Sunday seismometer #4

Wiechert vertical seismograph (1909)

The Wiechert vertical seismograph was built in Göttingen, Germany, in 1909, five years after the horizontal seismograph described in last week's post. The two instruments together formed a complete recording system, capable of determining the 3 components of ground motion.



The vertical seismograph has a mass of 1200 kg and a natural period of 5 seconds. Its design is more immediately recognizable by today's high-school students, as it is essentially a damped mass on a spring.



In order to reduce the temperature variations inside the instrument, which change the mechanical properties of the springs, the vertical seismometer is entirely enclosed by a metal casing, which makes it much less aesthetically pleasing than its horizontal companion. The small spring at the center of the instrument helps to correct for residual thermal variations.

The air-piston damping and the stylus and smoked-paper recording system are identical to those on the Wiechert horizontal seismograph.

The two instruments ran side by side in the Strasbourg Seismic Observatory until 1968. You can see working examples of both at the Wiechert Earthquake Station in Göttingen

Sunday seismometer #3

Wiechert horizontal seismograph (1904)

The Wiechert horizontal seismograph (built in Göttingen, Germany, in 1904) has an unusual and striking design: it is essentially an inverse pendulum weighing 1 ton, in unstable equilibrium about a universal pivot at its base. Its natural period is 8 seconds.


The horizontal motion of the mass with respect to the casing is decomposed into its two perpendicular components, North-South and East-West, as we are used to seeing in modern-day instruments. The particularity of the Wiechert is its use of the 2D motion of a single mass to measure the two horizontal components of ground motion.

The motion of the mass is damped by air pistons (see schematic drawing). Damping is used in all modern seismometers to permit recording and interpretation of seismic energy after the first arrival. In undamped instruments, the later arrivals are drowned out by the oscillations caused by the first arrival.


The recording system is mechanical: two fine points scratch out the seismograms for each component on a roll of smoke blackened paper that rotates and translates in order for a full day of recording to be contained on a single sheet. This system is not dissimilar to the drum recordings used by the World-wide Standard Seismograph Network (WSSN) in the 1970s and 1980s.

The Wiechert seismograms contain minute marks made by lifting the two recording styluses in response to an electrical impulse that could be given by a contact switch on a precision pendulum, or by any other time-keeping device.

The Wiechert horizontal seismograph was kept working in Strasbourg Seismic Observatry from 1904 to 1968. It is now visible in the Seismology Museum, which is housed in the original observatory building.

Should you wish to see a working Wiechert seismograph, you should visit the Wiechert Earthquake Station in Göttingen.

Japan earthquake : an early warning failure?

The earthquake that occurred last Friday June 13th in Eastern Honshu, Japan - a M6.8 event according to the USGS, a M7.0 event according to JMA - occurred in a relatively lightly populated area, and caused few fatalities. Most of the damage seems to have been caused by landslides following the event (see Dave's landslide post).

In October 2007, Japan launched its Earthquake Early Warning system, meant to give the general population advance warning of the arrival of strong shaking. The system works by picking up the fast-arriving but non-destructive P-waves, producing an estimation of the earthquake location and magnitude, and sending out a warning to the region likely to be affected by significant shaking due to S- and surface waves.

How well did the system work for the June 13 earthquake? The following information comes from a NatureNews piece that appeared on June 16th.

The early warning system signalled a powerful quake 3.5 seconds after detecting the p-waves, but at places such as Oshu within 30 kilometres of the epicentre, the s-waves had already arrived. Residents of Kurihara, one of the cities hardest hit, received only 0.3 seconds of warning. Farther away, at a distance of 50 kilometres, the warnings were issued 5 seconds before the violent shaking; residents at 80 kilometres' distance were given 15 seconds. Those relying on television, radio and mobile-phone systems to relay the message would have had to have waited an extra second longer than those with an independent terminal that can receive the broadcast warnings directly.

The NatureNews article sees the Early Warning performance as a failure, saying the "controversial" system was "beaten" by the earthquake. I would not be so negative. It will always be difficult if not impossible to give adequate warning to the area immediately surrounding the epicenter for a shallow event, as the S-waves follow the P-waves to closely for even the fastest warning system to act (and I believe 3.5 seconds from first P-wave arrival to warning is not bad at all in terms of speed). This point is indeed conceded towards the end of the piece, but by then the damage is done.

I wonder if Nature has a beef with EEW (Earthquake Early Warning)? The three news articles published on the subject since the system was launched are all negative. From a purely scientific and technical point of view, I would say the system functioned properly, within its inherent limitations. These limitations are pointed out explicitly in the documentation given to the general public. I quoted the JMA explanation of these limitations in my first EEW post. Here is a brief summary:

1. Timing: because of the time required to process the seismic data and generate a warning, areas close to the focus of an earthquake may not receive the warning before the strong shaking occurs.
2. False alarms: these can occur when using data from only one seismograph.
3. Magnitude: magnitude estimates are notoriously inaccurate, especially before all the waves generated from the earthquake have arrived.
4. Seismic intensity: intensity estimates are limited in accuracy due both to the limitations in magnitude estimation and residual uncertainties in the local amplification due to land structure.
   

The acceptance / confidence problems currently plaguing the Japanese EEW system are in my opinion less due to failures of the system itself than to the inherent difficulties that occur any time one deals with warnings to the general public. Even though the limitations of an automated warning system may be explained clearly, the public is rarely generous of spirit when it comes to fearful occurrences such as earthquakes. Tsunami warning systems such as that run by the PTWC are confronted with similar difficulties, especially close to the focus of tsunamigenic earthquakes.

How should one deal with the general public? What is the actual tolerance of false alarms? What is the tolerance of underestimations of damage? I do not believe there is a cut-and dried answer to any of these questions. Moreover, the answer may change with time and will probably depend on the performance history of the system. We are still in the early days of Earthquake Early Warning. Maybe a few years from now we shall have a greater handle on how to deal with these issues...

CASE-IPY wake-up poll

As you can see from the state of health information for the CASE-IPY stations, the last station to fall asleep for the winter was CASE03 on April 18th. Our group at the lab has opened a pseudo-spread-bet on its wake-up date (pseudo, as in no money changes hands).

You too can participate in the betting using this online poll. The poll is open starting today and will run until July 30th (so you have six weeks to vote). I shall publish the poll results sometime in August, and as soon as the stations actually wake up, we shall have our winners.

Best of luck to all those who decide to play!

Opinion Polls & Market Research

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Sunday seismometer #2

Reuber-Ehlert (1895)

In 1895 Reinhold Ehlert - continuing the work of Reuber-Paschwitz whose instrument provided the first recording of a distant earthquake (see Sunday siesmometer #1) - modified the original Reuber-Paschwitz design to come up with a new instrument.



The Reuber-Ehlert seismometer has not one but three undamped horizontal pendulums weighing 200g each, and has a natural period of 12 seconds.



A mirror is attached to each pendulum, and reflects a light beam back out through the windows at the front of the instrument. The reflected light can then be recorded on a roll of photographic paper. The amplification depends directly on the distance between the mirror and the recording apparatus.



The use of three horizontal pendulums to record two orthogonal directions of motion may seem curious to us today, as does the lack of damping in both this and the original Reuber-Paschwitz seismometer.

The Reuber-Ehlert seismometer was installed at the Astronomical Observatory in Strasbourg in 1895, then moved to the newly built Seismological Observatory in 1900, where it was kept running until 1906.

Earth Systems teaching

There is an interesting discussion on Kim Hannula's blog about the pros and cons of teaching Earth Systems science as opposed to more hard core Geology topics. Essentially the debate centers on the purpose of such courses for the students that take them. If these students end up in careers outside Geology, then having the kind of overview an Earth Systems course could offer would be more than enough. However, for students intending to continue to a Geology based job (in the oil industry for example) or to a master's or doctorate degree, such courses are wholly inadequate.

A similar debate is happening at the moment in the Geophysics circles at our lab. We are being asked to open up geophysics teaching to students from a variety of backgrounds, including a number who have little or no mathematical training. The question is how to follow these courses with a master's or PhD program that necessarily involves a great deal of mathematical analysis? As for the Earth Systems debate, the jury is still out on this one.

Earthquake cleared of causing Lusi mud volcano

It seems earthquakes have been cleared of causing the Lusi mud volcano in Indonesia:

The two-year old mud volcano called Lusi spews huge volumes of mud and has displaced more than 30,000 people and caused millions of dollars worth of damage. An international team of scientists has now concluded that it was caused by the drilling of a gas exploration well and not by an earthquake that happened two-days before the mud volcano erupted in East Java, Indonesia.

This result, contested at first and now confirmed, comes shortly after the publication in Nature Geoscience of a study suggesting earthquake triggering is a ubiquitous phenomenon, at least for earthquakes of magnitude 7. The main argument against earthquake triggering being the cause of the Lusi mud eruption is precisely a magnitude argument:

Prof Michael Manga, of University of California, Berkeley, said: “We have known for hundreds of years that earthquakes can trigger eruptions. In this case, the earthquake was simply too small and too far away.”

Read more about Lusi and its possible collapse here.

Tangjiashan quake lake

The M7.9 Sichuan earthquake of May 12th triggered a massive landslide that created a lake at Tangjiashan.

Landslide created dams are notoriously unstable, and the Chinese authorities have been trying to limit the possible damage from an uncontrolled breach of the Tangjiashan dam by cutting a sluice to evacuate the ever rising water.

It seems now that these efforts have been in vain... See Dave Petely's great series of posts on the subject.

[Thanks to Andrew and Chris for useful links into this subject.]

One year of blogging

Sismordia is one year old today. I shall mark the anniversary with a look back at my original hopes for the blog and how things have evolved over the past year.

In my very first post Starting out I wrote:

The Concordia Seismic Experiment - as we have lovingly named it - has gone from residing somewhere in my brain, to being present also in my gut. It's a daunting thing, especially for me, as this will be both my first complete seismic experiment, and my visit to Antarctica.

I've set up this blog partly in order to relieve some of the tension that's building up as we prepare to make this experiment a reality, but also to share the experience with as many people as possible. I hope to convince some of my colleagues working with me on the experiment to participate in writing the blog, and I hope to entice you all to read it and send in your comments.

So my stated goals were

1. to share the experience of working in Antarctica;
2. to elicit the participation of my colleagues;
3. to entice you all to read my blog and send in comments.
   

Of these three goals, the first was for me the most important one, and I believe I have reached it. Blogging from the field in Antarctica was a great experience, and led to me compiling a Blog book about the campaign. I consider this document my greatest single blogging achievement.

In the second goal, I failed most miserably: Sismordia is still essentially a one-woman blog. My colleague JJL provided much needed copy-editing of the posts I emailed in from Concordia, but declined to write for Sismordia himself. As for those colleagues that have participated in the Antarctic seismology project (and continue to participate, for the project will live a few years yet), my greatest thanks go to JY.

My third goal was to be read. I was skeptical at first about the interest that a blog such as Sismordia could elicit in the blogging world. The subject matter is somewhat arcane, I tend not to deal in political controversy, and have not spent much effort in publicizing the blog.

Looking back over the statistics of the past year from Google Analytics, my 170 blog posts have received just over 5000 visits from about 3700 unique visitors who viewed a total of over 8600 pages. The graph below shows the number of visits per week over the past year. The peak in December-January corresponds to my Concordia field trip, which seems to have been the most successful time for Sismordia.



A few months after I started the blog, I linked my RSS feed through Feedburner. The number of subscriptions to my feed has risen gradually over the past year, and is now oscillating around 40.



So what can be said to sum all this up? Well for starters, I am still blogging, so the experience cannot have been too traumatic! As for the numbers, they are not Earth shattering, but are respectable all the same. I think I shall give my self a pat on the back as I settle into my second year of blogging.

Sunday seismometer #1

When I first started this blog nearly a year ago, I ran a short miniseries on my favorite historical seismometers from those on display at the Strasbourg Seismology Museum. The original series petered out after on three posts on the Wiechert, Galitzin and Ewing-Press instruments.

I am starting up the series again as a weekly feature (the Sunday seismometer) that will run over the summer months. I hope you enjoy it!

Reuber-Paschwitz (1889)

The seismogram above represents the first recording of a distant earthquake. It was made on April 17th 1889, in Potsdam, Germany, of an earthquake that occurred in Japan.

The instrument that made this first historical recording was built by Ernst von Reuber-Pashwitz.

Three years later, an identical instrument installed in the Astronomical Observatory in Strasbourg recorded another distant earthquake that occurred in Baloutchistan (a region that is shared by modern day Iran, Afganistan and Pakistan).

These two recordings mark the beginning of modern seismology.

The Reuber-Paschwitz was small (about 40cm in diameter) with a single horizontal pendulum. It seems that none of the original instruments have survived. We do, however, have a schematic drawing that you can see below.

Many of the physical characteristics of this seismometer are recognizable to modern-day seismologists: the horizontal pendulum, the three adjustable feet for leveling, the glass dome for protection from atmospheric perturbations.



The Baloutchistan seismogram recorded at Strasbourg:

Cool observations of glacial earthquakes

Glacial earthquakes have been one of the hot topics in seismology over the past few years. As they slide past asperities in the bedrock, certain glaciers emit long period seismic waves that can be detected by relatively distant seismic stations. These slip events are what we call glacial earthquakes.

Wiens et al. (2008) have recently published in Nature a set of beautiful observations of this phenomenon. They have put together information from regional seismic observations and a local GPS survey to constrain the nucleation, slip velocity and duration of glacial earthquakes occurring on the Whillans ice-field in West Antarctica.


They find that glacial earthquakes occur on this glacier twice a day, and seem to be triggered by tides in the Ross Sea. When the ice starts moving, it generates a seismic signal. Some minutes later, a second signal is generated when the moving ice hits the side of the glacier, and a third signal is generated when the ice hits the grounding line and stops moving.



The timing of these signals and the horizontal velocities measured by a temporary GPS network installed on the glacier give information on the amount of ice that moves in each event. Wiens et al. have estimated the energy released by the glacial earthquake to be equivalent to a M7 earthquake, only spread over the 20 minutes it takes the ice to stop moving.

For more publicly acessible information, read the early releases from ScientificBlogging and ScienceNOW.

Wiens, D.A., Anandakrishnan, S., Winberry, J.P., King, M.A. (2008). Simultaneous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream. Nature, 453(7196), 770-774. DOI: 10.1038/nature06990

Concordia film available from CNRS

During the 2007-2008 summer campaign at Concordia, a team from CNRS-Images filmed a number of the experiments being carried out on the site in the fields of glaciology, astronomy and seismology. The images have now been edited into a 34-minute documentary available directly from CNRS.

The seismology sequences were shot during the field installation of the CASE01 prototype station, and during our descent into the seismic vault (see the Seismology in the Freezer post). Watching the film made the whole experience come back to JY and me very vividly indeed.

The film is entirely in French (no dubbing or subtitles). Here is my translation of the film description:

The French-Italian station Concordia sits in the heart of the Antarctic plateau, on the site of Dome C, and at an altitude of 3233 meters of which more than 3200 meters are made of ice. It is a unique place, totally devoted to sciences. The 3270 meter ice-core project Epica permitted the decryption of up to 800 000 years of past climate. Other activities are in development at the site. The CNRS-Images team has followed the astronomers and the seismologists during their labors. At the start of February, the doors of Concordia will close to the summer technicians and researchers. Only 13 over-winterers will remain, both French and Italian. They will be there almost 10 months, far from everything, out of time, in this world of extremes.

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Keep up to date with the latest developments at http://sismordia.blogspot.com

Blogging hiatus is over

Apologies to my regular readers for the prolonged blogging hiatus. I have been somewhat absent from the whole blogging world for the past couple of months. It proved to be too difficult to blog from my last field-trip, and I have been working hard on a paper since getting back last month (the paper was submitted a few days ago, details soon).

In the meantime, a lot has been going on regarding CASE-IPY and other seismology at Concordia:

• The stations we installed during the last Antarctic field trip (CASE01, CASE02, CASE03) have now all gone to sleep for the duration of the Antarctic winter. The updated state of health plot for the stations shows that the longest running station CASE03 went into hibernation on April 18th. Bets are open as to when the first station will awaken in the spring.
• The permanent station CCD is running nicely, with both the heated and the unheated seismometers performing well. You can look up the state of health for the station, as well as journal plots for the data and snapshots of events (including the recent M7.9 Sichuan earthquake) on the Concordia Seismology website.
• We are planning both the 2008-2009 and the 2009-2010 summer campaigns at Concordia. The first campaign will be dedicated to recovering the full data from the CASE prototype stations, re-installing these prototypes for a second year of measurement, and upgrading the permanent station (CCD). We hope to stay longer than two weeks at Concordia this time, which should give us enough time to complete the essentials of the campaign and run a number of extra tests. The 2009-2010 campaign will be dedicated to installing 7 new autonomous seismic stations between Concordia and Vostok. Given the logistical constraints on the transport of all the material required for this deployment, we are building the stations this year. They will be shipped to Antarctica in the fall, will over-winter at Dumont d'Urville station, and will travel up to Concordia on the first land transport of the 2009-2010 season.
  

All in all, plenty to keep me busy and out of mischief! Now that my work load has simmered down to manageable proportions again (!), you can expect to see more of me in the blogging arena, and specifically many more Sismordia posts.


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Keep up to date with the latest developments at http://sismordia.blogspot.com