5:15 PM - 6:30 PM
[SEM14-P03] Detection of magnetic field variation induced by tsunami
1. Introduction
When a tsunami moves in the geomagnetic field, an electromagnetic field is induced by the dynamo effect (tsunami induced electromagnetic field, hereafter TIEM). Tyler (2005) shows that the vertical component of TIM is approximately linearly correlated with sea level. TIM appears in the vertical component before tsunami arrive (Tatehata et al., 2015) which shows a possibility to contribute to the tsunami warning. However, there are few reports on TIEM, and their characteristics are known little. Therefore, in this study, we try to investigate the characteristics of TIM at magnetic stations in the Pacific.
2. Analysis
We compared 1-minute values of the vertical geomagnetic field at a seaside station with 1-minute values of the tide level nearby. There are 9 pairs of those stations whose data are available online. Magnetic field data was provided by the JMA, GIS, EIR, and WDC, and tide level data was provided by the JMA, GIS, IOC, and NOAA for the 14 earthquakes which tsunamis have been observed in Japan since 2006.
The trend was removed by 60-minute moving average. Geomagnetic changes originating from the ionosphere and magnetosphere in the vertical component were estimated and removed using the horizontal magnetic field of the same observatory with a Kalman filter.
The significance of the correlation between the vertical geomagnetic field and the sea level change was judged by statistics. The criteria are (1) that both the first-order correlation coefficient in the time domain and the squared coherence in the frequency meets the 95% confidence limit of the uncorrelated test, and (2) that the first-order correlation coefficient has periodic change due to the predominant period of the tsunami.
3. Results
Significant correlation between vertical geomagnetic field and sea level were confirmed in 44 cases out of 98. When the maximum sea level change is 20 cm or less, the correlation tends to be insignificant. That suggests that 20 cm for the maximum wave height is a thresh hold to stably observe TIM. In 19 significant cases whose maximum sea level change are larger than 20 cm the amplitude of the vertical magnetic field is approximately proportional to the amplitude of the sea level change. Furthermore, there are regional differences in the stability of the correlation between geomagnetic field and sea level changes. The significant correlation is often seen on isolated islands in the ocean.
4. Calculation of TIM using tsunami numerical model
To explain causes of regional differences in the correlation, we theoretically calculate TIM from the sea level change and flow velocity obtained from a tsunami numerical model. In this study, we calculated for the 2010 Chile earthquake, which tsunami was observed at many stations.
One-minute values of the sea level change and flow velocity at every 0.025 degree throughout the Pacific were provided by a two-dimensional plane tsunami calculation model (Minami, personal communication).
Then, TIM were computed around 9 magnetic stations by using Biot-Savart’s law (Tatehata et al., 2015).
The computed vertical magnetic field was correlated to the modeled sea level change at 7 stations, except 2 in the Southern Hemisphere.
At the 7 stations, the correlation is high for changes at periods longer than 20 minutes and for the tsunami arrival time. The amplitude ratio of TIM to sea level change was large at isolated islands in the Pacific, probably because the current density was available on many grid points, and the tsunami flow around the island tend to be a simple long wave. These imply that topography and land-sea distribution affect the regional differences in correlation. The amplitude ratio is larger than that of the observation. Imposed conditions and assumptions may cause the difference.
When a tsunami moves in the geomagnetic field, an electromagnetic field is induced by the dynamo effect (tsunami induced electromagnetic field, hereafter TIEM). Tyler (2005) shows that the vertical component of TIM is approximately linearly correlated with sea level. TIM appears in the vertical component before tsunami arrive (Tatehata et al., 2015) which shows a possibility to contribute to the tsunami warning. However, there are few reports on TIEM, and their characteristics are known little. Therefore, in this study, we try to investigate the characteristics of TIM at magnetic stations in the Pacific.
2. Analysis
We compared 1-minute values of the vertical geomagnetic field at a seaside station with 1-minute values of the tide level nearby. There are 9 pairs of those stations whose data are available online. Magnetic field data was provided by the JMA, GIS, EIR, and WDC, and tide level data was provided by the JMA, GIS, IOC, and NOAA for the 14 earthquakes which tsunamis have been observed in Japan since 2006.
The trend was removed by 60-minute moving average. Geomagnetic changes originating from the ionosphere and magnetosphere in the vertical component were estimated and removed using the horizontal magnetic field of the same observatory with a Kalman filter.
The significance of the correlation between the vertical geomagnetic field and the sea level change was judged by statistics. The criteria are (1) that both the first-order correlation coefficient in the time domain and the squared coherence in the frequency meets the 95% confidence limit of the uncorrelated test, and (2) that the first-order correlation coefficient has periodic change due to the predominant period of the tsunami.
3. Results
Significant correlation between vertical geomagnetic field and sea level were confirmed in 44 cases out of 98. When the maximum sea level change is 20 cm or less, the correlation tends to be insignificant. That suggests that 20 cm for the maximum wave height is a thresh hold to stably observe TIM. In 19 significant cases whose maximum sea level change are larger than 20 cm the amplitude of the vertical magnetic field is approximately proportional to the amplitude of the sea level change. Furthermore, there are regional differences in the stability of the correlation between geomagnetic field and sea level changes. The significant correlation is often seen on isolated islands in the ocean.
4. Calculation of TIM using tsunami numerical model
To explain causes of regional differences in the correlation, we theoretically calculate TIM from the sea level change and flow velocity obtained from a tsunami numerical model. In this study, we calculated for the 2010 Chile earthquake, which tsunami was observed at many stations.
One-minute values of the sea level change and flow velocity at every 0.025 degree throughout the Pacific were provided by a two-dimensional plane tsunami calculation model (Minami, personal communication).
Then, TIM were computed around 9 magnetic stations by using Biot-Savart’s law (Tatehata et al., 2015).
The computed vertical magnetic field was correlated to the modeled sea level change at 7 stations, except 2 in the Southern Hemisphere.
At the 7 stations, the correlation is high for changes at periods longer than 20 minutes and for the tsunami arrival time. The amplitude ratio of TIM to sea level change was large at isolated islands in the Pacific, probably because the current density was available on many grid points, and the tsunami flow around the island tend to be a simple long wave. These imply that topography and land-sea distribution affect the regional differences in correlation. The amplitude ratio is larger than that of the observation. Imposed conditions and assumptions may cause the difference.