Introduction

Regional moment tensors were determined for some earthquakes in Vietnam. For earthquake in the northwestern part of the country, two velocity models were considered: the CUS model which is appropriate for a stable continental craton, and the Northwest Vietnam model (NWVN) which was developed for the northwest.

The links below give details on the solutions.

Solutions

Summary of solutions

The moment tensor solutions are tabulated here. The Date and Time are given in UTC. The velocity models are tabulated at the bottom of the individual event pages, and also in the next section. 
  Date    Time    Lat     Lon   H(km)  Mw  Stk Dip Rake  Model  Fit
20101230_185016  20.84  103.40   9.0  4.67 210  90   10   CUS   0.8194
20101230_185016  20.84  103.40  11.0  4.51  30  90  -10   NWVN  0.6584

20121115_072404  15.38  108.10  10.0  4.31 170  40  -70   CUS   0.6165

20130903_000820  15.36  108.12   7.0  3.22 335  55   85   CUS   0.6984

20180107_201418  21.38  103.28   7.0 3.91   25  85  -35   CUS   0.6028
20180107_201418  21.38  103.28   8.0 3.74   25  85  -40   NWVN  0.5099

20180108_232121  21.43  103.31   7.0  3.94  15  75  -35   CUS   0.7291
20180108_232121  21.43  103.31   9.0  3.80  15  75  -35   NWVM  0.6019

20180209_124938  21.38  103.33   8.0  3.83  20  80  -35   CUS   0.7288
20180209_124938  21.38  103.33   8.0  3.67 210  85   35   NWVN  0.5601
Two velocity models were considered. The CUS model developed in for the central United States, but which also works very well for the southern Korea Peninsula. The statement "works very well" means that the fits to the details fo regional waveforms is very good in the filter band used. The fine details include the shape of the waveform.

The NWVN (Northwest Vietnam) model was developed using local data.

For the events in the northwest, the mechanisms and depths are very similar. The seismic moments for the NWVN model are smaller than those for the CUS model because the NWVN model has slightly lower velocities.

Regional stress context

An extensive study of earthquake in North America shows definite patters in the type of faulting and the direction of the estimated maximum compressive stress axis. These pattern can be used to assess the reasonableness of a solution, especially for smaller earthquakes.

The next figure compares the meahanisms and the directions of maximum compressive stress for the earthquakes processed so far. The CUS model solutions are used, although the pattern will not change is the NWVN solutions are used.

Earthquake context figure. Left) focal mechanism showing the compressional (solid color) and dilational quadrants (white). Right) direction of maximum compressive stress axis. The colors indicate strike-slip (green), thrust (blue) and normal (red) faulting.

In the figure, the only outlier is the Mw=3.2 earthquake of 2013/09/03 at 00:02. This was poorly recorded and should only be used for the moment ant not the mechanism.

Velocity models

The S-wave velocities of the two models are displayed in the next figure:

The models used are tablulated here in the format for use in Computer Programs in Seismology.
CUS Model
NWVN Model
WUS Model
 
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

MODEL.01
NWVN Model 
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
  6.0000  4.94  2.91  2.50    0 0 0  0 1 1
  2.0000  5.26  3.09  2.55    0 0 0  0 1 1
 13.0000  5.86  3.45  2.66    0 0 0  0 1 1
  3.0000  6.09  3.58  2.70    0 0 0  0 1 1
  5.0000  6.21  3.65  2.73    0 0 0  0 1 1
  4.0000  6.48  3.81  2.79    0 0 0  0 1 1
  0.0000  7.50  4.41  3.07    0 0 0 0  1 1

MODEL.01
Model after     8 iterations
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
  1.9000    3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
  6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
 13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
 19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
  0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00

Test of velocity models

On way to test whether a velocity model is the proper one to use for source inversion is to compare the observed group velocities to those predicted using the different models. The group velocities are determined using multiple filter analysis using the program do_mft which calls sacmft96. The only requirement is that the location and origin time be well determined. If the origin time is not correct, then there will a a systematic error in the disperion. The origin time depends on the velocity model used, but also depends on the distribution of the stations.

Group velocities were measured for the four earthquakes in the northwestern part of the country. The next figure compares the observed dispersion to the dispersion predcted by the three velocity models.

Comparison of the observed group velocities for Love waves (left) and Rayleigh waves (right) to the theoretical prediction of the three velocity models: CUS (blue), NWVN (green) and WUS(red)

The disperson comparison shows that the WUS model cannot be used for regional moment tensor inversion. The CUS model has the correct shape but seems a little too fast. Of course if the NWVN model was used for location, then the origin time may be a little too early.

the current NWVN model seems to have the proper shape, but is too slow comapred to the observed dispersion.

Discussion

The regional moment tensor inversion permits a time shift to account for location error and an incorrect velocity model. This permits the shape of the filtered waveform to be better fit. The ability of the different velocity models to fit the waveforms shapes is an indication of whether the velocity model is the correct one to use.

For all four earthquakes in northwest Vietnam, the CUS velocity model provides better fits than the NWVN model. The significant difference is that the CUS model does not have the thick zone of low velocities in the upper 10 km that the NWVN model does.

On the basis of waveform shapes and the measured dispersion, I believe that the NWVN model is too slow in the upper crust to be sued in this region.