Analysis and modelling of tsunami-induced tilt for the 2007, M = 7.6, Tocopilla and the 2010, M = 8.8 Maule earthquakes, Chile, from long-base tiltmeter and broadband seismometer records

Abstract

We present a detailed study of tsunami-induced tilt at in-land sites, to test the interest and feasibility of such analysis for tsunami detection and modelling. We studied tiltmeter and broadband seismometer records of northern Chile, detecting a clear signature of the tsunamis generated by the 2007 Tocopilla (M = 7.6) and the 2010 Maule (M = 8.8) earthquakes. We find that these records are dominated by the tilt due to the elastic loading of the oceanic floor, with a small effect of the horizontal gravitational attraction. We modelled the Maule tsunami using the seismic source model proposed by Delouis et al. and a bathymetric map, correctly fitting three tide gauge records of the area (Antofagasta, Iquique and Arica). At all the closest stations (7 STS2, 2 long-base tiltmeters), we correctly modelled the first few hours of the tilt signal for the Maule tsunami. The only phase mismatch is for the site that is closer to the ocean. We find a tilt response of 0.005-0.01 µm at 7 km away from the coastline in response to a sea level amplitude change of 10 cm. For the Maule earthquake, we observe a clear tilt signal starting 20 min before the arrival time of the tsunami at the nearest point on the coastline. This capability of tilt or seismic sensors to detect distant tsunamis before they arrive has been successfully tested with a scenario megathrust in the southern Peru-northern Chile seismic gap. However, for large events near the stations, this analysis may no longer be feasible, due to the large amplitude of the long-period seismic signals expected to obscure the loading signal. Inland tilt measurements of tsunamis smooth out short, often unmodelled wavelengths of the sea level perturbation, thus providing robust, large-scale images of the tsunami. Furthermore, tilt measurements are not expected to saturate even for the largest run-ups, nor to suffer from near-coast tsunami damages. Tiltmeters and broadband seismometers are thus valuable instruments for monitoring tsunamis in complement with tide gauge arrays.

Numerical tsunami simulation including elastic loading and seawater density stratification

“…Many studies have modeled the waveforms of tsunamis generated by such earthquakes using the shallow water theory that describes waves having wavelengths much longer than water depth over a rigid ocean floor. However, it has been known for some time that the solid Earth deforms in response to the surface load of the tsunami water mass, and this deformation has been observed in several studies [ Nawa et al , ; Okal , ; Boudin et al , ].…”

Slow slip events precursory to the 2014 Iquique Earthquake, revisited with long-base tilt and GPS records

“…The noise level at 1 hour to 1 month period was extremely low, between 10 −9 rad and several 10 −8 rad (Boudin et al 2013). Coseismic steps of moderate, nearby earthquakes were well recorded, and correctly modelled with a shear dislocation in elastic half-space.…”

“…Long-base tiltmeters have been installed for many years for monitoring transient tectonic strains in active fault system (USA, (Bilham et al 1979) and Wyatt et al (1982); Japan, Inochi et al (1987); Mexico, Kostoglodov et al (2002); Chile, Boudin et al (2013)), but also for tracking hydrological strain sources (Longuevergne et al 2009). They are usually tens to hundreds meters long, installed in long trenches a few meter deep, or in tunnels hundreds of meter deep (Boudin 2004;D'Oreye & Zürn 2005;Boudin et al 2008).…”

“…Our team has long experience in the conception, construction and installation of such long-base tiltmeter in these various geophysical contexts (Boudin et al 2013;Longuevergne et al 2009;Boudin 2004;D'Oreye & Zürn 2005;Boudin et al 2008). Our instruments consists of a long horizontal tube 6 F. Boudin et al (between 30 and 50 m long in Chile), filled with water and connecting two pots made of fused silica (Boudin et al 2008).…”

“…When reported on a 50 m long tiltmeter, this provides a possible tilt drift of several 10 −7 rad/year. We calibrated the LVDT sensitivity to temperature, typically 0.3 mm/ • C (Boudin et al 2008(Boudin et al , 2013, and, considering an uncertainty on the horizontallity of the instrument of a few mm, a temperature-induced sensitivity of tilt of 10 −7 rad/ • C (Boudin et al 2008).…”