Friday, May 5, 2017

Well, how big WAS it?





It is human nature to want to measure things, or at least calibrate big things against other big things. The big and destructive fairly beg quantifying, in fact, so we have for instance the Saffir-Simpson hurricane wind scale (with a top level of 5 for winds above 156 mph/250 kph). This depends only on wind velocities, and doesn’t take into account rain or storm surges (Allaby, 2008). We also have the Fujita tornado intensity scale (Fujita, 1971), which for winds above 261 mph/420 kph can reach a level of F5. The following question asks about measuring earthquakes and volcanoes, which are much harder to quantify than wind-speed velocities.


Q: Hi I am an 8th grade student and I was wondering what determines the magnitude of an earthquake or what determines the power of a volcano...
- Caleb Le M.


A: Your question has two parts, which I will answer in order:

1. Earthquake magnitudes are calculated many different ways, but ultimately it comes down to measuring the amplitude of the actual ground motion (up-down, side-to-side, front-back) on multiple seismometers, and correcting for the varying seismic velocities and the distance separating these seismometers from the earthquake epicenter. Of course you have to calculate the distance to the epicenter first by triangulation from three or more seismometers (and also correct THOSE results by different velocities of sound in the different rocks between the hypocenter [the actual source] and the different measuring seismometers). 

Asking a seismologist how big an earthquake was is like asking a friend to describe how big someone is? Do you mean tall? Wide? Heavy? Some combination of all of these? Does this dress make me look fat? Seismologists do NOT like being asked how they calculate a magnitude, because it will generally require a 30-minute explanation. Therefore, their first reply is often which magnitude are we talking about here?

The original earthquake magnitude scale (Richter, 1935) was the first coherent attempt to define something that is ultimately very three-dimensional and complex. The original Richter scale  measured only the energy in the low frequency end of the seismic energy spectrum, standardized to the particular type of Wood-Anderson seismometer available at the time. Today a modified Richter magnitude is called the “local magnitude” or ML, and is tuned for the rocks and sediments of a local region. For southern California, the equation to calculate this magnitude (Spence et al., 1989; Bormann and Dewey, 2014) is:
ML = Log (A) + 0.00189*r - 2.09,
…where A = amplitude of maximum ground movement in nanometers measured at the seismometer, r = distance from the seismometer to the epicenter in kilometers, and – 2.09 is a correction factor. This equation works only for southern California, and doesn’t work for Cascadia, Japan, the Mediterranean, or Indonesia, which are each served better by different numerical factors.

Another way to calculate an earthquake local magnitude is to work off of an analog log-scale diagram such as in this link:

Though relatively easy to understand and use, the Richter Scale is no longer commonly used.

There are also Mb (the body-wave magnitude), MS (the surface-wave magnitude), and Mw (the moment magnitude). Most of these track closely together for magnitudes of M = 2 to M = 5, but diverge for larger and smaller earthquakes. In part this is because some wave-types strongly influence a short-period or broadband seismometer (which are sensitive to higher frequencies) while other wave-types (for example, surface waves) more strongly affect a seismometer designed to optimally measure low-frequency energy in the 1 – 2 Hz range.

For large earthquakes, MW (Moment Magnitude) is the preferred magnitude, because it more fully represents everything emanating from the earthquake hypocenter. The “moment” MO is calculated as a product of ยต (the shear strength of the rocks) times S (the surface area of the fault tear), and d (the displacement – how far did one side of the fault move with respect to the other side). The largest ever recorded earthquake was the Great Chilean event of May 1960, which had a moment magnitude Mw = 9.5

Confused yet? There is also Me (the energy magnitude – a measure of the potential damage to man-made structures), and Intensity (the measure of surface-shaking damage observed). They are related. Energy release is generally proportional to the shaking amplitude raised to the 3/2 power, so an increase of 1 magnitude corresponds to a release of energy 31.6 times greater than that released by the next lower earthquake magnitude. In other words,
Magnitude 3 = 2 gigajoules
Magnitude 4 = 63 gigajoules
Magnitude 5 = 2,000 gigajoules
Magnitude 6 = 63,000 gigajoules
Magnitude 7 = 2,000,000 gigajoules

These numbers dwarf the puny power of hydrogen bombs, by the way,  

Both Intensity and Magnitude depend on many local variables, including surface geometry and velocities of various underlying rock and sediment units. For example, the 1985 Mexico City earthquake had a surface-wave magnitude MS of 8.1 However, because of resonant focusing of seismic waves as the partially-dried-up Lake Texcoco basin lapped onto bedrock, some buildings on one side of a city boulevard had ground motions 75 times greater than the other side (Moreno-Murillo, 1985; see also http://earthquake.usgs.gov/learn/topics/measure.php ). A friend (Mauricio de la Fuente, a Mexican geophysicist) who lived through this event told me that it was amazing to stand in that street and see everything on one side standing, and everything on the other side flattened. Over 8,000 people died, many in buildings on that (Texcoco ancient lake) side.

Intensity is based on the Mercalli scale (https://en.wikipedia.org/wiki/Mercalli_intensity_scale). It is a twelve-level scale designed to fit to differences in observed damage. The name Mercalli is attached to a scale that Giuseppe Mercalli revised from an earlier Rossi-Forel scale, and which has been further modified multiple times since then (http://pubs.usgs.gov/gip/earthq4/severitygip.html ). On the Modified Mercalli scale, the 1985 Mexico City event scored an intensity level of IX (“Violent”). There are higher levels (and scarier words) than that, by the way.

One more thing to think about: seismologists estimate that only 1% to 10% of the energy of any given earthquake is released as seismic waves. Almost all the rest of the energy is released as heat (http://earthquake.usgs.gov/learn/topics/measure.php ). This figures indirectly into models designed to emulate the complex breaking process of a fault tear, because at some points, wall-rocks are literally welded together by the intense heat, forcing complex movements around these focal points (Dieterich, 1978; James Dieterich, personal communication 2016).

Moment magnitudes are calculated by complex equations that take into account a number of factors including different velocities and different attenuation of seismic energy in different rocks.

An earthquake on the San Andreas fault system will almost certainly be smaller than an earthquake where I live in the Pacific Northwest. This is because the San Andreas fault plane (at least the earthquake shears visible from the surface) can only go down vertically 10 to 15 kilometers before the crust turns plastic. A subduction earthquake, however (think of the Great Tohoku Earthquake of Japan in 2011) occurs on a SHALLOWLY DIPPING fault plane. The depth-direction part (dipping in the direction of the Japanese Archipelago) of the fault-tear actually extended over 200 kilometers! It has been estimated that the surface rip was at least 200 km x 300 km!  By comparison, a major earthquake on a part of the San Andreas fault system might be "just" 100 km x 15 km. 


2. The "power of a volcano" is generally characterized by scientists as Volcano Explosivity Index or VEI. This is a relative measure of explosiveness of volcanic eruptions, and is open-ended with the largest supervolcano eruptions in pre-history (Yellowstone, Toba, Taupo) given a magnitude of 8 in this classification system. The 79 AD eruption of Vesuvius and the 1980 eruption of Mount St Helens in Washington State are both rated a VEI 5 on this scale. The VEI number attached to a volcanic eruption depends on (a) how much volcanic material (dense rock equivalent) is thrown out, (b) to what height is it thrown, and (c) how long the eruption lasts. There is no equation to calculate this scale (it is like the Mercalli scale based on visual observations), but it is considered logarithmic from VEI 2 upwards. In other words a VEI = 5 event represents approximately 10 times more energy than a VEI = 4 event. Follow this link for more information on how to assess the VEI magnitude (from Newhall and Self, 1982):


References:

Allaby, Michael, 2008, Saffir-Simpson scale, in: A dictionary of earth sciences (3rd ed.): Oxford University Press, 1672 pp. ISBN 978-0-1992-11944

Bormann, Peter; and James W. Dewey, 2014, The new IASPEI standards for determining magnitudes from digital data and their relation to classical magnitudes:
doi: 10.2312/GFZ.NMSOP-2_IS_3.3

Dieterich, James H., 1978, Time-dependent friction and the mechanics of stick-slip: Pure and Applied Geophysics 116, issue 4, p. 790–806. doi: 10.1007/BF00876539

Fujita, Tetsuya Theodore, 1971, Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity: Satellite and Mesometeorology Research Paper 91. Chicago, IL: Department of Geophysical Sciences, University of Chicago.

Moreno-Murillo, Juan Manuel, 1995, The 1985 Mexico Earthquake: Geofisica Colombiana. Universidad Nacional de Colombia 3, p. 5–19. ISSN 0121-2974.

Newhall, Christopher G.; and Self, Stephen, 1982, The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism (PDF): Journal of Geophysical Research 87 (C2), p. 1231–1238. doi: 10.1029/JC087iC02p01231.

Richter, C.F., 1935, An instrumental earthquake magnitude scale (PDF): Bulletin of the Seismological Society of America. Seismological Society of America 25 (1-2), p. 1–32.

Spence, William; Stuart A. Sipkin; and George L. Choy, 1989, Measuring the size of an earthquake, in: Earthquakes and Volcanoes 21, Number 1, 1989.
http://earthquake.usgs.gov/learn/topics/measure.php

Friday, April 7, 2017

What happens to oil?



Oil is pretty ubiquitous in our lives, right? All the kayaks in the Willamette River protesting the movement of a Shell drilling platform in 2015... were almost all derived from hydrocarbons. It's literally everywhere around us... and beneath us.

Q: What does unused oil become (it can't stay a liquid forever right?) when not drilled from the Earth and does it play some important function in the Earth's geological process?

- A.W

A: There are actually three possible answers to this question, and I'll attempt to address each one:

1. Oil still sequestered in the ground: 
     Hydrocarbons still in the ground are likely at some sort of equilibrium. After burial of the carbon-rich components (mostly ancient forests and swamps but yes, some dinosaurs also), the carbonaceous material will "mature" with heat and pressure into several final forms: coal, oil, gas. If these cannot escape to the atmosphere (there is some sort of seal, like a salt dome or impermeable sedimentary layer) they tend to stay where they are. If oxygen can get into the reservoirs where the hydrocarbons are lurking, it could lead to further evolution or change of those hydrocarbons, generally an increase in viscosity. Likewise, if the volatile components of crude oil can somehow escape their entombment, what remains becomes heavy crude, tar sands, or coal.

2. Oil that has been taken out of the ground: 
    Fresh crude, exposed to water and atmosphere, tends to oxidize and self-convert (bio-degrade) to a more sludge-like material. In other words, liquid oil tends to turn thicker or even solid with time and exposure to oxygen and bacteria. When I was a child, my working single mom was so poor that she couldn't afford to change the oil in her car for six years - until the engine seized. The oil pan and engine were full of solid and tar-like hydrocarbons that had to be scraped out mechanically. 

    There are natural seeps of hydrocarbons in the Gulf of Mexico (that's what clued geologists to start drilling there in the first place). These seeps tend to have evolved benthic communities form around them. This begins with bio-degradation via bacteria. In other words, the sea-life close to a natural seep is different from what you might encounter some distance away. 



    Keep in mind that there are MANY different kinds of crude oil (API Gravity >10 will float, and API gravity <10 will sink in water, for instance), and they all have different high-viscosity (long-carbon-chain) and volatile (low-carbon-chain) contents, plus assorted poly-aromatic hydrocarbons (PAH's). That API gravity differential leads to an initial separation of the crude oil, if it somehow gets away and flows into water: some of it sinks, some floats, some drifts along in the current. The multi-vis you put in your car has a limited range of carbon-chain molecules compared to the stuff that comes out of the well-head. There are many different exposure environments also, so the speed and degree of change can vary wildly. API > 10 oil from the 2010 Deepwater Horizon well blow-out accumulating at the Louisiana coastline evolves differently than denser oil accumulating at 6,000-meter, near-freezing depths in the deep ocean. Temperature also has a lot to do with how the oil changes with time: higher temperature encourages faster bacterial activity (bio-degradation). There is some evidence that natural seeps on the floor of the Gulf of Mexico have led to different benthic communities based upon the oil and bacterial by-products.



3. Oils that are already used and need to be disposed of:
    There are different ways to recycle oil products, but these are as varied as the people doing it. The clean-up of an oil-spill in the Kalamazoo River in 2010 is now estimated to be in the $1.2 billion range. Recycling and clean-up in rivers, sounds, and estuaries may include dredge-and-cap efforts, and may involve storage-in-place, off-site storage, and possibly re-refining or even combustion. A friend collects used cooking oil from restaurants and recycles it; his old Volvo has a sticker on the back that reads "Bio Fuel". The reserves of heavy crude and tar sands in the western hemisphere (mainly Venezuela and Canada) were once estimated to be sufficient to power the industrial world for centuries - if they could be extracted economically. They must to be converted from the solid (or high-viscosity) form first, of course, and this involves vast amounts of heat and water that cannot be used for much of anything else subsequently. 

Friday, March 10, 2017

Will a M = 4.8 Earthquake Wake Me Up?



Is geology useful? Well, yes - if you are reading this message or drive a car or have a smartphone. Without geology, we would be squatting around campfires beating rocks into chips and arguing philosophy until the Sun goes nova.  The following question may fall in the "actually useful to me" realm.


Q: My question is: So if an earthquake hits during nightime, since people are sleeping, and lets say the earthquake is 4.8, then would people feel it sleeping or feel a shake and wake up immediately?๐Ÿ˜Š
- Melanie G

A: From sleepdex.org: "Usually sleepers pass through five [sleep] stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes."

As a six-year-old, I was awakened by a 7.3 magnitude earthquake - because it physically threw me out of my bed at 3am and onto the floor. My Mom told me that she called for me to come to her bedroom (she was trying hard to stay in her own bed at the time) and that I replied "I can't. The walls keep hitting me."

Depending on:

  •  where you are in the sleep cycle, 
  •  how deep the hypocenter of the earthquake is (deeper = more attenuation = less sensation), and
  •  how far away the epicenter is (more distant = more attenuation = less sensation)...                ...you may sleep right through an event of that magnitude. I have two sons living in the LA area, and sometimes they are not aware of an earthquake during the night, and at other times they are hyper-aware and send me text messages to find out what it was that they felt. If you are driving you may not be aware of an earthquake of that magnitude (again, depending on rupture depth and distance), taking the sensations you feel as just a few more bumps in the road. If you happen to notice trees waving around, you may not easily realize that the movement is not being caused by wind (are they randomly waving around, or do they all wave back and forth at the same time and rhythm?). I personally know one person who was driving and did not realize that an earthquake had happened until he got home and his family asked if he had felt it?


I hope this give you a few more parameters to think about (and answers your question).

Friday, February 3, 2017

Simple Answers to Complex Problems Are a "Misteak."



One more question: How will the world end? Probably NOT with a whimper, but instead with a bang.

Q: Wow, a lot to un-pack there (by no means a criticism.) In fact, thanks very much for the extensive informative responses! My next question (maybe last, if I'm not pushing your patience too much here,) regards the truly catastrophic's.  I'm 29 years old, which can't even be considered a mote in time when considering numbers like 13.7B or 4.5B, but nevertheless, here we are. Growing up it was accepted via the direction of our science teachers that dinosaurs were wiped out by an asteroid, and it seems to make sense. But alternative views like supervalcano's have been touted on science sounding TV channels  as an alternative and I wonder about your thoughts on that. And minus dinosaurs, would we more statistically face worldwide threat from geology, a comet/asteroid, or a biological problem? We can leave out human stupidity towards ourselves for the sake of the argument. I guess cosmic factors too.
--Joe A

A: Most of humanity seems to gravitate towards a simple solution or answer to a complex problem. It's mentally easier. This is really obvious in the current political "debates" going on (Build a wall! Cut taxes! Increase spending on X!). The most difficult scientific problems to solve are the ones with more than one poorly-understood variable. Most science consists of trying to constrain down those variables to just one for your experiment. MOST problems, however, have complex causes. If my microwave stops working, I think oh: it must be the power is out. Closer inspection shows that the power is there. Darn, no simple answer. OK, what's next to check then? Heck, I’m gonna have to disassemble it…

The Chicxulub event certainly had a big impact (pun intended) on saurian life when it hit 66M years ago, but there IS evidence that Life for Large Saurians was getting more and more difficult before the major extinction event, with environmental degradation due to several things already underway, including volcanism (the extinction may have been accelerated by the formation of the Deccan Traps in India). However, make no mistake: a 2-cm layer of ash full of 1000-times-normal iridium in Gubbio, Italy, came from the Gulf of Mexico. A 10-km-diameter asteroid carries a ginormous amount of kinetic energy with it. The Earth's gravity well is pretty strong (a rock dropped from the Lagrangian point between Earth and Moon reaches about 8 km/second before it hits atmosphere). This is also the speed of a minimum Low Earth Orbit. The Chicxulub object certainly had a much higher velocity than that, because it came from the Asteroid Belt or the Oort Cloud, and energy is mass times velocity squared. Thus, double the velocity and you quadruple the energy delivered (it's a principle I teach to my Jujitsu students: the speed of a palm-heel strike is more important than putting your entire body-weight into it). From several studies there is a consensus that the Chicxulub object’s kinetic energy before atmospheric entry was about 5.4 x 10^23 Joules, or 130,000,000 Megatons of TNT equivalent. By comparison, the Tsar Bomba, the largest hydrogen bomb ever detonated (by the Soviets, at Novaya Zemlya in 1961), had a yield of "just" 55-60 Megatons.The bomb itself weighed 27 metric tons!

As far as future devastation goes, biologic threats tend to be self-limiting. Volcano and earthquake threats are pretty much steady state (with minor fluctuations) over time. The two unlimited threats are human interference (climate change is just one consequence) and asteroid/comet impacts. If there is one certainty, it is that there WILL be change.