When a 6.2 magnitude earthquake hit near Taumarunui last week, it was felt throughout New Zealand.
But what it did in the Waikato was more revealing.
University of Waikato’s Dr Seokho Jeong has been recording earthquakes with a seismometer installed at the Hamilton campus. Right in the heart of what’s referred to as the Waikato Basin. He says that the soft sedimentary soils in the Basin amplify earthquakes. And he’s got images from last week’s quake to illustrate the point.
Dr Jeong’s analysis:
On the 30 October, 2018 the seismometer on the Hamilton campus recorded a magnitude 6.2 earthquake, 25km south-west of Taumarunui. The recorded ground motion immediately drew my attention; the duration of shaking was much longer than usual and it lasted about three minutes. It is well-known that the ground motions are amplified on the soft ground; the duration also increases due to the reverberation of the waves trapped within the soft sediments. This phenomenon is called the “basin effect” among the earthquake engineers and seismologists. The prime example of such effect is the 1985 Mexico City earthquake, which killed thousands of people in the Mexico Valley, more than 350 km away from the fault; the same phenomenon was also observed during the 2010-2011 Canterbury earthquakes in New Zealand. Comparing the ground motion recorded on the Hamilton campus with what was recorded at the nearest (20 kilometres away) GeoNet Tahuroa Road broadband station in the mountains clearly demonstrates the so-called basin effect; the motion recorded on the Hamilton campus lasted much longer and had a three times higher amplitude.
Larger and closer earthquakes will be similarly amplified. The waves also tend to get trapped within the basin, which means the shaking would be stronger and longer-lasting. Despite its relatively low regional seismicity, the impact of a future large earthquake in the Waikato region would be high, considering the region has a population of 460,000, the fourth largest regional economy in New Zealand (Stats NZ, 2017), and critical national infrastructure, such as the Waikato expressway, high-pressure gas pipes, and high-voltage electricity transmission lines. Any damage to those lifelines will also slow down the recovery. The geology of Waikato Basin is characterised by soft, challenging soils. But the quantitative knowledge on the dynamic characteristics of Waikato soils is lacking and the region is heavily under-instrumented compared with other regions in New Zealand.
The seismic hazard was considered “low” in the Canterbury region before the deadly earthquakes, but we still suffered tremendous damage. A reason for the underestimated hazard was that the active faults buried under the sedimentary soils were unidentified. Even though there is no evidence that a large earthquake will occur in the Waikato region any time soon, we do have evidence suggesting that a moderately large earthquake will have a high impact in this region, which stresses the need for the improved prediction capability of seismic hazard and the associated risk in the Waikato region.