A local tremor on a distant wave
Recorded by CHARNET stations in Armenia
- Date2 May 2025
- Record15:27:07–15:30:27 UTC
- Local tremorP at 15:28:10
- Long waves15–20 s period
Two signals from different distances, written by the same instrument at the same time
On the afternoon of 2 May 2025 all our stations were swaying slowly and evenly, as if the ground were breathing. Each swing lasted 15–20 seconds. These were long waves that had travelled from far away across the planet. And right in the middle of this slow swell, at 15:28 UTC, a small local earthquake struck nearby.
On the raw record the two are tangled together: the short sharp tremor is almost lost on the large slow wave. Can we pull them apart? Seismologists do it with filters, separating the long periods from the short ones, much as you can pick out a single voice in a noisy room.
Antarashat station (ANTS), vertical component, 100 seconds of record (15:27:37–15:29:17 UTC), shown 3× faster. Row 1 — the long waves and the local earthquake together, as the station wrote them (the local earthquake is magnified 2.5× so that it is not lost); row 2 — only periods longer than about 3 seconds; row 3 — only frequencies of 1–15 Hz. Red — the long waves; blue and gold — P- and S-waves of the local earthquake.
- Row 1The raw record: a slow swell with a small tremor hidden in it
- Row 2Keep only the long waves: a smooth swell with a period of 15–20 s, arriving from far away. The local earthquake disappears
- Row 3Keep only the short waves: the swell vanishes, and the local earthquake stands out clearly, with its P- and S-waves 14 seconds apart
Records: CHARNET, IGES
The same instrument hears the whole planet and the ground under its feet at the same moment. Separating such overlapping signals is everyday work for the institute’s seismologists, and it is how no tremor, near or far, goes unnoticed.