Felt Intensity – What You Feel is What You Get!

How do we size an earthquake?

A few earthquakes over the last few months have got me thinking about how we talk about earthquake size. In the western world we are quite focused on magnitude, but that only gives a starting point. I have already bored you enough about earthquake magnitude in a few previous blogs (see What’s the Magnitude?; Deep Earthquakes and Magnitudes; and Deep Earthquakes and Magnitude – Again!) – but to recap, an earthquake magnitude is an estimate of the true size of an earthquake independent of the observer (or where the observer is). The magnitude is only the start of the story if you want to understand the likely impacts of an earthquake. Not all earthquakes are created equal; some are more energetic than others even if they have the same magnitude. Some direct shaking energy towards where people live, and where you are compared to the earthquake source is very important. All this leads to the idea of shaking intensity – a mapping of the levels of shaking caused by an earthquake rather than a single number like magnitude.

Modified Mercalli Shaking Intensity

  

If you want to characterise how you feel an earthquake, then felt intensity is what you are after. This is a measure of the shaking where you are, and is given (at least in New Zealand) by the Modified Mercalli (MM) scale which covers the range from not felt (MM 1: Imperceptible) to complete destruction (MM 12: Completely devastating). Obviously an earthquake’s impact and the level of damage it causes are related to the intensity. The MM value at a particular place depends on the distance from the earthquake, its size and depth, the kind of rocks between you and the source and material you or the building you are in rests on.

ShakeMap (or where did the map go?)

On the old GeoNet website we had a display on the front page based on the shaking at each recording station (Figure 1). Although this gave a good indication of the where maximum shaking levels were being recorded by our instruments (and I know some of you want it back!), it could be biased by instrument issues and was often misinterpreted. So it was good for a quick look, but not really very useful for characterising the potential damage in detail.

Figure 1: The "ShakeNZ" plot for the Christchurch  Earthquake  of 22 February 2011. This map shows the shaking  levels as squares around the sensor sites which change colour and get larger as the shaking level increases.

We are working towards having a ShakeMap (as developed by the United States Geological Survey, see the USGS Shakemap site) available for larger earthquakes which will indicate the distribution of shaking (see Figure 2 for an example). This map will be produced within a few minutes of an earthquake occurring and be based on data from the sensor sites and a knowledge of how the earthquake waves travel through the Earth (tailored for New Zealand conditions). The map will show MM intensity but we will also be able to provide information in forms that are suitable for use by engineers interested in the level of shaking experienced by buildings or other structures in the region. This can include shaking accelerations at different periods of oscillation – different size structures are more susceptible to different shaking oscillations caused by earthquakes.

Current planning should see ShakeMap on the new GeoNet website within the next few months.

Figure 2: An example ShakeMap for the Christchurch Earthquake of 22 February 2011. This is  an example of what the ShakeMap on the new GeoNet website may look like.

So what about shaking duration?

This is a hard one as the perceived duration will depend both on the size of the earthquake and where you are (a bit like intensity) but is also very dependent on the near surface structure under your feet. For example, if you live in a valley the shaking waves will “bounce around” in the valley and the shaking will go on for much longer than if you were on a hard rock site. We can estimate how long the fault takes to rupture (by studying the earthquake waves recorded on our instruments), but how long the Earth shakes depends on the size and distance, and how many ways the earthquake waves reach you (some waves “bounce” around in the Earth so the shaking goes on much longer than the fault break time). For these reasons we do not usually use duration as a measure of earthquake size.

To put this in terms of recent experience the fault-break of the Christchurch Earthquake (22 February 2011) was over in just a few seconds, but the shaking went on longer because of the near-surface structure under the city. But the total duration in areas of maximum damage was only around 10 - 15 seconds. Compare that to a possible Alpine Fault earthquake much further away from Christchurch where the shaking intensity would be much less in the city (in fact even much less than the Darfield Earthquake of 4 September 2010) but the shaking would go on for minutes. Duration is not a good indication of likely earthquake impacts. 

GeoNet – Past, Present and Future

GeoNet needs your input …. But first some background:

Why do we need GeoNet?

New Zealanders live on the edge - astride the Pacific-Australia plate boundary, a part of the Pacific “Ring of Fire”. The level of earthquake hazard in New Zealand is similar to that of California and most communities have some exposure to this hazard. Additionally there is a significant volcanic hazard, both from the cone and caldera volcanoes of the central North Island and the volcanic field underlying its largest city, Auckland. Throughout New Zealand, landslides may be triggered by extreme weather or earthquakes, and the coastal areas are prone to tsunami, both from distant and local sources.

The case for GeoNet

In 2000 at the invitation of the New Zealand Earthquake Commission (EQC), GNS Science proposed the establishment of GeoNet, a geological hazards monitoring system. GeoNet would facilitate the detection, data gathering and rapid response to New Zealand earthquakes, volcanic activity, landslides, tsunami and the slow deformation that precedes or follows large earthquakes. This followed more than five years of equipment trials, capability reviews and widening concern about national geophysical infrastructure, the purpose and renewal of which had been largely overlooked during a major restructuring of the Government science sector in the early 1990s.

EQC launched GeoNet in 2001 through its research and education programme. In partnership with Land Information New Zealand (LINZ) and the Department of Conservation, EQC’s long-term support and direction of GeoNet has facilitated the creation of world-class capabilities.  GeoNet now has sensor networks throughout New Zealand (over 550 sites), distributed data collection, processing and distribution capabilities and a programme of continual improvement. In 2009, EQC renewed its commitment to GeoNet for a further decade, with the strategic focus shifting from delivery of minimum geographic coverage, to more sophisticated management of data and information to meet evolving user needs.

The GeoNet Review

Every four years an international strategic review of GeoNet is conducted to ensure its performance and map future directions and the next one will take place in late October this year. Earlier reviews took place in 2004 and 2008.

Since the 2008 review, the earthquakes in Canterbury have provided an extreme test of all GeoNet systems.  It is therefore timely to consider how GeoNet might be enhanced or extended to maximise the value of investment in the system.  This contemplates wider use of the collected information beyond the core geological hazards area.  For example, the current networks could be adapted to support country-wide, high-accuracy real-time positioning applications for many different sectors.

GeoNet Needs Your Input ….

If you regularly use GeoNet data and information for your work or analysis, or have used GeoNet data in a major project, we want to hear about it.  For example, we are aware that many people have used GeoNet strong-motion data in the analysis of the impacts of the Canterbury earthquakes and in published research papers, but the source of the data has not always been attributed, so it is hard for us to identify all related work without your help.

We are particularly keen to hear how GeoNet might be significantly improved in future. Please submit brief (maximum one page) summaries on either or both of these topics as soon as possible, not later than  21 September to geonet-review@gns.cri.nz.Your experience and ideas will inform the planning and direction for the next few years, so please help make a difference.