How to read the Icelandic borehole strain and seismicity plots and NtV Riddle

In this post I will elaborate on how to understand the Icelandic borehole strain and seismicity graphs. For the experts I might just be stating the obvious, but for the more general public (like myself) this might be a guide on how to understand all these enigmatic waves and ripples.

This map shows the locations of three kinds of instrument that monitor earthquake and volcanic activity around Hekla volcano. SIL stations (of the South Iceland Lowland automatic earthquake data acquisition and evaluation system; black triangles), GPS stations (yellow) and volumetric borehole strainmeters (green squares).

Location of the SIL and GPS stations and borehole strainmeters.
Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/Stadsetning_stodva_31052011.jpg

Strainmeters can be of various design. In Iceland we are dealing with Sacks-Everton volumetric strainmeters. Wikipedia reveals: “a design that uses specially shaped volumes to measure the strain tensor.” In other words, changes to the volume of a fluid filled chamber anchored in the borehole.

Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/thensla_burfell.html

The sample rate of the volumetric strainmeter data is one second (1 sps = samples per second, i.e. 1 Hz). The unit “strain counts” on the vertical axis is arbitrary, because a gain is manually set to determine what amount of relative change in strain or stress is one count. Strainmeters indicate ground velocity (displacement per time). Positive strain values mean volume increase in the bedrock (extension due to tension force, i.e. strain), negative values decrease of volume (contraction due to compressive force, i.e. stress). If you think of driving a vehicle, this plot shows your velocity relative to the starting velocity, since the start of the trace is always set to zero. A massive drop or rise might for example indicate you came to full stop at a tree or reached escape velocity for space travel.

Whether a strainmeter shows extension or contraction during an eruption depends on its relative position to the conduit/rift, see the opposite reactions during the Hekla 2000 eruption.

Búrfell darkblue,
Saurbær blue, Skálholt red, Geldingaá yellow, Stórólfhvoll violet, Hella light blue. Image courtesy of IMO
http://hraun.vedur.is/ja/englishweb/heklafigure1.html

Besides the Hekla strainmeter Búrfell is the second closest to Hekla, roughly 15 km at a perpendicular angle to the rift direction. The huge strain drop (i.e. massive stress increase) at Búrfell was interpreted as magma forcing it’s way up, opening a conduit. On the other hand, the simultaneous strain increase (decreased stress) at the other stations was due to emptying of the magma chamber. Here is further (paywalled) read on the strain during the 1991 Hekla eruption. The unit nanostrain indicates a change by a billionth part of the volume, i.e. 10^-9. Earthtides are known to have an amplitude of about 50 nanostrains. The 2000 eruption caused a sudden drop about an order of magnitude larger.

A seismometer literally measures shaking, i.e. motion of the ground, which can be recorded as a seismogram.
The seismometers of the SIL array can both measure ground displacement (unit is meters per second, m s^-1) or be used as accelerometers (unit meters per square second, m s^-2).
Most Icelandic seismometers are 5 sec (0.2 Hz resonant frequency, limiting the frequency range) Lennarz seismometers. The sampling frequency is 100 Hz. The Haukadalur seismometer (63°58´08.4´´ N / 19°57´54.0´´ W, appr. 10 km West of Hekla) is a LE-3D/5s, measures oscillations in three dimensions (“transverse”, North-South; “radial”, East-West; “vertical”, Up-Down).

Tremor amplitude time series with different frequency bands. Vertical axis: One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/oroi_hau.html

First of all, this graph does not show the raw seismogram, but is a spectral analysis. You remember the colorful spectrograms from the El Hierro stations? A spectral analysis is performed on the waves of the seismogram to extract oscillations of different frequencies. Several algorithms can be used to create a spectrogram, for example STFT, short-time Fourier transformation, or CWT, continuous wavelet transform. For El Hierro the amplitudes are given over the whole frequency range while in Iceland they show averages of three frequency bands.

This example is a tremor amplitude time series showing averages of the frequency bands 0.5–1.0 Hz (red line), 1.0–2.0 Hz (green line) and 2.0–4.0 Hz (blue line), of the vertical component (Z) for the station HAU. Unfortunately the vertical axis is not labelled, but is presumably representing the amount of bedrock displacement in micro meters per second multiplied by a variable scaling factor (x). The values are presumably one-minute averages. An example for this analysis is described e.g. in this thesis, see p. 564 ff.

The blue trace (high frequency band, fast shaking) mainly represents earthquakes and the green and red traces (low frequency bands, slow shaking, harmonic tremor) tremor from magma movement, which for Hekla is usually in a well-defined spectral band at 0.5–1.5 Hz (see the thesis).
Based on previous observations, the  following scenario might occur when the next eruption is about to happen: First there will be more earthquakes opening a fissure, showing as an increase of the blue earthquake trace amplitude by an order of magnitude. When the fissure is opened earthquake activity seizes and the blue trace will go back to normal. Meanwhile the magma starts spilling out and a sudden increase in the red and green tremor trace amplitude by at least an order or magnitude will be seen, which gradually decays with decreasing pressure. What we should actually look for in this graph is not the width of the traces, which only indicates how much the shaking amplitudes vary, but a really really strong rise of the curves as seen in 2000:

Tremor amplitude time series with different frequency bands. Vertical axis:  One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/hau20000226.gif

Lastly, the following graph is a composite of data derived from the volumetric borehole strainmeters and from the Haukadalur seismometer, plus information on local earthquakes determined by the SIL system.

Upper panel: Volumetric strain rate.
Lower panel: earthquake magnitude (left), horizontal components of tremor amplitudes (right)
Image courtesy of http://hraun.vedur.is/ja/hekla/borholu_thensla.html

The upper part shows the “two-minute median from one-second data” of borehole strain rate (strain counts per second) measured by the four stations Búrfell, Hekla, Hella and Stórólfhvoll. See the green squares on the map. A change of the strain rate means the bedrock is compressed or extended faster or slower than before. The cause of this is a change in the pushing or pulling forces. Think of it as your vehicle being accelerated or decelerated when pushing your gas or brake pedal. This graph shows what your feet do. When Hekla erupted in 2000 the strain rate looked like this:

The rate of strain changes in Búrfell (blue, 15 km from Hekla) and Skálholt (red, 45 km from Hekla) (nanostrain per hour)
Image courtesy of IMO http://hraun.vedur.is/ja/englishweb/heklafigure3.html

The minimum in the strain rate indicates the time of the surface breakout of the magma, along with the visual observation of the eruption at 18:17.

Because the ground is moved by several variable sources, mainly earth tides (very slow change in strain counts rate) and microseismicity (very fast change in strain counts rate) the above mentioned two-minute time range is chosen by which these events are filtered out. Then the median, the mean value separating the higher half of a data sample from the lower half, is plotted.

The left axis in the lower part shows the magnitude (in Ml) of local earthquakes. Since most of the time there are no earthquakes (counted in the lower right corner) no trace appears.

The right vertical axis in the lower part indicates the bedrock displacement, i.e. velocity in micro meters per second. The data is derived from the horizontal components (North and East) of the Haukadalur tremor amplitude time series data, which are 60-sec averages. Short-lasting shaking, for example caused by single earthquakes or a sledge hammer, are averaged out by plotting the the three-minute median. When an eruption is imminent, the blue (high frequency) trace will rise first indicating fissure opening and the green and red traces will follow when the eruption starts.

Standard VolcanoCafé disclaimer: I am not an expert on this topic, just read a few papers while researching for the post. Please excuse me if I jumped to false conclusions and feel free to post corrections!

chryphia

Many thanks to the dragons who read the draft and special thanks to Geolurking for helpful comments!

Other links:
-The SIL seismological data acquisition system – As operated in Iceland and in Sweden. Abstract only (2003)
-How a seismometer works from Sep 25, 2012 by Geolurking
-Summary about long and short period and broadband seismometers in this blog post
-ON THE USE OF VOLUMETRIC STRAIN METERS TO INFER ADDITIONAL CHARACTERISTICS OF SHORT-PERIOD SEISMIC RADIATION
-Seismometers of the SIL used as accelerometers
- Earthquake engineering research center, University of Iceland operating the Icelandic strong-motion network since 1984.
-Sturkell et al., 2005, Volcano geodesy and magma dynamics in Iceland
-Description by IMO of the Hekla 2000 eruption.
-Visualizing Stress is a good site, even if you are not into the math aspects of it, it has some really good narative data in the tutorials.

Name those Volcanoes Riddle

1 point for each volcano … enjoy!

No 1 - Did it crash in the Gobi Desert during CE3K? SOLVED COTOPAXI

No 2 - Volcanic group associated with siblings and satelites. SOLVED LES PLEIADES

No 3 - In English it can be added to seal, crow and mantis. SOLVED HEKLA

No 4 - Bruce and Nigel’s buddy studies this one. SOLVED Axial Seamount

KILGHARRAH

Name that Lava, Herdubreid Scientific Challenge and Volcano Riddle

Photograph courtesy of Dr Carmen Morataya.

Having the brain rebuilt from top down every Friday never get’s old. I do not know how you guys are doing since I am bad at both riddles and recognizing lavas and volcanoes.

But it is fascinating to witness how the blood hounds of the Volcano Café unerringly zeroes in on the pray. This week I want the name of the lava, the volcano and the name of the local fast food. So, 3 points to be had all in all. I will of course ding when someone has scored.

Tomorrow, or the day after I will post you an explanatory article.

Herðubreið

Photograph by Eggert Nordahl under permission.

Herðubreið is a tabletop volcano (tuya) in Iceland that has been active during the last few years with repeated earthquake swarms and magmatic emplecements. Supposedly the volcano is a defunct part of the Askja volcano. Evidently it is not as defunct as previously believed, and there is quite a lot pointing to it not being a part of Askja.

So, last night I proposed that we should do something with the volcano since it is rather poorly investigated. Ontop of the two questions above we can also have a bit of fun with the youngish looking strato volcano on top of the tabletop mountain. Here is the question, have it erupted after the last glacial period?

I thought we could do this as a case of crowd science. We have access to some equipment, we have access to a lot of people with various expertise, so we could actually collectively make a fairly nice piece of scientific work on Herðubreið.

So, in another word, I propose that Volcano Café officially adopt Herðubreið as our project. Now and then we should post status reports on our progress, if anyone feel up to it, being the collector and editor of what has been said please do not hesitate to come forward! Herðubreið and Volcano Café needs your services.

Have a nice weekend everyone!

CARL

Name Those Volcanoes Riddle

The riddle contained the mixed up clues to 2 volcanoes … 1 point awarded for each …

At the intersection of Rts 36 and 119 Peter was totally over come by the glowing clouds; then Bill, Andie and Chris arrived to cover the celebrations!

1 point Sa’ke for Mount Pelee … “Peter was totally over come by the glowing clouds”

Glowing clouds was the English translation of a French term nuée ardente first used to describe the pyroclastic flows of Mount Pelee that engulfed the town of St Pierre in 1902 killing 30,000 people.

http://en.wikipedia.org/wiki/Pyroclastic_flow

http://en.wikipedia.org/wiki/Mount_Pel%C3%A9e

1 point KarenZ for Candlemass Island … “At the intersection of Rts 36 and 119; then Bill, Andie and Chris arrived to cover the celebrations!”

The clue pointed to the movie Groundhog day (1993) set in Punxsutawney, starring Bill (Murray), Andie (MacDowell) and Chris (Elliott). Groundhog Day is the US term for Candlemass Day.

http://www.fisheaters.com/customstimeafterepiphany3.html

http://www.volcanolive.com/candlemas.html

KILGHARRAH

Countdown to Hekla

It has been a long time since I wrote about Hekla. But, I guess nobody is surprised at what I am about to write.

Everyone with a genuine interest in volcanoes have their favorite volcano. As many in here know Hekla is my favorite volcano to bang my head against. Few volcanoes are as intricate as Hekla, and few have such a short run up before an eruption as Hekla. Normal run up time to an eruption is between 30 – 80 minutes from the first sign.

This time around has been different. But let us first recapitulate what has happened since the last eruption. For those who are curious about how Hekla works I would like to recommend my own post about her innards:

http://volcanocafe.wordpress.com/2012/01/10/deconstructing-hekla-hells-gate-revisited/

Background

In 2004 Hekla had received as much new magma as was discharged during the 2000 eruption and sometime during late 2008 to early 2009 that figure had doubled. After that the inflation stagnated and no real uplift was measured at the GPS-stations with the exception of what was most likely magma moving between the different magma chambers.

During the summer of 2011 earthquakes was registered and a public safety alert was issued stating that Hekla was close to erupting. From then on Hekla has had earthquakes ranging from miniscule to 2M+ without erupting. For those who are not familiar with Hekla one should notice that she normally is aseismic, or in other words, that she does not have a lot of earthquakes.

From 2010 and onwards Hekla started to show a new feature that I dubbed “transients”. The transients are sudden rapid drops in the strain measured at the borehole strainmeters. These transients have only been seen before as Hekla erupted. They had before 2010 never been seen without an eruption occurring. A transient is in short happening as the mountain strains to open up.

On the 13^th of March and onwards Hekla had a swarm of earthquakes and once again transients were noticed on the strain-meters. There was also harmonic tremor measured indicating rapid movement of magma. This caused IMO to issue a public safety warning, and the London VAAC issued a flight code warning Orange. This followed the exact pattern of how all the previously instrumentally monitored eruptions had started so far.

As we all know nothing happened in the end. We can now safely say that we know even less than we did before about how Hekla acts before an eruption. Because now we have to figure out why Hekla did not erupt when she should have. I guess someone will have a research career out of it in the end.

Present

After this Hekla entered into a new phase never seen before, this time a phase of very rapid and unbroken inflation started. What happened is most likely that the earthquake swarm removed blockages inside the deep feeder tubes of Hekla enabling fresh magma to flow into the volcanic system.

The rate of inflation varies a lot depending on where the GPS station is placed. The big exception is Mjóaskard situated to the west of Hekla. It has only suffered an uplift of 5mm in the last 5 weeks. For the other stations the rate of inflation is between 15mm in Hestáalda and 32mm at ISAK. Average uplift is 16mm, and 21mm if MJSK is not counted. This type of rapid inflation has so far never been measured at Hekla.

http://strokkur.raunvis.hi.is/~sigrun/HEKLA.html

During the entire inflation phase there have been scattered earthquakes and micro-quakes.

If the current rapid inflation continues there is a very low chance of Hekla not erupting. Yes, we do not know what is happening with Hekla now since we have never seen this type of behavior. But Hekla is constructed in such a way that she can’t take a huge increase in pressure without erupting.

Image by GeoLurking based on data by the University of Iceland and Professor Sigrún Hréinsdóttir. All areas are showing uplift on this image covering the period from 4th of April up untill now. The area with the highest uplift are due north of Hekla.

If the inflation continues at the current rate Hekla will erupt. When? Well I am not going to make any bets, but any time from 1 hour from when you read this to 4 weeks. Remember that 4 weeks into the future the combined uplift in 2 months will have exceeded 50mm at many GPS stations. As seen on the image above the largest uplift is happening on the northern slopes. This is a known site for one of Heklas primary magma chambers. The area to the northeast are not showing correctly, there is uplift there too, but due to lack of a GPS station there the model get scewed.

Image courtesy of the University of Reykjavik and Professor Sigrún Hréinsdóttir.

I personally would not at any cost get closer to Hekla then 10 km from now on. And then I would stay in the car on the road. If you are closer the chance of you surviving is not good and 5 km the chance of you surviving the initial blast is pretty much nill.

What will the eruption be like? Here I will be guessing since Hekla has changed her behavior compared to the last eruptions. I would say that Hekla has remobilized old evolved magma during all that moving of magma, and this latest inflation phase seems to fill up a lot of old magma chambers. This causes me to fear a rather explosive start of the eruption. I would also say that there is quite a high likelihood of there being more lava erupted then was seen during the last 3 eruptions. I will hedge my bet by saying that I would expect it to be anything between a VEI2 and a VEI4 on the volcanic explosivity index, and that Hekla will effuse between 0.1 to 2 cubic kilometers of lava. Based on the GPS plot above my best judgement is that the eruption will start at the top of Hekla proper and then open up the fissure both to the south, but mainly to the north. Most probably the Hekla fissure will open over all of Hekla proper with a fissure extending to the Northeast.

For those who wish to follow the eruption, here is the Hekluvöktun page:

http://hraun.vedur.is/ja/hekla/

CARL

Stromboli and Etna

Stromboli showed heightened activity during the last days and weeks.

Carls screenshot saved from tinypic

Sunday nights action as seen through a thermo cam.

IngeB found an article on this behavior:
“The stronger eruption intensity of Stromboli has also been subject to an article in Italian webnews. Because of the explosions at its summit, the civil protection organisation has hightened the alert level. http://www.meteoweb.eu/2013/04/non-solo-letna-anche-lo-stromboli-sta-eruttando-con-forti-esplosioni-avviso-di-elevata-criticita-dalla-protezione-civile/200183/ “

Siencia Obsoleta found a wonderfull video of stromboli 17 to 19 april 2013

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Etna:  Here is the report by our EYEWITNESS to make us all turn green with envy.

Schteve: “Mornin` All,

I know you are all dead jealous, but I don`t mind

I left the apartment at a run about 5 minutes after my comment at 17.17… The locals on Via Etnea who were casually going about their Saturday evening business ( posing in their best clothes and eating ice cream) must`ve thought I was crazy ( 6feet6inches of wild eyed English man going full tilt and saying wow over and over…) It was just about dark when I got to Villa Bellini ( a nice park with the best free viewpoint in town.) What can I say, it was just awesome; I couldn`t hear anything, (too much background noise) but what a show…My breathing eventually returned to normal, but my heart carried on pounding throughout. I took pictures until my battery went flat, but nothing really worth sharing (my camera is a little schnap schot job with insufficient zoom.) Lizzie came and met me after her snooze and we adjourned for cocktails and the second half on the roof terrace of the Palace Hotel…

Somehow I have got around Sod`s Law for once, we are off for 5 days in Palermo tomorrow and I was fully expecting Etna to go boom then…

I have some ash from the paroxysm of 20th April but I will pick up some more from this’n if I get chance…
“

The next passage is a repost from yesterdays addition to the Sunday post. We wont test today so we need a new post.

Etna Paroxysm # 13 (?) 2013 summary as seen from the point of view of the VC crowd:

Quite some of us had been noticing that the tremmor rose higher and higher over the last few days and quite some, including Carl, had expected /hoped for it to happen a night earlier. We also may have an Eye-witness very close to the action this time. Schteve is spending his holidays in Sicily. We are hoping he watched it and will give us a report when he returns.

The last few paroxysms have been different to those last year. Last year the tremor rose within a very short period of time, an hour or hours at maximum, and then Etna went BOOM, to go back to slumber a few hours later. This year it is different. The tremmor contiually rises over a period of days, during this uprise, lava is already been splashed out occasionally. Incandescent glow is visible on the cams and so people do watch cameras for 1 or 2 days expecting the real action to appear any minute. But Etna is in no hurry and fooled us for at least a day this time. After the show seems to be over it is not. Lavaflows can still be seen and they last for up to 3 days.

With the last 4 or 5 paroxysms the fountain grew bigger and bigger. Each time the media and THE expert Dr Boris B announced afterwards: the event was stronger than the one before. (though yesterdays eruption column is said to be 500 meters, it was almost twice as high with paroxysm #12 2013)

Because of Etna´s different behavior, people here start coming up with new names for the events: Paroxysmal, and other words were suggested. (help me here, I forgot)

Whenever a VC crowd is on the watch and it is also Sheepy Dalek time, people start funny discussions.

Can you see a face in this pic? Or is it a rabbit to you? The “face” ( or the rabbit? ) even got a name: Rudolph!
Carls offical Party Over pic:

A lava flow which seems to emerge not from the top crater but from a side fissure was still visible on the Lave cam which was also seen by Granyia. http://tinypic.com/view.php?pic=xm67t5&s=5

I can only quote Renato Rio: “It was great “paroxysming” together.“

Now that was our POV. Here comes the media, the expert and Youtube:

IngeB found this: “In this Italian webnews they say the paroxysm seems to be bigger than the last ones. The noise of explosions is so intensive that the window glass is rattling. This noise can even be heard in Taormina. Wind is from S/SW and the ash cloud directed to Messina and the Calabrian coast.“ http://www.meteoweb.eu/2013/04/forte-eruzione-delletna-con-forti-boati-avvertiti-fino-a-taormina/200229/

Of course! Do not miss Dr Boris B.´s Flickr report: http://www.flickr.com/photos/etnaboris/

Arjanemm found this: “For those that haven’t seen enough of Etna, try a search in youtube for “etna 27″. You will find dozens of movies shot yesterday.“
A small selection:

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Alans Riddle is still not solved:

Alan C’s Evil Riddle:

My true dusky identity was hidden! My secret lies in my blackness! Confused?
What am I?
Who are my relatives?
Where can I be found?

Clue:

Pay an old Greek a visit! He may let the cat out of the bag!

Spica

Etna’s paroxysm overview

Since the beginning of this year, Inge B. has kept track of all of Etna’s activity this year and has submitted a summary to keep track of all this for everyone. By now, we have seen a dozen of paroxysms and several other instances of activity at various craters, so it is easy to see people get lost there. For that reason, a table has been made by Inge B. (thanks for that!) which I have sort of converted to wordpress-language.

Etna summit craters map. Image by INGV

Bigger eruptions	Date	Summit craters involved	Characteristics of activity
	9-15 Jan 2013	Bocca Nuova (BN)	Vigorous Strombolian eruption after 3 months quiescence at this crater which had had its first magmatic activity after 10 years again in 2011.
	16 Jan 2013	BN	Strombolian activity
	18 Jan 2013	BN	Strombolian Activity
	20 Jan 2013	New SouthEast Crater (NSEC)	Strombolian activity, first magmatic activity after 9 months quiescence at NSEC
	22 Jan 2013	NSEC	Strombolian activity, more intense than the last ones, bombes rose up to 100 m above crater rim
	28-29 Jan 2013	BN   NSEC	Vigorous Strombolian activity, rumbling noises, to be heard in vicinity of craters Weak Strombolian activity; rhythmic degassing with slight ash content. First time since 12 years, simultaneous activity from 2 summit craters
	30 Jan 2013	BN	Intensive Strombolian activity, launching bombs to about 120 m above crater rim
Paroxysm 1	19 Feb 2013	NSEC	Lava fountaining with production of lava flows, pyroclastic flows, lahars and ash cloud
Paroxysm 2	20 Feb 2013	NSEC	Lava fountaining, lava flows, ash
Paroxysm 3	20 Feb 2013	NSEC	Lava fountaining, lava flows, ash. Second paroxysm that day
Paroxysm 4	21 Feb 2013	NSEC	Lava fountaining, lava flows, ash; 4 paroxysms in 48 hours is very rare
Paroxysm 5	23 Feb 2013	NSEC	Lava fountaining, more intense than before, fountains up to 800 m high, lava flows, ash cloud containing scoriae
	27 Feb – 4 Apr 2013	BN   Voragine (VOR)	Strombolian activity, weak lava fountains continuing through 28 Feb 2013 Explosive activity, first activity of VOR since 1999. Strombolian activity in the night before Paroxysm 6. Still ongoing Strombolian activity with stronger explosions in between, small ash emission, volcanic bombs during Paroxysm 7. Weak Strombolian activity still ongoing; after activity ceased at NSEC at Paroxysm 8, strong explosions at VOR. Weak Strombolian activity still ongoing at Paroxysm 9.
Paroxysm 6	28 Feb 2013	NSEC	Lava fountaining; eruptive fissure opened in the saddle between the Southeast Crater and the New Southeast Crater, lava flows; ash plume, scoriae fallout
Paroxysm 7	5-6 Mar 2013	NSEC	Spattering from new fissure in the saddle between the SEC and the NSEC; lava flows then lava fountains up to 800 m high; intense fallout of scoriae in adjacent towns and villages down to Taormina on the Ionian Coast
Paroxysm 8	16-17 Mar 2013	NSEC	Slow increase of Strombolian activity; lava fountains, ash and scoriae fall, rather violent paroxysm with several loud bangs heard in population centers around the volcano, gas rings; lava overflow, up to 800 m high lava fountains, ash plume 2.000 m high, lightening within the plume; downpour of heavy material, volcanic bombs and big scoriae in the summit area; after explosive activity ceased: small collapses and slides of still hot material
Paroxysm 9	3-4 Apr 2013	NSEC	violent paroxysm, only short time lava fountains, but very loud explosions, noise heard tens of kilometers from the craters, opening of new vents on the NSEC, production of small pyroclastic flows in the summit area (summit area declared “off limits” by the authorities after this episode); ash and scoriae fall in adjacent towns and villages
Paroxysm 10	8-14 Apr 2013	NSEC	Long run-up phase beginning on 8. April with loud Vulkanian (?) explosions (heard tens of km away), some rather heavy explosions with ash emission, followed by a period of Strombolian activity of increasing intensity culminating in lava fountaining on April 12, emission of tephra, ash and lapilli as well as lava bombs, less ashfall though than during Paroxysm 9 and the ones in March 2013; lava flow into Valle del Bove; repeated collapse at NSEC, leading to formation of a depression in the cone lava flow from “saddle” between SEC and NSEC in direction of Belvedere activity also from 2 vents at the base of NSEC on the same day; landslide; small pyroclastic flow two days following of declining activity with some production of weak lava flows
Paroxysm 11	18 Apr 2013	NSEC	From 16 April on some explosive activity at NSEC, ash emissions, small Strombolian eruptions, slowly increasing paroxysm with lava fountaining in the morning of 18. April; eruption cloud with ashfall and lapilli dispersion in direction S-SW; big lava flow into Valle del Bove, two smaller ones from the “saddle” between NSEC and SEC in directions N and S weaker Strombolian activity still ongoing since morning of 19 April
Paroxysm 12	20 Apr 2013	NSEC	Violent activity, ramping up quickly. Fountaining up to 1000 m high with column of gas, ash and lapilli and heavy tephra fall on the east flank. Downpoor of lapilli on the highway Catania-Messina near Giarre. Intensity approximately 30% higher than previous paroxysms.

Sources:
INGV Catania (also for the pictures), reports http://www.ct.ingv.it/en/ (English)
for the 9th paroxysm: http://www.ct.ingv.it/it/component/content/article/11-notizie/news/868
(Italian)
for the 10th paroxysm: http://www.ct.ingv.it/en/component/content/article/11-
notizie/news/875 (English)
for the 11th paroxysm (as per April 20, 2013): http://www.ct.ingv.it/en/ (English) and
http://www.ct.ingv.it/it/component/content/article/11-notizie/news/880 (Italian)

Photo of Paroxysm 10 at the 12th of April 2013. Photo by etnawalk.it

El Nathan