El Hierro Volcano activity report – New seismic crisis – more deformation and shallower earthquakes

Last update: December 24, 2013 at 4:56 pm by By Armand Vervaeck

2013-12-24 16:36 UTC
– It took a while before the last interesting (low frequency) earthquakes were listed, but now we see another of these earthquakes at 9 km depth (not dangerous but certainly one that IGN will zoom into).
To give our readers an idea of the epicenter of both 9 km depth earthquakes, we generated a map of the epicenter of both quakes. We are unsure about the depth (no good seafloor maps on the internet. Google earth mentions 500 to 600 meter.

The epicenter of the 9 km 06:17 earthquake

The epicenter of the 9 km 14:58 earthquake

2013-12-24 15:20 UTC
A typical phenomenon happened only a couple of minutes ago. A seemingly calm and weaker seismic period ends with another strong(er) earthquake (no details yet but we expect in the M3 proximity). How can this be explained : quiet simple – pressure buildup with almost no release of energy which (within an active period) ens with a stronger than usual earthquake. Also this is a low frequency earthquake! (lucky to have explained it in the 14:13 update )

2013-12-24 14:13 UTC
We think it should be a good idea to explain to our readers the difference in between the different types of earthquakes, as soo many earthquakes are currently occurring below the island. The (scientific) text was initially written by Tilling, et.al., 1987, Eruptions of Hawaiian Volcanoes, but is also to be used everywhere else in the world.

During inflation (lifting of the island as discussed in previous updates) the rocks become stressed, and this stress is partly relieved by increasing numbers of earthquakes, too small to be felt, but easily recorded by seismometers at Kilauea summit. These earthquakes (called short-period or tectonic) are recorded as high-frequency features on a seismograph. During deflation the stress is completely relieved. The short-period earthquakes stop, but their place is taken by low-frequency earthquakes (called long-period or volcanic), which reflect adjustments related to the movement of magma from the reservoir to finds its way to the surface. The long-period earthquakes are related to harmonic tremor, the continuous seismic record of underground magma movement. (small adjustments to the text to reflect the El Hierro event).

A low frequency earthquake which occurred this morning at 06:17

2013-12-24 12:49 UTC
– New stronger earthquakes and a thicker tremor line (magma intrusion)

2013-12-24 12:29 UTC
– The strength and frequency of the earthquakes has weakened the last couple of hours. Microseismicity is still present abundantly.

Purple square : the most recent – red squares: less than 12 hours

2013-12-24 09:50 UTC
– Relatively few earthquakes today (less than yesterday)
– some information for both ends of readers. Those on the island who merely want to live their safe live and want to enjoy tourist spending : less earthquakes. Those (almost all from other places in the world : it is not over yet.
– The seismograph image below shows a strong earthquake at 06:17. If you are looking in the list for this earthquake, you will be surprised that it is only a 2.4 Magnitude. Why such a big line on the graph then? Because of the depth of 9 km. The seismograph is the confirmation that the depth must have been true (in seismology error margins are normal – in preliminary readings).
– Some activity is taking place offshore at the east of the island.
– Magma wint gradually into a more shallow layer. It went up approx. 2 km (from 15 to 17 km towards 12 to 15 km (last 5 earthquakes).
– The seismograph image shows at the end some more tremor indicating a further intrusion of magma. These quakes are not yet showed on the list.
– De El Pinar GPS deformation data is showing a Northern deformation of nearly 2 cm. More lifting of a bout 1 cm.
– The IGN deformation data are showing a further lifting of 2 cm! on the HI09 (La Restinga) GPS and a 1 cm lifting of the El Pinar HI08 GPS. this means lesser lifting in El Pinar and more lifting in La Restinga than the prior 24 hours. These readings are confirming what we see on the seismicity map.
– In terms of safety on the short term, we are convinced that a PEVOLCA will meet soon and will publish a report afterwards. (PEVOLCA safety board of the island consisting of IGN, INVOLCAN and the local and national authorities)

from:    http://earthquake-report.com/2011/09/25/el-hierro-canary-islands-spain-volcanic-risk-alert-increased-to-yellow/