.