El Hierro : a tectonic compression/relaxation mechanism?

Hypothesis: the earthquake swarms report relaxation of accumulated, compressive tectonic stresses

Tectonic forces
The crust under the Canaries, part of the African tectonic plate, is moving north eastwards at 24mm annually in response to drag from the upper mantle.

The crust/upper mantle boundary under Hierro plunges to at least 30km deep below the island . Gorbatikov et.al.: www.gradient-geo.com/library/storing.php?doing=T91oVy5lrThe MOHO on the ‘bulk plate’ is at ca. 10km depth between the islands. Watts: http://www.earth.ox.ac.uk/~tony/watts/Oceanislandsandseamounts.htmSo the island presents a 30-40km tall obstruction that distorts mantle flow and so reduces the propulsive force on the the island compared with the bulk plate. This results in a compressive force, directed NE-wards, being exerted on the SW arc of the island by the bulk plate. So what might be the response of Hierro’s crust to these forces?

Compressive crustal strain
Compressive stress will accumulate in the country rock throughout an arc around the south west of the island. The initial eruptions that formed Hierro occurred in the north east of El Hierro (Tinor, SSW of Valverde). The site is revealed in Gorbatikov’s microseismc survey as a massive, 5km diameter, 30km deep towering structure. So a NE-wards directed loading will lead to accumulation of compressive stress between this Valverde stack and the bulk plate to the SW of the island. This compressive stress accumulation must be almost silent to seismometers: very few earthquakes have been recorded at Hierro over 25 years prior to 2011. The relentless force from plate motion has the desired property to sustain compressive loading. Note that for simplicity we ignore mantle flow forces directed downwards and around the island. We can now offer explanations for the patterns of the earthquake swarms.

Swarm location where stress relaxation is maximal
It is the sudden relaxation of accumulated compressive stress that causes the earthquakes. The earthquake swarms are proposed to result from relaxation of compressive stress. So the stress gradient is important. We can see this gradient in simplified form by looking at the flow of an incompressible fluid around a cylinder representing Hierro:

Source en.wikipedia.org/wiki/Potential_flow_around_a_circular_cylinder
Fig 1: The diagram shows the pressure field, red being high, blue low. The steepest gradient in pressure is
around the sectors at 7o’clock and 10 o’clock. (ignore the pressures to right of cylinder)

Fig2: By superimposing the earthquake swarms on this diagram and making the fluid (=bulk plate) flow
NE-wards like the African plate, we can see the principle of the compression-relaxation mechanism.

While compression will be maximal ( red) on the central axis ( SW-NE) it is only when the compression can relax into less compressed rock that we detect earthquakes in elongated swarms.
The stress gradient ( red going to blue) will be maximal around the western arc (and southern arc) of the island. The two 2011 swarms follow the stress gradient. The 2012 swarm is on the central axis and did not appear to follow stress gradient (see below). Note that the precise pressure distribution around Hierro will be much more complex than depicted and will be maximal at the 10km depth of the MOHO, so the outline of the the island is merely a very rough guide to locations.

Patterns in swarms
Several patterns support a stress relaxation hypothesis.

  1. The first two swarms showed distinct north-south linearity, which is at 45 degrees to the SW-NE central axis and plate motion. The orientation is what would be expected if earthquakes occur where the stress gradient is greatest ( red-blue on diagrams above). The swarms’ southern limits are where the gradient reduces, despite the absolute level of compression being greatest on the central axis (dark red)
  2. The 2012 swarm was orthogonal to the first two swarms and may not have followed the circumferential pressure gradient (see 6 below).
  3. The distinct conical shape in vertical section to the first swarm suggest a stress-relaxation failure process,with a greater gradient at shallower depths permitting relaxation upwards. The conical pattern is clearly seen in Geolurking’s plot of earthquakes onto the microseismic image.
  4. Fig 3: From Gorbatikov, see image for full reference.

  5. The maximum compression from the bulk plate impinging the island would occur at MOHO depthof ca 10km, coinciding with the maximum width of the first swarm. This region, of all, would be expected to be exposed to the greatest compression and this, combined with it the shallowest swarm, may explain why it was the first zone to fail. The trend to events spreading southwards during the first and second swarm likewise could reflect an ‘unzipping’ of stress, with prior relaxation influencing subsequent events.
  6. The gently curving upper surface of the first swarm, deepening to the south, putatively follows the lower bound of the sedimentary layer. The sedimentary layer in this model does not accumulate stress, but rather accommodates compressive strain by sliding in shear. So few earthquakes arise in the sedimentary layer, which also insulates the overlying erupted edifice from strain and hence explains its lack of stress accumulation and earthquakes.
  7. The curved lower bound to the base of the second swarm may reflect the transition from colder crustal rock that has accumulated compressive stress, compared with underlying more plastic upper mantle that deforms and accommodates loading without accumulating stress. The second swarm was deeper, ca 20-25km, and again elongated N-S. It was separated from the base of the first swarm by an unusual aseismic layer at ca 15-20km depth. It can be seen as the clear band here http://www.02.ign.es/ign/resources/volcanologia/jpg/Eventos_HIERRO_2012.jpg 
    The south end of this aseismic layer became active in the third swarm suggesting that stress had accumulated there but was unable to be relaxed during the second swarm. Interestingly the azimuths in this region differed in the two swarms- see 7 later)
  8. The third swarm (2012) produced rapid GPS uplift and strong NE-wards horizontal motion. Daniel’s plot gives a clue to the geometry of stress relaxation horizontally: source: http://earthquake-report.com/2011/09/25/el-hierro-canary-islands-spain-volcanic-risk-alertincreased-

    Image by Daniel at Earthquake-Report, full link above.

    The geometry of the GPS motions suggests an on-axis origin for the driving force, corresponding to the high pressure zone (Fig2, dark red) and directed NE-wards across the SW of the structure The GPS motion suggests the entire island edifice relaxed to the NE, as shown by Valverde H100 – the massive stack – shifting in precisely the direction of African plate motion. In 2011 H100 was immobile. Other motions suggest a a radial relaxation towards low pressure zones from the geometric centre. The alarmingly rapid GPS uplifts ( not shown) may result from a component of the stress relaxation being directed upward.

  9. Data on earthquake azimuth in the two 2011 swarms are compatible with azimuth reflecting the direction of the relaxations.

    (source: Chryphia). As seen at 1-34min in the video the azimuths in the first and second swarms were either northwards or southwards ( red or dark blue) with few intermediate angles (pale bluegreen). This alignment, predominantly north or south, is in agreement with the compression gradient. The third swarm, 2012 started with eastward azimuths (Chryphia’s plot: yellow, green) followed by southward (blue) as activity spread westwards The temporal relationship between the eastward azimuths in the 2012 swarm and the eastward GPS motion deserves investigation.
  10. Conventionally the swarms are believed to be generated by magma intrusion. Interestingly IGN’s deployable seismometer array recorded no magmatic signatures such as harmonic tremor or LP events during the 10,000 earthquakes of the first swarm prior to the submarine eruption. http://meetingorganizer.copernicus.org/EGU2012/EGU2012-11986.pdf
  11. Tenerife has shown a low intensity linear eq swarm with N-S orientation to the west of Teide, the island’s central volcano, as seen in Schteve’s plot:

    The mechanism may resemble Hierro’s except that the gradient of compression is less due to the larger size of the island, hence the low intensity of earthquaking. The swarm under Emedio shows no linearity, which would be predicted as its erupted mass is considerably less than Hierro’s and compression does not accumulate, and magmatic processes dominate.

Hypothesis and implications
The earthquake swarms are envisaged as reflecting the sudden relaxation of tectonic compressive stress that had accumulated over several hundred years. The 2011-2012 swarms may be a repeat of the swarm in 1793. However, the model predicts that swarms will also occur as a mirror image, extending at 45 degrees south of the central axis. If the 1793 activity was focussed to the south then the period of stress accumulation could be longer for the recent swarms. If the repose period is say 200 years, a NE-wards GPS motion of 15cm would represent about six years’ lost motion. So Hierro would lag the bulk plate motion by just 3%.
If the hypothesis is valid, it means that considerably less magma (if any) has arrived under El Hierro than indicated by the conventional interpretation of seismic and GPS data. Relaxation of compressed rock will involve some increase in volume as reflected in the GPS motion many km above. We have no knowledge on the motions at depth, but the relaxation of a column of compressed rock will create space in neighbouring rock that allows decompression degassing and melting of emplaced magma.
Without detailed analogue or in silica modelling of the compressive stresses, or knowledge of deep structure on which to base those models, we cannot predict with any confidence if the swarms are complete. Swarms in a mirror image configuration south of the axis are a possibility. Fluid flows past obstructions often oscillate side-to-side,so perhaps in another 200 years the Restinga swarms will recur?
The best initial test of the hypothesis might lie in the 2004 microseismic study by Gorbatikov at al. The location of the microseismic sources, reflecting compresional stressing, would be predicted to be greatest in an arc of compression extending across the south western arc of the island.

Mercedes Star.
The 2011- 2012 swarms were north of the central, SW-NE compressive axis. The model predicts similar swarms to its south, running W-E ( El Julan-Las Playas) under La Restinga. So the hypothesis presented here can potentially explain the ‘Mercedes star’ shape, together with its orientation on the plate. Those zones of the island that have been repeatedly fractured by cycles of compression and relaxation-fracturing will provide the network of small conduits and sills needed to explain rapid magma ascent. So the Mercedes star and its orientation on the plate are proposed to be the result of tectonic forces. The sedimentary layer, possibly unique to the Canary Islands, may play a pivotal role in allowing tectonic earthquake swarming by removing restrictions from the erupted edifice on deeper compression-relaxation motions. Alternatively, the mechanism proposed here may be rare by being dependent upon the size of the initial eruption being small enough to create a compression gradient that permits stress/relaxation cycling. The gradient around larger initial eruptive edifices (eg putatively Gomera) may not permit relaxation, so intense swarming and fracturing and the star shape do not occur.
This hypothesis should in no way used to negate the safety measures promoted by the authorities. It is speculation by an amateur.

PETER COBBOLD, Professor in cell biology, not the professor of geology of similar name.


317 thoughts on “El Hierro : a tectonic compression/relaxation mechanism?

      • I have been living in Iceland for some years and don’t remember having seen such swarms very often then. And IMO anyways says that they intend to “v e l fylgjast med”. 😉

        • Yes, Inge, they are not every month or every-half-year, but over long time they are rather common. Having long history of watching news in general, and I know thrugh eye-to-eye conversation with the good folk that are (were) on IMO quake department some years ago. IMO always is watching, thats their PRIMARY job. Swarm tonight is larger, more “unrest” on all meters in Iceland. I do not rule out larger ones or swarm extending to other areas.

    • @Luisport – A SIL re-acting to a 4 Richter quake. We ( I and some other regulars ) have discussed this all day in previous comments – ok?
      FLA (Flatey) SIL is on an small Island that sits right on the faultline, so, likely shows more there, than on other in-plate stations. I likely calms down again.

      • I am back to saying that Flatöy is picking up something else.
        A 4M quake is just not psychic. The first HT started an hour before the previous swarm. So there is something else going on somewhere. I momentarilly thought it was that landslide that had been picked up, but I do not think so any longer.

        I think what is happening is that the quakes (which for a neat stack) have opened a path from depth, and some of that molten magma that sloshes about under Iceland has started to find a path up. I think this is the start of a rifting event, even though under an unusual place. Will there be an eruption? Most likely not, but if this continues we will see wetter and wetter quakes.

        • Yes, possibly. Flatöy (Flatey) sits smack on the rift – check map – Possibly intrusion in all or part of that rift (due suction). Intriguing thought.

          • I think something like that is happening.
            Only problem I am having is that there are no known parent volcanoes for a fissure swarm there. But, on the other hand the hotspot leaves quite a lot of freefloating magma down there below the crust.

          • Next “Swarm Episode (TM)” might resolve if new “Surtseyja” emerges … or better schleep on it. It might be thou. Do not rule out the “weird” happening. I am more expecting a traditional mountain blowing.

    • Checked other info I have. FLA SIL is off-line. It will not go down again. Seems this last swarm is LARGER and shows on all meters in Iceland.

  1. This ongoing swarm at Tjornes raises my attention. It is the third major swarm that starts with a 4.3 ish earthquake, followed by several M3 aftershocks.

    There is a lot of stress at Tjornes. In 1976 a major earthquake occurred linked to the Krafla rifting events. Could these swarms be connected to something else? I don’t know.

    From what I heard, there is no volcano at that spot. But it’s a spot where the transform zone turns into a rifting zone again. Just like near Hengill or Hekla.

    • Yuo should watch. But the 7 Richter quake you predict in Þingvellir area will not come. Such stress is not there. That requres so much drastic changes in Iceland overall tetonics, to be near impossible. Quakes, yes, but not over 5 or 6 generally.

      • Well, at least I don’t desire any earthquakes in Thingvellir, because I live so close there.

        Always best that stress is release through lots of 4.5s, than a major 6.0. However, this whole 3 swarms in Tjornes at most is equivalent to a 5.5 earthquake, followed by some aftershocks.

        • Tiny swarm = less than 10
          Smallish Swarm = less than 50
          Small Swarm = less than a hundred
          Medium Swarm = 100 to 250
          Large Swarm = running into hundreds
          Mega Swarm = runing into thousands

          *my ranting ; we could call this “Islander Scale”!??*

          • Nah Carl. Not complicate with size, then quality etc. etc.. Think in blog comments, they do not count size do they; A simple LOL may not be much to some, but huge to others. Heard of KISS method? 😉

          • Keep It Simple, Stupid 🙂
            Yes, I have heard of that and try to practice it to ill effect.
            You are right, let’s keep it simple.

            (it is snowing outside… sigh, I would have been happy if it could have waited for a month or two)

          • Oh, snow!!!! 🙂 Darn, this thing never arrives on time. Oh. Wait. We had snowstorm in the north last week. Killed off several sheep. They are still digging them out (some alive) others not. Guess autumn is over at your place too.

          • Normally we have autum well into october, this is a bit early for first snow. But we had a cold snowy winter, a long cold spring, and a so-so summer, so an early autumn fit the bill. This year has left a lot to wish for.
            The trees are still green. Or was yesterday, cold snow will make short order of that, guess when sun comes up all will be yellow and red.

  2. I particularly like Islander explanation. Maybe we should see this on the bigger picture.

    We know that a pulse of the hotspot is just happening at this period (years). The IMO has been speaking on this for several times. Grimsvotn had a large eruption.

    In 2000 and 2008, the south had major earthquakes. Now, it seems to be more activity to the north. It might be that the pulse from the hotspot pushed the rifting at the mainland of Iceland, and that causes large stress to the north and southwest of the hotspot center. However, except for Grimsvotn, Eyjafjallajokull and increase activity at Katla, we have not seen yet anything large over the center of Iceland.

    Perhaps, the dead zone might rift eventually, resulting in a fissure eruption.
    Or Hekla, if it erupts, it will have a larger eruption.
    Perhaps Vatnsfjoll or anything else weird (near Tindfjoll or Tungafellsjokull) might erupt.
    Or Hamarinn.

    We simply do not know, but let’s see how the stress at the different spots plays out during the next few days, over the rest of Iceland.

    • Hmmmmmm! It’s all my fault for muttering something about Iceland more interesting that El Hierro at the moment! Now I am watching because I cannot sleep and the sheep are less interesting that the SILs from North to south.

        • In last hours, I feel there are quakes (in the north), one in Borgarfjörður area in last update. Truly. The earth must transmit quite well tonight (no wind outside to speak of). I have noticed that quakes in “American Plate” I feel more (cause I set on that side).

    • Answer abowe – I old days logged one 0,5 R at 15 km distance – usually not under 2. Threes I have felt often, as well as 5,5 sitting in car in Reykjanes (it dipped corner to corner), several sixes over long distances.

      • I am amazed at Tjörnes fracture zone. It appears to peak, fall off, peak, fall off.
        Never have seen anything quite like that. Even El Heirro.. Right now we have a huge Forest fire in our area, and I have ash on my car. No Danger to my home but it is
        not something I like. Rather be fighting it….
        But my powers are weakened under a red sun 🙂

  3. Heh… data is cool.

    I hate the term, but what would you consider to be the lower end of a “supervolcano” eruption?

    A resulting caldera larger than 4km across? Yeah, that would be pretty large. Somewhere on the order of 28 km³ of ejecta. (28 times larger than Mt St Helens) but thats only VEI-6.

    How about a caldera 27km across? Roughly 104 km³, or VEI-7.

    Dunno about you, but Pinatubo came in at about 10 km³, much larger than that and its… um, rough. Of the calderas I’ve peeked at so far… 95% are larger than that. That is heavily skewed though, I haven’t been looking for small ones. The count of them is up around 65 or so.

    “only a handful” my ass arse. (BBC “Supervolcano” Program, circa 2000)

    • I think we should start with thinking about what is a decade event, what is a centennial event, and what is a millenium event.
      That is how I see most risk charts here at least. GeoLoco is the pro on this though, so chim in if you wish.
      Having 1 km^3 jumping out of the ground is pretty much your decade event. It might not be exactly one per decade, but around that.
      10km^3 is your centennial event. We had at least 2 of those the last century, Novarupta and Pinatuba both fit the bill.
      And for the millennial eruption we have Tambora this millenia, and Baekdu the millenia before that.

      So, I would say that for anything to be measured as “super” it should be at least a millenium event. So, go for 100km^3 for the divider.

    • I agree with the “only a handful” being bullshit comment.. I think there are a couple of reasons why the frequency of megascary eruptions is underestimated:
      1. multiple event calderas .. see one, get a dozen. (Rabaul etc.) I don’t think these have been exhaustively studied/counted
      2. buried calderas (NZ has got loads, e.g. Kapenga)
      3. shallow submarine calderas (e.g. was Kolombo near Santorini included in their count?) and there are literally hundreds more of these around.

      • There are a few other factors to consider as well. What about volcanoes that have potential to go caldera, but have yet to do so? There is a first for everything, so you can’t disclude volcanoes with a potential to go “big” from doing it just because they haven’t done it before.

        Second, some volcanoes can potentially go bigger than they have in the past.

        Third, the information that’s been compiled already is far far far from comprehensive. As Bruce mentioned, there are a lot of “hidden” calderas I’d imagine, and while countries like New Zealand are on top of it, I wouldn’t doubt there are quite a few very large eruptions in areas like the high Andes, Vanautu, or Central america that have not been documented or discovered (and moreso, listed in the GVP database).

        Also, if we’re going off the VEI 7 being a millenium thing, we’ve had three vei 7 events in the 1200 years if we’re including the 1257 mystery volcano & baekdu (changbaichan)

        • Good point. I remember Boris saying he was convinced the next big one was going to be somewhere we hadn’t even heard of.

          • Though I think it is fair to exclude all deeper calderas that don’t produce any ash into the atmosphere. But there are loads of shallower calderas like Raoul (it has two) and Macauley Island that should be included. And there’s that other one NE of Rabaul, Tavui I think it’s called.

          • I agree, I just wanted to point out that this years big one, was indeed very unexpected. And Boris original comment was about next big one (4+) eruption, not where the next caldera event would be.

          • Do you think Havre was that big? Kind of hard to tell with these submarine eruptions. The pumice raft was certainly extensive but it was pretty spread out and, from a 3d perspective, all the tephra concentrated on the ocean surface. I had a feeling that pumice raft by Tonga a couple of years ago was much bigger.

          • Trust me, the Tonga raft was much smaller. It is another of my nearly missed volcanic eruptions. Technically I was there, but I didn’t get to see anything except the raft. But it was intriguing sailing into it.
            Here is a page that was made by one of the other boats sailing there at the same time, they got to see the island being born, I did not.

            And, this was much closer to the surface to beginn with, so all the pumice ended up floating whereas Havre was just a small part due to depth.

          • And, it got really slow.
            They guys on the boat that shot the photos ruined their engine. We used sails instead, slow going indeed. But instead we went through it all. Took some time, but worth it.

          • ouch, losing your engine when you have to navigate into harbours lined with kilometers of coral shoals would be pretty nerve-wracking.
            But you are certainly one lucky guy to have experienced that raft. Must have been awesome.

          • I didn’t loose my engine 🙂

            But, I pretty much never use the engine. I a long time ago perfected the art of sailing into port. Joshua Slocum circumnavigated the world without engine, so I should be able to get into a port without one.
            Not everyone is stunningly happy about this habbit though…

    • Another really rough way of looking at it is this:
      NZ alone (excluding the Kermadecs) has one VEI7 or more every 50,000 years (34 ignimbrite eruptions in the last 1.6 million years). Now, NZ is currently the most productive site of these beasts but there are a number of other places that are not far behind:
      Indonesia, Japan, Kamchatka, Chile, oh stuff it, the entire ring of fire. Than add in Iceland, the Med and Africa. This must put the chance down to well below 1 every 10,000 years, which as I recall, was the stat cited in that Royal Society paper they used in the Supervolcano documentary.
      Even taking the last two thousand years we have already had Taupo, Baedku, Tambora in the VEI 7 category. If you extend the timeframe to 5000 years, you get at least Santorini and I would also add Pago and Long Island (cited as VEI 6 in the GVP but I strongly suspect these were bigger, just look at the size of Long Island caldera, certainly bigger than Pinatubo) and possibly a couple of others, which gives us at least four in 5000 years, possibly as many as six. And I count 29 VEI 6 eruptions in the GVP database over the last 5000 years which is most certainly not complete. Together this gives us at least 33 confirmed big eruptions in the last 5000 years. I suspect the real number is probably around 40, or or one big eruption (think Pinatubo or bigger) every 125 years.

  4. yawn…. morning everyone. I see the TFZ sequence is not stopping. Pattern seems to be main shock, aftershock sequence, rinse and repeat on an 8 hour cycle (very roughly). The main shock/after schock sequence looks tectonic. The 8 hour cycle makes me go, hmmm…

    • Agreed Bruce, that cycle is intriguing me too…
      Going to bed now in smokey,ashy,NE Oregon….
      Smoke was so thick tonight you could not see the
      sun set….

      • yikes.. doesn’t sound good. I wonder how much all these fires are adding to the carbon loading of the atmosphere? There seem to have been an awful lot of them lately. Australia last couple of years, China, USA…

        • The plants, they be loving it. Especially the C3 carbon fixation plants… Rice, Soybean, Wheat, Rye, Oats, Millet, Barley, Potato (Food, Beer, Whiskey, Scotch, Vodka and Sake)

          From WUWT.

        • Hello Bruce!
          It does not add anything to the Carbon load, since the same amount of trees will grow up again. Also, soot vs CO2 ratio makes it even neutral in short term. Nothing to worry about, it is natural.

        • Fires are needed by some plants and should be viewed as a natural cycle of regeneration. Much as it damages human property and is seen as a disaster by our species. No fire is a disaster to some plants and suppressing the occurrences of smaller fires can lead to bigger fires which ARE damaging.
          In habitats that are prone to fires, such as a Maquis in France and the bush in Australia, the plants and animals that live there have developed strategies for survival.
          The ash from fires adds essential Potash to the soil which feeds and encourages new root growth and stronger shoots.

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