Amateur Geology

October 18, 2006 | In Maps, Neighbor Islands | 2 Comments

An updated map from USGS shows the large aftershock which didn’t appear as large on the first map. The magnitude of both has been revised upwards, now listed as 6.7 at 7:07am and 6.0 at 7:14am.

Source (dynamic): http://earthquake.usgs.gov/eqcenter/recenteqsus/Maps/HI2/19.21.-157.-155.php

Both earthquakes were deep (18 and 12 mi–29 and 19 km–below sea level I imagine, so somewhat less below the sea floor), and I’ve heard them attributed to the earth’s crust being strained and crackling beneath the weight of the volcano. That may sound surprising until you realize that Mauna Loa (”Long Mountain”) is more massive than many entire mountain ranges. One study estimates the volume of the island (or maybe just the one volcano–it’s not clear which) to be in the range of “68,750 to 79,343 cubic kilometers, or 16,494 to 19,035 cubic miles.”

And not only does the volcano rise from the sea floor which averages around 18,000′ (very roughly 6000 m) deep, its weight depresses the sea floor even deeper at the center of the island (just like you sink into a water bed). So the same study estimates the tip-to-tip height of Mauna Loa at 10.56 miles (17km), significantly more than the 6 miles (10km) usually measured from the sea floor.

Source: http://oregonstate.edu/dept/ncs/photos/mauna.jpg

This whole explanation of the mass of the volcano is a sort of tangent, but it all makes sense if you look at where the earthquakes happened on the graphic above, as indicated by the yellow squares I added (those are the epicenters, I imagine the hypocenters were located on the downward sloping curve of the crust under the volcano mass). The earthquakes occurred below an underwater extension of the island, furthest away from the hotspot now at Kilauea volcano and in the direction that the plate is moving and forming the Hawaiian chain. If you imagine all the plate tectonics and volcanism as an interaction of fluids with differing surface tensions, this is the point where the “fountain” of volcanic rock is falling back towards the crust. So it is here that the mass of the volcano is most unsupported and weighing down on the crust away from the upswelling at the hot spot.

All this is pure speculation on my part, an educated guess if you will. However, in researching and thinking about all these geological processes, it has made me realize that there are two factors which “form” a Hawaiian island after it moves off the hot spot. The first that everybody knows about is the erosion of the atmosphere and of the ocean on the volcano’s pile of rock. The second is the interaction of the volcano’s mass with the crust beneath it and with itself, causing the volcano to subside and flatten out. This second one is often beyond our imagination because it deals with dimensions and time scales beyond our usual perception.

One reason I am fascinated with all this amateur geology is that I’d love to have an idea of what the proto-Kauai looked like when it was the “big island” of the chain. One hypothesis is that all islands looked similar to the current Big Island when they were over the hot spot. I rather suspect that the interaction of the hotspot and the crust movement and the ocean is akin to the weather: some days you get a lot of rain others not much. That would mean some islands had more mass to start with than others and maybe took longer to erode.

I suspect Kauai was one of the larger ones, because it survives larger and more monolithic than Oahu (one mountain mass versus Oahu’s two main ridges) despite being older. Another clue is the fact that “beyond” Kauai, all the islands are tiny. There is nothing intermediate unless you consider Niihau, but it may have been part of the proto-Kauai just as Lanai and Molokai were likely part of the proto-Maui. So my theory is that Kauai “hogged” the hotspot for a long time, building up a massive volcano, perhaps larger than the current Big Island. We all have dreams of gradeur.

Earthquake!

October 15, 2006 | In Maps, Neighbor Islands | No Comments

We were woken up this morning by the earthquatke centered just off the Big Island’s NW Kohala coast. The house shook gently for maybe 10 seconds, the wall hangings banged, one cupboard swung open, but nothing fell or broke. As I was waking up, I thought it was maybe a strong wind buffeting the house but then I realized there was no wind sound. There were actually two earthquakes, we felt the first at 7:11am the second weaker at 7:23am. Those times are approximate, but they are definitely delayed from the “official” times (which I don’t have offhand). We are located 260 miles (415 km) from the epicenter.

Source: http://earthquake.usgs.gov/eqcenter/recenteqsus/Maps/special/Hawaii.php

After the first one, we got up, figured there was no danger and went back to bed—since we sleep in on Sundays. I wasn’t quite asleep when I felt what I thought was a tiny aftershock: just a jitter and the neighbor dogs barked once. I told my wife there was an aftershock, and a few seconds later the house shook a little bit for about 5 seconds, much less than the first time. We didn’t feel any of the true aftershocks, and none of them rattled the wall hangings.

Unless there was some strange localized effects, I doubt there was any damage on Kauai. The electricity never went out, so it was a normal day for us. Maybe some rocks fell somewhere, loosened by the recent rains, but we’ll have to wait for tomorrow’s newspaper to find out. All the local TV stations from Oahu were not broadcasting at first due to a complete power outage on all the other islands. CNN and Fox news had around the clock coverage at first, but not much to say until reporters from KITV (channel 4) on Oahu were back on the air.

So far, there are reports of rock slides on the Big Island, some bridges out on Maui isolating the town of Hana, and a 100 year-old chimney falling on Oahu, but no deaths. Lots of food spoiled and dishes were broken on the Big Island, bug significant damage was limited to a hospital building, some big roofs on stores, and a few blocked or cracked roads. There were a lot of dry rock walls (no mortar, just stacked rocks) and retaining walls that collapsed, damaging some houses and cars. KITV has more of the story and lots of photos on their website.

We’re not really worried about earthquakes here on Kauai, we know they happen at least every decade on the Big Island because of the volcanism, but like today’s they are not felt stronly here. I think they have tiny quakes on Kauai probably due to the island settling, but I think we have nothing to worry about. Of course, a tsunami is always a threat, but we bought our house uphill about 150′ (50 m) and a mile (1.6 km) from the ocean, on purpose.

Just as an aside, I wonder how long it will take FEMA to update their website:

Source: fema.gov, 14 hours after today’s earthquake