ultra-microearthquake
Technical term in seismology for an extremely small earthquake, typically with a magnitude below 0.0.
Ultra-microearthquakes are frequently observed in this region.
微小地震 (microearthquake) refers to small earthquakes generally below magnitude 2.0, while 極微小地震 is even smaller, typically below 0.0.
Compound of 極 (ごく, 'extremely') + 微小 (びしょう, 'minute') + 地震 (じしん, 'earthquake').