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2013 Cook Strait earthquake sequence

InSAR imaging of displacement on flexural-slip faults triggered by the 2013 Mw 6.6 Lake Grassmere earthquake, central New Zealand (Kaneko et al., 2015)

Interferometric Synthetic Aperture Radar (InSAR) data reveal surface slip on multiple faults triggered by the 2013 Mw 6.6 Lake Grassmere earthquake, New Zealand. Surface offsets of 1–2 cm occurred on previously inferred flexural-slip faults located ∼4 km from the epicenter. 

Coulomb stress analysis indicates that slip was not triggered by the static stress change of the main shock but was likely caused by dynamic shaking during the passage of seismic waves. Our finding also provides an important clue on how some slip on shallow flexural-slip faults takes place.

KaekoCookStraitInSAR

Line-of-sight (LOS) displacement from a TerraSAR-X interferogram, the best fitting LOS displacement from inverted slip distributions, and the residual for (a) the entire region and for (b) a subset (within a dashed box) that includes Faults B and C. (Kaneko et al., 2015)

Sources, ground motion and structural response characteristics in Wellington of the 2013 Cook Strait earthquakes. (Holden et al., 2013).

The Cook Strait earthquake sequence occurred in a region of known high seismicity. However, this was the strongest shaking felt in decades for the Wellington region and the top of the South Island. The location and size of the earthquake meant that the ground shaking was of rather short duration and moderate intensity, except for the epicentral region of the Lake Grassmere earthquake where a PGA of 0.7g was recorded, and for part of the Wellington foreshore where up to 0.2g was recorded in both earthquakes.

Amplification and polarization in the NE-SW direction at approximately ~1 s period at many Wellington stations is likely due to basin resonance effects, whereas dominant polarization in the NW-SE direction at shorter periods is consistent with a directivity effect, and is particularly evident in the Lake Grassmere earthquake.

Figure 19 The 2013 Cook Strait earthquake sequence, to date, comprising a Mw 5.5 foreshock on 19 July, 2013, the Mw 6.6 Cook Strait earthquake on 21 July, the Mw 6.6 Lake Grassmere earthquake on 16 August and many accompanying aftershocks. The epicentral region is characterized by historically high levels of seismicity and several major active fault structures including the west-dipping subduction interface at ~25 km depth. The seismicity to date has largely occurred on an unmapped structure at depths of 6 – 18 km, and generally follows a northeast-southwest trend.

The 2013 Cook Strait earthquake sequence, to date, comprising a Mw 5.5 foreshock on 19 July, 2013, the Mw 6.6 Cook Strait earthquake on 21 July, the Mw 6.6 Lake Grassmere earthquake on 16 August and many accompanying aftershocks. The epicentral region is characterized by historically high levels of seismicity and several major active fault structures including the west-dipping subduction interface at ~25 km depth. The seismicity to date has largely occurred on an unmapped structure at depths of 6 – 18 km, and generally follows a northeast-southwest trend.

Figure 20: Preliminary slip history for the Cook Strait earthquake resolved onto a 18 by 12 km near-vertical fault plane striking NE-SW with up to 1 m of slip at a depth of about 12 km and a bilateral rupture. Fault parameters are strike 234, dip 75 degrees, rake 164 degree, rupture velocity 2.2 km/s. The black and red arrows are measured and observed ground displacement respectively following the Cook Strait earthquake (Maximum of displacement at WITH moving 5 cm to the East). The inset shows the rupture history on the fault plane looking from the North: the rupture time contours are in white isochrones, the red star is the GeoNet hypocentre location and colours represent slip amplitudes (scale in metres). This model does not suggest any particular directivity effect (Holden et al., 2013)

Preliminary slip history for the Cook Strait earthquake resolved onto a 18 by 12 km near-vertical fault plane striking NE-SW with up to 1 m of slip at a depth of about 12 km and a bilateral rupture. Fault parameters are strike 234, dip 75 degrees, rake 164 degree, rupture velocity 2.2 km/s. The black and red arrows are measured and observed ground displacement respectively following the Cook Strait earthquake (Maximum of displacement at WITH moving 5 cm to the East). The inset shows the rupture history on the fault plane looking from the North: the rupture time contours are in white isochrones, the red star is the GeoNet hypocentre location and colours represent slip amplitudes (scale in metres). This model does not suggest any particular directivity effect (Holden et al., 2013)