Earthquake vs. Volcano Early Warning: How USGS’s Two Systems Differ

Every year in the United States, natural hazards threaten lives and livelihoods, causing thousands of casualties and billions of dollars in damage. The U.S. Geological Survey (USGS) runs two warning efforts that sound alike but work very differently: earthquake early warning and the National Volcano Early Warning System.

Earthquake early warning: seconds count

Earthquake early warning (EEW) alerts people when damaging shaking is about to reach them. On the West Coast, the USGS-managed system is ShakeAlert. Its alerts go out automatically through emergency channels, including push notifications to phones.

How ShakeAlert works. Credit: U.S. Geological Survey.

How it works:

1. An earthquake generates two kinds of waves. P-waves, the first felt, travel faster through the Earth; S-waves, shear waves, arrive later and do more damage.
2. A regional grid of seismometers picks up the P-waves, which reveal the quake's location, depth and magnitude.
3. ShakeAlert processing centers instantly send an automated message to users and the public that damaging S-waves are coming.

The warning is only seconds to tens of seconds — the most advanced form of earthquake early warning — but that is often enough for people and automated systems to take protective action.

USGS geologists measure fault displacement in Searles Valley, California, during the 2019 Ridgecrest earthquake sequence. Credit: Ben Brooks, U.S. Geological Survey.

Volcano early warning: minutes to months

The National Volcano Early Warning System (NVEWS) is a national plan to monitor each volcano at a level that matches its threat. USGS ranks potentially active U.S. volcanoes as very high, high, moderate, low or very low threat, weighing each volcano's hazards against the people, property and infrastructure exposed. Many of the country's 161 potentially active volcanoes have too little monitoring, and others have outdated equipment.

The goal is to equip the most hazardous volcanoes with a range of modern instruments well before unrest begins, so that forecasts can come minutes, days or even weeks before an eruption. NVEWS also aims to add staff and automation for round-the-clock monitoring and rapid response, and to integrate computer systems that get data to scientists, response agencies, researchers and the public more efficiently.

The monitoring network on Mount St. Helens, Washington: broadband seismometers, GPS, webcams, infrasound and multi-gas sensors and more, designed to catch ground deformation, swarms of small earthquakes, changes in gas and steam explosions before an eruption. Credit: U.S. Geological Survey.

A varied array of sensors lets scientists "fingerprint" a volcano's normal background activity and spot departures from it as early as possible, so observatories can warn land managers, the public and aviation.

Side by side

The two systems have different missions and technical needs and are run independently. But broadband seismometers and webcams can be added at some EEW sites to serve volcano monitoring too.

Rapid-deployment stations: a strong-motion sensor for felt shaking and a broadband sensor for small quakes, sending real-time data by cellular link. Image from USGS Fact Sheet 2023–3033.

Sharing data

Seismic data from both systems are shared in real time — important in California, the Pacific Northwest and Alaska, where large, tsunami-generating earthquakes can strike near active volcanoes. Data also go to NOAA's tsunami warning centers in Hawaii and Alaska. In remote places such as the Aleutian Islands, American Samoa and the Northern Mariana Islands, the first sign of a big tsunami-generating quake or eruption may come from Global Seismic Network stations or volcano-monitoring seismometers — far from population centers, but vital for remote communities, civil and military aviation and ships.

The USGS Volcano Hazards Program already uses seismic and geodetic instruments run by the Earthquake Hazards Program, universities, states and private groups; during unrest at under-monitored volcanoes, those networks are critical until volcano-specific monitoring arrives. Together the programs work out whether an earthquake swarm near a volcano is volcanic, using seismic data, ground sensors and satellite radar and gas measurements. They also share communications networks, staff expertise, logistics and bulk equipment contracts to save money.

Measuring gravity and precise position at Kīlauea's summit, January 7, 2023. Credit: A. Flinders, Hawaiian Volcano Observatory, U.S. Geological Survey.

Sources

Based on "Comparison of Earthquake Early Warning Systems and the National Volcano Early Warning System at the U.S. Geological Survey," USGS Fact Sheet 2023–3033; a work of the United States government in the public domain. Full text.