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[SSS07-P21] Characteristics of source process on volcano tectonic earthquakes during the earthquake swarm off the east of Izu Peninsula in 2006 and 2009
Keywords:East off Izu Peninsula, Source process, earthquake swarm
1. Introduction
Earthquake swarms have been occurring off the east of Izu Peninsula intermittently. These swarms are volcano tectonic earthquakes associated with the intrusion of volcanic fluids. In this study, we estimated source process of four (Mw3.9-5.6) and two (Mw4.8, 4.9) earthquakes in 2006 and 2009, respectively, by using Empirical Green’s Functions (EGFs).
2. Data and Method
We used waveforms observed at K-net stations. Waveforms of smaller earthquakes (Mw3.4-3.9) in the same swarm sequence were used as EGFs. All of the analyzed and EGF earthquakes had strike-slip focal mechanisms. We carried out the waveform inversion with 40 time windows for two horizontal components of 10-s-long waveforms. Individual time windows had a duration of 0.2 s and a spacing of 0.05 s. We fixed the total seismic moments to those determined by National Research Institute for Earth Science and Disaster Resilience of Japan (NIED) and investigated temporal characteristics in the moment rate functions.
3. Results and Discussion
Figure 1 shows the source time functions of the analyzed six earthquakes. Hereinafter, we call analyzed earthquakes with the origin year and A-D in order of size, such as 06-A and 09-B. 06-A (Mw5.6) initiated a large rupture followed by continuous smaller ruptures. Both 06-B (Mw4.5) and 06-C (Mw4.3) had two subevents and they indicated different characteristics. The two subevents had similar seismic moments in 06-B. On the other hand, the first subevent was five times larger than the second one in 06-C. 06-D (Mw3.9) consisted single moment release with a duration of 0.2 s. In 2009, 09-A (Mw4.9) had a large subevent and consecutive subevents. 09-B (Mw4.8) consisted of distinct two ruptures with a separation of 0.2 s approximately.
These characteristics of source process would be associated with locations of hypocenters. Earthquakes of 06-A and 06-B took place in an inactive region for 20 years, but 06-C was close to the area of the swarm activity in 1998, where many fresh fractures would be included. These facts implicate that earthquakes of 06-A and 06-B ruptured faults with a high shear strength, resulting in complex rupture characteristics, whereas the rupture of 06-C expanded smoothly in the area with a lower shear strength. In other words, different characteristics of source process would be caused by the existence of fractures due to a previous swarm activity.
Earthquake swarms have been occurring off the east of Izu Peninsula intermittently. These swarms are volcano tectonic earthquakes associated with the intrusion of volcanic fluids. In this study, we estimated source process of four (Mw3.9-5.6) and two (Mw4.8, 4.9) earthquakes in 2006 and 2009, respectively, by using Empirical Green’s Functions (EGFs).
2. Data and Method
We used waveforms observed at K-net stations. Waveforms of smaller earthquakes (Mw3.4-3.9) in the same swarm sequence were used as EGFs. All of the analyzed and EGF earthquakes had strike-slip focal mechanisms. We carried out the waveform inversion with 40 time windows for two horizontal components of 10-s-long waveforms. Individual time windows had a duration of 0.2 s and a spacing of 0.05 s. We fixed the total seismic moments to those determined by National Research Institute for Earth Science and Disaster Resilience of Japan (NIED) and investigated temporal characteristics in the moment rate functions.
3. Results and Discussion
Figure 1 shows the source time functions of the analyzed six earthquakes. Hereinafter, we call analyzed earthquakes with the origin year and A-D in order of size, such as 06-A and 09-B. 06-A (Mw5.6) initiated a large rupture followed by continuous smaller ruptures. Both 06-B (Mw4.5) and 06-C (Mw4.3) had two subevents and they indicated different characteristics. The two subevents had similar seismic moments in 06-B. On the other hand, the first subevent was five times larger than the second one in 06-C. 06-D (Mw3.9) consisted single moment release with a duration of 0.2 s. In 2009, 09-A (Mw4.9) had a large subevent and consecutive subevents. 09-B (Mw4.8) consisted of distinct two ruptures with a separation of 0.2 s approximately.
These characteristics of source process would be associated with locations of hypocenters. Earthquakes of 06-A and 06-B took place in an inactive region for 20 years, but 06-C was close to the area of the swarm activity in 1998, where many fresh fractures would be included. These facts implicate that earthquakes of 06-A and 06-B ruptured faults with a high shear strength, resulting in complex rupture characteristics, whereas the rupture of 06-C expanded smoothly in the area with a lower shear strength. In other words, different characteristics of source process would be caused by the existence of fractures due to a previous swarm activity.