●第40回IFREE　YESセミナーのお知らせ
日　時：2013年6月12日（水）14：00〜15：30
場　所：横浜研究所　IT棟４F　会議室
講演者：高橋　栄一（東京工業大学　教授）

タイトル：
How to predict future volcanic activity in Japan?

概要：
In Hokkaido, very large earthquake (M9?) took place in 1611AD, which may triggered eruption of Komagatake(1640AD), Usu(1663AD) and　Tarumae(1667AD) after more than 3000yr of dormant period. Simultaneous start in the activity of these volcanoes may be due to the change in crustal stress field discovered after M9 earthquake (Hasegawa et al, 2011). New injection of basalt magma into crustal magma chamber would result in eruption in a few years (Chokai erupted basalt lava 2yr after Jogan 869AD earthquake) or 30-50 years in stagnant silicic system (above three volcanoes plus Towada which has erupted 46yrs after Jogan earthquake). In this seminar, I will discuss the possible future volcanic activity in Japanese arcs as the result of March 11, 2011 Tohoku-oki earthquake. Special emphasis is on the deep structure of Fuji volcano (unpublished tomography by J.Nakajima) and its eruption mechanism. Comments from various disciplines which help our understanding of future activity of Japanese volcanoes are highly appreciated.

●第39回IFREE　YESセミナーのお知らせ
日　時：2013年5月29日（水）14：00〜15：30
場　所：横浜研究所　IT棟４F　会議室
発表者：塩原　肇（東京大学地震研究所准教授）

タイトル:
BBOBS-NX: practical observation tool for broadband seismology at the seafloor

概要
Since 1999, we have already developed the mobile broadband ocean bottom seismometer (BBOBS), and many practical observations have been conducted.
But, the noise level of BBOBS's horizontal components in long periods is rather high in average and its variation in time is also large. The reason of this high noise level is assumed as the small tilt variation of the large housing sphere that contains the broadband sensor, recorder and batteries, due to the bottom current. To clear this problem, one idea of observation without the tilt variation is the use of small and low profile broadband sensor that enables to penetrate into the sediment, that is apart from the large housing. This next generation type of BBOBS (BBOBS-NX) has been tested since 2008. The observation procedure of the BBOBS-NX is as following; (1) free-fall the BBOBS-NX to the seafloor aiming enough penetration of the sensor unit with the recording unit attached above it, (2) detach and move the recording unit from the sensor unit by the ROV to start the observation, (3) pull-up the recording unit and the sensor unit connected with the umbilical rope to recover the whole BBOBS-NX by the ROV. The averaged noise level of the BBOBS-NX is below the new high noise model for all three components in periods longer than 20 s, which is more than 20 dB of noise reduction in horizontal components compared to the BBOBS. So, the BBOBS-NX's data is suitable for analyses using horizontal component waveforms, such as the receiver function analysis. From a rough estimation, one year observation by the BBOBS-NX is long enough to get a reliable receiver function at a single station, although at least three years long observation was required for same quality by the BBOBS. Recently, we have started the normal oceanic mantle project since 2010 around the Shatsky Rise in the northwestern Pacific by using several BBOBS-NX and other instruments to research the nature of the LAB and the water content in the MTZ until 2014.
In this seminar, I will present the instrumental development of the BBOBS in these 10 years at first. Then, some new experimental developments of our OBS will be also introduced, those are designed to extend our observation range in several dimensions, such as in time, space and more.

●定例第38回IFREE　YESセミナーのお知らせ
日　時：2013年4月24日（水）14:00-15:30
場　所：横浜研究所　IT棟4F　会議室
発表者：ロイ　ハインドマン（SEOS教授）

タイトル:
Subduction Thrust Earthquakes: Controls on the Maximum Size

概要
The maximum size of subduction zone earthquakes, Mx, is an important scientific question and a critical earthquake hazard issue. The 2011, M9 NE Japan megathrust emphasized that this question needs more study. Historical Mx for different subduction zones range from greater than M9 (i.e., Japan, Chile, Sumatra, Alaska, etc.) to Mx less than M7.5-8.0 (i.e., Marianas and several other SW Pacific island arcs). An important problem I will not discuss is the short instrumental earthquake record of ~100 years (several 100 years historical data for a few subduction zones), compared to M9 return periods that may be more than 1000 years. We could be more confident of Mx if we had physical explanations for Mx. Many associations have been made between subduction zone characteristics and Mx but most have important exceptions. (I note the problem of tsunami earthquakes with "slow" slip, and ETS). Related to Mx is Seismic Coupling or Efficiency, i.e, the fraction of plate convergence accommodated by thrust earthquakes compared to aseismic motion. "Coupling" estimates involve down-dip width, commonly trench to 30 or 40 km, but much narrower downdip seismogenic widths exist.
Laboratory data indicate that faults are expected to be seismic for common crustal rocks, so special compositions or conditions are required for aseismic motion. There are two possible origins of low coupling (and small Mx): (1) Deep updip seismogenic limit and shallow downdip limit, (2) Areas on thrust that are seismic and other areas that are aseismic. There may be complete coupling with many moderate size megathrusts with stress concentrators from rough crustal topography (seamounts, fractures, etc.) stopping long ruptures, but usually low coupling is inferred for small Mx.
For the updip seismogenic limit, I will mainly discuss dehydration of stable-sliding clays, smectite (e.g., saponite) to illite/chlorite at 100-150C. Saponite is very weak (i.e., creeping part of San Andreas Fault). For the downdip seismogenic limit, there may be a temperature limit for hot subduction zones (SW Japan, Cascadia etc.), at ~350C; with a transition to ~450C, for velocity weakening to velocity strengthening (related to brittle-ductile transition), and for cold subduction zones, the limit may be the forearc mantle corner, downdip of which there is aseismic serpentintite and talc in forearc mantle due to rising dehydration fluids. For aseismic patches, I will discuss the importance of aseismic serpentinite/talc in the upper oceanic crust of the incoming plate.

●定例第37回IFREE　YESセミナーのお知らせ
日　時：2013年4月5日(金）16:00-17:30
場　所：横浜研究所　IT棟4F　会議室
発表者：マニュエル・ホビガー(BGR)

タイトル:
Coseismic velocity changes caused by large crustal earthquakes in Japan - Comparison and depth estimation

概要
Using Passive Image Interferometry, i.e. by cross-correlating ambient seismic noise recorded by Hi-net sensors, we measured coseismic and postseismic velocity changes for several earthquakes (Mw > 6.5) which occurred in Japan (Fukuoka 2005, Noto Hanto 2007, Iwate-Miyagi Nairiku 2008, three earthquakes in Niigata prefecture 2004, 2007 and 2011). For each of these earthquakes, we cross-correlated ambient seismic noise recordings of several years in four different frequency ranges between 0.125 and 2.0 Hz. Using a simple tomography algorithm, the observations of the different station pairs can be reprojected onto the actual station locations. For the analyzed earthquakes, the observed coseismic velocity changes are systematically larger at higher frequency. As the analyzed seismic noise is mainly composed of surface waves, these findings suggest that the coseismic velocity changes are concentrated in the shallow layers of the ground structure. The cross-correlation analysis fails at higher frequencies, as the distance of a station pair becomes too large for well-correlated signals. Auto-correlations of the signals at a single sensor might work better at higher frequencies, but they are prone to changes in the seismic noise field or human activities. We can overcome these problems by correlating the different components of a single sensor (self-correlation). In this way, the frequency range of velocity variations can be increased to about 4 Hz. Using the coseismic velocity changes at different frequencies, the actual depth distribution of the coseismic velocity changes can be modeled. Starting from a reference shear and pressure wave velocity profile, the corresponding Rayleigh wave dispersion curve is modified according to the measured velocity changes in the different frequency ranges. By changing the original velocity profile in a simple way (constant percental velocity change between given depths), the depth distribution of the coseismic velocity changes can be constrained. Using this modeling, we find that for stations close to the epicenter of the Iwate-Miyagi Nairiku earthquake, the coseismic velocity change is most likely to be of the order of - 3 % to - 5 % and to be concentrated in the shallowest several hundred meters, whereas a stationat at the southern end of the fault zone shows a more complicated damage which is most likely to reach several kilometers of depth.