11:30 〜 11:45
[PPS06-04] Quantitative evaluation of seismic scattering of the Moon and implications for future seismic explorations
キーワード：惑星地震学、地震波散乱、将来探査、月、アポロ
For the purpose of the direct exploration of the lunar interior, seismic observations were carried out in Apollo 12, 14, 15, 16, and 17 missions. The observed seismic waves not only provided us with a ticket to the internal structure but also opened a way to the comparative planetary seismology. Since the internal structure strongly reflects the formation process and/or the subsequent evolution, it is significant to constrain the inner structure and compare it among various planets.
In this study, we pay special attention to the subsurface structure which tells us how a planetary surface evolved until today since its formation. One of the surprising facts brought by the Apollo seismic observation is that the lunar seismic signals show a spindle-shaped form with an extremely long duration (typically 1 – 2 hours). While this characteristic waveform reflects a large difference in the subsurface structure between the Moon and our planet, it is still an open issue how these differences are made and what kind of structure essentially characterizes the wave propagation on the Moon. According to previous works, it is considered that the very heterogeneous structure within the upper crust (so-called megaregolith) plays a dominant role in the influence on wave propagation. However, the detailed structure of the region remains uncertain. Since the intense seismic scattering due to heterogeneous structure makes it difficult to precisely identify the seismic phases (such as P and S), understanding the scattering structure is a crucial step towards the improvement of the lunar internal structure model.
We tackled this long-standing and most complicated problem (namely “seismic scattering”) from the numerical approach, which was barely performed in this field before. Making full use of the recent computational technology allowed us to perform a full 3D simulation of seismic wave propagation, which led to the first direct comparison between synthetics and the Apollo data at exactly the same frequency band for all three components (radial, transverse, and vertical). As a result, we succeeded in reproducing the Apollo seismic data, resulting in the quantitative evaluation of the scattering properties of the megaregolith. This achievement opened a quantitative comparison of the scattering environment among various planets, which we suppose would invoke the further investigation of surface evolution of solid bodies.
In the presentation, we will show forward modeling results of lunar seismic waves, followed by the quantitative comparison of the scattering environment between the Earth, the Moon, and Mars. Also, we will give constructive implications for future planetary seismic explorations.
In this study, we pay special attention to the subsurface structure which tells us how a planetary surface evolved until today since its formation. One of the surprising facts brought by the Apollo seismic observation is that the lunar seismic signals show a spindle-shaped form with an extremely long duration (typically 1 – 2 hours). While this characteristic waveform reflects a large difference in the subsurface structure between the Moon and our planet, it is still an open issue how these differences are made and what kind of structure essentially characterizes the wave propagation on the Moon. According to previous works, it is considered that the very heterogeneous structure within the upper crust (so-called megaregolith) plays a dominant role in the influence on wave propagation. However, the detailed structure of the region remains uncertain. Since the intense seismic scattering due to heterogeneous structure makes it difficult to precisely identify the seismic phases (such as P and S), understanding the scattering structure is a crucial step towards the improvement of the lunar internal structure model.
We tackled this long-standing and most complicated problem (namely “seismic scattering”) from the numerical approach, which was barely performed in this field before. Making full use of the recent computational technology allowed us to perform a full 3D simulation of seismic wave propagation, which led to the first direct comparison between synthetics and the Apollo data at exactly the same frequency band for all three components (radial, transverse, and vertical). As a result, we succeeded in reproducing the Apollo seismic data, resulting in the quantitative evaluation of the scattering properties of the megaregolith. This achievement opened a quantitative comparison of the scattering environment among various planets, which we suppose would invoke the further investigation of surface evolution of solid bodies.
In the presentation, we will show forward modeling results of lunar seismic waves, followed by the quantitative comparison of the scattering environment between the Earth, the Moon, and Mars. Also, we will give constructive implications for future planetary seismic explorations.