The Mw 6.6 Gisborne earthquake of 2007: Preliminary records and general source characterisation

Caroline François-Holden; Stephen Bannister; John Beavan; Jim Cousins; Bryan Field; Rob McCaffrey; Graeme McVerry; Martin Reyners; John Ristau; Sergey Samsonov; Laura Wallace

doi:10.5459/bnzsee.41.4.266-277

Authors

Caroline François-Holden GNS Science, Lower Hutt, New Zealand
Stephen Bannister GNS Science, Lower Hutt, New Zealand https://orcid.org/0000-0002-2125-0506
John Beavan GNS Science, Lower Hutt, New Zealand
Jim Cousins GNS Science, Lower Hutt, New Zealand
Bryan Field GNS Science, Lower Hutt, New Zealand
Rob McCaffrey GNS Science, Lower Hutt, New Zealand
Graeme McVerry GNS Science, Lower Hutt, New Zealand
Martin Reyners GNS Science, Lower Hutt, New Zealand
John Ristau GNS Science, Lower Hutt, New Zealand https://orcid.org/0000-0002-7363-5205
Sergey Samsonov GNS Science, Lower Hutt, New Zealand
Laura Wallace GNS Science, Lower Hutt, New Zealand

DOI:

https://doi.org/10.5459/bnzsee.41.4.266-277

Abstract

Gisborne city experienced recorded peak ground accelerations exceeding 0.25g for the third time since 1966 in the magnitude M_w 6.6 earthquake at 075516 UT (8:55 pm local time) on 20 December 2007. The earthquake was at a hypocentral distance of 64 km from Gisborne at a depth of 40 km, well within the mantle of the subducted slab of the Pacific plate as it dips beneath the North Island of New Zealand. At this location the plate interface is about 10-15 km deep. The main event was followed by sparse aftershocks consistent with a rupture of the subducted plate, with the largest aftershock of magnitude 4.6 occurring on December 22nd. The GeoNet website received 3,257 felt reports, with a strongest intensity of MM8 (heavily damaging) assigned to the main shock.

The 122 strong motion records of this event show a clear regional directional variation in the wave propagation, as well as a distinct 2 Hz peak widely observed throughout the country. At a local scale, three sites in the Gisborne region recorded accelerations around 0.2g. Recordings in Gisborne city also revealed a predominant displacement direction, parallel to the main street where most of the damage occurred.

Source studies from moment tensor solution, aftershock relocations, GPS and strong motion data showed that the earthquake occurred within the subducted plate on a 45 degree eastward dipping fault plane. The mainshock rupture area is about 10 km² reaching a maximum slip of 2.6 m. The computed high stress drop value of 17 MPa is as expected for an intraslab event and consistent with observations of very energetic seismic waves as well as heavy structural damage.

GPS data recorded by continuous GPS instruments have also shown that slow slip occurred for about three weeks after the main shock. The slow slip was triggered on the subduction interface, rather than on the same fault plane as the aftershocks. This is the first clear-cut case worldwide of triggered slow slip, although three non-triggered slow-slip events have occurred in the same region since 2002.

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