Thursday, December 1, 2016

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, November 4, 2016

Is Our Atmosphere Dynamic?

It may surprise readers to learn that the American Geophysical Union has divisions named Atmospheric and Space Electricity, Global Environmental Change, and Atmospheric Sciences. In short, the geosciences world include a very large element of atmospheric science.  The following query has several different elements, all of which suggest an awareness of how dynamic our atmosphere really is.

Q: Hello. I wasn't sure which category to inquire within but this seemed appropriate. Since Earth is not a perfectly spherical object, nor any other planetary bodies we know of, how does that affect the gaseous layers of atmosphere surrounding us? My question stems from an uneducated assumption that our atmosphere is not a perfect bubble around us but must be dynamic given the amount of energy factors associated with it, the terrain beneath it and which locations have the greatest gravitational pull. Can the sphere of air around us ever dissipate into space? Are there higher or lower points that exist because of geography that make our categories of layers more ambiguous?
-Joe A

A: As you suggest, the atmosphere is indeed a very dynamic thing, and yes all rotating solar system bodies are oblate spheroids because of centrifugal force at the equators (and none at the poles). Jupiter rotates at a phenomenal rate (it has 9.8 hour days!) and is thus is the most oblate planet of all.

If you think about oceans, however (the ocean surface is at the same elevation above the spheroid datum over the Marianas Trench as it is in Pamlico Sound), then mountain ranges will similarly have little to do with atmospheric height over the globe (there IS a small amount of isostacy). The most common exception to this are called storm surges - the low-pressure cores of hurricanes and typhoons will literally lift up the (warmed and expanded) ocean water. With Hurricane Katrina, the storm surge reached an astonishing 8.5 meters (27.8 feet!) at Pass Christian, Mississippi. That's above the normal tides!

There is atmospheric thinning with altitude, however, and the upper reaches can still be detected at 100+ kilometers, which is why satellites must fly at 250+ kilometers. Even at those altitudes there is measurable drag that over time will bring down low-flying satellites and launch vehicle debris. Most of the upper atmospheric variation has to do with solar wind and solar heating activity, however. Because of Earth’s gravity, most of our original atmosphere remains - unlike Mars, where the original atmosphere and water were stripped over time by solar winds. When you see clouds over mountain tops (pretty common over our volcanoes in the Pacific Northwest), it is because winds trying to get around the mountain send some of their components up and OVER the mountain. This leads to a drop in temperature with increasing altitude, which contributes to dissolved moisture precipitating out into what we call orographic clouds - cloud caps. As the air moves past and back down to lower elevations the water re-dissolves back into the atmosphere and the clouds disappear... but the same AMOUNT of water remains.

Q: I had to do a little research to understand a few of the terms you used but I definitely feel like I came away with a better understanding. Thank you for your insightful response. I shouldnt be surprised I guess that avenues of inquiry like this are out there given the ubiquity of websites, but I never tried something like this before. I had a thought, did some googling and found you. It's awesome to get answers from professionals as if I was back in school and could pick the brains of my professors after hours. So thanks again, despite my questions being kind of convoluted!
- Joe

A: I'm glad I could help. I suppose I am technically a professional, in that I get paid to do research in geophysics, but I'm just a very ordinary person with the same level of curiosity that you have. I personally don't divide the world into professional vs. non-professional, but instead into interested vs. non-interested. I plumber who asked some really deep questions about the lithosphere and upper mantle told me he spends a lot of "windshield time" thinking about the physical world as he drives from job to job.

THAT meets my definition of a scientist. You and I fit in there also. That goes for anyone reading this chapter, too.

Monday, October 3, 2016

When Was North Carolina Last Under Water?

We often get queries that ask about local geology that we do not have easy access to. However, it’s fairly easy to sleuth things in the broad brush by locating state geologic maps. I can’t say much about a rock found in someone’s backyard, because glaciers and rivers could have moved that rock hundreds of kilometers from its original source. The following is a local-geology question that I CAN reasonably respond to.

Q: Can you tell me when was the last time North Carolina was under water? I'm finding fossil seashells yet I live nowhere near any ocean. I live in Jacksonville, NC (Onslow County)
- Brandon F

A: You live on the Outer Coastal Plain of North Carolina; Onslow County runs all the way to the ocean. The Outer Coastal Plain, or Tidewater is extremely flat, averaging less than 20 feet above sea level. It contains large swamps and lakes indicative of poor drainage conditions, which have hosted both freshwater and marine mollusks at different times. The coastal margin north of Cape Lookout is a “drowned coast,” in which sea level rise associated with the end of the last Ice Age, and continual melting of the ice caps, has caused the ocean to invade the lower reaches of river valleys including where you live. This drowning has produced large embayments such as Albemarle and Pamlico Sounds. New River (where you live) lies between this region and the Cape Fear uplift.

You might wish to look at the North Carolina geologic map for more detail:

(https://ncdenr.s3.amazonaws.com/s3fs-public/Energy%20Mineral%20and%20Land%20Resources/Geological%20Survey/1985_state_geologic_map_500000_scale_600dpi_rs.pdf)

To your east you have the Belgrade Formation, with oyster shells embedded in sand. To your north and west you have the River Bend Formation, also fossiliferous with limestone among other rocks. Both formations are listed as Tertiary in age (66 million to 2.6 million years ago). However, the shells you are seeing could conceivably be from the last several tens of millennia if I read your elevations and location correctly.

I hope this helps. You have some excellent geologists in your state, both at the state and university levels. It should be fairly easy to contact one - perhaps visit the closest university and ask to talk with a geologist there.
~~~~~

Monday, August 22, 2016

Hunting Asbestos

This question opens the door to several issues: indirect detection when direct detection is not possible, and unanticipated down-sides (such as death) to some mineral exploration projects.

Q: Hello Sir,
Could you please tell which is the best method for locating asbestos, pyrrhotite and manganese? Whether airborne magnetic survey or ground magnetic survey?
Regards, Ahtisham ul-H.

A: Asbestos (an aggregate of six different but related silicate minerals) and manganese are not magnetic. Pyrrhotite (FeS) is sometimes weakly ferromagnetic if there are iron deficits in the ideal FeS lattice. However, these minerals are all usually associated with serpentinization, a hydration and metamorphic transformation of ultramafic (dark, iron and magnesium-rich) rock. Serpentinization usually has significant magnetite associated with it, and THIS is strongly magnetic. I've worked in ultramafic rocks where I have personally encountered 3-cm-thick veins of pure magnetite. It doesn’t take much more than a percent or two of this to make a rock really magnetic.

The usual method for mapping these sorts of deposits is ground magnetics and geochemistry, but keep in mind that you are only indirectly imaging the minerals you are interested in. Airborne magnetics are often used for regional surveying, to outline target areas for later follow-up with ground magnetics.

As an aside, please keep in mind that mining asbestos, or fabrication of asbestos products, is dangerous. My father died of mesothelioma-related lung cancer. In his 80's he was an avid bicyclist in San Francisco. The pipes in the basement ceiling of his apartment building, where he stored his bike, were insulated with blown-in asbestos. When he developed a persistent cough, a biopsy showed his lungs to be poisoned with asbestosis.

Saturday, July 16, 2016

Immediacy or Temporal Myopia

Immediacy or Temporal Myopia
Much of how we view our world is filtered – by time, by previous experiences, by others’ reactions. This is one reason why scientists keep careful records of their observations and experiments. Otherwise, we would be subjected to the most strongly held, no-basis-in-scientific-fact opinions of those around us. Sort of like the Internet.

Q: There seems to be alot of Earthquake and Volcano activity going on lately within the pacific ring of fire. Is there more earthquakes and active volcanos this year then there has been in a while? Seems rather worrying :(     Thank you!
- Jessica M

A: Not to worry - it has always been this way - there is nothing particularly unusual going on. Please understand that earthquakes and volcanic eruptions do NOT occur in a steady rhythm*. If you average over a 10-year cycle it evens out and looks similar from cycle to cycle. From a much larger window of time - something research seismologists can see because they have decades-long databases to work with - you can see the synoptic view. The larger picture shows these apparent surges in events are just the Earth system puttering along as usual.

* There is a tendency among human beings that for lack of a better expression I call temporal myopia. We tend to remember only the most recent attention-grabbing events, and focus on them. We also tend NOT to remember as well older similar events, nor the periods of quiescence between them. Immediacy might be another word for this tendency.

Sunday, April 17, 2016

This is a question that I actually answered in some detail three months ago, so when it came in I thought I would just point the individual at my earlier response. However, I was struck by HOW I was asked this time. To give readers a sense of what we sometimes encounter in our email in-boxes, I share this, but I'm disguising the name and identity of the questioner. You can't make this stuff up, to paraphrase Dave Barry.

Q: Hello sir

In the defination of earthquake

Eathquake is the sudden terror or shaking of earths crust which lasts for the short time. But  in 2015 the earthquake in nepal not lasted for a short time . So why we use that sentance , " which last for the short time".But Generaly in most cases it not lasted for shot time. Sir please answer this question in 24 hrs please sir i sent this question you not answered please answer . Please tel me if you want to hep ,me  or not. I f you want to help, if you want to make a bright student please help me . aSK YOUR OWN SOUL AND HELP.

tHANKYOU

A:

Volunteer geoscientists in the US Geological Survey do not see questions that arrive during western hemisphere weekends. Please do not blame us for not instantly replying to your questions from <Asia>.

I have no idea what definition you are referencing, since you did not provide that information. The simple answer is the larger the moment magnitude (Mm) of an earthquake, the longer the coda. In other words, the greater the energy released, the longer the apparent shaking will last. In fact, you can get a rough idea of how big a regional earthquake is by timing the shaking.

Friday, December 11, 2015

Terraforming Mars

Here's a Q&A that has nothing to do with earthly geology, but may have some instructive content for future geologists. There is usually at least SOME science in SciFi novels!

Q: What would happen to the Martian atmosphere, over the course of the next 100 years, if we could build a machine on Mars that could output the equivalent quantity and composition of greenhouse gasses as are released on earth (approximately) every year?  Thanks for your time, hopefully this has not already been answered!
- Kyle R

A: That's a rather unique question, but it begs several critical assumptions.

According to a recent Science article, Mars lost its original atmosphere billions of years ago because the planet lost (if it ever had) its magnetic field. As a result the solar wind (high energy charged particles blasted out from the Sun) stripped most of Mars' atmosphere away. So one assumption is that the planetary magnetic field is somehow restored.

Another assumption is a bit more obvious: where would the carbon and oxygen come from? Certainly not the planet's crust, as it has been degassing for billions of years and is a depleted desert now. Hundreds of trillions of tons of material would have to be brought to Mars' surface. This is actually not as unreasonable as it may sound: comets can do (and have done) this in the past... but it would require a number of pretty large comets. A colliding planetary body from the Oort Cloud on the scale of Sedna could bring the mass as well as restart the magnetic dynamo, however. A collision like that is thought to be the reason why we still have a magnetic field here on Earth... and a Moon as big as the one we have.

A final assumption is also necessary: a weaker planetary gravity field would make it easier for gases to escape the planet. So another assumption would be that somehow the planet became much more dense. A comet impact couldn't solve this one. A collision with something like Sedna would only marginally increase the gravity field of the planet. Weak gravity -> easier for atmospheric gases to escape.

I'm not a specialist in atmospheric dynamics, so I don't want to speculate what would happen if all three of these conditions were somehow met. I suspect that Mars' currently pink sky might end up a different color, however.

Q: Thanks for the thoughtful reply Jeff, I appreciate you taking the time.  The thrust behind my question was basically to get an understanding of the scale of the terraforming humans have engineered on Earth and what the impact would be if that same process was applied to another planet of similar size.  I guess looking back I should have simply asked what the impact might be of 'magically' pumping 7,000 million metric tons of  carbon dioxide into the Martian atmosphere every year (7000 million metric tons being an approximate average volume created by human factors on Earth).  Thanks again and enjoy your weekend!
-Kyle

A: Yes, I was fascinated by the book Dune and the movie Total Recall, but the physicist in me kept slapping me on the back of the head: There's no evidence of sequestered carbon on Mars except frozen CO2 at the poles. There is only rare (indirect) evidence of water - it's a desert world. Water being low density, it would be hard to hide it on a planet like Mars or Arrakis. THAT said, I participated in several expeditions across the Empty Quarter of Saudi Arabia. Ambient humidity there is about 2% (in an Arizona summer it is around 20%). It is so dry that you have to "snuff" a handful of water every hour all night long because your mucous membranes are on fire - and cracking from the desiccation. However, I did some geo-electrical soundings along our two routes to the Wabar Impact site and found evidence of conductors - probably above-bedrock water - in several locations at about 60 - 100 meter depths.