Strong attenuation of shear waves in the focal region of the 1997 northwestern Kagoshima earthquakes, Japan

Yutaka Mamada, Hiroshi Takenaka

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13 Citations (Scopus)

Abstract

In 1997, two moderate earthquakes (MJMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (QS-1) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of QS-1 and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated QS-1's have significantly larger values compared to other previous studies in the crust. From our results, QS-1 can be approximately represented as a function of frequencyf by QS-1 (f) = (9.93 × 10-2)f-0.95. In order to check the significance of the large values of QS-1 in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated QS-1 outside the focal region can be approximately represented as a function of frequency by QS-1(f) = (1.03 × 10 2)f-0.59. This is similar to the crustal QS-1 in the surrounding area obtained by Kato (1999), suggesting that the values of QS-1 in the focal region are correctly estimated. Comparison of the values of QS-1 demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.

Original languageEnglish
Pages (from-to)464-478
Number of pages15
JournalBulletin of the Seismological Society of America
Volume94
Issue number2
Publication statusPublished - Apr 2004
Externally publishedYes

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Shear waves
S waves
S-wave
Japan
Earthquakes
earthquakes
attenuation
earthquake
Frequency bands
aftershock
crusts
stations
coda
radiation
rupture
crust

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

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title = "Strong attenuation of shear waves in the focal region of the 1997 northwestern Kagoshima earthquakes, Japan",
abstract = "In 1997, two moderate earthquakes (MJMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (QS-1) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of QS-1 and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated QS-1's have significantly larger values compared to other previous studies in the crust. From our results, QS-1 can be approximately represented as a function of frequencyf by QS-1 (f) = (9.93 × 10-2)f-0.95. In order to check the significance of the large values of QS-1 in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated QS-1 outside the focal region can be approximately represented as a function of frequency by QS-1(f) = (1.03 × 10 2)f-0.59. This is similar to the crustal QS-1 in the surrounding area obtained by Kato (1999), suggesting that the values of QS-1 in the focal region are correctly estimated. Comparison of the values of QS-1 demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.",
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AU - Mamada, Yutaka

AU - Takenaka, Hiroshi

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N2 - In 1997, two moderate earthquakes (MJMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (QS-1) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of QS-1 and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated QS-1's have significantly larger values compared to other previous studies in the crust. From our results, QS-1 can be approximately represented as a function of frequencyf by QS-1 (f) = (9.93 × 10-2)f-0.95. In order to check the significance of the large values of QS-1 in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated QS-1 outside the focal region can be approximately represented as a function of frequency by QS-1(f) = (1.03 × 10 2)f-0.59. This is similar to the crustal QS-1 in the surrounding area obtained by Kato (1999), suggesting that the values of QS-1 in the focal region are correctly estimated. Comparison of the values of QS-1 demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.

AB - In 1997, two moderate earthquakes (MJMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (QS-1) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of QS-1 and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated QS-1's have significantly larger values compared to other previous studies in the crust. From our results, QS-1 can be approximately represented as a function of frequencyf by QS-1 (f) = (9.93 × 10-2)f-0.95. In order to check the significance of the large values of QS-1 in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated QS-1 outside the focal region can be approximately represented as a function of frequency by QS-1(f) = (1.03 × 10 2)f-0.59. This is similar to the crustal QS-1 in the surrounding area obtained by Kato (1999), suggesting that the values of QS-1 in the focal region are correctly estimated. Comparison of the values of QS-1 demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.

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