2:15 PM - 2:30 PM
[SEM12-03] How about using the magnetic field for the tsunami warning? Comparison of the tsunami magnetic field with the sea level change
Keywords:Tsunami generated magnetic field, Observed tsunami magnetic signals, 3-D time-domain simulation, Phase leads of the tsunami magnetic components
Particle motions of the conductive seawater associated with tsunamis can generate induced magnetic fields by cutting the geomagnetic main field in the course of their propagation. There are two important properties of the tsunami magnetic field in terms of tsunami early warning:
1. The tsunami vertical magnetic component Bz has a phase lead with respect to the tsunami sea level change. The amount of the phase lead depends mainly on the ocean depth.
2. The tsunami horizontal magnetic component Bh has two peaks: One corresponding to the leading Bz, which has approximately 90-degree phase lag to Bz, and the other corresponding to the initial rise (Minami et al., 2015) that appears earlier than the leading Bz.
These properties have been revealed by the 2D (Minami and Toh, 2013; Minami et al., 2015) and 3D simulations (Zhang et al., 2014; Minami et al., 2017). However, they have never been confirmed by direct comparison of simultaneous observation of magnetic fields and sea level changes.
Here we show the result of the direct comparison of the two different physical quantities acquired by a geophysical observation on the French Polynesian seafloor in the south Pacific Ocean (TIARES Project: Suetsugu et al., 2012). They observed significant electromagnetic fields of the 2009 Samoa tsunami (Mw 8.1) and the 2010 Chile tsunami (Mw 8.8). The questions that we will address using this data are as follows:
1. Is the phase lead of the tsunami Bz really present in the observation?
2. Does the initial rise in the tsunami Bh really exist?
The TIARES experiment deployed an observation array consisting of 9 Ocean Bottom Electro-Magnetometers (OBEMs) and a Differential Pressure Gauge (DPG) on the 4000~5000m deep seafloor for investigating the electrical conductivity structures of the mantle beneath the Society hotspot. The OBEMs recorded three components of the seafloor magnetic field with a 1-minute sampling rate. In addition, a DPG was installed at Site SOC8 for the observation of ocean bottom pressure. When the earthquake tsunamis passed over the TIARES area, on the 29th of September in 2009 and the 27th of February in 2010 at the times of the Samoa earthquake of Mw 8.1 and the Chile earthquake of Mw 8.8, the observation array captured the signals of the tsunami-generated magnetic field and the ocean bottom pressure change associated the tsunamis.
First, we compared the observed magnetic field with the observed sea level change. The sea level change was successfully deconvoluted from the raw data recorded by the DPG. The sea level change and magnetic time-series were filtered by the same high-pass filter, and then external magnetic fields were further removed from the magnetic time-series by the remote magnetic reference method by Minami et al. (2017). The comparison of the two observations revealed the following facts:
1. The tsunami Bz has clear phase leads to the sea level change for both events.
2. The initial rise in the observed Bh is too small to detect. We need further confirmation by 3-D numerical simulation.
Then the simulation of the tsunami magnetic field was done by two steps. First, the simulation of the tsunami velocity field was conducted. In this study, JAGURS tsunami simulation code by Baba et al. (2017) was used for calculation of the tsunami velocity field including the effects of Boussinesq dispersion and elastic loading. The tsunami magnetic field was then simulated using the estimated velocity field in the second step. The 3-D time-domain code by Minami et al. (2017) was used to calculate the tsunami magnetic field. The simulated tsunami magnetic field showed the following two features:
1. The simulation confirms the phase lead of the tsunami Bz with respect to the tsunami sea level change.
2. The simulation implies the presence of the initial rises of the tsunami Bh in both events.
1. The tsunami vertical magnetic component Bz has a phase lead with respect to the tsunami sea level change. The amount of the phase lead depends mainly on the ocean depth.
2. The tsunami horizontal magnetic component Bh has two peaks: One corresponding to the leading Bz, which has approximately 90-degree phase lag to Bz, and the other corresponding to the initial rise (Minami et al., 2015) that appears earlier than the leading Bz.
These properties have been revealed by the 2D (Minami and Toh, 2013; Minami et al., 2015) and 3D simulations (Zhang et al., 2014; Minami et al., 2017). However, they have never been confirmed by direct comparison of simultaneous observation of magnetic fields and sea level changes.
Here we show the result of the direct comparison of the two different physical quantities acquired by a geophysical observation on the French Polynesian seafloor in the south Pacific Ocean (TIARES Project: Suetsugu et al., 2012). They observed significant electromagnetic fields of the 2009 Samoa tsunami (Mw 8.1) and the 2010 Chile tsunami (Mw 8.8). The questions that we will address using this data are as follows:
1. Is the phase lead of the tsunami Bz really present in the observation?
2. Does the initial rise in the tsunami Bh really exist?
The TIARES experiment deployed an observation array consisting of 9 Ocean Bottom Electro-Magnetometers (OBEMs) and a Differential Pressure Gauge (DPG) on the 4000~5000m deep seafloor for investigating the electrical conductivity structures of the mantle beneath the Society hotspot. The OBEMs recorded three components of the seafloor magnetic field with a 1-minute sampling rate. In addition, a DPG was installed at Site SOC8 for the observation of ocean bottom pressure. When the earthquake tsunamis passed over the TIARES area, on the 29th of September in 2009 and the 27th of February in 2010 at the times of the Samoa earthquake of Mw 8.1 and the Chile earthquake of Mw 8.8, the observation array captured the signals of the tsunami-generated magnetic field and the ocean bottom pressure change associated the tsunamis.
First, we compared the observed magnetic field with the observed sea level change. The sea level change was successfully deconvoluted from the raw data recorded by the DPG. The sea level change and magnetic time-series were filtered by the same high-pass filter, and then external magnetic fields were further removed from the magnetic time-series by the remote magnetic reference method by Minami et al. (2017). The comparison of the two observations revealed the following facts:
1. The tsunami Bz has clear phase leads to the sea level change for both events.
2. The initial rise in the observed Bh is too small to detect. We need further confirmation by 3-D numerical simulation.
Then the simulation of the tsunami magnetic field was done by two steps. First, the simulation of the tsunami velocity field was conducted. In this study, JAGURS tsunami simulation code by Baba et al. (2017) was used for calculation of the tsunami velocity field including the effects of Boussinesq dispersion and elastic loading. The tsunami magnetic field was then simulated using the estimated velocity field in the second step. The 3-D time-domain code by Minami et al. (2017) was used to calculate the tsunami magnetic field. The simulated tsunami magnetic field showed the following two features:
1. The simulation confirms the phase lead of the tsunami Bz with respect to the tsunami sea level change.
2. The simulation implies the presence of the initial rises of the tsunami Bh in both events.