Saturday, May 30, 2009

Earthquake, but not quite here

Roatan earthquake 7.1 Did you feel it?
What we got from May 28th's earthquake 39 miles northeast of Roatan, about 300 miles from us, was a weird little shaking of the rig, as if the wind had suddenly come up or a squall were about to begin. It was more of a curiosity than an event. When I, or my zombie, got up to close the ports and hatches, I was surprised to see that the night was starlit and absolutely still at about 2:25 AM. The distant sound of a car alarm in Fronteras was a puzzle - alarms and sirens are hardly part of the soundscape here.

We did hear of damage in Belize, in Placencia, to the fuel dock and water tower, in Mango Creek and Independence, and in Monkey River Village, where liquifaction of the earth dropped buildings eight to ten feet. Superficial soil cracks were reported, and water being pushed up out of the earth.

Liquefaction would be an issue here as well. Seismic waves passing through loose, unconsolidated coastal alluvium makes it 'liquid' for a few brief moments, long enough for pilings to slip through and slabs to sink.

Just the other day we were looking at a ramshackle marina where this very phenomenon had occurred in a 1989 earthquake. They just spliced stubs on top of sunken pilings to hold up the roof, and relaid a new floor over the old one. Walking down some of the docks is like trying to balance on a barrel. True bedrock is about 32 feet down, and of course reaching it is the ideal, not the standard.

Here's a picture of a dock being built. These guys are pounding the piling in with a wooden block they manually lift and drop. Manually driving pilings

A lot of pilings seem to be set this way. I've yet to see a barge with a pile driver, but the houses and boat sheds of the rich are done 'to code' so such equipment must sometimes be available. Even a block and tackle, or a see-saw arrangement, would help, but no one asked me!

Update: a month later that dock is almost done and looking plumb and level and quite professional. The pounding block that I thought was wood is a metal box with handles on the side, so heavy (concrete inside?) that Doug couldn't budge it, although admittedly, he only tried with one hand. And yesterday we drove past a piling the height of a telephone pole, two short ?transepts?, each with a single brace. The guys were just quitting for the day. There was only room for two to stand, and their pounding block was smaller. It really gives me pause to see such manual labor thrown at a problem mental labor could have solved. At the same time, I can sort of understand it, myself being impatient to get things done. My father once told me that 'smart' was getting someone to do something for you. By that definition, I've never been 'smart'!

The same earthquake in 1989 also damaged the Rio Dulce bridge and it was 'glued and screwed' back together by the US Army Corps of Engineers.Rio Dulce bridge repair

We walked to the top of the bridge, two days before the quake, to see what was going on up there. It was a blazing hot day, after lunch, and we passed the man with the Polaroid camera heading down. But the ice cream vendor and friends were still there, cooler in the breeze than they'd have been in town.

People are always gathered in the shady park under the bridge, bathing and doing laundry.Photobucket

Here's a picture looking downstream, towards Mario's Marina, where we'll shortly be installed for the season. Photobucket

Guatemala Seismicity Map

More about earthquakes here:

About May 28, 2009, they say:

At least 6 people killed, 40 injured and more than 130 buildings damaged or destroyed in northern Honduras. The central span of a major bridge at El Progreso was destroyed. At least 5 buildings destroyed and 25 damaged in Belize. Felt in much of Belize, El Salvador, Guatemala and Honduras. Also felt in the Cayman Islands and in parts of Colombia, Costa Rica, Cuba, Jamaica, Mexico, Nicaragua and Panama. Seiches were reported in swimming pools at La Ceiba and Roatan and ground cracks and possible liquefaction was observed at Monkey River, Belize.

The location and focal mechanism of the Honduras earthquake of May 28, 2009, imply that the shock occurred as the result of left-lateral strike-slip faulting on the Swan Islands Transform Fault, a segment of the boundary between the North America and Caribbean plates. In this region the plate boundary accommodates about 20 mm/y slip.

1 comment:

curtw said...

I too am beginning to see passing thundershowers stretch out and linger or even back slide. Today while waiting near the beltway with all aerial gear ready a thick dark smokey gray thundershower came up. It billowed line the collapse of the North building at suction was as much a force as expansion. I did not need the salt water washed off....