noun
imperceptible earthquake
A seismological term for an earthquake too weak to be felt by people; detected only by instruments.
昨夜、無感地震が観測された。
An imperceptible earthquake was detected last night.
有感地震 is an earthquake that can be felt by people, in contrast to 無感地震 which is only detected by instruments.
Compound of 無感 (mukan, 'no sensation') + 地震 (jishin, 'earthquake').