A brief tsunami warning was issued and lifted within an hour. (Image credit: National Tsunami Warning Center (ATWC), Pacific Tsunami Warning Center (PTWC), USGS National Earthquake Information Center, PDE)The USGS measurement is different from the magnitude 7.1 reported by the Japan Meteorological Agency , "primarily because the agencies are reporting different magnitude types," the USGS reported. These measurements are helpful, as "different magnitude types capture different characteristics of earthquake source processes," the USGS noted. Local magnitude, for example, reflects how strongly the ground shook nearby, while moment magnitude reflects the total energy released across the fault. Earthquake countrySeveral earthquakes have already clustered in the area, and numerous aftershocks are expected over the coming days.