Scientists are monitoring a swarm of earthquakes on Mount Hood in the past 24 hours, but there's no cause for alarm, a geophysicist said Monday.
The approximately 77 minor earthquakes registered less than 2.0 magnitude and began about 6 p.m. Sunday, according to the seismic network website.
Most of the tremors are clustered east of Timberline Highway and north of U.S. 35 in Mount Hood Village, an area designated by the U.S. Census that included a population of 4,864 in 2010.
Mount Hood has experienced at least 89 earthquakes in 30 days, according to the seismic network.
John Vidale, director of the seismic network, said his data indicates the tremors have been occurring along a fault 3 miles deep on the south of the volcano.
"Swarms are pretty common around here, and this is probably the biggest one in Hood for a few years," Vidale said.
Earthquake swarms tend to happen on volcanoes as magma and gases navigate beneath the surface, said Joan Gomberg, a research geophysicist with the survey's Seattle-based field office.
"There's a lot of active changes going on in a volcano," Gomberg said. "All the Cascade volcanoes are active in some sense."
Mount St. Helens was recently the source of about 130 tremors in a matter of weeks. The Oregonian/OregonLive reported May 9 that quakes ranging from 0.5 to 1.3 were felt beginning March 14.
"The magma chamber is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stress drives fluids through cracks, producing the small quakes," the USGS reported at the time.
The mountain erupted on May 18, 1980, losing 1,300 feet in elevation and killing 57 people. Recharging, similar to what is being seen now, can happen for years and was previously observed from 1987-2004, USGS reported.
The most recent eruption on Mount Hood happened in 1865, according to the survey.