Often slow slip is too slow to create seismic waves. But sometimes the rock on either side of the fault may move quickly enough to generate seismic waves that are just large enough to be above background noise. In that case, the slow slip generates what scientists call tremor.
A confusing point is that sometimes tremor occurs before or after the movement of the slow slip. Sometimes slow slip occurs with no tremor at all. The reasons for these facts are not currently understood.
The longest period of slow slip yet detected anywhere started last August under Vancouver Island. It began as tremors there, then moved south. It crossed the international border, moving to and then beyond the Seattle region. All together, the event lasted 42 days.
At first it might seem that slow slip relieves stress on faults and could help us avoid major quakes. But some geologists think that slow slip events transfer stress to areas that then are more likely to rupture when a mega-quake occurs in a region.
So even when it comes to slow motion earth movement, we’ve got to hang onto our hats.
Dr. E. Kirsten Peters was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.