Long-Period Disturbances in Broadband Records of Weak Events at Short Epicentral Distances
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Long-Period Disturbances in Long-Period Disturbances in Broadband Records of Weak Broadband Records of Weak
Events at Short Epicentral Events at Short Epicentral DistancesDistances
J. ZahradnJ. Zahradniikk and A. Plesinger and A. Plesinger
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Long-Period Disturbances in Long-Period Disturbances in Broadband Records of Broadband Records of Weak Weak
Events at Events at ShortShort Epicentral Epicentral DistancesDistances
J. ZahradnJ. Zahradniikk and A. Plesinger and A. Plesinger
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Czech Republic, West Bohemia swarm 2008 NKC station Mw 3.6 event, STS-2.
tilt-like
New(U-shaped)
~0.2e-6 rad
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Step response STS-2
CMG-40
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STS-2measured tilt0.2e-6 rad
CMG-40T5-times less than STS-2
Tilt ?
Source model: 1.0e-8 rad
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Collocated STS-2 and CMG-40T clearly prove that
the tilt-like disturbances are (at least partially) due to instrumental effect.
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STS-20.5e-5m/s
CMG-40T0.5e-5 m/s
U-shaped disturbance on Z
No explanationby source model
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U-shaped disturbance: purely instrumental effect due to
internal saturation
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(a) (b) (c)
(d) (e) (f)
Groundmotion
No saturationin direct path
Withsaturationin direct path
Normaloutput U-disturbed
output
System simulation
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Records without disturbances - still ‘strange’: static displacement
Synt.(permanentoffset 2e-5 mproved)
Obs. =
Synt. + instr.
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Synthetics with
near-field ramp and
static offset(stable with
foc-mech variations)
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Static displacement: two instruments
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Japan 2002-03-11 a shallow crustal event of Mw 3.9, ISI station, STS-1 sensor at epicentral distance 1 km.
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Japan 2006-05-14 a shallow crustal event of Mw 4.3, NOK station, STS-2 sensor at epicentral distance 13 km.
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Greece 2009-02-20 a shallow ML 4.2 event, RGA station, KS2000M sensor at epicentral distance 10 km.
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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).
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L’Aquila Mw 6.2 earthquake: TEO station, 250 km (Irpinia network), Trillium40
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40 50 60 70 80tim e (sec)
-8E-005
-6E-005
-4E-005
-2E-005
0
2E-005
inte
gra
ted
rec
ord
(m
)
28. 10. NKC-ZBlack: STS - velstepYellow: synth. displ., no STSBlue: synth. + STSRed: synth. + STS + veloc. step
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