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日時：7月25日（月）14:00
場所：本館 E-232D

担当：Wiebke Heise
題目：Imaging plate coupling at the northern Hikurangi subduction margin, New Zealand
概要：
   Inter-plate coupling on of the Hikurangi subduction margin along the east coast of New Zealand’s North Island changes from weakly coupled in the northern part to strongly coupled in the southern part. The northern part of the margin is characterized by shallow (10-15 km depth) slow slip events that occur at roughly 2 year intervals. The conditions needed for slow slip are poorly understood but the presence of fluid and/or clay rich sediments may play an important role in controlling the frictional strength of the interface and thus inter-seismic plate coupling.
   Magnetotelluric (MT) measurements from the northern and southern part of the Hikurangi margin have shown that a dipping conductor is present above the subduction interface in the North but is absent in the South. This conductor is interpreted to mark fluid and/or clay rich sediments within the subduction -interface-shear-zone. The correlation between plate-coupling and resistivity structure seen in the northern and southern parts of the Hikurangi margin supports the idea that fluid and/or clay rich sediments play an important role in frictional processes at the interface.
   3-D inverse modelling of new MT measurements from a 90 km long segment on the northern margin shows that the conductivity of the interface is heterogeneous. Higher resistivity areas of the interface correlate with the location of tremor observed during a large slow slip event in 2010 and with seismicity occurring within a few km of the interface. This correlation suggests that more resistive regions correspond to regions with greater frictional-strength.
   This correlation is tested by collecting a total of 160 MT data across the transition from weakly to strongly coupled plate interface 3-D modeling of the northernmost 80 sites is being carried out.

担当：Ted Bertrand
題目：Links Between Volcanic, Tectonic and Geothermal systems revealed with array MT data in the Taupo Volcanic Zone, New Zealand
概要：
   The Taupo Volcanic Zone (TVZ) is an actively rifting volcanic arc, characterised by recent rhyolitic volcanism, large collapse calderas and more than 20 high-temperature liquid-dominated geothermal systems that convectively discharge ~ 4 GW of heat. During the past 6 years, over 500 broadband magnetotelluric (MT) measurements (2 km site spacing) have been made that form two arrays in the northern and southern parts of the TVZ. 3-D inversion models of these data provide a detailed picture of the electrical resistivity structure of the brittle upper crust, which show interaction between the underlying magmatic system and the overlying system of hydrothermal (convective) heat transport.
   A key result of this work is obvious correlation between the locations of geothermal fields, deep-seated low-resistivity zones, and the margins of low-gravity anomalies that suggest geological structure (in particular collapse calderas) and shallow magmatic intrusions play a more direct role in the pattern of convective heat transport than previously envisaged. In addition, while several of the geothermal fields appear to be underlain by shallow (~3 km depth) bodies of crystallising magma, others are not. A model of episodic intrusion in a region focused around the geothermal fields is consistent with the resistivity models, and with geological evidence for temporal changes at the geothermal fields in response to nearby magmatic events.

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