Deformations of Land, Sea and Gravity Levels by the 2009 Samoa Earthquake

The Samoa Islands were struck by the September 2009 earthquake of Mw 8.1. We study the effect on the land level by means of GPS monitoring and on ocean level by tide-gauge records. This allows us to present a new picture of the interaction of crustal movements, gravitational adjustment and sea level changes. The land level exhibits a co-seismic uplift followed by a post-seismic crustal subsidence. The ocean level records a fall, significantly larger than the uplift and delayed by several months, followed by a significant rise, by far exceeding the crustal subsidence and delayed by several months. This indicates a significant contribution from changes in gravity (geoidal eustasy), besides relative sea level changes due to crustal movements. High amplitude, and high frequency changes in tidal range provide evidence of changes in gravity and geoidal eustasy.