legends · 4 min read ·
Station KLD, Seismic Log
A seismologist records a footstep wave that follows his instrument to a new site five kilometres away.
**Kildrum Seismic Observatory — Station KLD (broadband, 20 m borehole)**
**Analyst:** Dr. R. Ito · **Period:** October
*The station records ground motion from earthquakes, quarry blasts, traffic, wind and ocean microseisms. Analysts review traces daily.*
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**3 Oct.** Quiet. Microseismic background normal. M 4.1 regional earthquake at 07:20. No local events.
**4 Oct.** Quiet. Regional quarry blast at 12:00. No local events.
**5 Oct.** Trace shows a series of small repeated pulses at 02:11 to 02:19, amplitude 0.2 µm/s, period about 0.6 s, lasting eight minutes. Possible cultural noise. Logged.
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**6 Oct.** Same pattern 02:09 to 02:21. Amplitude greater, period 0.6 s, duration twelve minutes. The pulses are asymmetric, with a sharp onset and a slow decay. I examined the polarisation. The source is local, within 200 m of the station. The pulses travel from the south-west, in a line, towards the borehole.
**7 Oct.** Pulses begin at 02:03, end at 02:25. Closer.
Analyst's comment: the pattern looks like footsteps. I do not say this lightly. The waveform is that of a heel strike on a hard surface. A walker, at a pace of about one step per 0.6 seconds, approaching the station along a straight line.
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**8 Oct.** Pulses begin 01:55, end 02:28. The station's three-component record shows motion first on the south-west sensor, then centred, then reversed. The walker has passed the borehole and is receding to the north-east.
**9 Oct.** Pulses begin 01:47. Peak amplitude at 02:15: 1.1 µm/s. The footsteps passed within 15 m of the borehole. The depth of the source is zero. They are on the surface.
I walked the area at dawn. It is open moor. A track runs south-west to north-east about 60 m from the station. There was dew on the heather and no marks in it.
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**10 Oct.** Same. Pulses begin 01:40.
**12 Oct.** Pulses begin 01:20. I analysed the stride length from the arrival times at the three sensors. The walker takes steps of about 0.7 m at 0.6 s intervals. A slow, deliberate walk. A man, I think. A heavy one. Or a man carrying something heavy.
**14 Oct.** Pulses begin 00:45. They continue without a break until 03:10. Each pass is a little longer than the one before. The walker is not going away. He is circling.
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**16 Oct.** I installed a second seismometer on the surface, 20 m to the east. If he is walking, it will tell me in which direction he is facing as he passes.
**17 Oct.** The second seismometer recorded the passes. Both stations show the same sequence of pulses. The two records are exact copies in time, to within a millisecond. But the second station is 20 m from the first. A walker cannot be at both places at once.
I have asked a colleague to review the records. She says the pulses are not footsteps. They are not at a single point. They are travelling as a wave. The source is not moving. The ground is.
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**20 Oct.** Pulses begin at 23:30 and continue all night. The wave crosses the station and comes back. Each pass takes about ten minutes. I counted the number of passes in a night. Fifty-eight.
I think the ground is breathing. I mean this in a literal sense. The trace has the shape of a slow respiration: a long inward wave, a pause, a long outward wave, a pause. A rate of about six breaths an hour. For a living thing of this size that is a very slow rate, but it is regular, and it has been getting slower since the 5th.
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**26 Oct.** The station has been closed. I recommended that the station be moved to a site 5 km north. The move was completed on the 25th.
The new site is a granite outcrop on a hill with no soil. The ground is hard. The instrument was installed in a 20 m borehole, the same depth. It started recording at 18:00.
At 02:05 on the 26th the pulses began.
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**Analyst's final comment (R. Ito, 27 Oct)**
The new station shows the same signal, the same period, the same amplitude. The wave arrives from the south-west and passes beneath the station. It is not the same thing as before. It cannot be the same thing, because it is 5 km away. Unless it has followed us.
I am an analyst. I do not make claims about causes. I note that the wave has been recorded at both sites on the same nights, at the same times, in the same phase. Which means it is a single wave, 5 km across. Which means whatever is walking is 5 km tall, and its stride is half a metre, and it has been, patiently, for three weeks, getting nearer.