Example of the output provided for each focal mechanism in Lamontagne (1998). The 971028 11:44 U.T. event is used: 1) Text; A) The FOCMEC results in five focal spheres (Figure 12A). Starting in the upper left and going clockwise, the first sphere shows the series of solutions obtained with P and SH first motions (entitled "P, SH and P-Nodal"). The second shows the solutions obtained using the P first motions only (entitled "Solution with P FM only"). Unless well constrained with P first motions only (rather rare), the increment for the b axis is 5. The lower right sphere shows the SH first motions and the SH "nodal" lines computed with the P and SH solutions (entitled "SH motions and Lines"). The lower left one shows all first motions (entitled "First Motions Only"). Finally, the central mechanism is the average solution using the P and SH families of solutions (entitled "Best Solution"). One should also note that the exact position of the first motion on the focal sphere is not in the centre but in the lower left portion of the C and D first motion symbols; B) First motion picks; C) Location map of the earthquake (focal sphere) and background seismicity.
Earthquake of 19971028 1144 Magnitude 4.7mN 1- Earthquake information and list of first motions.
Date Time Latitude Longitude Depth Mag Sta/Pha hhmm ss (km) 971028 1144 18.64 47.6721 -69.9051 11.42 4.7mN 6/012 COMMENTS: CHARLEVOIX SEISMIC ZONE, QUE. Felt. Felt in Charlevoix, Riviere-du-Loup, and in the Quebec City, Saguenay and Lac St-Jean regions. 18 km E from LA MALBAIE, QUE. $ LMQ; SLQ; QCQ down. $ CIQ FM from R. Du Berger. $ A11 starts with weak C, which is followed by an impulsive D. $ Isoseismals to come. $: REV is F. Revetta, Potsdam, N.Y. $ESDC SZ P XB1152 49.700 L $ARCE SZ P XB1152 58.300 L $LPAZ SZ P XB1154 51.300 L $DBIC SZ P XB1155 21.000 L $ZAL SZ P XB1156 9.800 L $ The Sg lines of some stations include the SH FM. $ The convention used is the Virginia Tech convention used in FOCMEC: $ <: SH first motion to left (back to event) impulsive $ >: SH first motion to right (back to event) impulsive $ Preliminary Focmec computed by ML. $ Note: most CLTN Sg FM are unclear.
Sta Pha Dist Azim Take-off FM (km) Angle A61 PG 14 280 -51 D A61 SG 14 280 -51 > A21 PG 17 78 -55 D A64 PG 17 3 -56 D A16 PG 24 199 -64 C A16 SG 24 199 -64 > SHQ PG 38 259 -73 D A54 PG 45 238 -76 D A11 PG 53 205 -78 C CIQ PG 94 316 -83 C DAQ PG 105 288 -84 D JOQ PG 129 310 0 C DPQ PG 244 244 -87 D
Number of P first motions: 11
Number of SH first motions: 2
2- Most Representative Solution.
A total of 247 solutions were found using a b axis increment of 3 degree. The
following parameters describe the most representative solution, which was derived
from the average B, P, and T axes listed below.
dip,strike,rake 85.99 111.73 41.84
dip,strike,rake 48.28 18.15 174.62 :auxiliary plane
lower hem. trend, plunge of a,n 288.15 41.72 21.73 4.01
lower hem. trend & plunge of b 116.20 48.00
lower hem. trend, plunge of p,t 237.24 24.90 343.65 31.33
Average B, P, and T axes:
B P T Mean trend: N 137 N 246 N 348 Mean Plunge: 49 16 37 Vector Magnitude: 242 244 245 Mean length of resulting vector: 0.981 0.987 0.991
3- Misfits.
All 11 P first motions agreed with the solution.
Out of 2 SH first motions, a total of 1 did not fit the solutions (50/100).
Station Number Percentage
Name of misfits of wrongs
A16 247 100%
4- Rating of the focal mechanism.
The solution has a C rating based on the poor constraints on the P, T and B axes (value of 0.986).
(Blank lines separate multiplets).
yyyymmdd hhmm ss.ddd Lat Lon Depth Mag Distance (km) Sub-zone DNE 19900907 0406 32.800 47.7910 -69.8170 21.87km 0.6ML 19920113 0904 11.800 47.7900 -69.8190 22.91km 0.4ML 1.057 19920716 0328 06.100 47.7920 -69.8330 22.10km 0.8ML 1.225 19961230 0246 23.479 47.7985 -69.8120 21.62km -0.2ML 0.947 19970329 0653 59.603 47.7897 -69.8172 21.78km 1.2mN 0.171
19911208 0300 30.994 47.7796 -69.8640 23.20km 4.3mN 19911208 0631 04.400 47.7850 -69.8640 22.77km 0.9ML 0.738 19911208 0857 00.800 47.7850 -69.8640 22.56km 0.7ML 0.877 19911227 2324 37.400 47.7820 -69.8650 22.70km 1.3mN 0.572 19920218 0149 45.811 47.7946 -69.8454 23.34km 1.7mN 2.177 19951209 2205 37.713 47.7940 -69.8614 23.26km 1.6mN 1.613
19921011 1859 02.900 47.8140 -69.9480 20.30km 0.2ML 19930216 0842 57.300 47.8200 -69.9690 19.52km 0.2ML 1.877
19940602 1445 37.118 47.7178 -69.9481 25.44km 2.4mN 19950615 1559 17.215 47.7155 -69.9275 25.39km 2.7mN 1.567
19961228 1704 01.809 47.8257 -69.7970 21.81km -0.2ML 19970309 0655 11.523 47.8227 -69.7947 20.90km -0.7ML 0.984
19970217 1947 43.077 47.8069 -69.7711 23.40km 1.7mN 19970219 2056 24.113 47.8094 -69.7641 23.89km -1.0ML 0.769
Sub-zone IRE
19891123 1308 56.900 47.5420 -70.2980 11.34km 0.4ML 19960813 2237 10.375 47.5468 -70.2956 11.67km 1.2mN 0.653
19900429 0014 29.320 47.6185 -70.1676 11.80km 1.7mN 19960403 0434 24.519 47.6174 -70.1687 11.34km -0.5ML 0.483
19950311 1119 04.600 47.5280 -70.3170 9.43km 0.5mN 19951228 2139 01.771 47.5372 -70.3003 9.41km -0.3ML 1.621
19950820 0416 10.309 47.4998 -70.2956 14.09km 0.8mN 19951117 0757 08.213 47.5173 -70.2949 15.33km 1.2mN 2.307
19960105 0425 57.522 47.6003 -70.2264 6.88km 1.5mN 19960105 0440 30.215 47.6017 -70.2251 5.85km 2.0mN 1.046
19961004 2216 37.536 47.5965 -70.2076 14.41km 1.3mN 19970422 1100 45.076 47.6065 -70.2164 15.14km 2.4mN 1.485
19900618 0700 16.600 47.5870 -70.2440 12.69km -0.2ML 19961115 2329 01.620 47.5989 -70.2453 12.28km -0.3ML 1.388
19921203 1002 07.911 47.6099 -70.1753 11.84km 2.0mN 19951108 1646 05.796 47.6045 -70.1832 10.65km -0.4ML 1.459 19960403 0434 24.519 47.6174 -70.1687 11.34km -0.5ML 1.091
19960924 2341 02.882 47.5475 -70.2417 12.83km 3.1mN 19961003 1024 46.893 47.5515 -70.2373 12.83km 1.3mN 0.554 19961003 1032 24.470 47.5442 -70.2396 13.14km 1.3mN 0.506
Sub-zone M5
19890309 0941 32.262 47.7170 -69.8568 10.51km 4.3mN 19890310 0329 59.500 47.7160 -69.8540 9.23km 1.2mN 1.302 19890311 0831 52.168 47.7181 -69.8700 10.36km 4.4mN 1.009
19900305 0025 12.100 47.6540 -69.9020 7.60km 0.1ML 19970316 1104 16.007 47.6608 -69.8939 8.61km 1.6mN 1.400
19910221 1702 48.400 47.7020 -69.8830 9.09km 0.9ML 19940311 0836 33.510 47.6929 -69.8959 9.45km 1.7mN 1.446
19920311 0120 57.600 47.7180 -69.8570 9.45km 0.2ML 19920314 2128 43.602 47.7170 -69.8570 9.77km 1.6mN 0.339
19930330 1235 28.300 47.6740 -69.9080 10.31km 0.0ML 19930407 1402 29.500 47.6770 -69.9100 10.70km 1.3mN 0.535
19930807 2125 31.304 47.6654 -69.8857 4.87km 3.1mN 19931013 0143 13.507 47.6808 -69.8784 13.96km 0.4ML 9.266
19950812 1742 55.813 47.6680 -69.9035 10.83km 2.5mN 19950918 1609 47.386 47.6678 -69.9033 10.63km 2.6mN 0.202
Sub-zone A61
19891208 1720 34.446 47.7009 -70.0644 10.37km 2.6mN 19891221 1211 28.000 47.7010 -70.0730 10.79km 0.2ML 0.770 19940614 2339 58.000 47.7040 -70.0700 10.12km 1.4mN 0.598 19960215 0005 39.828 47.7011 -70.0454 16.27km 0.0ML 6.070
19930706 0633 33.200 47.6950 -70.0730 9.02km 0.6ML 19950303 2334 37.800 47.6960 -70.0750 9.73km -0.4ML 0.734 19950820 0241 42.030 47.6946 -70.0687 9.03km 0.1ML 0.326
19960803 1827 03.119 47.6744 -70.1431 9.79km 1.9mN 19960804 0918 55.206 47.6763 -70.1457 9.45km 0.9mN 0.445
Sub-zone CEN
19881220 0246 18.800 47.5310 -70.0840 8.81km 0.5ML 19970130 2150 08.704 47.5299 -70.0695 10.36km -0.8ML 1.900
19890203 1355 39.800 47.5720 -70.0210 11.08km 1.1ML 19910608 0338 49.800 47.5720 -70.0230 10.64km 0.4ML 0.465
19900522 1221 09.700 47.5350 -70.0290 7.45km 0.5ML 19920130 1042 33.400 47.5389 -70.0349 5.99km 2.0mN 1.586 19920130 1352 11.615 47.5403 -70.0277 7.20km 1.9mN 0.647 19930801 1935 29.717 47.5355 -70.0390 6.69km 1.7mN 1.071
19901018 0603 28.467 47.4817 -70.0868 15.22km 2.8mN 19920817 1255 26.600 47.4830 -70.0910 14.19km 0.4ML 1.087
19910711 0858 11.600 47.4730 -70.0400 7.27km 0.4ML 19910711 1451 05.600 47.4750 -70.0400 7.20km 0.3ML 0.233
19911008 0411 02.800 47.5950 -69.9900 8.47km 0.2ML 19920627 2343 25.900 47.5910 -69.9940 8.03km 0.5ML 0.694 19940207 0609 01.442 47.5903 -70.0005 8.40km 2.0mN 0.949 19970509 0906 28.311 47.5624 -69.9974 8.76km 1.3mN 3.677
19920404 1230 28.560 47.4325 -70.1733 18.92km 2.9mN 19920420 0855 27.505 47.4333 -70.1748 18.90km 1.9mN 0.145
19920430 0401 41.000 47.4980 -70.0490 11.01km 0.3ML 19961222 0030 32.038 47.5059 -70.0637 10.83km 2.3mN 1.425
19920816 1851 25.300 47.5730 -70.0410 10.48km 0.4ML 19960901 0517 18.200 47.5745 -70.0434 10.84km 1.4mN 0.436
19930605 0449 40.991 47.4931 -70.1126 11.03km 2.6mN 19930606 0152 11.800 47.4940 -70.1130 10.41km -0.1ML 0.629
19950718 1306 38.413 47.5706 -69.9979 6.62km 1.9mN 19950724 0328 01.314 47.5662 -69.9969 5.09km -0.3ML 1.608 19951019 0316 07.603 47.5844 -70.0009 9.04km 2.1mN 2.874 19970319 0953 12.399 47.5885 -70.0023 7.77km 1.5mN 2.322
19950719 0953 01.613 47.5675 -70.0007 13.66km -0.2ML 19970401 1203 12.904 47.5652 -70.0017 13.81km 1.6mN 0.306
19950918 1322 03.731 47.4471 -70.1084 1.90km 0.6ML 19970112 0523 24.741 47.4357 -70.0907 5.76km -0.7ML 4.276
19901221 0940 32.115 47.4583 -70.0715 8.52km 1.9mN 19901221 0957 51.500 47.4560 -70.0680 8.75km 0.3ML 0.433
19910414 0508 35.300 47.5460 -70.0300 7.94km 0.1ML 19930518 0017 41.713 47.5432 -70.0329 7.60km 2.4mN 0.510 19940711 0935 41.143 47.5620 -70.0513 4.91km 2.8mN 3.862
Sub-zone COU
19920914 2038 14.800 47.4230 -70.4210 5.97km 0.8ML 19970212 0541 31.702 47.4289 -70.4167 7.46km -0.2ML 1.660
19960824 0245 08.522 47.4013 -70.3200 11.47km 2.0mN 19960824 0247 22.525 47.4017 -70.3196 11.63km 1.8mN 0.169
19961001 0049 03.173 47.4136 -70.3322 11.66km -0.6ML 19961007 0950 11.871 47.4293 -70.3315 11.69km -0.4ML 1.746
19930814 0755 20.100 47.4170 -70.3380 10.29km -0.3ML 19930814 1456 13.700 47.4170 -70.3320 9.28km 0.1ML 1.107 19960518 0816 32.273 47.4208 -70.3143 9.84km 0.7mN 1.891
19910312 0657 08.946 47.3910 -70.4832 10.25km 1.9mN 19950911 1410 21.046 47.3983 -70.4805 9.98km 1.7mN 0.879
19891122 2302 51.700 47.4558 -70.3430 7.61km 3.4mN 19891231 0234 53.900 47.4510 -70.3430 7.63km 0.1ML 0.534 19900111 0715 12.208 47.4544 -70.3480 8.36km 1.8mN 0.854
19901026 1755 58.000 47.4290 -70.4090 6.86km 0.2ML 19970212 0541 31.702 47.4289 -70.4167 7.46km -0.2ML 0.835
19901021 1338 43.209 47.3975 -70.3640 15.82km 3.3mN 19901102 0427 50.100 47.3970 -70.3610 14.93km 0.5ML 0.920
19960113 0732 50.103 47.4160 -70.4406 4.81km 1.9mN 19960618 0144 24.387 47.4169 -70.4364 4.59km 0.8mN 0.399
19920311 1305 50.015 47.4578 -70.3570 7.03km 1.9mN 19950502 1954 28.823 47.4493 -70.3766 7.72km 0.9ML 1.885
19920222 2155 32.800 47.4280 -70.3570 7.95km 0.3ML 19920816 2106 58.000 47.4270 -70.3560 7.61km 0.2ML 0.366
19911102 0254 28.800 47.3850 -70.4410 9.24km 0.4ML 19970409 0244 17.495 47.3785 -70.4647 11.35km 2.1mN 2.859
Sub-zone EBO
19900506 0204 43.500 47.5140 -70.3770 3.21km 0.9mN 19940901 0444 43.300 47.5170 -70.3870 4.95km 0.5ML 1.925
19930417 0535 42.519 47.5293 -70.3672 7.55km 1.8mN 19950401 0210 06.539 47.5435 -70.3978 6.33km 0.7mN 3.047
19950806 0436 28.823 47.5554 -70.3777 9.44km 0.8mN 19961027 0647 37.598 47.5501 -70.3548 9.94km -1.0ML 1.889
19951120 2111 25.907 47.4922 -70.4024 5.37km 1.0mN 19960403 0123 52.212 47.4994 -70.3948 5.35km 1.4mN 0.984
19960518 0505 19.606 47.4687 -70.3891 5.82km 1.2mN 19960530 0836 58.713 47.4632 -70.3824 5.54km 0.5ML 0.841
Sub-zone KAM
19900728 1437 10.000 47.6090 -69.9660 10.48km 0.4ML 19920417 1330 24.600 47.6050 -69.9690 9.45km 0.6ML 1.144
19900916 2113 25.300 47.5630 -69.9580 11.04km 0.7ML 19930823 0931 26.819 47.5603 -69.9423 11.89km 2.0mN 1.486
19901213 0350 53.100 47.5690 -69.9060 14.32km 0.4ML 19911011 0659 02.600 47.5790 -69.9150 13.69km 0.3ML 1.446
19920526 0854 44.300 47.5940 -69.9270 11.64km 0.6ML 19961224 1045 55.518 47.5904 -69.9320 11.99km -1.0ML 0.651
19920609 1145 09.200 47.6050 -69.9210 12.20km 0.9ML 19960920 1543 06.202 47.6065 -69.9199 12.24km 1.5mN 0.190
19910919 1106 53.170 47.5384 -69.9324 13.64km 2.6mN 19920903 0704 54.400 47.5240 -69.9520 12.67km 0.5ML 2.383 19940910 2342 03.500 47.5270 -69.9520 12.60km 0.7ML 2.205 19960523 0939 27.217 47.5283 -69.9395 13.66km 1.7mN 1.243 19960607 0941 42.840 47.5299 -69.9417 13.31km 3.1mN 1.221
19950112 0324 08.120 47.6066 -69.9523 11.75km 2.4mN 19960726 1438 45.149 47.6097 -69.9468 10.36km 2.3mN 1.491
Sub-zone POC
19900731 1510 35.938 47.3866 -70.2493 11.99km 0.6ML 19921109 1757 13.600 47.3860 -70.2490 12.13km 0.1ML 0.157 19960205 0810 27.122 47.3849 -70.2561 13.47km 1.4mN 1.578 19960601 0201 42.306 47.3844 -70.2564 12.18km -0.1ML 0.619 19960720 1649 21.345 47.3835 -70.2521 13.87km 0.2ML 1.923 19970315 1303 00.310 47.3853 -70.2520 12.38km 0.7mN 0.463
19940723 0734 25.500 47.3820 -70.1350 13.66km 1.4mN 19940723 0946 41.208 47.3804 -70.1365 15.16km 2.0mN 1.515 19960625 0600 12.684 47.3778 -70.1384 14.57km 1.6mN 1.054 19960702 2359 31.804 47.3792 -70.1397 13.02km 1.0mN 0.795 19960803 0136 34.329 47.3812 -70.1387 12.94km 1.4mN 0.777 19960803 0159 20.822 47.3707 -70.1343 13.22km 0.9mN 1.332 19960814 2110 54.634 47.3816 -70.1371 13.90km 2.2mN 0.291 19961022 0247 22.775 47.3808 -70.1450 13.33km 1.7mN 0.835 19970101 1233 06.148 47.3783 -70.1342 15.55km 1.6mN 1.935
19891218 1455 35.616 47.3866 -70.1543 18.20km 2.8mN 19970118 0149 25.479 47.3704 -70.1445 18.61km -0.9ML 1.989
The current CSZ seismograph network can be used to compute focal mechanisms for earthquakes as small as magnitude mN 3.0. These solutions, however, are strongly dependent on the correctness of every first motion reading. In order to make the solutions more robust, the number of stations within 100 km of the seismic zone should be increased especially in the NE and SW quadrants. The 1996 summer field survey showed that with about six additional stations, focal mechanisms can be computed for events as small as magnitude 2.0. Clearly, additional CSZ stations are needed to derive well defined focal mechanisms from which seismo-tectonic conclusions can be drawn. The calculations of focal mechanisms with Sv/P ratios should be examined carefully to test its reliability. One could test how consistent the Sv/P ratios are in focal mechanisms well constrained with P first motions. For events below magnitude 4.0, P first motions are emergent on stations at regional distances and the velocity model between the CSZ and these stations is at best approximate. This causes problems in using Pn first motions to constrain CSZ mechanisms.
It is possible to resolve more accurately the relative arrival times at a station using the cross-correlation technique (Deichmann and Garcia-Fernandez, 1994; Li et al., 1995). The relative arrival times can be resolved at the sub-sample rate level. In the CSZ case, this method can only be applied to the data recorded between November 1988 and August 1994, i.e. when the three-component data was time-tagged at a central site. The current scheme of GPS time tagging at the various CSZ sites does not provide the consistency required for such an analysis. In the future, the time series files for multiplets recorded between 1988 and 1994 could be correlated.
The data from the current seven local stations do not permit resolution of a 3-D velocity model, which could yield important seismotectonic information, such as lateral velocity contrast across geological faults. An estimate of the network requirements needed to infer a 3-D velocity model for the CSZ follows. The CSZ is a zone 40 km wide, 70 km long and 25 km deep. A velocity model with 2.5 km-sided blocks implies the determination of some 4480 blocks for P and for S velocities. The number of unknowns can be divided in half by assuming a Vp/Vs ratio. For each earthquake, four additional unknowns have to be defined: latitude, longitude, depth and origin time. The observed data, on the other hand, are the total number of stations, times the number of phases used (P and S) times the number of events. The minimum number of stations and events to solve a CSZ 3-D model can be approximated as follows
Number of observed data = number of unknowns
Sta * Pha * Ev = NB + 4 Ev
Where Sta: Number of Stations; Pha: Number of Phases; Ev: Number of Events; and NB: Number of blocks.
Assuming Sta = 12; Pha = 2; and NB= 4680, the minimum number of events is 224. If one assumes an annual rate of 75 events with magnitude mN 1.5 (the minimum level for events to be recorded across the CSZ), then one would have to run the 12 station network for a minimum of about 3 years. Naturally, additional stations mean a shorter time period to reach the objective, while providing a better sampling of the near surface layers. A better network also means more focal mechanisms and better hypocentre locations.
Therefore, the inversion of a local 3-D velocity model for the CSZ requires an expansion of the CSZ seismograph network from the current 7 stations, to 12 or more. During the 1996 summer experiment, up to 8 seismograph stations were operated simultaneously during a 6 month period (Lamontagne et al., 1997). A total of 142 events was recorded, of which 40 met the mN 1.5 selection criteria. The biggest drawback of the portable seismographs was the drift of the internal clocks between the weekly servicing. The drift was generally less than ± 0.05 s, which is still too high for local velocity inversion purposes. The expansion of the CSZ network should make use of portable seismographs with accurate timing. Fortunately, the next generation of portable seismographs will use continuous GPS timing. The joint inversion of the hypocentre-velocity structure is imperative to infer velocity variations and therefore imaging zones of possibly enhanced earthquake activity.