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Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances

Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances. J. Zahradn i k and A. Plesinger. Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances. J. Zahradn i k and A. Plesinger.

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Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances

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  1. Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances J. Zahradnik and A. Plesinger

  2. Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances J. Zahradnik and A. Plesinger

  3. Czech Republic, West Bohemia swarm 2008 NKC station Mw 3.6 event, STS-2. New (U-shaped) tilt-like ~0.2e-6 rad

  4. STS-2 Step response CMG-40

  5. Tilt ? CMG-40T 5-times less than STS-2 STS-2 measured tilt 0.2e-6 rad Source model: 1.0e-8 rad

  6. Collocated STS-2 and CMG-40T clearly prove that the tilt-like disturbances are (at least partially) due to instrumental effect.

  7. U-shaped disturbance on Z CMG-40T 0.5e-5 m/s STS-2 0.5e-5m/s No explanation by source model

  8. U-shaped disturbance: purely instrumental effect due to internal saturation

  9. System simulation With saturation in direct path Ground motion No saturation in direct path (a) (b) (c) (d) (e) (f) Normal output U-disturbed output

  10. Records without disturbances - still ‘strange’: static displacement Obs. = Synt. + instr. Synt. (permanent offset 2e-5 m proved)

  11. Synthetics with near-field rampandstatic offset(stable with foc-mech variations)

  12. Static displacement: two instruments

  13. Japan 2002-03-11 a shallow crustal event of Mw 3.9, ISI station, STS-1 sensor at epicentral distance 1 km.

  14. Japan 2006-05-14 a shallow crustal event of Mw 4.3, NOK station, STS-2 sensor at epicentral distance 13 km.

  15. Greece 2009-02-20 a shallow ML 4.2 event, RGA station, KS2000M sensor at epicentral distance 10 km.

  16. Conclusions Broadband observations of small earthquakes at short epicentral distances = mixture of near-field effects and instrumental artifacts. STS-2 and CMG-40T sensors at the same station measured different tilts. The tilt of STS-2 exceeded more than 10-times the values predicted by source model. A new type of disturbance also found: apparent step in ground velocity (pulse in ground acceleration). Likely explanation - asymmetric saturation of high-frequency signal peaks in the forward path of the feedback velocimeters. Users may numerically simulate the disturbances and subtract from records (which is necessary in source studies at frequencies below 0.1 Hz). The 'decontaminated' records still may have a strange, bow-shaped form, related to the near-field ramp and the static displacement (of the order of 1x10-5 m in this paper). The effects have a general character, for apparently any feedback-controlled broadband velocimeter (STS-1, STST-2, CMG-3T, Trillium40, KS-2000).

  17. L’Aquila Mw 6.2 earthquake: TEO station, 250 km (Irpinia network), Trillium40

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