So called "cascading" blackouts can be triggered by a fallen tree or as in the case of Sunday's outage, an earthquake-induced shutdown of just two of 13 power generators. While rare, such systemwide electricity shutdowns can happen, according to experts.

Just why the earthquake, which did not cause any significant structural damage, caused such a major blackout is still unclear. However, the blackouts likely were exacerbated by the state's geographically isolated power grid and the tremors associated earthquakes.

Power outages typically follow large earthquakes, said Dennis Murphy, a spokesman or GE Energy in Atlanta, Georgia, which sells power generation and management systems.

"If there's a large enough tremor, (power generators) shutdown. That's by design," Murphy said. "You're dealing with something that's the size of a bus that runs with the precision of a wrist watch."

On the Mainland, a similar systemwide shutdown may not have had as significant an impact because electricity can be brought in from other states, Murphy said. That's not an option in Hawai'i.

Sunday's outages resembled cascading blackouts that have occurred sporadically on the Mainland, said James Bushnell, research director for the University of California Energy Institute in Berkeley, Calif.

Cascading blackouts are caused when an electrical grid is unable to handle a unexpected large imbalance between available power and electrical demand. They often the result when sudden network problems cannot be immediately isolated or contained, causing a successive series of power generators to automatically shutdown to prevent permanent damage.

"A generator has to take itself out to protect itself," Bushnell said. "Once a power plant shuts down, it can take 12 hours to restart because they have to go through a thorough checklist. If that happens in some cases it can take several days to restore power."

Among the major cascading power outages in recent years was a major blackout in the Northeastern United States in 2003 that knocked out power to 50 million U.S. and Canadian citizens. Closer to home, an O'ahu-wide blackout in 1991 was caused when a fallen tree took out a power line.

Sunday's O'ahu-wide blackout was caused when Hawaiian Electric Co. was unable to adjust to the unexpected shutdown of two power generators that represented just 12 percent of generating capacity. Rather than cut power to 12 percent of customers, the utility shutoff power to the whole island.

Utility grids are designed with automated systems capable of adjusting to withstand such incidents, Bushnell said. However, as Sunday's blackout illustrates, these systems don't always work as planned.

"It doesn't always work and it could be quite expensive to make the network so resistant that you never have these kinds of blackouts every five or 10 years," Bushnell said.