Yosemite Rock Fall

At 6:52 pm Wednesday 10 July a 400' slab of granite fell about 2500' onto the
Happy Isles area of the valley floor in Yosemite National Park.  This event
was recorded by three broadband stations (CMB, KCC, & MLAC) within a radius of
80 km.  


The source parameters are:

Location:		37.733, -119.563	(Happy Isles)

Origin Time:		96.193,01:52:54		(Inferred from 1.16 s period
			6:52 pm local time	Lr waves at CMB & KCC)
			Wed 10 July

ML:			2.15 +/- 0.027 		(Average of CMB, KCC, & MLAC)

Mo:			1.9E19 dyne-cm		(Inferred from ML)

Potential Energy:	2.E20 ergs		(Estimated from ~500k cubic
						meters of granite falling 750
						meters)


The origin time was determined by measuring the arrival time of the dominant
1.16 sec period Rayleigh waves at CMB and KCC:

Stn	Delta	 Time

CMB	79.9	53:14.43

KCC	50.3	53:06.90

The propagation paths from Yosemite to CMB and KCC are entirely within the
Sierran block.  Assuming that the propagation velocity is the same to both
stations, we get v(Lr) = 3.9 km/sec.  The propagation time is thus 20.4 sec to
CMB and 12.9 sec to KCC, inferring an origin time of 01:52:54.  Note that MLAC
was not used in this calculation because of the propagation path complexity
(it traverses the boundry between the Sierran and the Basin and Range
provinces).


As usual, ML was determined from Wood-Anderson seismograms synthesized from 
the broadband recordings:

Stn	Delta	Azim	ML

CMB	79.7	295.1	2.195

KCC	50.3	154.4	2.12

MLAC	65.1	 99.9	2.14

Where a -0.2 ML station adjustment (appropriate for a soft alluvial site) was
assumed for MLAC.


Mo is about 10 percent of the estimated potential energy released by the rock
fall.  Thus the seismic wave generation for this source is relatively 
efficient.  Also, the observed Rayleigh wave excitation and relative lack of
Love wave excitation at CMB and KCC is consistant with excitation of seismic
waves by a point downward force.  The MLAC seismograms are relatively noisy and
also more complex than are the CMB and KCC seismograms.

Bob Uhrhammer