5:15 PM - 6:45 PM
[SVC25-P07] Hazard evaluation of ballistic projectiles from Vulcanian eruptions with infrasound observation: consideration for the pressure integral and velocity relation
Keywords:Volcanic ballistic projectiles, Vulcanian eruptions
Volcanic ballistic projectiles (VBPs) are a common hazard factor at active volcanoes worldwide. The maximum reaching distance of fragments from the vent defines the hazard area of VBPs. Hence, evaluating the maximum distance is an essential challenge for geophysical observations. Since larger fragments are less affected by drag in the air, the launching velocity at the vent is a dominant factor for the maximum distance of VBPs. Based on the equation of motion, the launching velocity is related to the time integral of the pressure term (pressure integral). Vulcanian explosions are a typical eruption style with VBPs and accompanying impulsive pressure waves originating from the failure of the gas pocket beneath the crater bottom. The present study evaluates the launching velocity of VBPs with infrasound observation. To achieve a generalized method, we focus on Vulcanian eruptions at different volcanoes (Minamidake and Showa craters at Sakurajima and Suwanosejima).
We adopt the maximum distance data of VBPs reported by JMA with visual observations, except for one event in which the lava fragments had reached 3.3 km from the Minamidake vent at Sakurajima (June 4, 2020). Following the equation of motion of VBPs (Wilson, 1972), we convert from the reaching distance to the launching velocity at the vent. Several assumptions are adopted for this conversion, such as the launching angle of 63° (Iguchi et al., 1983), the drag coefficient of 0.6, and representative fragment size (2 m for Sakurajima and 1 m Suwanosejima). We analyze continuous infrasound data recorded by infrasound microphones (SI 104, Hakusan) at Sakurajima and Suwanosejima volcanoes by DPRI, Kyoto University.
One of the essential features of observed infrasound waveforms is that both the peak amplitude and pulse width are correlated with the initial vertical velocity of VBPs, Vmax (m/s). This feature suggests the overpressure P at the gas pocket and the volatile emission amount are critical for Vmax. We set the peak amplitude of the integrated infrasound waveform Imax as a proxy. To evaluate the distribution of Vmax and Imax (Figure a), we examine the relation of Vmax and the pressure integral based on the equation of motion with a 1-D model (Alatorre-Ibargüengoitia et al., 2010).
The primal input parameters of the model by Alatorre-Ibargüengoitia et al. (2010) are overpressure P (Pa), porosity φ at the gas pocket, and explosion depth d (m). Following the previous studies and the strength of volcanic rocks, the maximum range of P is assumed to be 25 MPa. We set the range zth that the fragments move in the framework of the 1-D model as zth = zth0 + A(ma), where zth0 is 2 m, A is a coefficient (2.0×10-5) to explain the observed Vmax and Imax range, m (kg) is a block weight, and a is the acceleration of the block when going through zth0. The largest value of zth in our considerations is 270 m, almost equivalent to the Minamidake crater depth at Sakurajima.
Figure a shows the relations of Vmax and the pressure integral with different d values. Based on the model, observed Imax and Vmax distributions imply an apparent correlation between Vmax, P, and d. Since heavier fragments need more time to accelerate, the pressure integral correlated with d, which links to the block weight. We plot a representative relation of the upper limit of Vmax as Vmax=31Imax0.14 with a solid line in Figure a. According to the model, the Vmax-Imax region near the upper limit follows a relation of Imax=1.4×10-8P2.01. The above discussion implies a relation of Vmax=2.47P0.28 behind the upper limit line. To evaluate the nature of the P-Vmax relation, we refer to the model by Wilson (1980), which considers the relations between P and Vmax. Figure b shows that our P-Vmax relation has characteristics of P and volume fraction of volatiles n correlated with Vmax. This result supports our P and volatile discharge amount hypothesis on Vmax from infrasound observation.
We adopt the maximum distance data of VBPs reported by JMA with visual observations, except for one event in which the lava fragments had reached 3.3 km from the Minamidake vent at Sakurajima (June 4, 2020). Following the equation of motion of VBPs (Wilson, 1972), we convert from the reaching distance to the launching velocity at the vent. Several assumptions are adopted for this conversion, such as the launching angle of 63° (Iguchi et al., 1983), the drag coefficient of 0.6, and representative fragment size (2 m for Sakurajima and 1 m Suwanosejima). We analyze continuous infrasound data recorded by infrasound microphones (SI 104, Hakusan) at Sakurajima and Suwanosejima volcanoes by DPRI, Kyoto University.
One of the essential features of observed infrasound waveforms is that both the peak amplitude and pulse width are correlated with the initial vertical velocity of VBPs, Vmax (m/s). This feature suggests the overpressure P at the gas pocket and the volatile emission amount are critical for Vmax. We set the peak amplitude of the integrated infrasound waveform Imax as a proxy. To evaluate the distribution of Vmax and Imax (Figure a), we examine the relation of Vmax and the pressure integral based on the equation of motion with a 1-D model (Alatorre-Ibargüengoitia et al., 2010).
The primal input parameters of the model by Alatorre-Ibargüengoitia et al. (2010) are overpressure P (Pa), porosity φ at the gas pocket, and explosion depth d (m). Following the previous studies and the strength of volcanic rocks, the maximum range of P is assumed to be 25 MPa. We set the range zth that the fragments move in the framework of the 1-D model as zth = zth0 + A(ma), where zth0 is 2 m, A is a coefficient (2.0×10-5) to explain the observed Vmax and Imax range, m (kg) is a block weight, and a is the acceleration of the block when going through zth0. The largest value of zth in our considerations is 270 m, almost equivalent to the Minamidake crater depth at Sakurajima.
Figure a shows the relations of Vmax and the pressure integral with different d values. Based on the model, observed Imax and Vmax distributions imply an apparent correlation between Vmax, P, and d. Since heavier fragments need more time to accelerate, the pressure integral correlated with d, which links to the block weight. We plot a representative relation of the upper limit of Vmax as Vmax=31Imax0.14 with a solid line in Figure a. According to the model, the Vmax-Imax region near the upper limit follows a relation of Imax=1.4×10-8P2.01. The above discussion implies a relation of Vmax=2.47P0.28 behind the upper limit line. To evaluate the nature of the P-Vmax relation, we refer to the model by Wilson (1980), which considers the relations between P and Vmax. Figure b shows that our P-Vmax relation has characteristics of P and volume fraction of volatiles n correlated with Vmax. This result supports our P and volatile discharge amount hypothesis on Vmax from infrasound observation.