Map depicting the potential path of a tsunami that could theoretically destroy Honolulu, Hawaii. It would originate with an eruption of Hualalai Volcano, which has not erupted since 1801, although is expected to erupt again. 
One of five shield volcanoes that comprise the Big Island
Hualalai is one of five shield volcanoes that comprise the Big Island of Hawaii. This volcano last erupted in 1801, and is considered the third-most active of the five, after Kilauea and Mauna Loa.
The mountain’s peak reaches 8,721 feet (2,521 meters) above sea level, and is expected by geologists to erupt again sometime within the next century. Vacation resorts, businesses, and homes that have been built on old lava flows could be in danger the next time an eruption takes place. Large earthquakes damaged buildings in 1929 and 2006, providing evidence that the magma beneath the volcano continues to move and position itself for the next eruption.
Hualalai is the westernmost of the five volcanoes of the Big Island. Like Kilauea, Mauna Kea, and extinct Kohala, it is dwarfed in size by Mauna Loa, which makes up most of the island and is the largest active volcano on Earth. Hualalai comprises about 7% of the island.
The mountain is popular for hiking and has several beautiful nature reserves. Dozens of cinder cones are popular to visit, found all around the mountain, and mostly on the northwest, north, and east sides.
For more about basic volcanology terms, see my article here on InfoBarrel on the topic.
Honolulu, Hawaii is the capital and largest city in the state, and located on the island of Oahu. The metro area has nearly one million residents. This is a view as seen from Diamond Head Crater, which is an old extinct volcano popular for hiking. 
Hualalai poses a potential major threat to Honolulu
Whenever Hualalai erupts again, scientists see potential for a massive landslide into the ocean. This could result in a massive tsunami wave, which would travel northwest away from the Big Island and directly toward the island of Oahu (as seen on the map at the top of this page) at terrifying speed.
The fear is that a 100-foot (30-meter) wave could slam into Pearl Harbor and the city of Honolulu, an area with almost one million residents, busy beaches, and many thousands of tourists. It would take a tsunami wave only about 30 minutes to travel the distance. If this were to occur, tens of thousands of people could potentially perish, and economic damages would certainly be in the billions of dollars. It would be an enormous disaster.
For more information about tsunamis, how they work and what can be done about them, plus additional potential major threats identified by scientists around the world, see my InfoBarrel article on the topic.