When it comes down to it, volcanoes spend most of their existence not erupting. If you look at almost any volcano, it might have a bout of eruption for days to months at a time, then go quiet for decades, centuries or more. So when you think about the activity at any given volcano, you should not only concern yourself with what might be happening when the volcano is actually coughing stuff up (erupting), but also when, at the surface, things look perfectly calm.
More than 130 small tremors have been documented in the Mount St. Helens region in Washington, according to the US Geological Survey. The quakes began in mid-March and have ramped up to nearly 40 localized earthquakes a week.