Electrical conductivity structure of the Alpine Fault and its relationship to seismicity and seismogenesis
Authors: T V Caldwell, S Bannister, S Bourguignon, G J Hill, E A Bertand, S L Bennie, W Heise, Y Ogawa, H M Bibby (GNS Science)
Paper number: 3771 (EQC 10/595)
Abstract
The occurrence time of an earthquake on a fault ultimately depends upon the build-up of stress on the fault overcoming the strength of the frictional force resisting the fault rupturing. Since we are unable to measure these forces directly, we must use surface geophysical measurements to provide information about other properties that are indirectly related to these forces. In this EQC funded project we have used information gathered from an array of seismometers and from measurements of natural fluctuations in the Earth’s magnetic field to study a 50 km long segment of the Alpine Fault on the west coast of the South Island.
From this information we have been able to determine the depth at which rocks close to the Alpine Fault become too soft or ductile to allow micro-earthquakes to occur to creep in response to plate-tectonic forces driving the fault. The depth at which the rocks become ductile is one of the key factors that will control the ultimate size of a future Alpine Fault earthquake. We have also shown that the rocks are electrically conductive in the ductile region beneath the fault. The high conductivity indicates that high pressure fluids are present in the fault’s root zone. These fluids will reduce the frictional strength above, especially if they leak upwards. Comparison with other measurements 40 km further to the south show that the electrical conductivity structure there is different, perhaps reflecting a change in the frictional strength of the fault. Our work provides a glimpse into the inner workings of the Alpine Fault that will help us better understand the hazard it poses.
Technical Abstract
We have collected and analysed new magnetotelluric (MT) data and determined the 3-D distribution of seismicity and seismic properties from a ~60 km long segment of the Alpine Fault between Whataroa and Ross. MT data from 85 new measurements sites show that a south-eastward dipping electrically-conductive zone is present in the mid-crust about 10 km south-east of the Alpine Fault, in good agreement with the conductivity structure inferred from an earlier MT survey along the Whataroa. This structure is interpreted to be a ductile shear zone at the down-dip extension of the Alpine Fault below the frictionally locked part of the fault. The conductive zone continues northwards to at least Ross; the high electrical conductivity indicating the presence of (electrically) interconnected fluid within the inferred shear zone.
Earthquake seismic data recorded by the GeoNet network and two temporary seismometer arrays: ALFA08 and ALFA09 provide complementary information on the seismicity distribution and seismic properties of the crust from the northern part this segment of the fault, between Harihari and Ross. Double-difference analysis of the seismological data using a detailed 3-D velocity model was used to relocate 1062 earthquakes concentrated southeast of this part of the fault. The relocated seismicity shows that more than 70% of the events are concentrated in a 12 km wide band centred 16 km southeast of the fault. Almost all of the earthquakes shallower than ~4 km occur within this band. Ninety five per cent of the seismicity occurs above 8 km and this depth limit does not appear to change significantly with distance south-east of the fault. If we assume that the 95% depth limit of seismicity can be identified with the transition to ductile behaviour and that this depth is controlled by temperature alone, our results suggest that the thermal structure southeast of this part of the doesn’t vary significantly with distance from the fault.
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