5:15 PM - 6:30 PM
[SVC28-P08] Banded Tremors Activity at Miyakejima Volcano
Keywords:Miyakejima Volcano, Banded Tremors
Introduction
Since the 2000 eruption, volcanic tremors (banded tremors) that occur intermittently in 2000-2006, 2011 (September-October) and 2015 (October-December) have been observed on Miyakejima Volcano. Banded tremors are characterized by volcanic tremors with a maximum duration of about 20 minutes and a predominant cycle of about 0.2 seconds, which occur repeatedly at intervals of about 30 minutes (some events exceed 1 hour). The maximum amplitude (maximum during in the Oyamahokutou) was 16 μm / s from 2000 to 2006, 30 μm / s in 2011, and 62 μm / s in 2015. We considered the relationship between the changes in the location of banded tremors since 2000 and volcanic activity.
Discussion
Otsuka et al. (2012) estimate that the location of banded tremors from 2000 to 2011 is about 2 km near the crater. Similarly, when estimating the location of occurrence in 2015, the depth near the crater was about 1 km. It is possible that these occur in shallower areas than before 2011 (Fig.1). Slightly low-frequency earthquakes (BH-type earthquakes) tend to be shallower than 2 km in 2004, shallower than 1.5 km in 2011, and shallower than 1 km in 2015. Low-frequency earthquakes (BL-type earthquakes) have a small number of events, but are often found shallower than 1.5 km.
In order to consider the occurrence of banded tremors, we divided the transition of volcanic activity into three periods from seismic activity and crustal deformation after the 2000 eruption. The period (1) "2000-2007", the banded tremors that had been observed since the start of the eruption disappeared after April 2006 as the thermal activity decreased. However, from around this time, the pressure source in the deep part turned to inflation. The period (2) "2008-2012", eruptions accompanied by BH-type and BL-type earthquakes were repeated until July 2010. The inflation of the deep part has continued since around 2006, but the deflation of the shallow part has been stagnant since about October 2010, and banded tremors have been observed since then. The period (3) "2013-present", deep inflation continues. The stagnation in the shallow part changed to inflation from around September 2014, but banded tremors occurred during the temporary stagnation from around May 2015 (Report of Coordinating Comittee for Prediction of Volcanic Eruptions, 2018). After the banded tremors subsided in the first half of 2016, crustal deformation showing rapid inflation were observed. After that, volcanic gas ejection event with tilt change occurred in May 2016 (Nagato et al., 2017).
Since all banded tremors occur during the period when the temperature inside the crater and the volcanic fume index (Q) are high, it is considered to be related to thermal activity (Fig.2). The epicenter of the BH-type earthquakes became shallow from around 2013. The GNSS baseline associated with the shallow pressure source began to shift from deflation to stagnation and then inflation from around 2013. From these facts, it is possible that the location of thermal activity, which is the cause of banded tremors, has moved in the shallow direction due to seismic activity and movement of the pressure source. In addition, the period (1) is the period when volcanic activity such as deflation of the pressure source is low, the period (2) is the period when the deflation of the shallow part is temporarily stagnated, and the period (3) is the period when the gradual inflation of the shallow part is temporary. After stagnation, banded tremors occur. From these facts, it is possible that the supply balance of volcanic fluid underground changes. On the other hand, in the 1962 and 1983 eruptions, continuous tremors occurred for several days, but no records of volcanic tremors have been observed since then. From this, banded tremors are considered to be a peculiar phenomenon of the 2000 eruption.
Since the 2000 eruption, volcanic tremors (banded tremors) that occur intermittently in 2000-2006, 2011 (September-October) and 2015 (October-December) have been observed on Miyakejima Volcano. Banded tremors are characterized by volcanic tremors with a maximum duration of about 20 minutes and a predominant cycle of about 0.2 seconds, which occur repeatedly at intervals of about 30 minutes (some events exceed 1 hour). The maximum amplitude (maximum during in the Oyamahokutou) was 16 μm / s from 2000 to 2006, 30 μm / s in 2011, and 62 μm / s in 2015. We considered the relationship between the changes in the location of banded tremors since 2000 and volcanic activity.
Discussion
Otsuka et al. (2012) estimate that the location of banded tremors from 2000 to 2011 is about 2 km near the crater. Similarly, when estimating the location of occurrence in 2015, the depth near the crater was about 1 km. It is possible that these occur in shallower areas than before 2011 (Fig.1). Slightly low-frequency earthquakes (BH-type earthquakes) tend to be shallower than 2 km in 2004, shallower than 1.5 km in 2011, and shallower than 1 km in 2015. Low-frequency earthquakes (BL-type earthquakes) have a small number of events, but are often found shallower than 1.5 km.
In order to consider the occurrence of banded tremors, we divided the transition of volcanic activity into three periods from seismic activity and crustal deformation after the 2000 eruption. The period (1) "2000-2007", the banded tremors that had been observed since the start of the eruption disappeared after April 2006 as the thermal activity decreased. However, from around this time, the pressure source in the deep part turned to inflation. The period (2) "2008-2012", eruptions accompanied by BH-type and BL-type earthquakes were repeated until July 2010. The inflation of the deep part has continued since around 2006, but the deflation of the shallow part has been stagnant since about October 2010, and banded tremors have been observed since then. The period (3) "2013-present", deep inflation continues. The stagnation in the shallow part changed to inflation from around September 2014, but banded tremors occurred during the temporary stagnation from around May 2015 (Report of Coordinating Comittee for Prediction of Volcanic Eruptions, 2018). After the banded tremors subsided in the first half of 2016, crustal deformation showing rapid inflation were observed. After that, volcanic gas ejection event with tilt change occurred in May 2016 (Nagato et al., 2017).
Since all banded tremors occur during the period when the temperature inside the crater and the volcanic fume index (Q) are high, it is considered to be related to thermal activity (Fig.2). The epicenter of the BH-type earthquakes became shallow from around 2013. The GNSS baseline associated with the shallow pressure source began to shift from deflation to stagnation and then inflation from around 2013. From these facts, it is possible that the location of thermal activity, which is the cause of banded tremors, has moved in the shallow direction due to seismic activity and movement of the pressure source. In addition, the period (1) is the period when volcanic activity such as deflation of the pressure source is low, the period (2) is the period when the deflation of the shallow part is temporarily stagnated, and the period (3) is the period when the gradual inflation of the shallow part is temporary. After stagnation, banded tremors occur. From these facts, it is possible that the supply balance of volcanic fluid underground changes. On the other hand, in the 1962 and 1983 eruptions, continuous tremors occurred for several days, but no records of volcanic tremors have been observed since then. From this, banded tremors are considered to be a peculiar phenomenon of the 2000 eruption.