The ZH ratio method for long-period seismic data: Inversion for S-wave velocity structure

Tomoko E. Yano, & Toshiro Tanimoto

Published 2009, SCEC Contribution #1166

Particle motion of surface waves, in addition to phase and group velocities, provides useful information for S-wave velocity structure in the crust and upper mantle. In this study, we develop a method for the ZH ratio, the ratio between vertical and horizontal surface amplitudes of Rayleigh waves, using data from GEOSCOPE network.

We measure the ZH ratios for frequencies between 0.004 Hz to 0.05 Hz (period 20-250s) and invert them for S-wave velocity structure beneath each station. Our analysis shows that the resolving power of the ZH ratio is limited and final solutions display dependence on starting models; in particular, the depth of Moho is an important parameter to get reliable results. Thus, initial models for the inversion need to be carefully constructed. We adopted PREM and CRUST2.0 in this study for all but one station (ECH). The eigenvalue analysis of the least-squares problem that arises for each step of iteration shows the dominance of one eigenvalue and explains the cause for the initial-model dependence of inversions. On the other hand, the sensitivity of the ZH ratio to near-surface structure is unique and thus the ZH ratio provides complementary information to phase or group velocities. Application of this method to GEOSCOPE data show that low velocity zones exist beneath some stations near hotspots. While the formal depth-resolution analysis does not warrant the existence of low-velocity anomalies for depths around 50-200 km, our tests with different starting models confirm that the models with low-velocity anomalies fit the ZH ratio data better. Such low velocity zones are found near Hawaii (KIP), Crozet Island (CRZF) and Djibuti (ATD) but not near Reunion Island (RER). It is also found near Echery (ECH) due probably to a geothermal activity in the region.

Citation
Yano, T. E., & Tanimoto, T. (2009). The ZH ratio method for long-period seismic data: Inversion for S-wave velocity structure. Geophysical Journal International, 179, 413-424. doi: 10.1111/j.1365-246X.2009.04293.x.