Saturday, March 30, 2002

I Feel the Earth Move Under My Feet

It's just about impossible to live in California without, at some point, becoming inquisitive about earthquakes. Major fault lines run up the western coast passing within a mile or two of us, and a few run inland. Together these faults produce a plentiful supply of mild tremors - 314 in the last 7 days - and the occasional, more attention-grabbing, certified earthquake. The speed with which these tremors and quakes are analysed, catalogued and made available for online review is stunning. As Debbie mentioned in her previous blog, when we experienced a tremor measuring just over 3 on the Richter scale, the event was recorded online within a minute. By then, we knew the following information about the tremor:

  • where it was located (latitude and longitude) and its proximity to nearby populated areas
  • the depth from which it emanated
  • its measurement on the Richter scale (and, consequently, its official designation as a minor quake)
  • the time at which it occured, to the nearest second
  • the quality of the data about the tremor that had just been captured

This from the same country that took a couple of months to decide who had won arguably the most important election in the free world. Maybe next time the US could get people to vote by jumping up and down at predetermined times and then declare the election based on the resulting seismic activity.

Anyway, this plethora of real-time data led me to wonder about what other, historical data might be available to help answer questions about the relative magnitude of the minor quake we experienced. What was a 'typical' quake for this area? What's the largest quake that's been experienced here in recent history? What did this area experience during the big quake of 1989?

Sniffing around the web, I found The Northern California Earthquake Center, which carries extensive information about quakes in this part of the world and which allows visitors to query online its extensive quake database. I decided to analyse all the quakes since 1970 that had epicentres within about 1 degree of latitude and 1 degree of longitude of here, and which were of magnitude 2.0 or greater on the Richter scale. Here's a summary of the information:

  • In a typical year, this area will experience between 25 and 40 tremors of magnitude 2.0 or greater - about 2 or 3 per month.
  • The relatively quake-free years have been 1974 (16 quakes) and, ominously as you'll realise from the next bullet point, 1988 (17 quakes).
  • In 1989 - the year of the disastrous Loma Prieta quake - this area experienced 92 quakes, including an amazing 38 quakes at the time of and just after the Loma Prieta quake (4pm on October 17th). One of the quakes on October 17th was measured here at 5.1 on the Richter scale, the largest quake in this area for the time period I analysed, but still less than the magnitude which is the minimum usually associated with damage (namely, 5.4). Nearer its epicentre, around Santa Cruz, this same quake measured 7.1. This difference - 7.1 compared to 5.1 - means that the seismic energy produced near the epicentre was over 1000 times that produced here.
  • The year 1990 produced a further 100 quakes. Subsequent years have resumed the pattern of 25-40 quakes per annum.
  • This year (to March 30) we've had 12 quakes already, only slightly above what you might expect in a typical year.
  • Our recent quake ranks only 91st among the 1,155 quakes that have been recorded here since January 1st 1970.

All this talk of earthquakes leads nicely into a discussion of our next trip.

Calistoga & Sonoma - 26th March 2002

Old Faithful Geyser

Our first stop for the day was at the Old Faithful Geyser in Calistoga. Originally this was to be our second stop, the first stop instead being at the Sterling Vineyard, also in Calistoga. What had attracted us to the vineyard was the opportunity to ride its aerial tramway up a steepish hill to its tasting rooms but, when we arrived, the tramway was under repair and the alternative offer of a minivan ride to the top was insufficiently appealing to prise us from the car. As Debbie so astutely pointed out, with so many vineyards in the Napa, each tries desperately to craft a point of differentiation. One vineyard, for example, offered wine-tasting and beeswax candles, the thinking I suppose being that this doubled their chances of putting something on your table that night. Well, a Sterling Vineyard without an aerial tramway was like a month without a minor quake, and we were having none of it.

Old Faithful Geyser The Old Faithful Geyser in Calistoga (pictured at left) represents an interesting blend of nature and marketing. There's no doubt that the 350°F jets of water that spurt out 60 ft into the air at regular intervals are heated from deep within the earth and are ejected by natural forces. But the hole through which the water is expelled is not at all a natural phenomenon : it was drilled in the 1800s and has only remained open through subsequent interventions by humans intent to preserve the geyser's tourist-attracting and hence money-producing abilities.

On average, throughout the year, the geyser erupts every 30 minutes. On the day we were there it was erupting every 8 minutes due, apparently, to recent rains. Upon paying the entrance fee we were informed that these more frequent eruptions were not cause for concern, but that the opposite phenomenon - a cessation of eruptions - is often the harbinger of a "significant quake" in the next "2 days to 2 weeks". This sounds somewhat impressive until you read on the web site the qualifying phrase that the quake will occur somewhere "within 500 miles or more from the area" of Calistoga. Given the level of seismic activity here in California, this claim seems to me a bit like saying that there will be sunshine tomorrow somewhere in Brazil. But, to be fair, the claim does seem to be receiving some serious scientific scrutiny which, perhaps, might narrow the predictive window's timespan and geographic expanse.

Petrified Forest

Petrified Forest - The Queen Just a few miles from Old Faithful lies another natural marvel, this one indisputably authentic, called The Petrified Forest. Here lie the petrified remains of giant redwoods, the tallest, The Monarch, is 105ft in length and 6ft around, making it "the largest intact petrified tree in the world". The Queen, pictured at right, is 65ft long but a massive 8ft around.

So, how did the trees come to be petrified? Well, 3 million years ago (although I suppose its now 3 million and one since the brochure I'm reading was probably published a year ago), Mt. Saint Helena erupted and covered the forest - and a bunch of other stuff besides - in boiling hot ash and water. Over time, the logs rot and molecules of dissolved silicon and oxygen from the ash are carried down to the logs by the water. The molecules of silica eventually replace all the molecules of wood and what's left is a block of solid silica that looks exactly like the tree whose molecules it replaced. Amazing what you can get done when you've 3 million years to do it.


Sonoma : Cow On the way home we thought we should stop at Sonoma, a city we had first visited a few years earlier whilst on a coach tour. Well, things had changed. The stores that we expected to find weren't where we expected to find them or, in one instance, had been totally remodelled. And cows were everywhere. An example is the ugly specimen at left, recently relocated to the city's otherwise picturesque plaza, whose creation was sponsored by the City's Cultural and Fine Arts Commission. It seems that cows have taken on some sort of mascot status for the area, though I'm unable to find any online acknowledgement of this. On the wall of a shop across the road from the central plaza, I saw a sign heralding the marvellous qualities of the "meadow muffins" produced by local cows. Perhaps Mad Cow's Disease has spread a little further around the globe than scientists first thought ...

Originally posted by TC

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