Anchorage earthquake disrupts clock in Seattle

Friday, 30-Nov-2018

Well, this was an unexpected surprise for someone like me that collects and measures clocks as a hobby. Why on earth is did my pendulum clock jump 0.2 milliseconds today?

Earthquake near Anchorage, Alaska

USGS says "a magnitude 7.0 earthquake struck north of Anchorage, Alaska on November 30, 2018 at 8:29 a.m. local time (17:29:28 UTC)". Although it was a large earthquake, Alaska is about 1500 miles away from Washington. We live in the greater Seattle area and felt absolutely nothing.

Synchronome pendulum clock near Seattle, WA

But I have a Synchronome pendulum clock and it definitely "felt" something! The Synchronome is running in the basement and recently I have been continuously logging its performance down at the microsecond level. The effect of an earthquake, even if very far away and undetectable by humans, is dramatic to a precision pendulum clock. The clock was disturbed for as much as 20 minutes and lost nearly 200 microseconds due to the event. Ok, that's only 0.2 milliseconds, or 0.0002 seconds. Invisible to the human eye. But don't laugh, we pendulum guys take our clocks seriously!

What is a Synchronome pendulum clock?

A Synchronome is an English pendulum clock that was commonly used in the 20th century for precise timekeeping. Most people would just call it a "grandfather clock". But what made the Synchronome different from a traditional mechanical wind-up grandfather clock was the use of batteries, relays, and magnetic coils to keep it running. The hands of the dial were controlled with electrical impulses from the clock instead of gears. This and other design factors made the clock a better timekeeper. Below is a random internet photo of the inside and outside of a typical Synchronome clock:

Certainly many other pendulum clocks are better looking, but the Synchronome was designed by engineers for engineers. Consequently it lacks most of the artistic frills of the typical grandfather clock. Note that this model of generic Synchronome clock (similar to the one I own) is not the exotic vacuum chamber Shortt-Synchronome clock (which I could never possibly afford).

Some plots

I have been logging data from my Synchronome clock during much of 2018, that is, comparing the time of the Synchronome against atomic time every second. When I looked at the plots Friday evening I noticed a fairly large glitch in the data.

It never occured to me that an earthquake up in Alaska would affect a pendulum clock here in Washington. But with some quick internet searches, I could see that the glitch in the pendulum data was closely correlated with the quake. How cool is that.

Below is a plot of pendulum error (aka time offset, phase difference):

Below is a plot of pendulum period (aka pendulum 1/rate, 1/frequency). The typical variation in 2 second period is 2 µs RMS but the earthquake caused peak-to-peak excursions approaching ±150 µs. If this looks like a seismogram, yes, it should. Some seismometers are in fact based on a pendulum, but designed for vibration recording rather than timekeeping.

Below is a plot of pendulum amplitude (in degrees, mrad, or cm):

Comments

Seismometers around the world picked up the signal; that is what they are designed to do. Simple physics says a pendulum clock should also be disturbed by seismic activity. In fact some seismometers are based on pendulum designs. Still we normally think of pendulum clocks only as timekeepers and not seismometers. So to actually see it happen in the clock data is quite remarkable. Few people would notice this because the error was only 2 thousandths of a second. But it speaks to the accuracy of the clock, the precision of the measurements, and the power of the earthquake in spite of the vast distance of 1500 miles.

Synchronome photo

My Synchronome pendulum clock is installed in the basement / crawl space, bolted into a solid concrete corner wall. The precise time of the pendulum is sensed using an IR photogate sensor located under the bob. An assortment of high-resolution electronic timers do the timing. A laptop logs all readings for later analysis.

A dim view into the clock showing the count wheel and gravity arm escapement:

At the time of the quake I happened to be collecting data using multiple timers. One has ~5 µs resolution; the other two are 400 ns and 10 ns. The consistency of the timers confirms the erratic data was not due to optics or electronics but the pendulum clock itself. The magnitude, shape, and UTC timing of the disturbance suggest an earthquake origin.

Technical details

Preliminary data; not calibrated yet.
Measurements made with IR photogate sensor below the bob.
Timings collected with Microset timer.
Timings also collected with picPET timer, sharing same optical trigger signal.
Amplitude derived from "flag width" (aka blink time, dark interval, shadow width).
Data logged via USB/serial adapters.
One sample per beat (1 Hz).
Each Microset sample contains leading edge interval (e.g., 1.000 second) and pulse width (e.g., 25 ms).
Each picPET sample contains timestamps of leading edge and trailing edge.
Resolution ~5 us (Microset) and ~400 ns (picPET).
Plots generated with one point average per 30 seconds (Synchronome impulse interval).
Plots span two hours, an hour on either side of the first hint of disturbance.

Seismographs

Since the day of the earthquake I periodically captured a few images from the usgs.gov web site.

Below are two side-by-side plots showing seismic activity in Fairbanks, Alaska. On the left is the day of the earthquake (30-Nov). On the right is a typically quiet day (3-Dec).

Click for full-res: usgs-2018-11-30-08h-COLA.png and usgs-2018-12-03-12h-COLA.png

Much closer to home, below are two side-by-side plots showing seismic activity at Hanford, Washington. On the left is the day of the earthquake (30-Nov). On the right is a typically quiet day (3-Dec).

Click for full-res: usgs-2018-11-30-08h-HAWA.png and usgs-2018-12-03-12h-HAWA.png

To appreciate what it looks like at the end of the earth, below are two side-by-side plots showing seismic activity at Scott Base, Antarctica. On the left is the day of the earthquake (30-Nov). On the right is a typically quiet day (3-Dec).

Click for full-res: usgs-2018-11-30-08h-SBA.png and usgs-2018-12-03-12h-SBA.png

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