Wells Earthquake.   March 2, 2008. 

 

Some Thoughts

 

John Anderson.  Nevada Seismological Laboratory, University of Nevada, Reno, Nevada 89557

jga@seismo.unr.edu

775-784-4265

775-784-4975

775-741-9299 (cell)

 

Contributions from Ken Smith, Glenn Biasi, Ileana Tibuleac, and many others at the Nevada Seismological Laboratory.  Maps prepared by Yui Miyata of the Nevada Seismological Laboratory.

 

 

Prior seismic activity

 

Figure 1 shows locations, and Figure 2 shows magnitudes of earthquakes from 40-42N, and 114-116 W for the entire Nevada Seismological Laboratory catalog.  This shows that our catalog shows four events with magnitude about 5.0 prior to the Wells earthquake.  These occurred on July 26, 1901, March 12, 1934, March 29, 1970, and June 4, 2000.  These were located between 56-119 km from the epicenter of the Feb 21 earthquake, and thus are presumably on unrelated faults.

 

Our catalog shows two events on February 28, 2007 with epicenters located within 1.0 km of the Wells main shock epicenter.  These events might be very early foreshocks to February 21 mainshock.  Thus there may be a foreshock sequence; a preliminary review of seismograms for February 2007 indicated that these two events were the largest of a swarm, in which other events were too small to locate based on timed phases at nearby stations.

 

Figure 3 shows the empirical occurrence rate of earthquakes in the northeast quadrant of Nevada, for 1970-2007.  This shows that in that time interval there was on average about one earthquake per year with magnitude of 3.0 or larger.  Earthquake occurrence rates are often fit with a Gutenberg-Richter relation of the form:

 

                                           (1)

 

The occurrence rates in Figure 3 are poorly constrained.  For a typical value of b=1.0, and with M1=3.0, one expects an earthquake with M=6 once every 1000 years.  The catalog behind Figure 3 lacks the coverage needed for high confidence in the data points, but if one insists on fitting a line to them, that line can be fit with a b-value as low as 0.64.  This would anomalous, as well constrained values are usually greater than 0.85.  Extrapolated to M=6, it would predict that an M=6 earthquake would occur at an annual rate of once every 84 years.  A low slope is often the result of incomplete coverage at small magnitudes.  Incomplete coverage is a known condition in northeastern Nevada.  To try to come to some conclusion, we suggest that while poorly constrained, the indication is that an event this size in northeastern Nevada is expected at an annual rate of between once per 100 years and once per 1000 years.

 

Wells Sequence

 

Figure 4 shows a map of the well-constrained epicenters of aftershocks for the 8 days after the main shock.  Faults on either side of this valley are not named on the USGS Quaternary Faulting database.  This shows that the aftershock sequence has tended to fill in the space between the main shock epicenter and Wells. 

 

Figure 5 shows the focal mechanism as determined by regional broad-band seismic stations (ref: Ekstrom/Harvard CMT).  The mechanism shows that the earthquake is almost purely a normal-faulting earthquake.  Therefore the aftershock locations will tend to outline the ruptured area of the fault.  The aftershock locations suggest that Wells is on the hanging wall, or just beyond the hanging wall, of the fault in a forward-directivity direction. However, as of writing this (Sunday, 3-2-08) I don’t think we have the confidence to determine which plane was active.  There are indications that it is more likely that the southeast-dipping plane is the active plane, but the evidence is not yet totally conclusive.  The available evidence includes a preliminary model for a finite fault by Dreger and Ford (UC Berkeley: http://seismo.berkeley.edu/~dreger/wells_nv.htm) which fits the ANSS seismograms better if the southeast-dipping plane is assumed, and some aftershock locations which are still scattered by suggestive that the aftershocks are shallower on the west side of the aftershock zone.

 

Figure 6 shows the aftershock magnitudes, as a function of time after the main shock.  These show that the magnitudes and rates of aftershocks have tended to decrease over time.  Aftershock numbers for the first day were fit to an empirical curve called the modified Omori’s Law: 

 

                                                             (2)

 

Figure 7 shows a cumulative number of aftershocks, and a fit to those numbers using Equation 2.  The parameters are again poorly constrained, and in any case extrapolation using a fit to an early part of an earthquake sequence is unreliable.  However the value of p used here is not unusual, and the figure suggests that the rate of earthquake occurrence is decreasing at a normal rate, at least so far.

 

Further notes.

 

The Nevada Seismological Laboratory, the University of Utah, and the US Geological Survey have field teams and are gathering aftershock locations and seismograms that should be critical for constraining the depths of aftershocks and thus determining the orientation of the fault. 

 

Ground motions in the main shock were recorded on scale on several USArray Stations.  We have converted these to acceleration and velocity.  The nearest of these stations is 36 km from the epicenter. Preliminary results suggest several readings of peak velocity in the range of 0.1-1 cm/s on these stations. These will be added to an updated version of this report when they have been adequately reviewed.

 

Figure 1.  Epicenters of earthquakes in northeast Nevada, 1900 through 2007.

Figure 2.  Time and magnitude of earthquakes in the northeast quadrant of Nevada.

Figure 3.  Gutenberg-Richter recurrence curve for earthquakes in the northeast quadrant of Nevada, for 1970-2007.

Figure 4.  Epicenters of aftershocks through February 28, 2008.

February 21, 2008, NEVADA, MW=6.0

 

Goran Ekstrom

 

CENTROID-MOMENT-TENSOR  SOLUTION

GCMT EVENT:     C200802211416A 

DATA: II IU CU IC G  GE

L.P.BODY WAVES: 92S, 209C, T= 40

MANTLE WAVES:   83S, 120C, T=125

SURFACE WAVES:  99S, 252C, T= 50

TIMESTAMP:      Q-20080221151936

CENTROID LOCATION:

ORIGIN TIME:      14:16:10.1 0.1

LAT:41.23N 0.01;LON:114.86W 0.01

DEP: 14.1  0.2;TRIANG HDUR:  2.5

MOMENT TENSOR: SCALE 10**25 D-CM

RR=-1.230 0.010; TT= 0.245 0.008

PP= 0.990 0.009; RT=-0.078 0.018

RP= 0.125 0.018; TP= 0.628 0.007

PRINCIPAL AXES:

1.(T) VAL=  1.350;PLG= 2;AZM=300

2.(N)      -0.098;     7;    209

3.(P)      -1.247;    83;     43

BEST DBLE.COUPLE:M0= 1.30*10**25

NP1: STRIKE= 36;DIP=44;SLIP= -81

NP2: STRIKE=203;DIP=47;SLIP= -99

 

            ###########          

        ###########--------      

      ##########------------#    

     #########--------------###  

   T #######-----------------### 

     ######------------------####

  ########-------------------####

 ########--------   ---------#####

 ########-------- P --------######

 #######---------   -------#######

 #######-------------------#######

  ######-----------------########

  ######----------------#########

   #####--------------########## 

    ####------------###########  

      ###--------############    

        --#################      

            ###########          

 

 

 

Figure 5.  Focal mechanism of the main shock.

Figure 6.  Times and magnitudes of aftershocks with M>3, for the the first 9 days after the main shock.

Figure 7.  Comparison of the cumulative numbers of aftershocks with a typical model for aftershock occurrence numbers.  The model (thin black line) was developed on February 24 (day 4), and calibrated to fit the first three days of the sequence.