金曜日セミナーのトップへ戻るIn geodynamics, a view has emerged that plate tectonics is simply the surface expression of mantle convection and the lithosphere-convecting mantle must be understood as one continuous system. However, there is a fundamental problem: all numerical models designed to self-consistently produce plate tectonic-like behavior require very very weak plate boundaries, but the mechanical behavior of mantle materials produced in the laboratory are much stronger than those required by models. One possible resolution to this problem is that plate boundary strength is controlled by the development ofheterogeneous structures of melt-rich shear zones. We have produced such structures in laboratory deformation experiments, and are working to understand their applicability to conditions in the Earth with two approaches: 1) a physical understanding of the process occurring in experimental materials and the scaling properties from continuum mechanics, and 2) predictions for their seismic properties and spatial distribution of melt-rich networks in various geodynamic settings. I will discuss both of these efforts, with an illustration of seismic data from the East African Rift in Ethiopia. This talk is a work-in-progress towards a unified understanding of rheological and seismic properties as clues to the dynamics of plate boundaries.
Caldera collapse is the failure and subsidence during a volcanic eruption of the rocks overlying a crustal magma reservoir. Caldera-forming eruptions have major environmental impact. Collapse happens because magma is removed from the reservoir, but all eruptions remove magma, yet not all cause collapse. We wish to explain why some eruptions form calderas,while others stop before doing so, and why the largest non-caldera eruptions are larger than the smallest caldera eruptions. The seminar will discuss these problems and try to outline a physical framework to address these questions based on eruption of volatile-saturated magma from a magma reservoir of simple shape embedded in elastic crustal rocks. Thekey control parameters that emerge are initial volume and burial depth of the reservoir, giving hope that geophysical observation prior to eruptionscan be used to improve hazard assessment.
Convective stirring acts to reduce the size of the heterogeneities introduced into the mantle at subduction zones. These heterogeneities reappear as isotopic anomalies both in mid-ocean ridge and ocean basalts, which modern techniques of mass spectrometry allow us to sample with a much improved density. Comparing the wavenumber spectrum of mid-ocean ridge basalts, or the frequency spectrum of volcanic series, with the spectrum predicted by the fluid mechanics of tracer dispersal provides new insights into the dynamics of mantle stirring by convection and plume ascent. This approach allows us to relate mantle geochemistry with thermal dissipation, to discuss how fast heterogeneities disappear, and to decide whether the mantle is mixed by the global circulation rather than by the drag of sinking plates.
Water enters the Earth‘s mantle at trenches by subduction of oceanic lithosphere. Most of water immediately returns to the atmosphere through island arc volcanism, but part of it, which could be retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected as deep as the Earth‘s mantle transition zone (410-660km depth). Water carried by subducted slabs is the only source of water of the deep mantle. It makes the understanding of the processes of water transfer from a deep slab to the mantle a key element to estimate the water content and distribution in the Earth‘s mantle, especially in the transition zone. Here we present results from numerical modeling of two possible processes for slab dehydration and discuss them in regard to the Transition Zone water content: i) Within the Earth‘s mantle transition zone by diffusion of water out of the slab, and ii) at the base of the upper mantle (660km depth) by percolation of hydrous fluids formed by water exsolution from the slab. Regarding stagnant slab dehydration by diffusion, we consider temperature dependent water solubility in transition zone olivine polymorphs (Wadsleyite, Ringwoodite). We show that if water solubility is decreasing with increasing temperature, the expected water flux reduction is mitigated by a self adjustment of the water concentration gradient. This feed-back effect is induced by the coupling of heat diffusion (cooling of a stagnant slab) and water diffusion (dehydration of a wet stagnant slab into a dry mantle above it). The water concentration is displaying a local maxima just below the slab-mantle interface that could reach the solubility limit of the mineral and trigger melting. When slab eventually reaches the lower mantle (660km depth), due to its low solubility in lower mantle minerals, the remaining water is likely to be released as anhydrous fluid, during the spinel-postspinel phase change. The dynamics of this fluid phaseis investigated through a 1-D model using two-phases flow theory, in which a source term has been introduced to take the fluid precipitation into account. The competition between the advective transport by the descending slab and the buoyant rise has been explored for the all range of possible properties of solid and of fluid phases. The most likely behavior is an effective compaction resulting in an accumulation of fluid at and below the phase change. The pressure difference between the fluid and the matrix increases continuously, exceeding the yield strength of rocks. As a result, cracks would initiate and evolve toward the formation of dykes. Thus, it is expected that water comes back to the upper mantle by the way of dykes propagating along the direction of the maximum compressive stress. Those two processes are likely to make the transition zone rather wet in the vicinity of a subducting slab.
In the continental upper crust, deformation is brittle and faults produce earthquakes. In contrast, deformation in the lower ductile crust corresponds to larger shear zones. If it is relatively easy to measure in situ deformation rates at the surface of the crust using for example GPS data; it is much more difficult to determinate in situ values of strain rate in the ductile crust. Such strain rates can only be estimated in paleo-shear zones.
We propose a new method to measure local strain rates in paleo-shear zones, coupling quantification of deformation and geochronology. We applied that method on the Ailao-Shan - Red River Shear Zone (ASRRsz) in SE Asia for which we dispose of large-scale estimates of the bulk strain rate. In outcrops where several generations of syntectonic dykes are present, the strain rate can be measured by quantifying the deformation and the age of each dyke. On one hand, we used a method of surface restoration to quantify the stretching and the minimum shear strain γ of boudins trails corresponding to the deformed dykes. On the other hand, we determined the emplacement age of each dyke from Th-Pb ages of monazites measured by ion microprobe.
In rocks from the same shear zone, we calculated the paleo shear-stress using the mean sizes of recrystallised quartz grains (Stipp and Tullis, GRL, 2003). If these estimates are paired with the corresponding temperature of deformation estimated from quartz LPO’s, and introduced in a classical power flow-law strain rates can be calculated.
We discuss the results of the two methods for measuring local strain rates with respect to the large-scale bulk strain rate constrains and thermo-mechanical numerical models of lithospheric strike-slip shear zones taking into account shear heating both in the brittle and ductile deformation fields (Leloup et al., 1999, G.R.L.). We stress out the importance of taking into account the ductile deformation in main fault zones in order to built accurate thermo-mechanical models.
火山では通常の地震とは異なる多様な地震波形が観測され、それはそのまま火山内部の物理現象の多様性を反映している。火山で発生する地震波を解析することは火山内部を覗く有力な手法のひとつである。これまでの火山地震に関する研究を振り返ると、観測された複雑な地震波形が十分に解析・解釈されてきたとは必ずしも言えない。このことは、火山で発生する地震の震源について理解が十分に進んでおらず、断層モデルのように確立された震源モデルが火山地震の分野には未だに存在していないことに端的に現れている。震源の理解という意味では、火山地震学は普通の「地震学」に大きく遅れをとっており、通常の「地震学」に追いつき、追い越すために何をすべきか、何ができるかを考える時がきていると思われる。
セミナーでは、火山ではどのような地震波が観測されてきたか、それらはどのような物理現象として理解されてきたのか、をレビューする。更に、通常の地震学における「断層モデル」に対応する、火山地震の震源モデルの複数の候補について議論する。また、観測波形から、より多くの情報を引き出すために必要な手法開発についても述べたい。
アポロ計画終了から 30 年以上経過し、月にはもう不思議なところはないのでは・・・と多くの方が考えているのではないでしょうか？しかし、 90 年代の米国の 2 機の無人月探査機クレメンタインとルナプロスペクタによって、月の内部構造の複雑さや、水資源の眠る可能性が示唆されはじめました。近年、米中対立の影響か、アポロ時代以来かつてなかった規模での宇宙開発競争が始まろうとしています。そんな中で日本も有人探査を視野に入れた月開発に参加しようとしています。セミナーでは、最近明らかになってきた月の科学の情報と月開発をめぐる世界情勢、そして、いよいよ来年度に打ち上げられる予定の日本の月探査機 SELENE（セレーネ）の紹介をします。また、演者が開発運用に係わっているセレーネ搭載分光カメラで月の地下構造を探る試みと、その結果が地球科学に与える影響について議論します。
地震研究所にアウトリーチ推進室が 2003 年に発足して以来、 4 年目を迎える。今春、専任教員が気象庁出身者から国土地理院出身者に交代したのを機に、今までのアウトリーチ推進室の活動を振り返るとともに、今後の地震研究所のアウトリーチ活動のあり方について考察と提案を行う。
まず、自らのアウトリーチとして、 GPS 等に関する過去の業務経験及びそれらを通して得られた研究スタイルや学際分野への関心について簡単に述べる。また、着任直後に感じた地震研究所の印象についても触れる。
アウトリーチを「研究開発を行う組織・機関が一般社会に向けて教育普及・啓発活動等の働きかけを行うこと」と定義した場合、研究所の全構成員がアウトリーチに関係することになる。この中で、アウトリーチ推進室は、(1) 研究成果の還元の効率化と、(2) 情報発信のフィードバックを基本的な役割としてきた。(1)については、一般的な広報に加え、所外対応の一元化、マスコミ関係者との月例の懇談の場、出前講義、中高生等の訪問・見学の受け入れ、公開講座・一般公開の実施、報道のモニタリングなど多数の実績がある。一方、(2)はアウトリーチとしても新しい視点であるものの、その実施は今後の課題となっている。
セミナーでは、国内外の関係機関や隣接分野におけるアウトリーチ活動の事例も踏まえ、地震研究所のアウトリーチの目的、対象、アプローチを再整理し、今後、所全体として、アウトリーチ推進室として、そして専任教員として取り組むべき課題と活動について提案する。所全体としてのアウトリーチには、各自のアウトリーチの心が不可欠であり、専任教員によるインタビューを早期に実施したい。今後、自治体を含む行政機関や海外の研究機関に対するアウトリーチも重要である。また、アウトリーチを行う他の機関との連携も深める必要がある。
Rock salt is a candidate material for construction of a telescope detecting ultrahigh-energy neutrinos by acoustic emission measurements. These ultrahigh-energy neutrinos are generated, for instance, by the collision of galaxies or supernova explosions. Interaction of these ultrahigh-energy neutrinos with matter is extremely seldom. Therefore, the telescopes have to have dimensions of kilometers in all directions and should be placed in the ocean, or in the polar ice, or in salt domes. The economical feasibility of an acoustic neutrino detector strongly depends on the spacing between the acoustic sensors. In this paper we will report on our experience of acoustic wave propagation and wave attenuation in rock salt in the frequency range of 1 to 100 kHz and some conclusions with respect to the usefulness of rock salt as a neutrino detector. The experience bases on long-term acoustic emission measurements in a salt mine.
Cape Verde islands, located in the Atlantic around 450 km west of Senegal, is a classic hot spot. It has long-term volcanism (~20 Myr), a bathymetric swell (2 km shallower than seafloor of a similar age), and a geoid anomaly (8 m). The archipelago is also volcanically active, one peak (Fogo) last erupting in 1995, and with reports of felt seismicity on the island of Santo Antao.
The islands emit hydroacoustic signals that are detected across the Atlantic Ocean basin. We investigated the source of these by correlating hydroacoustic detections with the seismic network operating on the islands. We find that volcano-tectonic events in the volcanic edifice correlate with the hydroacoustic events. The volcanic edifice slope and the position of the source relative to the SOFAR channel plays a role in the long-range hydroacoustic detectability of Cape Verde events.
The islands are also fixed in the hot spot reference frame because they stand on the African plate. Consequently, plume buoyancy source that causes the bathymetric swell (and presumably the geoid anomaly as well) is also in a fixed relation to the lithosphere. This makes Cape Verde an ideal place to test the prevailing ideas of what processes forms the hot spot swell. We report on a two-year long seismic deployment on Cape Verde to investigate its structure, focusing on the shallow mantle structure. Our study shows that the mantle beneath the islands consists of a seismically fast, low density layer.This provides strong support for the model that bathymetric swells are formed by changes in mantle composition due to the melting process that formed the volcanic edifice, and that lithospheric reheating and dynamic pressure are not involved in raising the bathymetry in the vicinity of the islands.
2004 年 12 月 26 日歴史的に M8 クラスの地震しか発生していなかった場所でスマトラ・アンダマン巨大地震が発生した。津波波形の解析結果をこれまでに発表されている地震波形の解析結果や地殻変動の解析結果と合わせて説明すると伴に、大地震の繰り返しについて考察する。
北海道太平洋沿岸から南千島にかけて見ると、 1963 年エトロフ沖地震の繰り返し地震として 1995 年エトロフ島地震が発生したが、 1963 年の地震の南側だけしか破壊しなかった。 1968 年十勝沖地震の繰り返し地震として 1994 年三陸はるか沖地震が発生したが、この地震も 1968 年の地震の南側の一部を破壊しただけだった。 1952 年十勝沖地震の繰り返し地震として 2003 年十勝沖地震が発生したが、地震波解析はともかく津波を見る限り、 1952 年十勝沖地震の震源域は 2003 年のそれよりも大きいと言わざるを得ない。 1973 年根室半島沖地震の 1 つ前の繰り返し地震と言われる 1894 年根室半島沖地震は津波波形から 1973 年の震源域よりも明らかに大きいことが分かった。さらに津波堆積物の研究から北海道では 500 年に一度、巨大地震が発生していたと言われている。
宮城県沖でも 2005 年宮城県沖地震は1978年宮城県沖地震の一部だけしか破壊しなかった。さらに 1 つ前の宮城県沖地震（ 1936 年、 1937 年）も 1978 年の震源域を分けて破壊していたと言われている。南海トラフでも昭和の東南海・南海地震よりも安政の東南海・南海地震が大きいのは津波の波高をみても確かであり、 1707 年宝永地震では東南海と南海が同時に破壊している。この様に、記録に残る繰り返し大地震を多く解析・研究してきたが、破壊域までそっくり同じだと言える大地震は経験していないようだ。つまり、繰り返し地震であってもその最終的な大きさを規定することはかなり難しいことが容易に想像できる。
Since grain and interphase boundaries (i.e., interface) are the only structural defects that connect three dimensionally in Earth’s interior, they play important roles on many dynamic geologic phenomena that would not appear if the Earth were made of a single crystal! How do these amazingly small zones affect geodynamics? The boundaries are central to diffusion, grain growth, grain boundary sliding, fracture, fluid production and migration, source and sink for dislocations and vacancies, and hence control the fundamental micro-processes for dynamic motion in the Earth interior.
In this talk, I will present my work on investigation of interfaces in rocks mainly focused on width (w) of interfaces and solute enrichment (=solute concentration at interface / crystal lattice). These two values are critical to evaluate the role of interfaces in the Earth. Further, I will show how we can use these quantities to solve geochemical and geophysical problems.
Since the 2002 Denali earthquake, my research group at the University of Colorado has been working to improve the precision and accuracy of high-rate (10-0.03 Hz) GPS. Unlike traditional GPS applications where one position is estimated each 24 hours, in high-rate GPS a new position is estimated at each time interval. The advantage of high-rate GPS is that it it opens up new geophysical problems for investigation: fault rupture for large earthquakes, improved determination of static offsets, volcano monitoring, and immediate measurements of afterslip. A disadvantage of high-rate GPS is that it is very sensitive to the geometry of the satellites in the sky and to multipath sources on the ground. We have developed new methods to reduce these geometric systematic errors. I will show high-rate GPS results for the Denali, San Simeon, Tokachi-Oki, Sumatra, and Parkfield earthquakes, followed by discussion of the scientific impact of these new data.
最近の地震観測および GPS を用いた測地観測から、海溝型地震のアスペリティ分布、およびその周辺の非地震性滑り領域が明らかにされている。アスペリティの分布を見ると、その多くは海域下の深さ 30 km 程度におよぶプレート境界面に存在すると考えられる。アスペリティの形成要因については、プレート境界面上における構造物の存在（例えば海山の沈み込み）、あるいは物性の不均質（例えば含水量の不均質性）などが挙げられているところである。この形成要因を直接的に把握するためには、海域における地震調査・観測が適しているが、これまでは定性的な議論にとどまっている。ここでは、構造調査の波形解析によって、プレート境界面の構造的、物性的なアスペリティ形成要因の定量的な解明に向けた試みについて紹介をする。
火山噴火に伴い発生する高温の火砕物粒子とガスからなる流れの中では、何が起こっているのだろうか？現在の火山学は、この流れの中を直接観察することに成功していない。しかし、その堆積物や痕跡に残された情報をもとに、粒子の振る舞いを推定し、大気や水の中を拡散する密度流の動きを復元することは可能になりつつある。
我々が現在観察できるものは、密度流の内部で起こる粒子の浮遊・衝突・沈降過程、場合によっては溶結現象の結果生じた堆積物である。私の研究では、堆積物の多様な構造・粒度分布・組成をヒントにして、こうした火砕物粒子の振る舞いに深く関わっていると考えられる物理量（粒子濃度、温度、流速、accumulation rate など）を推定し、陸上及び水底噴火のダイナミクスをできるだけ定量的に理解することを目指している。
セミナーでは、火砕物密度流の堆積構造-粒子の振る舞い-物理量の相互関係、密度流とその水域流入現象のモデル、そしてこの分野の現状や将来像について、国内最新のカルデラ噴火の復元を例に述べたい。
What happens in particle-driven density currents generated by volcanic eruptions? Nobody has yet succeeded to directly observe the interior of moving density currents, but we will be able to estimate the behavior of particles and reconstruct the motion of the density currents, based on geological data of eruptive deposits and traces.
The deposits are resulted from complex particle-motion, as suspension, collision, sedimentation, and welding deformation, and are critical clues that we can observe today. The first step of my study is to find significant physical parameters controlling particle-motion (as concentration, temperature, velocity, accumulation rate, and so on), and the goal is to reconstruct, model, and quantitatively understand the dynamics of subaerial and marine volcanic eruptions.
I will talk about how we can understand the relationship between sedimentary structures, particle-motion, and physical parameters of particle-driven density currents, and how we can model a density current and it entering water. I will give an example study for understanding a dynamics of a giant marine caldera-forming eruption.
四国沖南海トラフはフィリピン海プレートが西南日本弧下に沈み込むプレート収束域である．過去南海トラフ周辺域では，東南海地震や南海地震などの巨大地震が繰り返し発生している．この地域での地震発生様式を考える上で，沈み込むフィリピン海プレートの形状や西南日本弧の地殻構造を詳細に把握することは重要である．四国沖南海トラフから四国東部においては平成 11 年に海陸統合地殻構造探査が実施され， 1946 年南海地震の発生・破壊過程に関係したと考えられている海山の沈み込み構造や，四国東部下に沈み込むフィリピン海プレートの形状が明らかになった．また平成 14 年には，平成 11 年海陸統合地殻構造探査測線上とその延長上にあたる四国から中国地方にいたる地域で地殻構造探査を実施し，西南日本弧や沈み込むフィリピン海プレートの全体像を得ることができた．これらの探査で得られたデータで最も特徴的なことは，沈み込むフィリピン海プレート上面からの明瞭な振幅を持った反射波が観測されたことである．観測されたプレート境界からの反射波に対して振幅解析を行なうことで，沈み込むプレート上面における反射強度の深さ方向における変化を，反射係数の値で定量的に示すことができた．本セミナーでは．これら構造探査データの解析によって得られた西南日本弧の地殻構造と沈み込むプレート境界面上の物理特性の変化について紹介する．
Repeating microearthquakes on the San Andreas fault: Stress drop, asperity dimensions, and frictional properties of the SAFOD target earthquakes. The source properties and recurrence time of repeating microearthquakes give insight into the physics of earthquake rupture. The San Andreas Fault Observatory at Depth (SAFOD) is a 3-km-deep borehole that crosses the San Andreas fault near Parkfield, California. The project represents the combined efforts of dozens of scientists from around the world. I will begin this presentation with a brief background summary of the project and summarize some recent findings from this global team. In summer 2007, the borehole will drill through the source area of a M2 repeating earthquake, so understanding the source properties of this specific earthquake and the surrounding creeping portions of the fault are a high priority. In addition to advancing our understanding of rupture dynamics, it is important to fully characterize the dimensions of the drilling target asperity so that the borehole can be drilled in the correct location to intersect the asperity.
We focus on characterizing microearthquake source properties around SAFOD, including asperity dimensions and stress drop, using the spectral ratio method. The spectral ratio method allows us to determine the relative moment release and corner frequency of earthquakes located close to one another by removing common propagation-related effects such as attenuation. We combine seismic data from the SAFOD pilot hole (32 geophones from 850-2000 m depth) with a regional network (12 near-surface boreholes, typically 250 m deep). We find stress drop for many of the microearthquakes to be fairly high, usually 5-15 MPa. We find no systematic variation in stress drop for events from M0 to M2.5 at a given depth interval, but we do observe higher stress drops at deeper depths. We also compare several events in a repeating earthquake sequence, evaluating the changes in moment release and spectral content of each event. Using this method, along with the recurrence time between repeating events, we find that the local slip rate accelerates following the 2004 Parkfield M6 earthquake, but exponentially decays back to the pre-earthquake rate over several years. This post-earthquake recovery can help constrain the frictional properties of the fault inside and outside the repeating earthquake asperity.
Constraints on present temperature are essential for understanding mantle dynamics at present and in the past. Mantle temperature can be estimated from seismic tomography models and laboratory experiments on properties of minerals and rocks and through the extrapolation of surface heat flow observations. We (S.Honda-san and myself) map the thermal state of the mantle beneath the Japanese islands and surrounding area by inverting P-wave velocity anomalies of the high-resolution global seismic tomography model by Y.Fukao-san and his school. In the forward modeling of synthetic seismic velocity anomalies the effects of anharmonicity, anelasticity, and partial melting on seismic velocities are considered. We analyze a sensitivity of the temperature model with respect to dry and wet solidus and estimate regions of possible melting. In general the model of seismic temperatures are consistent with those inferred from surface heat flow. We estimate temperature anomalies beneath Japanese islands and show that mantle is hot at the depths of 350 to 550 km. Temperature anomalies with respect to the mantle adiabatic geotherm (Katsura et al. 2004) reach up to 300 K at depths of 348 to 410 km. This confirms the recent qualitative estimates for temperature anomalies at the depths in the region by Obayashi et al. (2006). The uncertainties in the inversion of seismic wave anomalies to temperature will be discussed. In the second part of the talk I explain the principal aim for the development of the present temperature model and discuss briefly the ongoing research and numerical modeling on an assimilation of the present temperature model to the geological past in order to understand regional geodynamics in the past.
To cool down hot debates on the hot mantle beneath the Japanese islands cold beer will be available after the seminar.
マントル内部の熱物質輸送を担うプルームは、どのように生成され、どのような振る 舞いをするのであろうか？そして、それらによってもたらされる火山活動は、どのように観測されるのであろうか？
近年の地震学的・地球化学的研究の進歩により、古典的な「 head-tail プルーム」では LIPs やホットスポット火山に関する観測量をうまく説明できなくなってきている。それらのモデルでは均質なマントルを仮定しているが、現実の地球では、マントル最下部に組成的に不均質な領域の存在が指摘されており、それらはマントルプルームの生成やその後の熱物質輸送過程に大きな影響を及ぼすはずである。本講演では、熱組成プルームに関する流体実験の結果を紹介し、冒頭の疑問について考える。またケーススタディとしてアイスランドプルームの振る舞いについて議論する。アイスランドプルームは、 reconstruction の結果などから、少なくとも ～100 Maから活動があると考えられ、～60 Maに picrite マグマを含む高温かつ大規模な火山活動、その前後に高温のマグマではない断続的な火山活動をひき起こしたと考えられる。現在のアイスランドでは、マグマの温度はさほど高温ではないが、プルーム起源の物質も見つかっている。一方、アイスランド直下の地震波低速度域は、 CMB まで繋がっているようには見えない。こうしたアイスランドプルームに関する観測事実は、古典的な「 head-tail プルーム」モデルでは説明することができない。我々の流体実験で観察された熱組成プルームの振る舞いは、 initial Buoyancy ratio と時間に強く依存する。それらの結果を考慮することで、先に記したアイスランドプルームに関する puzzling な観測事実を調和的に説明できるかもしれない。
Taiwan Chelungpu-fault drilling project drilled two holes, 39m apart of hole-A and hole-B, with one branch all crossing the Chelungpu fault to retrieve the fresh slip zone associated with the 1999 Chi-Chi earthquake. The TCDP hole-A is 2 km deep, and a 12-cm primary slip zone (PSZ) at the depth of 1111km was identified. A 2-cm major slip zone (MSZ) in the PSZ with less deformed texture was considered as the slip zone related to the most recent ruptured fault for the 1999 Chi-Chi (Mw7.6) earthquake. Considering the entire history of the Chelungpu fault, the slip thickness for a single event similar to the Chi-Chi earthquake is about 3mm. The comparison of the seismic break down work, calculated from the dense strong motion networks, and the observations of the grains in the MSZ suggest that only 2% of the earthquake breakdown work was contributed to the formation of the fault gouge. Most of the energy was dissipated in different forms (Tanaka et al., 2007). After the successful drilling of the TCDP, the TCDP borehole was used as an in-situ fault zone dynamic observatory. A state-of-the-art 7 level seismometer was installed in the borehole. The 7-level borehole seismometers (TCDP 7-level BHS) were placed crossing the fault zone with three seismometers in the hanging wall, and footwall, respectively, and one seismometer close to the slip zone related to the 1999 Chi-Chi earthquake. A fluid injection test (FIT) was carried out after the drilling and the completed installation of the seismometers to understand the in-situ hydraulic behavior of the fault zone. A high pressure fluid (~4MPa) was injected in hole-B with chemical and gas observations and monitoring in hole-A. The high resolution of TCDP 7-level BHS recorded several different types of events. A significant feature is the observation of three repeating events in 10 sec with almost identical waveforms in S-waves. The travel time differences of S-wave to P-wave (ts-tp) are 1.60sec, 1.57 sec and 1.38 sec, respectively, for the three events, suggesting a possible propagating crack. The waveform simulations of the observed repeating events with harmonic wave trains after S-wave suggest these events were probably from the fault zone, which is about 190m below the major slip zone. Another observed feature is a pulse like wave, which have an apparent velocity of about P-wave velocity (4km/sec), was observed in TCDP 7-level BHS. No S-waves were observed in this event. Whether this event is an association of a new open crack after high pressure Fluid Injection Test (FIT) was examined. With the high resolution TCDP 7-level BHS and FIT in an active fault zone, we try to construct a physical model for the nucleation and rupture behavior within the fault zone.
The seismic stress drop (EG) was estimated by dynamic wave inversion of slip produced by the September 21, 1999 Chi-Chi, Taiwan earthquake (Mw 7.6) at large slip region (10 m) of northern part of the fault [Ma et al., 2001], which show a large value around 40 MJ/m2. On the other hand, two data sets of temperature loggings both from shallow borehole in 2000 [Tanaka et al., 2002, 2006] and deep borehole in 2005 [Kano et al., 2006] penetrating Chelung-pu fault zone show positive thermal anomaly right on the slip zones. Considering the thermal conductivity directly measured from the retrieved core of the TCDP, we suggest that the temperature anomaly observed in the deeper hole might be resulted from the thermal conductivity fluctuation [Tanaka et al., in submission]. Thus, only the temperature logs from shallow hole was used to estimate dynamic friction during faulting [Tanaka et al., 2006]. Strength drop during faulting is estimated by thermal pressurization modeling of the slip zone materials recovered from deep borehole. Most of the parameters necessary for thermal pressurization calculations are obtained directory from recovered core and geophysical logging. Assuming some parameters, complete strength drop is estimated at the slip distance of 5 m, implying dynamic friction is close to zero after 5 m slip. In most of the calculation, dynamic energy for friction (Edf) is estimated to be extremely low, 1 to 10 MJ/m2 at the slip zone of 1 km depth. The amounts of stress drop from seismic wave inversion and strength drop from thermal pressurization is almost identical, around 12 MPa. Comparison between the two dynamic models lead to a conclusion that, if we consider the real value of fracture energy is presented as EG - Edf, more than 70% of the energy was consumed by fracturing in the slip zone at the depth of 1 km.