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

Eifel Volcanic Field II

Recently Nathan took you on a comfortable journey into the Eifel volcanic field. But what is the origin of this intraplate volcanism and where will the journey go?

About 400 million years ago during the Devonian, the Age of Fish, when only plants and insects roamed the land, Laurussia and Gondwana converged into the supercontinent of Pangaea forming the European Variscan Belt. It includes vast mountain ranges stretching from Portugal to Turkey. The Rhenish Massif in central Europe is one of the outcrops of this period, others are the Massif Central in France or the Bohemian Massif in Czech Republic and Poland.

The Rhenish Massif is mainly made of highly folded sedimentary metamorphic rocks, mostly slates, hence the name “Rheinisches Schiefergebirge” or “Rhenish Slate Range”.

Geological map of the Rhenish Massif. Author Jo Weber (Wikimedia Commons)

When the Age of the Mammals dawned and Africa started to collide with Eurasia, a whole lot of volcanic activity started north of the rising Alps. This belt was termed European Cenozoic Volcanic Province by Meyer and Foulger. In the Alpine forelands extensional rift systems developed with the Rhine graben as a prominent feature. Volcanic activity of that period can be found in France (Massif Central), Germany (High Eifel, Westerwald, Vogelsberg, Rhön), The Czech Republic (Eger graben) and Poland (Lower Silesia).

Figure 1 from Meyer and Foulger http://www.mantleplumes.org/Europe.html

The ductile and tough shale and slate bedrock of the Rhenish Massif presumably was incompatible with extensional rifting. Instead the region acted as a hinge between shear rifting along the Upper Rhine Graben and extensional rifting at the Lower Rhine Basin (Illies et al. 1981).

Tectonic situation in central Europe (from this thesis, modified from Illies and Fuchs, 1983)

The Eifel volcanic field is situated west of the Rhine river near Koblenz in the center of the Rhenish Massif. Fluvial deposits prove that this area was uplifted up to 300 m since the Pliocene epoch 5 million years ago and that the uplift had accelerated during the last 800,000 years with maximal elevation around the Eifel volcanic field. Since then the Rhine river and its tributaries were forced to cut deep valleys through the Rhenish Massif, flowing past Hunsrück and Taunus, Eifel and Westerwald, Ardennes and Süder Uplands.

Uplift in the Rhenish Massif, from Meyer and Stets (2002)

The most recent volcanic activity in the West and East Eifel volcanic fields coincides with this uplift which amounts to 0.35 mm per year on average. The dome building may be a combination of widespread uplift of the so-called Rhenish Shield due to horizontal deformation from Alpine orogeny (Illies et al., 1979 and 1981; Meyer and Stets, 2002) and more locally by uplift due to the Eifel mantle plume (Schmincke, 2007).

To study the deep structures of the Eifel region the Eifel Plume project temporarily deployed a large network of seismic stations in 1997. A shear wave velocity model suggested a 100 km wide low-velocity structure extending down at least 400 km into the upper mantle which could indicate an area of increased temperature and partial melting. It remains debated whether this anomaly caused the Eifel volcanism. Other volcanic areas of the European Cenozoic Volcanic Province lack clear evidence of deep mantle plumes and the spacial distribution and timing of eruptive phases is not consistent with movement of the European plate over a fixed hot spot.

Alternative models could be a magma source derived from previous Alpine subduction or local decompression melting from passive rifting caused by tectonic deformation of the crust. Notably, the Mohorovičić discontinuity (Moho) is only 30 km deep below the Eifel while under the Alps it goes down to about 50 km which could give rise to some mantle turbulence and convection.

South-North section of the Moho beneath Europe between 6 and 9° longitude. Depth is highly exaggerated (Image by chryphia). Data from www.seismo.helsinki.fi/mohomap/

There is an overwhelming amount of literature about the recent quaternary activity of the 300+ volcanoes in the Eifel, sadly most of it paywalled or even without online access, because published in books or exotic German journals. So the following is taken from secondary literature. The eruptive history was e.g. summarized by Schmincke in Mantle Plumes (2007), Schmitt et al. (2010) (see Fig. 1 here for a map of geological map of the East Eifel volcanic field) and is nicely illustrated in this German blog post.

In summary, there seem to have been at least four main eruptive phases:

700,000 to 450,000 years before present: the main bulk of monogenetic volcanoes, small cinder cones and short lava flows erupted in the West Eifel and late some in the East Eifel. Their lava contained leucite (potassium rich) basalts, poor in SiO₂, indicating an upper mantle source.

The West Eifel then fell dormant for several hundred thousand years.

430,000 to 360,000 years before present: In the East Eifel the Rieden complex (“Riedener Kessel”) west of the Laacher See had its most productive episode sputtering out several cubic km of lava in larger cinder cones and kilometer long phonolithic lava flows out of a 4 km diameter caldera system.

215,000 to 190,000 years before present: In the East Eifel the Wehr volcano (“Wehrer Kessel”, a 2 km diameter depression) west of the Laacher See and many large scoria cones in the Neuwieder tectonic basin erupted several cubic km of dense rock equivalent. The lava was highly differentiated phonolitic and rich in SiO₂, indicating that country rock had been partially melted. During this time the first Maars were blasted out of the West Eifel volcanic field.

100,000 to 10,000 years before present: the West Eifel field was peppered with Maars still erupting the original lava, the last one to be the Ulmener Maar. Simultaneously, a new kind of lava, basanites, poor in potassium, hence leucite free, presumably from the asthenosphere, created large cinder cones and lava flows sometimes right next to the Maars (e.g. Meerfelder Maar next to the Mosenberg).

In the East Eifel only the Laacher See erupted 12,900 years ago, without doubt the most powerful eruption of all time in the Eifel probably equalling the total output of the West Eifel volcanic field. The Laacher See erupted more than 6 cubic km of magma within days, with an at least 25 km high eruptive column spreading tephra from Italy to Sweden. The magma is thought to have differentiated over several thousand, possibly tens of thousands of years, showing zonation from mafic to evolved phonolite and carbonatite. Pyroclastic flows temporarily built a dam in the Rhine river which eventually broke unleashing torrential floods, illustrated here (in German). Finally the emptied magma chamber collapsed leaving this recreational lake.

The “Loch Lochy” of Germany, the Laacher See. Image by USEBlackbird (Wikimedia Commons)

So the Eifel volcanism occurred in tens to hundred thousand years periods intermitted by hundred thousand years of dormancy. There was a general trend of eruptions starting in the NW progressing to the SE. Eruptions became increasingly voluminous and explosive with time and there was a shift of lava from an upper mantle source to partially melted crust.

Today the Eifel volcanism is dormant. As already featured in Nathan´s post abundant CO₂ emission is a sign that the Eifel volcanic field is not extinct. But also seismically the region is active. Earthquakes during the past 36 years are almost exclusively confined to the upper 15 km. There is no indication of magmatic origin so far. The highest earthquake density is east of the Laacher See and west of the Neuwieder basin along the Ochtendunger fault zone on a NW to SE axis, aligned to the general tectonic setting in the Rhenish Massif.

Recent earthquakes (Sep 2012 to Jan 2013, green, enlarged) and earthquakes dating back 36 years recorded by the seismic station Bensberg, University of Cologne. Image by chryphia.

And here a 3D plot:

Since 1975 up until January 2013 over 1180 local earthquakes were reported by the seismic station Bensberg (University of Cologne) with some increased frequency in the last years.

Earthquake data from the seismic station Bensberg from 1975 to 2013 (between 5.21 and 5.472° lat and 7.25 and 7.65° lon, as in 3D plot). Image by chryphia

Helium and other noble gases that are found in high concentrations around the Laacher See are indicators of the volcanic origin of the Mofettas. Helium isotope 4 (⁴He) is naturally formed in earth´s crust. Another rare Helium isotope, Helium 3 (³He), is produced by fission and bombardement with high-energy cosmic rays, so what we find on earth was created before our solar system formed. In the atmosphere it escapes into space. Looking at the ³He to ⁴He ratio in volcanic gases relative to the ratio in earth´s atmosphere (Ra) gives a clue about the source of the magma. If it´s of deep origin, it still should contain relatively high ³He. The ³He/⁴He ratio measured from Mofettas from the Laacher See is 5.5 Ra, indicating an upper mantle source, but it is less than measured at mid oceanic ridges (8 Ra), thus there is mixing with ⁴He from the crust.

So there we are today. Was this the end of it for the next 100,000 years? As long as the Brubbel squirts and the earth rumbles occasionally we can´t be sure of it. Maybe the ants will tell us one day.

And just in case: a list of webcams

chryphia

Many thanks to Nathan for discussion and support!

Katla and volcanic weekend Riddles!

Today commenter Islander brought to my attention some news that the Icelandic volcano Katla seems to be picking up activity. But what is actually all the hoopla about really?

Katla is as most of you know one of the larger volcanoes on Iceland, but not the largest. The largest volcano on Iceland is the far less known Bárdarbunga volcano. Katla has been dormant since 1918, with the possible exceptions of small hydro magmatic events during 1955, 1999 and 2011. The hydro magmatic events are though not entirely proven to even have happened.

Katla has a 10 by 14km caldera that seems to have been formed during a minimum of two large VEI-6 eruptions. The volcano is covered by Myrdalsjökul glacier. During eruptions the volcano emits copious Jökulhlaups (water discharged as the volcano melts the glacier) and they are the single greatest threat from this volcano. Normally Katla has eruptions that range between VEI-3 to VEI-4, with larger eruptions happening after longer times of dormancy. Katla has had two large flood basalts after the ice age ended, the 5 500BC Hólmsá Fires and the Eldgjá eruption, both of them where following NW rifts trending roughly towards the larger volcano of Grimsvötn.

Earlier today (2012-09-29) the geologist Kristin Vogfjörð appeared on Icelandic Radio reporting that during the 24^th of September a set of deep earthquakes had happened under Katla and that is considered as a sign of an increase of the risk for an eruption. She carefully avoided making any predictions.

So what happened then? Late on the 23^rd 6 earthquakes ranging between 0 and 0.5M happened at depths ranging from 30 to 20 kilometers. On the 24^th two more earthquakes occurred at 26 kilometers depth at the same location, those two where 0.4 and 0.7M respectively. The area is in the eastern part of the caldera.

Normally I have a deep respect for Icelandic geologists, but this is just so much hot air blowing. First of all the earthquakes are so minute that the amount of in-fluxed magma can comfortably be counted in wheelbarrows. It is like taking a piss in the ocean.

There was no associated tremor pointing towards magmatic movement of any significant scale. There is also absolutely no visible motion on the GPS stations surrounding Katla. Even at worst and these earthquakes where just precursors of a later influx we are still talking about years before an eruption taking place, and one should remember that Katla has had no less than two significant influxes of magma during the last 13 years without erupting. This so called “news” will only bring out the Katla fearmongerers, without anything having happened really.

http://strokkur.raunvis.hi.is/~sigrun/KATLA11.html

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Katla with trap formation in mid-ground. (behind the lake)

Image Copyright Eggert Norddahl

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The weekend volcanic Riddles

GeoLurking posted a riddle yesterday. I am reposting it as the official volcanic riddle of the week so it will not get lost.

A Bald Hiker disappeared, the other ran away.

Who or what am I talking about? The answer should be one volcano, and one volcano-related name. Two points to be had.

Return of the Evil One’s Riddle

This time Alan has outdone himself to create a brain-wrecking geological riddle for us. I must admit that it is beyond my limited capacity.

Ek jou gesonde-of-tuin se gesig-gesentreerde kubieke “, maar my hoppers sou wees van geen nut in ‘n stope

What am I? What do I look like? What is the mining jargon? 3 points to be had.

Have a nice weekend everybody!

CARL

How a seismometer works

Image from Wikimedia Commons. An old seismometer from Lick Observatory.

Earthquakes are a release of energy due to rock failing from the amount of stress and strain that are placed on it. This energy is released as a series of waves, or vibratory motion as the energy dissipates away from the source of the quake. Simplifying it a bit, there are S-waves, P-waves, Love Waves, and Rayleigh waves. Together, along with all the reflections and refractions that these waves can experience (due to what sort of material the wave encounters on its path) they generate a signal that can be measured. By measuring the signal and performing in depth analysis on it, geophysicists come up with ideas and theories about the specifics of what caused the earthquake.

One of the earliest seismic detection devices consisted of an ornate urn with copper balls that would fall off of the urn into the mouth of an awaiting toad (part of the ornate structure). Crude by today’s standard, but pretty brilliant considering that it was 132 AD. It provided a rough direction in which the quake originated.
Later designs incorporated a suspended mass that etched out a path on a lamp black coated drum with a stylus.(1) (lamp black – the soot from an oil lamp, almost pure carbon) It didn’t take long for paper and ink to take over the recording task.
With the advent of electronics, eventually seismographs consisted of a mass suspended on a pendulum structure, and a coil of wire moving in a magnetic field. The early ones used mechanical dampers in order to slow the movement if the mass. Failing to do so would result in the seismograph oscillating far beyond the time period of the quake and yield faulty data. (Remember, the mass is on a pendulum, and pendulums by their nature oscillate or swing back and forth.)

Enter the the torsion seismometer.

In 1925, Harry O. Wood and J.A. Anderson published a seismograph design that eliminated many of the problems with the pendulum siesmos.(2) It consisted of a wire under tension and a mass attached to one side of it. As the wave passed, the mass would swing back and forth along an arc beside the wire. In order to provide damping, magnets would counteract the movement and bleed off momentum from the moving mass. This design became the predominant tool for seismographs world wide. Highly sensitive, it had minimal resistance due to mechanical damping.

Years later, other designs came along, but in order to keep the seismological record consistent, the output of the newer gear had to be adjusted to match the sort of response that a Wood-Anderson seismograph would provide.

But that is only part of the problem.

How a quake is measured really depends on the structure of the crust, the type of gear measuring it, what size it was, and even what agency is doing the measuring. Here are a few of the scales listed in “Magnitude Scale and Quantification of Earthquakes” Hiroo Kanamori, 1983, Tectonophysics, 93.

ML Local magnitude, Richter (1935)
Ms Surface-wave magnitude, Gutenberg (1945a)
mB Body-wave magnitude, Gutenberg (1945b), Gutenberg and Richter (1956)
mb Short-period body-wave magnitude reported in “Earthquake Data Reports” and “Bulletin of International Seismological Center”
mbLg Lg-wave magnitude, e.g., Nuttli (1973)
MGR Magnitude used in Gutenberg and Richter (1954)
MR Magnitude used in Richter (1958)
MD Magnitude used in Duda (1965)
Mz Surface-wave magnitude determined from the vertical-component seismograms (e.g., Earthquake Data Reports)
Mv Surface-wave magnitude defined by Vanek et a!. (1962)
MJMA Magnitude scale used by the Japan Meteorological Agency
MM Moment magnitude by Brune and Engen (1969)
Mw Kanamori (1977)
ME Purcaru and Berckhemer (1978)
Mi Tsunami magnitude regressed against Mw ‘ Abe (1979)
Mc Coda (or duration magnitude), e.g., Bisztricsany (1959), Tsumura (1967), Real and Teng (1973)
MI Magnitude determined from intensity data and macro-seismic data, e.g., Nuttli and Zollweg (1974),
Nuttli et a!., (1979), Utsu (1979).
MK Kawasumi (1951).

A simple drawing for “Do It Yourself” building of a seismometer.

As you can see, there a lot of different scales. The ones that we see most often are Mw, M, and ML. mblg is used by IGN quite a bit. For the most part, they track along with each other in the mid scale, but at the low or high end they tend to diverge. Me was not mentioned in the list, but it figures prominently in trying to convert from one scale to the other… it’s the energy magnitude and is directly related to “A” which is the energy release in several of the formulas.
I’ll not bore you with the nitty gritty details about getting from one scale into another. I have been fighting this task for quite some time and have yet to find a reliable, reproducible method listed anywhere.

How I deal with energy release is to find a plot or published data by the agency that has the energy release and the magnitude listed, and then calibrating a conversion curve that matches what that agency uses. It’s a kludge, but it works and is stable across many quake reports within that agency. That’s the method that I used to come up with my cumulative released energy plots, which is different than using a canned formula published on a website. At least what I produce matches whatever methods the agency has adopted and has incorporated their adjustments.
Remember that “A” value that I mentioned? That’s the total energy of the quake. When spread across the fault face (where the fracture actually occurred) that will determine what the quake magnitude value is. (however it gets measured).
Some scales use the total amplitude of the trace movement; some measure the coda (how long it lasts). It’s part art, part science, and part nuanced thinking by some brilliant researchers.

GEOLURKING

1) Popular Mechanics Aug 1946
2) http://www.data.scec.org/Module/s3inset3.html
3) http://www.gps.caltech.edu/uploads/File/People/kanamori/HKtect83b.pdf

Lusi – The “man made” volcano?

Not all natural phenomena that carry the name “volcano” are caused by the eruption of some kind of magma. A mudvolcano is one of them. Since the 29th of may, 2006, mud and steam are spewing from the earth on the island of Java, Indonesia, leaving  thousands of people homeless. Today, it is still “erupting”. At that time, a drilling rig was in the direct vicinity, and the drilling rig had experienced some problems with the well the days before. On the 26th of may, a M6.3 earthquake occured 250 km to the southwest of the location, with several aftershocks, leading to those problems with the well. So….. Who did it? What caused the Lusi Mud Volcano to form? Was it the oil company, was it nature, are they both to blame?

Let’s start off by mentioning that despite the general idea you might get from watching the Discovery Channel oil and rig shows, drilling a well for oil and gas nowadays is in general a very delicate, well planned and carefully managed operation. Countless rules, regulations, industry standards, best practices and company regulations have to be followed in order to begin or continue drilling operations.

Two of the key things to understand what could have caused this disaster are the management of formation pressure and the design of a casing scheme.

When we are thinking about volcanism, we are in general talking about very high pressured ‘fluids’ that are stored in a magma reservoir, that crack, break, melt and blow their way through overlying formations from their source, often tens of kilometers down. Those fluids can be almost continuous (undeveloped hot fresh stuff, like basalt) or fluids in a sort of matrix of crystals (viscous mush, like rhyolite). When we talk about oil, gas or water reservoirs, we are talking about fluids and gasses that are stored inside the pore spaces of existing rock at some 2 to 3 kilometers down (in this case). Those pore spaces can be very small, like in shale/claystone, or even nonexistent, as in a homogeneous rock salt. Formations that have a large percentage of pore space are generally called reservoirs, because they can contain gas, oil or water that can flow through the rock. Sandstone and carbonate-rocks are the main types of reservoirs that are of interest for oil&gas, geothermal or drinkwater prospectors, despite the recent trends in shalegas.

When we drill down, all pore spaces we encounter have something in them, usually fresh or salt water, sometimes oil or gas, with a certain pressure. This pressure is caused by the weight of the overlying rocks pushing down on it, and sometimes by local geological stress. The deeper you go, the higher the pressure. This pressure can be expressed as an absolute measure, for example 400 bar at a depth of 3000 meters (let’s stick with metric for ease of understanding and calculation). We can also express this as a pressure gradient, a number that stays somewhat constant no matter how deep you go. We know that just fresh water (which has a specific density/gravity or SG of 1.00) will give you a hydrostatic pressure of 9.81 bars at 100 meters depth, so we can calculate that the 400 bar at 3000 meters is roughly equivalent to a hypothetical fluid which is 1.31 times heavier than water. If we would fill our hole from top to bottom with a fluid of 1.31 SG, we would nicely balance the pore pressure at 3000 meters depth to 400 bar, so that no fluids enter or exit the wellbore into the formation. This is the stable situation that is needed for drilling the formation. If the hydrostatic pressure caused by the column of fluid is increased too far above the pore pressure of the rock, the drilling fluid will leak into the pores of the formation, which is called “losses”. If the hydrostatic pressure is lowered too far, formation fluids and/or gas can start rushing into the wellbore, which is called a kick or influx. If a kick goes on without controlling the pressure down in the hole in some way, it can evolve into a uncontrolled flow or even a blowout at some point.

Pressure gradients in formations lie almost always between a hydrostatic gradient (1.00 SG) and a lithostatic gradient (+/- 2.6 SG). In rare cases, the pressure can exceed those gradients. A gas or oil reservoir that has been depleted will generally have a gradient lower than 1.00 SG, down to practically 0.00 SG in some places. On the other hand, some geological processes can cause a gradient greater than 2.6 SG in some extreme cases.

The area seen from the sky. This used to be a place where you could drive through villages and meet people.

It often happens that drilling proceeds through various formations with quite different formation pressure gradients from formation to formation. This poses a problem, because drilling cannot be resumed. If two different formations need fluids with very different specific gravities, there is no right number to choose. The stable condition that is required for continuing the drilling is not possible anymore.  If a formation is expected where the pressure gradient will be a lot higher or lower than the previously penetrated formations, preventative measures have to be taken. A steel pipe, called casing, with a diameter slightly smaller than the wellbore, is lowered into the well all the way to bottom. Cement is then pumped into the void between the casings outer wall and the wellbore to isolate and seal off all the formations that have been penetrated up (down?) to that point. If everything went according to plan, the casing and the cement will serve as a barrier to allow drilling to continue with a smaller drillbit and with a fluid that has the required density for the upcoming formation.

Before a well is drilled, there has to be a complete plan in place regarding the drilling fluids to be used and the casings that are planned to be run. Some wells need only 1 or 2 casing intervals, others need up to 10 different casing intervals because of the changing conditions down in the hole. If several wells have been drilled in the area already, things get a bit simpler, because you have a very good idea which formations you will encounter and you can look at the data from previous wells to see if they had problems with specific formations.

Now, with all of this information in mind, we can start looking at what actually happened in Indonesia.

A drilling spot was picked by the oil company to target a reservoir. The area is seismically active (as everything in Indonesia) and there is an active conventional volcano at 15 km from the drilling site. The area is particularly known for the existence of mud volcano’s, several can be found in the area, all getting their feed of water/mud from the same overpressured carbonate formation that stretches the area. Drilling started and a casing was cemented in place. Drilling commenced with a smaller drillbit and just below the casing, the strength of the formation and quality of the casing and cement was tested by performing a so-called leak-off test. The cement and the formation were confirmed to be strong enough to continue safe drilling.

Construction workers building a dam around the area to direct the flow of the mud.

On the 26th of may, a M6.3 earthquake shook Yogyjakarta, some 250 km away. Right after the quake, 7 minutes later, it was noted that some drilling fluid was lost to the formation, most likely into a fracture or fault. Two aftershocks followed, leading to the immediate total loss of all drilling fluid in the hole. The losses were cured using a special blend of chemicals to seal off the fracture or loss zone, which is a very common occurence in the industry. After this, drilling resumed. A day later, overpressured limestone was encountered, leading to the influx of formation fluid into the well. The influx was “killed” by increasing the density of the drilling fluid until the flow stopped. Barely a day later, mud, steam and gasses started spewing from the earth less than 200 meters away from the drilling site.

So, who or what is to blame? A recent statement of geologists and drilling engineers in the court case points to a human cause. In the statement it is mentioned that the drilling program for the well had not been followed. A casing was planned to be set some 250 meters above the formation that later on provided the catastrophic influx that cracked and broke through the overlying rocks that had limited strength. After breaking through the final sealing formation just above the high-pressured zone, the cap was off the shaken bottle.

Many of the other arguments can be found on the wikipedia page of this event: http://en.wikipedia.org/wiki/Sidoarjo_mud_flow but it goes beyond this post to go into them in detail. Have a read through if you are curious.

The main thing is, that it is very well possible for human beings to have a devastating impact on the surroundings if great care is not taken. This example is probably the best candidate for being a truly man made volcano, although it doesn’t erupt any real lava. As this shows, it doesn’t take magma at 1000 degrees Celsius oozing or blowing out of the earth to do great damage to a village, just a bit of water and mud will have a devastating impact as well when driven by anomalously high pressure.

The main “vent”. It certainly does remind of a shield volcano.

The eruption started off with flowrates in excess of 100.000 m3/day, or more than 0.1 m3 km in 3 years. That flowrate has gone down a bit now, but geologists expect that it can take up to 35 years longer before the flow ceases. Meanwhile, all this material is being extracted from just below the eruption site. Subsidence is already measurable and even the formation of a small caldera is expected the next couple of years. Some 40.000 people have already had to flee their homes, a staggering number that even most large conventional eruptions cannot match.

El Nathan