17:15 〜 17:30
[G03-5-04] Seasonal water mass variation in the Japan Sea from satellite gravimetry: Comparison with GNSS and seasonality in earthquake occurrences
Since the launch in 2002, most researches with the GRACE satellites have focused on gravity changes on land, but its increasing time span enables us to discuss time-variable gravity in the ocean, e.g. seasonal water mass changes in the Red Sea (Wahr et al., 2014). Here, we study seasonal mass changes in the Japan Sea (the East Sea of Korea) using the GRACE data 2002-2016 (Level-2 RL-5 data from CSR). By analyzing the GSM files, we found the gravity maximum and minimum in November and June, respectively, with the peak-to-peak amplitude ~10 cm in equivalent water depth. Then we added back the GAD files to recover the original gravity changes in the ocean. Oddly, GSM + GAD resulted in a smaller seasonal change with amplitude of ~4 cm. This means that the non-tidal ocean mass change model used for the GAD files is incorrect in the Japan Sea.
We compared the GRACE data with the tide gauge records in the Japan Sea, and found that tide gauge showed amplitudes several times as large and the maximum in late summer. This suggests that the seasonal sea surface height changes mainly reflect thermal expansion of warm water above the thermocline rather than the real water mass changes. Winter snow also causes seasonal gravity changes in NE Japan with a sharp peak in February. GNSS stations in NE Japan show complicated seasonal movements coming partly from the sea water and partly from the snow. We further discuss the causal relationship between seasonal water mass changes in the Japan Sea and seasonal changes in seismicity in the plate boundary running along the eastern margin of the Japan Sea, i.e. major earthquakes occur during three months May, June and July there.
We compared the GRACE data with the tide gauge records in the Japan Sea, and found that tide gauge showed amplitudes several times as large and the maximum in late summer. This suggests that the seasonal sea surface height changes mainly reflect thermal expansion of warm water above the thermocline rather than the real water mass changes. Winter snow also causes seasonal gravity changes in NE Japan with a sharp peak in February. GNSS stations in NE Japan show complicated seasonal movements coming partly from the sea water and partly from the snow. We further discuss the causal relationship between seasonal water mass changes in the Japan Sea and seasonal changes in seismicity in the plate boundary running along the eastern margin of the Japan Sea, i.e. major earthquakes occur during three months May, June and July there.