An ML = 4.4 earthquake occurred near Yucca Mountain, the potential site of high-level nuclear waste repository, on 14 June 2002 at 12:40:44 UTC (05:40:44 PDT). Using a 30-station seismic network surrounding Yucca Mountain, the NSL determined its location to be 36.7150 N, 116.3003 W, with a depth of 12 kilometers. This location is approximately 20 km southeast of the potential repository at Yucca Mountain and just north of US Hwy 95 between Indian Springs and Beatty, Nevada. View the location of this earthquake in relation to Las Vegas, Nevada.

The 14 June 2002 earthquake occurred in the aftershock zone of the M 5.6 Little Skull Mountain (LSM) earthquake of 29 June 1992. Its epicenter approximately abutts the southwest side of the LSM rupture area outlined by Smith et al. (2001), and its depth is essentially the same as the 1992 hypocentral depth of 12 km. This suggests that the recent event is a continuation of rupture on the southeast-dipping, high-angle fault plane preferred by Smith et al. Additional aftershock recordings from the recent event should help to clarify this relation. Hundreds of aftershocks from the ML = 4.4 event have already been recorded, with the largest being an ML = 3.0 event approximately one minute after the main shock and the second largest an ML = 2.7 event on 15 June 2002 at 06:03:24 UTC.

The LSM aftershock area has been active since the 1992 earthquake, but this is the largest event in the aftershock zone since September 1992. There have been 5 M > 4 aftershocks of the LSM earthquake recorded and 102 M > 3 aftershocks. The Nevada Seismological Laboratory has continued to record numerous small earthquakes in this aftershock zone throughout the past 10 years, and these aftershocks have comprised roughly one-half of the earthquakes located by the Yucca Mountain digital seismic network over the past six years. View a map of the most recent year's events located by this network in relation to the 14 June 2002 event. Because the recent ML 4.4 event comes 10 years after the LSM earthquake, it represents a stretch of the term "aftershock".

The 14 June 2002 event was preceded by a steady increase in activity within the LSM aftershock zone. In the preceding year, aftershocks of the LSM earthquake were recorded at the rate of roughly 5 per day by the station LSC; but this rate had increased to around 15 per day in the last month before the 14 June 2002 event. Whether or not these should be considered "foreshocks" is unclear.

The focal mechanism of the 14 June 2002 event was determined with P-wave first-motion polarities. This focal mechanism, representing the lower hemisphere projection, shows either a northeast-striking fault plane with a dip of 55 degrees to the southeast or a south-striking fault plane with a dip of 45 degrees to the west. A small component of strike-slip is suggested by this mechanism, but it is dominantly dip-slip. In accord with Smith et al. (2001), the plane that dips to the southeast would be the preferred solution. The dip here (55 degrees) is somewhat less than their 70 degree dip but sufficiently close, given some error in both focal mechanism solutions, that it is reasonable to infer that the recent earthquake slightly extended the rupture zone of the LSM earthquake. The tension axis of the new mechanism agrees almost exactly in azimuth with the mean of those for focal mechanisms determined for early LSM aftershocks (Harmsen, 1994). The seismotectonic interpretation is presented in a separate report.

The 3-D view of aftershock hypocenters of the 14 June 2002 event shows the spatial distribution of over 200 of the larger aftershocks. These hypocenters were computed using P arrivals only from the stations LSC, SPC, STH, and SYM. Only events whose arrivals at all four stations were clear and impulsive were used. The double-difference location program HYPODD was used to compute the hypocenters. There is an apparent alignment of hypocenters which mark the inferred SE-dipping plane of the main shock, but these are limited to the northern part of the aftershock zone. To the south of the zone, the picture seems more complex, with a possible N-S plane.

The NSL operates 19 strong-motion sites around Yucca Mountain in addition to the 30-station seismic monitoring network. Ten (10) of these strong-motion sites are collocated with regular seismic stations, and therefore the data is telemetered in real-time to the NSL. The data from the remaining 9 non-telemetered sites was retrieved shortly after the earthquake. Peak horizontal ground accelerations, in cm/s/s, were read from the recordings of the strong-motion sites and plotted on a map of all 19 strong-motion sites. (One of the 10 telemetered sites - YCW - failed to provide data.) The largest acceleration was recorded at station LSC, which is roughly two km horizontally from the epicenter but over 12 km from the hypocenter. One of the two horizontal components at this station was unfortunately malfunctioning. The available peak accelerations are plotted versus epicentral distance to show their falloff with distance. The accelerations at the two telemetered stations just west of the NTS boundary are at Yucca Mountain itself. The station with 3.6 cm/s/s is inside the Exploratory Surface Facility (ESF) at Alcove 5 (called AL5). The station with 9.4 cm/s/s is a surface station (called RPY). The amplification of ground motion at the surface, relative to the tunnel, inferred from these two readings is corroborated by many previous weak-motion recordings at these same two sites. A cluster of three non-telemetered sites just east of the NTS boundary is near the ESF north portal, and one of these (PGA = 32 cm/s/s) is at the location of future waste-handling facilities. This station is actually inside a building and may not be representative of true ground motion.

The horizontal strong-motion recordings were used to determine a moment for the 14 June 2002 event. The preliminary average moment was estimated from the low-frequency asymptote of the Fourier amplitudes by fitting an omega-square spectrum with a single corner. The analysis used 17 components at 9 sites, and found an average seismic moment of 2.0 * 10^22 dyne-cm, which translates to an MW of 4.1 based on the formula used by Harvard University for their global moment tensor calculations (originally proposed by Hanks and Kanamori, 1979): 

MW = 2/3 log M0 - 10.73

A preliminary moment tensor analysis by the University of California Berkeley seismic station gives a significantly larger moment of about 7.86 * 10^22 dyne-cm, corresponding to MW=4.5 using the above formula. The strong motion estimate of the seismic moment is predominantly influenced by frequencies near 1 Hz, while the Berkeley value uses much lower frequencies. The difference may represent significant energy that was treated as potential noise in the first look at the preliminary strong motion spectra. The discrepancy strongly suggests that the spectrum deviates significantly from an omega-square spectrum with a single corner frequency. Further refinement of the recent event's magnitude and moment is planned: 1) most importantly, the data from the weak-motion stations needs to be included in the moment estimation, 2) the SGBDSN magnitude estimate of ML=4.4 does not include distant NSL stations not in the Yucca Mountain network (SGBDSN); these NSL stations give an average ML of 4.7.

The strong motion data will be useful for estimating the earthquake's stress drop, and for comparison with models used in the seismic hazard analysis. Assuming that the spectral amplitudes from frequencies substantially lower than 1 Hz may be treated as signal, the moment and stress drop dominating the strong motion may be a measure of a subevent that dominates the high frequencies of the spectrum, rather than an average characterization of the entire faulting process. This type of phenomenon was recognized by Anderson et al (1986) in a much larger earthquake.
 

This preliminary report was initially prepared by John Anderson and David von Seggern, with further contributions by Ken Smith and Glenn Biasi, all with support from Diane dePolo, Arturo Aburto, Tom Rennie, Aasha Pancha, and Robin Courts.

References:

Anderson, J. G., P. Bodin, J. Brune, J. Prince, S. Singh, R. Quaas, M. Onate, and E. Mena, (1986). Strong ground motion and source mechanism of the Mexico earthquake of Sept. 19, 1985, Science 233, 1043-1049.

Hanks, T. C., and H. Kanamori, 1979. A moment magnitude scale, J. Geophys. Res., 84, 2348-2350.

Harmsen, S., 1994. The Little Skull Mountain, Nevada, earthquake of 29 June 1992: Aftershock focal mechanisms and tectonic stress field implications, Bull. Seism. Soc. Am., 84, 1484-1505.

Smith, K. D., J. N. Brune, D. dePolo, M. K. Savage, R. Anooshehpoor, and A. F. Sheehan, 2001. The 1992 Little Skull Mountain earthquake sequence, southern Nevada Test Site, Bull. Seism. Soc. Am., 91, 1595-1606.