ON THE USE OF DEEP-SEA GRAVITY-FLOW DEPOSITS TO UNRAVEL SEISMIC HAZARD: TURBIDITE PALEOSEISMOLOGY OF THE NORTHERN HIKURANGI SUBDUCTION MARGIN OF NEW ZEALAND
Résumé
Deep-marine depositional systems are of particular interest for hazard assessment as they
provide a well preserved and continuous sedimentary record of past climate changes, sea level
fluctuations, oceanographic conditions and tectonic activity over long time periods. While climate
changes and glacio-eustatic fluctuations are well recorded at 104 time-scales, episodic tectonic
deformation or short-lived extreme climatic events appear predominant in fostering turbidite
production at 101-103 time-scales. The turbidite paleoseismology approach aims at using this
specificity and seeks to characterise the signature of moderate-to-great earthquakes in the geo- logical
record.
Quaternary turbidites preserved in deep basins along active margins have proven to be powerful
tools for establishing calendars of paleo-earthquakes. In New Zealand, sediment cores collected in
strategic locations along the plate boundary present a stack of cm-thick turbidites interbed- ded
with hemipelagite and air-fall tephra beds. Age control is provided by exceptionally dense set of
radiocarbon dating and tephrochronology. Core-to-core correlations using similarities in
sedimentary facies, petrophysical properties and ages indicate the synchronicity of 19 turbidites across
the 200 km-long margin since 7500 yr BP. High-resolution sedimentological, chronostrati- graphic,
petrophysical, geochemical and micropaleontological analyses of the material indicate that 17 are the
distal expression of synchronous earthquake-triggered submarine landslides that occur on the
continental slope at 150 – 1000 m water depths. Well-established empirical relationships that combine
Peak Ground Acceleration (PGA) and earthquake characteristics (hypocentre, magnitude,
mechanism), classically used to evaluate slope stability, were adapted to deduce the source and
magnitude of those paleo-earthquakes. Isomagnitude maps based on a local slope stability threshold
PGA 0.08 constrain the average location and minimum magnitude of paleo- earthquakes. Compared
to the well-established active fault catalogue, this approach suggests the 17 synchronous turbidites
deposited from 390±170 to 7480±120 yr BP record the rupture of three active faults, including the
subduction interface, which all triggered major-to-great earthquakes Mw 7.3.
In term of seismic hazard assessment, the 7500 year-long paleo-earthquake catalogue extracted
from New Zealand deep-sea turbidites outreaches the 200 year-long historical record and comple- ments
the seismotectonic studies undertaken of the continental shelf and plate boundary. More specifically,
10 out of the 17 synchronous turbidites were interpreted as the sedimentary record of subduction
earthquakes. Their temporal distribution suggests an erratic tectonic regime of the subduction
interface with periods of high earthquake frequencies (~1860 yrs) comparable to pre- dictions made in
seismotectonic studies and periods of unexpected low earthquakes frequencies (~490 yrs). Such
clustering of subduction earthquakes was suggested in other active margins and contradicts the steady
seismic cycle hypothesis commonly used to build seismic hazard models