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

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]