Characteristics of Shallow Structure Obtained from the Inversion of Colocated Pressure and Seismic Data for Frequencies below 0.05 Hz

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We have developed a method for deriving shallow elasticity structure from colocated pressure and seismic instruments for frequencies between 0.01 Hz and 0.05 Hz. In this paper, we summarize the main characteristics of this inversion method using examples from various stations.

First, the depth resolution of this method is typically limited to the upper 50-100 m from the surface. But this is only a typical depth range we find at most stations. The depth resolution could vary with pressure-wave speed on the Earth’s surface, which in turn should be closely related to surface wind speeds. This (moving) surface pressure is the excitation source of seismic noise. Its speed can be estimated from the ratios between seismic horizontal data and vertical data. At locations with strong winds, we find that the data sense deeper structures; for example, at MLAC (Mammoth Lakes) we see the depth resolving kernels that reach 200m.

Second, colocated data are sensitive to rigidity structure (shear-modulus structure), not seismic P-wave or S-wave structure. Effects from density structure are very small. This is related to the fact that we analyze slow, almost static deformation of the medium.

Third, because the analysis is in a low frequency band, the depth resolving kernels are smooth and the main constraint on structure turns out to be its average rigidity value in the upper 50-100m. This smoothness does not pose a problem for retrieving Vs30. Our estimates for Vs30 are robust and agree with Vs30 derived from other approaches. This is because Vs30 is an averaged property in the top 30 m. But some features in Earth structure such as sharp discontinuities cannot be retrieved by our approach.

Fourth, we actually take advantage of a situation in which the ground tilt effects dominate seismic horizontal data for the chosen frequency band (<0.05 Hz). In higher frequency bands, the dominance of tilt disappears and the basic formulation needs to be modified.

Presenting Author: Toshiro Tanimoto

Authors