The recent earthquake off the west coast of Hawai'i has sparked curiosity about the unique geological phenomena that shape our island home. While the magnitude-4.5 earthquake on July 9th was indeed a lithospheric flexure event, it's just the tip of the iceberg when it comes to understanding the complex dynamics beneath our feet. In this article, I'll delve into the fascinating world of deep earthquakes in Hawai'i, exploring their causes, implications, and the broader context that makes our island chain so geologically extraordinary.
Unraveling the Mystery of Deep Earthquakes
What makes Hawai'i's deep earthquakes particularly intriguing is their departure from the typical volcanic activity we associate with our islands. Unlike the day-to-day seismic activity within volcanic systems, these widely felt earthquakes are not directly linked to magmatic processes. The magnitude-6.0 earthquake on May 22nd, for instance, occurred beneath the west side of the Island of Hawai'i, southeast of Captain Cook, at a depth of 14 miles (23 km) below the ocean surface. This event, along with the magnitude-4.6 and magnitude-4.7 earthquakes that followed, raises the question: Why do these deep earthquakes happen in Hawai'i?
The Unique Geometry of the Hawaiian Island Chain
The answer lies in the unique geometry of the Hawaiian Island Chain. As one of the most geographically isolated landmasses on Earth, Hawai'i is far from continental landmasses and tectonic plate boundaries, which are responsible for most global seismic activity. Instead, Hawai'i's volcanism is driven by a mantle plume, a column of hot rock originating near the core-mantle boundary, thousands of miles deep in the Earth. This mantle plume has created a chain of undersea volcanoes that grow into islands as the Pacific plate creeps northwest. Over millions of years, the weight of these islands has caused the lithosphere, the brittle upper layer of the Earth's mantle, to sag downward toward the center of the Earth.
The Weight of the Islands: A Bowling Ball on a Bed
To visualize the impact of the islands' weight on the lithosphere, imagine a bowling ball on a bed. The bowling ball sinks down into the blankets, and the blankets create folds radially outward from the bowling ball. These blanket folds represent the deep faulting resulting from plate flexure. Since the lithosphere is relatively cool and brittle, it can only bend so much before the strain is released as an earthquake. These earthquakes, known as flexure events, are much deeper and less common than most Hawaiian earthquakes, which are the result of magma movement and pressure changes or faulting within the island itself.
The Impact of Deep Earthquakes
What makes these flexure events particularly noteworthy is their potential for widespread impact. Unlike typical Hawaiian earthquakes, which are often localized within volcanic systems, these deep earthquakes occur offshore and at greater depths. This means that the dense lithosphere allows seismic waves to impact a larger footprint without attenuating as much energy as they propagate, potentially leading to damage. The magnitude-6.0 earthquake on May 22nd, for instance, rattled residents statewide, highlighting the potential for these events to have far-reaching effects.
Distinguishing Flexure Events
It's important to distinguish between flexure events and other deep offshore earthquakes. For example, the Pāhala seismic swarm regularly produces earthquakes larger than magnitude-4 and more than 18 miles (30 km) deep off the southeast coast of the Island of Hawai'i. These are not flexure earthquakes but are thought to be related to hot spot magma transport pathways. As the authoritative seismic network in the State of Hawaii, the USGS Hawaiian Volcano Observatory (HVO) is tasked with reviewing all large local earthquakes, regardless of whether they are flexure events or occurring within volcanic systems.
A Call to Action: Drop, Cover, and Hold On
While deep earthquakes in Hawai'i may not be directly related to magmatic processes, they are a reminder of the dynamic forces that shape our island home. As residents of the Island of Hawai'i, it's crucial to remember that these earthquakes can be felt statewide and may even generate tsunamis. So, when you feel shaking, it's essential to drop, cover, and hold on, and to move away from the coast in case a tsunami is generated. By understanding the causes and implications of these deep earthquakes, we can better appreciate the complexity of our island's geology and the ongoing efforts of organizations like HVO to monitor and study these phenomena.
In my opinion, the recent earthquake off the west coast of Hawai'i is a powerful reminder of the interconnectedness of our island's systems and the ongoing efforts of scientists and organizations like HVO to unravel the mysteries beneath our feet. As we continue to explore and understand the unique geology of Hawai'i, we gain a deeper appreciation for the forces that shape our home and the importance of being prepared for the unexpected.
