The inner structure of the Moon provides us some important constraints on its bulk composition, thermal state and thermal history. To infer the origin and the evolution of the Moon, information on its inner structure is essential. Various methods were applied for its estimation and the observation of moonquakes was one of the most successful methods carried out. Observations of moonquakes were carried out in the Passive Seismic Experiment of Apollo missions. It carried out about 5 and half years of network observation with 4 seismic stations and succeeded in estimating some important geophysical parameter such as seismic velocity model of the Moon or the thickness of the crust. This gave us various implications on the lunar science and its contribution is remarkable. However, limited observation of the Apollo seismic network could not fully uncover the seismic features of the Moon and many important questions in the lunar seismology are yet to be solved. On the other hand, since the termination of the Apollo observation, no seismic observation was carried out on the Moon and the Apollo seismic data is still one of the most important sources of information of the lunar seismology. In this study, I will re-analyze the Apollo seismic data in new points of view to overcome some remaining questions of the lunar seismology.

In this study, I will discuss 3 topics.

First, I tried to extend the lunar seismic network by using the data of Apollo 17 Lunar Surface Gravimeter as seismic data. The limited number of seismic stations and the limited seismic network has been a major problem in the lunar seismology. All the seismic stations were on the lunar nearside and the observable area of the network could not fully cover the lunar surface. Thus, we are missing seismic sources on the lunar farside and this is one of the reasons that the deep inner structure of the Moon is poorly constrained seismically. Additional seismic data are desired for further investigation. To obtain additional seismic information, I focused on the data of the Lunar Surface Gravimeter and succeeded in extracting seismic information from the data. I evaluated the gravimeter data in a seismological point of view and used them in seismic analyses with other Apollo seismic data. With this additional data, I examined the unlocated deep moonquakes reported in the previous study. I succeeded in identifying 5 new deep moonquakes out of 60 unlocated deep moonquakes. One of the newly located deep moonquakes was located on the lunar farside. Only 8 farside deep moonquakes were reported so far and the new farside deep moonquake can pose a new constraint on the seismic feature of the Moon. In this study, I used this deep moonquake to evaluate the deep moonquake activity of the lunar farside. Weather the seismicity differ between the lunar nearside and the farside has been an important question in the lunar seismology and it is meaningful to pose a new constraints with newly identified sources. The size-frequency distribution of the deep moonquake on the nearside and the farside of the Moon implies that the deep moonquake activity on the farside is comparable or higher than that of the lunar nearside.

Second, I extended the effective frequency range of the observation to investigate the source parameters and the source mechanisms of the lunar seismic events through spectral analyses. Apollo seismic observations were carried out with Long Period (LP) and Short Period (SP) seismometers but previous studies used the data of the two seismometers independently. Thus, their analyses were limited to narrow frequency range of each seismometer. In this study, I combined the data of the two seismometers numerically and estimated continuous spectra that cover the both frequency ranges. The combined spectrum enables us spectral analyses with wider frequency range. From the estimated spectral feature, source parameters of lunar seismic events were estimated. Source parameters of seismic events are closely related to their source mechanisms. Since the details of the source mechanisms of the lunar seismic events are still unclear, the quantitative estimations of the source parameters are meaningful. I estimated the corner frequencies and seismic moments of lunar seismic events from the spectra. The obtained corner frequencies showed wider range of values compared to previous studies and distributed between 1-10 Hz. On the other hand, the seismic moments were estimated to be smaller than the previous estimation. These results imply small stress drops of about 0.01 MPa, which is comparable with the previous estimations. For deep moonquakes, the stress drops were estimated to be 0.005~0.02 MPa. This is comparable with the stress variation caused by the tidal stress between the Moon, the Earth and the Sun. This result implies that the tidal stress is responsible of the occurrence of the deep moonquake.

Finally, I regarded the seismic data as a lunar bombardment record and attempted their new application. When we view the seismic data as an impact records, it is notable in 3 points. First, they provide 8 years of continuous observation of the lunar surface that does not suffer from the observation or lunar illumination condition like terrestrial observation of impact flashes. Second, they focus on relatively small events compared to those studied in crater countings or numerical simulations. Finally, they are bombardment records of current Earth-Moon system, unlike crater records, which are records of impact events within a geological time scale. Thus, it is meaningful to investigate the impact records left in the seismic data. In this study the spatial distribution of the impact events was examined. It is theoretically predicted that the cratering rate on the Moon is asymmetric because it is in the state of rotation-revolution synchronization. The cratering rate on the leading side of the Moon is predicted to exceed that of the trailing side. I examined whether such feature is detected with the seismically identified impact events. From the spatial distribution, I obtained leading/trailing asymmetry of 1.8±0.4. I also evaluated the observation bias of the seismic network and concluded that the cratering rate of the leading side is significantly higher than that of the trailing side even if we consider the effect of the observation bias. The higher asymmetry I obtained may imply that the feature of the source of impactors for small, meter-sized crater that is examined in this study, differ from that for kilometer size craters studied in the previous studies.

In all 3 topics, I succeeded in obtaining new understandings of the lunar seismic activity by re-analyzing the Apollo seismic data. It is true that the various limitations of the seismic data make it difficult to investigate the lunar seismic feature more in depth. However, with special care and careful data processing, the seismic data can still be used as an important source of information.

本論文は6章からなる。第1章はイントロダクションであり、月震データ解析には(1)観測点数・(2)周波数帯域・(3)解析対象の限界があるという現状を指摘するとともに、これらに改善を加えた月震イベント解析を本研究の目的であると位置づけている。第2-4章では解析方法及び解析結果について述べてあり、第2章では観測点数の改善、第3章では周波数帯域の改善、第4章では解析対象の拡大をそれぞれ試みている。第5章は得られた結果から得られる示唆を述べ、第6章で結論を述べている。

本論文では、(1)不具合のあった月面重力計データの地震計としての活用、(2)長周期・短周期計データの同時活用による解析周波数帯域の広帯域化の実現、(3)隕石衝突速度を見積もる道具としての月震データの活用、という3つの新たな手法を用いて、既存データ(アポロデータ)の再解析を行っている。そしてデータ解析を通じ、(1)深発月震活動の地域性に対する制約、(2)深発月震の応力降下量の制約、(3)小隕石の平均衝突速度の制約を行った。それぞれの解析の特長及び主たる成果は以下のようにまとめられる。

(1)アポロの月震計アレイは口径が小さいため、グローバルの月震活動分布の制約が困難であった。本論文では、重力計データを加えることによりこの困難を改善した。解析の結果、月の裏側に活発な活動をしている深発月震の巣を新たに同定した。また、活発な深発月震活動が表側に偏在しているという従来の描像は根拠が乏しいこと、表側の深発月震活動がベルト状に局在化している(深発月震ベルトの存在)という従来の描像は妥当であることを示した。

(2)アポロの月震計の帯域は狭いため、月震の震源過程を制約することは困難であった。本研究では狭帯域の長周期計・短周期計の双方のデータを同時活用することにより、この困難を改善した。様々な大きさの月震イベントに対して震源スペクトル及び応力降下量を見積もった。深発月震では、小月震になるほどコーナー周波数が増大する傾向があること、大月震から小月震に至るまで深発月震の応力降下量がおおよそ一定であること、応力降下量は地球の地震の1/100程度(0.01-0.1MPaのオーダー)であることを示した。

(3)アポロの月震計は、メートルサイズの比較的小さな隕石の衝突を数多く記録している。大きな隕石の衝突の跡であるクレーターの解析結果と比較することにより、隕石衝突の性質に関し新たな情報が得られる。隕石衝突地点の地域性の解析から、小隕石は大隕石よりも低い衝突速度を持つことを示した。

これらの成果は月科学に重要な貢献をもたらすものと考えられる。深発月震が潮汐応力のみにより発生しているか否かは、現在も論争中の問題である。本研究で示された深発月震の地域性の存在、さらには応力降下量の見積もりは、この問題に新たな制約条件を与える。大隕石と小隕石に系統的な衝突速度の違いが存在することは独創性のある発見であり、それぞれの起源や集積過程に新たな制約条件を与える。さらにこれらの成果は、将来の月ミッションの立案にも重要な示唆を与える。月震活動度の地域性は、月震ネットワークのデザインの際の最も重要な指標と言える。裏側に活発な深発月震の巣を発見したことは、未解明の月深部構造の制約につながる。

月震学はデータの量や質の制約が大きな障害となっており、既存データを有効活用することや、効果的なミッションを立案することは、多くの研究者が認める重要課題である。本研究はこの双方に本質的な貢献を与えたと認められる。

なお、本論文の第4章は、諸田智克氏・小林直樹氏・田中智氏との共同研究であるが、論文提出者は筆頭著者として、解析及び解釈において中心的な役割を果たしていると判断する。

したがって、博士(理学)の学位を授与できると認める。