At tidal frequencies, recordings of closely-spaced horizontal seismometers, even if separated only by a few meters, can present large discrepancies. These perturbations are due to local effects such as topography, local geology or cavity effects (when instruments are installed in tunnels or cavities), which imply strain-tilt coupling. These local effects are well observed in the Earth tide recordings and have been well understood since the 70s. At very low frequencies (tide), the horizontal seismometers are mainly sensitive to tilt. As the frequency increases, the inertial sensitivity becomes larger and competes with tilt, until at high frequency the local effects mentioned above should eventually disapear. The question arises then about the importance of such local effects in the seismic band (T < 1h). This is the problem we address here by studying horizontal recordings of the Earth's normal modes from co-located instruments. The gravest free oscillations of the Earth are excited by large earthquakes but are often masked by the high noise level induced by atmospheric activity. The long period recordings of the giant 2004 Sumatra earthquake (Mw ~ 9.3) offer an excellent opportunity to study these local effects on the Earth's gravest free oscillations (f < 1.2 mHz). In order to better understand the problem, we also compare the observations with synthetic spectra that include the modeling of Earth's global 3D structure. Using records from several seismic stations with high signal/noise ratio, we demonstrate for the first time the presence of these local effects for the Earth's gravest free oscillations. Our observations indicate that the fundamental modes are the most affected. These local effects can be characterized by 3 coefficients which express the coupling between the tilt and the strain field. For each station and each instrument, we estimate a set of coefficients by comparing the numerical predictions with the observations. Once estimated, these coefficients can be directly used to reduce the local effects in the normal modes data. The obtained coefficients for the free oscillations are comparable to those found in previous analyses at tidal frequencies.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1734 Seismology;
- 7255 Surface waves and free oscillations