Ground Shaking (Volcanic Earthquake)

Primary reference(s)

UN-SPIDER, no date. Knowledge Portal. United Nations Platform for Space-based Information for Disaster Management and Emergency Response (UN-SPIDER). Accessed 19 October 2020.

USGS, no date. Earthquake Glossary. United States Geological Survey (USGS). Accessed 19 October 2020.

Additional scientific description

Seismic activity is a common feature of volcanic eruptions. Often, there are many thousands of earthquakes recorded during an eruption. Most volcanic earthquakes are small but significant (moderate and large) volcanic earthquakes do occur (Zobin, 2001). Volcanic earthquakes, like all earthquakes, can cause shaking, damage to buildings and other structures, as well as changes in the surrounding environment. This shaking depends on the size of the earthquake, the distance from the source and the soil conditions (Bormann et al., 2013).

Metrics and numeric limits

The size of a volcanic earthquake is measured using the same magnitude scales as other earthquakes.

Magnitude is not a direct measure of ground shaking but, along with the distance from the earthquake source and geological conditions, decides the shaking at any point. There are many magnitude scales, but they should all yield approximately the same value for any given earthquake (USGS, no date). During the 20th century there were three large (magnitude greater than 7) earthquakes directly associated with volcanic eruptions (Zobin, 2001).

The effect of ground shaking on people and buildings is characterised by its macroseismic intensity. The three most important intensity scales in current use are the European Macroseismic Scale (EMS-98), the Modified Mercalli Scale (MM or MMI) and the JMA scale (Musson and Cecić, 2013). These scales rate the shaking at a given point by the observed effects, ranging from not felt to total damage (e.g., Grünthal, 1998). A magnitude 7 earthquake would be expected to have an intensity, near to the epicentre, of about EMS-98 9 (normally written IX to avoid confusion with magnitude). This is described by the scale as ‘Destructive’ with the description ‘Many weak constructions collapse. Even well-built ordinary buildings show very heavy damage’ (Grünthal, 1998).

Instrumental measures of shaking include peak ground velocity (PGV) and peak ground acceleration (PGA). Although it has been found that earthquake damage is much more closely correlated with PGV than with PGA (Wu et al., 2003), PGA continues to be the more used of these parameters. An often used relationship between intensity and PGA and PGV (Wald et al., 2019) suggests that shaking below 0.0005 g or 0.002 m/s will not be felt and that above 0.4 g or 0.4 m/s structural damage can be expected. A magnitude 7 earthquake could be expected to cause ground shaking of over 0.8 g or 0.9 m/s near the epicentre.

Examples of drivers, outcomes and risk management

Volcanic seismicity is often the first sign of a volcanic eruption. It can also occur at a volcano which does not subsequently erupt (Sparks, 2003).

Ground shaking on volcanoes is more likely to result in secondary hazards than elsewhere. These include landslides, lahars and pyroclastic density currents. Aggravating factors are the time of the event and the number and intensity of aftershocks. Compound hazards include fire and tsunami (WHO, no date).

A community can mitigate ground shaking damage by adopting and enforcing a building code with appropriate seismic design and construction standards (FEMA, 2010).