In my first post I noted that Thames Coromandel Council/Civil Defence had failed to take heed of a May 2016 GNS Science which documented a potentially greater earthquake risk to Thames from the Kerepehi fault.
Well it gets worse. An earlier GNS Science report of February 2014 has also gone unheeded by TCDC and Waikato Regional Council. This 2014 report suggests that even greater levels of damage to parts of the Thames Coromandel District could result from an earthquake in the offshore part of the fault in the Firth of Thames.
The scenario in the 2014 report has the fault rupturing in the Firth offshore from Wilsons Bay, causing a magnitude 7.2 earthquake. A map in the report shows that areas around Wilsons Bay could suffer (“destructive”) MM 9 damage, and all of the Western seaboard of the Coromandel Peninsula, including Coromandel town and Thames could suffer MM8 “very damaging” impacts.
The 2014 report deals only with potential damage to Auckland city. The projections are based on the research available in 2014 that the Kerepehi fault might rupture at 6000 – 8000 year or even longer intervals. The latest 2016 GNS report suggests, the interval could be as frequent as every 1000 years.
Although the 2014 report suggests there is a very low probability of a major earthquake from the Kerepehi fault, were it to occur there would be substantial damage to Auckland buildings and infrastructure of approximately $1 billion, and some loss of life . The map shows Auckland within the lesser MM 7 zone of “damaging” intensity. However areas around Wilsons Bay are within the “destructive” MM 9 zone, and most of the Western seaboard of the Coromandel Peninsula, including Thames is in the “heavily damaging” MM 8 zone. What would be the extent of damage and loss of life in Thames/ West Coast from this scenario? We don’t know because no one has asked.
In spite of having all of this Auckland information available to it from early 2014, and having a draft copy of the 2016 Report since November 2015, TCDC/Civil Defence seem to have failed to have given either of these reports the serious attention they command, and has failed to meet its obligations to inform the public.
There seems to have been no public engagement/education program on earthquake risk to people living on the western side of the Peninsula. This is simply not good enough, and the Councils needs to get their act together pronto.
In my first blog I noted how an earthquake could cause greater than expected damage on softer soils which are subject to liquefaction. The earthquake hazard map from WRC shows that parts of Thames and Coromandel town are on “most hazardous” soft soils.
This graphic from the 2014 GNS report shows clearly how “normally expected” damage impacts can be intensified into “heavily damaging” and “destructive” impacts because a structure is situated on softer soils. Note – the diagram refers not only to greater damage from liquefaction but also from landslides.
Also noteworthy is that the epicentre chosen for the 2014 report is in the northern part of the Firth. Should the rupture of the fault occur in the southern part of the Firth opposite Thames, – just 8 km away – this would possibly bring the town and the Thames Coast into an area subject to “destructive damage”.
Meanwhile Dr John Ristau from GNS Science says if the subduction zone that runs beneath the entire North Island were to rupture it would produce an enormous earthquake in the North Island of at least magnitude 8.5 or greater. No city, (or Town) including Auckland, would be spared.
The possibility of a large local earthquake is still low and there is no need for alarm. But that must not hinder the District and Regional Councils from:-
- initiating an immediate a public information program.
- undertaking research similar to that done for Wellington and Auckland to assess the potential for property damage and loss of life on the western side of the Peninsula.
- mapping areas of Thames and Coromandel town which have soft soils and are particularly susceptible to liquefaction and landslide, and
- investigating whether the existing planning and building code rules and enforcement are fit for purpose.
These are potentially difficult and distressing issues, but that is not an excuse for looking the other way.