Analisis Pembangkitan Medan Magnetik Sekunder Akibat Perambatan Gelombang Tsunami Pada Kasus Hunga Tonga-Hunga Ha'apai, Tonga 2022

Analysis of Secondary Magnetic Field Generation Due to Tsunami Wave Propagation in the Case of Hunga Tonga-Hunga Ha'apai, Tonga 2022

Mekanisme pembangkitan medan magnet sekunder akibat tsunami passage pada kasus tsunami HTHH Tonga 2022 dapat dipelajari melalui persamaan analitik dan data magnetogram. Saat terjadi tsunami, aliran fluida konduktif air laut memotong garis-garis gaya medan magnet utama Bumi dan membangkitkan sinyal magnetik dengan intensitas lemah yang terukur oleh instrumen magnetik beberapa nano tesla, jauh lebih kecil dari intensitas medan magnet utama Bumi (2,5–6,5 × 10⁴ nT). Meskipun relatif jauh lebih kecil, namun anomali lokal medan magnet tersebut dapat dibedakan dari variasi temporal medan magnet utama Bumi. Penelitian ini menganalisis mekanisme pembangkitan medan magnetik sekunder (anomali lokal medan magnet Bumi) akibat tsunami passage lintas Samudera Pasifik saat terjadi erupsi vulkanik HTHH Tonga pada tanggal 15 Januari 2022. Tujuan penelitian ini adalah menguji kesesuaian antara medan magnetik sekunder yang diturunkan dari persamaan analitik dan observasi berbasis data magnetogram. Asumsi penting yang bersifat fundamental, yaitu pendekatan frozen-flux di mana proses adveksi air laut mendominasi proses difusi. Pendekatan inilah yang menjadi basis rumusan estimasi komponen vertikal medan magnet sekunder  yang dipengaruhi tiga parameter sekaligus, yaitu  (komponen vertikal medan magnet utama Bumi) diperoleh dari model magnetik global IAGA (IGRF-13),  (kedalaman laut) diperoleh dari Bathymetric Data Viewer (NCEI) dan  (elevasi muka laut) diperoleh dari NOAA Center for Tsunami Research (NCTR) untuk kasus tsunami HTHH Tonga 2022. Hasil-hasil penelitian berupa estimasi analitik  yang selalu lebih besar dari nilai  yang diperoleh dari magnetogram. Selisih nilai  hasil estimasi dan observasi berkisar antara 0,03–0,44 nT yang dikategorikan tidak signifikan dan membuktikan kesesuaian antara hasil perhitungan analitik dan yang diperoleh dari magnetogram adalah relatif tinggi. Temuan penelitian ini dalam hal keberlakuan teori frozen-flux sesuai dengan temuan terdahulu pada beberapa kasus trans-oceanic tsunamis yang dipicu oleh gempa tektonik bawah laut. Implikasi temuan penelitian ini adalah sumber pemicu tsunami tidak memengaruhi keberlakuan teori frozen-flux.

Kata Kunci: HTHH Tonga 2022, tsunami passage, medan magnet sekunder

The mechanism of secondary magnetic field generation due to tsunami passage in the case of the 2022 HTHH Tonga tsunami can be studied through analytical equations and magnetogram data. When a tsunami occurs, the flow of conductive seawater fluid cuts the lines of force of the Earth's main magnetic field and generates a magnetic signal with a weak intensity measured by magnetic instruments of several nano tesla, much smaller than the intensity of the Earth's main magnetic field (2.5–6.5 × 104 nT). Although relatively much smaller, the local magnetic field anomaly can be distinguished from the temporal variation of the Earth's main magnetic field. This study analyzes the mechanism of secondary magnetic field generation (local anomalies of the Earth's magnetic field) due to the tsunami passage across the Pacific Ocean during the HTHH Tonga volcanic eruption on January 15, 2022. The purpose of this study is to test the agreement between the secondary magnetic field derived from analytical equations and observations based on magnetogram data. An important fundamental assumption is the frozen-flux approach where the seawater advection process dominates the diffusion process. This approach is the basis for the formulation of the estimation of the vertical component of the secondary magnetic field  which is influenced by three parameters at once, namely  (the vertical component of the Earth's main magnetic field) obtained from the IAGA global magnetic model (IGRF-13),  (sea depth) obtained from the Bathymetric Data Viewer (NCEI) and  (sea surface elevation) obtained from the NOAA Center for Tsunami Research (NCTR) for the 2022 Tonga HTHH tsunami case. The research results are in the form of analytical estimates of  which are always greater than the  value obtained from the magnetogram. The difference in the estimated and observed  values ​​ranges from 0.03–0.44 nT which is categorized as insignificant and proves that the agreement between the analytical calculation results and those obtained from the magnetogram is relatively high. The findings of this study in terms of the validity of the frozen-flux theory are in accordance with previous findings in several cases of trans-oceanic tsunamis triggered by underwater tectonic earthquakes. The implication of the findings of this study is that the source of the tsunami trigger does not affect the validity of the frozen-flux theory.

Keywords: HTHH Tonga 2022, tsunami passage, secondary magnetic field