Tuesday, April 7, 2015

That Earthquake Felt Bigger

... is something we often hear at the office.

Now while an earthquakes magnitude is the true size of that particular quake and the energy it released at the source, it does not describe how that energy was directed,  and how the quake was felt on the surface.

So you can have two magnitude 5 quakes in different towns, with very different results:
~ one causes no damage and not all people felt it,  while the other caused damage in most houses,was felt my most people, and many were shaken and scared.
So why the difference? Read on to see

This is where Intensity comes in

You can see each quakes intensity on our website - its based on a number scale but we have added in words and colours to make it easier/quicker to see. You can see in the table below, the scale ranges from MM1 - unnoticeable (which may be felt by some people)  to Severe - which covers damaging up to worse case scenario MM12.

The two Christchurch quakes this Easter Weekend, both M3.8, were both MM5/Moderate - which if you read below in the table, were not small quakes!

So next time you look at an earthquake, check out its intensity - it will give you a better idea of the effects the quake had on people/buildings etc. on the surface.  If you look at your regions quakes you will see two intensities given, so how the earthquake was felt at its location, and how the earthquake was felt at your location. The Auckland page is a good example of this, as they don't frequently get quakes in the Auckland region, so you can see the quakes listed are all of a larger intensity, but only felt 'weak' in Auckland.

Full page here

So those two Magnitude 5 quakes...
~ The first had an intensity of MM4/ light  and the second was MM6/Strong
So you cannot take a quake at its magnitude/face value, you really need to know the MMI level to truly know how the people on the surface felt it.

Some Intensity examples of our quakes: 

The 2014 Eketahuna quake was MM8

The 2013 Lake Grassmere quake was MM9  and the Cook Strait quake was MM8

Both the 2010 Darfield earthquake, and the 2011 Christchurch earthquake were MM9

The 1855 Wairarapa, and 1931 Hawkes Bay earthquakes both had an intensity of MM10!

Check out the boss' blog with more on felt intensity.