Location

Location ANSS

The ANSS event ID is nm600379 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm600379/executive.

1976/03/25 00:41:20 35.59 -90.48 15 4.62 Arkansas

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 1976/03/25 00:41:20:0 35.590 -90.480 15.0 4.6 Arkansas Stations used: Filtering commands used: Best Fitting Double Couple Mo = 1.07e+23 dyne-cm Mw = 4.62 Z = 12 km Plane Strike Dip Rake NP1 220 65 150 NP2 324 63 28 Principal Axes: Axis Value Plunge Azimuth T 1.07e+23 38 181 N 0.00e+00 52 4 P -1.07e+23 1 272 Moment Tensor: (dyne-cm) Component Value Mxx 6.59e+22 Mxy 5.60e+21 Mxz -5.22e+22 Myy -1.07e+23 Myz 1.17e+21 Mzz 4.10e+22 ############## ###################### -----#####################-- ----------############-------- ---------------######------------- ------------------#----------------- ------------------###----------------- -----------------#######---------------- ----------------#########--------------- -------------############--------------- P ------------###############------------- ----------##################------------ -----------####################----------- ---------#####################---------- ---------######################--------- -------########################------- -----########### ###########------ ----########### T ###########----- --########### ###########--- -#########################-- ###################### ############## Global CMT Convention Moment Tensor: R T P 4.10e+22 -5.22e+22 -1.17e+21 -5.22e+22 6.59e+22 -5.60e+21 -1.17e+21 -5.60e+21 -1.07e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/19760325004120/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 220
      DIP = 65
     RAKE = 150
       MW = 4.62
       HS = 12

The NDK file is 19760325004120.ndk

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Reference

Older earthquake were studied using analog seismograms, which were digitized and analyzed. The original data are not longer available. However focal mechanism solutions based on the waveforms are given in published literature. At the same time, a given earthquake may have been analyzed independently, and the reference is given here.

For this earthquake the source reference is the following:

Herrmann, R. B. (1979). Surface wave focal mechanisms for eastern North American earthquakes with tectonic implications, J. Geophys. Res.84, 3543-3552.

Velocity Model

The CUS model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

MODEL.01
CUS Model with Q from simple gamma values
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
  H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC)   QP   QS  ETAP  ETAS  FREFP  FREFS
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00

Last Changed Wed Mar 27 15:36:33 CDT 2024