What’s Shaking Deep Under Hawaii?
The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:
Most people know that the Island of Hawaiʻi has many earthquakes, including several every year that are strong enough to be felt. They are caused by three dominant forces. The first force is volcanic. When magma moves underground, it stresses the rocks around it, creating earthquakes. These typically occur in the shallowest 5 km (3 mi) of the volcano, although some can occur deeper. Volcanic earthquakes usually do not exceed magnitude 5 in size.
The second force results from the settling of the Hawaiian Islands. As the islands grow larger over time, they tend to flatten due to gravitational forces. This causes the volcanoes to spread under their own weight. The spreading affects the entire volcano but the stress is most easily released on the interface between the spreading volcanic island and the underlying ocean floor on which the volcanoes are built. Sediment, which accumulated on the ocean floor before the Hawaiian volcanoes began to grow, acts as a lubricant for spreading, but the boundary is still a sticky one. The largest and most damaging earthquakes that take place in Hawaiʻi—including the 1975 magnitude-7.7 Kalapana earthquake—occur along this interface.
The third force is the weight of the volcanoes on the Earth’s surface which can cause earthquakes around each of the Hawaiian Islands. Such earthquakes usually occur in the Earth’s mantle, beneath the 10-km-thick (6-mi-thick) oceanic crust on which the volcanoes are built. The 2006 magnitude-6.7 Kīholo Bay and 1973 magnitude-6.2 Honomū earthquakes are examples of this force in action.
As many of us learned in primary school, the Earth is made up of a solid inner core floating within a molten outer core. The outer core is surrounded by mantle, which makes up the bulk of our planet, and the crust is the very thin outermost shell on which we all live.
Pressures are already high in the mantle because of all the rock that lies above. Additional stresses arise from the weight of the islands pushing down on the crust below. It is these additional stresses that build in the mantle and create the environment where an earthquake can happen. Earthquakes that occur in the mantle can be very large, as nearly everyone who was in the state on October 15, 2006, can appreciate (that was when the magnitude-6.7 Kīholo Bay earthquake caused heavy damage on the Island of Hawaiʻi, closed roads on Maui, and knocked out power to Oʻahu). Thankfully, the fact that mantle earthquakes occur at such deep levels means that the shaking is less intense at the surface than it would be if the earthquakes were much shallower.
Mantle earthquakes can occur anywhere under the Hawaiian Islands, although they are most common on the younger islands. The most recent strong mantle earthquake occurred just last week, when an magnitude-5.3 event was recorded at 40-km depth off the south coast of Hawaiʻi Island. The earthquake was widely felt but caused no damage. Still, it serves as a good reminder of just how strong these mantle earthquakes can be.
Even to the northwest of the Island of Hawaiʻi, the mantle is still being stressed by the weight of the older volcanoes that make up the island chain. As a result, mantle earthquakes can cause damage on older islands, like Oʻahu and Kauaʻi, even though the volcanoes that make up those islands haven’t been active in many thousands of years. For that reason, it is important that all residents of the State of Hawaii, and not just the Island of Hawaiʻi, know what to do in case of an earthquake. Stay tuned to this column for more details on earthquake preparedness in the months to come.
For more information about earthquakes and volcanic eruptions in Hawaii, visit the Hawaiian Volcano Observatory website!