# Geometry, segmentation and stress regime of the Spitak (Armenia) earthquake from the analysis of the aftershock sequence

L. Dorbath, C. Dorbath, Luis Rivera, A. Fuenzalida, A. Cisternas, R.

Tatevossian, J. Aptekman, S. Arefiev

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## To cite this version:

L. Dorbath, C. Dorbath, Luis Rivera, A. Fuenzalida, A. Cisternas, et al., Geometry, segmentation and stress regime of the Spitak (Armenia) earthquake from the analysis of the aftershock sequence. Geophysical Journal International, 1992, 108 (1), pp.309-328. 10.1111/j.1365-246X.1992.tb00860.x. hal-03186267

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# Geometry, segmentation and stress regime of the Spitak (Armenia) earthquake from the analysis of the aftershock sequence

L. Dorbath,<fnref n="1" />,<fnref n="2" /> C. Dorbath,<fnref n="1" />,<fnref n="2" /> L. Rivera,<fnref n="1" /> A. Fuenzalida,<fnref n="1" /> A. Cisternas,<fnref n="1" /> R. Tatevossian,<fnref n="3" /> J. Aptekman<fnref n="3" /> and S. Arefiev<fnref n="3" />

Accepted 1991 July 17. Received 1991 July 16; in original form 1991 April 8

## SUMMARY

The aftershock sequence of the Spitak earthquake, as recorded by a dense portable network deployed around the source region, is analysed in order to obtain a precise description of the mechanics of the rupture. A collection of 708 well-recorded events corresponding to a period of about two weeks is studied, their hypocentres are accurately located, and their focal mechanisms are calculated individually as well as by a joint procedure that permits us to estimate the stress regime.

The epicentral distribution of these aftershocks indicates that the fault at depth extends well beyond the surface rupture towards the west and northwest. Neotectonic and seismic observations permit us to identify five segments broken during the earthquake. The southeastern one, oriented N140°, corresponds to the surface ruptures along the Alavar right lateral shear fault. The main surface ruptures, between Spitak and Gekhasar, correlate well with aftershocks showing a N120° trending fault surface, dipping 50° to the NE and acting as a thrust with a right lateral component. Two similar segments towards the west, are offset and hidden under active folds. A fifth segment, towards the NW, corresponds to a right lateral blind shear fault buried in depth, in the vicinity of the large Pambak-Sevan fault. 14 vertical sections across the fault, including focal mechanisms, illustrate these features.

A relocation of the main shock and aftershocks for the period before the installation of the portable network, confirms the spatial extent of the seismicity. The foreshock, the main shock and the strongest aftershock (4 min 20 s after the main shock) have approximately the same epicentre. Thus the rupture started at the crossing of the Pambak-Sevan and Alavar faults and propagated bilaterally from there, although the seismic moment associated to the Alavar branch is only about one sixth of the total moment.

The stress regime is one of triaxial compression with a $σ_1$ axis oriented N344°. The same orientation is given by the microtectonic observations made on the central segment of the surface ruptures, a result that agrees with previous estimates for the Georgian Caucasus.

**Key words:** aftershock, Armenia, earthquake, stress.

## INTRODUCTION

The Armenian earthquake of 1988 December 7 (40.987°N, 44.185°E, 5 km depth, origin time: 07:41:24.2 UTM, $M_S$ = 6.9 after NEIC) produced widespread destruction in the region around the cities of Spitak, Leninakan and

Kirovakan. This region, located within the Lesser Caucasus, is of particular interest because it represents an early stage of continent-continent collision (Philip et al. 1989). At the same time, the joint occurrence of impressive surface faulting and intense aftershock activity, constituted a natural laboratory to study reverse faulting in a tectonic

<footnote n="1">¹Institut de Physique du Globe de Strasbourg, 5 Rue René Descartes, 67084 Strasbourg Cedex, France</footnote>

<footnote n="2">²ORSTOM, 213 rue La Fayette, 75480 Paris Cedex 10, France</footnote>

<footnote n="3">³Institute of Physics of the Earth, Soviet Academy of Sciences, Bolshaya Gruzinskaya 10, Moscow 123242, USSR</footnote>
environment different from that of the San Fernando (1971), the El Asnam (1980), the Coalinga (1983) or the Whittier Narrows (1987) earthquakes. The study following the event, included teleseismic and near-field seismic observations, neotectonics, geodesy, levelling, palaeoseismicity, radon content and other physical observations.

12 days after the earthquake, a French-Soviet seismic network was installed in the epicentral area, and preliminary results from this expedition have been published elsewhere (Cisternas et al. 1989; Dufumier 1989; Jimenez, Cara & Rouland 1989). Another field team from the USA also obtained similar results (Pacheco et al. 1989). The portable networks complemented the near-source coverage of the permanent regional network of 27 Armenian, Georgian and Azerbaydzhan stations, some of which were damaged and failed to operate.

The field work experiment can be divided into three periods:

(1) from December 19 to December 24 the portable network was deployed and adjusted to provide the best possible coverage of the aftershock zone;

(2) from December 24 to January 8 the portable network of 26 sites distributed over an area of 1500 km² operated continuously;

(3) from January 8 to the end of February seismic monitoring continued but with a reduced network (20 sites).

The present paper discusses the aftershock distribution and focal mechanisms, the estimation of the stress regime and the construction of a detailed and accurate source model, from data obtained during the second period, when the information was of the best quality.

## INSTRUMENTATION

The network, during the period under consideration, consisted of three different types of recorders (Fig. 1 and Table 1).

### 1 Analogue seismographs

10 of the sites were equipped with smoked paper recording seismographs (Sprengnether MEQ 800) and one component

<table><caption>**Table 1.** Temporary network.</caption><thead><tr><th>Code</th><th>Name</th><th>Latitude</th><th>Longitude</th><th>Elevation Type</th></tr></thead><tbody><tr><td>AID</td><td>Aiderly</td><td>40° 46.70N</td><td>44° 22.80E</td><td>1550 D</td></tr><tr><td>COL</td><td>Col Krashen</td><td>40° 52.80N</td><td>43° 57.90E</td><td>1720 A</td></tr><tr><td>DAR</td><td>Darpas</td><td>40° 51.10N</td><td>44° 25.80E</td><td>1655 T</td></tr><tr><td>GOG</td><td>Gogaran</td><td>40° 54.10N</td><td>44° 11.70E</td><td>1900 T</td></tr><tr><td>KAP</td><td>Kaps</td><td>40° 52.30N</td><td>43° 45.20E</td><td>1700 D</td></tr><tr><td>KAT</td><td>Katnakhpior</td><td>41° 01.10N</td><td>44° 11.10E</td><td>1750 D</td></tr><tr><td>KET</td><td>Keti</td><td>40° 52.70N</td><td>43° 51.00E</td><td>1730 D</td></tr><tr><td>KIR</td><td>Kirovakan</td><td>40° 47.20N</td><td>44° 29.50E</td><td>1525 A</td></tr><tr><td>KML</td><td>Chigdamal</td><td>40° 52.60N</td><td>44° 15.80E</td><td>1820 A</td></tr><tr><td>KM2</td><td>Chigdamal2</td><td>40° 53.30N</td><td>44° 16.75E</td><td>1900 T</td></tr><tr><td>KYB</td><td>Kuibishev</td><td>40° 58.40N</td><td>44° 15.90E</td><td>1596 A</td></tr><tr><td>LER</td><td>Lernavan</td><td>40° 46.70N</td><td>44° 09.10E</td><td>2020 T</td></tr><tr><td>LRN</td><td>Lernansk</td><td>40° 46.80N</td><td>44° 16.00E</td><td>1940 T</td></tr><tr><td>LYE</td><td>Lernapar</td><td>40° 44.80N</td><td>44° 09.80E</td><td>2150 A</td></tr><tr><td>MET</td><td>Metsparni</td><td>40° 52.60N</td><td>44° 07.60E</td><td>2200 T</td></tr><tr><td>MOL</td><td>Molakishlag</td><td>40° 44.20N</td><td>44° 25.00E</td><td>1880 A</td></tr><tr><td>NB2</td><td>Nalband</td><td>40° 48.90N</td><td>44° 10.30E</td><td>1740 A</td></tr><tr><td>SAR</td><td>Saral</td><td>40° 50.80N</td><td>44° 20.90E</td><td>1640 T</td></tr><tr><td>SAT</td><td>Sarapat</td><td>40° 56.60N</td><td>44° 03.00E</td><td>2100 T</td></tr><tr><td>SPZ</td><td>Spitak</td><td>40° 49.90N</td><td>44° 15.70E</td><td>1700 T</td></tr><tr><td>SRR</td><td>Saraart</td><td>40° 52.10N</td><td>44° 13.30E</td><td>1680 D</td></tr><tr><td>ST1</td><td>Stepanavan1</td><td>40° 59.90N</td><td>44° 23.90E</td><td>1405 D</td></tr><tr><td>ST2</td><td>Stepanavan2</td><td>41° 01.30N</td><td>44° 15.60E</td><td>1640 A</td></tr><tr><td>TOR</td><td>Torosgiukh</td><td>40° 56.10N</td><td>43° 52.40E</td><td>1980 D</td></tr><tr><td>TUN</td><td>Tunnel</td><td>40° 55.40N</td><td>44° 26.80E</td><td>1900 A</td></tr><tr><td>XYB</td><td>Khinkoyan</td><td>40° 50.10N</td><td>44° 01.20E</td><td>1920 A</td></tr></tbody></table>

vertical sensors (Mark Product L4C, $T_0 = 1$ s). The gains were set at 66, 72 or 78 dB according to the background noise, and the filters were set at the position 'out' for the low-pass, and at 30 Hz for the high-pass. At some places, the gains were temporary reduced during windy conditions. All of the instruments functioned at a speed of 60 mm per minute with a separation of 1 mm between consecutive traces, allowing for 48 hr of continuous operation. The seismographs included temperature compensated crystal clocks whose drifts were measured by comparing every two days (and simultaneously recording) the internal time signal with the reference time radio-transmitted by Moscow (14 994 kHz). Despite the severe temperature conditions, the clocks did not drift more than 0.05 seconds per day. Internal time marks were placed at the second thus permitting a reading precision of 0.05 s for P-waves.

### 2 Digital recording systems

Six Geostras digital recorders built at the Institut de Physique du Globe de Strasbourg were used to record signals from three-component seismometers (Mark Product

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Locations of the temporary portable stations in the Spitak earthquake region (Table 1). Triangles: telemetric array. Circles: analogue stations. Squares: digital stations. The shaded contours are those of 2000 and 4000 m.</figcaption></figure>
L22, 0.5 s natural period). The sampling rate was fixed at 150 samples per second. The recording process was triggered by a STA/LTA type algorithm, with the amplification being set automatically. Because of the limited dynamic range of the system (110 dB) some of the records of the strongest aftershocks were clipped.

Time signals were obtained from the worldwide Omega system. At some sites the drift of the receiving unit (Omegarec) crystal due to the very low temperatures (down to minus 20 °C) was faster than the built-in compensation process designed to correct the phase difference with respect to the incoming Omega signal. In these cases, the drift might have reached several seconds per day and absolute time is not available.

## 3 Telemetric network

Eight sites were equipped with a vertical component velocity transducer L4C, whose signals were transmitted via FM radio link to a central receiving station, where an additional three-component seismometer L22 was operating. The seismic signals together with the internal clock and the reference DCF time signals were digitized at a rate of 150 samples per second, mixed and then recorded on magnetic tape. The system is essentially the same as that of the autonomous Geostras stations. In this case, however, the recording process was activated only when any four of the stations jointly received a signal greater than a selected threshold within a given time period, to reduce the non-seismic triggering that occurs sometimes with the autonomous stations.

The locations of the telemetric stations were prescribed by the topography since they have to be within direct sight of the central station. The Spitak television tower was selected as receiving centre, so that the telemetric stations made up the kernel of the temporary network. Tapes were played back, arrival times of P- and S-waves were read and aftershocks automatically located every day. A subset of the hypocentre determinations thus obtained on a routine basis has been published in a previous paper (Cisternas et al. 1989). This knowledge of the aftershock distribution was very useful in selecting the sites of the autonomous stations to provide a uniform coverage in order to optimize hypocentral locations and constrain focal mechanisms. Except for the northwestern region to the north of the station SAT (Fig. 1), where the only existing road had been closed and access was impossible, the portable network of stations covers the aftershock area well.

## DATA ANALYSIS

In the first days we recorded about 200 aftershocks per day with magnitude greater than 0.5 at the analogue stations; this number decreased slightly during the time interval of the present study (December 24 to January 8), staying nevertheless over a hundred by the end of the recording period. We studied only those events recorded at least by the telemetric network, though keeping in mind that this set of aftershocks is not really complete mainly due to short operational cut-offs and to difficulties in receiving the DCF time signals. We could have supplemented the gaps with events recorded only by the autonomous analogue stations

and by Geostras, however we preferred to maintain the homogeneity of quality for the data and results since, as we have said, the telemetric network was the essential part of our dispositive. About 750 aftershocks were thus located, 708 among which were selected on the basis of the quality of the solution.

### 1 Velocity model

First, we tried to find a reliable velocity model. For this purpose, and in order to ensure high-quality readings, we used data from the telemetric network. The $V_p/V_s$ ratio was determined by using a composite Wadati diagram obtained from about one hundred aftershocks. We found this ratio to be 1.78. We checked that this value did not change significantly in the course of time, daily values ranging from 1.76 to 1.82.

With this $V_p/V_s$ ratio of 1.78, we located about one hundred well-recorded aftershocks in a half-space with a P velocity ranging from 5.0 to 6.4 km s⁻¹. The minimum value of mean RMS was obtained for a velocity of 5.6 km s⁻¹. When we observed the minimum in the mean RMS as a function of hypocentral depth we realized that the P velocity increased with depth and then we chose a velocity model with two horizontal layers. Our best model was made of a thin layer of 4 km with a velocity $V_p = 5.3$ km s⁻¹, over a half-space with $V_p = 6.0$ km s⁻¹. The Soviet geophysicists exploded about 100 kg of TNT on January 23, in a drill-hole close to the epicentre of the main shock. This explosion was recorded by our network and analysis of the data confirmed that the upper crust velocity is about 5.3 to 5.4 km s⁻¹.

### 2 Hypocentre determinations

The aftershocks were determined by using the HYPOINVERSE program (Klein 1978) and good locations were selected on the basis of: RMS < 0.20 s and conditioning factor < 100. Some Geostras stations did not have correct absolute time for some periods as stated above, but we used S-P times as data in those cases. Most of the locations rely on several S readings. The RMS is lower than 0.12 s for most of the locations (87 per cent), and 70 per cent correspond to conditioning factors less than 30 (Fig. 2), while 80 per cent of the aftershocks have standard errors on epicentral

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Histograms of (a) depth, (b) magnitude, (c) arrival time mean error, RMS, (d) epicentral error, ERH and (e) depth error, ERZ for the 708 selected events (1988 December 20 to 1989 January 8) (Klein 1978).</figcaption></figure>
position and depth lower than 0.5 and 1 km respectively (Fig. 2). The latter values are known to be underestimates of the errors; however, when taken together with the RMS and the conditioning factor, they provide a reliable estimate of the quality of the locations. We located the whole set of aftershocks both, in a half-space model ($V_P = 5.6 \text{ km s}^{-1}$), and in the selected two-layer model. The results do not differ significantly, as previously observed (Lyon Caen et al. 1988) when a network is dense and adequately covers the seismic zone; depths are tightly constrained by nearby stations. The epicentres differ by no more than 200 m on the average, and 83 per cent change by less than 500 m. The depths, which are more sensitive to model perturbations, differ by about 700 m on the average, 70 per cent of them changing by less than 1 km. Hence, we believe that the standard errors given by HYPOINVERSE are quite realistic.

## 3 Focal mechanisms

The polarity of each seismograph system was checked before the departure to Armenia, in the field by the recording of an explosion, and checked again upon the return to Strasbourg. Using a half-space model all the rays are direct ones, namely they have take-off angles greater than 90°. On the other hand, rays coming from hypocentres within the upper layer of a two-layer model, and diffracted at the lower interface, may hit a station as first arrivals. It follows that incidence angles, and therefore the distribution of polarities on the focal sphere, may change significantly. For these shallow events, we kept only those focal solutions for which the nodal planes did not vary much in azimuth and dip, when passing from the half-space to the two-layer model.

In this way, we have constructed individual fault-plane solutions for events with more than 10 *P*-wave polarities. The data set is of excellent quality and we could determine 412 focal solutions without contradictory polarities, and with an average of 14 readings. Nodal planes fitted by eye and maximum likelihood solutions (Udías et al. 1982) are in good agreement, and the nodal planes appear well constrained. Only a subset of these mechanisms is plotted on the cross-sections (Fig. 4), but all of them are to be considered in the section related to the fault mechanism. Table 2 shows the parameters defining nodal planes, azimuth, dip and rake, following Aki's convention (Aki & Richards 1980).

## SPACE DISTRIBUTION OF AFTERSHOCKS AND FAULT-PLANE SOLUTIONS

The epicentres of the 708 well-located aftershocks are shown in Fig. 3. They depict a long (50 km) band, narrow in the eastern extremity, which widens to the west up to 10 km. The seismic activity appears to be weak in the upper 3 km. The hypocentres are superficial or shallow at the eastern end, but their depths increase to the west where they reach 14 km. It looks as if the relative amount of large magnitude aftershocks is higher at the western end, but this result may be due to the geometry and triggering system of the telemetric network. It seems convenient to define five main segments on the basis of the following seismological and tectonic considerations.

### 1 The Alavar southeastern segment

(Fig. 3; Fig. 4 AA', BB', CC', DD'; Table 2). The seismicity along this segment forms a very narrow zone, about 2 km wide and 15 km long, striking N140°. These general orientation and space distribution may be correlated to the Alavar fault (Philip et al. 1991), since the surface breaks pass through the epicentres at the southern tip, and they are slightly shifted west of the seismicity near the northern tip. Most of the shallow activity concentrates near the ends of this segment, seismicity at the centre being deeper. This distribution suggests that no further slip occurred on the area broken during the main shock, and that aftershock activity concentrated along the edges of the ruptured zone.

The seismicity seems to separate into two nearly vertical branches towards the southern edge of the segment and depths do not exceed 6 km (Fig. 4, a, b). Most of the focal mechanisms show stike-slip faulting on an almost vertical nodal plane striking at about the same direction as the general trend of the surface ruptures and the seismicity (N140°). Some reverse faulting occurs usually outside the main clusters.

Hypocentral depths increase up to 9 km towards the northern edge of the segment, the aftershocks lying on a single plane dipping about 65° to the northeast (Fig. 4CC', DD'). Cross-section CC' still shows dominant strike-slip faulting, however the relative amount of reverse faulting increases. The last cross-section (DD') suggests that the aftershocks define a clear fault plane. This cross-section, southeast of, but next to Spitak, is beyond the observed surface ruptures and shows as many strike-slip mechanisms as reverse ones, thus characterizing the transition to the next segment situated to the west of Spitak.

Hence, the Alavar segment exhibits shallow seismicity along a nearly vertical fault. The deformation is rather simple, most of the focal mechanisms being compatible with right lateral strike-slip faulting, if the nodal planes oriented along the general direction of the seismicity are selected as fault planes. This result is in agreement with the observation of surface ruptures which show oblique en échelon right lateral shears with a maximum displacement of 0.50 m (Philip et al. 1991). The seismic moment of this segment depends on the area of the broken surface (≈66 km²) and on the relative offset across the fault (≈50 cm), and amounts to 1/6 of the total seismic moment released during the main shock (Haessler et al. 1991), and therefore much less important than the value given by Pacheco et al. (1989).

### 2 Central segments

(Fig. 3; Fig. 4, FF' to KK'; Table 2). The east central segment corresponds to the most important and continuous surface dislocations between Spitak and Gekhasar. These breaks extend for about 8 km (Fig. 3). They show reverse faulting dipping to the north with a right-lateral offset. The maximum vertical and lateral displacements were measured near Spitak: 160 and 40 cm respectively. On the other hand no surface breaks were observed along the western central segment where the fault is hidden under an anticline whose axis has about the same direction as the surface ruptures west of Spitak. This suggests that the rupture is hidden at depth, as indicated by the seismicity, and that the
<table><caption>Table 2. Aftershock parameters.</caption><caption>CROSS SECTION AA'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890107</td><td>175712.2</td><td>40° 46.20</td><td>44° 22.81</td><td>3.45</td><td>2.6</td><td>268</td><td>50</td><td>43</td><td>50</td><td>1</td></tr><tr><td>2</td><td>890103</td><td>072026.9</td><td>40° 45.91</td><td>44° 22.48</td><td>3.16</td><td>2.7</td><td>281</td><td>69</td><td>33</td><td>51</td><td>2</td></tr><tr><td>3</td><td>881226</td><td>155420.8</td><td>40° 45.77</td><td>44° 22.25</td><td>3.30</td><td>1.5</td><td>117</td><td>80</td><td>216</td><td>72</td><td>3</td></tr><tr><td>4</td><td>890105</td><td>161449.8</td><td>40° 45.05</td><td>44° 22.68</td><td>3.65</td><td>0.7</td><td>312</td><td>80</td><td>42</td><td>70</td><td>4</td></tr><tr><td>5</td><td>890105</td><td>153026.4</td><td>40° 45.08</td><td>44° 22.74</td><td>3.77</td><td>2.5</td><td>304</td><td>70</td><td>43</td><td>73</td><td>5</td></tr><tr><td>6</td><td>890106</td><td>205323.0</td><td>40° 46.06</td><td>44° 22.16</td><td>3.91</td><td>2.7</td><td>311</td><td>61</td><td>53</td><td>75</td><td>6</td></tr><tr><td>7</td><td>881230</td><td>203557.4</td><td>40° 45.71</td><td>44° 22.46</td><td>4.03</td><td>1.2</td><td>330</td><td>60</td><td>65</td><td>80</td><td>7</td></tr><tr><td>8</td><td>881225</td><td>230724.8</td><td>40° 45.93</td><td>44° 22.03</td><td>4.61</td><td>1.9</td><td>310</td><td>54</td><td>209</td><td>84</td><td>8</td></tr><tr><td>9</td><td>890103</td><td>230330.5</td><td>40° 45.68</td><td>44° 22.00</td><td>5.34</td><td>1.6</td><td>330</td><td>60</td><td>83</td><td>55</td><td>9</td></tr><tr><td>10</td><td>890103</td><td>174712.4</td><td>40° 45.81</td><td>44° 21.93</td><td>5.46</td><td>1.1</td><td>326</td><td>70</td><td>64</td><td>60</td><td>10</td></tr><tr><td>11</td><td>881228</td><td>215607.5</td><td>40° 45.08</td><td>44° 22.78</td><td>8.49</td><td>1.0</td><td>321</td><td>62</td><td>216</td><td>72</td><td>11</td></tr><tr><td>12</td><td>890101</td><td>223846.9</td><td>40° 45.89</td><td>44° 23.22</td><td>4.32</td><td>1.2</td><td>290</td><td>70</td><td>192</td><td>70</td><td>12</td></tr><tr><td>13</td><td>881225</td><td>212250.9</td><td>40° 46.28</td><td>44° 23.34</td><td>4.25</td><td>1.9</td><td>291</td><td>72</td><td>33</td><td>61</td><td>13</td></tr><tr><td>14</td><td>890102</td><td>035014.4</td><td>40° 46.18</td><td>44° 23.33</td><td>4.16</td><td>1.8</td><td>253</td><td>71</td><td>148</td><td>60</td><td>14</td></tr><tr><td>15</td><td>881228</td><td>085132.9</td><td>40° 46.17</td><td>44° 23.47</td><td>5.46</td><td>1.6</td><td>297</td><td>82</td><td>36</td><td>70</td><td>15</td></tr><tr><td>16</td><td>881226</td><td>150430.5</td><td>40° 45.96</td><td>44° 23.82</td><td>4.78</td><td>1.9</td><td>318</td><td>80</td><td>60</td><td>62</td><td>16</td></tr><tr><td>17</td><td>890106</td><td>231443.1</td><td>40° 46.55</td><td>44° 23.37</td><td>4.10</td><td>2.8</td><td>318</td><td>78</td><td>57</td><td>66</td><td>17</td></tr><tr><td>18</td><td>890101</td><td>001023.3</td><td>40° 46.13</td><td>44° 23.88</td><td>4.01</td><td>2.0</td><td>305</td><td>50</td><td>88</td><td>51</td><td>18</td></tr><tr><td>19</td><td>881229</td><td>004237.1</td><td>40° 46.74</td><td>44° 23.70</td><td>4.32</td><td>1.3</td><td>122</td><td>75</td><td>17</td><td>53</td><td>19</td></tr><tr><td>20</td><td>881226</td><td>221126.0</td><td>40° 46.91</td><td>44° 25.56</td><td>4.73</td><td>2.3</td><td>263</td><td>50</td><td>92</td><td>40</td><td>20</td></tr><tr><td>21</td><td>881226</td><td>222756.8</td><td>40° 47.01</td><td>44° 25.42</td><td>4.05</td><td>1.7</td><td>266</td><td>62</td><td>7</td><td>79</td><td>21</td></tr><tr><td>22</td><td>890105</td><td>120225.3</td><td>40° 46.56</td><td>44° 23.05</td><td>3.48</td><td>2.1</td><td>102</td><td>87</td><td>192</td><td>79</td><td>22</td></tr><tr><td>23</td><td>890103</td><td>034014.1</td><td>40° 46.38</td><td>44° 23.25</td><td>3.43</td><td>1.9</td><td>128</td><td>78</td><td>226</td><td>74</td><td>23</td></tr><tr><td>24</td><td>890101</td><td>063129.3</td><td>40° 47.29</td><td>44° 25.01</td><td>2.92</td><td>2.3</td><td>120</td><td>60</td><td>232</td><td>45</td><td>24</td></tr><tr><td>25</td><td>890101</td><td>132048.0</td><td>40° 45.82</td><td>44° 24.14</td><td>2.52</td><td>1.5</td><td>260</td><td>50</td><td>108</td><td>44</td><td>25</td></tr></tbody></table>

<table><caption>CROSS SECTION BB'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>881226</td><td>82737.12</td><td>40° 47.06</td><td>44° 22.25</td><td>3.10</td><td>2.3</td><td>118</td><td>72</td><td>220</td><td>72</td><td>26</td></tr><tr><td>2</td><td>890107</td><td>144444.6</td><td>40° 47.44</td><td>44° 21.87</td><td>3.18</td><td>2.0</td><td>262</td><td>50</td><td>84</td><td>40</td><td>27</td></tr><tr><td>3</td><td>890101</td><td>124122.9</td><td>40° 46.91</td><td>44° 22.44</td><td>3.30</td><td>1.5</td><td>293</td><td>70</td><td>30</td><td>70</td><td>28</td></tr><tr><td>4</td><td>890106</td><td>184913.3</td><td>40° 46.72</td><td>44° 21.86</td><td>3.42</td><td>1.5</td><td>274</td><td>40</td><td>112</td><td>52</td><td>29</td></tr><tr><td>5</td><td>890106</td><td>230325.6</td><td>40° 46.16</td><td>44° 22.18</td><td>3.64</td><td>2.1</td><td>284</td><td>51</td><td>83</td><td>42</td><td>30</td></tr><tr><td>6</td><td>881228</td><td>202618.9</td><td>40° 45.72</td><td>44° 21.40</td><td>5.80</td><td>1.2</td><td>338</td><td>75</td><td>78</td><td>64</td><td>31</td></tr><tr><td>7</td><td>890107</td><td>075321.3</td><td>40° 47.11</td><td>44° 21.42</td><td>4.48</td><td>2.8</td><td>286</td><td>82</td><td>190</td><td>67</td><td>32</td></tr><tr><td>8</td><td>890107</td><td>030327.5</td><td>40° 47.11</td><td>44° 21.42</td><td>4.72</td><td>3.3</td><td>296</td><td>74</td><td>193</td><td>62</td><td>33</td></tr><tr><td>9</td><td>881229</td><td>002313.7</td><td>40° 46.11</td><td>44° 21.96</td><td>5.65</td><td>2.4</td><td>307</td><td>69</td><td>57</td><td>54</td><td>34</td></tr><tr><td>10</td><td>890104</td><td>185514.8</td><td>40° 47.05</td><td>44° 22.14</td><td>3.70</td><td>2.2</td><td>131</td><td>86</td><td>20</td><td>66</td><td>35</td></tr><tr><td>11</td><td>890104</td><td>024702.4</td><td>40° 47.17</td><td>44° 21.95</td><td>3.80</td><td>2.3</td><td>133</td><td>80</td><td>33</td><td>56</td><td>36</td></tr><tr><td>12</td><td>890107</td><td>150402.9</td><td>40° 47.37</td><td>44° 21.89</td><td>3.86</td><td>1.9</td><td>136</td><td>80</td><td>42</td><td>62</td><td>37</td></tr><tr><td>13</td><td>881226</td><td>033458.8</td><td>40° 47.12</td><td>44° 22.07</td><td>3.82</td><td>1.4</td><td>133</td><td>83</td><td>34</td><td>60</td><td>38</td></tr><tr><td>14</td><td>881226</td><td>113444.7</td><td>40° 46.72</td><td>44° 22.69</td><td>4.13</td><td>1.7</td><td>287</td><td>76</td><td>25</td><td>76</td><td>39</td></tr><tr><td>15</td><td>881227</td><td>120918.6</td><td>40° 46.92</td><td>44° 22.53</td><td>5.33</td><td>1.9</td><td>292</td><td>61</td><td>30</td><td>80</td><td>40</td></tr><tr><td>16</td><td>890105</td><td>214824.5</td><td>40° 47.05</td><td>44° 22.56</td><td>4.87</td><td>2.4</td><td>286</td><td>78</td><td>20</td><td>70</td><td>41</td></tr><tr><td>17</td><td>881227</td><td>000405.9</td><td>40° 46.64</td><td>44° 23.06</td><td>4.80</td><td>1.1</td><td>282</td><td>70</td><td>24</td><td>60</td><td>42</td></tr><tr><td>18</td><td>890104</td><td>223422.8</td><td>40° 47.37</td><td>44° 22.72</td><td>4.25</td><td>1.5</td><td>104</td><td>70</td><td>200</td><td>70</td><td>43</td></tr><tr><td>19</td><td>890104</td><td>140233.0</td><td>40° 47.39</td><td>44° 22.65</td><td>4.10</td><td>2.3</td><td>107</td><td>70</td><td>354</td><td>50</td><td>44</td></tr><tr><td>20</td><td>881227</td><td>031844.7</td><td>40° 46.71</td><td>44° 22.91</td><td>3.63</td><td>1.4</td><td>112</td><td>80</td><td>209</td><td>67</td><td>45</td></tr><tr><td>21</td><td>890101</td><td>231121.8</td><td>40° 46.97</td><td>44° 22.90</td><td>3.28</td><td>1.0</td><td>258</td><td>80</td><td>351</td><td>70</td><td>46</td></tr><tr><td>22</td><td>890101</td><td>225938.2</td><td>40° 46.76</td><td>44° 22.93</td><td>3.13</td><td>4.2</td><td>276</td><td>84</td><td>4</td><td>70</td><td>47</td></tr><tr><td>23</td><td>890105</td><td>025318.8</td><td>40° 46.98</td><td>44° 22.30</td><td>2.24</td><td>2.5</td><td>280</td><td>82</td><td>19</td><td>70</td><td>48</td></tr></tbody></table>

<table><caption>CROSS SECTION CC'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890102</td><td>003002.2</td><td>40° 49.37</td><td>44° 19.34</td><td>3.40</td><td>1.6</td><td>250</td><td>60</td><td>70</td><td>30</td><td>49</td></tr><tr><td>2</td><td>890107</td><td>040030.0</td><td>40° 48.92</td><td>44° 19.88</td><td>3.51</td><td>1.1</td><td>330</td><td>80</td><td>62</td><td>70</td><td>50</td></tr><tr><td>3</td><td>881229</td><td>174327.2</td><td>40° 49.19</td><td>44° 19.20</td><td>3.73</td><td>1.9</td><td>333</td><td>80</td><td>64</td><td>70</td><td>51</td></tr><tr><td>4</td><td>890102</td><td>184737.2</td><td>40° 49.37</td><td>44° 19.07</td><td>4.80</td><td>0.7</td><td>321</td><td>70</td><td>57</td><td>70</td><td>52</td></tr><tr><td>5</td><td>881229</td><td>221000.3</td><td>40° 49.83</td><td>44° 18.75</td><td>4.66</td><td>1.3</td><td>345</td><td>89</td><td>180</td><td>2</td><td>53</td></tr><tr><td>6</td><td>890105</td><td>045923.9</td><td>40° 49.52</td><td>44° 19.28</td><td>5.57</td><td>1.2</td><td>317</td><td>86</td><td>56</td><td>40</td><td>54</td></tr><tr><td>7</td><td>890108</td><td>130138.3</td><td>40° 49.45</td><td>44° 19.20</td><td>5.64</td><td>3.5</td><td>311</td><td>67</td><td>53</td><td>72</td><td>55</td></tr><tr><td>8</td><td>890105</td><td>191703.1</td><td>40° 49.56</td><td>44° 19.14</td><td>5.99</td><td>3.0</td><td>324</td><td>57</td><td>72</td><td>68</td><td>56</td></tr><tr><td>9</td><td>890104</td><td>235619.2</td><td>40° 49.61</td><td>44° 19.21</td><td>6.21</td><td>3.4</td><td>312</td><td>68</td><td>54</td><td>73</td><td>57</td></tr><tr><td>10</td><td>890105</td><td>081513.6</td><td>40° 50.12</td><td>44° 19.17</td><td>6.46</td><td>4.2</td><td>287</td><td>61</td><td>27</td><td>81</td><td>58</td></tr><tr><td>11</td><td>890104</td><td>015404.9</td><td>40° 49.43</td><td>44° 20.43</td><td>6.86</td><td>1.7</td><td>294</td><td>30</td><td>70</td><td>68</td><td>59</td></tr><tr><td>12</td><td>890105</td><td>164824.8</td><td>40° 50.15</td><td>44° 19.19</td><td>7.24</td><td>0.8</td><td>238</td><td>46</td><td>95</td><td>50</td><td>60</td></tr><tr><td>13</td><td>881230</td><td>165916.2</td><td>40° 49.98</td><td>44° 19.40</td><td>8.18</td><td>1.0</td><td>131</td><td>83</td><td>34</td><td>77</td><td>61</td></tr><tr><td>14</td><td>890104</td><td>001804.4</td><td>40° 49.79</td><td>44° 20.14</td><td>5.98</td><td>1.2</td><td>238</td><td>60</td><td>358</td><td>50</td><td>62</td></tr></tbody></table>
<table><caption>Table 2. (continued)</caption><caption>CROSS SECTION CC'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>15</td><td>890107</td><td>150227.7</td><td>40° 50.38</td><td>44° 19.57</td><td>6.29</td><td>1.6</td><td>247</td><td>41</td><td>24</td><td>59</td><td>63</td></tr><tr><td>16</td><td>890102</td><td>203855.5</td><td>40° 49.76</td><td>44° 20.53</td><td>6.51</td><td>1.2</td><td>288</td><td>60</td><td>36</td><td>60</td><td>64</td></tr><tr><td>17</td><td>881231</td><td>194551.2</td><td>40° 50.14</td><td>44° 19.80</td><td>5.75</td><td>1.5</td><td>246</td><td>60</td><td>5</td><td>50</td><td>65</td></tr><tr><td>18</td><td>890101</td><td>091228.5</td><td>40° 50.38</td><td>44° 20.57</td><td>6.80</td><td>1.9</td><td>294</td><td>61</td><td>35</td><td>77</td><td>66</td></tr><tr><td>19</td><td>881229</td><td>170905.7</td><td>40° 49.87</td><td>44° 20.96</td><td>6.34</td><td>1.3</td><td>276</td><td>61</td><td>35</td><td>51</td><td>67</td></tr><tr><td>20</td><td>890107</td><td>175751.4</td><td>40° 49.88</td><td>44° 19.97</td><td>5.05</td><td>1.1</td><td>268</td><td>51</td><td>42</td><td>50</td><td>68</td></tr><tr><td>21</td><td>890101</td><td>005905.7</td><td>40° 49.52</td><td>44° 19.98</td><td>4.80</td><td>1.8</td><td>250</td><td>58</td><td>93</td><td>34</td><td>69</td></tr><tr><td>22</td><td>890102</td><td>234247.3</td><td>40° 51.95</td><td>44° 21.72</td><td>4.66</td><td>1.6</td><td>110</td><td>70</td><td>272</td><td>20</td><td>70</td></tr><tr><td>23</td><td>881230</td><td>161637.8</td><td>40° 49.46</td><td>44° 19.10</td><td>3.78</td><td>1.2</td><td>100</td><td>77</td><td>359</td><td>66</td><td>71</td></tr><tr><td>24</td><td>881230</td><td>050346.8</td><td>40° 49.67</td><td>44° 19.08</td><td>3.58</td><td>0.8</td><td>283</td><td>84</td><td>14</td><td>80</td><td>72</td></tr><tr><td>25</td><td>881230</td><td>050235.7</td><td>40° 49.57</td><td>44° 19.14</td><td>3.42</td><td>3.0</td><td>306</td><td>80</td><td>37</td><td>86</td><td>73</td></tr></tbody></table>

<table><caption>CROSS SECTION DD'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>881231</td><td>042655.2</td><td>40° 50.13</td><td>44° 17.37</td><td>3.12</td><td>2.0</td><td>300</td><td>62</td><td>42</td><td>80</td><td>74</td></tr><tr><td>2</td><td>881226</td><td>071439.9</td><td>40° 49.69</td><td>44° 18.14</td><td>3.23</td><td>1.1</td><td>330</td><td>69</td><td>80</td><td>54</td><td>75</td></tr><tr><td>3</td><td>881225</td><td>203533.8</td><td>40° 50.24</td><td>44° 17.67</td><td>3.91</td><td>0.7</td><td>352</td><td>74</td><td>88</td><td>75</td><td>76</td></tr><tr><td>4</td><td>890101</td><td>165116.2</td><td>40° 49.78</td><td>44° 18.28</td><td>4.05</td><td>1.1</td><td>292</td><td>60</td><td>34</td><td>70</td><td>77</td></tr><tr><td>5</td><td>881231</td><td>162619.9</td><td>40° 50.30</td><td>44° 18.74</td><td>4.95</td><td>1.6</td><td>288</td><td>50</td><td>24</td><td>80</td><td>78</td></tr><tr><td>6</td><td>881229</td><td>175938.6</td><td>40° 50.29</td><td>44° 19.19</td><td>6.04</td><td>1.5</td><td>284</td><td>49</td><td>34</td><td>71</td><td>79</td></tr><tr><td>7</td><td>890102</td><td>015004.1</td><td>40° 50.32</td><td>44° 19.19</td><td>6.07</td><td>0.6</td><td>318</td><td>40</td><td>214</td><td>80</td><td>80</td></tr><tr><td>8</td><td>890101</td><td>134025.6</td><td>40° 50.37</td><td>44° 19.15</td><td>6.16</td><td>0.9</td><td>278</td><td>37</td><td>22</td><td>80</td><td>81</td></tr><tr><td>9</td><td>881225</td><td>174943.9</td><td>40° 51.00</td><td>44° 17.96</td><td>6.83</td><td>1.5</td><td>232</td><td>62</td><td>51</td><td>28</td><td>82</td></tr><tr><td>10</td><td>881225</td><td>165117.6</td><td>40° 51.05</td><td>44° 18.05</td><td>6.85</td><td>1.6</td><td>252</td><td>76</td><td>19</td><td>26</td><td>83</td></tr><tr><td>11</td><td>881227</td><td>001503.6</td><td>40° 51.19</td><td>44° 18.00</td><td>6.97</td><td>1.8</td><td>255</td><td>64</td><td>55</td><td>28</td><td>84</td></tr><tr><td>12</td><td>890105</td><td>113058.1</td><td>40° 50.88</td><td>44° 18.25</td><td>7.77</td><td>1.7</td><td>96</td><td>60</td><td>208</td><td>61</td><td>85</td></tr><tr><td>13</td><td>890101</td><td>130211.4</td><td>40° 50.61</td><td>44° 18.93</td><td>7.60</td><td>1.7</td><td>270</td><td>61</td><td>49</td><td>38</td><td>86</td></tr><tr><td>14</td><td>881231</td><td>061526.8</td><td>40° 50.83</td><td>44° 17.87</td><td>12.52</td><td>0.9</td><td>308</td><td>70</td><td>62</td><td>40</td><td>87</td></tr><tr><td>15</td><td>890108</td><td>034339.7</td><td>40° 51.01</td><td>44° 18.65</td><td>8.47</td><td>1.9</td><td>295</td><td>80</td><td>40</td><td>40</td><td>88</td></tr><tr><td>16</td><td>890107</td><td>155058.9</td><td>40° 50.81</td><td>44° 18.84</td><td>7.27</td><td>1.1</td><td>256</td><td>35</td><td>76</td><td>55</td><td>89</td></tr><tr><td>17</td><td>890101</td><td>063510.5</td><td>40° 50.43</td><td>44° 19.30</td><td>6.45</td><td>2.6</td><td>296</td><td>65</td><td>41</td><td>67</td><td>90</td></tr><tr><td>18</td><td>881229</td><td>257513.9</td><td>40° 50.47</td><td>44° 19.36</td><td>6.27</td><td>2.2</td><td>246</td><td>61</td><td>358</td><td>61</td><td>91</td></tr><tr><td>19</td><td>881225</td><td>193617.2</td><td>40° 50.57</td><td>44° 19.42</td><td>6.47</td><td>1.4</td><td>232</td><td>60</td><td>48</td><td>30</td><td>92</td></tr><tr><td>20</td><td>890101</td><td>164744.7</td><td>40° 51.57</td><td>44° 19.83</td><td>7.65</td><td>1.2</td><td>264</td><td>50</td><td>104</td><td>42</td><td>93</td></tr><tr><td>21</td><td>890104</td><td>012708.3</td><td>40° 51.77</td><td>44° 19.48</td><td>7.50</td><td>1.5</td><td>327</td><td>70</td><td>69</td><td>60</td><td>94</td></tr><tr><td>22</td><td>881229</td><td>182512.8</td><td>40° 50.27</td><td>44° 19.11</td><td>5.82</td><td>1.0</td><td>275</td><td>80</td><td>22</td><td>46</td><td>95</td></tr><tr><td>23</td><td>881229</td><td>175542.9</td><td>40° 50.26</td><td>44° 19.16</td><td>5.75</td><td>1.1</td><td>304</td><td>27</td><td>189</td><td>81</td><td>96</td></tr><tr><td>24</td><td>890107</td><td>115555.2</td><td>40° 50.90</td><td>44° 17.74</td><td>4.09</td><td>1.8</td><td>298</td><td>40</td><td>149</td><td>54</td><td>97</td></tr><tr><td>25</td><td>890101</td><td>110202.3</td><td>40° 50.44</td><td>44° 18.26</td><td>3.44</td><td>1.0</td><td>265</td><td>36</td><td>108</td><td>56</td><td>98</td></tr><tr><td>26</td><td>881226</td><td>075210.2</td><td>40° 50.57</td><td>44° 17.44</td><td>3.56</td><td>1.6</td><td>209</td><td>46</td><td>41</td><td>45</td><td>99</td></tr></tbody></table>

<table><caption>CROSS SECTION EE'</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890104</td><td>170946.2</td><td>40° 51.10</td><td>44° 17.52</td><td>2.86</td><td>1.9</td><td>274</td><td>70</td><td>14</td><td>66</td><td>100</td></tr><tr><td>2</td><td>890104</td><td>185453.9</td><td>40° 51.44</td><td>44° 16.70</td><td>3.47</td><td>1.5</td><td>88</td><td>70</td><td>355</td><td>80</td><td>101</td></tr><tr><td>3</td><td>881229</td><td>101721.2</td><td>40° 50.96</td><td>44° 16.99</td><td>3.56</td><td>1.2</td><td>254</td><td>80</td><td>163</td><td>90</td><td>102</td></tr><tr><td>4</td><td>890104</td><td>232546.1</td><td>40° 51.05</td><td>44° 17.36</td><td>3.57</td><td>1.3</td><td>232</td><td>50</td><td>52</td><td>40</td><td>103</td></tr><tr><td>5</td><td>890103</td><td>165334.0</td><td>40° 51.46</td><td>44° 16.57</td><td>3.83</td><td>1.9</td><td>225</td><td>46</td><td>70</td><td>47</td><td>104</td></tr><tr><td>6</td><td>881227</td><td>041400.2</td><td>40° 50.99</td><td>44° 17.62</td><td>4.81</td><td>1.8</td><td>188</td><td>68</td><td>55</td><td>33</td><td>105</td></tr><tr><td>7</td><td>881228</td><td>180753.8</td><td>40° 51.65</td><td>44° 16.42</td><td>5.23</td><td>2.3</td><td>290</td><td>40</td><td>110</td><td>50</td><td>106</td></tr><tr><td>8</td><td>890101</td><td>050055.4</td><td>40° 51.59</td><td>44° 16.08</td><td>5.72</td><td>2.1</td><td>340</td><td>59</td><td>235</td><td>71</td><td>107</td></tr><tr><td>9</td><td>890102</td><td>080520.7</td><td>40° 51.31</td><td>44° 17.11</td><td>6.02</td><td>2.2</td><td>285</td><td>28</td><td>99</td><td>62</td><td>108</td></tr><tr><td>10</td><td>881226</td><td>145126.0</td><td>40° 52.29</td><td>44° 16.69</td><td>5.56</td><td>1.4</td><td>88</td><td>61</td><td>192</td><td>76</td><td>109</td></tr><tr><td>11</td><td>890104</td><td>233930.7</td><td>40° 50.92</td><td>44° 17.56</td><td>9.00</td><td>1.1</td><td>288</td><td>70</td><td>20</td><td>80</td><td>110</td></tr><tr><td>12</td><td>890102</td><td>205505.2</td><td>40° 50.78</td><td>44° 17.20</td><td>9.70</td><td>0.7</td><td>126</td><td>80</td><td>34</td><td>80</td><td>111</td></tr><tr><td>13</td><td>890103</td><td>185338.3</td><td>40° 51.67</td><td>44° 16.92</td><td>10.88</td><td>0.9</td><td>312</td><td>48</td><td>157</td><td>45</td><td>112</td></tr><tr><td>14</td><td>890101</td><td>144548.2</td><td>40° 51.95</td><td>44° 16.70</td><td>7.45</td><td>1.0</td><td>330</td><td>70</td><td>150</td><td>20</td><td>113</td></tr><tr><td>15</td><td>890104</td><td>060134.3</td><td>40° 52.33</td><td>44° 17.83</td><td>8.02</td><td>1.1</td><td>263</td><td>15</td><td>63</td><td>76</td><td>114</td></tr><tr><td>16</td><td>890101</td><td>094047.7</td><td>40° 53.24</td><td>44° 18.68</td><td>6.70</td><td>1.4</td><td>109</td><td>80</td><td>204</td><td>60</td><td>115</td></tr><tr><td>17</td><td>890102</td><td>211358.7</td><td>40° 53.80</td><td>44° 18.16</td><td>7.25</td><td>1.6</td><td>122</td><td>74</td><td>232</td><td>48</td><td>116</td></tr><tr><td>18</td><td>881225</td><td>220442.7</td><td>40° 53.79</td><td>44° 18.32</td><td>7.08</td><td>2.0</td><td>249</td><td>77</td><td>148</td><td>70</td><td>117</td></tr><tr><td>19</td><td>881231</td><td>153519.6</td><td>40° 52.98</td><td>44° 18.60</td><td>4.64</td><td>3.6</td><td>265</td><td>70</td><td>75</td><td>20</td><td>118</td></tr><tr><td>20</td><td>881231</td><td>191028.8</td><td>40° 52.94</td><td>44° 18.77</td><td>3.99</td><td>1.7</td><td>295</td><td>40</td><td>106</td><td>50</td><td>119</td></tr><tr><td>21</td><td>881229</td><td>005447.5</td><td>40° 53.19</td><td>44° 18.54</td><td>4.08</td><td>1.0</td><td>279</td><td>25</td><td>105</td><td>66</td><td>120</td></tr><tr><td>22</td><td>881231</td><td>102639.6</td><td>40° 53.13</td><td>44° 18.52</td><td>3.94</td><td>2.3</td><td>283</td><td>45</td><td>91</td><td>46</td><td>121</td></tr><tr><td>23</td><td>881231</td><td>174036.8</td><td>40° 52.92</td><td>44° 18.67</td><td>3.93</td><td>1.1</td><td>100</td><td>80</td><td>208</td><td>30</td><td>122</td></tr><tr><td>24</td><td>890105</td><td>195437.5</td><td>40° 52.91</td><td>44° 17.89</td><td>3.62</td><td>1.3</td><td>100</td><td>46</td><td>244</td><td>50</td><td>123</td></tr></tbody></table>
<caption>Table 2. (continued)</caption>

CROSS SECTION FF'

<table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>881231</td><td>155434.1</td><td>40° 52.28</td><td>44° 15.71</td><td>3.43</td><td>4.2</td><td>222</td><td>36</td><td>59</td><td>56</td><td>124</td></tr><tr><td>2</td><td>881226</td><td>144303.3</td><td>40° 52.18</td><td>44° 15.43</td><td>3.38</td><td>2.7</td><td>289</td><td>70</td><td>28</td><td>82</td><td>125</td></tr><tr><td>3</td><td>890101</td><td>044235.9</td><td>40° 50.55</td><td>44° 14.33</td><td>3.23</td><td>0.8</td><td>298</td><td>80</td><td>28</td><td>84</td><td>126</td></tr><tr><td>4</td><td>881230</td><td>015330.9</td><td>40° 50.94</td><td>44° 14.04</td><td>3.34</td><td>1.3</td><td>94</td><td>90</td><td>184</td><td>70</td><td>127</td></tr><tr><td>5</td><td>881227</td><td>002609.9</td><td>40° 51.32</td><td>44° 14.88</td><td>3.85</td><td>0.9</td><td>289</td><td>83</td><td>35</td><td>83</td><td>128</td></tr><tr><td>6</td><td>881230</td><td>025318.7</td><td>40° 51.76</td><td>44° 15.30</td><td>4.43</td><td>2.0</td><td>332</td><td>62</td><td>227</td><td>68</td><td>129</td></tr><tr><td>7</td><td>890101</td><td>050055.4</td><td>40° 51.59</td><td>44° 16.08</td><td>5.72</td><td>2.1</td><td>340</td><td>59</td><td>235</td><td>71</td><td>130</td></tr><tr><td>8</td><td>881225</td><td>205758.3</td><td>40° 51.89</td><td>44° 15.76</td><td>6.98</td><td>2.4</td><td>310</td><td>34</td><td>115</td><td>57</td><td>131</td></tr><tr><td>9</td><td>890105</td><td>141522.7</td><td>40° 52.09</td><td>44° 15.85</td><td>6.83</td><td>1.2</td><td>270</td><td>50</td><td>111</td><td>42</td><td>132</td></tr><tr><td>10</td><td>890101</td><td>001521.7</td><td>40° 51.67</td><td>44° 16.04</td><td>7.87</td><td>1.1</td><td>338</td><td>56</td><td>158</td><td>34</td><td>133</td></tr><tr><td>11</td><td>890102</td><td>073722.8</td><td>40° 52.50</td><td>44° 15.81</td><td>6.31</td><td>1.5</td><td>265</td><td>40</td><td>85</td><td>50</td><td>134</td></tr><tr><td>12</td><td>881229</td><td>101506.1</td><td>40° 52.77</td><td>44° 15.09</td><td>7.54</td><td>0.5</td><td>323</td><td>61</td><td>84</td><td>51</td><td>135</td></tr><tr><td>13</td><td>881225</td><td>224127.1</td><td>40° 52.82</td><td>44° 15.00</td><td>8.27</td><td>1.5</td><td>290</td><td>69</td><td>42</td><td>50</td><td>136</td></tr><tr><td>14</td><td>890101</td><td>042011.3</td><td>40° 52.66</td><td>44° 15.67</td><td>6.93</td><td>0.7</td><td>324</td><td>50</td><td>144</td><td>40</td><td>137</td></tr><tr><td>15</td><td>890101</td><td>202929.0</td><td>40° 52.76</td><td>44° 15.79</td><td>6.45</td><td>1.5</td><td>265</td><td>50</td><td>85</td><td>40</td><td>138</td></tr><tr><td>16</td><td>890101</td><td>004901.6</td><td>40° 53.12</td><td>44° 15.40</td><td>6.81</td><td>1.2</td><td>288</td><td>40</td><td>108</td><td>50</td><td>139</td></tr><tr><td>17</td><td>890107</td><td>162442.9</td><td>40° 53.50</td><td>44° 16.30</td><td>5.26</td><td>1.1</td><td>95</td><td>85</td><td>186</td><td>80</td><td>140</td></tr><tr><td>18</td><td>890105</td><td>231830.2</td><td>40° 53.73</td><td>44° 17.22</td><td>5.95</td><td>1.2</td><td>292</td><td>71</td><td>36</td><td>71</td><td>141</td></tr><tr><td>19</td><td>890105</td><td>135457.4</td><td>40° 53.47</td><td>44° 15.52</td><td>3.67</td><td>2.7</td><td>246</td><td>35</td><td>69</td><td>56</td><td>142</td></tr><tr><td>20</td><td>890107</td><td>184907.4</td><td>40° 53.49</td><td>44° 15.51</td><td>3.98</td><td>2.7</td><td>220</td><td>40</td><td>62</td><td>53</td><td>143</td></tr><tr><td>21</td><td>880201</td><td>163319.2</td><td>40° 53.00</td><td>44° 17.07</td><td>4.16</td><td>3.3</td><td>316</td><td>70</td><td>185</td><td>30</td><td>144</td></tr><tr><td>22</td><td>881230</td><td>101559.1</td><td>40° 53.11</td><td>44° 16.77</td><td>3.93</td><td>1.5</td><td>92</td><td>72</td><td>194</td><td>69</td><td>145</td></tr><tr><td>23</td><td>881231</td><td>203301.5</td><td>40° 53.47</td><td>44° 16.37</td><td>3.48</td><td>1.6</td><td>281</td><td>36</td><td>104</td><td>54</td><td>146</td></tr><tr><td>24</td><td>890102</td><td>060359.4</td><td>40° 53.37</td><td>44° 17.25</td><td>3.38</td><td>1.3</td><td>240</td><td>40</td><td>74</td><td>50</td><td>147</td></tr><tr><td>25</td><td>890108</td><td>012844.0</td><td>40° 53.46</td><td>44° 16.66</td><td>3.20</td><td>1.2</td><td>112</td><td>79</td><td>270</td><td>10</td><td>148</td></tr><tr><td>26</td><td>890104</td><td>135842.9</td><td>40° 53.23</td><td>44° 15.96</td><td>3.59</td><td>2.2</td><td>308</td><td>68</td><td>203</td><td>62</td><td>149</td></tr></tbody></table>

CROSS SECTION GG'

<table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>881226</td><td>154337.5</td><td>40° 52.32</td><td>44° 13.43</td><td>4.03</td><td>1.9</td><td>289</td><td>70</td><td>186</td><td>60</td><td>150</td></tr><tr><td>2</td><td>890102</td><td>011820.1</td><td>40° 51.70</td><td>44° 14.02</td><td>3.84</td><td>0.3</td><td>205</td><td>50</td><td>55</td><td>44</td><td>151</td></tr><tr><td>3</td><td>890108</td><td>052846.6</td><td>40° 52.14</td><td>44° 12.66</td><td>4.26</td><td>2.0</td><td>340</td><td>80</td><td>74</td><td>70</td><td>152</td></tr><tr><td>4</td><td>881225</td><td>215511.0</td><td>40° 51.79</td><td>44° 12.88</td><td>4.27</td><td>1.1</td><td>316</td><td>75</td><td>64</td><td>44</td><td>153</td></tr><tr><td>5</td><td>881225</td><td>163205.2</td><td>40° 52.03</td><td>44° 12.36</td><td>4.56</td><td>0.8</td><td>274</td><td>64</td><td>166</td><td>64</td><td>154</td></tr><tr><td>6</td><td>890108</td><td>090456.9</td><td>40° 51.93</td><td>44° 13.13</td><td>4.47</td><td>2.3</td><td>243</td><td>40</td><td>66</td><td>50</td><td>155</td></tr><tr><td>7</td><td>890103</td><td>134847.1</td><td>40° 52.06</td><td>44° 12.47</td><td>5.33</td><td>3.6</td><td>332</td><td>72</td><td>230</td><td>56</td><td>156</td></tr><tr><td>8</td><td>881226</td><td>025409.1</td><td>40° 51.89</td><td>44° 12.64</td><td>5.77</td><td>1.1</td><td>239</td><td>47</td><td>41</td><td>45</td><td>157</td></tr><tr><td>9</td><td>890103</td><td>081954.8</td><td>40° 52.17</td><td>44° 12.76</td><td>5.19</td><td>4.2</td><td>264</td><td>48</td><td>59</td><td>46</td><td>158</td></tr><tr><td>10</td><td>890103</td><td>083619.7</td><td>40° 52.31</td><td>44° 12.70</td><td>5.84</td><td>1.3</td><td>242</td><td>60</td><td>76</td><td>30</td><td>159</td></tr><tr><td>11</td><td>881231</td><td>142548.8</td><td>40° 52.96</td><td>44° 13.14</td><td>5.05</td><td>1.0</td><td>227</td><td>40</td><td>76</td><td>54</td><td>160</td></tr><tr><td>12</td><td>881226</td><td>175605.9</td><td>40° 52.83</td><td>44° 14.29</td><td>7.33</td><td>1.5</td><td>312</td><td>62</td><td>128</td><td>28</td><td>161</td></tr><tr><td>13</td><td>890103</td><td>213421.2</td><td>40° 53.02</td><td>44° 13.66</td><td>8.13</td><td>1.7</td><td>120</td><td>42</td><td>11</td><td>75</td><td>162</td></tr><tr><td>14</td><td>890107</td><td>142005.8</td><td>40° 53.04</td><td>44° 14.00</td><td>8.03</td><td>1.3</td><td>112</td><td>60</td><td>9</td><td>70</td><td>163</td></tr><tr><td>15</td><td>890106</td><td>235401.3</td><td>40° 53.22</td><td>44° 13.66</td><td>10.43</td><td>2.2</td><td>111</td><td>67</td><td>357</td><td>39</td><td>164</td></tr><tr><td>16</td><td>881229</td><td>094952.8</td><td>40° 53.40</td><td>44° 13.54</td><td>10.45</td><td>1.7</td><td>130</td><td>77</td><td>38</td><td>83</td><td>165</td></tr><tr><td>17</td><td>890103</td><td>175211.8</td><td>40° 53.35</td><td>44° 14.12</td><td>9.68</td><td>1.1</td><td>318</td><td>74</td><td>134</td><td>16</td><td>166</td></tr><tr><td>18</td><td>881230</td><td>052330.7</td><td>40° 53.40</td><td>44° 13.43</td><td>9.05</td><td>0.3</td><td>336</td><td>58</td><td>88</td><td>62</td><td>167</td></tr><tr><td>19</td><td>881229</td><td>144842.5</td><td>40° 53.34</td><td>44° 13.67</td><td>6.80</td><td>2.6</td><td>284</td><td>65</td><td>38</td><td>53</td><td>168</td></tr><tr><td>20</td><td>890108</td><td>113330.1</td><td>40° 53.40</td><td>44° 13.82</td><td>6.44</td><td>1.5</td><td>160</td><td>60</td><td>58</td><td>70</td><td>169</td></tr><tr><td>21</td><td>881230</td><td>190730.6</td><td>40° 53.42</td><td>44° 13.59</td><td>6.21</td><td>1.8</td><td>320</td><td>80</td><td>222</td><td>70</td><td>170</td></tr><tr><td>22</td><td>890102</td><td>223441.0</td><td>40° 53.19</td><td>44° 13.56</td><td>6.08</td><td>2.2</td><td>214</td><td>26</td><td>38</td><td>64</td><td>171</td></tr><tr><td>23</td><td>890102</td><td>020327.4</td><td>40° 54.09</td><td>44° 14.69</td><td>5.43</td><td>1.3</td><td>112</td><td>88</td><td>292</td><td>2</td><td>172</td></tr><tr><td>24</td><td>890104</td><td>072940.4</td><td>40° 53.32</td><td>44° 14.96</td><td>4.94</td><td>4.8</td><td>81</td><td>77</td><td>182</td><td>65</td><td>173</td></tr><tr><td>25</td><td>881229</td><td>100544.9</td><td>40° 53.19</td><td>44° 14.33</td><td>4.94</td><td>1.5</td><td>80</td><td>66</td><td>244</td><td>26</td><td>174</td></tr></tbody></table>

CROSS SECTION HH'

<table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>881227</td><td>022319.8</td><td>40° 51.88</td><td>44° 11.73</td><td>3.90</td><td>0.6</td><td>286</td><td>40</td><td>109</td><td>50</td><td>175</td></tr><tr><td>2</td><td>890102</td><td>055608.7</td><td>40° 51.35</td><td>44° 11.35</td><td>4.37</td><td>0.7</td><td>252</td><td>63</td><td>104</td><td>30</td><td>176</td></tr><tr><td>3</td><td>881231</td><td>163003.7</td><td>40° 52.61</td><td>44° 11.92</td><td>4.96</td><td>0.8</td><td>214</td><td>50</td><td>101</td><td>64</td><td>177</td></tr><tr><td>4</td><td>890107</td><td>203718.0</td><td>40° 52.57</td><td>44° 12.53</td><td>5.82</td><td>1.7</td><td>255</td><td>50</td><td>75</td><td>40</td><td>178</td></tr><tr><td>5</td><td>881226</td><td>183007.1</td><td>40° 53.44</td><td>44° 12.22</td><td>6.40</td><td>2.3</td><td>219</td><td>62</td><td>85</td><td>39</td><td>179</td></tr><tr><td>6</td><td>890101</td><td>193649.5</td><td>40° 53.33</td><td>44° 12.58</td><td>6.43</td><td>1.4</td><td>235</td><td>40</td><td>91</td><td>57</td><td>180</td></tr><tr><td>7</td><td>881227</td><td>000622.1</td><td>40° 53.43</td><td>44° 11.99</td><td>6.67</td><td>1.7</td><td>294</td><td>51</td><td>46</td><td>69</td><td>181</td></tr><tr><td>8</td><td>881226</td><td>131103.0</td><td>40° 53.93</td><td>44° 11.99</td><td>6.37</td><td>1.7</td><td>295</td><td>72</td><td>34</td><td>80</td><td>182</td></tr><tr><td>9</td><td>881226</td><td>131124.3</td><td>40° 53.74</td><td>44° 12.08</td><td>6.44</td><td>2.6</td><td>295</td><td>50</td><td>39</td><td>80</td><td>183</td></tr><tr><td>10</td><td>881229</td><td>211619.8</td><td>40° 53.50</td><td>44° 12.82</td><td>6.86</td><td>0.9</td><td>255</td><td>28</td><td>130</td><td>75</td><td>184</td></tr><tr><td>11</td><td>890105</td><td>030755.1</td><td>40° 54.08</td><td>44° 11.78</td><td>7.00</td><td>1.4</td><td>254</td><td>40</td><td>66</td><td>50</td><td>185</td></tr><tr><td>12</td><td>890102</td><td>234639.1</td><td>40° 53.67</td><td>44° 12.68</td><td>7.13</td><td>1.2</td><td>204</td><td>50</td><td>21</td><td>40</td><td>186</td></tr><tr><td>13</td><td>881229</td><td>143210.8</td><td>40° 54.11</td><td>44° 11.99</td><td>7.70</td><td>3.0</td><td>235</td><td>54</td><td>66</td><td>37</td><td>187</td></tr></tbody></table>
<table><caption>**Table 2. (continued)**</caption><thead><tr><th colspan="12">CROSS SECTION HH'</th></tr><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th><th></th><th></th></tr></thead><tbody><tr><td>14</td><td>890102</td><td>014216.7</td><td>40° 54.05</td><td>44° 12.76</td><td>7.98</td><td>1.5</td><td>270</td><td>60</td><td>90</td><td>30</td><td>188</td><td></td></tr><tr><td>15</td><td>881230</td><td>023041.3</td><td>40° 54.08</td><td>44° 12.27</td><td>7.29</td><td>1.9</td><td>252</td><td>51</td><td>63</td><td>40</td><td>189</td><td></td></tr><tr><td>16</td><td>881229</td><td>220656.4</td><td>40° 54.08</td><td>44° 12.13</td><td>6.76</td><td>1.3</td><td>226</td><td>41</td><td>53</td><td>49</td><td>190</td><td></td></tr><tr><td>17</td><td>890102</td><td>145123.3</td><td>40° 53.54</td><td>44° 11.86</td><td>5.89</td><td>1.3</td><td>238</td><td>50</td><td>62</td><td>40</td><td>191</td><td></td></tr><tr><td>18</td><td>890103</td><td>170537.4</td><td>40° 53.33</td><td>44° 12.40</td><td>6.09</td><td>1.6</td><td>244</td><td>40</td><td>55</td><td>51</td><td>192</td><td></td></tr><tr><td>19</td><td>890107</td><td>023015.6</td><td>40° 53.29</td><td>44° 12.11</td><td>5.71</td><td>1.9</td><td>274</td><td>30</td><td>98</td><td>60</td><td>193</td><td></td></tr><tr><td>20</td><td>881229</td><td>112303.5</td><td>40° 53.49</td><td>44° 12.90</td><td>4.82</td><td>2.2</td><td>43</td><td>83</td><td>177</td><td>12</td><td>194</td><td></td></tr><tr><td>21</td><td>890103</td><td>012050.6</td><td>40° 52.91</td><td>44° 11.30</td><td>4.31</td><td>1.2</td><td>255</td><td>60</td><td>48</td><td>34</td><td>195</td><td></td></tr></tbody></table><table><thead><tr><th colspan="12">CROSS SECTION II'</th></tr><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th><th></th><th></th></tr></thead><tbody><tr><td>1</td><td>890105</td><td>043100.5</td><td>40° 53.48</td><td>44° 10.02</td><td>3.84</td><td>1.6</td><td>324</td><td>70</td><td>233</td><td>85</td><td>196</td><td></td></tr><tr><td>2</td><td>890102</td><td>095835.5</td><td>40° 52.52</td><td>44° 9.75</td><td>3.89</td><td>1.7</td><td>271</td><td>46</td><td>98</td><td>44</td><td>197</td><td></td></tr><tr><td>3</td><td>890101</td><td>132241.4</td><td>40° 52.78</td><td>44° 9.91</td><td>3.90</td><td>1.5</td><td>254</td><td>38</td><td>96</td><td>52</td><td>198</td><td></td></tr><tr><td>4</td><td>890107</td><td>043650.3</td><td>40° 53.50</td><td>44° 10.45</td><td>4.56</td><td>1.7</td><td>264</td><td>60</td><td>4</td><td>70</td><td>199</td><td></td></tr><tr><td>5</td><td>890105</td><td>170413.3</td><td>40° 53.70</td><td>44° 11.45</td><td>5.99</td><td>1.2</td><td>270</td><td>20</td><td>60</td><td>72</td><td>200</td><td></td></tr><tr><td>6</td><td>890106</td><td>095218.0</td><td>40° 54.46</td><td>44° 10.33</td><td>6.59</td><td>1.5</td><td>302</td><td>80</td><td>42</td><td>40</td><td>201</td><td></td></tr><tr><td>7</td><td>890105</td><td>201306.2</td><td>40° 54.08</td><td>44° 11.73</td><td>6.69</td><td>1.2</td><td>222</td><td>26</td><td>46</td><td>64</td><td>202</td><td></td></tr><tr><td>8</td><td>890103</td><td>232726.0</td><td>40° 54.29</td><td>44° 11.14</td><td>6.97</td><td>2.0</td><td>254</td><td>26</td><td>92</td><td>66</td><td>203</td><td></td></tr><tr><td>9</td><td>881230</td><td>064222.1</td><td>40° 54.68</td><td>44° 10.19</td><td>7.18</td><td>2.0</td><td>287</td><td>26</td><td>135</td><td>67</td><td>204</td><td></td></tr><tr><td>10</td><td>890101</td><td>170706.7</td><td>40° 54.06</td><td>44° 10.48</td><td>8.06</td><td>2.3</td><td>311</td><td>70</td><td>50</td><td>77</td><td>205</td><td></td></tr><tr><td>11</td><td>881230</td><td>153850.3</td><td>40° 54.32</td><td>44° 10.95</td><td>7.89</td><td>0.9</td><td>298</td><td>50</td><td>188</td><td>78</td><td>206</td><td></td></tr><tr><td>12</td><td>890105</td><td>160717.6</td><td>40° 54.10</td><td>44° 10.61</td><td>9.30</td><td>1.0</td><td>304</td><td>70</td><td>40</td><td>70</td><td>207</td><td></td></tr><tr><td>13</td><td>890105</td><td>202021.7</td><td>40° 54.25</td><td>44° 11.44</td><td>8.38</td><td>1.3</td><td>276</td><td>60</td><td>50</td><td>40</td><td>208</td><td></td></tr><tr><td>14</td><td>881226</td><td>072524.4</td><td>40° 54.41</td><td>44° 10.58</td><td>8.47</td><td>1.4</td><td>279</td><td>55</td><td>34</td><td>61</td><td>209</td><td></td></tr><tr><td>15</td><td>881230</td><td>054309.9</td><td>40° 54.39</td><td>44° 11.44</td><td>8.88</td><td>0.3</td><td>270</td><td>42</td><td>33</td><td>66</td><td>210</td><td></td></tr><tr><td>16</td><td>890101</td><td>154608.5</td><td>40° 54.78</td><td>44° 10.71</td><td>8.22</td><td>1.8</td><td>267</td><td>56</td><td>64</td><td>37</td><td>211</td><td></td></tr><tr><td>17</td><td>881229</td><td>214521.7</td><td>40° 54.63</td><td>44° 11.59</td><td>7.45</td><td>1.0</td><td>284</td><td>86</td><td>194</td><td>80</td><td>212</td><td></td></tr><tr><td>18</td><td>890107</td><td>115635.3</td><td>40° 54.75</td><td>44° 11.53</td><td>7.13</td><td>0.9</td><td>229</td><td>50</td><td>96</td><td>50</td><td>213</td><td></td></tr><tr><td>19</td><td>881227</td><td>013240.1</td><td>40° 55.00</td><td>44° 11.27</td><td>6.77</td><td>1.3</td><td>294</td><td>80</td><td>200</td><td>70</td><td>214</td><td></td></tr><tr><td>20</td><td>881230</td><td>124752.1</td><td>40° 54.97</td><td>44° 11.12</td><td>6.34</td><td>2.9</td><td>311</td><td>74</td><td>58</td><td>87</td><td>215</td><td></td></tr><tr><td>21</td><td>890101</td><td>173038.0</td><td>40° 54.15</td><td>44° 9.98</td><td>4.50</td><td>0.9</td><td>308</td><td>70</td><td>214</td><td>80</td><td>216</td><td></td></tr><tr><td>22</td><td>890108</td><td>064811.3</td><td>40° 53.86</td><td>44° 10.17</td><td>3.83</td><td>2.5</td><td>341</td><td>60</td><td>230</td><td>60</td><td>217</td><td></td></tr><tr><td>23</td><td>890102</td><td>042241.1</td><td>40° 53.97</td><td>44° 9.97</td><td>3.68</td><td>1.5</td><td>123</td><td>62</td><td>233</td><td>60</td><td>218</td><td></td></tr><tr><td>24</td><td>890105</td><td>162612.4</td><td>40° 53.73</td><td>44° 10.24</td><td>3.41</td><td>1.2</td><td>130</td><td>70</td><td>38</td><td>80</td><td>219</td><td></td></tr></tbody></table><table><thead><tr><th colspan="12">CROSS SECTION JJ'</th></tr><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th><th></th><th></th></tr></thead><tbody><tr><td>1</td><td>890105</td><td>140957.8</td><td>40° 53.99</td><td>44° 8.75</td><td>3.33</td><td>2.0</td><td>254</td><td>31</td><td>46</td><td>63</td><td>220</td><td></td></tr><tr><td>2</td><td>890102</td><td>185744.4</td><td>40° 53.84</td><td>44° 9.50</td><td>3.73</td><td>2.1</td><td>313</td><td>74</td><td>214</td><td>86</td><td>221</td><td></td></tr><tr><td>3</td><td>890103</td><td>144509.1</td><td>40° 53.64</td><td>44° 9.35</td><td>3.90</td><td>2.6</td><td>327</td><td>73</td><td>225</td><td>70</td><td>222</td><td></td></tr><tr><td>4</td><td>890102</td><td>083701.4</td><td>40° 53.10</td><td>44° 8.75</td><td>5.39</td><td>0.9</td><td>223</td><td>40</td><td>46</td><td>50</td><td>223</td><td></td></tr><tr><td>5</td><td>881231</td><td>045459.3</td><td>40° 54.89</td><td>44° 9.63</td><td>6.56</td><td>0.8</td><td>281</td><td>19</td><td>101</td><td>71</td><td>224</td><td></td></tr><tr><td>6</td><td>881228</td><td>234849.3</td><td>40° 55.21</td><td>44° 8.69</td><td>8.57</td><td>1.7</td><td>315</td><td>38</td><td>62</td><td>81</td><td>225</td><td></td></tr><tr><td>7</td><td>890107</td><td>005242.0</td><td>40° 54.25</td><td>44° 8.47</td><td>4.86</td><td>2.2</td><td>357</td><td>73</td><td>257</td><td>81</td><td>226</td><td></td></tr><tr><td>8</td><td>890108</td><td>102647.7</td><td>40° 54.18</td><td>44° 9.35</td><td>4.20</td><td>3.8</td><td>313</td><td>72</td><td>212</td><td>65</td><td>227</td><td></td></tr><tr><td>9</td><td>890105</td><td>142733.3</td><td>40° 54.34</td><td>44° 9.61</td><td>3.71</td><td>2.5</td><td>312</td><td>88</td><td>222</td><td>70</td><td>228</td><td></td></tr><tr><td>10</td><td>890101</td><td>020850.6</td><td>40° 54.04</td><td>44° 9.50</td><td>3.56</td><td>0.5</td><td>329</td><td>74</td><td>72</td><td>62</td><td>229</td><td></td></tr><tr><td>11</td><td>890105</td><td>223438.4</td><td>40° 54.17</td><td>44° 9.02</td><td>3.50</td><td>1.3</td><td>301</td><td>67</td><td>201</td><td>76</td><td>230</td><td></td></tr></tbody></table><table><thead><tr><th colspan="12">CROSS SECTION KK'</th></tr><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th><th></th><th></th></tr></thead><tbody><tr><td>1</td><td>881229</td><td>095238.4</td><td>40° 54.36</td><td>44° 7.25</td><td>3.79</td><td>2.2</td><td>323</td><td>79</td><td>74</td><td>87</td><td>231</td><td></td></tr><tr><td>2</td><td>881226</td><td>141244.0</td><td>40° 54.41</td><td>44° 6.41</td><td>3.68</td><td>2.1</td><td>315</td><td>83</td><td>61</td><td>83</td><td>232</td><td></td></tr><tr><td>3</td><td>890102</td><td>071647.6</td><td>40° 53.92</td><td>44° 5.94</td><td>3.45</td><td>1.7</td><td>280</td><td>35</td><td>104</td><td>55</td><td>233</td><td></td></tr><tr><td>4</td><td>881226</td><td>232132.9</td><td>40° 54.03</td><td>44° 6.38</td><td>3.99</td><td>1.8</td><td>285</td><td>81</td><td>25</td><td>73</td><td>234</td><td></td></tr><tr><td>5</td><td>881230</td><td>132251.6</td><td>40° 54.32</td><td>44° 6.94</td><td>4.56</td><td>1.2</td><td>327</td><td>82</td><td>65</td><td>82</td><td>235</td><td></td></tr><tr><td>6</td><td>890107</td><td>144651.5</td><td>40° 54.01</td><td>44° 5.91</td><td>4.50</td><td>2.3</td><td>273</td><td>50</td><td>65</td><td>44</td><td>236</td><td></td></tr><tr><td>7</td><td>890106</td><td>221618.9</td><td>40° 54.19</td><td>44° 7.52</td><td>4.97</td><td>1.7</td><td>301</td><td>55</td><td>56</td><td>61</td><td>237</td><td></td></tr><tr><td>8</td><td>881226</td><td>220310.6</td><td>40° 54.74</td><td>44° 6.50</td><td>5.37</td><td>0.7</td><td>257</td><td>40</td><td>142</td><td>74</td><td>238</td><td></td></tr><tr><td>9</td><td>890107</td><td>132509.8</td><td>40° 55.35</td><td>44° 8.16</td><td>7.33</td><td>1.8</td><td>281</td><td>40</td><td>65</td><td>57</td><td>239</td><td></td></tr><tr><td>10</td><td>890101</td><td>013132.6</td><td>40° 55.36</td><td>44° 7.94</td><td>8.25</td><td>0.9</td><td>285</td><td>51</td><td>80</td><td>43</td><td>240</td><td></td></tr><tr><td>11</td><td>881225</td><td>235736.3</td><td>40° 55.44</td><td>44° 8.65</td><td>7.64</td><td>0.8</td><td>320</td><td>40</td><td>77</td><td>71</td><td>241</td><td></td></tr><tr><td>12</td><td>881226</td><td>114550.2</td><td>40° 56.10</td><td>44° 7.90</td><td>10.30</td><td>1.80</td><td>286</td><td>50</td><td>109</td><td>40</td><td>242</td><td></td></tr><tr><td>13</td><td>890104</td><td>224525.4</td><td>40° 56.41</td><td>44° 8.13</td><td>9.85</td><td>2.2</td><td>292</td><td>88</td><td>22</td><td>75</td><td>243</td><td></td></tr><tr><td>14</td><td>890108</td><td>133241.0</td><td>40° 54.67</td><td>44° 7.14</td><td>4.44</td><td>2.2</td><td>303</td><td>59</td><td>41</td><td>82</td><td>244</td><td></td></tr><tr><td>15</td><td>881230</td><td>233004.0</td><td>40° 55.11</td><td>44° 8.20</td><td>3.65</td><td>1.9</td><td>328</td><td>68</td><td>78</td><td>57</td><td>245</td><td></td></tr></tbody></table>
<table><caption>**Table 2. (continued)**</caption><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>16</td><td>890108</td><td>142801.6</td><td>40° 55.58</td><td>44° 8.19</td><td>3.62</td><td>0.3</td><td>284</td><td>51</td><td>94</td><td>40</td><td>246</td></tr><tr><td>17</td><td>890105</td><td>185103.0</td><td>40° 59.89</td><td>44° 10.19</td><td>3.22</td><td>2.0</td><td>164</td><td>45</td><td>22</td><td>53</td><td>247</td></tr><tr><td>18</td><td>890106</td><td>192731.3</td><td>40° 55.01</td><td>44° 8.20</td><td>3.46</td><td>1.3</td><td>135</td><td>66</td><td>34</td><td>77</td><td>248</td></tr><tr><td>19</td><td>881226</td><td>153622.7</td><td>40° 54.71</td><td>44° 8.08</td><td>3.67</td><td>1.3</td><td>324</td><td>82</td><td>62</td><td>82</td><td>249</td></tr></tbody></table><table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890105</td><td>013646.2</td><td>40° 53.78</td><td>44° 5.26</td><td>3.93</td><td>1.4</td><td>275</td><td>80</td><td>174</td><td>40</td><td>250</td></tr><tr><td>2</td><td>890108</td><td>073151.0</td><td>40° 54.34</td><td>44° 4.31</td><td>5.36</td><td>2.6</td><td>302</td><td>50</td><td>78</td><td>50</td><td>251</td></tr><tr><td>3</td><td>890105</td><td>101039.9</td><td>40° 55.20</td><td>44° 5.32</td><td>6.88</td><td>1.4</td><td>324</td><td>84</td><td>56</td><td>80</td><td>252</td></tr><tr><td>4</td><td>881229</td><td>044245.8</td><td>40° 55.48</td><td>44° 4.78</td><td>6.98</td><td>1.9</td><td>290</td><td>31</td><td>68</td><td>66</td><td>253</td></tr><tr><td>5</td><td>890102</td><td>030406.5</td><td>40° 55.08</td><td>44° 5.62</td><td>7.01</td><td>2.0</td><td>292</td><td>28</td><td>34</td><td>84</td><td>254</td></tr><tr><td>6</td><td>881231</td><td>023047.1</td><td>40° 55.33</td><td>44° 5.15</td><td>7.44</td><td>1.6</td><td>289</td><td>41</td><td>51</td><td>67</td><td>255</td></tr><tr><td>7</td><td>881226</td><td>103526.2</td><td>40° 54.81</td><td>44° 4.58</td><td>9.79</td><td>1.7</td><td>283</td><td>70</td><td>22</td><td>77</td><td>256</td></tr><tr><td>8</td><td>881229</td><td>043543.4</td><td>40° 55.29</td><td>44° 4.74</td><td>9.76</td><td>1.1</td><td>279</td><td>60</td><td>118</td><td>32</td><td>257</td></tr><tr><td>9</td><td>881230</td><td>002208.7</td><td>40° 55.76</td><td>44° 6.04</td><td>12.64</td><td>2.5</td><td>272</td><td>50</td><td>148</td><td>59</td><td>258</td></tr><tr><td>10</td><td>881230</td><td>114617.1</td><td>40° 56.33</td><td>44° 6.31</td><td>11.35</td><td>0.7</td><td>280</td><td>60</td><td>100</td><td>30</td><td>259</td></tr><tr><td>11</td><td>881229</td><td>121613.6</td><td>40° 55.41</td><td>44° 6.24</td><td>8.11</td><td>1.5</td><td>276</td><td>76</td><td>14</td><td>76</td><td>260</td></tr><tr><td>12</td><td>890102</td><td>181108.8</td><td>40° 55.17</td><td>44° 5.99</td><td>6.57</td><td>1.9</td><td>103</td><td>80</td><td>212</td><td>30</td><td>261</td></tr><tr><td>13</td><td>890106</td><td>105053.1</td><td>40° 55.50</td><td>44° 5.83</td><td>6.27</td><td>1.4</td><td>308</td><td>50</td><td>45</td><td>80</td><td>262</td></tr><tr><td>14</td><td>890102</td><td>114809.7</td><td>40° 55.00</td><td>44° 5.72</td><td>5.87</td><td>0.8</td><td>270</td><td>56</td><td>54</td><td>40</td><td>263</td></tr><tr><td>15</td><td>890107</td><td>031117.5</td><td>40° 55.46</td><td>44° 5.19</td><td>5.49</td><td>1.0</td><td>310</td><td>60</td><td>50</td><td>70</td><td>264</td></tr><tr><td>16</td><td>881226</td><td>053057.4</td><td>40° 54.97</td><td>44° 5.62</td><td>5.50</td><td>1.6</td><td>300</td><td>56</td><td>32</td><td>84</td><td>265</td></tr><tr><td>17</td><td>881231</td><td>222356.9</td><td>41° 00.24</td><td>44° 8.71</td><td>3.60</td><td>2.1</td><td>168</td><td>84</td><td>71</td><td>71</td><td>266</td></tr><tr><td>18</td><td>881230</td><td>014615.1</td><td>40° 55.22</td><td>44° 6.22</td><td>3.45</td><td>2.5</td><td>304</td><td>71</td><td>45</td><td>79</td><td>267</td></tr></tbody></table><table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890104</td><td>053758.5</td><td>40° 55.02</td><td>44° 2.78</td><td>3.03</td><td>1.7</td><td>298</td><td>60</td><td>40</td><td>70</td><td>268</td></tr><tr><td>2</td><td>890103</td><td>142625.0</td><td>40° 54.63</td><td>44° 3.3</td><td>3.09</td><td>1.9</td><td>294</td><td>60</td><td>190</td><td>70</td><td>269</td></tr><tr><td>3</td><td>881229</td><td>72431.74</td><td>40° 54.93</td><td>44° 3.11</td><td>3.71</td><td>1.5</td><td>121</td><td>69</td><td>224</td><td>64</td><td>270</td></tr><tr><td>4</td><td>881226</td><td>103049.0</td><td>40° 54.41</td><td>44° 2.52</td><td>5.32</td><td>2.5</td><td>268</td><td>51</td><td>115</td><td>43</td><td>271</td></tr><tr><td>5</td><td>881227</td><td>021909.8</td><td>40° 55.57</td><td>44° 2.89</td><td>7.07</td><td>1.4</td><td>286</td><td>50</td><td>37</td><td>70</td><td>272</td></tr><tr><td>6</td><td>890103</td><td>074714.3</td><td>40° 55.19</td><td>44° 4.04</td><td>7.45</td><td>2.2</td><td>262</td><td>40</td><td>102</td><td>52</td><td>273</td></tr><tr><td>7</td><td>890101</td><td>091744.4</td><td>40° 53.96</td><td>44° 2.68</td><td>11.14</td><td>1.4</td><td>242</td><td>50</td><td>76</td><td>40</td><td>274</td></tr><tr><td>8</td><td>881231</td><td>115938.9</td><td>40° 53.84</td><td>44° 2.58</td><td>12.1</td><td>2.1</td><td>244</td><td>55</td><td>92</td><td>40</td><td>275</td></tr><tr><td>9</td><td>890102</td><td>145926.4</td><td>40° 56.97</td><td>44° 3.78</td><td>10.8</td><td>3.2</td><td>278</td><td>51</td><td>119</td><td>42</td><td>276</td></tr><tr><td>10</td><td>881229</td><td>222016.3</td><td>40° 55.76</td><td>44° 4.07</td><td>8.78</td><td>1.9</td><td>340</td><td>60</td><td>240</td><td>70</td><td>277</td></tr><tr><td>11</td><td>881229</td><td>141424.6</td><td>40° 55.82</td><td>44° 4.68</td><td>8.62</td><td>1.1</td><td>330</td><td>80</td><td>236</td><td>70</td><td>278</td></tr><tr><td>12</td><td>890107</td><td>145639.3</td><td>40° 55.62</td><td>44° 4.35</td><td>7.94</td><td>2.3</td><td>321</td><td>64</td><td>56</td><td>80</td><td>279</td></tr><tr><td>13</td><td>890102</td><td>002424.2</td><td>40° 55.8</td><td>44° 3.94</td><td>7.38</td><td>1.5</td><td>287</td><td>50</td><td>31</td><td>80</td><td>280</td></tr><tr><td>14</td><td>881225</td><td>155347.8</td><td>40° 58.1</td><td>44° 6.17</td><td>5.69</td><td>2.1</td><td>270</td><td>70</td><td>174</td><td>70</td><td>281</td></tr><tr><td>15</td><td>890108</td><td>061147.1</td><td>40° 58.12</td><td>44° 6.49</td><td>5.41</td><td>2.4</td><td>241</td><td>82</td><td>150</td><td>70</td><td>282</td></tr><tr><td>16</td><td>890101</td><td>223343.2</td><td>40° 55.44</td><td>44° 2.75</td><td>4.91</td><td>1.3</td><td>314</td><td>80</td><td>44</td><td>80</td><td>283</td></tr><tr><td>17</td><td>881227</td><td>020857.9</td><td>40° 55.18</td><td>44° 3.32</td><td>4.45</td><td>0.9</td><td>322</td><td>70</td><td>52</td><td>70</td><td>284</td></tr></tbody></table><table><thead><tr><th>N</th><th>DATE</th><th>To</th><th>Latitude</th><th>Longitude</th><th>Depth</th><th>Mag</th><th>Strike</th><th>Dip</th><th>Strike</th><th>Dip</th></tr></thead><tbody><tr><td>1</td><td>890107</td><td>131735.5</td><td>40° 53.91</td><td>43° 56.76</td><td>3.11</td><td>2.4</td><td>298</td><td>60</td><td>150</td><td>34</td><td>285</td></tr><tr><td>2</td><td>890105</td><td>004642.9</td><td>40° 52.37</td><td>43° 55.33</td><td>3.18</td><td>3.7</td><td>340</td><td>60</td><td>82</td><td>70</td><td>286</td></tr><tr><td>3</td><td>881231</td><td>214446.8</td><td>40° 55.67</td><td>43° 56.64</td><td>7.16</td><td>2.1</td><td>312</td><td>70</td><td>50</td><td>70</td><td>287</td></tr><tr><td>4</td><td>881227</td><td>010440.1</td><td>40° 55.34</td><td>43° 56.47</td><td>6.68</td><td>1.4</td><td>275</td><td>75</td><td>17</td><td>72</td><td>288</td></tr><tr><td>5</td><td>890104</td><td>172418.1</td><td>40° 54.33</td><td>43° 57.08</td><td>7.43</td><td>2.3</td><td>265</td><td>61</td><td>11</td><td>70</td><td>289</td></tr><tr><td>6</td><td>881231</td><td>041052.4</td><td>40° 55.02</td><td>43° 56.62</td><td>7.50</td><td>1.9</td><td>257</td><td>60</td><td>0</td><td>70</td><td>290</td></tr><tr><td>7</td><td>890102</td><td>010212.4</td><td>40° 54.74</td><td>43° 57.47</td><td>7.69</td><td>1.9</td><td>218</td><td>50</td><td>99</td><td>60</td><td>291</td></tr><tr><td>8</td><td>881231</td><td>060339.0</td><td>40° 54.28</td><td>43° 56.17</td><td>8.87</td><td>1.5</td><td>264</td><td>70</td><td>100</td><td>70</td><td>292</td></tr><tr><td>9</td><td>881231</td><td>143251.8</td><td>40° 54.87</td><td>43° 57.6</td><td>8.44</td><td>2.8</td><td>245</td><td>34</td><td>46</td><td>56</td><td>293</td></tr><tr><td>10</td><td>881230</td><td>133111.2</td><td>40° 54.74</td><td>43° 57.43</td><td>8.79</td><td>3.1</td><td>258</td><td>60</td><td>75</td><td>30</td><td>295</td></tr><tr><td>11</td><td>881230</td><td>165938.9</td><td>40° 54.91</td><td>43° 57.1</td><td>9.45</td><td>1.2</td><td>255</td><td>60</td><td>38</td><td>52</td><td>296</td></tr><tr><td>12</td><td>881231</td><td>040709.4</td><td>40° 55.13</td><td>43° 56.66</td><td>9.71</td><td>4.7</td><td>140</td><td>89</td><td>225</td><td>24</td><td>296</td></tr><tr><td>13</td><td>881229</td><td>184349.8</td><td>40° 55.63</td><td>43° 57.32</td><td>13.17</td><td>3.6</td><td>260</td><td>80</td><td>358</td><td>50</td><td>297</td></tr><tr><td>14</td><td>881231</td><td>052819.6</td><td>40° 54.92</td><td>43° 57.7</td><td>9.73</td><td>1.3</td><td>226</td><td>38</td><td>46</td><td>52</td><td>298</td></tr><tr><td>15</td><td>881231</td><td>101608.6</td><td>40° 55.13</td><td>43° 57.51</td><td>9.59</td><td>1.9</td><td>260</td><td>45</td><td>75</td><td>46</td><td>299</td></tr><tr><td>16</td><td>881231</td><td>045900.1</td><td>40° 55.37</td><td>43° 57.01</td><td>9.75</td><td>2</td><td>337</td><td>81</td><td>221</td><td>24</td><td>300</td></tr><tr><td>17</td><td>881229</td><td>140205.1</td><td>40° 55.06</td><td>43° 57.71</td><td>9.66</td><td>0.8</td><td>253</td><td>47</td><td>32</td><td>52</td><td>301</td></tr><tr><td>18</td><td>881226</td><td>200245.2</td><td>40° 57.84</td><td>43° 59</td><td>12.02</td><td>1</td><td>302</td><td>71</td><td>151</td><td>22</td><td>302</td></tr><tr><td>19</td><td>881226</td><td>184116.4</td><td>40° 55.23</td><td>43° 57.6</td><td>10.11</td><td>2.3</td><td>250</td><td>72</td><td>142</td><td>56</td><td>303</td></tr><tr><td>20</td><td>881231</td><td>043412.2</td><td>40° 55.28</td><td>43° 57.61</td><td>9.50</td><td>1.9</td><td>252</td><td>31</td><td>105</td><td>64</td><td>304</td></tr><tr><td>21</td><td>890108</td><td>000809.9</td><td>40° 56.4</td><td>43° 57.26</td><td>8.39</td><td>2</td><td>270</td><td>50</td><td>76</td><td>40</td><td>305</td></tr><tr><td>22</td><td>881230</td><td>024932.5</td><td>40° 59.38</td><td>44° 0.99</td><td>6.94</td><td>2.6</td><td>319</td><td>80</td><td>50</td><td>70</td><td>306</td></tr><tr><td>23</td><td>881228</td><td>224936.6</td><td>40° 58.99</td><td>44° 0.34</td><td>6.40</td><td>2.9</td><td>305</td><td>68</td><td>45</td><td>83</td><td>307</td></tr><tr><td>24</td><td>881228</td><td>224657.5</td><td>40° 58.86</td><td>44° 0.24</td><td>6.50</td><td>2.9</td><td>308</td><td>71</td><td>48</td><td>71</td><td>308</td></tr><tr><td>25</td><td>890101</td><td>221925.5</td><td>40° 59.17</td><td>43° 59.87</td><td>4.73</td><td>1.4</td><td>264</td><td>60</td><td>24</td><td>50</td><td>309</td></tr><tr><td>26</td><td>890101</td><td>094807.5</td><td>40° 59.21</td><td>44° 0.04</td><td>3.29</td><td>1.8</td><td>310</td><td>80</td><td>42</td><td>70</td><td>310</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Map of the epicentres of the aftershocks from 1988 December 25 to 1989 January 8. The cross indicates our relocation of the epicentre of the main shock. Five distinct segments are shown. The main tectonic features are included: thicker lines show surface ruptures and the indentation shows the thrust dip, parallel segmented lines indicate the anticline axis, the indented segmented line is the unbroken thrust fault, the long broken line bordering the seismicity to the north is the Pambak-Sevan major fault.</figcaption></figure>

deformation behaves in a plastic way at the surface (Stein & King 1984).

The aftershock distribution (Fig. 3) shows that the seismicity pattern changes abruptly to the west of Spitak: the azimuth shifts to N120°, the epicentre cluster widens, surface breaks are to the south of the epicentres, depths reach 10 km and more. Shallow activity is mainly concentrated near the edges of the surface breaks. Some of these near-surface events form elongated clusters transverse to the general direction of the seismicity (Fig. 4, FF' to KK') and might be related to transform faulting connecting adjoint segments. It is worth noticing that important shallow aftershocks take place just above the relocated hypocentre of the main shock (see below) and at the bend of the fault between the N120° and N140° oriented segments. Distribution of hypocentres with depth is not uniform (Fig. 3). The dip of the fault, about 55°, is in agreement with the GEOSCOPE average solution (Haessler *et al.* 1991). This dip, together with the greater depth of the hypocentres, explains the widening of the epicentral distribution with respect to the Alavar segment.

Fault-plane solutions in the transition regions between segments divide between strike-slip and reverse faulting, like those located on the main fault (Fig. 4 FF'). Cross-section HH', near the middle of the segment, shows a simple feature: hypocentres delineate a single fault plane and almost all focal mechanisms are reverse faulting (Fig. 4 HH'). Thus, mechanisms are dominantly reverse dip slip

in the central part, a right lateral component being frequent on the sides. Strike-slip faulting indicates left-lateral movement along the transverse lines of shallow seismicity which bound the segment at each end. The dominant feature of aftershock focal mechanisms, namely reverse faulting with pronounced right lateral component, is in fairly good agreement with the surface breaks.

We have to emphasize that while there are strong differences in the surface expression of the fault between the region Spitak-Gekhasar and the fold west of Nalband (Fig. 8), it is more difficult to resolve noticeable changes in the aftershock pattern near the transition.

### 3 Western segments

(Fig. 3; Fig. 4, MM', PP'; Table 2). West of the central segment the seismicity divides into two branches; one to the southwest and another one to the northwest.

Aftershocks extend over a zone approximately 10 km long with a general E-W elongation on the southwestern branch. The seismicity appears to be more scattered than along the previous segments, shallow activity is weak, and many aftershocks reach 15 km in depth. No surface ruptures have been observed along this segment. Although the general trend of the activity appears to be E-W on the map of epicentres, cross-sections with the same N30° azimuth as those of the central segment, exhibit a fairly well-defined plane (Fig. 4, PP'). Nevertheless, a geological fault
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Cross-sections (AA' to PP') through the hypocentres of the aftershocks along the lines shown in the insets. No vertical exaggeration. Depth estimations have 1400 m above sea-level as reference level. The focal mechanisms are shown in the Schmidt equal area back projection roughly looking NW. An inverted triangle at the surface shows the trace of the fault, while the filled half circle indicates the position of an anticline. The coordinates of the central point of the section, its width and its azimuth are shown.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 4.** (continued)</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure 4.** (continued)</figcaption></figure>

<figure><img src="image_19.png" /></figure>

apparently not active during the main shock borders the seismicity to the south, is well seen on satellite images, and was already known by the geologists before the earthquake. This fault, at least 10 km long and dipping to the north, has the same orientation as the surface breaks of the central segment, namely about N120°. From this evidence, we conclude that most of the seismic activity along the southwestern segment takes place on a fault plane parallel to the central segment fault plane and offset by about 4 km to the south; thus forming an 'en échelon' system. The dislocation did not reach the surface during the main shock. Focal mechanisms are comparable to those of the central segment, namely strike-slip faulting and reverse faulting, the latter being dominant.

The north western activity defines an elongated cluster of aftershocks (Fig. 3), whose depths range from 3 to more than 10 km. This branch is also prominent on cross-sections (Fig. 4 PP'), where it appears as a narrow vertical band. Practically all of the focal mechanisms in this segment are pure right lateral strike-slip.

Whereas all aftershocks clusters corresponding to the four previous segments end against the Pambak-Sevan fracture zone, which is the main tectonic feature of the area, this last branch extends beyond this structure. The depths vary from shallow to deep, and although several events have depths of less than 4 km, no surface breaks have been found in this northwestern region.

## STRESS TENSOR AND FAULTING MECHANISM

The quality of the data gathered and the density of the network raised our expectations of mapping stress variations along the fault. The method developed by Rivera & Cisternas (1990) was used to determine the stress tensor. First we assumed that a unique stress tensor could explain the whole set of data, and performed the calculation using 155 aftershocks with more than 15 polarities. A normalized likelihood function of 97 per cent was obtained, and 94 per cent of the polarities were explained by a single tensor with

almost horizontal principal axes $σ_y$ and $σ_x$ ($σ_z$ is the principal value with axis closer to the vertical, $σ_y > σ_x$, and the three are oriented to form a dextral reference frame) with strike N344° and N74° respectively, $σ_z$ being nearly vertical (Fig. 5a, total data set), the shape factor, $R = (σ_z - σ_x)/(σ_y - σ_x) = -0.7$, being typical of a triaxial compression regime. This is a remarkable result that leaves very little room for variations of the stress.

Next, we determined a separate stress tensor for difference regions. The stress tensor corresponding to the southeastern segment is the same, but the uncertainty on the directions of $σ_x$ and $σ_y$ is large due to the lack of variety of the fault planes which are dominantly vertical and strike in the same direction. This is also why the shape factor is poorly resolved (Fig. 5b). The central segments offer more variety in azimuths and dips and the tensor is well constrained (Fig. 5c). The solution is the same as the one given by Philip *et al.* (1991) from striae measured on the exposed fault scarp between Spitak and Gekhasar. The same tensor is found again towards the west, but only the direction of $σ_1$ is well defined, since the shape factor is near zero indicating that $σ_z ≈ σ_x$ and that the stress regime corresponds to uniaxial compression (Fig. 5d). The change is shape factor passing from the central ($R = -0.9$) to the western segment ($R = 0$) may be a real effect since the fault planes have different orientations and should contain the tensor well.

The major result is that the area where the earthquake occurred is subjected to a N344° compressive regime. A similar conclusion was obtained in the study of Philip *et al.* (1989) from microtectonic data collected in Georgia and at a much larger regional scale. However, these authors found that the minimum principal stress was horizontal and roughly oriented E-W. The reason for this difference is that they worked north of the Spitak area, in a region which contains NS oriented volcanic alignments and the corresponding normal faulting, among other tectonic features. This region, where EW extension is clear is connected to a similar one (also in EW extension) which is found at the southeast of Spitak, the Spitak fault acting as a transform
<figure><img src="image_20.png" /><figcaption>**Figure 5.** Principal axes of the stress tensors in lower hemisphere Schmidt equal area projection. One standard deviation ellipses around the axes are shown. The shape factor $R = (σ_y - σ_x)/(σ_y - σ_x)$ indicates the stress regime (Rivera & Cisternas 1990). (a) Total data set of the aftershocks of Table 2. (b) South western segment. (c) Spitak-Gekhasar central segment. (d) Alavar eastern segment. Negative values of $R$ correspond to triaxial compression, but the tensor has symmetries in cases (b), where two different faults are mixed, and (d) where we have almost one single fault plane.</figcaption></figure>

fault between them (Philip *et al.* 1991). This explains the lack of normal faulting in the Spitak fault zone and the more compressive character of the resulting stress tensor.

Going back to the focal mechanisms of the aftershocks, the stress-tensor obtained from the whole set of data allows us to resolve some details related to the geometry and the process of faulting. A large number of focal mechanisms has been presented on the cross-sections, but the complete set will be used now. It is clear from the focal solutions shown earlier (Fig. 4) that the mechanisms range from strike-slip to pure reverse dip-slip. However, the rake is not evenly distributed, and a statistical study over the complete set shows that most of the mechanisms are either almost pure strike-slip (like in the sourtheastern segment), or nearly pure dip-slip (like in the central part of the central segment). More precisely, we divided the rake interval [0°, 90°] into four equal sectors and verified that 33 per cent of the mechanisms were almost pure strike-slip (rake within the [0°, 22.5°] interval), and 33 per cent almost pure dip-slip (rake within the [67.5°, 90°] interval).

In both cases, strike-slip faulting or reverse faulting, we chose as fault plane the one which was the closest to the general trend of seismic activity. Fig. 6 shows the histograms of the azimuths and dips of the chosen fault planes in polar

representation. A large majority of fault planes corresponding to strike-slip mechanisms have an azimuth range of 100°-150°, and dip steeper than 70°. A few of these aftershocks might actually have the other nodal plane as fault plane, as was suggested for the shallow alignments at the extremities of the central segment. On the other hand, dips are steeper than those of the planes observed on the cross-sections, except for the Alavar southeastern segment which shows complete agreement between the individual mechanisms and the fault plane obtained from the general trend of the seismicity. In addition to this we observe that the azimuth range for reverse mechanisms varies from 60° to 110°, this is about 40° less than the values obtained from the hypocentres for the general structural directions even though most dips range between 40° and 60°, namely about the same value as that defined by the cross-sections. These apparent discrepancies for the segments west of Spitak may be explained by the picture suggested by Scholz (1990, p. 27) and by King & Yielding (1984) where the ruptured surface is growing through a system of 'en échelon' smaller scale faults (aftershocks) along its rim, which are not on the same plane as the main fault.

The region experienced a long tectonic history (Philip *et al.* 1989) and the medium should be highly fractured. Then,

<figure><img src="image_21.png" /><figcaption>**Figure 6.** Azimuths and dips of the fault planes for the 'reverse like' and 'strike-slip like' mechanisms of aftershocks respectively. The histograms are obtained after the sorting described in the text. Circles, in polar coordinates, indicate percentage of the total number of aftershocks. Azimuth varies between 180° and 360°, and dip between 0° and 90°, in steps of 10°.</figcaption></figure>
under the actual stress regime, fracturing should occur along the most favourable surfaces: the pre-existing Alavar and Pambak-Sevan great faults (Philip *et al.* 1991). But as they intersect each other forming a sharp bend, it is likely that the Spitak-Gekhasar fault and the system of ‘en échelon’ blind folds that continues the deformation to the west, act in such a way as to smooth the transition from one fault to the other.

## RELOCATION OF THE MAIN SHOCK AND EARLY AFTERSHOCKS

The epicentre of the main shock is 40.987°N, 44.185°E according to the National Earthquake Information Center (NEIC), 40.99°N, 44.25°E after the Euro-Mediterranean Seismological Center (EMSC) and 40.91°N, 44.25°E according to the seismological centre of Obninsk. Although these locations are close to each other, they differ by more than 10 km, and the uncertainty is too large to undertake detailed studies such as defining the sense of rupture, for example. In a similar way, location errors for the largest aftershocks that occurred on the days following the main shock, until the setting up of our temporary network, do not allow a detailed study of the evolution of the activity in space and time. In particular, the relation between the largest aftershock and the main shock is not clear on the basis of these locations.

Nevertheless, enough information was available from the local permanent networks to improve the epicentral determinations. We relocated the main shock and the principal aftershocks up to December 25 using two variants of the master event technique (Dewey 1972). The master events were strong aftershocks accurately located with the

temporary network and for which we could say that we knew their absolute position with an accuracy of 0.5 km in epicentre and 1 km in focal depth. In the first technique, these master events were used to obtain station delays for the regional stations including Armenian, Georgian and Azerbaidjanian stations (Fig. 7). In the second variant, the main shock and early aftershocks were relocated relative to the master events by using an inversion algorithm (Tarantola & Valette 1982; Besse 1988).

To apply the first method we collected *P* arrival times from 23 regional stations (Δ < 350 km) for 29 master events. *S*-wave station delays were obtained from *P*-waves delays simply by multiplying by 1.7. Data from stations ALG, ART and SHN were not available before 1988 December 11, when they were set up by the Moscow Institute of Physics of the Earth. Except for the nine Armenian stations with direct readings, all of the data was taken from bulletins. The data from the Stepanavan station (STE) were particularly useful because of its vicinity to the seismic zone, its continuous recording and the impulsive character of the onsets. After subtracting mean *P*- and *S*-wave station delays from the observed arrival times, we relocated the main shock and early aftershocks using the HYPOINVERSE program with a fixed focal depth (7 km).

In the second method, the data set consisted of *P* arrival times to nine of the Armenian stations read by one of us, plus *P* arrival times at three Georgian stations to the northwest of the area taken from bulletins after controlling their quality against our records of common events (Table 3, Fig. 7). The quality of the seismograms of Soviet stations is excellent and the service homogeneous. One minute of coded Moscow time is recorded once a day in the photographic traces of all stations, thus permitting a reading accuracy at least as good as a tenth of a second.

<figure><img src="image_22.png" /><figcaption>**Figure 7.** Topographical map of Caucasus with the sites of regional (Armenian, Georgian and Azerbaydhanian) stations used for relocations. The rectangle shows the region surveyed by the portable network after the Spitak earthquake.</figcaption></figure>
<table><caption>**Table 3.** Permanent stations. Asterisks denote Armenian stations.</caption><thead><tr><th>Code</th><th>Name</th><th>Latitude</th><th>Longitude</th><th>Elevation</th></tr></thead><tbody><tr><td>ABS *</td><td>Abastoumani</td><td>41° 45.00N</td><td>42° 49.00E</td><td>1700</td></tr><tr><td>AKH *</td><td>Akhalkalaki</td><td>41° 24.00N</td><td>43° 28.80E</td><td>1703</td></tr><tr><td>ALG</td><td>Alaguiaz</td><td>40° 40.80N</td><td>44° 17.20E</td><td>2010</td></tr><tr><td>ARR *</td><td>Ararat</td><td>39° 50.40N</td><td>44° 42.00E</td><td>850</td></tr><tr><td>ART</td><td>Artik</td><td>40° 34.40N</td><td>43° 59.90E</td><td>2280</td></tr><tr><td>BKR *</td><td>Bakouriani</td><td>41° 43.80N</td><td>43° 31.20E</td><td>829</td></tr><tr><td>ERE *</td><td>Erevan</td><td>40° 10.80N</td><td>44° 30.00E</td><td>1000</td></tr><tr><td>GOR</td><td>Gori</td><td>41° 59.00N</td><td>44° 08.00E</td><td>640</td></tr><tr><td>GRS</td><td>Goris</td><td>39° 31.20N</td><td>46° 19.80E</td><td>1398</td></tr><tr><td>IDZ *</td><td>Idjevan</td><td>40° 53.00N</td><td>45° 12.50E</td><td>1500</td></tr><tr><td>ISK *</td><td>Issaakian</td><td>40° 36.00N</td><td>43° 42.00E</td><td>1435</td></tr><tr><td>KRM</td><td>Karmrakar</td><td>39° 19.20N</td><td>46° 28.20E</td><td>1524</td></tr><tr><td>LEN *</td><td>Leninakan</td><td>40° 48.60N</td><td>43° 50.90E</td><td>1550</td></tr><tr><td>MCR *</td><td>Metsamor</td><td>40° 11.30N</td><td>44° 08.30E</td><td>850</td></tr><tr><td>ONI</td><td>Oni</td><td>42° 35.00N</td><td>43° 27.00E</td><td>810</td></tr><tr><td>PAA *</td><td>Parakar</td><td>40° 10.20N</td><td>44° 24.60E</td><td>810</td></tr><tr><td>SHI</td><td>Sheki</td><td>41° 13.00N</td><td>47° 12.00E</td><td>750</td></tr><tr><td>SHN</td><td>Shenavan</td><td>40° 51.60N</td><td>44° 13.20E</td><td>1400</td></tr><tr><td>STE *</td><td>Stepanavan</td><td>40° 59.90N</td><td>44° 23.90E</td><td>1390</td></tr><tr><td>TBS</td><td>Tbilissi</td><td>41° 44.00N</td><td>44° 45.00E</td><td>609</td></tr><tr><td>VAD *</td><td>Vardenis</td><td>40° 11.40N</td><td>45° 43.80E</td><td>1930</td></tr></tbody></table>

Six master events covering the aftershock area were selected. Each master event gave one hypocentre for each one of the aftershocks as output of the relative location routine. Out of the six hypocentres determined for each aftershock we kept the one with the minimum RMS value, checking that this solution was close to the hypocentre of the corresponding master event. The problem is almost one-dimensional due to the elongated shape of the aftershocks area and the two stations ISK and IDZ, respectively to the southwest and east of the seismic zone controlled the epicentral positions fairly well, when both P arrival times were available. Depths were poorly controlled and generally fixed to the depths of the master events.

About 80 aftershocks with magnitude greater than $M_L \approx 3.0$ were located using the second method (master events) (Table 4), but only half of this number by using station corrections, due to lack of data in the bulletins.

<caption>**Table 4. Relocations.**</caption>

1. Relocation of the main shock with respect to the aftershock of 04-01-1989 at 07h 29mn

<table><thead><tr><th>Y M D</th><th>To</th><th>LAT N</th><th>LON W</th><th>DEPTH</th><th>RMS</th><th>ERX</th><th>ERY</th><th>ERZ</th><th>Mag</th></tr></thead><tbody><tr><td>881207</td><td>7 41 24.61</td><td>40N53.14</td><td>44E15.65</td><td>5.46</td><td>.58</td><td>.22</td><td>.17</td><td>2.83</td><td>7.0</td></tr></tbody></table>

2. Relocation of the foreshock with respect to the main shock

<table><thead><tr><th>Y M D</th><th>To</th><th>LAT N</th><th>LON W</th><th>DEPTH</th><th>RMS</th><th>ERX</th><th>ERY</th><th>ERZ</th><th>Mag</th></tr></thead><tbody><tr><td>881206</td><td>15 27 7.80</td><td>40N49.41</td><td>44E11.83</td><td>5.46</td><td>.89</td><td>.15</td><td>.19</td><td>3.80</td><td>3.</td></tr></tbody></table>

3. Relocation of the largest aftershock with respect to the main shock

<table><thead><tr><th>Y M D</th><th>To</th><th>LAT N</th><th>LON W</th><th>DEPTH</th><th>RMS</th><th>ERX</th><th>ERY</th><th>ERZ</th><th>Mag</th></tr></thead><tbody><tr><td>881207</td><td>7 45 45.49</td><td>40N54.09</td><td>44E15.70</td><td>5.46</td><td>.52</td><td>1.41</td><td>.63</td><td>9.98</td><td>6.0</td></tr></tbody></table>

4. Relocation of the early aftershocks with respect to the following master events:

<table><thead><tr><th>Date</th><th>Origin time</th><th>Lat</th><th>Long</th><th>Depth</th></tr></thead><tbody><tr><td>• 31-12-1988</td><td>04h 07mn 09.45s</td><td>40°55.13'N</td><td>43°56.66'E</td><td>9.71</td></tr><tr><td>• 01-01-1989</td><td>22h 59mn 38.23s</td><td>40°46.76'N</td><td>44°22.93'E</td><td>3.13</td></tr><tr><td>• 03-01-1989</td><td>13h 48mn 47.10s</td><td>40°52.06'N</td><td>44°12.47'E</td><td>5.33</td></tr><tr><td>• 04-01-1989</td><td>07h 29mn 40.44s</td><td>40°53.32'N</td><td>44°14.96'E</td><td>4.94</td></tr><tr><td>• 05-01-1989</td><td>08h 15mn 13.64s</td><td>40°50.12'N</td><td>44°19.17'E</td><td>6.46</td></tr><tr><td>• 06-01-1989</td><td>07h 35mn 58.54s</td><td>40°53.85'N</td><td>43°56.67'E</td><td>4.92</td></tr><tr><td>• 08-01-1989</td><td>10h 26mn 47.71s</td><td>40°54.18'N</td><td>44°09.36'E</td><td>4.20</td></tr></tbody></table>

<table><thead><tr><th>Y M D</th><th>To</th><th>LAT N</th><th>LON W</th><th>DEPTH</th><th>RMS</th><th>ERX</th><th>ERY</th><th>ERZ</th><th>Mag</th></tr></thead><tbody><tr><td>881207</td><td>0806 26.39</td><td>40N50.83</td><td>44E00.64</td><td>4.89</td><td>0.61</td><td>0.37</td><td>0.24</td><td>09.10</td><td>4.9</td></tr><tr><td>881207</td><td>0857 03.49</td><td>40N53.59</td><td>44E01.65</td><td>4.20</td><td>0.44</td><td>0.22</td><td>0.18</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>0928 37.49</td><td>40N48.36</td><td>44E22.85</td><td>6.34</td><td>0.48</td><td>0.40</td><td>0.34</td><td>08.55</td><td>3.0</td></tr><tr><td>881207</td><td>1020 26.94</td><td>40N55.06</td><td>44E05.69</td><td>5.33</td><td>0.69</td><td>0.61</td><td>0.28</td><td>09.39</td><td>3.0</td></tr><tr><td>881207</td><td>1056 50.39</td><td>40N52.99</td><td>44E12.82</td><td>6.34</td><td>0.61</td><td>0.21</td><td>0.16</td><td>02.11</td><td>4.2</td></tr><tr><td>881207</td><td>1101 04.07</td><td>40N56.21</td><td>44E08.73</td><td>6.34</td><td>0.96</td><td>0.20</td><td>0.20</td><td>02.37</td><td>3.9</td></tr><tr><td>881207</td><td>1103 32.24</td><td>40N54.79</td><td>44E09.76</td><td>4.20</td><td>0.53</td><td>0.16</td><td>0.14</td><td>10.00</td><td>3.9</td></tr><tr><td>881207</td><td>1209 26.46</td><td>40N53.10</td><td>44E13.16</td><td>4.20</td><td>0.42</td><td>0.20</td><td>0.17</td><td>10.00</td><td>3.8</td></tr><tr><td>881207</td><td>1212 55.96</td><td>40N54.51</td><td>44E02.50</td><td>4.20</td><td>0.90</td><td>0.21</td><td>0.16</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>1223 06.63</td><td>40N51.62</td><td>44E12.70</td><td>4.20</td><td>1.00</td><td>0.39</td><td>0.35</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>1230 45.13</td><td>40N53.59</td><td>44E11.96</td><td>5.33</td><td>0.32</td><td>0.27</td><td>0.70</td><td>09.98</td><td>3.0</td></tr><tr><td>881207</td><td>1247 58.33</td><td>40N54.13</td><td>44E08.05</td><td>6.34</td><td>0.56</td><td>0.43</td><td>0.29</td><td>08.35</td><td>3.0</td></tr><tr><td>881207</td><td>1404 39.23</td><td>40N55.30</td><td>44E00.78</td><td>4.20</td><td>0.58</td><td>0.27</td><td>0.25</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>1410 14.46</td><td>40N56.40</td><td>44E13.01</td><td>6.34</td><td>0.83</td><td>0.66</td><td>0.30</td><td>08.19</td><td>4.3</td></tr><tr><td>881207</td><td>1437 14.16</td><td>40N48.88</td><td>44E24.29</td><td>6.34</td><td>0.60</td><td>0.22</td><td>0.15</td><td>01.76</td><td>4.0</td></tr><tr><td>881207</td><td>1446 29.28</td><td>40N52.03</td><td>44E14.34</td><td>5.46</td><td>0.43</td><td>0.22</td><td>0.16</td><td>03.13</td><td>3.0</td></tr><tr><td>881207</td><td>1503 00.87</td><td>40N53.80</td><td>44E12.22</td><td>5.46</td><td>0.78</td><td>0.23</td><td>0.21</td><td>09.41</td><td>3.0</td></tr><tr><td>881207</td><td>1554 52.73</td><td>40N53.25</td><td>43E58.67</td><td>5.46</td><td>0.90</td><td>0.33</td><td>0.22</td><td>08.33</td><td>3.0</td></tr><tr><td>881207</td><td>1647 48.19</td><td>40N56.26</td><td>44E03.54</td><td>4.89</td><td>0.53</td><td>0.14</td><td>0.18</td><td>05.47</td><td>3.0</td></tr><tr><td>881207</td><td>1650 02.36</td><td>40N55.34</td><td>44E02.80</td><td>4.20</td><td>0.80</td><td>0.46</td><td>0.35</td><td>10.00</td><td>3.0</td></tr></tbody></table>
<table><caption>**Table 4. (continued)**</caption><thead><tr><th>Y M D</th><th>To</th><th>LAT N</th><th>LON W</th><th>DEPTH</th><th>RMS</th><th>ERX</th><th>ERY</th><th>ERZ</th><th>Mag</th></tr></thead><tbody><tr><td>881207</td><td>1805 41.82</td><td>40N52.54</td><td>44E09.73</td><td>4.20</td><td>0.35</td><td>0.86</td><td>0.80</td><td>10.00</td><td>4.4</td></tr><tr><td>881207</td><td>1826 55.97</td><td>41N00.19</td><td>43E51.63</td><td>4.20</td><td>0.49</td><td>1.71</td><td>1.01</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>1917 51.49</td><td>40N51.60</td><td>44E08.31</td><td>6.34</td><td>0.43</td><td>1.97</td><td>1.32</td><td>09.99</td><td>4.1</td></tr><tr><td>881207</td><td>2007 28.93</td><td>40N54.83</td><td>44E12.95</td><td>6.34</td><td>0.28</td><td>0.22</td><td>0.21</td><td>06.82</td><td>4.4</td></tr><tr><td>881207</td><td>2035 08.88</td><td>40N50.33</td><td>43E58.27</td><td>6.34</td><td>0.27</td><td>0.78</td><td>1.03</td><td>09.30</td><td>3.0</td></tr><tr><td>881207</td><td>2203 21.98</td><td>40N52.88</td><td>44E13.26</td><td>4.20</td><td>0.55</td><td>1.06</td><td>0.31</td><td>10.00</td><td>3.0</td></tr><tr><td>881207</td><td>2350 53.85</td><td>40N56.30</td><td>43E58.45</td><td>4.20</td><td>0.19</td><td>0.30</td><td>0.24</td><td>10.00</td><td>3.0</td></tr><tr><td>881208</td><td>0014 54.81</td><td>40N49.91</td><td>44E13.70</td><td>6.34</td><td>0.80</td><td>0.24</td><td>0.21</td><td>06.69</td><td>3.0</td></tr><tr><td>881208</td><td>0114 59.47</td><td>40N49.90</td><td>44E19.46</td><td>6.34</td><td>0.61</td><td>1.14</td><td>0.84</td><td>09.99</td><td>4.4</td></tr><tr><td>881208</td><td>0149 38.15</td><td>40N54.56</td><td>43E59.92</td><td>4.20</td><td>0.48</td><td>0.26</td><td>0.37</td><td>10.00</td><td>4.3</td></tr><tr><td>881208</td><td>0209 29.81</td><td>40N53.42</td><td>44E18.60</td><td>6.34</td><td>0.15</td><td>1.54</td><td>1.33</td><td>09.99</td><td>4.2</td></tr><tr><td>881208</td><td>0604 50.37</td><td>40N57.08</td><td>44E04.87</td><td>5.46</td><td>0.87</td><td>2.85</td><td>1.86</td><td>09.36</td><td>4.0</td></tr><tr><td>881208</td><td>0746 00.43</td><td>40N46.51</td><td>44E24.57</td><td>6.34</td><td>0.55</td><td>0.23</td><td>0.14</td><td>02.21</td><td>4.6</td></tr><tr><td>881208</td><td>1014 14.72</td><td>40N48.55</td><td>44E19.79</td><td>6.34</td><td>0.43</td><td>0.59</td><td>0.55</td><td>07.21</td><td>4.3</td></tr><tr><td>881208</td><td>1246 54.43</td><td>40N53.40</td><td>43E57.19</td><td>4.20</td><td>0.27</td><td>1.83</td><td>2.22</td><td>10.00</td><td>4.1</td></tr><tr><td>881208</td><td>1441 36.28</td><td>40N55.90</td><td>44E11.44</td><td>6.34</td><td>0.54</td><td>0.18</td><td>0.17</td><td>06.10</td><td>3.0</td></tr><tr><td>881208</td><td>1759 15.76</td><td>40N51.80</td><td>44E10.86</td><td>6.34</td><td>0.64</td><td>0.96</td><td>1.41</td><td>08.93</td><td>4.2</td></tr><tr><td>881208</td><td>2032 05.78</td><td>40N53.42</td><td>44E11.81</td><td>6.34</td><td>0.43</td><td>0.19</td><td>0.16</td><td>02.05</td><td>4.5</td></tr><tr><td>881209</td><td>0112 13.34</td><td>40N52.45</td><td>44E12.66</td><td>3.13</td><td>0.44</td><td>0.16</td><td>0.17</td><td>09.99</td><td>3.0</td></tr><tr><td>881209</td><td>0450 27.89</td><td>40N45.27</td><td>44E21.96</td><td>5.33</td><td>0.32</td><td>0.15</td><td>0.14</td><td>04.21</td><td>3.0</td></tr><tr><td>881209</td><td>0831 07.15</td><td>40N54.38</td><td>44E08.92</td><td>6.34</td><td>0.38</td><td>0.14</td><td>0.15</td><td>02.14</td><td>3.0</td></tr><tr><td>881209</td><td>0854 26.09</td><td>40N50.27</td><td>44E11.44</td><td>6.34</td><td>0.06</td><td>0.42</td><td>0.49</td><td>07.91</td><td>3.0</td></tr><tr><td>881209</td><td>1401 25.62</td><td>40N54.49</td><td>44E08.42</td><td>6.34</td><td>0.45</td><td>0.24</td><td>0.17</td><td>02.72</td><td>4.1</td></tr><tr><td>881209</td><td>1413 57.21</td><td>40N54.97</td><td>44E02.00</td><td>4.20</td><td>0.26</td><td>0.19</td><td>0.16</td><td>10.00</td><td>3.0</td></tr><tr><td>881209</td><td>1423 01.21</td><td>40N51.72</td><td>44E18.06</td><td>5.46</td><td>0.30</td><td>0.20</td><td>0.16</td><td>02.39</td><td>3.0</td></tr><tr><td>881209</td><td>1448 23.60</td><td>40N55.63</td><td>44E07.86</td><td>4.20</td><td>0.74</td><td>0.74</td><td>0.54</td><td>10.00</td><td>3.0</td></tr><tr><td>881209</td><td>1516 15.00</td><td>40N51.71</td><td>44E13.54</td><td>5.46</td><td>0.26</td><td>0.19</td><td>0.35</td><td>05.33</td><td>3.0</td></tr><tr><td>881209</td><td>1916 33.35</td><td>40N47.32</td><td>44E21.83</td><td>5.46</td><td>0.50</td><td>0.23</td><td>0.18</td><td>04.22</td><td>4.1</td></tr><tr><td>881210</td><td>0000 46.28</td><td>40N44.52</td><td>44E23.03</td><td>5.33</td><td>0.29</td><td>0.35</td><td>0.43</td><td>08.83</td><td>3.0</td></tr><tr><td>881210</td><td>1913 57.76</td><td>40N48.92</td><td>44E18.61</td><td>5.46</td><td>0.35</td><td>0.14</td><td>0.18</td><td>07.03</td><td>4.2</td></tr><tr><td>881210</td><td>2125 02.74</td><td>40N57.11</td><td>44E04.53</td><td>3.13</td><td>0.32</td><td>0.49</td><td>0.32</td><td>09.99</td><td>3.0</td></tr><tr><td>881211</td><td>0355 10.90</td><td>40N51.70</td><td>44E13.60</td><td>5.46</td><td>0.95</td><td>0.16</td><td>0.18</td><td>03.13</td><td>4.4</td></tr><tr><td>881211</td><td>0458 11.56</td><td>41N 0.72</td><td>44E04.52</td><td>4.20</td><td>0.37</td><td>0.22</td><td>0.19</td><td>10.00</td><td>3.0</td></tr><tr><td>881211</td><td>0936 19.14</td><td>40N47.87</td><td>44E19.94</td><td>5.46</td><td>0.34</td><td>0.15</td><td>0.14</td><td>03.46</td><td>3.0</td></tr><tr><td>881211</td><td>1220 07.97</td><td>40N48.25</td><td>44E20.65</td><td>5.46</td><td>0.30</td><td>0.13</td><td>0.13</td><td>02.96</td><td>3.0</td></tr><tr><td>881211</td><td>2308 17.64</td><td>40N56.00</td><td>44E06.98</td><td>6.34</td><td>0.25</td><td>0.18</td><td>0.17</td><td>02.56</td><td>3.0</td></tr><tr><td>881212</td><td>0115 00.33</td><td>40N53.52</td><td>44E08.92</td><td>4.20</td><td>0.64</td><td>0.21</td><td>0.15</td><td>10.00</td><td>3.0</td></tr><tr><td>881212</td><td>0845 54.54</td><td>40N51.08</td><td>44E17.00</td><td>4.20</td><td>0.53</td><td>0.59</td><td>0.21</td><td>10.00</td><td>3.8</td></tr><tr><td>881212</td><td>1536 17.19</td><td>40N53.96</td><td>44E17.39</td><td>5.46</td><td>0.37</td><td>0.25</td><td>0.23</td><td>02.29</td><td>4.2</td></tr><tr><td>881212</td><td>1713 08.53</td><td>40N48.73</td><td>44E22.74</td><td>4.20</td><td>0.80</td><td>0.30</td><td>0.15</td><td>10.00</td><td>3.0</td></tr><tr><td>881212</td><td>1722 33.27</td><td>40N54.02</td><td>44E03.43</td><td>3.13</td><td>0.59</td><td>0.21</td><td>0.14</td><td>09.99</td><td>3.6</td></tr><tr><td>881212</td><td>1803 41.89</td><td>40N44.78</td><td>44E21.07</td><td>4.89</td><td>0.16</td><td>0.26</td><td>0.24</td><td>08.59</td><td>3.0</td></tr><tr><td>881213</td><td>0421 37.33</td><td>40N54.58</td><td>44E13.18</td><td>6.34</td><td>0.45</td><td>0.17</td><td>0.17</td><td>01.74</td><td>4.1</td></tr><tr><td>881213</td><td>1112 36.39</td><td>40N54.22</td><td>44E01.01</td><td>4.20</td><td>0.41</td><td>0.27</td><td>0.19</td><td>10.00</td><td>3.2</td></tr><tr><td>881213</td><td>2135 21.73</td><td>40N55.29</td><td>44E05.87</td><td>3.13</td><td>0.69</td><td>0.19</td><td>0.16</td><td>09.99</td><td>3.5</td></tr><tr><td>881214</td><td>2049 08.76</td><td>40N52.98</td><td>44E09.28</td><td>4.20</td><td>0.44</td><td>0.17</td><td>0.15</td><td>10.00</td><td>3.9</td></tr><tr><td>881215</td><td>0414 08.89</td><td>40N46.89</td><td>44E21.66</td><td>4.20</td><td>0.29</td><td>0.35</td><td>0.13</td><td>10.00</td><td>3.2</td></tr><tr><td>881215</td><td>1543 44.74</td><td>40N53.74</td><td>43E59.00</td><td>4.89</td><td>0.17</td><td>0.22</td><td>0.18</td><td>07.55</td><td>4.5</td></tr><tr><td>881216</td><td>1853 02.58</td><td>40N53.70</td><td>44E05.54</td><td>4.20</td><td>0.80</td><td>0.18</td><td>0.22</td><td>10.00</td><td>4.3</td></tr><tr><td>881217</td><td>2333 18.08</td><td>40N53.38</td><td>44E09.09</td><td>5.33</td><td>0.08</td><td>0.26</td><td>0.20</td><td>04.14</td><td>3.3</td></tr><tr><td>881218</td><td>0408 52.66</td><td>40N44.69</td><td>44E22.62</td><td>6.34</td><td>0.33</td><td>0.18</td><td>0.26</td><td>04.82</td><td>3.3</td></tr><tr><td>881218</td><td>1307 02.19</td><td>40N57.34</td><td>44E03.55</td><td>4.20</td><td>0.94</td><td>0.25</td><td>0.17</td><td>10.00</td><td>3.0</td></tr><tr><td>881218</td><td>1749 33.39</td><td>40N51.01</td><td>44E12.09</td><td>6.34</td><td>0.66</td><td>0.31</td><td>0.61</td><td>09.07</td><td>3.6</td></tr><tr><td>881218</td><td>1952 06.00</td><td>40N56.22</td><td>44E02.76</td><td>4.20</td><td>0.22</td><td>0.23</td><td>0.19</td><td>10.00</td><td>3.3</td></tr><tr><td>881218</td><td>2000 39.47</td><td>40N48.92</td><td>44E17.30</td><td>6.34</td><td>0.58</td><td>0.21</td><td>0.41</td><td>07.52</td><td>3.3</td></tr><tr><td>881220</td><td>0004 54.09</td><td>40N47.31</td><td>44E25.28</td><td>5.46</td><td>0.74</td><td>2.03</td><td>0.78</td><td>10.00</td><td>4.5</td></tr></tbody></table>

Solutions with RMS > 1 s were disregarded in all cases. An overall comparison between the two methods for the 40 common events shows that 90 per cent of them do not differ by more than 5 km. Based on this result, we trust the epicentre locations from the regional permanent network with station corrections to be accurate within 5 km. In the following, we present only the results obtained by the second method (master events) since this sequence is more complete.

The main shock was preceded by a foreshock of magnitude $M_L = 3$ on December 6 at 15 hr 27 min, namely

16 hr 14 min before. We located this foreshock relative to the main shock by using arrival times from seven common stations. Its epicentre lies at about 8 km to the southwest of the main shock and slightly outside the aftershock cloud. Nevertheless this location is not well constrained since the RMS value is of 0.9 s.

The main shock was relocated by the second method at 40.886°N, 44.261°E, less than 3 km from the Obninsk determination which includes arrival times from regional and distant stations. The epicentre relocated by the first method, using station delays from 17 regional stations, is
<figure><img src="image_23.png" /><figcaption>**Figure 8.** Map of epicentral relocations (Table 4) from the data of the permanent network of stations of Fig. 7, from 1988 December 6 to 1988 December 24.</figcaption></figure>

found to be at 40.881°N, 44.265°E, so our two independent determinations differ by 650 m only.

Careful examination of broad-band waves from distant records shows that the main energy burst was preceded by a small signal (foreshock) a few seconds before (Haessler *et al.* 1991). It was impossible to identify the initial time of the large amplitudes on the records of the Armenian stations, thus the relocated epicentre corresponds to the weak initial phase. However Kondorskaya (personal communication) performed a relative location of these two events (the arrival time of the first small phase with respect to the arrival of the large amplitudes) by using records at teleseismic distances and found that they are at the same place within error bars, and thus the epicentre of the main shock has the position we computed with an accuracy probably better than 3 km. These considerations confirm the place of initiation of the main shock to be just a few kilometres north of Spitak, at the eastern edge of the central segment and near the intersection with the northern extension of the Alavar fault.

The strongest aftershock [4 min 20 s after the main shock, 40.974°N, 44.246°E, $m_b = 5.9$ (NEIC); 40.80°N, 44.16°E, $m_b = 6.0$ (Obninsk)] caused extensive damage to buildings already weakened by the main earthquake and numerous casualties. Unfortunately, it was not possible to read the P-wave onset at more than four Armenian stations: BAW, LEN, ERE and ARR. Differences of P arrival times relative to those of the main shock are the same at all of the stations (within 0.2 s) but the BKR bulletin indicates that this difference is 1.2 s smaller. We verified that the epicentre is

close to the one of the main shock, and slightly to the north by computing its position relative to the main shock (Fig. 8). Although this location is weakly constrained we do not believe that the error is more than 15 km. We may ask whether this strong aftershock was responsible for the rupture of the southeastern segment. The latter argument suggests that the aftershock did not take place on that part of the southeastern segment as we postulated in a previous paper (Cisternas *et al.*, 1989). And another piece of information, the modelling of broad-band seismograms (Haessler *et al.* 1991), also implies that the Alavar segment very likely ruptured a few seconds after the initial shock and not at the time of the strong aftershock.

No other aftershock of the Spitak earthquake exceeded the magnitude $M_L = 5.0$. The epicentres (Fig. 8) exhibit the same overall pattern already seen on Fig. 3, but with more dispersion. A large number (40 per cent) of the located aftershocks occurred during the first 24 hr after the main shock. Nevertheless, not all of the aftershocks could be adequately located during the first day due to the high density of events and the consequent inference of the recordings, and some of the early aftershocks may be absent from Fig. 8.

We do not observe any evident clustering in time or space. Therefore, as we pointed out above, the whole aftershock zone of Fig. 8 was active after the main shock, in agreement with the broad-band modelling that shows that the five segments ruptured all along within about 15 s (Haessler *et al.* 1991).
## CONCLUSIONS

Aftershocks of the Spitak earthquake have been recorded by a dense network of 26 portable seismic stations, thus providing a detailed observation of the aftershock sequence with high-quality numerical records.

Focal mechanisms of aftershocks and their space distribution, together with the mapping of surface ruptures and observations of deformation of active folds (Philip et al. 1991), led us to propose a five segment fault model of the main earthquake (Fig. 3). Two of them break up the surface, namely the Alavar (N140°) and the Spital-Gekhasar (N120°) segments. Two other segments are hidden under 'en échelon' blind folds west of Ghekhasar. The fifth segment is located to the northwest of the aftershock zone, and begins near the surface trace of the Pambak-Sevan fault. Maximum depths are very shallow (6 km) along the Alavar segment but they increase towards the west, reaching more than 15 km under the western and northwestern segments. The seismic moment of the Alavar segment is 1/6 of the total moment.

The set of polarity data from the first arrivals of the aftershocks is in agreement with a single stress tensor which is compatible with the northward motion of the Arabian plate. The focal mechanisms are well constrained, either when calculated individually by two independent methods, or when obtained at the same time as the stress tensor in a maximum likelihood procedure, the likelihood being 97 per cent in this latter case. The $σ_1$ axis is horizontal, oriented N344°, and the stress regime corresponds to triaxial compression. Right lateral strike-slip on an almost vertical plane dominates in the Alavar and the northwestern segment, while reverse motion with a smaller right lateral component on a surface dipping 50° to the NE, is typical of the Spitak-Gekhasar segment and of the blind folds.

The general aspect of the relocated events, corresponding to the period going from the occurrence of the main shock to the installation of the portable network, is very similar but less precise than that of the well-located aftershocks. The relocation of the main shock gives an epicentre at 40.886°N and 44.261°E a few kilometres to the north of Spitak. The relative locations of the foreshock, the main shock, and the strongest aftershock, place these three events near the intersection between the Alavar fault and the Pambak-Sevan fault. The implication is that the Spitak fault differs from the Alavar and Pambak-Sevan faults near the surface, but it is strongly related to them at depth, smoothing the sharp bend formed by the intersection of these two main tectonic features.

The above results make part of a comprehensive study that includes neotectonics, near-field local seismic stations and teleseismic broad-band recordings, geodynamic models of the region, deformation associated with the earthquake and palaeoseismology, thus permitting a global view of the geometry and time evolution of the source process (Philip et al. 1991; Haessler et al. 1991). The mechanism of the main shock and those of the aftershocks are in agreement with the stress regime previously obtained from a general study of the recent tectonics of the Caucasus. The fault activated by this earthquake is not easily recognized in satellite images, and appears as a secondary feature related to well-defined main accidents such as the Pambak-Sevan and Alavar

faults. Nevertheless, it gives precise complementary information about the recent tectonics of the southern border of the Lesser Caucasus, a region where approximately EW oriented reverse faulting and folding coexists with NS volcanic alignments and diagonal strike-slip faulting.

## ACKNOWLEDGMENTS

This work was performed within the cooperation program between the Centre National de la Recherche Scientifique (France) and the Soviet Academy of Sciences. The Institut National des Sciences de l'Univers (INSU), the Secrétariat d'Etat aux Risques Naturels Majeurs and the Ministère de la Recherche et de la Technologie provided financial support. This work could not be done without the kind support from the Armenian Academy of Sciences, and particularly that of Professors A. Karapetian and R. Djerbashian who helped us to overcome all of the difficulties of the field work. We are specially indebted to Michel Cara for his constant encouragement and advice and to Geoff King for his many comments and suggestions.

## REFERENCES

* Aki, K. & Richards, P. G., 1980. *Quantitative Seismology: Theory and Methods*, Vol. 1, Freeman, San Francisco.

* Besse, P., 1986. Relocalisation relative d'événements sismiques, appliqué à la région Sud du Lac Ghoubbet situé dans la République de Djibouti, Rapport DEA, Université Paris VII.

* Cisternas, A. et al., 1989. The Spitak (Armenia) earthquake of 7 December 1988: field observations, seismology and tectonics, Nature, **339**, 675-679.

* Dewey, J. W., 1972. Seismicity and tectonics of Western Venezuela, Bull. seism. Soc. Am., **62**, 1711-1751.

* Dufumier, H., 1989. Détermination de mécanismes au foyer à partir d'enregistrement d'ondes de volume et de surface, Diplome d'Ingénieur Géophysicien, Université L. Pasteur, Strasbourg.

* Haessler, H., Deschamps, A., Dufumier, H., Fuenzalida, H. & Cisternas, A., 1991. The rupture process of the Armenian earthquake from broad-band teleseismic body wave records, Geophys. J. Int., submitted.

* Jimenez, E., Cara, M. & Rouland, D., 1989. Focal mechanism of moderate size earthquakes from the analysis of single station three components surface wave records, Bull. seism. Soc. Am., **79**, 955-972.

* King, G. & Yielding, G., 1984. The evolution of a thrust fault system: processes of rupture initiation, propagation and termination in the 1980 El Asnam (Algeria) earthquake, Geophys. J. R. astr. Soc., **77**, 915-933.

* Klein, F., W., 1978. Hypocenter location program HYPO-INVERSE, Open-File Rept 78-694, US Geological Survey, Boulder, CO.

* Lyon-Caen, H. et al., 1988. The 1986 Kalamata (South Peloponnesus) earthquake: detailed study of a normal fault, evidences for east-west extension in the Hellenic Arc, J. geophys. Res., **93**, B12, 14967-15000.

* Pacheco, J. F., Estabrook, C. H., Simpson, D. W. & Nabelek, J. L., 1989. Teleseismic body wave analysis of the 1988 Armenian earthquake, Geophys. Res. Lett., **16**, 1425-1428.

* Philip, H., Cisternas, A., Gvishiani, A. & Gorshkov, A., 1989. The Caucasus: an actual example of the initial stage of continental collision, Tectonophysics, **161**, 1-21.

* Philip, H., Rogozhin, E., Cisternas, A., Bousquet, J. C., Borisov, B. & Karakhanian, A., 1991. The Armenian earthquake of
1988 December 7: faulting and folding, neotectonics and palaeoseismicity, *Geophys. J. Int.*, submitted.

Rivera, L. & Cisternas, A., 1990. Stress tensor and fault plane solutions for a population of earthquakes, *Bull. seism. Soc. Am.*, **80**, 600–614.

Scholz, C. H., 1990. *The Mechanics of Earthquakes and Faulting*, Cambridge University Press, Cambridge, UK.

Stein, R. & King, G. C., 1984. Seismic potential revealed by

surface folding: 1983 Coalinga, California earthquake, *Science*, **224**, 869–872.

Tarantola, A. & Valette, B., 1982. Generalized nonlinear inverse problems solved using the least-squares criterium, *Rev. Geophys. Space Phys.*, **20**, 219–232.

Udías, A., Buforn, E., Brillinger, D. & Bolt, B., 1982. Joint statistical determination of fault-plane parameters, *Phys. Earth planet. Inter.*, **30**, 178–184.