Gaye Downes

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Rupturing of the Awatere Fault during the 1848 October 16 Marlborough earthquake, New Zealand: Historical and present day evidence

Abstract Evidence from newspaper reports, diaries and journals, related first‐hand information, an 1854 survey map of the lower part of the Awatere Valley, and reports by geologists between 1856 and 1890, indicates that surface rupturing occurred on the Awatere Fault during the 1848 October 16 earthquake and not on the Wairau Fault as previously inferred. The rupture was initially described as a “fissure”, “crack”, and later as a “rent”, and although it extended for c. 105 km (from the coast to Barefell Pass), it was not termed a fault because displacement of the land surface or strata across the rupture could not be determined. The coincidence of the 1848 earthquake “rent” and the Awatere Fault was first demonstrated by Alexander McKay in 1885. Present day evidence of the 1848 Awatere Fault rupture is indicated by a depression between 0.6–1.5 m wide and c. 0.3 m deep that has the appearance of an infilled fissure similar to that described in early reports. The smallest and freshest displacements along th...

Pre-2010 historical seismicity near Christchurch, New Zealand: the 1869 M W 4.7–4.9 Christchurch and 1870 M W 5.6–5.8 Lake Ellesmere earthquakes

Abstract The 5 June 1869 (NZMT), 4 June 1869 (UT) Christchurch earthquake has long been known to have caused chimney and structural damage, to Modified Mercalli intensity MM7, in central Christchurch. On 31 August 1870, another strong earthquake shook the city causing widespread contents damage and the cracking and fall of a few chimneys. These two events represent the most significant near-field earthquakes in Christchurchs historical record prior to the 2010 M W 7.1 Darfield and 2011 M W 6.2 Christchurch earthquakes. Comprehensive data on the effects of 1869 and 1870 events now provide reasonable constraint on their locations and magnitudes. The 1869 earthquake, known as the New Brighton earthquake prior to 1999, occurred at shallow depth (upper crustal) about 3 kilometres southwest of Christchurch Central Business District, within the aftershock zone of the 2010 and 2011 events. The 1870 Lake Ellesmere earthquake had a larger magnitude, was deeper and was about 30 km south of Christchurch.