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...