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

The focal mechanism was determined from surface-wave spectral amplitudes using the CUS model. This model has an upper crust that is too fast fort the source and propagation region but should provide a good estimate of the focal mechanism, source depth and seismic moment. However the fit to the surface-wave waveforms will not be perfect because of the timing of the surface-wave arrivals.


  STK=      25.00
  DIP=      50.00
 RAKE=     -90.00
  STK=     204.99
  DIP=      40.00
 RAKE=     -90.01
DEPTH =  10.0 km
MW =  4.19

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
AHID	54.8	405.5900
ANMO	127.3	997.4000
BEKR	259.5	453.0200	e+
BMN	260.0	178.5100	e+
BOZ	26.5	616.5236
BW06	62.2	517.0494
CMB	238.1	541.2372
CTU	89.7	287.2894
DAC	204.1	538.2498
DUG	105.9	206.8900	e+
ELK	282	  7.7		i-
HAWA	332.1	721.6469
HLID	10.7	320.4500	i-
HVU	58.9	230.6600	X
HWUT	70.9	316.1800
ISA	208.4	633.2300
ISCO	94.2	817.3474
MNV	226.3	362.7468
MOD	288.5	450.6000
MPU	103.7	308.9976
MSO	7.8	684.6000
NEW	350.1	851.5769
PAHR	253.9	377.8679
PFO	188.7	798.7094
RSSD	63.9	988.1000
SDCO	108.6	895.8799
TPNV	193.2	430.4600	e+
WCN	249.6	423.5700	e+
WDC	270.9	625.1262
WUAZ	149.0	666.5076
WVOR	304.2	346.8790	i+
YBH	282.4	643.3400
YMR	36.7	555.7300

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 2
	lp c 0.10 np 2
The fit to ELK and especially the FOR to the radial component ande the PnL at DUG show that this is an acceptable foral mechanism.


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