Earthquake seismology great Inge Lehmann

The Mad Scientists of Etsy theme for the month of April is Earthquake Seismology. This was inspired to honour the victims of the Japanese earthquake and tsunami. I decided I did not want to make art about the destructive power of earthquakes. (I'm offering a copy of my moku hanga woodblock print with all proceeds to the Canadian Red Cross for relief of those effected by the disaster. I hope this is more constructive an approach.).

I had been thinking, rather, for some time, of doing a portrait of Inge Lehmann (1888-1993), a Danish (or, as she put it the Danish) seismologist who was at the forefront of the field in the early twentieth century. In 1936, she wrote a paper, entitled simply P', in which she laid out her arguments supporting her discovery of the inner core of the earth.



We now know, as she first postulated, that the earth has roughly three equal concentric sections: mantle, liquid outer core and solid inner core. The crust, on which we live is merely a thin, um, scum really, on top of this slowly boiling pot. The only way to probe deep into the earth's core is to employ massive earthquakes, the waves they generate and the paths they follow. There are two main types of seismic waves used for studies of the globe, unimaginatively named Primary (or P, or compressional) and Secondary (or S, or shear). Imagine a glass of water with a straw; the straw will appear broken at the air-water interface, because light bends as it enters the water. Just like light travelling through different media, these seismic waves can bend, reflect or be transmitted at any boundary. The difference in physical properties between the mantle and outer core causes a P-wave shadow. (For S-waves, the shadow zone is absolute because liquids, like the outer core, do not support shear - imagine trying to cut water with a pair of shears and you can see this for yourself. Thus, no shear waves can make it through the outer core, and thus we can be certain the outer core is fluid). That means, the compressional waves from an earthquake can be recorded at seismic stations out to 105^o from an epicentre and then there is a zone which is in the core's shadow. Lehmann found that there were some late-arriving P-waves are much larger angles (142^o to 180^o) which had been vaguely labelled 'diffractions'. She showed that these could be explained instead by deflections of the waves which travelled through the outer core at her postulated inner core boundary.

She later discovered a discontinuity in the mantle (confusingly also called the Lehmann discontinuity). She did important work well into her 70s and lived to be 105.

When she received the Bowie medal in 1971 (she was the first woman to receive the highest honour of the American Geophysical Union), her citation noted that the "Lehmann discontinuity was discovered through exacting scrutiny of seismic records by a master of a black art for which no amount of computerization is likely to be a complete substitute..." (1, 2).

I think her accomplishment is downright astonishing. To have the exactitude to work with the data and the daring to neglect the irrelevant and offer up a simple, elegant - correct! - explanation is a rare and marvellous thing. To be the top of her field in 1936, when she was a pioneer for women in science and had to compete in vain with incompetent men (her words,1) is heroic.

There are a grand total of two easily found photographs of Lehmann I was able to find on the internet. I based my portrait on the earlier one, to match the date of her phenomenal P' paper. I also show her model of the earth in red-orange ink, complete with mantle, inner and outer core, and travel paths for rays through the layers, including into the shadow zone.

8.9

I keep checking the news to see updates on the aftermath of the magnitude 8.9 event off Japan's Honshu Island last night (or afternoon locally, 2011 March 11 05:46:23 UTC). The initial estimates of lost of life were far too low for an event of this magnitude which triggered a tsunami. It's beyond Hawaii now, but tsunami warnings were issued throughout the Pacific, including coastal BC. The problem is the nature of the coastline, as people in Port Alberni, where a 1964 tsunami swept away homes and swamped businesses, know well.



Though I don't have any instruments in the water which could sense the wave, at the moment, people still called me at the crack of dawn. I don't envy friends who work in earthquake seismology or in disaster preparedness on days like these.* I'm glad the epicentre was not further south - closer to larger population bases and known significant gas hydrate deposits. There are fears that earthquake shaking could destabilize gas hydrates, which may both hinder slope stability (another tsunami risk) and release large amounts of methane.

*For the most seriously affected people: Médecins Sans Frontières and the Canadian Red Cross can be found at these links.

(figures from Bunting et al., The Science of Tsunamis, Oilfield Review, 2007)

Earthquake

So I'm working from home today because the University is closing at 6 pm. They are going to lock everyone out, so I figured why fight traffic only to get kicked out at 6 pm? It figures though... there's hardly ever noticeable earthquakes in Toronto and on the random Wednesday I'm not actually in the office the entire house shook for a good portion of a minute with a tangible coda. The closet door opened and a few things even fell off the shelf. I was not seeing anything on the USGS page (until just now) and the Earthquakes Canada page was down altogether (possibly because they run everything from Ottawa in typical government fashion).

It's now up:
a magnitude 5.5 event near the Ontario-Québec border, 61 km north of Ottawa
* Wednesday, June 23, 2010 at 17:41:41 UTC
* Wednesday, June 23, 2010 at 01:41:41 PM at epicenter
* Time of Earthquake in other Time Zones

Location 45.866°N, 75.457°W

That's a fairly large event and early felt reports came from Ottawa, Toronto and as far as Buffalo.

If you felt the event, you should fill in a felt report. By doing so you can help with earthquake safety and building codes. It is a straightforward thing to record ground shaking, but the specific effects in different areas can be greatly effected by local ground conditions. The Ottawa Valley, believe it or not, has some of the highest earthquake hazard in Canada.

You can fill in a felt repot with the USGS. Canadians should fill in felt reports with the GSC... I'll post a link as soon the Earthquakes Canada site is back up.

Edit: Earthquakes Canada felt report here

Interestingly, Minouette did not freak out like she did when she experienced a magnitude 3.9 event within 10 km of the epicenter.

Chilean earthquake and tsunami hazard

A friend pointed out that there was a magnitude 8.8 thrust-fault earthquake, where the Nazca plate is subducting below the South American plate, offshore Maule, Chile, and the subsequent tsunami watch. Another friend followed up with questions about tsunami hazard, so I thought I would try to shed some light on this.

This news saddened me; I spent one month there and made many friends. Shaking along the coast would have been severe and a long aftershock sequence is to be expected. We were in the twin cities Valparaíso and Viña del Mar, west of Santiago, and sailed south towards Concepcíon (115 km NNE of Maule). We were invited by the Catholic University of Valparaíso to do a controlled-source electromagnetic survey to map gas hydrate deposits. Their interest was tsunami hazard. Chile is prepared for earthquakes and tsunami. They still remember the magnitude 9.5 earthquake of May, 1960 – the largest earthquake worldwide in the last 200 years or more. (It's important to remember that the magnitude scale is logarithmic.) Gas hydrates are stable under certain pressure and temperature conditions. If there is upwards movement or shaking of the seafloor, it is possible that they might become destabilized and lead to submarine slumps and slides (imagine a landslide underwater). This in turn might increase the risk of tsunami. The good news was we found little evidence for gas hydrates in this area.

If you see stories about earthquakes or tsunami in the news and find the facts bewildering (or worse, of dubious veracity) a good place to look for information is the USGS website (or for events in Canada, the GSC website - the USGS has the best worldwide coverage combined with public outreach).

The animated image above is an Tsunami Model for Chilean earthquake produced by the Earthquake Research Institute, Tokyo (another place where I've spent one month of my life). You can see that the maximum amplitude of the the waves induced are on the order of centimeters. Tsunami (Japanese for 'habour wave') are defined by the source (a displacement of a large volume of water, due, for instance to earthquake - especially subduction earthquakes, volcano, land- or submarine slide), but they also have some consistent properties in terms of the physics of the waves produced in the oceans themselves. This includes a small amplitude and very long wavelength (like the gravity waves I use in my experiment). They are not defined by height. Heights of centimeters are most common. In fact, in the ocean basin, if you are not using very specialized, high precision instruments (pressure gauges and accelerometers, like those used in my research), you are unlikely to notice a tsunami. There are two main issues which can lead to death and destruction: harbour geometry and the seemingly relentless nature of the waves. As the seafloor shallows at a coast, the waves shoal - they are compressed, slow down and increase in amplitude. Depending on the geometry of the seafloor, the effect can be severe.

This second animation shows this effect schematically. The second problem is also evident: duration. Even a small increase in relative sealevel (run up) can cause a lot flooding of damage. Though these waves are slow moving in shallow water, they are propagating and some structures will fail with continual force of this nature.

We all remember the 2004 Indian Ocean Earthquake and tsunami, but this does not mean that is what you should imagine every time you here the word tsunami. The 2004 event was a magnitude 9.3 which is barely conceivable. During today's event 4.7±0.9 x 10¹⁶ Joules of energy was released. During the 2004 event, 20 x 10¹⁷ Joules which is forty times more (source: USGS). Wave heights were up to 250 centimeters in the ocean itself - clearly a entirely different situation. Further, there has been an active tsunami-monitoring system in the Pacific for decades - not so in the Indian ocean. The Indian ocean event was a megathrust earthquake. While we can expect megathrust earthquakes in other subduction zones around the Pacific, today's event was not one.

So while this event was large enough to warrant tsunami alerts, the real concern, as is generally the case, is the nature of the housing near the epicenter. The death toll will be considerably less (likely by a factor one thousand) than the much smaller Haitian earthquake six weeks ago, largely because of building standards.