[S-SS07] 地震波伝播：理論と応用

[SSS07-01] Unraveling Shallow Ice Structure with Active and Passive Seismic Surveys at West Antarctica

*Nakata Nori^1,2、Zhang Zhendong¹、Qin Lei²、Karplus Marianne³、Kaip Galen³、Walter Jake⁴、Tulaczyk Slawek⁵ (1.Massachusetts Institute of Technology、2.Lawrence Berkeley National Laboratory、3.University of Texas at El Paso、4.University of Oklahoma、5.University of California, Santa Cruz)

キーワード：雪氷圏地震学、南極、リーキーモード

Seismic signals are powerful to characterize ice and subsurface structures, and we image shallow ice structure near the West Antarctic Ice Sheet (WAIS) Divide with active and passive seismic data and the Thwaites Glacier eastern shear margin with passive data. The eastern shear margin has a planned active-seismic survey in the 23/24 field season. Passively recorded seismic ambient noise can be used to extract wave propagations between receivers and provide complementary information to the active seismic data. High-frequency energy is high in active-seismic data, but surface waves are often more prominent in ambient noise records due to their source mechanisms. We use a surface-wave dispersion analysis, an analysis of horizontal-versus-vertical (H/V) ratio, wave-equation-based seismic imaging, and reflection migration for imaging firn-air, firn and shallow ice layers. We identify the leaky modes following the direct waves by examining their behaviors of energy decay and particle motion. The leaky modes are guided P waves and contain P-wave velocity information and exist possibly due to the sharp velocity contrasts at the firn layer. Together with normal modes, we use a joint inversion of leaky and normal modes for P- and S-wave velocity estimation in the near-surface. Because seismic ambient noise always exists, we also use them for time-lapse analysis of the ice structure for understanding its dynamics.