The ground motion you feel in an earthquake, now in map form!

Published: Fri Oct 13 2017 3:15 PM

A couple of months ago we brought you our new geodetic data interface, and now we can also introduce our new strong motion map.

The freshly released strong motion map beta shows the peak ground motion measured at all of our strong motion sites in the last hour. You can find it on our website under the ‘Strong Motion’ tab in the Earthquake dropdown menu.

Each strong motion site is shown on the map as a square, and any earthquakes in the last hour are superimposed as circles. The two symbols are coloured and scaled to show the size of the earthquakes and the ground motions, so with a quick glance you can see how much shaking has (or hasn't) been produced by an earthquake.

If you click on one of the strong motion sites you’ll see the horizontal and vertical Peak Ground Velocity (PGV) and Peak Ground Acceleration (PGA) measurements, as well as the peak Modified Mercalli Intensity (MMI) value recorded at that site in the last hour.

A snapshot of our application interface shows the locations of strong motion sites identified by grey squares, with colour change illustrating the occurence of PGV movement following a quake located east of Palmerston North.

A snapshot of our application interface shows the locations of strong motion sites identified by grey squares, with colour change illustrating the occurence of PGV movement following a quake located east of Palmerston North.

If you click on one of the strong motion sites (grey squares) you'll see the horizontal and vertical PGA and PGV measurements, as well as the peak MMI value recorded at that site in the last hour.

If you click on one of the strong motion sites (grey squares) you'll see the horizontal and vertical PGA and PGV measurements, as well as the peak MMI value recorded at that site in the last hour.

So, what do these numbers mean?

An earthquake's magnitude only tells us how big an earthquake is at its source. Strong motion data - PGV, PGA and MMI - tell us how much the ground shook at different locations as the earthquake waves spread out from the source - in other words, the shaking intensity at different locations.

PGV and PGA are closely related measures of shaking intensity that are directly calculated from the movement a strong motion sensor records. PGV is measured in cm per second (so how fast the ground actually moved back, forth, up and down during the earthquake) and PGA is measured as a proportion of g (the force of gravity - when there isn't an earthquake happening, an acceleration of 1g is normally sticking you to the planet).

MMI (Modified Mercalli Intensity) is a different, more descriptive way of measuring shaking intensity and describes the effects that shaking has on people and infrastructure. When you fill out a felt report you are providing a MMI measurement and becoming a strong motion sensor (albeit one with a bit more personality).

Strong motion instruments are a little different to the seismometers GeoNet uses to locate earthquakes and determine earthquake magnitudes. Strong motion instruments are designed to measure ground motions close to earthquake epicentres, so need to be sensitive to ground accelerations up to and beyond 1g, as well as the smaller stuff seismometers record. Overall GeoNet’s strong motion instruments are pretty chilled out about little earthquakes and won’t show much until a big one hits.

Strong motion data are influenced by what you might expect: an earthquake’s magnitude, depth, and distance from the recording instrument, but also by what you might not expect: the rock type under an instrument, the soil structure near an instrument, seismic wave arrival angle, and much more.

Who uses this sort of information and why?

This strong motion information is very useful for our duty team. Following larger earthquakes the team needs to rapidly assess strong motion data to understand where the most intense horizontal and vertical ground motions have been experienced. While the ground shaking in the area around a large earthquake epicentre will often be too strong for seismometers to record accurately, strong motion sensors will hold fast, painting a bullseye for our duty officers to hone in on. In regions further from an earthquake epicentre strong motion sensors highlight potential damage zones, making hazardous areas and people in need easier to identify.

Strong motion instruments are deployed all over the country, but you’ll find them most intensely clustered in urban areas. These urban instruments are often installed inside buildings to see how they respond to seismic waves – valuable information for earthquake engineers, hazard-savvy developers, and anyone else interested in understanding how buildings move during earthquakes.

Why symbols might be absent from the map and why there might be odd-looking values

Sometimes strong motion sites will seem to fall off the face of the Earth (if you’re looking at the strong motion map, that is). These absences occur either because a strong motion sensor has gone offline or because the data 'noise' at the strong motion site is overwhelming.

Even without a large earthquake, you may still find a strong motion site showing appreciable ground motions in the last hour. As you might expect earthquakes aren’t the only thing that can throw the ground around: passing trucks, strong winds, and high country sheep stampedes can all produce strong seismic waves capable of influencing a strong motion sensor. If you see such anomalous ground motions try not to pay them too much attention, unless you’re looking for a few lost sheep near one of our sites.

This week is the Ministry of Civil Defence & Emergency Management’s annual Get Ready Week and this year it’s all about staying informed. Our new strong motion map is another way for you to stay informed after an earthquake, and to see which areas may have been affected by strong ground shaking.

Thanks to GeoNet's data-savvy whiz kid, Sam Taylor-Offord, for his technical input.