Directions: Read the passage given below and answer the question that follows.

Earthquakes happen all the time. In fact, nine thousand earthquakes occur around the world everyday. Most of them are so small that we do not even notice them. But at 5:46 a.m. on the morning of 17^th January, 1995, Japan was struck by its biggest earthquake in 72 years. It happened about 20 km. below the earth’s surface.

Up to nine thousand earthquakes happen everyday in the world but they are not noticed because they are ___________.