Mai-Linh Doan

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan

[1] Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the Mw = 7.6 1999 Chichi earthquake when pressurization may have reduced highfrequency shaking in the regions of large slip if the fault was sufficiently sealed. We investigate in situ hydraulic diffusivity which is the key parameter in such models through a cross-hole experiment. We find a diffusivity of D =( 7 ±1 )� 10 � 5 m 2 /s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity S lies between 10 � 7 and 10 � 5 ,s o that the transmissivity T along the fault zone is comprised between 10 � 11 m 2 /s and 10 � 9 m 2 /s. The corresponding permeability (10 � 18 –10 � 16 m 2 ) is at most one hundred times larger than the value obtained on core samples from the host rock. The fault zone is overpressurized by 0.06 to 6 MPa, which is between 0.2% and 20% of the lithostatic pressure. Citation: Doan, M. L., E. E. Brodsky, Y. Kano, and K. F. Ma (2006), In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan, Geophys. Res. Lett., 33, L16317, doi:10.1029/2006GL026889.

Thermal anomaly near the Aigio fault, Gulf of Corinth, Greece, maybe due to convection below the fault

A thermal profile has been measured in a 1000 m deep borehole intersecting the active Aigio fault, Corinth Rift, Greece. The heat flow is 53 mW/m 2 , indicating that the rifting process has no effect in heat flow. The temperature near the fault is higher than expected from a pure conductive model. This discrepancy is not due to fluid flow above the fault as shown by the long term monitoring of downhole pressure. Neither can it be attributed to the fault slip since the Aigio fault is a minor normal fault of the rift, with no very recent earthquake. We propose that the anomaly is due to the convection within the karst that constitutes the footwall. Numerical simulations give a correct estimate for the recorded temperature increase. This is an extreme case of thermal disturbance induced near a fault by local fluid circulation. The occurrence of convection outside geothermal area is very rare.