A weekly feature provided by scientists at the Hawaiian Volcano Observatory. The relatively shallow depth helps account for the notable damage in Hilo. Taking place only 13 minutes before the transfer of Hong Kong to China, the earthquake could be taken as a Big Island contribution to the festivities. Earthquakes occur when the earth's crust is stressed to the breaking point. The stress can be relieved, and stability momentarily achieved, only by breaking rocks in the crust.
Earthquakes are the signals of breaking rocks.
Several factors contribute to the buildup of stress and the overall instability of the south flank of Kilauea. Gravity acts on the volcano, forcing it to move away from the rest of the island. The weight of the island on the old sea floor causes internal movements and readjustments. Intrusion of magma into the rift zones pushes the flank outward. Hydrothermal alteration of rocks within and south of the rift zones weakens the edifice and makes it prone to failure.