The Mw 6.6 Gisborne earthquake of 2007

Abstract

Gisborne city experienced recorded peak ground accelerations exceeding 0.25g for the third time since 1966 in the magnitude Mw 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.

Investigating the Basal Shear Zone of the Submarine Tuaheni Landslide Complex, New Zealand: A Core‐Log‐Seismic Integration Study

“…Given the location of the Tuaheni Landslide Complex atop an active subduction margin, earthquake shaking and/or any associated pore pressure change may have destabilized a weak layer (or layers). Sources of significant ground shaking could come from shallow crustal faults, intraslab faults, and the subduction interface (e.g., Francois‐Holden et al., 2008; Reyners, 1998; Stirling et al., 2012). Notably, transient fluid expulsion has also been interpreted in the region of the Tuaheni Landslide Complex on the basis of seafloor heat flow measurements (Pecher et al., 2017).…”

A Repeating Earthquake Catalog From 2003 to 2020 for the Raukumara Peninsula, Northern Hikurangi Subduction Margin, New Zealand

“…Two of these earthquakes occurred within the area of interest around the Raukumara Peninsula. The first was the M W 6.6 Gisborne earthquake, which occurred on December 20, 2007, UTC, 64 km from Gisborne at a depth of 40 km, in the subducting Pacific Plate (Francois‐Holden et al., 2008). The second was the M W 7.1 TA earthquake, which occurred on September 1, 2016, UTC, at a depth of 19 km, also in the subducting Pacific Plate (Warren‐Smith, Fry, Kaneko, et al., 2018).…”

“…These tectonic components exhibit a wide range of transient seismic phenomena throughout the Raukumara Peninsula and surrounding region (Wallace, Reyners, et al., 2009), including moderate‐ to large‐magnitude earthquakes ( M ≤ 7.2) (Francois‐Holden et al., 2008; Koulali et al., 2017; Warren‐Smith, Fry, Kaneko, et al., 2018), repeated episodes of shallow and deep slow‐slip (e.g., Douglas et al., 2005; Wallace, 2020; Wallace & Beavan, 2010; Wallace, Beavan, Bannister, et al., 2012; Wallace, Hreinsdóttir, et al., 2018; Wallace, Webb, et al., 2016), tectonic tremor (Todd & Schwartz, 2016; Todd et al., 2018), triggered seismicity (Delahaye et al., 2009), and recently documented burst‐type repeating earthquakes (Shaddox & Schwartz, 2019) (Figure 1). Several large earthquakes have occurred across the region in the last century.…”

“…Several large earthquakes have occurred across the region in the last century. Of particular note are two M W 7.2 tsunamigenic earthquakes that occurred in March and May of 1947 off the coast of Gisborne and Tokomaru Bay (Bell, Holden, et al., 2014; Doser & Webb, 2003), a normal‐faulting intraslab M W 6.6 event offshore from Gisborne in December 2007 (Francois‐Holden et al., 2008), and the M W 7.1 Te Araroa (TA) earthquake of September 2016 (Koulali et al., 2017; Warren‐Smith, Fry, Kaneko, et al., 2018). These larger events appear to often involve interactions with other seismic phenomena: for instance, the TA earthquake was preceeded by slow‐slip events (SSEs) (Koulali et al., 2017; Warren‐Smith, Fry, Kaneko, et al., 2018).…”