> Brian <bcl...
@es.co.nz> wrote in
>> Just one question.
>> Over the last few years has there been an increase in earthquake
>> activity. Please don't give me a lot of data that I may not understand.
>> Just a few lines of text that indicates Yes, No or Maybe is all I ask.
> Short answer: No. Well, maybe, but probably not. Then again... ?
> Longer, better answer: The problem is with the short duration
> of earthquake record keeping relative to geologic timescales.
> The Earth is 4.5 billion years old and we only have detailed
> records for maybe the last 50 years? (all quakes globally
> down to mag 4.5 or so) That's not even a gnat's kidney stone
> in a drop of its pee in the ocean, if that.
> What I'm saying is, although there may be more quakes "in
> recent years" compared to the last few decades, we don't
> really know what that means in the grand scheme of things.
> Perhaps this is just an uptick in activity that happens
> from time to time. We don't know. We don't know what may be
> the long term pattern over hundreds or even thousands of years.
> Compare this to the weather (setting aside 'global warming').
> Some years you get more rain than average. Some years are
> hotter than average. There are fluctuations. But we've been
> recording the weather far longer than we've been recording
> quakes (in any detail) and thus can make a measured guess
> as to the long term average. We can't yet do that for quakes.
> Locally, that might be a different story. Paleoseismology
> techniques (digging trenches across faults to examine slip
> events) allows scientists to judge recent activity back
> hundreds of years. For example, the southern San Andreas
> is now known to have had 7 major events in the past 1100
> years, with an average recurrence interval of about 180
> years. It has been more than 300 years since the last event.
> (scary, since I'm in it's area)
> In another example, Parkfield has had ~mag 6 quakes in 1857,
> 1881, 1901, 1922, 1934, and 1966. This gives an average
> recurrence interval of 21.8 years, meaning the next quake
> should have occured in 1987 or 1988. It didn't happen until
> 2004, a gap of 38 years and almost 17 years 'late'.
> So even when we do have some data, it's still difficult
> to say.
> Since quakes happen so infrequently, especially large ones,
> we might need tens of thousands of years of data in order
> to get a meaningful 'average' or trend of global rates.