Location
Location ANSS
The ANSS event ID is pr2020175032 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/pr2020175032/executive.
2020/06/23 20:34:02 19.229 -67.069 9.0 4.63 Puerto Rico
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2020/06/23 20:34:02:0 19.23 -67.07 9.0 4.6 Puerto Rico
Stations used:
GS.PR01 GS.PR02 GS.PR03 GS.PR04 GS.PR05 GS.PR06 IU.SJG
PR.AGPR PR.AOPR PR.CELP PR.CRPR PR.ECPR PR.HUMP PR.MLPR
PR.PCDR PR.PRSN PR.UUPR
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.68e+23 dyne-cm
Mw = 4.75
Z = 31 km
Plane Strike Dip Rake
NP1 340 85 -60
NP2 79 30 -170
Principal Axes:
Axis Value Plunge Azimuth
T 1.68e+23 33 45
N 0.00e+00 30 157
P -1.68e+23 42 278
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.67e+22
Mxy 7.22e+22
Mxz 4.21e+22
Myy -3.15e+22
Myz 1.37e+23
Mzz -2.52e+22
##############
-----#################
---------###################
-----------###################
--------------########### ######
---------------########### T #######
-----------------########## ########
-------------------#####################
------- ----------####################
-------- P ----------####################-
-------- -----------###################-
-----------------------#################--
-----------------------################---
-----------------------##############---
#-----------------------############----
##---------------------##########-----
###--------------------#######------
####------------------####--------
#######-------------#---------
####################--------
##################----
##############
Global CMT Convention Moment Tensor:
R T P
-2.52e+22 4.21e+22 -1.37e+23
4.21e+22 5.67e+22 -7.22e+22
-1.37e+23 -7.22e+22 -3.15e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200623203402/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 340
DIP = 85
RAKE = -60
MW = 4.75
HS = 31.0
The NDK file is 20200623203402.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2020/06/23 20:34:02:0 19.23 -67.07 9.0 4.6 Puerto Rico
Stations used:
GS.PR01 GS.PR02 GS.PR03 GS.PR04 GS.PR05 GS.PR06 IU.SJG
PR.AGPR PR.AOPR PR.CELP PR.CRPR PR.ECPR PR.HUMP PR.MLPR
PR.PCDR PR.PRSN PR.UUPR
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.68e+23 dyne-cm
Mw = 4.75
Z = 31 km
Plane Strike Dip Rake
NP1 340 85 -60
NP2 79 30 -170
Principal Axes:
Axis Value Plunge Azimuth
T 1.68e+23 33 45
N 0.00e+00 30 157
P -1.68e+23 42 278
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.67e+22
Mxy 7.22e+22
Mxz 4.21e+22
Myy -3.15e+22
Myz 1.37e+23
Mzz -2.52e+22
##############
-----#################
---------###################
-----------###################
--------------########### ######
---------------########### T #######
-----------------########## ########
-------------------#####################
------- ----------####################
-------- P ----------####################-
-------- -----------###################-
-----------------------#################--
-----------------------################---
-----------------------##############---
#-----------------------############----
##---------------------##########-----
###--------------------#######------
####------------------####--------
#######-------------#---------
####################--------
##################----
##############
Global CMT Convention Moment Tensor:
R T P
-2.52e+22 4.21e+22 -1.37e+23
4.21e+22 5.67e+22 -7.22e+22
-1.37e+23 -7.22e+22 -3.15e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200623203402/index.html
|
Regional Moment Tensor (Mwr)
Moment 2.447e+16 N-m
Magnitude 4.86 Mwr
Depth 40.0 km
Percent DC 86%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 337 84 117
NP2 80 27 14
Principal Axes
Axis Value Plunge Azimuth
T 2.355e+16 N-m 45 274
N 0.176e+16 N-m 26 154
P -2.530e+16 N-m 33 45
|
Magnitudes
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula
for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
ML Magnitude
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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.
Waveform Inversion using wvfgrd96
The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the
stations used for (red) the waveform inversion are shown in the next figure.
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Location of broadband stations used for waveform inversion
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 120 45 90 4.03 0.1422
WVFGRD96 2.0 115 50 80 4.16 0.1960
WVFGRD96 3.0 110 50 75 4.21 0.2018
WVFGRD96 4.0 195 55 25 4.19 0.2056
WVFGRD96 5.0 180 60 20 4.18 0.2275
WVFGRD96 6.0 180 60 20 4.21 0.2525
WVFGRD96 7.0 180 65 20 4.25 0.2783
WVFGRD96 8.0 180 60 20 4.29 0.3028
WVFGRD96 9.0 80 90 30 4.30 0.3253
WVFGRD96 10.0 80 90 25 4.32 0.3470
WVFGRD96 11.0 70 65 -20 4.37 0.3688
WVFGRD96 12.0 70 65 -20 4.40 0.3901
WVFGRD96 13.0 70 65 -15 4.42 0.4096
WVFGRD96 14.0 70 65 -15 4.44 0.4284
WVFGRD96 15.0 70 60 -10 4.48 0.4460
WVFGRD96 16.0 70 60 -10 4.49 0.4622
WVFGRD96 17.0 75 50 10 4.54 0.4777
WVFGRD96 18.0 75 50 10 4.55 0.4908
WVFGRD96 19.0 75 50 10 4.57 0.5011
WVFGRD96 20.0 75 50 10 4.58 0.5086
WVFGRD96 21.0 75 50 10 4.60 0.5136
WVFGRD96 22.0 75 50 10 4.61 0.5155
WVFGRD96 23.0 75 50 10 4.62 0.5148
WVFGRD96 24.0 75 50 10 4.62 0.5115
WVFGRD96 25.0 340 80 -55 4.67 0.5134
WVFGRD96 26.0 340 80 -55 4.68 0.5152
WVFGRD96 27.0 340 80 -55 4.69 0.5157
WVFGRD96 28.0 340 80 -55 4.70 0.5150
WVFGRD96 29.0 340 80 -60 4.72 0.5148
WVFGRD96 30.0 340 85 -60 4.74 0.5161
WVFGRD96 31.0 340 85 -60 4.75 0.5167
WVFGRD96 32.0 340 85 -60 4.75 0.5163
WVFGRD96 33.0 340 85 -60 4.76 0.5150
WVFGRD96 34.0 340 85 -65 4.78 0.5132
WVFGRD96 35.0 340 85 -65 4.78 0.5111
WVFGRD96 36.0 340 85 -65 4.79 0.5075
WVFGRD96 37.0 340 85 -65 4.79 0.5028
WVFGRD96 38.0 340 85 -65 4.79 0.4975
WVFGRD96 39.0 340 85 -60 4.78 0.4920
WVFGRD96 40.0 340 85 -70 4.91 0.4901
WVFGRD96 41.0 340 85 -70 4.92 0.4866
WVFGRD96 42.0 340 85 -70 4.92 0.4807
WVFGRD96 43.0 340 85 -70 4.93 0.4730
WVFGRD96 44.0 340 85 -70 4.94 0.4639
WVFGRD96 45.0 340 85 -70 4.94 0.4536
WVFGRD96 46.0 340 85 -70 4.94 0.4424
WVFGRD96 47.0 340 85 -70 4.94 0.4306
WVFGRD96 48.0 340 85 -75 4.97 0.4185
WVFGRD96 49.0 340 85 -75 4.97 0.4068
WVFGRD96 50.0 340 85 -75 4.97 0.3951
WVFGRD96 51.0 335 80 -75 4.94 0.3840
WVFGRD96 52.0 275 40 30 4.89 0.3709
WVFGRD96 53.0 275 45 30 4.87 0.3657
WVFGRD96 54.0 275 45 30 4.88 0.3611
WVFGRD96 55.0 275 45 30 4.89 0.3563
WVFGRD96 56.0 275 45 30 4.89 0.3514
WVFGRD96 57.0 275 45 30 4.90 0.3464
WVFGRD96 58.0 270 50 25 4.88 0.3417
WVFGRD96 59.0 270 50 25 4.88 0.3374
The best solution is
WVFGRD96 31.0 340 85 -60 4.75 0.5167
The mechanism corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted.
Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off.
A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
|
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms.
Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure.
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A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
Velocity Model
The WUS.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
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
Last Changed Thu Apr 25 06:49:29 PM CDT 2024
