Mechanism of Explosive Eruptions from Moment Tensor Analyses of Explosion Earthquakes at Sakurajima Volcano, Japan
Mechanism of Explosive Eruptions from Moment Tensor Analyses of Explosion Earthquakes at Sakurajima Volcano, Japan
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日本樱岛火山爆发地震矩张量分析的爆发机制
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
K. Ishihara
中科院分区：
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作者：
Takeshi Tameguri;M. Iguchi;K. Ishihara
本文研究了安山岩型樱岛火山爆炸喷发伴随的爆炸地震的源过程，以阐明爆炸喷发的机理。爆炸地震的波形是由纵波第一运动（P相）后的振幅较大的扩张运动（D相）和振幅最大、周期较长的2 s运动（LP相）组成的，它出现在纵波第一运动到达后的2 ~ 3 s。对粒子运动、波衰减和传播速度的分析表明，P相和D相由纵波组成，LP相由瑞利波组成。这些相位的矩张量用波形反演法估计。爆炸地震的波形由2公里深处的2个震源和火山口下方的2个震源模拟。产生P相的第一个源的矩张量的三个对角线分量在所有事件中都具有相似的正值，范围在1.1 × 10 11到8.1 × 10 11 Nm之间。与对角分量相比，非对角分量小得可以忽略不计。D相对应的第二个源是一个垂直偶极子为负值的收缩源，其垂直偶极子的值在- 0.4 × 10 12 ~ - 6.1 × 10 12 Nm之间。水平偶极子约为10 - 12 Nm，大约是垂直偶极子的两倍，范围从-0.9 × 10 - 12到-11.8 × 10 - 12 Nm。这些结果表明，P相是由各向同性膨胀产生的，而D相的来源近似于陨石坑下2公里处圆柱体的收缩。柱状收缩震源的地震矩比各向同性膨胀震源大5 ~ 20倍，柱状收缩震源的起始时间比各向同性膨胀的起始时间延迟0.2 ~ 0.5 s。低相位是由火山口底部0.25-0.5公里深处的各向同性膨胀和随后的水平收缩引起的。浅层各向同性膨胀震源和水平收缩震源的地震矩范围分别为2.5 × 10 11 ~ 12.8 × 10 11 Nm和-2.9 × 10 11 ~ - 13.8 × 10 11 Nm。浅层各向同性膨胀的起始时间比爆炸地震的发生延迟了0.9 ~ 1.1 s，与火山口底部空气冲击的产生时间一致。浅层各向同性膨胀源的地震矩也与空气冲击的振幅相关。浅层各向同性膨胀和水平收缩的震源深度与火山导管最上部气穴爆发引起的快速收缩的震源深度一致。推测各向同性膨胀是由火山导管最上部形成的气穴瞬间体积增大引起的，气穴产生气震。水平收缩反映了气穴塌陷造成的压力下降所导致的体积减小。
The source processes of explosion earthquakes that accompany explosive eruptions at an andesitic volcano Sakurajima, are investigated to clarify the mechanics of explosive eruption. Waveforms of explosion earthquakes are composed of a compressional P-wave first motion (P phase), following a dilatational motion with larger amplitude (D phase) and the largest amplitude motion, with longer period of 2 s (LP phase), which appears 2 to 3 s after the arrival of the P-wave first motion. Analyses of the particle motion, wave attenuation, and propagation velocity show that the P and D phases are composed of P-waves and the LP phase is composed of Rayleigh waves. Moment tensors for these phases are estimated by a waveform inversion method. The waveforms of explosion earthquakes are simulated by 2 sources at depths of about 2 km and 2 sources just beneath the crater. The three diagonal components of the moment tensors of the 1st source generating the P phase have similar positive values for all the events and ranging from 1.1 × 10 11 to 8.1 × 10 11 Nm. The non-diagonal components are negligibly small compared with the diagonal components. The 2nd source corresponding to the D phase is a contraction source with vertical dipoles of negative values ranging from - 0.4 × 10 12 to - 6.1 × 10 12 Nm. The horizontal dipoles are about 10 12 Nm and about twice as large as the vertical dipoles, ranging from -0.9 × 10 12 to -11.8 × 10 12 Nm. These results show that the P phase is generated by an isotropic expansion and the source of the D phase is approximated by a contraction of a cylinder at a depth of 2 km beneath the crater. The seismic moment of the cylindrical contraction is 5 to 20 times larger than that of the isotropic expansion, and the origin time of cylindrical contraction source is delayed by 0.2 to 0.5 s from the onset of the isotropic expansion. The LP phase is excited by an isotropic expansion and subsequent horizontal contraction at depths of 0.25-0.5 km beneath the crater bottom. The seismic moments of the shallow isotropic expansion and horizontal contraction sources range from 2.5 × 10 11 to 12.8 × 10 11 Nm and -2.9 × 10 11 to - 13.8 × 10 11 Nm, respectively. Origin times of the shallow isotropic expansions are delayed by 0.9-1.1 s from the occurrence of explosion earthquakes and coincide with the generation time of the air-shock at the crater bottom. Seismic moments of the shallow isotropic expansion sources are also correlated with the amplitudes of the air-shocks. The source depths of the shallow isotropic expansion and horizontal contraction coincide with that of the rapid deflations that are caused by an outbreak of gas pocket at the uppermost part of the volcanic conduit. It is inferred that the isotropic expansion is caused by instantaneous volume increase of the gas pocket formed at the uppermost region of the volcanic conduit which generates the air-shock. The horizontal contraction reflects decrease in volume caused by pressure decrease due to collapse of the gas pocket.