Novel Geomagnetic Disturbances Triggered by the M 7.4 Earthquake in Taiwan region: Evidence of Electric Currents

Earthquake-triggered geomagnetic variations typically exhibit time delays, with disturbances detected first at near-source stations and later at more distant locations. On April 2, 2024, a destructive M 7.4 earthquake struck the eastern Taiwan region, China, triggering an intriguing set of geomagnetic disturbances. Leveraging a high-density, three-component geomagnetic observation network with a 1 Hz sampling rate, we analyzed data from eight stations ranging from 84 to 320 km from the epicenter. Our findings reveal a striking pattern of simultaneous geomagnetic disturbances in both the X and Z components at stations within 114 km of the epicenter, with no similar disturbances observed further away. These disturbances persisted for 350 to 413 seconds, with peak amplitudes of 0.45 nT and 0.3 nT in the X and Z components, respectively. Notably, the Z-component disturbances exhibited opposite phases across the affected stations, suggesting the presence of electric currents near the epicenter. The time delay between the earthquake and the geomagnetic disturbances aligns with the expected propagation of acoustic waves from the earthquake's epicenter to the ionosphere. Using the Biot-Savart law, we estimated the location and intensity of the electric currents responsible for these disturbances. Our calculations place the currents approximately 17 km north of the epicenter, at an altitude of ~80 km, with an intensity of ~120 A and an azimuth of 301.5°. Furthermore, high-frequency Doppler sounders confirmed that acoustic waves propagated to the lower ionosphere, generating electric currents that led to the observed simultaneous geomagnetic disturbances, and continued upward to perturb the higher ionosphere. This study reveals a novel class of earthquake-triggered geomagnetic variations, offering new insights into the interaction between seismic events and the ionosphere.