<Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (June 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (June 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> On March 15, 1991, a succession of earthquakes was felt by villagers on the northwestern side of the volcano . Further earthquakes of increasing intensity were felt over the next two weeks, and it became clear some kind of volcanic activity was likely . On April 2, the volcano awoke, with phreatic eruptions occurring near the summit along a 1.5 km (0.93 mi) long fissure . Over the next few weeks, small eruptions continued, dusting the surrounding areas with volcanic ash . Seismographs recorded hundreds of small earthquakes every day . </P> <P> Scientists immediately installed monitoring equipment and analyzed the volcano for clues as to its previous eruptive history . Radiocarbon dating of charcoal found in old volcanic deposits revealed the last three major explosive eruption periods in recent millennia, about 5500, 3500 and 500 years ago . Geological mapping showed that much of the surrounding plains were formed by lahar flood deposits from previous eruptions . </P>

What was the first sign that mt. pinatubo had awoken