Japan is on high alert for a potentially catastrophic megathrust earthquake, following a powerful 7.5 magnitude quake that injured at least 34 people and damaged infrastructure, prompting the JMA to issue its first-ever megaquake warning.The JMA also issued a tsunami advisory after two additional earthquakes, measuring 6.7 and 4.9 magnitudes, struck off Japan's northern and eastern coasts, though nuclear facilities like Onagawa Nuclear Power Plant and Tomari Nuclear Power Plant reported no anomalies and observed waves were minor.Experts warn of a 60 to 90 percent chance of a major Nankai Trough earthquake within the next 30 years, with predictions of tsunami waves exceeding 66 feet and a death toll potentially reaching 300,000, reigniting concerns about the region's history of severe seismic activity.