3He dating of rockfalls helps to distinguish between proximal and distal paleo-earthquakes in Christchurch, NZ

The 2011 Christchurch earthquake series had severe consequences and surprised scientists for many reasons. Ground motions were extremely strong despite the relative moderate magnitudes of the quakes (M_W 5.3-7.1). The events happened on a system of hitherto unknown faults, some of which are located directly below Christchurch. Earthquake environmental effects (EEE), especially liquefaction, were intense and widespread. It turned out that subsequent quakes reactivated the same feeder dikes of sand blows, showing that saturated sediments are susceptible of liquefaction no matter if they had been liquefied recently (also see the paper of Quigley et al. (2013) on the liquefaction effects). Another stunning lesson was the occurrence of intense rockfall in the vicinity of Christchurch. In a recently published study, Mackey and Quigley (2014) dated rockfall boulders with ³He and show that they allow to estimate the recurrence intervall of local seismic events like the 2011 series. This works is a very interesting way to use EEE for paleo-earthquake studies.

The authors sampled boulders from a site where rockfalls were observed during the 2011 quakes and where older boulders could be found, too. ³He dating was used to determine the age of the boulders, probably the first time this technique was applied on paleorockfalls. It turned out that the ages clustered around 7 ka BP and 14 ka BP. This observation suggests that seismic events could be a possible trigger, instead of (frequent) heavy rain events or other mechanisms.

Now a problem is that New Zealand has quite many faults and a number of them could serve as candidates for the necessary shaking. Local faults like the ones that were activated during the 2011 quakes could definitely act as a seismic source – they proved this 3 years ago. It is known that very strong earthquakes also occur at the major seismic sources like the Alpine and Porters Pass Faults. These faults have earthquake recurrence intervalls of a few hundred years only. That’s the first hint that these faults are not responsible for the observed rockfalls since there were no major rockfalls during the last 7 ka.

From the 2011 quakes it was known that very high peak ground velocities (PGV) triggered the mass movements, and that PGV of less than 12 cm/s did not trigger rockfalls. Simulations by Mackey and Quigley (2014) indicate that the PGVs caused by the Alpine Fault and similar sources would be too low in Christchurch to trigger the rockfalls they observed. They conclude that the local faults are the best candidates for the rockfall triggering quakes, and that they likely have earthquake recurrence intervalls of ~7 ka.

This study shows the value of EEE to reconstruct paleo-earthquake activity. Faults that do not reach Earth’s surface (blind faults) can simply be unknown and it is likely that we are going to find much more of those in the future. Moderate earthquakes may not rupture the surface, leaving no geomorphological imprint that could be used to detect them. EEE like liquefaction and rockfalls may, however, still occur and allow to get an idea on the earthquake history. This information can then be used to search for the seismic sources.

Good news is that Mark Quigley told me that he’d like to blog at paleoseismicity.org, too. Welcome, Mark! We are looking forward to reading more about New Zealand earthquake science here in the future.

References and further reading

• Gosse, J. C., & Phillips, F. M. (2001). Terrestrial in situ cosmogenic nuclides: theory and application. Quaternary Science Reviews, 20(14), 1475-1560.
• Mackey, B. H., & Quigley, M. C. (2014). Strong proximal earthquakes revealed by cosmogenic 3He dating of prehistoric rockfalls, Christchurch, New Zealand. Geology, G36149-1.
• Michetti, A. M., Esposito, E., Guerrieri, L., Porfido, S., Serva, L., Tatevossian, R., … & Roghozin, E. (2007). Environmental Seismic Intensity Scale-ESI 2007. Memorie Descrittive della Carta Geologica d’Italia, 74, 41.
• Quigley, M. C., Bastin, S., & Bradley, B. A. (2013). Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence. Geology, 41(4), 419-422.

Tweet