Earth's core less anisotropic than we thought?

One of the reasons for installing seismic stations in Antarctica is to improve our knowledge of the Earth's inner core. It would seem - from the data currently available - that seismic waves that propagate through the inner core parallel to the Earth's rotation axis (polar paths) are faster than those that propagate perpendicular to the Earth's rotation axis (equatorial paths). This speed difference implies the inner core is anisotropic, with a N-S fast axis.

This conclusion is based on a large number of equatorial paths, and only a small number of polar paths, most of which come from earthquakes in one particular region (South Sandwich islands) recorded in Alaska. Data from new stations in Antarctica are expected to increase the number of polar paths available, and improve our understanding of the inner core.

At EGU this morning, I heard one of the first talks on the inner core that actually uses data from newly deployed Antarctic stations:

D. Leykam, H. Tkalčić, and A.M. Reading : Core structure reexamined using new teleseismic data recorded in Antarctica: Evidence for, at most, weak cylindrical seismic anisotropy in the inner core (Abstract).

The data are from the SSCUA stations, deployed near Mawson station. The PKP(bc-df) measurements made on these data are all between 0 and 2 seconds, and imply that if there is N-S oriented anisotropy in the inner core, this anisotropy must be weak (and specifically, much weaker than implied by the South Sandwich data). Indeed, the weak anisotropy hypothesis seems to be consistent with all data except those from South Sandwich.

Should we therefore dismiss anisotropy in the inner core? Possibly. But before doing so we need more data and measurements from other Antarctic stations (the CASE-IPY and Concordia stations will contribute some of these data), and we need to understand why the PKP(bc-df) measurements from South Sandwich events are so large.

Sunday Seismometer #12

Rocard (1958)

From last week's prototype, let's move on to a more useful set of instruments, designed specifically to detect nuclear explosions in the context of Comprehensive Test Ban Treaty monitoring.



They are named after Professor Yves Rocard, the physicist who started to develop detection seismology in France in the 1960s, and who founded the division of the French Atomic Energy Commission (CEA) that is in charge of geophysical studies and activities associated with monitoring and the environment (LDG).



The Rocard is a classical electromagnetic seismometer, with a 1s natural period and electronic amplification. Rocard instruments were in operation at the Welschbruch station not far from Strasbourg.

Sunday Seismometer #11

Peterschmitt (1950)

Continuing our mini-series on electromagnetic seismometers (see the Galitzine and Press-Ewing posts), here is a seismometer you are unlikely to see anywhere else. The Peterschmitt was designed and built in Strasbourg in 1950, where it was in use until 1975.



This admittedly ugly looking beast is a prototype classical electromagnetic seismometer (you can see its coils on the near side, very similar to those on the Galitzine instrument) combined with a galvanometer. It has a natural period of 1s, and its amplification is provided by a resistance bridge. The most interesting feature of this instrument is its original inbuilt calibration system.

The design of this instrument is attributed to Elie Peterschmitt, who was recruited by Strasbourg in 1937, took charge of the Strasbourg historical seismological station as well as the stations of Besançon and Bagnères de Bigorre, and later helped develop the European-Mediterranean Seismological Center (EMSC).

Sunday Seismometer #10

Press-Ewing (1953)

Some 40 years after the Galitzine electromagnetic innovation, the same principles of operation are put to work in the Ewing-Press seismograph, built at the Lamont Geological Observatory of Columbia (now the Lamont-Doherty Earth Observatory) by Maurice Ewing and Frank Press.

In the photo below you can see the vertical Press-Ewing instrument on display at the Strasbourg Seismological Museum. It was in use in Strasbourg from 1963 to 1975.



It is an electromagnetic seismograph, coupled with a galvanometer, and has a natural period that can be selected and fixed up to 30s. Recording was optical, on photographic paper. The glass ball you can see on the near side of the instrument reduces the effect of variations in atmospheric pressure on the seismograph recordings, using the Archimedes principle.

This seismograph and its horizontal counterparts are very well adapted for the recording of surface waves. In 1957-58, Press-Ewing instruments were deployed in 125 locations around the globe to establish the World-Wide Standardized Seismograph Network, the first global earthquake monitoring system.

Sunday seismometer #9

Galitzine (1910)

All the seismographs we have discussed up to now (Reuber-Paschwitz, Reuber-Ehlert, Wiechert horizontal and vertical, Mainka, Vicentini, 19-Ton, Mintrop) have been mechanical, with either mechanical or optical recording. Today's instruments, built by Galitzine in St Petersburg (Russia) in 1910, are the first examples of electromagnetic seismometers.


In the above photograph of the vertical Galitzine (mass 10 kg, period 24 s) you can see the new element of this seismometer: the coil placed at the end of the pendulum's rod. This coil oscillates in a magnetic field, and creates an electric induction current which can be measured using a galvanometer.

A copper plate, fixed on the same rod as the coil, oscillates in the field of a second magnet and provides damping via a Foucault current.


The horizontal instrument (above, mass 7 kg, period 12 s) works using the same principle. The object placed in front of the seismometer is a galvanometer that is equipped with a mobile frame and a mirror for optical recording.



The Galitzine instruments amplify Earth motion in two successive stages: an electromagnetic amplification (the galvanometer mirror rotates more than the pendulum oscillates) followed by the optical amplification caused by the distance between the galvanometer mirror and the recording medium.

Sunday seismometer #8

Mintrop (built sometime after 1910)

From the very large (last week's 19-ton seismograph) to the relatively small : the Mintrop portable horizontal seismograph.



The Mintrop is an odd instrument, that measures horizontal motion using a damped inverted pendulum with a horizontal rotation axis. Its relatively small mass is coupled with a vertically oscillating mirror and an optical recording system.



Given the delicate nature of the recording system, the Mintrop must have been rather difficult to install. It is considered to be one of the first portable field instruments, and was used for early prospection studies by German oil companies.

Quake Catcher Network

Thanks to Julian over at Harmonic Tremors whose post about last Tuesday's M5.4 earthquake brought the Quake-Catcher Network to my attention again (I originally read about it on Geology News and Highly Allochthonous earlier this year, but did not have time to blog about it).

The Quake-Catcher Network is a collaborative initiative run jointly by Stanford and UC Riverside that aims to use acceleration detectors present in most modern laptops to form a low-cost strong motion seismic network.

Laptop users can download a client program that sits and monitors the motion of their laptops, sending information to the Network when any strong signals are detected. If strong signals are detected by many nearby laptops at the same time, the Network knows an earthquake is happening.

Laptops continuously move with the people who use them. So how does the Quake Catcher Network know where the laptop is? Users can give precise locations (using a GoogleMaps widget) of where they use their laptops most often. To choose between these locations, and also to deal approximately with undefined locations, the Network uses the laptop's current IP address. Nifty!!

As correct time is essential for earthquake location (just ask any observational seismologist or seismic network manager), the Network also checks the laptop's clock to make sure it is on time.

I have just signed up as a Quake-Catcher laptop client. The sign up procedure is completely painless (at least it was on my mac). Quake-Catcher uses a system called BOINC to interact with your computer. This is the same system used by other distributed computing projects you may have heard about, such as SETI@home or LHC@home.

Quake-Catcher aims to become a global strong-motion network, but it can only do so with your help. The more laptops connect to the system, the better. The accuracy of Quake-Catcher detections depends on the number of users located in any given region, so if you want your contribution to Quake-Catcher to be really useful, you should urge your friends, families and colleagues to sign up.

Another Antarctic Earthquake

In November of last year I wrote about an unusually large earthquake (M 5.8) that had occurred close to Casey Station in Antarctica. Earthquakes of this magnitude are rare in East Antarctica, except, it seems, in the Casey region...

Indeed, on July 23rd of this year, another large earthquake (M 5.3) occurred in the same region. The following image is from the USGS and shows the position of this event as an orange star.



This event was recorded on seismometers all over Antarctica. As examples, I have plotted the recordings of vertical ground velocity for this earthquake at stations CASY (Casey), MAW (Mawson) and PSP02 (a POLENET temporary station near South Pole).


The numbers under the station names on the above plot are distances in km from the earthquake. As you can see, the earthquake was well recorded even at distances over 2000 km.

We often say that the Antarctic plateau is virtually a-seismic, meaning there are few if any earthquakes. As you can see for this Casey event, it would be hard to miss an earthquake larger than M5 virtually anywhere on the continent. Smaller events may still be missed, however, and we do not have enough seismic stations in Antarctica (yet) to be sure that they do not occur.

Sunday seismometer #7

Great Pendulum or "19-Tons"

In the last Sunday seismometer post on the Vicentini seismograph, we mentioned that in order for a seismograph to overcome the friction caused by a purely mechanical recording system, it needs a large mass.

The Vicentini instruments actually have the smallest masses (100 kg for the horizontal and 50 kg for the vertical) of the mechanically recorded seismographs we have described so far. The Mainka instrument has a 450 kg mass, the Wiechert horizontal instrument has a 1-ton (1000 kg) mass, and the Wiechert vertical instrument has a mass of 1.2 tons.

The largest mass of all the seismometers in the Strasbourg museum is that of the Great Pendulum: an impressive 19 tons (that is 19 000 kg)!



Its construction was started before the First World War (1910), when the Strasbourg Observatory was part of Germany. The idea was to build an instrument that would be similar to one installed in Göttingen, a 17-Ton seismograph. After the war, Strasbourg became French, and it was the French director of the Observatory, Edmond Rothé, who completed the construction of the Great Pendulum in 1925.



The mass itself is essentially made up of scrap metal from the War, including 12 tons of axles from military trucks and 2 tons of weapon parts.

The 19-Ton has a natural period of 2 seconds, and records both the horizontal directions of motion, like the Wiechert horizontal instrument. Also like the Wiechert, its motion is damped by air pistons.



The smoked paper recording system was abandoned in 1970 in favor of galvanometric recording. In 1987 the recording system was changed once again to digital recording using displacement detectors.

The 19-Ton instrument is still in working order today, and is a great favorite with visitors to the Strasbourg Seismology Museum.

Sunday seismometer #6

Vicentini (1895 and 1899)

Let's go back to the early days, and take a look at a contemporary of the Reuber-Paschwitz and Reuber-Ehlert seismographs. The Vicentini instruments are simple, un-damped pendulums, one to record the two horizontal components of motion, and another to record the vertical component.


The clock at the back of the image above is a precision chronometer that was used to produce the time base for the seismic recordings at the Strasbourg Seismic Observatory.

Built in 1895 and 1899 in Padova, Italy, the Vicentini seismographs had low sensitivity and were designed to record strong motion from local earthquakes. Their natural period is 1-2 seconds.



Where the Reuber instruments were lightweight (200 g masses) and used optical recording to reduce friction, the Vicentini instruments had heavy masses (100 kg for the horizontal and 50 kg for the vertical) and used a mechanical recording system. In order for a seismograph to overcome the friction caused by a purely mechanical recording system, it needs a large mass.

We no longer have the Vicentini recording systems. The horizontal system was installed under the pendulum mass and recorded the two components of motion and a time mark on a single sheet of smoked paper.

The two Vicentini seismographs operated in Strasbourg from 1895 to 1907.

Sunday seismometer #5

Mainka (1910)

The Mainka seismograph is a large, single component horizontal pendulum (two instruments installed at right angles to each other are required to fully describe the horizontal ground motion).



Its 450kg mass is suspended in such a way that it oscillates around a near-vertical axis, with a natural period of 8-10 seconds. Damping is provided by a dash-pot system (a plate moving through a viscous fluid). Seismograms are recorded on smoked paper via a mechanical stylus, as was the case for the Wiechert seismographs.



Various models of the Mainka instrument with differing masses were built. The model shown in the photo above and visible in the Strasbourg Seismology Museum was manufactured by the Society for Optics and precision Mechanics of Paris (SOM). It is an improved model, of average sensitivity, robust and easy to tune, and was chosen in 1925 by the French Central Seismological Bureau (BCSF) for deployment at seismic stations throughout France and in the French colonies.

A Mainka seismograph operated at the Strasbourg Seismic Observatory from 1910 to 1960.

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...

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.

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

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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Using seismic waves to image Earth's internal structure

Life on board the Marion Dufresne continues uneventfully. I am taking advantage of the relatively clement sea conditions to work on a manuscript due for submission soon, and to read some scientific literature.

Earth science is a frustrating subject at times. It seems the more we investigate the Earth, the less we understand how it works. Controversy is rife, generating much confusion in the minds of students and researchers alike. Questions like 'How do subduction zones work?' and 'Where does hotspot volcanism originate?' are still hotly debated at international conferences and in print. One of the ways we are trying to address these open questions is by striving for clearer and higher resolution images of the Earth's interior.

Romanowicz (2008) gives a good three-page summary of past progress and outstanding issues in the use of seismic waves for this type of imaging.

To address these controversies, seismology has been brought to bear to image Earth's deep interior. From the construction of accurate models of Earth's one-dimensional radial structure to the current models of its three-dimensional structure, progress in seismic imaging has gone hand in hand with improvements in the design of seismic sensors, the capacity to record digitally increasingly massive quantities of data, theoretical progress in handling seismic-wave propagation through complex three-dimensional media and the development of powerful computers for simulating seismic waves and for the inversion of large matrices.

We are now at a point where there is a certain consensus regarding the long wavelength heterogeneities within the Earth. The next steps according to Romanowicz should be

characterizing the sharpness or fuzziness of the boundaries of the heterogeneous structures deep inside the planet, and detecting and mapping small-scale heterogeneity [...] This will mean extracting more information from seismograms than has traditionally been done.

Romanowicz goes on to describe some of the recent advances that are already leading to improvements in tomographic imaging techniques, including first order scattering theory, spectral element wavefield simulation methods and the extraction of structural information from the cross-correlation of noise records.

The problem of data coverage still remains:

A significant challenge is the limited distribution of seismic-wave sources and receivers. Ideally, one would want to sample the volume of Earth uniformly. But unlike other disciplines that use imaging, such as medical tomography or petroleum exploration, earthquake seismologists cannot optimize their experimental geometry.

Here we return the main theme of the last few research blogging posts: the need to obtain more raw data and to exploit them more fully to improve the imaging of Earth's interior.

Quoted text reprinted by permission from Macmillan Publishers Ltd: Nature, copyright (2008).

References

Romanowicz, B. (2008). Using seismic waves to image Earth's internal structure. Nature, 451(7176), 266-268. DOI: 10.1038/nature06583

Can better physics guarantee better tomographic models?

One of the key elements in discussing an inverse problem such as seismic tomography is the quality of the forward theory. The better the forward theory, the better synthetic data can be predicted from physical model parameters, and hence the better the solution to the inverse problem, right?

Unfortunately the issue is not so simple. Trampert & Spetzler (2006) come to the dual conclusions that better physics (in the form a finite-frequency formulation of the sensitivity kernels of seismic wave measurements) is a necessary but not sufficient condition for improvement of tomographic models, and that the null-space (due to uneven or insufficient data coverage) is currently too large to permit the improvements in resolution that better physics could provide.

Despite finite-frequency kernels being more accurate than the approximate sensitivity formulations of ray-theory, models constructed from either theory are statistically similar, i.e. one cannot construct a finite-frequency model (with a given data fit and horizontal resolution) which cannot also be obtained from ray theory by changing the regularization damping of the inversion accordingly. Regularization dominates the significant aspects of tomographic models, and affects both finite-frequency models and ray-theory models similarly. Data error propagation is worse for finite-frequency kernels, but given the large influence of regularization, this is a minor problem.

The authors maintain that in order to increase the resolution of tomographic inversions, we have to remove the ill posedness in the inverse problem (an ill posed inverse problem has more degrees of freedom than can be constrained by the available data) by increasing and/or homogenizing data coverage. I agree whole-heartedly with this statement! What can be done?

(1) The current distribution of seismic stations is in-homogeneous (see figure at bottom of post showing all FDSN seismic stations), and is limited by the accessibility of suitable installation sites. We should attempt to homogenize the distribution of seismic stations by installing more instruments in currently inaccessible locations such as the sea-floor (ocean-bottom seismometers) and my personal favorite, Antarctica. This solution requires lots of time, effort and a high level of funding that is becoming more and more difficult to obtain.

(2) So far we only use very little information from the complete seismogram (first arrival times of a few main waves, or the dispersion characteristics of surface waves). We should use more of the information available from the complete seismogram, given that modern adjoint methods have made it possible to associate a complete sensitivity kernel to each measurable wiggle in a seismogram. This solution is technically feasible given enough computing power and the development of new tools to automate the data selection and measurement processes.



References

Trampert, J., Spetzler, J. (2006). Surface wave tomography: finite-frequency effects lost in the null space. Geophysical Journal International, 164(2), 394-400. DOI: 10.1111/j.1365-246X.2006.02864.x

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