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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