A trial for monitoring temporal variation of seismic velocity using an ACROSS system

Citation
K. Yamaoka et al., A trial for monitoring temporal variation of seismic velocity using an ACROSS system, ISL ARC, 10(3-4), 2001, pp. 336-347
Citations number
12
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ISLAND ARC
ISSN journal
1038-4871 → ACNP
Volume
10
Issue
3-4
Year of publication
2001
Pages
336 - 347
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<336:ATFMTV>2.0.ZU;2-W
Abstract
The temporal variation of seismic velocity near the Nojima Fault, which rup tured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was det ected using an accurately controlled routine-operated seismic source (ACROS S). The source generates elastic waves by a centrifugal force of an eccentr ic mass rotating around an axis. The mass is driven with an AC servomotor w hose angular position is accurately controlled with reference to a very acc urate global positioning system (GPS) clock. The error of the mass' positio n is less than 0.002 radian and does not accumulate. As a result, the sourc e generates sinusoidal waves of very narrow spectral peaks enabling their d etection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variati on was 10% in amplitude and 0.1 radian in phase of the signal observed at t he 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made o f reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration o f the foundation was measured and modeled assuming a rigid body movement. T he correction successfully reduced the daily variation by approximately 90% , resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 mus and less than 0.1% i n velocity over 1000 m between the source and the receiver.