Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Focal Mechanism


  STK=     274.13
  DIP=      85.07
 RAKE=     -10.04
  STK=       5.00
  DIP=      80.00
 RAKE=    -174.99
DEPTH =  7.0 km
MW =  4.14

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

Data preparation

Digital data were collected, intreument response removed and traces converted to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms. The figure
Best mechanism fit as a function of depth. the preferred depth is 7 km

Focal mechanism seisitivity at a depth of 7.0 km. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. The Each solution is plotted as a vector at a given value of strike and dip witht he angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Station     Azimuth(deg) Distance(km)  P-first motion
BMN            12.6       251.9100       X
BOZ            30.1       973.2200
BW06           51.7       866.2262
CMB           265.5       221.5900       e-
DUG            61.8       488.6500
ELK            38.1       360.3618
HLID           25.0       660.8784
HOPS          282.4       461.1600
HVU            46.1       587.6804
HWUT           53.1       657.0565
ISA           190.9       288.7377       e-
MNV           313.9       34.2940       i+
MOD           333.9       458.6000
MPU            67.8       574.9000
MVU            84.6       495.4100
PFO           165.6       527.1752
TPNV          134.1       200.8900
TUC           133.4       918.9400
WDC           304.5       480.0956
WUAZ          115.4       652.0100
WVOR          352.3       472.5400
YBH           314.9       568.2575

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the osberved and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

	hp c 0.02 np 3
	lp c 0.05 np 3
The fit to the SH pulse on the T component is good. Note that this is the dominant signal at stations ISA, CMB and BMN. The Rayleigh pulse is larger at TPNV but the waveform fit is affected by the microseism level. The fit to the nearest station, MNV is very good and would not change much if the focal mechanism is modified by +-5 degrees.


The focal mechanism nodal planes are well developed. However since the P-wave first motion data are of poor quality and the local velocity model is poorly known, we cannot resolve whther this is a thrust or normal faulting event.

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.


Dr. Harley Benz, USGS, provided the USGS USNSN digital data.


The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The CUS earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

Last Changed 03/11/23