legends · 4 min read ·
Blast Record
A quarry's seismograph records tremors in the minutes before each blast, closing in on a later time.
**Eskerdale Limestone Quarry — Blast Record (Shotfirer: R. Hale, licence 41-88)**
*Record each blast: time, holes, charge, vibration at nearest receptor (PPV, mm/s), airblast (dB).*
- **Date · Time · Holes · Charge (kg) · PPV (mm/s) · Airblast (dB)**
- 1 Oct · 12:00 · 38 · 720 · 3.1 · 112
- 8 Oct · 12:00 · 40 · 760 · 3.3 · 113
- 15 Oct · 12:00 · 40 · 760 · 3.4 · 112
· · ·
**Seismograph trace (Receptor 2, Eskerdale village hall, 15 October)**
11:59:48 — Background.
**11:59:52 — Event begins. PPV 2.8 mm/s.**
12:00:00 — Blast fired.
12:00:01 — Event ends.
12:00:03 — Background.
*Shotfirer's note: the monitor recorded the blast at 11:59:52, eight seconds before the shot was fired. Clock error suspected. The monitor was recalibrated on 16 October.*
· · ·
**Blast record (continued)**
- **Date · Time · Holes · Charge (kg) · PPV (mm/s) · Airblast (dB)**
- 22 Oct · 12:00 · 40 · 760 · 3.2 · 112
**Seismograph trace (Receptor 2, 22 October)**
11:58:50 — Event begins. PPV 3.2 mm/s. Duration 1.1 s.
11:59:20 — Event begins. PPV 3.1 mm/s. Duration 1.0 s.
11:59:50 — Event begins. PPV 3.2 mm/s. Duration 1.0 s.
12:00:00 — Blast fired.
*Three events at 30-second intervals before the shot. The monitor clock has been checked and is correct. The events are shocks of the same size and shape as the blast, but with no source. They precede the blast by 70, 40 and 10 seconds.*
· · ·
**Licensing inspector's notes (A. Pryor, 24 October)**
I attended the quarry to review the unexplained records. The shotfirer is an experienced man, careful with his paperwork, and he had his records in order. He showed me the traces. I looked at the monitor myself. It is in good order.
What I cannot dispute is the shape. The events before the blast have the same waveform as the blast. A sharp onset, a decaying coda, a faint tail of reflections from the rock. They look like echoes. But echoes come after a sound. These come before it.
· · ·
**Blast record (continued)**
- **Date · Time · Holes · Charge (kg) · PPV (mm/s) · Airblast (dB)**
- 29 Oct · 12:00 · 40 · 760 · 3.3 · 113
**Seismograph trace (Receptor 2, 29 October)**
The trace shows twenty-one events in the 10 minutes before the shot. Intervals between them shorten as the shot approaches, from 60 s to 30 s to 15 s to 5 s, a series closing in on 12:00:00 from the past.
At 12:00:00 the blast was fired. The trace shows no event at that time. There is a flat line, for exactly 1.0 s, where the blast should be.
The ten minutes after the shot show a further twenty-one events at intervals that grow longer, from 5 s to 15 s to 30 s to 60 s, as if the quarry were ringing, slowly, in its own memory.
· · ·
**Shotfirer's note (R. Hale, 29 October)**
I fired the shot. I felt it in the ground and I heard it. It was a good shot. A clean face. But when I walked over to the blast monitor I saw that it showed only the echoes. The blast itself is not on the record.
I have been blasting for twenty-six years. I know what a shot looks like on a trace. It is not an echo. An echo is the quarry answering. The blast is the question. What I am seeing now is the quarry asking first, over and over, and my charge is the answer.
· · ·
**Licensing inspector's notes (A. Pryor, 31 October)**
I have recommended that blasting at Eskerdale be suspended. The quarry manager has asked me to delay the recommendation until after the next shot. He says the face is at its working limit and the contract cannot be broken.
I asked the shotfirer what he thought. He said he would not fire the next shot. I asked why. He said: "Because the pattern isn't closing in on the blast any more. It's closing in on a time after the blast."
I looked at the traces. The events after the blast are not growing longer. They are getting shorter. Intervals of 60 s, 30 s, 15 s, 5 s, 2 s, 1 s. A chain of tremors accelerating towards a point on the trace some minutes after the shot. The point is 12:07:43. There is a mark on the trace at that time. It is the largest event the monitor has ever recorded.
It has not happened yet.