2011
●IFREE HQ セミナー
日時：12月21日 (水) 16:00-17:00
場所：横須賀本部 本館５階国際交流室
発表者： 佐藤 佳子(IFREE 3C)
タイトル:
The friction experiment under the controlled gas environment
アブストラクト:
Noble gases very limitedly react other elements, recognized as inert nature. Therefore, isotope ratios of noble gases in certain minerals are believed to be well reflected to their ambient reservoirs when the minerals reach temperatures that are higher than their closure temperatures. We have reported the resetting of isotope signatures of noble gases under frictional experiment. It was only observed in completely melted glass limitedly found at the surface of frictional plane in friction experiment. However, the most part of the post experiment specimen were little or not melted possibly due to very short duration of the experiment. Instead, the samples were heated and deformed at a moment. The condition of the friction experiment is similar to the natural faults. Although the samples shaped as cylinders constantly rotated under constant pressure, the friction reached its maxima at around 5 seconds from the beginning of the rotation. This first fusion is limitedly occurred at the temperature maxima (correspond to the displacement of M-2~-3). Then, friction suddenly dropped down, and decreases gradually further in several seconds associated with moderate melting (second fuse, correspond to the displacement of the M5 or larger). Clearly Ar gas degassed at first fusion was detected when granite was used for the starting material. The granite used is significantly old including much radiogenic 40Ar that clearly emitted only in 5 seconds sliding. In case that the surrounding rocks are sedimentary rocks containing much higher K, liberation of 40Ar may be much faster and intense. Then, changing the atmospheres of the experiment from dry-air to He or Ar, the timings of melting also changed. This suggests that the dissipation of friction-induced heat depends on the mass numbers of the ambient gases. Thus, the ambient condition of the faults may control liberation of volatiles that could be detectable in some large earthquakes.
●IFREE HQセミナー
日時：6月15日 (水) 15:00-17:00
場所：横須賀本部 別館(山側)1F セミナー室
発表者1：出原 光暉（東京大学地震研究所）
発表者2：鈴木 勝彦（IFREE）
発表者1： 出原 光暉 （東京大学地震研究所）, 15:00-16:00
タイトル:
Large- and small-scale heterogeneities in the lowermost mantle beneath western Pacific: Implications for thermal and compositional anomalies and the existence of partial melt
アブストラクト: It has been widely recognized that the Earth's lowermost mantle is highly heterogeneous at a variety of scale lengths. In this presentation, I attempt to illuminate large- and small scale seismic heterogeneities in the lowermost mantle beneath western Pacific, where the extensive accumulations of ancient slab materials and large-wavelength velocity variations are indicated in the previous studies. By using the core-reflected body waves recorded at short-period and broadband seismic stations, strong constraints were placed on the large-scale low-velocity anomaly extended into the East of Philippine Islands and localized ultra-low velocity zones, as well as their short-wavelength lateral variations. I further discuss the seismic heterogeneities in terms of thermal and compositional variations and the existence of partial melt, and explore the dynamics in the lowermost mantle region.
発表者2: 鈴木勝彦 (IFREE), 16:00-17:00
タイトル: Toward new scientific reserach on genesis of resources
アブストラクト:
A new Project of Seafloor Resource Research has launched in JAMSTEC this April. The research groups are composed of four groups. Our Ore Genesis Research Group will try to figure out the genesis and mechamism of ore resource formation. I wish to talk about our research plan in the seminar. Comments and questions in the seminar would be greatly appreciated. Please come to dicuss.
●IFREE HQセミナー
場所 ： 横須賀本部別館(山側)1F セミナー室
日時 ： 2011年3月9日（水）16:00-17:00
発表者： Teh-Ru Alex Song
タイトル： Anisotropic mantle lid in young subducted slab underplating central Mexico: Implications on paleo spreading rates and the formation of sub-cratonic lithosphere
概要：
Modern plate tectonics involves several important components such as seafloor
spreading at mid-ocean ridges (MORs), generations of island arcs and subductions
of plates. In particular, spreading rates at MORs have been used to estimate
modern plate motions and are influential to long-term evolutions of global heat
flux, seawater chemistry,carbon cycle and sea level changes and sometimes linked
to mantle convection events such as superplumes, supercontinents and slab
avalanches across the 660 seismic discontinuity. While magnetic anomaly
lineations frozen in the seafloor allow estimates of spreading rates back
to the Mid-Mesozoic (~ 180 Ma),seismic anisotropy initially formed in the MORs
and frozen in the oceanic plates potentially also provides access to spreading
rates further back in time if it can be recognized in subductd plates. Here we
model local converted S-to-P waves and teleseismic P-to-S converted waves
to interrogate the interior of the young subducted Cocos plate beneath Central
Mexico. We find a strong peak-to-peak P-wave (10%) and S-wave anisotropy (10%)
localized within the topmost 2-6 km of the subducted oceanic mantle, with a fast
symmetric axis dipping at about 40 degrees toward subduction direction,
trending 30-40 degrees clockwise from the north. P wave azimuthal anisotropy
projected from such an anisotropic mantle lid (AML) is not only consistent
with observations offshore Central Mexico, but it also follows a linear
relationship between azimuthal anisotropy and spreading rates established
from observations in modern oceanic plates. Such an AML is probably composed
of dunites and depleted harzburgites assemblages that were originally
synthesized and strained at MORs and later subducted. We find that the AML
is analogous to dipping anisotropic layers that resemble fossil subductions
beneath sub-cratonic lithospheric mantle in North America, which allow us
to infer spreading rates in their parent MORs back to as early as the Archean.
●IFREE HQ specialセミナー
場所 ： 横須賀本部本館(海側)5F国際交流施設
日時 ： 2011年3月9日（水）14:00-16:00
発表者： Jean-Claude Sibuet (IFremer, National Central Univ., College de France, Europole de l'Arbois)
タイトル： A destructive earthquake and tsunami might occur in the near future, close to Ishigaki Island (Japan)
概要：
High-quality MCS data acquired with the M. Langseth across the Ryukyu subduction system in the vicinity of Ishigaki Island show the existence of a megasplay fault system branched from the summit of a ~1 km high ridge located on top of a ~5° landward dipping plate interface (Hsu et al., submitted). As evidenced by the existence of an outer ridge and widespread normal faults in the inner wedge, the megasplay fault has been active in the past and the stress is accumulating again along the fault plane and the plate interface. Strong negative-polarity reflections observed along the potential rupture zone indicate the existence of high-porosity fluids. The high fluid pressure can effectively reduce the rock strength and seismic coupling along the plate interface, facilitating the relaxation of the “locked” seismogenic zone. The persistence through time of these zones of high fluid pressure would help to maintain the succeeding ruptures along the same trend.
A possible rupture length of the megasplay fault is estimated to be up to 450 km long along the south Ryukyu forearc where high gravity anomalies exist. We infer that a great earthquake with a sudden slip of the megasplay fault and the plate interface beneath the inner wedge generated the 1771 great Meiwa earthquake and tsunami, which struck the south Ryukyu islands and killed ~12000 people. After a quiescence of ~240 years, a potential great earthquake might be expected in the near future; the associated tsunami could heavily strike the coastal regions of the Ryukyu islands, Taiwan, China, Korea, Japan and Philippines.
Though the surface rupture occurred in a complex morphologic environment with the presence of sedimentary ridges, it would be crucial to try to map the location of the emergence of the cluster of splay faults and to determine which splay faults were recently active and the nature of the deformation along them.
発表者： Chris Goldfinger (Oregon State Univ.)
タイトル： Long Marine Paleoseismic Records at Plate Boundaries: Clustering, Segmentation and Supercycles
概要：
Long paleoseismic records from plate boundary earthquake series afford uncommon opportunities to examine recurrence models, clustering, segmentation, interaction with other faults, and long term strain history. For example, the 10,000 year marine record in Cascadia suggests at least four seismic segments defined by correlation of turbidites along strike and compatible temporal links to onshore evidence. The linkage makes use of subsurface log correlation techniques, radiocarbon ages, and the similarity of records in isolated settings along the Cascadia margin. The temporal and spatial record reveals a pattern of decreasing recurrence time southward, from ~ 500-530 years in northern Cascadia, to ~ 240 years or less in southern Cascadia. The decrease in recurrence times southward is consistent with a southward tapering sediment supply, which in turn allows lower plate roughness to define segment boundaries. The temporal record also reveals clusters of 4-5 events, with gaps of 700-1200 years between clusters. These temporal clusters are supported by regional high-resolution seismic reflection records, and appear to extend to Late Pleistocene time, deeper than reachable with gravity and piston coring. Long temporal records also afford opportunities to test linkages between adjacent fault systems. Long turbidite records for Cascadia and the Northern San Andreas Fault suggest that these two faults have virtually the same average recurrence interval through the Holocene. The physical connection at the Mendocino Triple Junction may be augmented by a stress connection between the earthquake cycles of these two great faults. This connection is suggested by a close event by event temporal correlation. Finally, energy in plate boundary fault zones may have long term cycling involving many seismic cycles. Several lines of evidence, both direct and indirect suggest that the connection between interevent time and earthquake magnitude, and models predicated on this relationship, may be very weak.
If a measure of earthquake magnitude can be inferred from a paleoseismic record, then recurrence models and long term-cycling can be explored. In Cascadia, coseismic energy may be modeled as proportional to the mass of turbidites triggered in seismic shaking. We infer that turbidite mass is a suitable proxy for energy release because of its consistency along strike at multiple sites. If turbidite mass (energy release) is plotted to balance plate convergence (energy gain) the result is a 10ka energy time series for Cascadia. The pattern reveals that the earthquake clusters apparent in the time series have variable behavior. Some events appear to release less, while others release more energy than available from plate convergence (slip deficit). Those that are larger may have borrowed stored energy from previous cycles. Cycle variations may explain mismatches between deformation models based on interevent times in the last 4600 years and coastal paleoseismic data.
●IFREE HQセミナー
場所 ： 横須賀本部本館(海側)1F 大講義室
日時 ： 2011年2月16日 (水) 13:00-14:30
発表者： Dr. Nikolay Palshin (Shirhov Institute of Oceanology, Russian Academy of Sciences)
タイトル： Structure of the Central part of the Kuril Trench according to new magnetic data
概要：
The central part of the Kuril Trench is also known as an atypical tectonic unit where
the existence of the extension structures was suggested in 1980-ies. The anomalous
behavior of the Kuril Sliver is usually explained by oblique subduction.
The central part of the Kuril Trench is known as a “closed” seismic gap.
Before the gap was closed two expeditions of the RV “Akademik M.A.Lavrentiev”
(37 cruise in 2005 and 42 in 2006) had been carried out in the framework of the project
≪Studies of structural peculiarities of a seismic gap in the central part of Kurile-Kamchatka
Island Arc as a possible source of a great earthquake and tsunami≫.
A great amount of magnetic data was collected during these two expeditions.
In the period of September-October 2010 another expedition of the RV
“Akademik M.A.Lavrentiev”(54 cruise) was carried out. Besides other geological
and geophysical studies the scientific program of the cruise included continuation of
the detailed magnetic survey in the Kuril Trench forearc region, which was started
in 37 and 42 cruises. The analysis of the magnetic data obtained during the cruises
as well as from available magnetic data from public sources has been done and a combined
magnetic map has been constructed. The combined map was used for tectonic interpretation
based on spatial spectral technique. The results obtained confirm the atypical tectonic
structure of the Kuril Sliver. Observed elongated magnetic anomaly (magnetic belt) could
interpreted as indication of the the existence of the serpentinite belt, which could in
turn could be an evidence of intense hydration of subducting slab.
●IFREE HQセミナー
場所 ： 横須賀本部別館(山側)1F セミナー室
日時 ： 2011年1月26日 (水) 16:00-17:30
発表者： Hrvoje Tkalcic (The Australian National University)
タイトル： How Was the Moho Discovered?
概要：
At about the turn of the 20th century, a convincing description of Earth’s structure
was beyond even science fiction. Andrija Mohorovičić was a meteorologist and it is
not well understood what provoked him to turn his scientific career upside-down in
his mid forties, to embark on a relatively new and almost nonexistent scientific field -
seismology.
Exactly one hundred years ago, an earthquake that occurred in Kupa Valley, Croatia,
led Mohorovičić to the discovery of the crust-mantle boundary, popularly known as
Moho. He determined crustal thickness through one of the first known inverse theory
applications using a drawing board and travel-time data from about dozen analog
seismographs operating in Europe at that time.
Since then, changes in our paradigms about the Earth's crust and upper mantle
have been driven by results from seismological and other geophysical data. An
unprecedented recent global expansion of seismic instruments has resulted in
considerable progress in elucidating crustal structure and the depth of Moho. I will
briefly present results from recent studies with the emphasis on Moho from different
parts of the globe.
●IFREE HQセミナー
場所 ： 横須賀本部
日時 ： 2011年1月12日 (水) 16:00-17:30
発表者： 木下 正高 (海洋底ダイナミクス研究チーム)
タイトル： Heat flow estimated from BSR and IODP borehole data: Implication of recent uplifting of the imbricate thrust zone in the Nankai Trough off Kumano
概要：
New heat flow values were determined in the Nankai Trough forearc slope
region of Kii Peninsula from the BSR identified in the 3-D seismic data
volume and the thermal conductivity data measured on core samples obtained
nearby during IODP Expeditions 315/316. High-resolution images in the
shallower portion enabled detection of BSRs in the forearc slope. Comparison
of P-wave velocities at drilled sites C0001/ C0004/C0006 and the interval
velocity model constrains the uncertainty in BSR depth to <25m and error in
heat flow by 5%. Thermal conductivity values were inferred from Vp-K
relationship obtained in nearby borehole data, which would involve error of
0.1 W/m/K, or 10 % in terms of heat flow.
BSRs were identified only in the Imbricate Thrust Zone (ITZ) and no BSRs are
visible either around the mega-splay fault or in the toe of the accretionary
prism. Within the ITZ, the BSR near the axis of anticlines is significantly
shallower than those in the adjacent slope zones. We designate the shallow
BSR below the ridge axes “Anticlinal High Value Zone (AHZ)”, and designate
other BSR “Basal Low-value Zone (BLZ)”.
BSR-derived heat flow in the BLZ (60-70 mW/m2) is consistent with the
general heat flow trend, which gradually decreases landward, from 120-140
mW/m2 in the Shikoku Basin to > 55 mW/m2 in the Kumano Forearc Basin.
Locally, it is lower than values obtained from probe measurements in the
slope region (60-90 mW/m2), and is higher than ones obtained at IODP drill
sites in the forearc slope region (47-55 mW/m2). These differences may be
caused by the erosion or deposition of slope sediments. The anomalously
shallow BSR in the BLZ produces an apparent high heat flow anomaly of 70-90
mW/m2, and a significant discontinuity is identified across the thrust fault
complex. The most likely cause is the transient effect of thrust faulting
followed by uplift and subsidence, then by the erosion and sedimentation. A
one-dimensional time-dependent numerical model confirms that the relaxation
time for the BSR depth to be reequilibrated from such a disturbance is
10-100 kyr.
The age of the accretionary prism and overlying sediment would be > 1Ma or
older, as inferred from the ages obtained at mega-splay (Sites C0004/C0008)
and at frontal region (Site C0007). Three ridges in the ITZ have been
uplifted since > 1 Ma. Thus the thermal regime and the BSR depth in the AHZ
have not yet equilibrated after the uplift due to faulting, resulting in an
anomalously shallow BSR depth and higher heat flow. We propose that the
anomalously shallow BSR in the AHZ and the BSR discontinuity across these
fault zones are caused by thrust faulting activities since > 1 Ma, followed
by uplift and erosion.
