Despite the high-falutin’ title, this is not a complete guide nor is it compiled by a pro. It’s more of an introduction by someone who has made all the mistakes possible and, hopefully, learnt a thing or two in the process which I now try to pass on.
As amateur volcanophiles, we are limited to the information we can scrounge from open sources on the internet, each with its own risks of misinterpretation and set of limitations. Webcams when working give us a picture of the now but are relatively useless as a tool for prediction. Earthquakes can be very misleading, even when a seismologist has evaluated each registered quake. Not only does this not tell you the nature of the quake, you also have to have an intimate knowledge of the system in question in order to accurately assess its importance and what it might portend. One of the better ways for us volcanophiles to get an idea of what, if anything, is going on is to look at tremor. Even so, there’s a lot you have to be aware of before you can draw any conclusions from such charts.
At first, the tremor maps displayed at http://hraun.vedur.is/ja/oroi/allarsort.html show a bewildering array of squiggly lines and spikes in red, green and blue and it’s easy to assume that everything displayed is connected with volcanism in some way. In fact, most of what you see has nothing to do with volcanism whatsoever, but is attributable to weather, wind, waves, tides and human activity. The first thing we have to do is to learn how to identify these:
Grimsey is a small island located north of Iceland, about 50 km offshore, and you’d expect wind, waves and tides to show up in the tremor chart as they indeed do. To the right, you can see that from May 5th, there are peaks twice daily and if you check with the chart for tides, you’ll see that the peaks coincide with the time for high tide and the troughs with the time for low tides.
The reason we cannot see this during the period April 29th to May 4th is twofold. First, we had a Full Moon on the 6th which results in much higher tides, spring tide, whereas the Moon was in the First Quarter on April 29th, which results in much lower tides, a so-called neap tide. Second, with a spring tide of about 4 meters and neap tide at 2 meters, it’s easy to see that if you have bad weather such as you had in the area prior to May 3rd, wind and waves will tend to drown out the tremor pattern induced by the tide.
The third phenomenon visible in fig.1 is earthquakes. They show up in the tremor graph as thin, vertical lines predominantly in the blue, the taller the spike, the greater the intensity as experienced at the SIL-station. Most of these quakes are local and are too small to have shown up on the IMO earthquake maps. However, if a quake is powerful enough, it will show up despite being at a great distance from the SIL-station. As an example, the March 11th 2011 quake off Japan was powerful enough to be picked up all over the planet.
Another factor to be wary of is location. If the SIL-station is close to an area of human activity, this will be picked up and displayed. The tell-tale to look for is periodicity.
Here I have marked the Sandskeid SIL-station in purple and gold. Please note the location a scant 10 km from Reykjavik, almost on the main road leading out of Reykjavik to Southern Iceland, thus in an area where a lot of traffic, and in particular heavy traffic, passes through.
The tell-tale here is periodicity. April 30th was a Monday. May 1st, a Tuesday, was a national holiday. Other than that, it’s easy to see that on weekdays, Icelanders start work around 08 am and finish at 5 pm. From the dip around mid-day, you can see that they take their lunch break between 12 am and 01 pm.
You can also see the influence of weather on human activity. From our look at Grimsey, we already know that the weather was not good before May 3rd or so but very nice over the next weekend in particular. The chart shows that people mostly stayed at home over the weekend April 29th – 30th plus May 1st when weather was iffy, but last weekend the nice weather must have almost emptied Reykjavik to judge by the traffic.
As can be clearly seen, in this case human activity shows up on the tremor chart to such an extent that it almost certainly masks everything but the heaviest of weather and earthquakes in the blue or 2 – 4 Hz part of the tremor chart.
Reynihlid SIL station is located close to Krafla Power Station in the Myvatn area, North-East Iceland. Even if the SIL-station is picking up some naturally caused tremor here, it still shows the tremor induced by the geothermal power plant beautifully. Again, you can see that human need for electric power follows the weekly and daily pattern of human activity with the added bonus that you can tell that most Icelanders go to bed at about 10 pm on weekdays when the demand for power suddenly drops. Quite a well-regulated people, wouldn’t you say!
At noon on May 14th, a magnitude 3.4 earthquake signalled the beginning of a period of seismic unrest at Herðubreið that included well over 120 earthquakes. These are easily visible in the tremor chart, but as can be seen, there was a lot of similarly looking and just as frequent if not equally intense activity prior to that. There are no corresponding earthquakes registered to explain them, so what are they?
A human origin seems to be ruled out as there is no periodicity, nor is the SIL-station located in an area where tremor-inducing human activities occur. Some of them, the smallest and most insignificant peaks, may be local microseismic earthquakes, far too weak to register on the IMO network. Single boulders, freed from ice, tumbling down a mountainside might generate such a signal if they bounced on rock but such an intense and long-lasting event is not very likely. They may also be hydrothermal in origin as superheated water, i.e. water far above the boiling point of 100C but kept liquid by the high pressure several hundred meters to a few kilometres below the surface. The most likely explanation is a combination of ongoing hydrothermal activity with microseismic activity as a minor component.
I’ll leave you with two SIL-charts to ponder. Vatnsfell is located as close to equidistant from the four major Icelandic glaciers and thus most of its main volcanoes in an area where few live. The area is also known as the “Dead Zone” and is where the last three of Iceland’s major rift eruptions occurred – Eldgjá in 934, Veidivötn in 1477 and Laki or the “Skaftár Fires” in 1783. The first is the Vatnsfell tremor chart from May 2011, leading up to and including the initial eruption of Grímsfjall on May 21st. Note that when there is an eruption, it shows up distinctly and unmistakably on the tremor chart. If you think you can spot an eruption on one of the many webcams, it is a very good idea to check relevant SIL-stations in order to see that they tell the same story!
The second is the Vatnsfell tremor chart on May 11th 2012. As can be seen, there is little if anything to suggest human activity here as tremor does not follow any sort of periodicity. It has been suggested that the tremor may have been caused by the hydro-electric plant at Lake Thorisvatn (Þórisvatn) opening the gates to let out excess water, but Carl has checked it and found no evidence that this could be the case. Thus, it is almost certain that the tremor visible in the chart is natural in origin, but in order to determine its exact nature, expert knowledge is required.