Water, rocks, earthquakes, and weather: Part III: A dance of sky, mountain and deep
Dave Waters
Director/Geoscience Consultant, Paetoro Consulting UK Ltd. Subsurface resource risk, estimation & planning.
CONTENTS PART I:?Wet, and under pressure (Figures 1-29) - prior article
CONTENTS PART II: Into the crackling abyss (Figures 30-93) - prior article
CONTENTS PART III: A dance of sky, mountain and deep (Figures 94-112) - THIS ARTICLE
In this article – Part III - we look at how weather effects influence loading stresses in rocks and look at the effect this can have on seismicity, and whether this effect changes in any way how we prepare for damaging seismicity.
In Part I we introduced some basics of how rocks interact with water, at depths where porosity is still and thing, and where water can exist as a separate phase.?In Part II we explored into the greater depths of the deep Earth, in places where pressures are so great as to preclude any porosity, and water exists predominantly as either i) hydrous phases of minerals; ii) transient reaction induced diffusion events; or iii) dissolved in melt.?In Part III, we go in the opposite direction to Part II, and instead of investigating how water interacts at great depth interacts with rocks, we go now to water at great heights.?We investigate whether the atmosphere can interact with rocks at great depth.?While that might sound bizarre, and while any effects may be predominantly very subtle indeed, it is less fanciful than we might think.?
The size, time and energy distribution of earthquakes - and by implication faults
Can weather affect seismicity in any significant, important way??That’s one of the questions we look to address. It’s important here then to understand just what we mean by seismicity – earthquakes. As we see in Figure 94, the number of earthquakes in this example from Spain increases dramatically – exponentially, as we go to smaller magnitudes.?For every magnitude seven earthquake in Spain, there are about 6000 recorded magnitude 3’s.?What this tells us is that we see a whole lot more small earthquakes and they give us a much more useful sample set.?One that can help us look for subtle effects.
The caveat to that is that mostly these earthquakes are tiny. A magnitude 3 earthquake might only involve slip on a fault of a few cm to a few mm.?A magnitude 2 earthquake from less than a cm to less than a mm.?And the amount of energy release increase for every step in the magnitude scale is roughly 30x:?31.6x for Richter magnitude Ms and 27.5x for moment magnitude Mw.?That tells us that while these small earthquakes help us to discern effects and influences better because there are very many of them, they are really very different beasts to the ones we are most interested in understanding – namely the ones that cause damage.?A magnitude 6 involves roughly 27000 times more energy than a magnitude 3.?A magnitude 7 involves 24.3 million times more energy than a magnitude 2.?It is reasonable to expect that very subtle influences observed at a magnitude 2 scale may have fairly little relevance to the influences on events at a magnitude 6 & 7 scale.?Yet the domino effect where a small event can trigger a bigger one, which can trigger a bigger one, and so on, is never impossible.?Rare perhaps, but not impossible. We will return to this question.?
A visual sense of just how huge this energy discrepancy is, even between magnitude 6 and magnitude 9 events – the largest we know of, is shown in Figure 95.?Figure 96 does a nice job of showing the same thing for a larger range of magnitudes and mapping it against worldwide frequency of events in a typical thirty-day period.?Figure 97 shows expressly the relationship between energy release and magnitude size, with the appropriate trends and formulae for both Richter magnitude (Ms) and moment magnitude (Mw), slightly different ways of measuring earthquake energy.?
What to take away from all this??When we talk about weather influences on seismicity, we need to note that any statistically significant inferences are based on observations of earthquakes that are much, much, smaller than the ones that cause damage, and that we cannot assume by default that any effects on earthquakes that are millions of times more energetic will be the same.?We can perhaps infer that triggering foreshocks might be slightly more likely at certain times, but how that effect compares to other much more important influences is the key topic.?The perpetual crackling of the crust due to tectonic stresses for example, in seismically active areas, is always generating a healthy spread of magnitude 3 and 4 earthquakes, and they are far, far more likely to be triggers.?We will look at this in more detail. ?First though, we will visit & refresh the topic, also discussed in Part II of how fluid pressure changes can “induce” seismicity.
Induced seismicity
Induced seismicity is when some event can be seen to directly trigger seismicity to a very high degree of probability.?This is not always human related and not always a disaster – it is routinely used in “fracking” for example, particularly but not only, in the United States, to induce small earthquakes in hydrocarbon bearing shales and extract more of them.?Of the subset of induced seismicity that is humanity caused, a fascinating review was published in 2018 by Foulger et al.?We see there, in Figure 98, how for human induced seismicity (unrelated to scheduled operational procedures) mining was responsible for just over a third, hydroelectric dams just under a quarter, conventional oil and gas 15%, and geothermal about 8%, with a smattering of contributions from other.?Things may well have moved on since this 2018 review a bit, but it is worth highlighting that the things we do that change pressures in the subsurface the most, would appear to be the big three of mining, hydroelectric lakes, and conventional oil and gas.?The sheer number of operations in these activities rings true with the seismicity observed.
Figure 99 and 100 are some reminders from Part II (Figures 45 and 52) of how increasing pore pressure is on its own often sufficient to induce seismicity – this is why the filling of hydrolakes often creates induced seismicity.?Note though it is not new introduction of fluids that creates the seismicity, it is just the changes in pressure associated with a new higher water table and associated loading.?The geometry of rocks and faults as per Figure 100 can allow this pressure to communicate relatively quickly – and any shallow events induced on a fault may then propagate new post-seismic stress loads to other parts of the fault deeper, where larger events can be possible, if much less likely.?The situation is summarised in Figure 101.
Mining events are typically of a “rockburst” and implosive style, where creating large cavities either underground or opencast, induces stress changes in surrounding rock, so it is less related to fluid changes, but of course because humans are operating in close proximity to this induced seismicity, it is probably the most dangerous as well as the most common. A 1989 event in Germany at ML 5.6 killed three.?It may also have been triggered by waste fluid injection and increased pore pressure into the mine.
Oil and gas operations routinely influence pressures in the subsurface, often lowering pore pressures during extraction, but often also locally increasing them as fluids are injected to increase recovery.?While Fracking we already know puts induced seismicity to use, it typically creates its own length limited fractures and it is only when elevated pressure fluids (sometimes as much as 9000 psi above hydrostatic) reach into natural fault systems that problems typically occur.?Geothermal exploitation, similarly can changes pressures through production and injection, but typically the objective is to maintain pressures as close to original as possible, precisely to avoid these kinds of effects.?That is usually very achievable, but even subtle local transient changes can sometimes induce seismicity.?
An exception to that is EGS or enhanced geothermal systems, as illustrated in Figure 102 and Figure 103. In these cases, a tight (not very permeable naturally) rock is pressured up - not by as much as fracking though - more typically ~ 2000 psi above hydrostatic, to help increase connectivity along permeable pathways already in the rock.?These are typically faults and/or fracture systems.?EGS is more a tickling of existing fractures than outright creation of new ones as per fracking.?However that EGS often utilises existing fault system fractures of course raises a special degree of care needed to ensure those faults are not activated.?Induced seismic events in France and Switzerland over the past few decades has brought this into a spotlight.?Ironically one event was caused when hydrocarbons were encountered unexpectedly and it was in pressuring up the well to control this unexpected gas influx, that pore pressures were increased and an event resulted. Any EGS activities these days are intensely monitored and operations halted if any concerning signs appear.?
Diligent fault mapping prior to drilling is perhaps the key risk mitigation that can occur in this geothermal context.?The most damaging ever geothermal induced earthquake is described in Figure 103, and while arguable modest in size at M5.4, its shallow nature caused widespread damage in Pohang Korea in 2017.?A series of planned injection events induced seismicity along two well trajectories, but an unmapped fault between them was triggered by the injection.?Geothermal EGS is now recognised official by the Korean Government as the cause of the event after a long series of studies.??A key thing to recognise with such induced events is that they do not create new big events that an area would not be vulnerable to anyway, they can simply hasten their arrival.?The induced seismicity has as a natural ceiling of the worst-case scenario event of faults in the area, as evidenced from historical seismicity records, as far back as they can be taken.?The more that historical seismicity is understood of course, and the longer the record, the better.?
The moral of the story really is fivefold:
1)?????Map the faults carefully;
2)?????Know the historical seismicity record well;
3)?????As far as is possible maintain subsurface pressures during operations;
4)?????Where that is not possible, or a feature of operations is changing that – monitor closely;
5)?????If things start to ramp up on the activity front, stop and assess.
Can weather induce seismicity?
Having examined how easily it is possible for humans to induce seismicity through pore fluid pressure and loading/unloading changes, it is not an unnatural or stupid question to ask if weather events can also influence seismicity.?There are three main ways this can potentially occur:?
The place where this has been most thoroughly looked at is Taiwan – as a country susceptible both to significant seismicity, and regular typhoon events.?One event that was especially closely monitored was Typhoon Morakot in 2009.?Figure 104 shows at left both the rainfall with recording stations and the landslide events and key faults resulting from the storm.?At right we see changes in seismicity of shallow (<15 km) events in the two and a half years after the storm, and changes in “b-value” which is a measure of the ration of strong to weak earthquakes.?While the correlation is not exact, there does seem to be some correlation with the south-central part of the island where influence of the typhoon was greatest.?Note though also that the largest and relatively few large events show no obvious relationship.?If there is a relationship it seems concentrated in smaller events.?
But if there is a bit of ambiguity in the geographical relationship with the typhoon’s areas of greatest impact, the temporal relationship is a little bit more clear cut and shown in Figure 105.?At the top left we see various earthquake frequencies and the “b-value” again.?At time 0 is Typhoon Morakot and the changes in earthquake frequency and b-value, if not spectacular in magnitude, are relatively clear in incremental stepwise change. At lower right we also see a notable and interesting trend in the magnitude-frequency relationships of events pre and post the Typhoon.?Interestingly the number of smaller events below about magnitude 3 noticeable increases while the larger ones appear slightly “damped” and lower in frequency.?
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This perhaps tallies with the idea that these changes are indeed very small, and where there is noticeable effect, it is most noticeable in smaller events.
Moving on from Typhoon Morakot, but still in Taiwan, Figure 106 looks at how regular seasonal changes in water table height – of magnitudes between 5 and 15 m – affect earthquake frequencies, on opposing western and eastern sides of Taiwan.?The charts show both earthquakes above certain size thresholds for east and west, and the water table loading as ascertained by both surface based and satellite based (GNSS) measurements.
What is immediately clear is that there is that while the number of events is not always huge, a corresponding seasonal cyclicity occurs in both. What is more puzzling is the phase shifts of the peaks relative to maximum water table loading, and how these are different on different sides of the island. In the west there is a stronger case for seismicity correlating with maximum water loading, while in the east the authors argue for the opposite – an anti-correlation.?In truth, the peaks in either case are not exact, and why there should be such a difference in the island – that is dominated by west verging thrusts on both sides – is difficult.??Very little area of the thrusts dominating the eastern area are onshore, and a great deal of those on the west are, so maybe the western area is the key one to focus on, and perhaps the east is responding dynamically to those changes in the west, with some time lag. Like a seasonal water table induced seiche slopping back and forth in the onshore stress “pool” of the island.
Whatever the case, it does seem reasonable to conclude that we are seeing – albeit very subtle – influences of seasonal weather patterns and extreme weather events, in the Taiwanese seismic record.
Such water tale effects are not limited to seasonal cyclicity or Taiwan, and another closely monitored region is California, where various mountain-valley pairs have strong interdependence and hugely intensive agriculture and horticulture, as well as no shortage of active faults of interest.?Figure 107 shows some of the water table and subsidence changes there during drought from 2012 to 2015, inferred from GPS data.?There are no corresponding observations of seismicity as per Taiwan to match to, but it shows these kinds of effects are far from isolated and can be sub-regional in scale.????
Having made the observation that at the very least, in the case of Taiwan, there is some evidence for weather and seismicity interplay – it is also very clear that any effect is subtle, and most manifestly detected in very small events.?The large events documented in Figure 104, 105, and 106 don’t show any meaningful statistically significant correlation.??An attempt to visualise the minuteness of the effect is attempted in Figures 108 and 109.?In Figure 108, proceeding from left to right we have five load scenarios and the associated conceptual pressure depth curve alongside.
The point from this exercise is that the differences between the dashed lines in 4 and 5 are really very small, and these are drawn conceptually with little attempt to draw to true scale, so in practice the difference is even smaller.
In Figure 109, we take in a) a fuller section down to 15km, the kinds of depths where sizeable?earthquakes often nucleate, and throw in an anastomosing fault zone.?We have shown some of the depths where shallower earthquakes might nucleate and propagate stress and strain deeper, as per Figure 55 in Part II.?We also see there in b) a zoom at scale, into the top 3 km, including a deeply located water table at 200m.?In c) we investigate the effect of a 50m increase in the water table height, and we see from the consequent dotted blue line just how subtle the pressure changes associated with this are, compared to the pressures present at earthquake nucleation depths.?
That is the key point – there is a change, yes, and could it have an effect, yes, but crucially, the effect is so small, it is likely that almost all of the time, other effects are more important.?Most significantly the ongoing never ending stress repositioning that accompanies every small event and is driven far more by tectonic stresses than anything the weather can throw.?These effects are not really the cause of seismicity, but they might conceivably sometimes be “the straw that breaks the camel’s back” – i.e. the last tiny increment that takes over a failure threshold.?Sometimes.?Perhaps very rarely, but sometimes.?
If the effect is so weak, why do we discern it?
Although we have touched on this point previously, it is worth revisiting previously.?The suggestion I make here is that while an effect of weather on seismicity is present, it is a minor one and relatively unimportant, mostly dominated by other far greater and more significant effects.?Figure 110 and 111 talk to this.?In Figure 110 i) and ii) we see once again that there are simply so many more small earthquakes – the statistical sample is huge and our detection good – with an example from both the global number of earthquakes (i) and from southern Japan (ii) shown.
The lower rung from figure 110, shows another intuitive point – that the displacements on small earthquake faults are way, way smaller than those of big ones.?Recall in Taiwan we were talking of water table loading differences of the scale 5-15 m – so that is the weight of water we are talking about influencing these events.?It’s hard to imagine such a subtle weight difference affecting magnitude 5 and above earthquakes nucleating under the weight of 15 km of rock and more.?It’s like a feather landing on a supertanker.?
We see from iv) that events in the Magnitude 5.5 above above range might involve displacements of 20cm up to 10m for the biggest events (~8.5).?It’s hard to imagine 10m of water inducing anything like that.?However, we see from iii) that once we get down to magnitude 3, the displacements are much smaller – from one to several mm. If we look at figure 112, we see this embellished down to even smaller magnitudes, where we can see a magnitude 1 or 2 event – of the kind documented in Figure 105 for Taiwan, the offsets are on the scale of a tenth of a mm to 1 cm.??It is just about possible to imagine water loading weight effects, or erosion unloading, influencing earthquakes of that size.?It seems more reasonable.??
If there is any effect on bigger events, it can only really be if very occasionally, very rarely, there is a chain of dominoes effect where a small one triggers a bigger one and so on – but this would require several steps up that chain, and so be low probability.?There might though, if one could look at 10000 years or even 100000 years of events, be a very tiny slightly increased likelihood of larger events at those small effect loading times due to these kinds of effects, but we just don’t have the records to know for sure – and intuition tells us that all those magnitude three and magnitude four events popping off on an almost daily basis as a result of tectonic stresses, and with offsets in the multi-cm scale, totally unrelated to weather, are much more likely to trigger larger events than any weather will.
So we have to ask, even if there is some discernment of a weather/water-table/erosion loading effect on seismicity, what use is it to us actually, in a predictive sense with respect to very large damaging earthquakes.?The answer is probably not much. ?What we are seeing is a slightly elevated frequency of very small earthquakes as a function of seasonal changes and extreme weather events.?
Even if that does translate up into larger events, which we can’t be sure of, what does it really change in our need for earthquake preparedness??Does it changing anything in our behaviour, the fact that events might be ever so slightly more likely in one month or season over another, or after an extreme weather event??Not really.?It is an interesting insight into how our atmosphere, hydrosphere, and lithosphere, are living and breathing to a certain degree in unison, but in terms of earthquake hazard planning it is relatively academic.?In seismic prone areas, large earthquakes can strike at any moment, triggered by effects far more prevalent and dominant, and to suggest anything else would be a dangerous letting down of guard.
One of the more useful ways of visualising this perhaps, is in a water balloon analogy.?
A water balloon analogy
I like to use an analogy to compere these two competing elements of weather-related loading effects and tectonic stress.?Take a look at Figure 122.?Imagine a big pile of water filled balloons.?Unlike the picture, imagine they are all different sizes.?Now imagine that each balloon in the pile has its own individual water supply that is gradually, slowly filling it up.?This represents the gradually building tectonic stress loads in any particular fault bounded part of the crust.??
Now as well as this pile of water filled balloons, imagine, as per Figure 122, that each balloon has a finger that is ever so slightly pressing and relieving by a mm or two to a steady rhythm and occasionally a bit deeper at random.??These represent seasonal and extreme weather loading and unloading events.?Imagine that all these processes are happening constantly, but that the rate of filling is happening at a greater rate than the finger pressing protagonist.?
What we can imagine, for a start, is that the bigger balloons will take a long, long time to fill to a point before rupture becomes possible.?Meanwhile, smaller balloons will be nearing popping capacity all the time.?As they near that point where rupture is imminent, bear in mind that they are still filling all the while at a rate more important than our finger pressing weather effects – so that can go any time, but on average, because there are so many of them, that little extra tweak from the finger to help push them over the edge will mean slightly more rupture when being finger pressed than when not.?It’s not the most important effect, but we can see it.
Now remember too, that every time a balloon pops, it sends shock waves through our pile of multi size balloons, and at the same time there is a cavity left which means the other balloons slight adjust position to fill it.?Both effects are much greater in magnitude than any finger pressing and might induce additional pops.?And so it goes on, until perhaps some of the very biggest balloons are nearing rupture points – but meanwhile a whole lot of other smaller and medium size balloons are popping due their water intake and sending shock waves through the pile.?So when we get to that point when the biggest ones are nearest to go, it’s not impossible to think that our finger pressing might produce a chain reaction somewhere that sets the big one off, but it is far, far, more likely that the general filling of the balloons and the popping occurring as a result, does so way more often instead.?
Understanding the process to make the correct prioritisations
What this says, to my mind, is on a higher academic level – wow – isn’t it amazing how all areas of our planet interact with each other.?And yes it seems like weather can sometimes have an influence – at least on small scale seismicity – and we can’t rule out that this might sometimes have a knock on effect on larger events.?What we can also say though, is that other normal tectonic stress build up effects are likely far more important – and so understanding the fault geometries and histories and ongoing seismicity in an areas is far, far, more important to any seismicity hazard undertakings than any weather forecast.?As we saw in Taiwan, even what effects there are can vary locally from one side of the island to the other, so there is no very strong predictive element to the observation that an influence is sometimes present.
The priority in any earthquake prone area is to ensure that all the population, all the buildings, all the infrastructure, is ready all the time.?An awareness that strong water table variations or extreme weather events might sometimes be linked, doesn’t hurt, but to start implying damaging earthquake events are significantly more or less likely during or after such times isn’t at this stage warranted.?If we live in an earthquake prone area, the best thing we can do is be as ready as we can all of the time, and not kid ourselves we know more than we do.?
Overview parts I, II, III – water, rocks, weather, and earthquakes
We have come to the end of this trilogy tour of water, rocks, weather and earthquakes.?I hope for anyone that has managed to last even a part of the distance it has been informative.?I have to say that for me, the exercise of going through it has convinced me more than ever before of the wonderful and at times controlling role water has in the dynamics of our planet – atmosphere, ocean, cryosphere, lithosphere, and atmosphere. It raises the very question as to whether we would have plate tectonics of the kind we do at all were it not for the presence of liquid water over much of our surface.??Its influence is profound, both in pure water form, and in hydrous forms of various minerals, to depths that we are only now coming to realise.?
It is no exaggeration to say, that if we want to understand rocks on our planet, and the stresses within them, we need to understand water.?I hope this leaves us all a little further down that path.?