Mount St. Helens has been shaking, with 40 earthquakes – the most powerful of which was a 3.9 – in just four days. Since the largest earthquake, which struck Wednesday night around midnight local time, smaller earthquakes have shaken the area every half hour. Researchers believe they could be related to activity within the nearby volcano. Earthquakes are not unusual around Mount St. Helens; it has been seismically active since records began. And while the largest quake and most recent activity are likely due to volcanic activity, some of the rumblings from the first two days of the year are probably related to regional stress and not the volcano itself. Geologists can determine whether an earthquake is related to the volcano by looking at where the quakes originate. Rumbling on the Mount St. Helen's axis is usually caused by the active volcanic system, whereas other earthquakes are aligned with regional faults. Although Mount St. Helens is an active volcano, scientists don't expect