Magnitude and Intensity Explained
Earthquake strength is measured using two distinct scales: magnitude and intensity. Magnitude describes the total energy released by an earthquake—there is only one magnitude value per earthquake. Intensity, meanwhile, measures the effects and damage at specific locations—intensity values vary by distance from the epicenter. A magnitude 7.0 earthquake might cause intensity 7 damage near the epicenter, but only intensity 3 or 4 shaking far away.
Japan's Intensity Scale
Japan's Japan Meteorological Agency uses a 10-point seismic intensity scale: 0, 1, 2, 3, 4, 5-lower, 5-upper, 6-lower, 6-upper, and 7. Lower intensities (0-2) are barely or not felt. Intensity 3-4 causes noticeable shaking with some objects falling. Intensity 5 and above produces significant structural damage. Intensity 7 is extraordinarily rare, observed in the 1995 Kobe earthquake and 2016 Kumamoto earthquake.
Energy Comparison Across Magnitudes
Magnitude uses a logarithmic scale where each unit increase represents about 32 times more energy. A magnitude 8.0 earthquake releases 32 times more energy than a magnitude 7.0 event. The 2011 Great East Japan Earthquake measured magnitude 9.0, making it one of the largest earthquakes ever recorded.