What are the risks and likely impacts of an earthquake in Thames?

As our thoughts go out to the people suffering trauma from the North Canterbury earthquake it is natural to be thinking “could that happen here”?

It turns out that while the risk is still quite remote, and there is no cause for panic,  a “big one” in Thames is more likely than previously assumed.

Thames is just 10 km distant from part of an active fault – the Kerepehi fault.

New research has recently been published by GNS Science on the fault which runs for about 80 km from Matamata through the Hauraki Plains, and into the Firth of Thames.

Rather than a rupture interval of 6000 – 8000 years, as previously thought, the new research suggests the interval between any segment of the fault rupturing is more likely to be about 1000 years.

The fault has multiple segments.  The report concludes an earthquake of up to magnitude 6.8Mw to 7.4Mw could be induced depending on whether a single segment or multiple onshore segments ruptured together (less likely).  For comparison, the Seddon earthquake of 2013 was 6.6, and Christchurch 2011 was 6.3.

In November 2015, based on a draft version of the GNS report, the Hauraki District Council held a workshop with a scenario for a 6.7Mw earthquake near Awaiti.   A map presented to the workshop showed Thames/Coast falling within a zone having MM7 intensity (Modified Mercalli Intensity) damage from such a quake.

MM7 intensity would cause some unreinforced and brick walls to crack, some chimneys and tiles to fall, and water cylinders to move and leak.

If the epicentre was closer to Thames at Kopuarahi for example (10 km), the damage could possibly be more severe – moving Thames into the MM8 (heavily damaging) or MM9 (destructive) zones.

A complicating factor for Thames is that the low-lying parts of the town, including the CBD, are built on debris flows of soil and gravel, or on reclaimed land overlaying marine mud.  Waikato Regional Council’s earthquake hazard map confirms that parts of Thames are within the “most hazardous” area.  These “most hazardous” areas have high water tables and are liquefiable.

The WRC map notes that amplification of 2 MMI units and settlement/liquefaction is common in these areas.  This suggests that an earthquake which might normally cause moderate damage (eg. MM7), might cause more severe (MM8), or even destructive damage (MM9) due to the underlying soils.

The Thames-Coromandel District Council and WRC have done excellent community engagement work with east coast communities regarding tsunami hazard.  But since the publication of the new GNS report in May 2016, TCDC has been too slow to engage with the Thames community on the potentially heightened earthquake risk.  HDC proactively held a workshop a year ago.  It seems no one from TCDC attended.  The Council’s Plans, Strategies and website show only a passing reference to local earthquake risk, and there has been no obvious community engagement. The new Council should liaise with WRC and get cracking with a community education programme.

The experience from Christchurch and now Kaikoura/Wellington highlights the vital importance of not underestimating natural hazard risks, and of having a community fully informed about them, and properly prepared.