FAQ About Relationships Between Earthquakes and Volcanic Eruptions
Is there a relationship between earthquakes larger than magnitude 6 that occur along major fault zones and nearby volcanic eruptions?
After large earthquakes the question of whether such strong events can trigger nearby volcanic eruptions often comes up. The short answer to this question is: “not very often.” There are a few examples in the historical record that suggest a connection between large earthquakes and volcanic eruptions, but even these, on close inspection, do not show a simple causative relationship. The most unambiguous case of triggering is probably the November 29, 1975 magnitude (M) 7.2 Kalapana, Hawai‘i earthquake, which was immediately followed by a small, and short-lived eruption at Kīlauea volcano, Hawai‘i. However, in this case, the fault plane of the earthquake (i.e., the extent of the rupture) was directly beneath Kīlauea Volcano. Also, at the time of the earthquake, Kilauea was showing signs of pressurization and was likely poised to erupt soon anyway. Kīlauea is one of the most active volcanoes in the world, and erupted frequently in the decades before and after the 1975 earthquake.
Another example of possible triggering occurred after the M9.5 Chile earthquake on May 22, 1960. About 38 hours after the mainshock, Puyehue-Cordón Caulle in Central Chile erupted violently after being inactive for more than 25 years. In this instance, as at Kīlauea above, it is likely that the fault rupture extended beneath the volcano.
Establishing a statistical correlation between large earthquakes and subsequent volcanic eruptions requires a long and accurate record of both seismic and volcanic activity. The global record of such activity is sparse, especially for events that occurred before 1900. While there are hints of connections in some cases, and a few convincing anecdotes, statistical analyses to date have shown only a weak correlation. And, even if a statistical correlation is shown, working to establish and explain a causal connection is another matter entirely.
There are speculations in the literature, some backed by detailed experiments, about what mechanisms might be behind volcanic triggering by large earthquakes. Mechanisms under consideration include:
- Stress changes caused by large earthquakes may either compress or expand nearby magma reservoirs. In the former case, the compression could increase the reservoir pressure, while in the latter case, the expansion could cause tensile (opening) fractures, around the reservoir. Either way, conditions promoting eruption may become more favorable after a large nearby earthquake.
- High amplitude seismic waves passing through a magma reservoir may cause the nucleation of bubbles within the magma and/or the disturbance of previously stable layers within the reservoir. Bubble creation can increase magma pressure, and layer destabilization can cause reservoir “overturn” where dense layers of relatively gas-poor magma sink forcing gas-rich magma to rise. Either of these events could prompt an eruption.
- Violent shaking during large earthquakes can create landslides, fractures, and other major ground disturbances that can affect shallow magma reservoirs, such as Kilauea’s in 1975, that are poised to erupt anyway.
There is clear evidence that large, distant earthquakes can, and do, trigger swarms of small earthquakes in active hydrothermal systems, such as those in Yellowstone and Long Valley calderas. This phenomenon has been observed at many volcanoes following large earthquakes; the M7.9 Denali Fault earthquake in Alaska is a good example. However, small earthquake swarms such as these are typically only recorded by sensitive seismic instrumentation and certainly do not fall into the category of an eruption.
Conversely, volcanic activity is known to trigger earthquakes. For example, swarms of small earthquakes, rarely larger than M5, can accompany the upward movement of magma through the Earth's crust. Also, large volumes of magma that rise from deep within the lower crust to shallower depths are thought to perturb the stress field around a volcano, possibly triggering small earthquakes up to 25 km away.
In conclusion, while it is possible that large earthquakes can trigger volcanic eruption in some cases, this connection is not well understood and as such not very useful for predicting the behavior of volcanoes. Data from well-designed local volcano monitoring networks remain the only reliable means for detecting volcanic unrest and forecasting volcanic activity.
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