Earthquake magnitude not always a good indication of how strong it feels - GeoNet

Tuesday, 7 April 2015

Christchurch people might be feeling rattled by two Easter aftershocks, but there is nothing unusual about the shakes.

The first shake rolled through the area at 4.08pm on Sunday. The magnitude-3.8 quake was centred near Lyttelton at a depth of 11 kilometres.

Cantabrians were woken at 3.03am on Tuesday by another magnitude-3.8 shake near Halswell, this time 13km deep.

GNS Science duty seismologist Caroline Little said the two shakes were a normal part of the aftershock sequence after the 2010 and 2011 Canterbury earthquakes.

'There will be more quakes than there used to be in Christchurch for decades to come,' she said.

'We can still expect these moderate-sized quakes to happen every now and then.'

Little said there was a spike in 'felt reports' on GeoNet. for the Sunday and Tuesday shakes compared with recent activity.

She thought that might be because the aftershocks were closer to the city than other recent shakes.

'Over the last few months the other big ones have been out near Hanmer Springs and Oxford,' she said.

'Being just that bit closer, people felt them a bit bigger.'

SHAKING INTENSITY

GeoNet offered an explanation after some Christchurch residents queried the magnitude reading for Tuesday's quake.

Some of those who felt it expressed surprise on GeoNet's Facebook page that a higher magnitude wasn't recorded.

'There should be a bicycle vs car vs truck vs train scale: ie what did it feel like drove through your house. This morning was a truck,' Jamie Scoble said.

Another described the quake as a 'very violent jolt'.

GeoNet said responded by saying it often heard the comment that earthquakes felt bigger than the magnitude reading suggested.

While a quake's n event's magnitude was the true size of an earthquake and of the energy released at the source, it was independent of observers and did not describe how the quake was felt on the surface, GeoNet said.

The measure of what a quake felt like was its shaking intensity. That depended on a range of factors, including distance from a quake, its size and depth, the kind of rocks between an observer and the source, and the material an observer, or the building they were in, rested on.

Intensity was measured on the Modified Mercalli scale, which ranged from MM1 for imperceptible to MM12 for complete destruction.

The two recent Christchurch quakes had an intensity of MM5, described as moderate and meaning they were generally felt outside and by almost everyone indoors.

The intensity of the 2014 Eketahuna quake was MM8, while the 2013 Lake Grassmere quake was MM9.

The 2010 Darfield quake and the 2011 Christchurch quake were both MM9, while the 1855 Wairarapa and 1931 Hawke's Bay quakes had an intensity of MM10.

GNS Science continues to produce an earthquake forecast every month for Canterbury.

It estimates a 53 per cent chance of one or more aftershocks between magnitude 5.0 and 5.9 in the next 12 months.

The chance of a magnitude 6.0 to 6.9 aftershock in the next year has dropped to 6 per cent.

'These smaller ones will obviously happen more frequently,' Little said.

She said there had been 12 aftershocks around the same magnitude as the two 3.8 shakes in the past year.

'That's on average once a month,' she said. 'We can expect something like that for the next year at least.'