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New aftershock maps show daily risk from quake shaking - 07/03/2011

GNS Science has started issuing a daily graphic showing the probability of strong earthquake shaking in the mid part of the South Island between Cheviot and Timaru and across to the west coast.

It is the first time the 24-hour aftershock forecast graphics have been used in New Zealand. They have been used in California by the US Geological Survey since 2005.

GNS Science geological hazards modeller, Matt Gerstenberger, developed the forecasting graphics in California while completing his PhD. He returned to New Zealand in 2005 and has adapted the model for local conditions.

The two main components that make up the graphic are the National Seismic Hazard Model and all the aftershocks that have occurred in the wake of the magnitude 7.1 earthquake near Darfield in September 2010.

The National Seismic Hazard Model is held on computers at GNS Science. It includes the national database of earthquakes in New Zealand going back more than 150 years, and information on earthquake faults going back thousands of years.

“The colour-graduated graphics show the probability of strong ground-shaking throughout Canterbury during a 24-hour period,” Dr Gerstenberger said.

The graphics are now a permanent feature of the GeoNet website: http://www.geonet.org.nz/canterbury-quakes/aftershocks/

GNS Science has started producing daily aftershock forecast maps of the Canterbury region. The example illustrated shows   graduated range of probabilities of slightly damaging earthquake shaking across Canterbury for a 24-hour period. Over time, the daily colour-coded maps change as the risk from aftershocks decreases.

GNS Science has started producing daily aftershock forecast maps of the Canterbury region. The example illustrated shows graduated range of probabilities of slightly damaging earthquake shaking across Canterbury for a 24-hour period. Over time, the daily colour-coded maps change as the risk from aftershocks decreases.

“Right now the input data is dominated by the aftershocks that have occurred in Canterbury during the past six months. Historic earthquake patterns are secondary. This will change as the aftershocks gradually decrease in frequency and the hazard from them decreases.”

The graphic shows the probability of a certain strength of ground-shaking as described by the Modified Mercalli Intensity Scale.

It shows the probability of MM6-level shaking on a scale of zero to 12. This is shaking strong enough to knock objects from shelves and cause minor non-structural damage to buildings.

“This scale is not magnitude, which is a measure of the amount of energy released at the focal point of the earthquake. Instead it shows how strong shaking may be at different locations and also if damage may occur.”

MM6-shaking is roughly equivalent to what you would feel standing directly above a magnitude 4.0 earthquake, or standing 30km from the epicentre of a magnitude 6.0 quake.

By comparing different locations on the graphic, it is possible to see the relative chance of strong shaking occurring at a certain location.

“Also, by comparing the graphic day-to-day, it’s possible to see how the chance of damaging shaking is changing. As the aftershock sequence develops, this chance will typically decrease.”

Dr Gerstenberger said seismologists could never say exactly when the next aftershock would occur, but they could work out a probability than an aftershock of a given size would occur in a given time period.

“The main use of these graphics is educational. Watching the changes in probabilities helps people to understand the changing nature of aftershock sequences.”

The graphics have been displayed on the United States Geological website since 2005, where they show daily aftershock probabilities in California, which experiences a similar number of earthquakes to New Zealand.

“The public reaction in California has been very positive. One of the main things people have learned is if they live in a particular location, they don’t need to worry about aftershocks.”

New Zealand, California, and Switzerland are currently the only places where the graphics are made available to the public, and updated daily.

The Royal Society has posted a paper from the Office of the Prime Minister's Science Advisory Committee which answers many questions related to the Canterbury Earthquakes.