金曜日セミナー 2008 年度の予定／記録

We discovered significant changes in seismic anisotropy orientation, which changed by almost 90 degrees between two portable deployments of three-component seismometers that were deployed before and after the 1995-1996 eruption sequence on Mt. Ruapehu Volcano. The fast orientation recorded on shallow events changed back again by 2002, without any other obvious manifestations of volcanic activity. We proposed that intrusion of a dike or dike system overprinted the regional stress system and reversed the maximum principal stress orientation in a localised region. This is very exciting, because it suggests that, not only does anisotropy change with a huge signal of a volcanic eruption, but it can change in response to more subtle signals. It has the potential to be used to monitor stress changes.

Here at ERI I am working together with Dr. Takao Ohminato to test whether changes in stress can be monitored in this way, by measuring anisotropy and relating it to other indicators of the state of stress and its changes over time around volcanoes in Japan. We will thus measure the background stress changes volcanoes between eruptions, and also the stress changes leading up to eruptions and intrusions. These observations will provide the data that may eventually lead to prediction tools.

我々の宇宙の中では想像を絶する高エネルギー現象が起きている。そこで、加速した宇宙線は驚くほど高いエネルギーを得て、地球を容易に突き抜けられるものすらある。宇宙線は大きく２種類に分けられる。宇宙空間を飛び交う、主に陽子からなる「１次宇宙線」とそれが大気と衝突して発生する「２次宇宙線」である。地上で検出できる宇宙線の大半は後者である。これまで、宇宙線の研究はその発生場所やエネルギー獲得のからくりに絡んで、宇宙創成の謎解きの有力手段の一つとして、第一線の宇宙物理学者、天文学者らが取り組む重要課題であった。その一方で、近年の素粒子検出技術の発達により、高エネルギー宇宙線が地球内部を覗くプローブとして使える可能性が出てきた。特に、物質透過力の卓越したミュオン（ミュー粒子）成分やニュートリノ成分を利用すれば全く別の視点から現在の地球内部を知ることが出来るだろう。いわば、これら粒子の透過度の違いを利用したミュオンやニュートリノの地球のレントゲン写真である。ニュートリノを用いれば地球の厚いところすなわち、地球全体を透かし撮りでき、これまでの地震学的情報に対して全く独立な情報が得られるだろう。また、ミュオンを使えば、火山体の密度構造など新しい知見が得られるだろう。本セミナーでは火山への適用例を中心に宇宙から地球へと飛来する高エネルギー粒子を用いた新しい地球科学を紹介する。

Southeast Asia, that includes northeast India and the Indo-Burma-Sumatra-Java arc, is seismically one of the most active regions in the seismic map of the world. Several great (M 8.0) and large (M 7.0) earthquakes occurred in this region during the recent years, like the 1897 great Shillong and the 1950 great Assam earthquakes (Ms 8.7) and most recently the 2004 Sumatra-Andaman mega thrust earthquake (Mw 9.3) and the 2005 Nias earthquake (Mw 8.7).

A large amount of broadband digital data are now available with upgraded permanent local networks in northeast India and temporary networks in the Andaman-Nicobar islands. Precise locations, fault plane solutions and seismic tomography provide us a better understanding of the tectonics and earthquake processes in the northeast India region. Pop-up tectonics of the 1897 great earthquake, strike-slip mechanisms of the transverse/strike slip faults in northeast India, atypical subduction tectonics beneath the Indo-Burma ranges and typical oceanic subduction and back arc tectonics at the Andaman-Nocobar-Sumatra arc are discussed using these recent digital data. Large amount of data are now available for regional tomography in the Andama-Sumatra arc.

近年，地震学や物理探査などの分野で地震波干渉法（ Seismic Interferometry ）がさかんに利用されるようになっている。この手法によると，通常の地震観測や探査においてはノイズと考えられてきた常時微動やコーダ波から，相互相関解析や自己相関解析を通して，地下構造の推定に役立つグリーン関数が求められるといわれている。

本講演では，地震波干渉法の理論的背景に関して，講演者がこれまで証明を行ってきた 2 つのモデルを中心に紹介する予定である。 1 つは水平成層構造中をスカラー波が鉛直に伝わる場合で，地震波干渉法の端緒といわれる Claerbout (1968) と類似した問題設定である。もう 1 つは均質構造中をスカラー波が様々な方向に伝わる場合であり，このモデル化は微動の空間自己相関法を提唱した Aki（1957) を参考にしている。これら 2 つのモデルは，現実の複雑性を考えると，単純すぎるかも知れない。しかし，問題の本質を理解する上で，単純なモデルはとても有益である。

This talk focuses on the use of the seismic ambient noise to perform 'empirical synthesis' of seismograms with cross correlations functions. These time series can be used to image the Earth interior and to monitor slight changes of seismic velocities. We discuss the limitations of this technique due to the characteristics of the microseisms and emphasize the part played by scattering. We present improvements of the technique that could increase the available number of virtual seismograms. We present applications to the structure of the Alps, and to the monitoring of a volcano and of a fault zone.

最近、 2004 年スマトラ沖地震による余効変動がグローバルな GPS ネットワークや衛星重力計 GRACE などによって捉えられている。余効変動とは、地震時の瞬間的な地殻変動に引き続いて観測される transient な変動のことをいう。大地震に伴う余効変動は幅広い時空間スケールを持っており、地震直後から数 10 年以上も継続することが知られている。代表的なメカニズムには、地震直後からの短期的な変動を説明するための、地震断層の周囲がゆっくりすべる余効すべりと、地震時に蓄積した応力が地殻下部やマントルの粘性によって緩和することにより生じる、数年以上のスケールを持つ粘弾性変形の 2 つがある。両メカニズムによる応力変化の様式は全く異なるため、海溝型地震のサイクルを理解する上で、両者を適切に分離することは重要である。そのためには、両モデルをより現実的なものへ改良することが求められる。このため、発表者は、長期的な余効変動のメカニズムと考えられている粘弾性変形の理論モデルの改良に努めてきた。セミナーでは、発表者が行ってきた博士研究とその後、ドイツ留学中に行った理論研究の結果について述べる。博士研究は、球対称成層構造を仮定したモデルでの計算手法を世界で初めて確立したものであり、ドイツでは、さらにスラブによる粘性の水平不均質の取り込みを行った。これらのモデルは、全球をモデル領域とすることで重力場の時間変化も自然かつ精密に計算でき、 GRACE 衛星による重力データとの高精度な比較を可能とすることに成功したものでもある。発表では、途中、国土地理院での経験についても若干ふれる予定である。最後に、今後の研究テーマや地震研で行われている研究との関連について述べる。

In this presentation an overview will be given of the variety of volcanism along the Melanesian Volcanic Arc with special reference to the eruption mechanism, depositional processes and resulting volcanic landforms. The presentation will be focused on four regions along the arc located in central Vanuatu. Ambae is a NE-SW elongated island forms a characteristic rift zone with scoria cones, lava spatter cones, and lava flows developed over a basal sheet like basaltic lava shield. Ambae has complex nested caldera system but lack of widespread evolved pyroclastic succession suggesting that caldera formation was rather a result of gradual caldera subsidence driven by lateral drainage of the magmatic plumbing system through flank vents forming a chain of rift axis, monogenetic volcanoes. This caldera forming mechanism is inferred to be similar in Ambrym where two frequently active volcanoes in a 12 km across caldera provide huge volumes of tephra accumulating in the caldera. It has been proposed that ~2200 years ago a phreatomagmatic eruption formed the caldera and constructed a giant tuff cone. However, as in Ambae there is no supporting evidence for this and therefore caldera formation is likely to be resulted from drainage of degassed magma along lateral dykes. In Tanna an andesitic caldera gradually fill up by overlapping scoria/tephra cones. Kuwae caldera is a place where a major climate-modifying eruption in the mid-15th century inferred to be as the fourth largest eruption in the past 10 ka. The inconsistency between conclusions reached from oral traditions and geological data highlights the fact that good critical analysis of all types of evidence is needed before conclusions can be reached about past cultural "catastrophes" or also possible global influence on climate. The cultural diversity and the small geometrical scale of the presented islands require complex approach to volcanic hazard managements which is a main focus of present research in the region.

粉体の摩擦や流れなどの現象は地震や火砕流，地滑りなど多くの地球科学現象と関連があり，粉体材料の制御は工業的技術としての重要性も高い。しかしながら，その研究の多くは経験則の積み重ねとなっており，粉体の基礎的な物理には未解決部分が多く残っている。そこで我々はシンプルな粉体流の実験を行い，様々な計測方法で流れの様子をとらえ，粉体流の流れと停止の機構解明に取り組んだ。具体的には，擬二次元砂山流の表面流速と流速揺らぎをPIV(粒子画像相関法)とSVS(動的光散乱法)を用いて計測した。また，砂山流の表面と深部で流速が著しく変化している様子をPTV(粒子追跡法)や画像相関を基にした動的不均一性の計測を用いて定量的に議論した。講演では，粉体流を計測するために我々が用いたこれらの光学的手法の紹介を中心に，時間に余裕があれば粉体流が停止する現象をある種の相転移とみなすアプローチについても触れたい。

There is much evidence, from seismology, geochemistry and geology, for compositional heterogeneity at all lengthscales in Earth's mantle, and also for complicated structures at the core-mantle boundary that may have a combined thermal, chemical and post-perovskite phase change origin. Additionally, the dynamics of the mantle determines the heat flux out of the core through Earth's history, hence the history of the geodynamo, and lateral variations in CMB heat flux strongly affect the dynamics of the geodynamo. Here, these topics are investigated using numerical simulations of mantle convection and plate tectonics that include compositional differentiation caused by melting and crustal production, as well as a multi-mineralogical treatment of phase transitions. After billions of years, the mantle is compositionally heterogeneous at all lengthscales in both major- and trace-elements, consistent with observations, and compositional heterogeneity has a much flatter spectrum than thermal heterogeneity. If subducted mid-ocean ridge basalt (MORB) is substantially denser that other material at the CMB, then it settles into a partial layer of material above the CMB, which has a strong effect on seismic heterogeneity, on geodynamo evolution, and on the pattern of lateral heat flux anomalies. The post-perovskite transition has a small but significant effect on the dynamics, acting to destabilize the lower thermal boundary layer, and it can also have a strong contribution to seismic heterogeneity at that depth. The composition-dependent phase transitions around 660 km can act to filter chemical heterogeneity, causing crustal enrichment in the transition zone, and depletion at the top of the lower mantle. Results indicate the importance of somewhat uncertain mineral physics parameters on the evolution of the mantle and core.

『三次元地殻活動シミュレーション・システム』により，プレート運動に伴う大地震発生や多様な地殻活動現象をプレート間の力学的相互作用の帰結として統一的に理解することが可能となりつつある。これを発展させた『地殻活動予測システム』の構築とそれに基づく予測可能性の追求の為には，シミュレーション・システムの高度化と共に，多様な観測・データ解析を通した地殻活動モニタリングによる情報をシミュレーションに取り込むことが重要になる。セミナーでは，地殻活動シミュレーションの基本的な考え方を示し，さらに，大地震の発生予測に於いて，地殻内応力状態の推定が本質的に重要であることを示す。また，地殻活動モニタリングの最新の結果を示すと共に，現実的な大地震発生シミュレーションの実現の為には，如何なる観測・データ解析の情報を取り込む必要が有るかを述べる。

Several slow-slip events beneath the south flank of Kilauea Volcano, Hawaii have been inferred from transient displacements in daily GPS positions. To search for smaller events that may be close to the noise level in the GPS time series we compare displacement fields on Kilauea's south flank with displacement patterns in previously identified slow slip events. Matching displacement patterns are found for several new candidate events, although displacements are much smaller than previously identified events. One of the candidates, on June 2, 2000 is coincident with a microearthquake swarm, as are all of the previously identified slow-slip events. The microearthquakes follow the onset of slow slip, implying that they are triggered by stress changes during slip. The new slow-slip event brings the total number of events on Kilauea, between 1997 and 2007, to eight, the smallest having MW and the largest MW. While the recurrence time between the four largest events is 2.11 ± 0.01 years, the repeat time for the eight events is 0.9 ± 0.6 years. We invert for the fault geometry and distribution of slip during the slow-slip events. The optimal source depths (∼ 5 km), assuming uniform-slip dislocations in an elastic half-space, are considerably shallower than the accompanying swarm earthquakes (6.5 - 8.5 km), which would place the quakes in a zone of decreased Coulomb stress. Inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths, such that the quakes are now located in a region of increased Coulomb stress. We also invert for time-dependent fault slip directly from the 30 second GPS phase observations, constraining the source to the optimal uniform slip geometry. Based on these inversions, the larger events last between 1.5 - 2.2 days. The data are unable to resolve migration of slip along the fault. The temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate-and-state friction, making the swarm earthquakes coshocks and aftershocks of the slow-slip events.

「アウトリーチって何をやっているんですか？」と一般の方に聞かれた時の答えはもう持っています。ところが、所内の人に聞かれた時が困る。ズバリ一言で言えないどころか、本当に何をやっているんだろう、と問い続けてきた半年間．．．でしたが、本セミナーでお答え致します。

5 年半前に地震研にアウトリーチ推進室が発足した経緯は皆さんご存知の通りです。金森先生が委員長でらした外部評価での進言でした。一方で、科学史や科学技術社会学に目を転ずれば、アウトリーチ・科学コミュニケーションといった現象は、戦後科学の流れの中ではごく自然に発生したものと言えます。本セミナーでは、欧米と日本との戦後科学技術の流れを比較しつつ、なぜ科学コミュニケーションの時代が到来したのか、その背景を説明し、欧米諸国から学べることは何か、それらを踏まえた科学コミュニケーションの取るべきアクションは何か、提起致します。

また、同じく広報室を配置した東大理学部や工学部と比較して、地震学・地震研のアウトリーチについて半年間考察したことを提言致します。この機会に、その方向性でいいのか、地震研はアウトリーチ推進室に何をやらせたいのか、ご一考頂ければ幸いです。

The rifting of continents and eventual formation of ocean basins is a fundamental component of plate tectonics, yet the mechanism for break-up is poorly understood. The East Africa Rift system (EARS) is an ideal place to study this process as it captures the initiation of a rift in the south through to incipient oceanic spreading in north-eastern Ethiopia. A variety of seismic methods can be used to test models of rifting. I will summarise observations from recent seismic experiments in the Ethiopian Rift and Afar. Special attention is given to evidence of seismic anisotropy in the transitional part of the Ethiopian Rift. Collectively, these observations support models of magma assisted rifting, rather than those of simple mechanical stretching.

The waves generated by faulting represent the primary threat posed by most large earthquakes. The effect of complex geological structure, such as sedimentary basins, on earthquake ground motion is a source of particular concern. The usual approach to predicting such effects is to image the Earth's crustal structure, and then to propagate waves through that structure using numerical modeling techniques. At long periods it is possible to extract reliable phase and amplitude response for the elastodynamic Green's function from the ambient seismic field using deconvolution. These Green's functions can be used to improve elastic and anelastic structural models, which will improve ground motion prediction. We demonstrate the accuracy of the approach by predicting complex ground motion for a moderate (Mw = 4.6) earthquake in southern California as recorded in the Los Angeles Basin. This experiment suggests a novel approach to seismic hazard analysis in which ground motion from hypothetical future earthquakes may be simulated directly, without the need for modeling the detailed heterogeneity of the Earth's crust as an intermediate step.

In the last decade, several statisticalmodels for both short-term and long-term earthquake occurrence have been developed and tested, proving a fairly good performance in earthquake forecasting. In particular, a number of studies haveconsidered the ETAS model, for short-term earthquake clustering, and the BPT distribution, among the long-term renewal models. However, statistical models, describing the behavior of large sets of earthquake, fail in forecasting with high reliability the behavior of a single earthquakesource.

On the other hand, in the recent years,studies about earthquake mechanics and laboratory experiments onthe friction constitutive laws of faults have increased our knowledge of the earthquake nucleation process. Nevertheless, thesephysical studies allow only simulations of earthquake source behavior, and have never proved predictive capability in real cases.

It is hopeful that the integration of physical and statistical approaches, that up to now have been studied by different groups of seismologists, can improve our ability of describing the earthquake process with the aim of a better predictive capability.

The stress interaction among seismic sources is taken as an example of physical modeling of the earthquake process. This modeling requires two distinct ingredients: (a) the computation of the co-seismic Coulomb stress change produced by slip on a fault, on other faults that can potentially generate subsequent earthquakes; and (b) the Dieterich (1994) rate-and-state constitutive law, as a possible physical justification of the Omori law describing typical aftershock decay.

First, in this presentation I consider the Dieterich's rate-and-state model as a component that can be included in the ETAS model for the time decay of the triggered events, allowing a substantial reduction of the number of free parameters neededin the purely statistical model. Lastly, an example of a fully physics based model of short-term earthquake interaction with only one free parameter, based on the computation of the Coulomb stress change, is considered. The application of both models is made on aseries of moderate seismic events occurred in Central Apennines (Italy) in 1997.

• Console, R., Murru, M., and Catalli, F., 2006, Physical and stochastic models of earthquake clustering, Tectonophysics, 417, 141-153.
• Console, R., Murru, M., Catalli, F., and Falcone, G., 2007, Real time forecasts through an earthquake clustering model constrained by the rate-and-state constitutive law: Comparison with a purely stochastic ETAS model, Seis. Res. Lett., 78, 49-56.
• Catalli, F., Cocco, M., Console, R. and L. Chiaraluce, 2007, Modeling seismicity rate changes during the 1997 Umbria-Marche sequence (central Italy) through a rate- and state-dependent model. J. Geophys. Res., 113, doi:10.1029/2007JB005356.