●IFREE HQ seminar
Date: 2013/5/29 (Wednesday) 16:00 – 17:30
Place: International Exchange Room, 5th floor Yokosuka HQ
Speaker: Sebastian Hammerschmidt (MARUM, Uni. Bremen)

Title:
Long-term and Real-time Monitoring at the Nankai Trough, Japan: Implications for Sources, Pathways and Composition of Upward Migrating Fluids and Gases

Abstract:
The Nankai Trough, SE offshore Japan, is a highly earthquake-prone area. One of the key parameters influencing seismogenic processes are fluids and gases, their behavior under greater pressure (p) - temperature (T) - conditions and different stress regimes. Of particular interest are fluid pathways, especially faults, and if deep-sourced fluids and gases can migrate to more shallow levels in the subduction zone complex. In order to improve our knowledge about possible fluid migration pathways and their depth of origin, this project focusses on (I) real-time drilling mud gas monitoring during riser drilling in the Kumano forearc basin and (II) long-term monitoring of the Nankai Trough Megasplay fault system.
For (I), formation gases released during riser drilling are forwarded to the onboard field laboratory and analyzed. This provides valuable information about the parent beds, thermal maturity, permeability, and depth of origin. Preliminary drilling mud gas data from the Kumano forearc basin showed intervals, in which elevated permeabilities (indicated by 222Rn) correlate well with high 3He/ 4He ratios and total gas concentration, indicating the presence of deep-sourced gases. It seems that gases are able to migrate through the accretionary prism and accumulate close to the unconformity between forearc basin and accretionary prism. Further research on the isotope composition of (non-) hydrocarbon gases to be sampled during upcoming IODP Exp. 348 is necessary to verify this assumption. Regarding (II), long-term monitoring of fluid flow is accomplished by using SmartPlug (p- and T-sensors) and GeniusPlug (p- and T-sensors, fluid sampling device) borehole observatories. Being installed at the Nankai Trough Megasplay Fault, fluid flow along and hydrogeologic properties of the splay fault can be determined. In 2010, after 15 months of monitoring, the SmartPlug observatory was recovered. Analyses of the tidal variations in the pressure data suggest that hydraulic communication between borehole and formation is enhanced by the presence of fractures in the fault zone, and that the dimensions of the SmartPlug allow accurate monitoring of pressure changes at tidal frequencies. After removal of the tidal noise, minutes- to days-lasting fluid pressure oscillations became visible, which were mainly caused by the arrival of teleseismic waves, storm-induced microseism and tsunami-wave induced infragravity noise. Clear p and T transients from local earthquakes are absent, and no conclusions could be drawn for recent fluid migration. Some of the minutes- and days-lasting pressure transients remain unresolved, and might be related to episodic tremor and slip at the Nankai Trough thrust fault boundary. In order to clarify this possibility, waveform data from the Dense Ocean-floor Network (DONET) will be analyzed for low-frequent signals, and these will be compared to the unresolved pressure transients in the SmartPlug data.
In the end of 2013, data from the GeniusPlug will be obtained, which will include not only p and T measurements but also fluid samples taken directly from the splay fault zone. By consulting DONET, future research will focus on coseismic changes in the geochemistry of the fluids and their potential use as proxy for seismogenic activity.

●IFREE HQ seminar
Date: 2013/5/15 (Wedbesday) 16:00 – 17:30
Place: Seminar Room, 1th floor Yokosuka HQ
Speaker: Grant Caldwell (visitor)


Title:
From the arc to the fore-arc, a magnetotelluric view from New Zealand

Abstract:
The Taupo Volcanic Zone (TVZ) in New Zealand’s North Island is an active zone of continental extension which is perhaps better described as a rifted arc. This zone has formed in response to the oblique subduction of the Pacific Plate beneath the North Island along the Hikurangi subduction margin. Despite its subduction zone setting, recent volcanism in the central part of the TVZ is overwhelmingly silicic and is the focus of an exceptionally high heat flux (~4.2 GW).
At upper crustal depths, i.e. in the seismogenic part of the crust, this heat flux is transported to the surface convectively in ~20 liquid-dominated geothermal systems. Recent 3-D inversion modeling of MT data provides an image of the hydrothermal heat transport in the upper-crust and the underlying magmatic system that drives it.

GPS data from the Hikurangi subduction margin show that coupling between the upper and lower plates along the margin changes from weakly coupled in the north to locked in the south. MT data from the weakly coupled region shows that an electrically conductive zone is present at or above the plate interface in the weakly coupled region. MT data from a single line of measurements in the locked region suggests that the plate interface is resistive. A thick layer of fluid-rich subductedsediment in the north that is absent in locked region would explain the MT data and perhaps also the change in plate coupling.

●IFREE HQ seminar
Date: 2013/5/7 (Tuesday) 13:30 - 14:30
Place: International Exchange Room, 5th floor Yokosuka HQ
Speaker: Seiki Asari (Uni. Potsdam/GFZ, Germany)


Title:
Probing Earth's core with satellite magnetic field models

Abstract:
In the CHAMP satellite mission the Earth's magnetic field was measured densely throughout the whole globe for July 2001-September 2010. The immense amount of vector field data has made it possible to build temporally continuous models of the main field (originating from the Earth's core) with an unprecedentedly high accuracy. GRIMM3 (2000-2011) is the latest of such models, which is particularly designed to map the vertical field component down at the core-mantle boundary. Also, the CHAMP data have allowed to refine field models even prior to the mission period. C3FM2 is a main field model for 1957-2008, for which satellite data are supplemented by secular variation data from ground-based observatories to backward estimate the field at earlier epochs.
This seminar talk consist of three topics, each expected to take half an hour if fully presented (the talk shall be arranged according to attendee's interests and time allowed).
(i) Introduction --- Historical and satellite magnetic observations
(ii) Rapid variations --- Subdecadal variations of core field and Earth rotation
(iii) Quasi-steady structure --- Imaging toroidal magnetic field at the top of the core The first topic (i) is a review of the history of global geomagnetic field observations and modelling, especially focusing on contributions of German scientists. (ii) and (iii) concern recent works exploring the Earth's core using C3FM2 and GRIMM3, respectively.

●IFREE HQ seminar
Date: 2013/4/17 (Wednesday) 16:00 – 17:00
Place: International Exchange Room, 5th floor Yokosuka HQ
Speaker: Daisuke Suetsugu (Director of Deep Earth Dynamics Research Program), Azusa Shito

Title:
Seismic attenuation in the upper mantle beneath the Ontong Java plateau

Abstract:
We investigated attenuation structure in the upper mantle beneath the Ontong Java plateau (OJP) to identify the origin of this largest LIPs (Large Igneous Provinces) in the globe. We analyzed teleseismic broadband waveforms of transverse component from a deep earthquake beneath the Solomon Islands subduction zone. Spectral ratios of the sS and S waves were computed to extract information of the attenuation in the upper mantle beneath the OJP. The Qs values were estimated to be 30-50 for the sS waves of which bounce points are located in the OJP. The Qs values are well lower than those estimated in other back-arc regions (70-90). It suggests that the upper mantle beneath the OJP has higher temperatures than its surrounding regions.

● IFREE HQ seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:15-17:45 on 8, March (Friday)
Speaker: Dr. Yoichi Usui (Mantle and Core Dynamics Research Team)

Title:
Paleomagnetism of exsolved magnetite: the youngest and the oldest

Abstract:
Early planetary magnetic field may be crucial for water retention and habitability, but geodynamic theory is yet to be adequate to predict the nature of early geomagnetic field. Paleomagnetism of single silicate crystals are emerging as an effective probe to reveal the early history of the geodynamo. The keys to future development of this technique include (1) finding crystals with stable and (relatively) strong magnetization, and (2) verifying the single crystal records against more conventional measurements. In this presentation, I focus on silicates with magnetite exsolution. This kind of material is known to have stable and strong magnetization, and reported from some plutonic and metamorphic rocks up to ~2.7 Ga. I extend the occurrence to ~3.3 Ga. In the first half, I talk about the past intensity of geomagnetic field (paleointensity) recorded in oceanic gabbros. Some gabbros recovered by ocean drillings are exceptionally young (~1 Ma) as plutonic rocks. Detailed tests revealed that magnetic properties of oceanic gabbros examined here are dominated by magnetite exsolution in plagioclase. This offers unique opportunity to test the effect of slow cooling on the paleointensity record in magnetite exsolution.
Comparing our paleointensity data with volcanic records, magnetite exsolution revealed values close to the time-averaged field intensity with predictable offset. This endorses the use of magnetite exsolution to study ancient geomagnetic field. In the second half, I talk about paleointensity results from ~3.3 Ga Mt. Edgar Complex in the Pilbara craton. Magnetic and microscopic investigations revealed that some rocks contain magnetite exsolution in plagioclase. Although the bulk magnetic properties of those samples are dominated by igneous large magnetite, direct measurements of separated single crystals successfully yielded paleointensity estimates. Preliminary data suggest somewhat weak geomagnetic field: they are only ~60 % of the modern geomagnetic field at the equator; if the paleolatitude was higher, even weaker geomagnetic dipole moment would be suggested. This may be consistent with ~3.44 Ga record from the Barberton craton. To interpret this value further, however, more studies on paleolatitude, magnetic anisotropy and exsolution temperature should be done.

● IFREE HQ seminar
Place: Seminar Room at 1F, YOKOSUKA HQ
Time: 16:00-17:30 on 20, February (Wednesday)
Speaker: Dr. Maria Luisa Tejada (IFREE3C)

Title:
Geochemical insights from Lyra Basin basalts on the possible sources of post-emplacement volcanism on the Ontong Java Plateau

Abstract:
Our Pb-Nd-Sr-Hf-Os isotope investigation of the alkalic rocks recovered by dredging from the Lyra Basin reveal post-OJP extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts that possess (87Sr/86Sr)t = 0.704513-0.705105, (143Nd/144Nd)t = 0.512709-0.512749 (εNd(t) = +3.0 to +3.8), and Pb isotopic ratios (206Pb/204Pb)t = 18.488-18.722, (207Pb/204Pb)t = 15.558-15.577, and (208Pb/204Pb)t = 38.467-38.680 that are distinct from those of the OJP tholeiites. They have age-corrected (187Os/188Os)t = 0.1263-0.1838 that are similar to the range of values determined for the Kwaimbaita-type OJP lavas but their (176Hf/177Hf)t = 0.28295-0.28299 and εHf(t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match any of the Polynesian hot spots well either. Instead, the Lyra Basin lavas have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita. Although not directly related to the main plateau volcanism at 120 Ma, our geochemical modeling suggests that the origin of the Lyra Basin alkalic rocks maybe genetically linked to the mantle preserved in the OJP's thick lithospheric mantle root although a contribution from the Rarotongan hotspot cannot be ruled out.

● IFREE HQ seminar
Place: Seminar Room at 1F, YOKOSUKA HQ
Time: 16:00-17:30 on 30th, January (Wednesday)
*The seminar date has been changed.
Speaker: Junichiro Kuroda (IFREE3C)

Title: Desiccation history of the Mediterranean Sea

Abstract:
Mediterranean Sea has experienced an extremely event in which nearly 5% of the dissolved salt of the world's ocean was extracted in a geologically short time (< 1 million years) to form a series of evaporite deposits more than 1 million cubic kilometers in volume. A buried abyssal salt layer was identified with reflection profiling as a thick transparent seismic unit. The top of the abyssal salt layer was sampled during the previous drilling projects. The evaporites (i.e., dolomite, gypsum, anhydrite and halite) in the drill cores draw a new picture of an end-Miocene (~5.5 Ma) Mediterranean desert lying several kilometers below the Atlantic Ocean with brine pools that shrank and expanded. These finding have led scientists to propose ‘Messinian Salinity Crisis’, although the explanation of ‘deep-basin, shallow-water’ desiccation hypothesis is still debatable. In the presentation I will review the previous studies on this extraordinary event, and present our work on sedimentary Os isotope data that would trace hydrological behaviors of the Mediterranean Sea during the last 20 million years.