- Published on Tuesday, 21 February 2017 03:06
Despite being well used to earthquakes by now, the Kaikoura event of 14 November was a very different experience for us at CSI.
From a seismological point of view, the quake itself was extremely interesting and complex, with a series of multiple ruptures progressively moving north and east of the epicentre. An interesting view of the rupture sequence is here.
A CSI CUSP-3C Strong Motion Accelerograph (GeoNet Station WTMC) was closest to the epicentre. This station recorded an extremely high vertical PGA of over 3g – a world record we believe! Analysis by our team confirms the instrument was functioning correctly and the extremely high PGA, periodicity and strongly asymmetric character of the record are suggestive of a 'trampoline effect' (also observed in the Christchurch 22 February 2011 event).
Aftershock activity in the upper South Island remains significant. Latest aftershock probability analysis is here. We are certainly becoming well accustomed to living in an age of increased seismic activity.
In Christchurch, the surface shaking was characterised by long period (low frequency) motion with relatively low peak accelerations. This was a marked difference in the felt motion when compared to the many quakes generated during the Canterbury earthquake sequence. The quake itself was long enough for us to get out of bed and watch shaking information coming through as it happened – true real time!
The damaging effects have been unevenly distributed. Christchurch experienced PGA of around 50 mg, and no damage was reported. Conversely, due to the focus of energy to the North East, Kaikoura (PGA 400 mg), Blenheim (PGA 350 mg) and Ward (PGA 1400 mg) experienced much higher levels, despite greater distance from the epicentre.
The capital city of Wellington (epicentral distance 200 km) experienced noticeable variation in shaking. Stiffer soil sites were typically characterised by a periodicity of around 1 s, but with significant variation in PGA from 122 mg to over 266 mg. Conversely, the softer soil sites had periods of 1.5 – 2.0 s, with a PGA range from 200 mg to 350 mg. One certain thing that we notice is that as more strong motion instruments are installed, the greater the observed variability.
A key lesson, in terms of quake response, is that many people and businesses still rely on the old rule of thumb – that is, action is only needed if the quake exceeds M5.5 inside 50 km. The November 14 quake was a clear demonstration as any of the fallacy of this approach; it is an extremely unreliably guide to likely effects. On this basis, Christchurch would have been severely damaged, Hanmer Springs obliterated and Wellington untouched… The sooner this dubious practice is dispensed with, the better.
The reliance on this method gave people and decision makers in Wellington false confidence that the quake wasn’t so bad, when in fact our instrumentation told us immediately that the quake had exceeded NZ building design code in many parts of Wellington.
CSI’s EQ Response systems eliminates this uncertainty, and tells you instantly, in plain language what the shaking is on a particular site. Why estimate or guess when you can measure and be certain? Check out our website here. And case studies here.
Used in conjunction with appropriate, targeted, engineering inspection, it gives valuable site-specific information to allow you, the tenant or owner, to prioritize and target what is most critical.