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Re: 7.0 Landers Aftershock

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I would be very cautious when comparing the accelerations reported by 
trinet with the reported damage.  Because the array of instruments is 
so sparse (especially near the Hector Mine Earthquake), most of the 
data is simulated using the 1981 Joyner & Boore attenuation 
relationship.  (As an aside, I'm not sure why they are not using the 
more recent 1993 Boore & Joyner relationship, which is based on more 

The largest recorded values are significantly smaller than the 
simulated values, because the nearest instrument wasn't at ground 
zero.  The maximum recorded ground acceleration and velocity were 
32.5%g and 37 cm/s.  The simulated values at ground zero (from the 
small contour maps) appear to be about 60%g and 70 cm/s.  What is not 
discussed on the trinet site is the very significant scatter in the 
attenuation relationship.  For a M7.1 event, the 1993 B&J relation 
(which is reported to have less uncertainty than the 1981 
relation) has (random, horizontal) PGA values at ground zero (NEHRP 
Site Class D) as follows: 

mean + 2 sd:  190%g 
mean + 1 sd:  112%g
mean:         65.7%g
mean - 1 sd:  38.7%g
mean -2 sd:  22.8%g

Based on these statistics, it's fairly likely (say 90% chance) that 
the actual acceleration is between half and twice the calculated 
(mean) value.  However, that is an awfully big range for drawing 
meaningful conclusions.


> You may find it interesting to see how large the ground motions were in the
> populated areas, especially in the light of the lack of damage that has been
> reported so far.
> Chris Poland

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Michael Valley                                   E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201