17:15 〜 18:30
[SVC28-P24] Temporal Change in Coda-Q at the upper East Rift Zone of Kīlauea volcano, Island of Hawaiʻi
キーワード：キラウエア火山、コーダQ、散乱体密度
1. Introduction
Quality factor coda-Q, or Qc, is measured from the decay rates of coda-wave amplitudes for local earthquakes. The decay of the coda amplitude is independent of the distance and path between source and receiver (Aki, 1969) and can be used as an indicator of subsurface complexity, such as the density of cracks and other wave scatterers. Lower suggests higher density of cracks and scatterers.
Elevated volcanic activity at Kīlauea volcano has persisted for decades, with essentially continuous eruption in its middle East Rift Zone from the Pu‘u O‘o vent from 1983 to mid-2018. A vent within Halema‘uma‘u Crater in the Kīlauea summit region was formed in 2008, and a lava lake occupied the vent also until mid-2018. In May 2018, magma intruded into and erupted from Kīlauea’s lower East Rift Zone for much of the summer of 2018. Punctuating Kīlauea’s sustained eruptions from the vents at its summit and East Rift Zone were intervals when the summit lava lake formed, a fissure eruption between the summit and Pu‘u O‘o, and two overflows of the lava lake within Halema‘uma‘u. During such intervals, we presume that magma rising toward the surface or migrating through the volcano will activate and fill cracks and fractures in the shallow crust. This suggests that changes in Qc could be detected prior to visible changes in the eruptions.
In this study, we focused on the upper East Rift Zone, now being referred to as the east rift connector between the summit and rift zone, where occasional elevation of seismicity has been detected. We analyzed waveforms of 462 earthquakes (M ≧ 2.0) that occurred at the upper East Rift Zone from 2009 to 2015 (Figures 1a and 1b) and investigated temporal change in Qc.
2. Methods
We first applied band-pass filters with five different frequency bands for S-coda waves, which were 1-2, 2-4, 4-8, 8-16, and 16-32 Hz. We then calculated the root-mean-square amplitude Ac of a S-coda wave in a moving time window with a duration of 4/fc, where fc is the center frequency of each frequency band. From the single backscattering model (Aki and Chouet, 1975), Ac and Qc will obey the following equation, ln Ac + ln t = -π(fc/Qc)t + const., where t is the lapse time from the origin time of the earthquake. We finally estimated values of Qc for individual frequency bands and hypocenter-station pairs from waveforms of 462 earthquakes (M ≧ 2.0) and investigated temporal change in the values.
3. Results and Discussion
Figure 1c shows temporal change in Qc for the east-west component of the seismogram at a station (HV.PAU) for the five analyzed frequency bands. Figure 1d shows a temporally smoothed result in each month for a frequency band of 2 to 4 Hz. We observed no clear change associated with elevated seismicity in 2009, 2011, and 2015. On the other hand, a long-term decrease in Qc was detected at some stations and frequency bands. We interpret this as suggesting an increasing density of cracks and scatterers associated with a raised volcanic activity at the upper East Rift Zone.
Quality factor coda-Q, or Qc, is measured from the decay rates of coda-wave amplitudes for local earthquakes. The decay of the coda amplitude is independent of the distance and path between source and receiver (Aki, 1969) and can be used as an indicator of subsurface complexity, such as the density of cracks and other wave scatterers. Lower suggests higher density of cracks and scatterers.
Elevated volcanic activity at Kīlauea volcano has persisted for decades, with essentially continuous eruption in its middle East Rift Zone from the Pu‘u O‘o vent from 1983 to mid-2018. A vent within Halema‘uma‘u Crater in the Kīlauea summit region was formed in 2008, and a lava lake occupied the vent also until mid-2018. In May 2018, magma intruded into and erupted from Kīlauea’s lower East Rift Zone for much of the summer of 2018. Punctuating Kīlauea’s sustained eruptions from the vents at its summit and East Rift Zone were intervals when the summit lava lake formed, a fissure eruption between the summit and Pu‘u O‘o, and two overflows of the lava lake within Halema‘uma‘u. During such intervals, we presume that magma rising toward the surface or migrating through the volcano will activate and fill cracks and fractures in the shallow crust. This suggests that changes in Qc could be detected prior to visible changes in the eruptions.
In this study, we focused on the upper East Rift Zone, now being referred to as the east rift connector between the summit and rift zone, where occasional elevation of seismicity has been detected. We analyzed waveforms of 462 earthquakes (M ≧ 2.0) that occurred at the upper East Rift Zone from 2009 to 2015 (Figures 1a and 1b) and investigated temporal change in Qc.
2. Methods
We first applied band-pass filters with five different frequency bands for S-coda waves, which were 1-2, 2-4, 4-8, 8-16, and 16-32 Hz. We then calculated the root-mean-square amplitude Ac of a S-coda wave in a moving time window with a duration of 4/fc, where fc is the center frequency of each frequency band. From the single backscattering model (Aki and Chouet, 1975), Ac and Qc will obey the following equation, ln Ac + ln t = -π(fc/Qc)t + const., where t is the lapse time from the origin time of the earthquake. We finally estimated values of Qc for individual frequency bands and hypocenter-station pairs from waveforms of 462 earthquakes (M ≧ 2.0) and investigated temporal change in the values.
3. Results and Discussion
Figure 1c shows temporal change in Qc for the east-west component of the seismogram at a station (HV.PAU) for the five analyzed frequency bands. Figure 1d shows a temporally smoothed result in each month for a frequency band of 2 to 4 Hz. We observed no clear change associated with elevated seismicity in 2009, 2011, and 2015. On the other hand, a long-term decrease in Qc was detected at some stations and frequency bands. We interpret this as suggesting an increasing density of cracks and scatterers associated with a raised volcanic activity at the upper East Rift Zone.