kanaloa wrote:

augie wrote: kanaloa wrote: The USGS guy said the largest they'd had since the 60's was a 4.8 but I don't think that's right. I think in 1990 they had a 6.4 on the Big Island. The Big Island has hundreds of EQ's daily, but 90% are lava movement deep under the islands. These EQ's (it may have actually been two) were what me and Augie are talking about, subsidence, and they are very rare in Hawaii. Kohala is an extinct volcano. Mauna Kea and Hualalai (the next closest to the epicenter) are dormant. Mauna Loa and Kilauea are the two active volcanoes and they are on the other side of the island where the majority of EQ's happen. I sit corrected as subduction is where two plates are pushing against each other whereas subsidence is more akin to them pulling apart as I understand it. It's weird as Hawaii sits almost in the middle of the Pacific plate. Anyways, I'm learning a lot from this thread and Googling this, I see I need a few years to get the drift(of tectonic plates). As always, thanks for your expertise. OK let me clarify a few things. Plates aren't what's at work under Hawaii. Hawaii is a 'hot spot.' Subduction takes place near the Aluetian Islands and in a lot of places around the ring of fire in the Pacific. That's where the oceanic plate slips beneath a continental plate. Thus subduction and pretty nasty EQ's. You can also get volcanoes (typically explosive ones) on the continental side of the boundary (thus the volcanoes on the west coast of the US and Canada, like Mt. St. Helens). Subduction also can be a oceanic and oceanic plate, like the mariana trench. When continental plates collide like India and China you get the Himalayas. Hawaii is formed by a hot spot in the Pacific plate, or a place where a plume of lava comes through the moving plate. Thus the plate moves above the hot spot in the earth. The Hawaiian islands form as this plate moves over the hot spot, eventually piggy-backing off to the north and west. Thus the islands that are west and north of the current hot spot (Kilauea and Lo'ihi, a seamount) are much much older than these current two. The other major hot spot in the world is Yellowstone National Park. The forces at play that likely caused this EQ were caused by subsidence or sinking into the crust, not subduction. It'd be like taking a piece of plywood and spanning it between two supports, then dumping rocks on it in the middle. It would bend down in the middle. The Hawaiian islands mass makes them sink into the crust. That's likely what caused this EQ, bc the EQ occured near older volcanic mountains, and not near the currently active ones.

Thanks for that lesson Kanaloa, I see I think, much like underground erosion then. Though I won't be buying any land on Lo'ihi any time soon.

A brief history of moderate or strong earthquakes that have struck in Hawaii, from the U.S. Geological Survey:

_ 6.6 preliminary, Kailua Kona, Oct. 15, 2006. Caused power outages, landslides and some property damage, according to early reports.

_ 5.2, Naalehu, July 17, 2005

_ 5.3, Paauilo, July 15, 2005

_ 6.1, off coast near Volcano, Jun. 25, 1989.

_ 6.7, Kaoiki, Nov. 16, 1983

_ 7.2, Kalapana, Nov. 29, 1975. Tsunami killed two people in Halape.

_ 6.9, Kona, Aug. 21, 1951

_ 6.8, Maui, Jan. 22, 1938

_ 6.5, Holualoa, Oct. 6, 1929

_ 6.8, Lanai, Feb. 19, 1871

_ 7.9, Ka'u district, April 2, 1868. Hawaii's largest earthquake, caused 77 deaths and knocked down homes made of stone, wood or straw. Landslides buried homes, and a tsunami drowned dozens of the victims.

_ 7.0, Ka'u district, March 28, 1868. A foreshock of the April 2 earthquake.

_ 7.0, south flank of Kilauea, June 2, 1823

I found out why they said it was the "strongest." They meant for that specific side of the island, which makes sense afterall.