Tag Archives: physics

It's All About the Waves - 2014's First Magnitude 7+ Event in Chile

There's been a decent amount of chatter amongst Earth scientists that it has been a long time since the last magnitude 7 or greater earthquake.  In fact, there hadn't even been one in 2014 until last night.  The earthquake is currently rated an 8.2 (mww) and occurred in a well known seismic gap that has been published on a decent amount in recent years.  The last major earthquake in North Chile was an 8.6 in 1877! Many smaller earthquakes in the area over the last weeks have kept everyone on their toes.

This location in Chile marks a major plate boundary where the Nazca plate is subducting, or being pushed under the South American plate.  The idea of subduction is that the two plates are being forced together and one ends up getting pushed underneath the other.  In this case, the cold and dense oceanic crust gets pushed underneath the less dense continental crust.  As we would expect, this means that the earthquakes occur on a very shallow angle thrust.  Moment tensor solutions can tell us about the fault by analyzing many seismograms.  Turns out that the moment tensor solution looks like about a 12-18 degree dip on the fault, not out of line with our prediction.  There are a lot more of the advanced scientific products such as the moment rate function here.  It looks like the rupture lasted for around 100 seconds and slipped a maximum of 6.5 meters (21 ft.) at a depth of near 30 km (18.6 miles).  The earthquake started a little more shallow though, about 20 km (12 miles) down.

There have been many aftershocks with the event, some sizable.  At the bottom of the post I've provided a channel list that I'm using to watch the aftershock sequence on the EpiCentral app (for iPad).  What I want to show are the buoy data though!  When a large earthquake of this type occurs, waves are generated in the ocean and the folks at the Pacific Tsunami Warning Center go into action.  There were some significant waves near Chile (about 2m/6.5 ft.).  It looks like, for the time being, most other locations such as Hawaii may be in the clear.  As I'm writing this the remnants of the waves should reach Hawaii in the next few hours.  Below is a rough travel time map from NOAA.


A list of the observations from the  warning center can be found in their most recent statement.  We can actually access the buoy data and look at the wave propagating across the ocean though!

When waves propagate across the water (or many other media) they often experience a phenomena called dispersion.  The idea is that waves are actually made of many frequency components, or notes if you will.  Because of some physics funny business, the longer period (lower frequency) waves will actually travel faster than the short period (high frequency) waves.  We can see this in the data below.  I'm showing two stations for sealevel.  They have different types of sensors, but that's not too important.  Be sure to click on the plot to see it full size (the link will open in a new window/tab)!


We see exactly what theory predicts, long period waves coming in first, followed by progressively shorter period waves.  We also see that stations further out don't see the high frequency waves.  This is another phenomena in which the medium filters out high frequency waves over the travel.  We would say that the high frequency waves have been strongly attenuated.

That's all for now! Thank you for sticking with me through some interesting observations of predictions from math and physics!

Channel List:

Physics of a Beer Head- It's All About Tension Gradients

Who doesn't like a good beer? Geologists/geophysicists are always appreciate a nice cold glass of their favorite beverage poured by a skilled bartender that produces a nice frothy head that persists for the entire experience.  The question is why does the head not disappear quickly like soap bubbles?  Douglas Durian and Srinivasa Raghavan wrote up a 'quick study' in the May 2010 issue of Physics Today.  This article is based on that article with some additional information.  Durian and Raghavan also discuss soap bubbles and present high magnification photos of foam structures, but those will be ignored here as their method of persistence is quite different than that of beer foam.

When beer is produced proteins are present in the mix.  If you enjoy the cloudy wheat beers you are seeing proteins precipitate out! Actually something called the isoelectric point determines what happens to the proteins.  If the pH of the beer is too close to the IEP the proteins precipitate out.  The further the pH gets from the IEP the more soluble the proteins are.  Why does it matter?

Remember from high school biology that proteins have hydrophilic and hydrophobic parts to them.  This means that one part of the protein 'likes' water and will immerse itself, but the other end does not like water and tries to stay away from it.  When we look at a foam magnified there are small fluid sections in-between the air bubbles.  Proteins orient themselves in the bubble walls.  Say that a bubble begins to stretch thin and is in danger of bursting.  The stretching of the wall means there are fewer proteins in the middle where the wall is thin and more on the edges.  Proteins can change the surface tension and this gradient is surface tension causes liquid to flow to the thin section, restoring the stability of the bubble.  This effect is known as the Gibbs-Marangoni effect, and is in fact a strong example of the phenomena.

This Gibbs-Marangoni effect was in fact first observed in glasses of wine (pictured) and discussed by Lord Kelvin's brother James back in the mid 1800's.  Carlo Marangoni studied the idea for this dissertation and the solutions of the problem were formalized by Williard Gibbs.  (Yep, that the same guy that independently developed vector analysis, Gibbs free energy, etc)

So is there anything other than pH that can change the persistence of head? Absolutely! Due to gravity the fluid drains to the bottom over time which destabilizes the foam.  There isn't much to do about that, but we can combat Ostwald ripening.  Basically this mean that gas diffuses from smaller bubbles to larger ones.  Laplace described this knowing that the large bubbles have a smaller curvature and therefore lower pressure than small bubbles.  According to the article brewers can add about 20ppm of nitrogen to the beer to slow this process.

The items discussed in this article apply across many scientific items.  There is an everyday example that scientists would call Benard-Marangoni convection, but you probably call it boiling water.

Once again these pictures are from the interweb and not my property.