Common and uncommon seismic records

This is a starting point for a collection of special signals recorded on our instruments.

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Lightning

Observation of lightning and thunder on Landwüst borehole array.

Metadata:

discovered_by: Daniel Vollmer
observations:
- time: 2024-06-02 08:40:26.200
  codes: 'SX.LWS??.'

Comments:

Dnn habe ich noch einen um 8:43:55.950, der ist auch auf der lightningmaps.org Seite zu finden, allerdings dreimal.

• 8:33:54.300
• 8:36:08.050

References:

• Zhu & Stensrud 2019: Characterizing Thunder‐Induced Ground Motions Using Fiber‐Optic Distributed Acoustic Sensing Array
• Hong et al. 2023: Inversion of acoustic thunder source spectral model from thunder-induced seismic waves in megacity

The Missing S-wave (?)

Is this a rare observation of a nodal point of S-wave radiation? Two events that are almost at the same location (according to the bulletins) show very different P/S amplitude ratios at different recordings sites - the nearly missing observation of S at location LWS00 (3C borehole stations) for one of the events is quite astonishing. The plots are showing common scale for each 3C station.

Metadata:

discovered_by: Matthias Ohrnberger
observations:
- time: 2024-05-23 00:03:53.3 (ev1) / 2024-05-23 01:45:24
  codes: '6A.LWS00.??.DP?', 'GX.STC00.??.DP?', 
         '6A.AA??.CH?' (mobile Reftek stations installed during array excursion 2024)

Comments:

An impossible overtone (?)

Metadata:

discovered_by: Alea Joachim, Thoralf Dietrich
observations:
- time: 2024-04-27 00:10:40 
  codes: needs to be revised, some Iceland field data

Comments:

Usually, if periodic events merge into tremor that shows harmonic series frequencies, they all lie on integer multiples of the ground frequency. Here, the ground frequency of the helicopter is at ~20Hz. Higher frequencies are at 40, 60, and 80. But also, and that is puzzling: At 70 Hz.

So what happened? Probably the smaller rotor at the back may be coupled (in a sense of: stable ratio for stable flight) to the big one, and that with a higher frequency.

An inverse ocean wave dispersion(?)

Metadata:

discovered_by: Thoralf Dietrich
observations:
- time: 2022-10-08 00:00:00, for the next 12 days 
  codes: SKeidara array, SK5.HHZ

Comments:

In the frequencies (below 1Hz), "sweeps" with lowering of the broad frequency content can be observed, merging with the oceanic microseisms. It is most probably not distant storm waves/swells, also there the usual dispersion of "lower frequencies arrive sooner, not later" is true.

Instead, it is probably new waves emerging from wind fields crossing Iceland from the north. As the stations are in the south, the first generated waves are of small wavelength and higher frequency. With time, they grow and become lower in frequency content.

Beep Bob

Metadata:

discovered_by: Thoralf Dietrich
observations:
- time: 2022-10-03 13:40:00, 2 hours 
  codes: SKeidara array, SK5.HHE

Comments:

Signals goes Beep, Bop & wheeeee (3 Hz, 1.5 Hz short pulses & 4Hz continuous)