Research ArticleGEOPHYSICS

Spatial and temporal seismic velocity changes on Kyushu Island during the 2016 Kumamoto earthquake

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Science Advances  24 Nov 2017:
Vol. 3, no. 11, e1700813
DOI: 10.1126/sciadv.1700813
  • Fig. 1 Map of central Kyushu Island, Japan, with locations of Hi-net stations (yellow dots).

    Hypocenters of large earthquakes and aftershocks associated with the 2016 Kumamoto earthquake are shown by green stars and white dots, respectively. The Aso caldera is outlined by a dashed orange line. The black lines connect the seismometer pairs shown in Fig. 2 and figs. S3 and S4. The base map is a 10-m mesh digital elevation model published by the Geospatial Information Authority of Japan. We drew this figure with Generic Mapping Tools (53).

  • Fig. 2 Temporal variation of seismic traces and velocity between three seismometer pairs.

    (A to C) Typical seismic traces (cross-correlations) between seismometer pairs: (A) across the fault plane from A to A′ in Fig. 1, (B) far from the fault (from B to B′ in Fig. 1), and (C) across Mount Aso (magmatic body; from C to C′ in Fig. 1). The vertical axis shows travel time in seconds, and the horizontal axis shows dates from December 2015 to November 2016. The red arrows indicate the date of the mainshock and volcanic eruption (16 April), and the yellow arrows indicate the date of the large eruption of Aso volcano (7 and 8 October). (D to F) Typical seismic velocity variation between station pairs derived from the seismic traces displayed in (A) to (C). Background color indicates the cross-correlation coefficient obtained by trace stretching; black curves show daily variations of the estimated velocity changes Embedded Image with respect to changes before the Kumamoto earthquake, defined at the maximum value of the coefficient; and dashed black curves indicate the SD of the velocity change estimation (see Materials and Methods). White dashed lines show the time window (30 days) influenced by the mainshock and the largest eruption.

  • Fig. 3 Depth-dependent S-wave sensitivity kernels (partial derivatives) for fundamental mode Rayleigh waves with respect to S-wave velocity, which constitute a proxy for depth resolution.

    The sensitivity kernels were computed by using the DISPER 80 program (51) for a one-dimensional (1D) layered model near Aso volcano estimated by Nishida et al. (52). Sensitivity kernels were normalized by the maximum amplitude at 0.8 Hz.

  • Fig. 4 Spatial and temporal variation of seismic velocity in central Kyushu during the 2016 Kumamoto earthquake.

    The surface wave velocity changes Embedded Image within each time window (30 days) relative to the averaged pre-earthquake value are displayed (see Materials and Methods). Each panel shows the central date within the 30-day window: (A) 8 March 2016, (B) 1 May 2016, (C) 14 May 2016, (D) 1 June 2016, (E) 1 August 2016, and (F) 1 October 2016. Warm colors indicate regions where seismic velocity was decreased. White dots are Hi-net stations.

  • Fig. 5 Frequency dependence of temporal variations of seismic velocity between two typical seismometer pairs.

    (A) Across the fault plane from A to A′ in Fig. 1. (B) Across Mount Aso from C to C′ in Fig. 1. Curves show daily variations of the estimated velocity change Embedded Image with respect to changes before the Kumamoto earthquake. Vertical dashed lines show the time window (30 days) influenced by the mainshock. The frequency-dependent velocity variation is likely associated with surface wave dispersion (37), supporting our assumption that the coda waves were dominated by surface waves.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/11/e1700813/DC1

    fig. S1. Temporal variation of seismic velocity between each receiver pair in relation to earthquake activity.

    fig. S2. Spatial and temporal variation of seismic velocity in central Kyushu during the 2016 Kumamoto earthquake.

    fig. S3. Asymmetry of cross-correlation between station D and D′ (1-year stack) due to tremors from Aso volcano.

    fig. S4. Comparison of temporal changes of seismic velocity derived from the stretching interpolation and the MWCS.

    movie S1. Animation showing spatial and temporal variations of seismic velocity on Kyushu Island during the 2016 Kumamoto earthquake.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Temporal variation of seismic velocity between each receiver pair in relation to earthquake activity.
    • fig. S2. Spatial and temporal variation of seismic velocity in central Kyushu during the 2016 Kumamoto earthquake.
    • fig. S3. Asymmetry of cross-correlation between station D and D′ (1-year stack) due to tremors from Aso volcano.
    • fig. S4. Comparison of temporal changes of seismic velocity derived from the stretching interpolation and the MWCS.
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Animation showing spatial and temporal variations of seismic velocity on Kyushu Island during the 2016 Kumamoto earthquake.

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