Preliminary Analysis of the June 14, 2002,
M 4.4 Little Skull Mountain, Nevada, Earthquake and Strong Motion Recordings
University of Nevada Reno
Nevada Seismological Laboratory
July 11, 2002
The June 14, 2002, M 4.4 earthquake that occurred within the aftershock zone of the M 5.6 June 1992, Little Skull Mountain (LSM) earthquake may be the largest aftershock of that sequence. The event occurred approximately 20 km southeast of Yucca Mountain (Figure 1). To date 75 events from the June 2002 sequence have been located and the distribution of these preliminary locations can be interpreted within the context of the seismotectonics of the LSM sequence presented by Smith and others (2001) and Meremonte and others (1995).
Preliminary aftershock locations are of good quality due to the dense distribution of seismic stations surrounding the LSM area. In particular, a recently installed station on Paleozoic rocks at Stripped Hills south of LSM, shows especially good signal to noise and closes the location gap to the south. Figure 2 shows the distribution of post June 14 earthquake activity plotted on relocations of the 1992 LSM aftershock sequence from Smith and others (2001). The orientation for the perspective view (Figure 3) is also shown on Figure 2. The recent activity is confined to the western side of the 1992 LSM aftershock zone and overlays 1992 aftershock activity. From this relationship, the June 2002 sequence can be interpreted as additional LSM aftershocks. Note that the June 14 M 4+ event occurred near the hypocenter of the 1992 M 5.6 LSM mainshock. The NE striking SE dipping1992 LSM mainshock fault plane is defined by the NE alignment of aftershock activity extending NE from the 1992 LSM Mainshock epicenter (Figure 3).
A perspective view (Figure 2) of June 2002 activity implies that the recent sequence has involved two faults within the LSM aftershock zone. One structure strikes NE and may represent the western extent of 1992 mainshock faulting and another strikes SSW dipping at a high angle (60-70 degrees) to the east. This subsidiary structure to the mainshock fault place was suggested by Smith and others (2002) from the distribution of LSM aftershock aftershocks and the N-S striking normal faulting focal mechanism of the September 13, 1992, M 4.5 aftershock. The September 13, 1992, event appears to have taken place on this structure and the June 14, 2002, event may have involved this structure as well. This complex faulting process may result in the large CLVD component of the UC Berkeley moment-tensor solution (Figure 4). The sources used to construct the composite model below are also shown on Figure 2 and 3. The UNRSL short period mechanism for the June event strikes 15 degrees more northerly than the LSM mainshock mechanism of Meremonte and other (1995) and Smith and others (2001).
Therefore, the LSM mainshock and the June 14 event may have both initiated at the intersection of these two structures identified in the aftershock data. Also, the LSM mainshock may have also been a complex faulting event involving down-dip motion on the primary NE striking SE dipping fault plane with a small component of down-to-the-east dip-slip motion on this subsidiary structure. The left slip component in the LSM mainshock focal mechanism (Smith and others, 2001) and the June 14 mechanism (Figure 4-5) may reflect this faulting process. This faulting mechanism would result in difficulties in interpreting the source mechanism from the spectra; the source mechanism may involve two structures in a complex faulting process.
Summary of Source Mechanisms:
UC Berkeley Moment Tensor Solution Summary:
Plane Strike Rake Dip
NP1 2 -121 75
NP2 248 -28 34
Principal Axes:
Axis Plunge Azimuth*
T 24 115
P 50 237
Source Composition:
Type Percent
DC 65.7
CLVD 34.3
Iso 0.0
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Figure 4
Nevada Seismological Laboratory First-Motion Polarity Solution:
Figure 5
In comparing the waveforms from the strong motion station at Lathrop Well, Nevada (Figure 1), the June 2002 mainshock record (Figure 6) can be approximately constructed as a composite of the May 25 and June 18, 2002 M 2.8 events, also recorded at Lathrop Wells. Both of these M 2.8 earthquakes show simple waveforms whereas the June 2002 mainshock is a complex record (Figure 6). By time shifting Evt. #2 relative to Evt. #1 by 0.8 seconds with respect to the P-wave arrival and scaling the amplitudes appropriately for each, we can nearly reproduce the mainshock records at high frequencies (figure below). Table 1 is a summary of event locations.
Table 1.
Origin Time Longitude Latitude Depth ML
5/25/2002 (145) 12:03:32.824 -116.2996 36.7261 9.4025 2.79 #1
6/14/2002 (165) 12:40:44.589 -116.3028 36.7171 11.2158 4.4*
6/18/2002 (169) 16:24:31.533 -116.3090 36.7150 9.6773 2.80 #2
*UNR June 2002 mainshock location.
Figure 7 shows the June 2002 mainshock and composite records from Figure 5 bandpass filtered between 1 and 8 Hz.
The epicenter of the June 2002 mainshock is between Evt.s #1 and #2, although the mainshock depth is about 2 km greater in these preliminary locations. Therefore is appears that Evt. #1 was on the 1992 mainshock fault plane and Evt. #2 occurred on the subsidiary NS striking structure to the south. This interpretation is consistent with the June 2002 mainshock being a composite of faulting on two structures.
Strong Motion Records and Acceleration Spectra:
Three-component records with the station name shown in the plots that are not referenced to a channel orientation are in Vertical-North-East order.
References:
Meremonte, M. E., Gomberg, J. and Cranswick, E., (1995). Constraints on the 29 June 1992 Little Skull Mountain sequence provided by robust hypocentral estimates: Bulletin of the Seismological Society of America, v. 85, p. 1039-1049.
Smith, K.D., Brune, J.N., Savage, M., dePolo, D.M., and Sheehan, A. (2001). The 1992 Little Skull Mountain earthquake sequence, Bulletin of the Seismological Society of America 91, 1595-1606.