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Volcanic Earthquake Sources

Around the world, earthquakes usually accompany volcanic eruptions. In the Snake River Plain near INL, age dates of basalt lava flows indicate volcanic eruptions have occurred from about 3.2 million years ago to as recently as 2,100 years ago. Earthquakes most likely accompanied these volcanic eruptions. Volcanic earthquakes were included as a potential source of earthquakes for INL probabilistic seismic hazards assessments.

Basalt volcanic eruptions occur as a result of dike intrusion within volcanic rift zones. Dike intrusion occurs in response to tensional stresses in the earth’s crust of the Snake River Plain. It is a process where magma within a tabular shaped body ascends upward into or moves laterally within the upper crust. Distinct deformational land surface features form, such as sets of tension cracks, monoclines, and small normal faults that form small grabens. These features have been studied in detail to determine the type, size, and recurrence of volcanic earthquakes that may be associated with dike-intrusion.

Estimated maximum magnitudes range from 4.5 to 5.5 for potential volcanic earthquakes. This range was estimated in the following two ways:

1. Using empirical relationships between magnitude and rupture dimensions of the small normal faults and fissures observed in eastern Snake River Plain volcanic rift zones

2. By analogy to maximum magnitudes of earthquakes observed at active volcanic rift zones worldwide.

Earthquakes associated with basalt dike-intrusion usually have small maximum magnitudes (less than 4.0) because dike intrusion ruptures faults incrementally and at shallow depths (less than 4 km or 2.5 miles) where crustal rocks have lower strengths.

Ages of basalt flows within volcanic rift zones were used to calculate recurrence intervals (or repeat times) of volcanism. The volcanic recurrence intervals are greater than 16,000 years for volcanic zones near INL. Because of the longer recurrence intervals (greater than 16,000 years) and their small maximum magnitudes, earthquakes associated with volcanism contribute very little to ground motion levels at return periods less than 10,000 years.

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