Quakes: All you wanted to know but were afraid to ask

October 24, 2011 4:10 pm by Tracey Taylor

Map showing the Hayward Fault. Source: USGS

Last week’s series of earthquakes in Berkeley had Berkeleyans, including Berkeleyside readers, all a-twitter about the possible significance of the rash of tremors, their concentration and location. We spoke to geophysicist Paul Caruso at the National Earthquake Information Center to sort out fact from fiction.

What can you tell us about the recent quakes centered in Berkeley?
The magnitude 4:0 quake [which was felt at 2:41 pm on Thursday October 20] was followed by several aftershocks in the area of rupture over the next few days as the earth tried to come back into equilibrium. Aftershocks are defined as being smaller than the original quake.

Some Berkeleyside readers said they thought a series of small quakes was a good thing because it indicated a “release of pressure” on the Hayward fault line; others said it indicated a “build-up to a big one”. Are either of these ideas valid?
Both are legitimate theories. The truth is we don’t know whether earthquakes like these are relieving pressure or whether pressure is building.

Why don’t you know?
Earthquake science is very young — we only have 100 years’ worth of data for potentially 4 billion years’ worth of activity since the continents were formed. That’s not much to extrapolate back on.

Why can earthquakes with the same magnitude feel different?
Magnitude is not the be-all and end-all. Depth comes into it too. A large earthquake that is very deep may not be felt as much as one that is closer to the surface. For example, the 2001 earthquake in Seattle had a high magnitude but was very deep. [The Nisqually earthquake on February 28, 2001, measured 6.8 on the Moment magnitude scale and had a recorded depth of 32 miles.]

Are we any nearer to being able to predict earthquakes?
No. There have been no breakthroughs in earthquake prediction. There is a private company in California that claims to be able to make predictions. But nobody has ever got predictions right, be it the time, location or the magnitude.

It’s well documented that seismologists think the SF Bay Area is due a “big one”. How is “big one” defined?
The largest magnitude that a fault can generate is directly related to the size of the fault and the segment of it that ruptures. There are several large faults in California, so we think the largest quake that could happen in California is a magnitude 7.0 or 8.0. [The USGS states that the overall probability of a magnitude 6.7 or greater earthquake in the Greater Bay Area is 63%.] A large or big earthquake is where there is damage and death. A “great” earthquake is one that is a magnitude 8.0 or greater.

Are seismologists confident of that range for the fault, or is there a potential for a significantly larger quake?
It’s true that the thinking for Japan was that the largest quake they could experience was a 7.0 or 8.0. [The March 11, 2011 earthquake off the Pacific coast of Tohoku was a magnitude 9.0.]

What do people mean when they talk about “earthquake weather”?
We talk about earthquake weather here [at the National Earthquake Information Center]. It’s a joke. Earthquakes are totally independent of the weather!

For answers to every question you ever may have had about earthquakes — including how their intensity is measured and the incremental increases in their energy between magnitudes — check out the FAQs section of the USGS website.

Related:
Quake hits Berkeley: 4.2 downgraded to 3.9, then 4.0 [10.20.11]
Berkeley’s fourth quake of the day at 2307 Piedmont Avenue [10.20.11]
Trio of earthquakes gently shake Berkeley early Saturday [10.22.11]

12 Comments

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  • bblomer

    It’s unclear what Paul Caruso is saying about the strength of earthquakes possible on the Hayward fault.
    I do not know what the “there” relates to. The Hayward, a segment of it, the part of it running under Berkeley or any fault in California?

  • Candopan

    SF is screwed, when the big one hits.

  • Anonymous

    I live 2 blocks from the fault (Oxford/Los Angeles), less than 2 miles from the epicenters of last week’s quakes. The motion seemed impossible to judge, was it a vertical thrust followed by vibrations? Reading physics teacher Lee Trampleasure’s earlier comment that a 7 pt. quake would be 1000 times (!!!) more powerful a 4.0, I wonder what we might have in store. Will buildings above this fault always receive that vertical lift and fall?

  • Anonymous

    Thanks for the information! Although it is not very consoling, it’s the kind of things we Berkeley residents should know. And I didn’t see it in any other newspaper. Good work, Berkeleyside! Another thing I am wondering: was there much retrofitting in Berkeley after the ’89m quake,  especially to apartment buildings with basement garages. I live in one of those and the construction form does not seem the most stable!

  • Mineuhold

    Live in Aussie land, my dog is my earthquake alert at all times. Anything within and around Aussie land cheers

  • Mike Farrell

    Thats a mexican Vulcan, right?

  • Anonymous

    There may be no science behind earthquake weather but as a child my mother would send us to school and countless times in October she would say “be careful today it’s earthquake weather” and almost without fail she was correct. she called me late morning the day of the 89′ earthquake and said the same. So i strongly disagree once again there may be no “science” to confirm it but i know what i experienced. also why are there so many people who claim that there is a correlation? are they all just crazy? a coincidence i doubt it, maybe it’s the time of year( October )  and it just so happens to be warm and still in October here in the bay area but then why October for the increased activity?

  • Lhasa7

    Berkeleyside has a spam filter?

  • Harry

    Yeah, who you gonna believe, science or your mother?

    There’s no correlation between weather and earthquakes. Sorry about that. What causes earthquakes is quite well known — as is the human tendency to confuse correlation with causation.

    I highly recommend “Living with Earthquakes in California” by Robert Yeats, which comprehensively and readably covers the causes, history, geology, and geography of earthquakes, the state of earthquake science and prediction, and preparing yourself for the inevitability of a large quake in California. (PS: It’s 400 pages long, and doesn’t mention ‘earthquake weather’.)

    It’s going to happen sooner or later, and there almost certainly won’t be any warning, so obsessing about whether a ‘small’ quake is a foreshock or aftershock is fairly pointless. Just get yourself ready with water, food, and disaster training (offered by the city), and we’ll all make it ok.

  • Blanquitochulo81

    I can’t speak specifically to Berkely but I live in the Imperial Valley, near the Mexican border. We are on top of the plate boundary (Imperial Fault) and there are many other parallel fault strands nearby.

    We quite often experience quakes that have an epicenter in our immediate vicinity-mainly 3s and 4s. I would describe these as “explosive” movements. Sometimes it feels like a bang or explosion when you are very close to a quake’s epicenter or near the fault it generated on. Sometimes it’s just one giant or strong movement up and down or sideways. That will vary. What will be constant is that if a quake ocurrs near you, it will generally be shorter and stronger. When quakes happen at a distance, they usually give warning, start out slower and build up, or are just a slight nudge, depending on intensity. It seems like the recent quakes that the East Bay has had are similar to what we have down here somewhat regularly. When we get the rare rolling motion, we know it was from a quake with an epicenter “far” away…like near Palm Springs or in the Gulf of California.

    Good luck to you all up there, stay safe.

  • Anonymous
  • Anonymous