Jökull


Jökull - 01.12.1976, Blaðsíða 13

Jökull - 01.12.1976, Blaðsíða 13
tance did not record all the earthquakes re- corded by the other 4 stations. Therefore, to maintain a homogeneous internally consistent data set, only the 4 near stations were used for locating the earthquakes near the volcano. The two distant stations were used as an indepen- dent check on the reliability of locations of more distant events. The 4 near stations were used in two dif- ferent combinations of 3 stations to locate events by the tripartite array method. For events outside of the array, azimuth and apparent velo- city are calculated from the P-wave time dif- ferences between the stations, and distance is estimated from the apparent velocity and the S-P times. The limitations of this method are discussed by Ward and Gregersen (1973): 1. Small errors in reading the P-wave arrival times can cause large errors in calculated ap- parent velocities and azimuths. Depending on the geometry of the array, these errors are func- tions of the apparent velocities and the azi- muths. 2. Uncertainty in picking the S-waves corre- sponds to uncertainty in the distance of the earthquake from the array. In a complex vol- canic region the wave of an earthquake can be quite irregular, and identifying the S-wave constitutes a major problem. 3. Assuming a crustal structure with flat lying layers with constant velocities introduces a pro- blem of nonuniqueness for the location of an earthquake if the first arriving P-wave is cri- tically refracted. It is therefore necessary to assume a crustal structure of flat lying layers with constant velocity gradient and without any first order velocity discontinuities. The constant gradient model is chosen so as to approximate the constant velocity model determined by con- ventional seismic refraction methods. 4. The most severe error cause is any un- known irregularity in the crustal structure. To estimate the magnitude of this error it is neces- sary to set off explosions in a number of places at different azimuths and distances, preferably near the epicenters of the earthquakes to be located. In this study the main sources of P-time read- ing errors are considered to be the finite sam- pling frequency of the FM recorder and irre- gularities in the speed of the recording and play-back instruments. The center frequency of the magnetic tape recorder was 84.4 Hz, which corresponds to a reading error of ± 0.006 sec. Irregularities in the speed of the recorder and the play-back instruments can produce uncer- tainties as large as ± 0.01 sec in the interpola- tion between second marks on the records. The total error is therefore estimated to be ±0.01 sec, which corresponds to an error of approxi- mately ±8° in azimuth and ±0.3 km/sec in apparent velocity for most of the near earth- quakes located with this method. The magnitude of the error in location caused by the uncertainty of picking the S- wave is more difficult to estimate. If we assume that the S-wave is picked correctly the error of that reading is the same as the error in read- ing the P-wave. If the S-wave is not identified correctly, however, the error is difficult to pre- dict. An experienced interpreter would hardly mispick an S-wave by more than ± 0.3 sec corresponding to an error of ± 2.5 km in dis- tance for most of the earthquakes located in this study. The seismic velocity model of the crust used for locating the earthquakes was based on velo- city measurements by Pálmason (1971). One of his refraction profiles runs about 10—30 km northeast and east of Hekla. The velocity model adopted is shown in Table 1. This model differs from Pálmason’s results only in the thickness of layer 1. The necessity of this modification will be discussed below. Layer TABLE 1 P-velocity km/sec Thickness km 0 2.1 0.6 1 4.1 2.3 2 5.1 2.5 3 6.5 3.5 4 7.2 In order to calibrate the seismometer array three explosions were set off in a lake about 11 km to the northeast of the array. For these blasts apparent velocities of 3.8 to 4.0 km/sec were measured across the array. These velo- cities are not significantly different from the velocity in layer 1 measured by Pálmason (1971). Assuming a P-velocity of 2.1 km/sec for layer 0 and using the absolute P-wave travel time for JÖKULL 26. ÁR 1 1
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