Q&A: Dr. Boris Behncke answers your Italian volcano questions, Part 2

Here is Part 2 of the Q&A with Dr. Boris Behncke. You can also check out Part 1.

Undated image of Stromboli in eruption.

(Shirakawa Akira) 1. Would a large scale eruption (VEI 5 or greater) of Mt. Etna like the ones occurred in 122 BC and 1500 BC (disputed) be possible again in the foreseeable future? Or is the volcano slowly "dying" although the last eruption ended in July 2009 has been one of the longest ever recorded?
2. Not really related to volcanism, but close: does INGV plan to distribute seismic data from seismic stations of its Rete Sismica Nazionale, including stations from subsections like Sezione di Catania, to the public in real-time via IRIS DMC ? Earlier this year many people including me have been able to monitor in real-time the Redoubt Volcano (Alaska) eruption thanks to the open efforts of the people of Alaska Volcano Observatory (AVO) who made all data public. I think it would be great to be able to do the same for future eruptions of Mt. Etna in Sicily, not to mention very instructive. MedNet, an array of broadband seismic stations installed in various countries surrounding the Mediterranean Sea and maintained by INGV and other geophysical institutes, is already available for public real-time monitoring through IRIS. So I wonder if the same will happen soon for most of the other seismic stations maintained by INGV installed in Italy.

BB: Answer to (1): Etna's certainly not dying; if anything it is now in one of its most vigorous and intense moments during recorded history, and indeed some of us are well aware that eruptions like in 122 B.C. or - if we don't want to cite the disputed 1500 B.C. event - about 4000 years ago (Coltelli et al., 2005, Kamenetsky et al., 2007) can happen anytime at this volcano. The activity of the past few decades, and of the past 15 years in particular, has shown a surprising variety - including two subplinian events on 22 July 1998 and 4 September 1999 plus more than 150 slightly smaller paroxysmal events between 1995 and 2001, and again a few in 2006-2008, plus two unusually explosive flank eruptions in 2001 and 2002-2003 ... and it's once more coming back to life in these days. So the possibility of highly explosive eruptions is real. One intriguing thing at Etna is that the more explosive eruptions in recent millennia have often been related to the very fast uprise of primitive and volatile-rich magma (such as picrite in the 4ka B.P. eruption), and in the past 30-40 years, there is a clear influx of more primitive magma into the feeding system of this volcano. I fear, however, that no one here is really prepared for such an event. And, overall, I guess that the probability that we will see a major flank eruption from vents at very low elevation on the flank of Etna - like in 1669 - is much higher than that of a large and potentially devastating explosive eruption.
Answer to (2): I know that some seismic data recorded by the INGV is actually available on-line (see, for example: http://www.ov.ingv.it/index.htm?ufmonitoraggio/tempo_reale/segnali_t_r…), maybe not to the extent as at the AVO, though. This has to do in part with the fact that Italian Civil Defence is privileged in having access to the data, and possibly also based on the assumption that being in a country with an exceptionally high level of volcanic and seismic hazard, free access to a lot of data can lead to misinterpretation in the public. Already the possibility to view selected graphs of the seismic activity at Etna has led to a lot of misinterpretation, which everybody here is trying to avoid (although keeping data too "secret" can have a boomerang effect). On the other hand I saw that data related to the Abruzzo earthquake on 6 April 2009 were extremely rapidly produced and rendered public by the INGV staff.
Coltelli, M., Del Carlo, P., Pompilio, M., Vezzoli, L. (2005) Explosive eruption of a picrite: the 3930 BP subplinian eruption of Etna volcano (Italy), Geophysical Research Letters, 32, L23307, doi:10.1019/2005GL024271R.
Kamenetsky, V.S., Pompilio, M., Métrich, N., Sobolev, A.V., Kuzmin, D.V., Thomas, R. (2007) Arrival of extremely volatile-rich high-Mg magmas changes explosivity of Mount Etna. Geology, 35: 255-258.

(Bruce Stout) 1. Some volcanoes in a back arc setting initially eject rhyolite and/or andesite that slowly transitions into more basaltic ejecta as the magma chamber empties and the underlying mafic source reaches the surface. Etna is a shield volcano in a subduction zone setting. Is there any evidence of rhyolite or andesite in its early history or was it a typical shield (mafic) volcano from the beginning?
2. Connected to this, in recent years there was some mention that scientists saw some indication that Etna was evolving away from a typical hot spot volcano and becoming more like a typical subduction zone volcano. Is this the case? If so, what kind of expression would this have? Could we expect a transition to a higher silica content or (merely!) a higher concentration of volatiles in a steadily mafic profile?
3. The tectonic setting of Italy is complicated to say the least, yet Etna is often spoken of as a "hot spot" volcano akin to Hawaii or Reunion. What is your view? Are there any indications that Etna is a product of, or at least influenced by, the nearby subduction of the African plate under the Tyrrhenian Sea?
4. If Etna is a hot spot volcano and given that hot spots generally leave a trail in their wake, where are the proto-Etnas? Have they been subducted? If this is the case, will Etna also be a candidate for subduction in the distant future? What would the implications of this be for volcanism at Etna?
Answer to (1): Etna is a strange volcano in a strange setting. Rather than a shield volcano, I would describe it as a volcano that is a bit of all - because it has more than one caldera, hundreds of scoria cones, some areas that resemble a shield volcano but its upper portion is rather like a stratovolcano. Then, Etna sits next to a subduction zone but not on it, and it never has. (I have already explained a lot about this setting in the answer to the first question, by Bernard Duyck.) We definitely have evidence for Etna having gone through periods when its magma was more evolved, particularly during the late stage of its so-called "Ellittico" phase, which lasted from about 40 ka to 15 ka and ended with extremely violent explosive volcanism of benmoreitic and mugearitic composition. It is believed that in that period magma differentiated in a large, relatively shallow reservoir. During the past 15,000 years no differentiated magma has been erupted from Etna.

BB: Answer to (2): Subduction zone volcanism is generally associated with more silica-and volatile rich magmas and explosive activity. Most magmas are calc-alkaline, which is not the case at Etna, so things are not as simple. Furthermore, at Etna it has been found that particularly volatile-rich magmas are among the most primitive to be erupted from this volcano. So, there's currently no evolution toward more silica-rich magmas but there is quite a bit of volatile-rich magma - consider that Etna on the average emits roughly 200,000 tons of water vapor per day, plus some 20,000 tons of CO2 (plus some 5000 tons of SO2), every single day. The H2O and CO2 make it more explosive than one would normally believe a basaltic volcano to be. If this is subduction related or something else remains to be discovered; certainly the fact that a subduction zone is not far away from Etna might have to do with it.
Answer to (3): This is explained in my answer to the question of Bernard Duyck.
Answer to (4): IF (and that's a BIG if) Etna is a hotspot volcano or IF ever it has been one, one might suggest that the older volcanic area of the Monti Iblei to the south is the trace of the hotspot, especially because the volcanics of that area are older the further south you go. The problem with such a hypothesis is that the plate upon which the Monti Iblei lie is moving northward, and so volcanism should become younger southward, not older. So the hotspot hypothesis does not seem to work. I'd rather think that the authors of the "mantle window" hypothesis (Doglioni et al., 2001, cited in my response to Bernard Duyck above) have found a quite plausible explanation for why Etna is there.

(Me) 1. Are there any volcanoes in Italy that pose a threat that most people are not familiar (i.e., volcanoes other than Vesuvius, Campei Flegrei or Etna)?
2. How well prepared do you feel Italy is for a large-scale volcanic eruption, especially around the Bay of Naples?
3. What would you tell a student who might be interested in getting involved in monitoring volcanoes or writing about volcanoes on the internet?

BB: Answer to (1): There are a few Italian volcanoes that there's not much talk about in the public - such as the Colli Albani, just south of Rome, which is a restless caldera complex showing intense seismic activity and active uplift in the caldera center (Amato & Chiarabba, 1995; Riguzzi et al., 2009). While chances of a major eruption in the near future are remote, I fear the public is oblivious of the potential threat of renewed activity, which even in case of a small eruption would have devastating consequences due to the dense urbanization of the area (the Pope's summer residence of Castelgandolfo lies on the rim of the most recently active maar crater of the Colli Albani ...)
Answer to (2): Recent disasters such as the Abruzzo (L'Aquila) earthquake of 6 April 2009 and the landslide disaster in northeast Sicily on 2 October 2009 have shown that in Italy far too little effort is spent on disaster prevention - such as construction norms and limits, and people living in hazardous areas are often not very well informed. And yes, there is much talk about Vesuvius, but it's a little bit like the big global campaign against smoking - there a big effort is made to show how precious our health is to the governments, but why do they still allow us to drive cars and die in car crashes, and so on ... so at Vesuvius there's a big effort going on, but in nearby Campi Flegrei, for example, there is an emergency plan existing as well but the administrative area of Naples is excluded from this plan, although quite a large area of Naples lies within Campi Flegrei caldera. At Etna, I think there is sufficient preparation for the type of eruption we saw in 2001 and 2002, but I guess an eruption like that of 1669, which came from a new crater at only 700 m elevation (compared to about 3330 m elevation of the summit) and whose lava flow reached the sea near Catania, no one is really prepared.
There is one recent publication that for the first time describes a survey carried out among people living near Vesuvius, concerning their knowledge of volcanic hazards and of the emergency plan and asking how much confidence they put into the plan, the authorities, the volcanologists, and the media (
Answer to (3): If you envisage a career in the field of volcano monitoring, there would be three possible pathways at a university - geology, geophysics, or engineering/computer sciences. Writing about volcanoes in the internet requires much less ... if I had had internet as a child I would have filled it with volcano stuff I fear ⺠but then, you may want to cross the fine line to writing volcano-related things both accurate and riveting. If that's the case I'd recommend looking at how Erik Klemetti does it, and along with him a few others - the Volcanism Blog is another fine example (http://volcanism.wordpress.com).
Amato, A., Chiarabba, C. (1995) Recent uplift of the Alban Hills volcano (Italy): evidence for magmatic inflation? Geophysical Research Letters, 22: 1985-1988.
Riguzzi, F., Pietrantonio, G., Devoti, R., Atzori, S., Anzidei, M. (2009) Volcanic unrest of the Colli Albani (central Italy) detected by GPS monitoring test. Physics of the Earth and Planetary Interiors, 177: 79-87.

(Matt McGinn, in Pozzuoli) 1. Are most volcanologists of the opinion that Vesuvio is in a long period of dormancy and would not be expected to erupt anytime within the next century? Is it then likely that the next eruption will be highly explosive?
2. In the AD 79 eruption of Vesuvio, what were the eruptive materials that destroyed Herculaneum? Was it indeed a pyroclastic flow? I cannot seem to find multiple sources (in English) that agree on this.
3. On what scale are future eruptions of the Campi Flegrei volcano expected to be? Smaller ones such as the one that formed Monte Nuovo, or much larger ones such as the area experienced in the distant past?

BB: Answer to (1): It is well possible that the current repose period at Vesuvius (called Vesuvio in Italy) will continue for a long time, maybe for centuries. There is general consensus that the longer it lasts, the more violently explosive will be the next eruption.
Answer to (2): There are indeed a multitude of English-language sources on the AD 79 eruption and pyroclastic flows that devastated Herculaneum and Pompeii ... but most of them are in the scientific literature. The first significant publications were by Sigurdsson et al. (1982, 1985) but many more recent publications also deal with the probable impact of pyroclastic flows in case of a new eruption of Vesuvius. A few are indicated below.
Answer to (3): It is difficult to say what scale future eruptions in the Campi Flegrei will be of. Let's say that generally the more likely eruption scenarios are the smallest - and many scientists say the next eruption will likely be of the dimensions of the Monte Nuovo 1538 eruption. But with active volcanoes you never know. A volcano that once has produced a cataclysmic, caldera-forming eruption and is still active, in my opinion is still capable of producing another cataclysmic, caldera-forming eruption. But statistically the likelihood of such gigantic events is vanishingly small, and in our lives we should worry about many other things first (such as ending up in a car crash or slipping in the bathtub), rather than about those doomsday scenarios. In the Campi Flegrei the problem is that even a very small eruption would be highly disruptive and destructive. Same is true on Vulcano Island in the Aeolian Islands.
Barberi, F., Davis, M.S., Isaia, R., Nave, R., Ricci, T. (2008) Volcanic risk perception in the Vesuvius population. Journal of Volcanology and Geothermal Research, 172: 244-258
Capasso, L. (2000) Herculaneum victims of the volcanic eruptions of Vesuvius in 79 AD. The Lancet, 356: 1344-1346.
Carlino, S., Somma, R., Mayberry, C.G. (2008) Volcanic risk perception of young people in the urban areas of Vesuvius: Comparisons with other volcanic areas and implications for emergency management. Journal of Volcanology and Geothermal Research, 172: 229-243.
Giacomelli, L., Perrotta, A., Scandone, R., Scarpati, C. (2003) The eruption of Vesuvius of 79 AD and its impact on human environment in Pompei. Episodes, 23: 234-237.
Gurioli, L., Cioni, R., Sbrana, A., Zanella, E. (2002) Transport and deposition of pyroclastic density currents over an inhabited area: the deposits of the AD 79 eruption of Vesuvius at Herculaneum, Italy. Sedimentology, 49: 929-953.
Gurioli, L., Pareschi, M.T., Zanella, E., Lanza, R., Deluca E., Bisson, M. (2005) Interaction of pyroclastic density currents with human settlements: Evidence from ancient Pompeii. Geology, 33: 441-444.
Gurioli, L., Zanella, E., Pareschi, M.T., Lanza, R. (2007) Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): 1. Flow direction and deposition. Journal of Geophysical Research, 112, B05213, doi:10.1029/2006JB004444.
Luongo, G., Perrotta, A., Scarpati, C. (2003a) Impact of the AD 79 explosive eruption on Pompeii, I.Relations amongst the depositional mechanisms of the pyroclastic products, the framework of the buildings and the associated destructive events. Journal of Volcanology and Geothermal Research, 126: 201-223.
Luongo, G., Perrotta, A., Scarpati, C., De Carolis, E., Patricelli, G., Ciarallo, A. (2003b) Impact of the AD 79 explosive eruption on Pompeii, II.Causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties. Journal of Volcanology and Geothermal Research, 126: 169-200.
Mastrolorenzo, G., Petrone, P.P., Pagano, M., Incoronato, A., Baxter, P.J., Canzanella, A., Fattore, L. (2001) Herculaneum victims of Vesuvius in AD 79. Nature, 410: 769-770.
Nunziante, L., Fraldi, M., Lirer, L., Petrosino, P., Scotellaro, S., Cicirelli, C. (2003) Risk assessment of the impact of pyroclastic currents on the towns located around Vesuvio: a non-linear structural inverse analysis. Bulletin of Volcanology, 65: 547-561.
Pesaresi, C., Marta, M., Palagiano, C., Scandone, R. (2008) The evaluation of "social risk" due to volcanic eruptions of Vesuvius. Natural Hazards, 47: 229-243.
Sigurdsson, H., Cashdollar, S., Sparks, S.R.J. (1982) The eruption of Vesuvius in 79 A.D.: Reconstruction from historical and volcanological evidence. American Journal of Archaeology, 86: 39-51.
Sigurdsson, H., Carey, S., Cornell, W., Pescatore, T. (1985) The eruption of Vesuvius in 79 AD. National Geographic Research, 1: 332-387.
Solana, M.C., Kilburn, C.R.J., Rolandi, G. (2008) Communicating eruption and hazard forecasts on Vesuvius, Southern Italy. Journal of Volcanology and Geothermal Research, 172: 308-314.
Spence, R.J.S., Baxter, P.J., Zuccaro, G. (2004a) Building vulnerability and human casualty estimation for a pyroclastic flow: a model and its application to Vesuvius. Journal of Volcanology and Geothermal Research, 133: 321-343.
Spence, R.J.S., Zuccaro, G., Petrazzuoli, S., Baxter, P.J. (2004b) Resistance of Buildings to Pyroclastic Flows: Analytical and Experimental Studies and Their Application to Vesuvius. Natural Hazards Review, 5: 48-59.
Zanella, E., Gurioli, L., Pareschi, M.T., Lanza, R. (2007) Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): 2. Temperature of the deposits and hazard implications. Journal of Geophysical Research, 112, B05214, doi:10.1029/2006JB004775.
Zuccaro, G., Cacace, F., Spence, R.J.S., Baxter, P.J. (2008) Impact of explosive eruption scenarios at Vesuvius. Journal of Volcanology and Geothermal Research, 178: 416-453.

(Diane) Since I found out Etna has started erupting again before the earthquake, I now have a new question. Was the quake magmatic or techtonic, and has it affected the eruption and flank where it occurred? I am wondering if it was strong enough to cause a possible flank eruption. My guess is it was too weak to do much, though I remember one several years ago that did some damage to roads and a bridge.
BB: The earthquake that you refer to occurred two days after the new incandescent vent was seen at the summit of Etna, on 6 November 2009, in the Enna area west of the Nebrodi mountains, and quite a few tens of kilometers away from Etna. It was a typical tectonic earthquake, and with a magnitude of 4.4 it caused very little material damage. As far as we can say it was not directly related to the volcano and its activity. As a matter of fact, there has been no change so far (as of 20 November 2009) in the activity of Etna, although signs are there that the volcano is definitely waking up. But this is normal at Etna, earthquakes or not, maybe a nearby earthquake can accelerate things if the volcano is already on the brink of erupting, but if there's no earthquake it will eventually erupt anyway. What I can say is that Etna is entering into a new phase of activity, and probably more news will soon be reported on the Eruptions and Volcanism Blogs ... in the meantime, keep an eye on the on-line seismic graphs of Etna, such as can be found at http://www.ct.ingv.it/Etna2007/SegSismici.asp?Staz=ESVO_HHZ_IT&Pos=6 and the amplitude of volcanic tremor, at http://www.ct.ingv.it/Etna2007/Sismologia/Tremore.htm

Thanks again to Dr. Behncke for taking the time to answer questions here on Eruptions!

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Thanks for answering my questions!

About seismic data availability, though, I did not mean only webicorder views from official websites, but also seismic data streams in SAC or SEED formats (or even other ones) often used in this field. USGS is very open regarding data availability, and pretty much all seismic stations monitored in real-time on USA territory are publicly available in every aspect. Also, as data is archived, through IRIS it's possible to examine it detail even from past years, if available.

I understand that INGV might want to keep most data restricted for public security reasons as volcanically hazardous places of Italy are densely inhabited; keeping a public archive of delayed data (1-2 weeks from real-time?) however, should not do any harm, while still being useful and instructive.

Regarding seismic data from this year's L'Aquila earthquake, I think it propagated quickly also because there a public (available through IRIS) Mednet seismic station within very few kilometers of the epicenter.

By SHIRAKAWA Akira (not verified) on 27 Nov 2009 #permalink

Many many thanks Boris. Really appreciate your passion for the subject. I must apologize in retrospect for my questions. I read through your website AFTER I posted this first set to Erik and realized how off the mark they were so I sent a second set and they seem to have been eaten by the email eating machine!! Ha. But you have anyway responded to most of the issues I had in other responses.

But what an enigma the mountain is!
"at Etna it has been found that particularly volatile-rich magmas are among the most primitive to be erupted from this volcano"
How does that work? In general how does the mantle window theory work? Like a tear in the lithosphere allowing mantle to rise (decompressional melt?) and then where do the volatiles come from if not from subducted ocean floor/lithosphere?

By bruce stout (not verified) on 27 Nov 2009 #permalink

Thank you, Boris, for answering my question about quakes and Etna. One of the things I monitor is the possiblity of a quake triggering an eruption. I think this is more likely in the Indonesia, Vanuatu, and Solomons area. Recently, just an hour after the 7.6 quake off Sumatra, there was a swarm of small quakes at Mammoth Mountain in CA. Most of them were techtonic as there is a fault on the mountain. So there was the possibility of the Sumatra quake setting off the quakes at Mammoth. The people at the seizmology department in Berkely were going to have a seminar on just that subject-quakes triggering quakes.

I find all of this very facinating. While we are not really sure about it, I would think that very large quakes could trigger others and perhaps not right away.

Another area I watch is Nevada. There you have mining blasts that register below 1-3 mag though rarely are there any 3s. I believe that some of those blast cause small quakes of varying depths. Most are no more than 10 miles down.

Anyway, thanks for the answers and also the sites I can check out and learn more.

Diane: "Recently, just an hour after the 7.6 quake off Sumatra, there was a swarm of small quakes at Mammoth Mountain in CA. Most of them were techtonic as there is a fault on the mountain. So there was the possibility of the Sumatra quake setting off the quakes at Mammoth."

There's quite a distance between the two locations o_0'. What makes you think there is a direct link between the two events?


I've seen a number of articles in the past few years saying what Diane is saying. Unfortunately, I couldn't find any articles using Google that spelled out the short-term connection that I've read about. This one references connections between earthquakes far and wide.


Others have found a two-hour increase in earthquake activity on the opposite side of the world when a major 7 earthquake shakes.

There have been several articles and studies showing that strong historical earthquakes have often clustered in time. In the 1900's the four biggest earthquakes worldwide clustered within a 12 year time frame. The Mediterranean has also had a cluster of major earthquakes in the distant past that were for the longest time thought to have been one earthquake. I don't want to enclose a second link because in the past posting two links a year or two ago sent my comment into cyber hybernation - awaiting Erik to check out the suspect posting.

There is sometimes a connection between earthquake activity far around the world - but it is statistically determined - and it is hard to prove that the timing of each individual quake was the result of earthquake activity on the other side of the world.

By Thomas Donlon (not verified) on 28 Nov 2009 #permalink


The main reason I think the 7.6 quake could have caused the swarm on Mammoth is I emailed a seizmologist and asked him about the Mammoth swarm. He is the one who told me that it happened an hour after the quake. He also told me that it was good timing because they were having a seminar about that very subject--large quakes causing quakes in other places--that very morning he emailed me back. Since they, the people at USGS think it is possible, it probably is. Distance doesn't stop the seizmic waves from traveling all over the world. There is a zone, however, where the waves do not show up. Other than that, they can be picked up anywhere.

I had the priviledge of seeing a book, an old book, that had the seizmic graphs from all over the world of the SF '06 quake. It was very interesting indeed. They had graphs from Russia, Europe, Asia and some of them were what you might think we would see today and others were a series of circles. Just from different seizmic stations all over the world.

The possibility is there and probably has happened. For a swarm to happen just an hour after a major quake half way around the world could be just coincedence or it could have been that the 7.6 caused it. At any rate, I think it is an interesting topic along with volcanoes. As Thomas said, it is difficult to study and to prove it beyond a shadow of a doubt may not be posible with our current measurement systems.

I suspected that big earthquakes can cause other earthquakes to happen shortly after the 'first' quake on a regional scale. For example: look at all the 6.0+ earthquakes happening in Southeast Asia in the weeks to months following a big quake (Samoa on the 29th of September this year is just one case).

It doesn't surprise me that seismic stations all over the world can pick up earthquakes happening on the other side of the globe, and it also doesn't surprise me that strong earthquake can cause other (strong) earthquakes to happen all over the world in the years following (every quake is the result of stress, and if it lessens in one place, there's automatically more stress at another location), but what I'm wondering is if the seismic waves of a 7.6 event are still strong enough to cause direct seimicity after having travelled, let's say 10.000 km's through the Earth's mantle, core and parts of the crust.

It is a very interesting topic indeed. I hope that within a few decades computer generated models can be made of how much stress can be found in specific places in the Earth's crust (of some places these are already available, like of the North Anatolian Fault and of the San Andreas Fault, but they don't really show how their behavior is related to stress in other places in the world), to see how different quakes are related, and therefore give us a (little?) help in 'predicting' future earthquakes... and maybe some volcanic eruptions?

I found an article posted on www.breitbart.com Sept 30 09 titled:

Major quakes can weaken seismic faults far away, scientists say.

The study is published in the British weekly journal Nature. Here are a couple of quotes that give an idea of why those who did the study came to their conclusions.

"almost jfive days after Sumatra event--one of the biggest quakes in recorded history--sensors noted dynamic stress on the Parkefield fault at a depth of five kilometers(three miles)....

The long-range influence of teh 2004 Sumatra-Andaman earthquake on this patch of the San Andreas fault suggests that many of the world's active faults were affected in teh same way, thus bringing a significant number of them to failure," 'the study says.'"

Anyway, what they found is that the large quakes over a 22yr period have affected the Parkfield part of the San Andreas. One of the things it did affect was the fluid behavior in the area. They have that area of the fault instrumented to death. They do have quite a few quakes there so it is a good area to study the fault.

Just thought you might be interested in what is being studied in this area of geology. There are a lot of factors affecting the faults and if any strong quakes happen to hit just right, another quake could be triggered elsewhere. At least they have found weakening of part of a large fault system.

Interesting point of view.Thanks for the post. - Once, during prohibition, I was forced to live for days on nothing but food and water. - W.C.Filelds 1880-1960