Progress in understanding the paleoseismicity of the central and northern Alpine Fault, Westland, New Zealand

Abstract

Radiocarbon dates from trenching of the Alpine Fault trace at Crane Creek, between the Haupiri and Ahaura Rivers, demonstrate the last earthquake rupture at this location occurred between AD 1480 and 1645, with associated local river aggradation and terrace formation. A second trench 6 km farther north at Ahaura gives the same radiocarbon age for the last event.An enlarged record of radiocarbon ages for aggradation terraces and landslides in central and north Westland has a group of dates which are a reasonable match to the date range from the trenching. While this is compatible with regional aggradation and landsliding associated with an Alpine Fault earthquake during this period, this type of data cannot be used to demonstrate the dates were synchronous or coseismic.However, historical earthquakes have shown that, in steep forested terrain, an earthquake causes extensive forest mortality, with a corresponding synchronous period of regeneration. Recent information of forest age in Westland indicates two such periods in the last 600 yr, and the most recent of these reflects the earthquake rupture recorded in the trenches. This implies the earthquake occurred at the young end of the trench date range at c. AD 1600-1650.

Review Article: Potential geomorphic consequences of a future great (<i>M</i><sub>w</sub> = 8.0+) Alpine Fault earthquake, South Island, New Zealand

“…This suggests that the next Alpine Fault earthquake is likely to occur at a relatively shallow depth of 8-12 km. Studies by Bull (1996), Yetton (1998), Wells et al (1999), and Wells and Goff (2007) (Fig. 3) show that the northern and central sections of the Alpine Fault usually rupture during great earthquakes while the southern section may be less frequently activated.…”

“…The welldefined 200 km long central segment accommodates most of the ∼ 37 mm a −1 relative plate motion as a combination of dextral slip (∼ 70 %) and uplift (∼ 30 %). Recognised as a large-displacement source in the 1940s (Wellman and Willett, 1942) and generally accepted as a potential seismic source in the 1990s (Yetton, 1998), the Alpine Fault is now thought to be capable of generating great earthquakes (M w ≥ 8) with a recurrence interval (average time between events) of the order of 200-400 yr (Berryman et al, 2012). The most recent rupture was ca.…”

“…The most recent rupture was ca. AD 1717, and involved a 380 km long segment of the fault (Yetton, 1998;Wells et al, 1999). Currently, there is an annual likelihood of rupture on the Alpine Fault of 1-2 % (Rhoades and Van Dissen, 2003).…”

Late Holocene landscape change history related to the Alpine Fault determined from drowned forests in Lake Poerua, Westland, New Zealand

“…Although there have been some on-fault trenching studies (e.g. Yetton, 1998Yetton, , 2000Berryman et al, 2012), paleoseismic studies have been dominated by off-fault data that rely on dating landscape surfaces using dendrochronology (e.g. Yetton and Wells, 2010;Wells et al, 1999Wells et al, , 2001.…”

“…Yetton and Wells, 2010;Wells et al, 1999Wells et al, , 2001. Dendrochronolgic studies have been undertaken at geomorphic settings that include young landslide deposits and scars (Adams 1980;Yetton 1998), alluvial terraces and fans (Yetton, 2000;Cullen et al, 2003;Wells et al, 2001), and coastal sand dunes (Wells and Goff, 2007). Estimated ages for the four most recent major ruptures along the length of the Alpine Fault are ca.…”