Feasibility of Monitoring Rock Fall in Yosemite Valley using Seismic Methods

Myers, S ; Rock, D ; Mayeda, K

Lawrence Livermore National Laboratory

关键词: Seismic Noise;    Amplitudes;    Wave Forms;    01 Coal, Lignite, And Peat;    P Waves;   

DOI  :  10.2172/15001990
RP-ID  :  UCRL-ID-137890
RP-ID  :  W-7405-ENG-48
RP-ID  :  15001990

美国|英语

来源: UNT Digital Library

【 摘 要 】

Public awareness of rock-fall hazard in Yosemite Valley has heightened after events in 1996 and 1999. Reports of audible blasts prior to rock-fall events suggest that rock cracking may in some instances precede the detachment a block from the cliff face. Seismic methods may be used to detect and locate small, inaudible rock cracking events, resulting in a catalog that outlines active areas and quantifies the level of activity. In order to test the feasibility of monitoring rock-fall activity with seismic methods, the Test Yosemite Rock-Fall Network (TYRN) was operated in the late summer and fall of 1999. The TYRN included five stations in the vicinity of the 1999 rock fall events: 2 stations at the base of the cliff and 3 above. Location of events depends on an estimate of seismic-wave velocity. During the TYRN deployment, a septic tank near Glacier Point was demolished, allowing the velocity of seismic P-waves to be calibrated. P-wave velocity was found to be about 5.68 km/s. Recordings of the explosion also allow assessment of arrival time precision, which controls the precision of seismic locations. Explosion recordings suggest that P-waves can be picked with a precision of about 0.005 seconds, suggesting that a seismic monitoring system would be able to locate events on the cliff face with sufficient precision to be useful in rock-fall monitoring. We used the amplitude of seismic noise recorded on the test network to determine the smallest event likely to be detected by the TYRN . An event with equivalent earthquake magnitude of -2.6 would be detectable at a sufficient number of stations to afford a location. This magnitude is equivalent to about 1.8 centimeters of slip on a surface with area of 1 square meter. Smaller displacements would be detectable for larger slip surfaces. The vast majority of events recorded on the TYRN were from the Mammoth Lakes region. About 5 to 6 events from the Mammoth Lakes area were recorded per day, but considerably more events were recorded on active days. A number of ''suspect'' events were also recorded by the TRYN. We term these events suspect, because the seismic waveforms have the characteristics that we expect from an event originating on the rock face. However, these events were not recorded at enough stations to determine a location. Based on the calculations for the minimum detectable event, we believe that the suspect events are tiny. We conclude that seismic monitoring of rock fall activity is Yosemite theoretically feasible, but the detection of rock-fall-related events has not been verified. The lack of verifiable events occurring on the cliff face during the operation of the TYRN may be due to the warm, dry weather during the experiment. During the experiment there were no reports of rock fall or audible events. A deployment during the winter and spring, when snow melt and precipitation tend to increase activity, may produce more definitive results.