Dynamic Triggering Semi-Volcanic Tremor in Japanese Volcanic Region by The 2016 Mw 7.0 Kumamoto Earthquake

Transcription

1 Dynamic Triggering Semi-Volcanic Tremor in Japanese Volcanic Region by The 016 Mw 7.0 Kumamoto Earthquake Heng-Yi Su 1 *, Aitaro Kato 1 Department of Earth Sciences, National Central University, Taoyuan City, Taiwan, R.O.C Earthquake Research Institute, University of Tokyo, Tokyo, Japan Abstract Semi-Volcanic Tremor is a non-impulsive deep seismic signal detected in the volcanic region. It has been identified by previous studies that the 004 Sumatra Earthquake and the 011 Tohoku-Oki Earthquake triggered the semi-volcanic tremors in the Hokkaido. These triggered tremors are roughly located at epicenters of ambient tremors Therefore, we downloaded the seismic data from Hi-net and V-net and examined whether the 016 Kumamoto earthquake triggered the semi-volcanic tremor. After data analysis, we detected a burst of tremors triggered during the passage of surface waves at the central Hokkaido, which is slightly western and deeper than the location determined by the previous study. Furthermore, the triggered tremor is more relative to the Love wave and we infer the temporal shear stress loading caused by Love wave deformation prompted fractures of some pre-existing faults those might be weakened by infiltration of fluids expelled from the underneath magmatic body. Repetitive stress loading leaded to an excitation of a burst of low-frequency earthquakes. 1. Introduction Recently, the research of the triggered tremor is a good way to help us to better understand the physical mechanism of the earthquake. Knowing the mechanism between earthquakes can help us to improve the early warning system and the seismic hazard analysis. The dynamic triggering earthquake is well known in the volcanic and geothermal regions, which have the high background earthquake activities or the fault on the boundary of the plates (Hill et al., 1993). Recently, some studies found that we can still detect the signal of triggered earthquake in the relatively stable intraplate region (Gomberg et al., 004). Moreover, long-period non-volcanic tremor was found and located along the subduction zone in southwest Japan (Obara, 00). We can also identify the similar non-volcanic signal in the volcanic region (Obara, 01; Chao and Obara, 016). Based on the previous study, there are some triggered regular earthquakes have been detected after the Kumamoto 1

2 Figure 1. The map of Japan. The High Sensitivity Seismograph Network (Hi-net) and the V-net operated by the National Research Institute for Earth Science and Disaster Prevention NIED of Japan are shown as black crosses and blue circles. The red triangles represent the active volcano near the V-net. The inset shows an enlarged view of the rectangle with epicenter and focal mechanism of the 016 Kumamoto earthquake. earthquake and we can find that there are some high frequency components in the waveform from the study (Enescu et al., 016). Therefore, we are going to investigate that whether we can identify the triggered tremor by the Kumamoto earthquake or not and explain the mechanism of the triggered tremor if we find the semi-volcanic tremor. Furthermore, people inferred the triggered events are more correlated to Rayleigh wave in western of japan (Miyazawa et al., 008). However, the tremor found in Taiwan was triggered by Love wave (Peng and Chao, 008). Therefore, we will examine the correlation between the triggered tremor we found in Hokkaido and the surface wave.. Observation In this study, we used the seismic data recorded by the V-Net, and by the High Sensitivity Seismograph Network Hi-Net which

3 Figure. We can see the clearly Love wave on transverse component. After applying the filter, the tremor triggering by the Love wave is clearly visible during the 800 seconds to 100 seconds. The bottom picture is to plot the frequency component of the transverse wave with 0.5 Hz low-pass filters. are both operated by the National Research Institute for Earth Science and Disaster Prevention NIED of Japan. We have processed the seismic data from the short period seismometer of V-Net. To identify triggered tremor, we utilize the Butterworth band-pass filter in the frequency range of to 8 Hz, which is the dominant frequency components of triggered tremor, and search for the bursts of non-impulsive seismic signal. In order to correlate the triggered tremor with the surface waves, we first removed the instrument response, then using the filter under the frequency range of 0.01 to 0. Hz to the broadband seismic data. The triggered tremor is often well correlated to the surface waves passing. That is to say that after we remove the dominant frequency component of the surface waves, we can see the low-frequency earthquake phenomena afterwards. Therefore, we might see the peak of the triggered tremor that is relative to the phase of surface waves. Figure shows the surface wave compared with the seismogram filtered by band-pass filter in to 8 Hz recorded by the V-net station TKOV. In the spectrogram we can clearly see the frequency components of triggered tremor during the 800s to 100s, of which amplitudes extend up to around 10 Hz. 3. Tremor Location After detecting the triggered tremor, we picked the S wave arrival time of each burst 3

4 Figure 3. Observations of triggered tremor and ambient tremors in the volcanic region of central Hokkaido. The green circle is the triggered tremor detected in this study. The white circle is the triggered tremor detected in the previous study. The diamonds are the location of different bursts in the tremor. The yellow circles are the ambient low-frequency earthquakes determined by the JMA. The red triangle is the active volcano. The inverted triangles are the V-net station and the purple inverted triangles are the station we used to locate the triggered tremor. The crosses are the stations of Hi-net and the red crosses are the station we used to locate the triggered tremor. Color bar shows the time sequences of different bursts of earthquake. of the low-frequency earthquake and utilized the different travel time between different seismic stations to locate the triggered event. Figure 3 shows triggered tremor hypocentral distribution and the error of depth is within 10 km. The result was calculated from five Hi-net stations and two V-net stations. The triggered tremor source is located at E, N and 6 km depth. Comparing with previous study (Chao and Obara, 016), ours is deeper. Also, we shift the bursts of the triggered tremor back to the source region. We use the record of stations TKOV and shift the time with 10.7s, which is computed by Taup (Crotwell et al., 1999). In Figure 4, we want to determine the relationship between the surface wave and triggered tremor. If the triggered tremor is sensitive to the change of the stress field, it will be well correlate to each surface wave phase. 4

5 HY. SU et al.: Dynamic Triggering Semi-Volcanic Tremor in Japanese Volcanic Region 4 N.TKOV.BR.sac.new X N.TKOV.BT.sac.new X Bandpass-8Hz.new 1 X X 10+ Figure 4. Correlating the Rayleigh wave and Love wave with triggered tremor. The upper and middle waveforms are under 0.01 to 0. Hz filter and represent Rayleigh wave and Love wave, respectively. The lower waveform is filter by band-pass to 8 Hz and we can clearly see the tremor signal. We shifted each burst of low-frequency earthquake back to the tremor source region and correlated the peak of both Rayleigh wave and Love wave to triggered tremor. Figure 5. Depth section of the P-wave velocity structure superimposed with triggered and ambient tremors. A low-velocity body is imaged beneath these tremors. 5

6 Here the Love wave is more in phase with tremor bursts and the amplitude of Love wave is also larger than Rayleigh wave, suggesting that the tremor might be triggered by the Love wave. 4. Discussions and Conclusion We verify the triggered tremor source in the region of the deep volcanic low-frequency earthquake in central Hokkaido (Obara, 01; Chao and Obara, 016). Nevertheless, the depth of the triggered tremor source in this study is greater than previous one (Chao and Obara, 016). Also the average depth of the triggered tremor is greater than most of the ambient tremors. Yet, the determination of depth still need more evidences (different method) to support it. The deep low-frequency earthquakes occur near the low-velocity zone (Hasegawa et al., 009; Obara, 01). In this study, our research focuses on the volcanic region, and the low-frequency tremor we detected might be adjacent to the magma body. In addition, the tremor is more correlative to the Love wave. This correlation result indicates that the temporal shear stress loading caused by Love wave deformation prompted fractures of some pre-existing faults those might be weakened by infiltration of fluids expelled from the underneath magmatic body. Repetitive stress loading leaded to an excitation of a burst of low-frequency earthquakes. Figure 5 shows that there is a low-velocity zone beneath the ambient tremors and the triggered tremors, which is more likely a magmatic body or other fluid body. 5. Acknowledgement We thank the NIED provides the great and high quality seismic network data and the JMA provides the seismic catalog. The figures in this study were created by using the Generic Mapping Tools, SAC and MATLAB. The data processing was done in the Earthquake Research Institute and supervises by Prof. Aitaro Kato. Also, I appreciate to the help from Dr. Kevin Chao when he was in his short-term visiting in ERI. 6. Reference 1. Chao, K., & Obara, K. (016). Triggered tectonic tremor in various types of fault systems of Japan following the 01 Mw8. 6 Sumatra earthquake. Journal of Geophysical Research: Solid Earth, 11(1), Crotwell, H. P., T. J. Owens, and J. Ritsema (1999). The TauP Toolkit: Flexible seismic travel-time and ray-path utilities, Seismological Research Letters 70, Enescu, B., Shimojo, K., Opris, A., & Yagi, Y. (016). Remote triggering of seismicity at Japanese volcanoes following the 016 M7. 3 Kumamoto earthquake. Earth, Planets and Space, 68(1), Gomberg, J., Bodin, P., Larson, K., & Dragert, H. (004). Earthquakes nucleated by transient deformations a fundamental process evident in observations surrounding 6

7 the M7. 9 Denali Fault earthquake. Nature, 47, Hasegawa, A., Nakajima, J., Uchida, N., Okada, T., Zhao, D., Matsuzawa, T., & Umino, N. (009). Plate subduction, and generation of earthquakes and magmas in Japan as inferred from seismic observations: an overview. Gondwana Research, 16(3), Hill, D. P., Reasenberg, P. A., Michael, A., Arabaz, W. J., Beroza, G., Brumbaugh, D.,... & Ellsworth, W. L. (1993). Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake. Science, Miyazawa, M., & Brodsky, E. E. (008). Deep low frequency tremor that correlates 8. Peng, Z., & Chao, K. (008). Non-volcanic tremor beneath the Central Range in Taiwan triggered by the 001 M w 7.8 Kunlun earthquake. Geophysical Journal International, 175(), Obara, K. (00). Nonvolcanic deep tremor associated with subduction in southwest Japan. Science, 96(5573), Obara, K. (01). New detection of tremor triggered in Hokkaido, northern Japan by the 004 Sumatra Andaman earthquake. Geophysical Research Letters, 39(0). with passing surface waves. Journal of Geophysical Research: Solid Earth, 113(B1).. 7