●IFREE ALLセミナーのお知らせ
第65回　IFREE ALLセミナー
*今月のセミナーは第5週目の金曜日になります。
日時：2013年5月31日（金） 16:30-17:30
場所：横浜研究所三好記念講堂
講演者：田村 芳彦（地殻進化研究チーム)

題目：海洋性島弧を掘削して「安山岩問題」を解決する

要旨：The continental crust, the most differentiated end-member of the components of solid Earth, is andesitic in overall composition (e.g.Rudnick & Gao, 2003). Thus, it is widely thought the continental crust has been created, or at least recycled, in subduction zones for the last~3.5 Ga (eg. Taylor, 1967, Rudnick, 1995). However, how andesite is generated, the so-called "andesite problem", has long been a central question of igneous petrology.
At first glance, intra-oceanic arcs do not appear to be the right place to study the production of andesitic magmas, because (1) modern magmatism at the intra-oceanic Izu-Bonin-Mariana (IBM) arc is bimodal, with basalt and rhyolite predominating (Tamura & Tatsumi, 2002); and (2) turbidites sampled during Ocean Drilling Program (ODP) Leg 126 in the Izu-Bonin arc, which range in age from 0.1 to 31 Ma, are similarly bimodal (Gill et al., 1994), suggesting that the bimodal volcanism has persisted throughout much of the arc's history. Moreover, such bimodal magmatism is not unique to the Izu-Bonin arc, with the 30-36.5°S sector of the Kermadec arc, another example of an intra-oceanic arc, also exhibiting it (Smith et al., 2003; 2006; Wright et al., 2006). So why and how do we study the intra-oceanic arcs to solve the "andesite problem"?

● 平成24年度 地球内部ダイナミクス領域 研究成果報告会
日時：平成25年4月25日（木）　13：00〜17：30
場所：独立行政法人海洋研究開発機構　横須賀本部１階　大講義室

プログラム（予定）：

●IFREE ALLセミナーのお知らせ
第64回　IFREE ALL セミナー
（いつもより30分早いです）
日時：2013年2月15日（金） 15:30-16:30
場所：横須賀本部　本館1階セミナー室
講演者：藤 亜希子（海底ネットワークデータ解析チーム）

題目：Broadband near-source features of the shallow low frequency events in the northern Nankai trough, triggered by the 2011 Tohoku-Oki earthquake

要旨：
Low frequency events are seismic events, which have longer duration and less energy radiation compared to regular earthquakes. The low frequency events detected in the shallow part of the Nankai trough (depth<10km), reported by previous studies, can roughly be divided into two groups depending on the observable frequency ranges of the signal, where the frequency ranges actually depend on the observed instrument.

The events of the first group are very low frequency earthquakes (VLFE), which were originally detected by broadband seismographs on-land (Ishihara et al., 2003; Ito & Obara, 2005), dominant in the frequency around 0.1-0.05 Hz. More recently a close-in observation was successfully made by temporally deployed broadband ocean-bottom seismometers (BBOBS), which revealed many intriguing features of the VLFEs (Sugioka et al., 2012). The events of the second group are low frequency tremors (LFT), which are recorded by OBSs equipped with 4.5?Hz short-period seismometer sited close to the source regions. They are dominant in the frequency range of 2?8 Hz with a lack of energy above 10 Hz (Obana & Kodaira, 2009). The classification between LFTs and VLFEs must be an important step toward estimating the physical process of the shallow low frequency events.

After the 2011 Mw9.0 Tohoku-oki earthquake, many shallow low frequency events were recorded at a cabled network of ocean bottom broadband stations (DONET) deployed in the northern part of Nanakai trough. The characteristics of the events are similar to previously observed LFTs at the frequency range around 2-8Hz. In addition, some of the events are accompanied by a lower frequency signal, clearly visible around 0.02-0.05 Hz, whose features are similar to those previously observed as VLFEs by Sugioka et al.(2012). One of such features of VLFEs is the ramp-type motion of the instrument-corrected seafloor displacement, which corresponds to a subsidence of up to 0.04 mm with a rise time of 10-20 s.

In order to examine whether the events, which are accompanied by the 0.02-0.05Hz signal, are intrinsically different from those without the 0.02-0.05Hz signal, the amplitudes of each event measured at 2-8Hz and 0.02-0.05Hz are compared. The comparison shows that the events without the 0.02-0.05 Hz signal tend to have lower amplitude in 2-8Hz than those accompanied by the 0.02-0.05 Hz signal. The result indicates that there is no such event, which is intrinsically missing the 0.02-0.05Hz components but has large amplitude in 2-8Hz. In other words, the events without the 0.02-0.05Hz signal are likely to be either smaller in size or occurred further away from the stations, compared to the events accompanied by the 0.02-0.05Hz signal. Our dataset shows that the two types of low frequency events are likely the same phenomenon.

●IFREE ALLセミナーのお知らせ
第63回　IFREE ALL セミナー
＊今月のセミナーは第4週目の金曜日になります。
日時：2013年1月25日（金） 16:30-17:30
場所：横浜研究所三好記念講堂
講演者：多田 訓子（地球深部構造研究チーム)

題目：Three-dimensional electrical conductivity structure of the upper mantle beneath the Philippine Sea from seafloor observation

要旨：
The Pacific slab penetrating into the mantle beneath the back-arc regions was imaged as high-velocity anomaly by seismic tomography, and the high-velocity anomaly tends to be distributed horizontally in the mantle transition zone, which is called stagnant slab (Fukao et al., 2001). The stagnant slab plays important role in mantle dynamics, but this mechanism is not fully understood. Electrical conductivity is an independent parameter from seismic velocity and strongly depends on temperature, composition (including the degree of mantle hydration and the fraction), and connectivity of melt, all of which are important parameters in understanding the mantle dynamics. Therefore, electrical conductivity structures of upper mantle and transition zone play an important role for understanding mantle dynamics by collaborating with seismic tomography.

We have performed three-dimensional (3-D) inversion analysis to image electrical features of the stagnant slab and the surrounding mantle in the Western Pacific area, where the old Pacific Plate subducts at Kurile-Japan, Izu-Bonin, and Mariana trenches.　 We used MT data obtained using ocean bottom electromagnetometers (OBEMs) at 25 sites (Shiobara et al., 2009). For inversion analysis, we have developed new 3-D inversion scheme for marine MT data (Tada et al., 2012, Baba et al, submitted), which can treat complex seafloor topography and seafloor data. This is because at the seafloor electric and magnetic signals are attenuated due to highly conductive seawater and are distorted by the rugged seafloor topography and the distribution of land and ocean.

From 3-D electrical conductivity model, we find four significant features so far. (1) The conductivity of the Philippine Sea mantle is much higher than that of the Pacific mantle at the depth shallower than around 150 km. This is consistent with 1-D conductivity models by Baba et al. (2010). (2) The model indicates conductive anomaly beneath Shikoku and Parece-Vela basins. (3) There are resistive anomalies along the trenches, which is good agreement with the high velocity area in the shear-wave velocity structure model (Isse et al., 2009). It demonstrates that this anomaly indicates subducting slab. (4) There is a resistive anomaly at the depth deeper than 200 km beneath Daito ridge. In this presentation, I will discuss features 3-D electrical conductivity model.