T022-01
Plate coupling at the Hikurangi margin: insights from magnetotellurics
Plate coupling at the Hikurangi margin: insights from magnetotellurics
Wednesday, 9 December 2020: 19:00
Virtual
Abstract:
At the Hikurangi subduction margin along the east coast of New Zealand’s North Island, plate coupling changes from weakly coupled in the northern part of the margin to locked in the south. Shallow slow slip events occur quasi-regularly in the northern weakly coupled part of the margin. In the last decade, magnetotelluric (MT) measurements have been collected at 408 locations along the margin to explore the relationship between the electrical resistivity of the subduction interface shear zone and plate coupling.
Our most recent field campaign (57 new measurements) covers the northernmost part of the Raukumara peninsula. These data show that the dipping conductor above the subduction interface imaged previously to the south is present beneath the entire Raukumara Peninsula and correlates with a large area of extensional strain rate consistent with weak plate coupling. The paucity of seismicity within 3km of the interface in the conductive areas of the plate interface corroborates our earlier interpretation that fluid and/or clay rich sediments are consistent with an area with a decreased density of asperities and stored strain.
While the conditions that lead to slow slip and changes in plate coupling are not fully understood, the presence of fluids within the subduction-interface-shear-zone are believed to play an important role. This belief is supported by the correlation we see along the Hikurangi margin between the resistivity at the depth of the plate interface in the MT data and the areal strain rate derived from GPS measurements. The correlation suggests that where plate coupling is weak, fluid and/or hydrated-clay rich sediments are abundant while where the plate coupling is strong or locked, the shear zone is thin and/or fluid and sediment poor.
Our most recent field campaign (57 new measurements) covers the northernmost part of the Raukumara peninsula. These data show that the dipping conductor above the subduction interface imaged previously to the south is present beneath the entire Raukumara Peninsula and correlates with a large area of extensional strain rate consistent with weak plate coupling. The paucity of seismicity within 3km of the interface in the conductive areas of the plate interface corroborates our earlier interpretation that fluid and/or clay rich sediments are consistent with an area with a decreased density of asperities and stored strain.
While the conditions that lead to slow slip and changes in plate coupling are not fully understood, the presence of fluids within the subduction-interface-shear-zone are believed to play an important role. This belief is supported by the correlation we see along the Hikurangi margin between the resistivity at the depth of the plate interface in the MT data and the areal strain rate derived from GPS measurements. The correlation suggests that where plate coupling is weak, fluid and/or hydrated-clay rich sediments are abundant while where the plate coupling is strong or locked, the shear zone is thin and/or fluid and sediment poor.