Rock Fall in Yosemite National Park, CA

General Information from the UC Berkeley Seismographic Station
Region:	Happy Isles Area of Yosemite Valley
Event:	First	Second
Date:	96/07/11	96/07/11
Impact Time (UT):	01:52:28.0	01:52:41.6
Latitude:	37.729	37.729
Longitude:	-119.561	-119.561
ML:	2.15+/-0.028	~1.55

At 6:52 p.m. PDT Wednesday, July 10, 1996, a large block of granite, with an estimated volume of 35,000 to 80,000 cubic yards (27,000 to 62,000 cubic meters), detached from the cliff between Washburn Point and Glacier Point, in Yosemite Valley. The rock mass subsequently launched from the cliff, free-fell ballistically an estimated 1800 feet (550 m) prior to impacting about 200 feet (60 m) from the base of the cliff in the Happy Isles area of the valley floor in Yosemite National Park (personal communications, Gerald Wieczorak and Robert Norris, 1996). The rock fall was well recorded by 3 UC Berkeley (BDSN) and Caltech (TERRAscope) broadband seismographic stations and 15 short period seismographic stations (operated by the UGSG in Menlo Park and the University of Nevada, Reno). In fact, it is the largest vertical rock fall ever recorded seismically and it registered on seismographs up to 200 km away.

The estimated rock mass of the granite is 80,000 to 184,000 tons (73,000 to 167,000 metric tins). Under ideal free-fall conditions, the rock would take 10.6 seconds to fall 550 m and it would impact with a velocity of approximately 100 m/s (230 mph) and release a gravitational potential energy of 3.9E11 Nm to 9.9E11 Nm (120 - 280 Megawatt-hours).

A local (Richter) magnitude (ML) of 2.15+/-0.028 was determined from Wood-Anderson seismograms synthesized from broadband recordings at the three closest broadband stations:

where: a dML of -0.2 for MLAC, appropriate for a soft alluvial site, was assumed.

This map, centered on the impact site, shows the locations of the stations, located within 50 km, which were used in the analysis. The epicenter, inferred from phase onset times, does not differ significantly from the impact coordinates (the uncertainty in the solution is +/- 1.5 km).

This figure shows vertical component seismograms at the 12 closest stations band-pass filtered from 0.55 Hz to 0.90 Hz to emphasize the Rayleigh (Lr) waves generated by the rock impact. The stations are arranged in order of increasing distance from bottom to top. Note that there are two envelop maxima in the recorded waveforms approximately 13.6 seconds apart, with the second maxima approximately four times the size of the first. THe implication is that there were two prominent rock impacts, 13.6 seconds apart and that the second one contained about 80 percent of the total rock mass. Scaling the amplitudes, we infer that the equivalent ML for the first impact is 0.6 units smaller of approximately ML 1.55. The apparent phase velocity of the Lr(z) Rayleigh wave is 2.94+/-0.114 km/s.

This figure shows the broadband recordings from the Berkeley Digital Seismic Network (BDSN) stations KCC, located in a granite tunnel 49.9 km SE of the Happy Isles impact site. The records have been band-pass filtered, from 0.55 Hz to 5.0 Hz (the dominant frequency band excited by the rock fall) and the horizontal components have been rotated to radial and transverse. The prominent phases recorded on this record are also consistent with two impact events separated by 13.6 seconds:

where: 1 and 2 refer to the impact number. The corresponding detachment generated seismic waves for the two events are not apparent in the record (the first is below the noise level and the second is obscured by the waves generated by the first impact. From the phase arrival times of the two Lr(z) phases, and the measured Lr(z) velocity of 2.94 km/s. we infer that the two impact times were 01:52:28.0 UT and 01:52:41.6 UT, respectively.