The question: Is there a useful dataset to be derived from earthquake point density as opposed to earthquake energy density, or do both have a role to play?
First let me define these two terms in order to be clear.
The point energy is the amount of energy released by a number of earthquakes in a given area over a specified period of time.
The point density is the number of earthquakes in a given area over a specified period of time.
Why do we need either? Well point energy is an indication of the amount of seismic activity is an area which could either be an indicator of an impending major or great earthquake, or could give an indication of the rampdown after such a quake.
Applying the energy daily/weekly/monthly etc in a given area will certainly give an indication of changes but, and it is a BIG but if we were to take an extreme example, say Argentina, then the results of point energy could be very misleading.
Take for example this event:
- Date/Time: 2012-05-28T05:07:23.450Z
- Latitude: -28.043
- Longitude: -63.094
- Mag/Type: 6.8 Mw
- ~GJ Energy: 1,000,000
- Depth (km): 586.9
- Location: Cordoba Province. Argentina
So here we have 1 Million Terajoules of energy release and a Magnitude 6.8 earthquake so nearly a major quake. On an energy density map it should give quite a bit of colour. Let’s look at the resulting aftershock energy. On seconds thought why bother? There was only one. Here is the circular search of ANSS data for 200 kilometres from the epicentre from 2000 to present.
Even years are blue and odd years are grey, and the epicentral event and its aftershock are in red.
If I reduce the radius to 100 km to get everything into the one province I get this:
The data is all from ANSS. That 1 6.8 could represent 170 or more Magnitude 5 earthquakes, yet that one single event would swamp 20 or 30 Mag 5 events in the area.
What is not shown is the depths and most of these quakes are deep or deepish. There are never many aftershocks, as is usually the case from deep quakes, so an energy density map of this area would be of little point. Apart from the Mag 7 in 2000 and the Mag 6.7 in 2011, the 6.8 would weigh in well in an energy density and completely swamp other years other than those two. Ignoring the reasons for the moment, the point density would show as greater in the later years.
The output of energy density is always going to swamp any mapping of such, in exactly the same manner that it does when doing linear charts of energy and the lesser quake just get lost as blips on the bottom line. If 1000 magnitude 3 earthquakes are the same energy as one magnitude 5 lesser number like 100 or so will be completely lost. 200 magnitude 3 however might represent a significant output.
Whilst energy density has its uses it is worthless where, as is mostly the case, an aftershock sequence is of earthquakes of Magnitude 1 or greater less than the main shock. Unless you apply a logarithmic graduation to the colouring of the energy density the whole area is going to be more or less one colour.
The Japanese quake in 2011 put out ~2,818 petajoules of energy. All the other quakes in the world in that year of Mag 5+ amounted to just 212 petajoules. So despite the fact that there were 1,750 or so earthquakes Mag 5+ in 2011, just 1 single earthquake would completely and utterly outweigh all the others.
I accept that this is perhaps not fair in a global situation but I think I have shown that even in a local situation energy may not have all the answers.
Whilst I always maintain that the amount of energy being released is what matters, and this is a view held by most seismologists, there is also a very good case for looking at point density.
The nature of swarms is to have many similar quakes in a tight area over a short period of time. In a swarm what matters is not so much the energy but the number of quakes. Here point density wins. As an example if a swarm develops it is far easier to see the migration of the swarm.
Such was the case at Yellowstone 2008/2009 where the swarm migrated northwards up the lake. The distribution of energy and points in a ‘heat map’ would be different.
Here is a graph of the energy and a graph of the counts:
(The approximate area)
It is perhaps worth considering that whilst seismology may favour energy output, swarms are almost invariable reported as number of events. If the numbers are not significant why is this the case?
My database only goes down to Mag 1.0 for this area and there will have been many many more of < Mag 1 that would be completely lost in an energy point map.
In this area of Sweden for example there are quakes up to 4.5 and many below Mag 1.0
It is easy to see that an energy based heat map would considerably skew the map to the left hand side where the larger magnitude quakes are found.
The Chinci World Atlas entry states
Strömstad can have strong (vi) earthquakes (on average one every 50 years), with occurances at 5-6 Richter. When a strong earthquake occurs, it will most likely be felt by everybody; people may be frightened and run outdoors, walk unsteadily. Windows, dishes, glassware may be broken, books fall off shelves. The damage will be slight.
It is quite obvious from the numerical distribution that there is a lot more going on than one strong earthquake every 50 years. The point density map correctly shows the greater concentration numerically around the town.
In my humble opinion as a citizen scientists there are good arguments for both methods.