Jökull


Jökull - 01.06.2000, Side 7

Jökull - 01.06.2000, Side 7
Ground deformation at Katla The station Kötlukriki is definitely of lower quality than the station Jökulkvísl as the computed standard error of tilt is higher and increases with time between the measurements used to compute the tilt. Tilt computed from observations made one year apart usually has standard error of about 0.5 /rrad while tilt computed from measurements made more than 10 years apart has standard error of slightly more than 1.0 /trad. This suggests that the long time stability of indi- vidual markers in this tilt station is not very good, or that gradual irregular ground deformation takes place within the area of the tilt station. This tilt station lies on lava which has flowed from the Mýrdalsjök- ull glacier. Beneth the lava is probably a thick layer of sand, similar to that found outside the lava. This sand layer may be subject to continuing compaction. The station Höfðabrekkuheiði is of far inferior quality relative to the two tilt stations east of Mýr- dalsjökull. Two or more markers of the tilt station were unstable during the first years of observations, probably because frost action moves sizeable slabs of the tuff in which the markers are cemented. Later, one of the markers became loose because of erosion. These problems at the station Höfðabrekkuheiði make it of limited value in interpreting the 1967 to 1995 tilt observations near Mýrdalsjökull. INTERPRETATION The computed tilt exceeds in a number of cases the computed standard error by a factor of more than three, suggesting more than 99 per cent probability that the observed tilt is real. If tilt between two successive observations is considered, this occurred in 5 out of 12 cases at the station Kötlukriki and in 7 out of 13 cases at the station Jökulkvísl. There is quite good correlation between tilt at these two stations. If tilt at one station is up in an easterly or westerly direction, the other station is also tilted in the same general direction (Figure 3). This suggests that the observed tilt at these two stations is caused by a common source. The observed tilt varies significantly from one per- iod to another, although tilt azimuth either toward east or west appear most common. This points towards the glacier Mýrdalsjökull to the west of these stations as a possible source, either as a source of uplift which would cause tilt up in westerly direction, or, at other times, a source of subsidence, which would cause tilt down towards a westerly direction. Prior to an eruption of Katla, an increase of pressure is expected below the volcano as magma moves toward a magma reservoir which supplies material for the eruptions. This process will cause more or less continuous uplift of the volcano and tilt up towards a westerly direction at the tilt stations east of Mýrdalsjökull. This has not been observed. The observations indicate periods of uplift and periods of subsidence of the crust beneth the Mýrdals- jökull glacier. This alternating uplift and subsidence has been interpreted as caused by variable ice mass of the glacier (Tryggvason, 1973). It is well known that a great amount of snow accumulates on the glacier each winter and a similar amount melts each sum- mer. Tilt observations were made in the summer of 1967 to 1973, sometimes as early as mid June, someti- mes as late as mid August. Estimates of glacier load (Figure 4) at times of tilt observations correlate fairly well with the observed east component of ground tilt at the two tilt stations east of the glacier. After mea- surements were resumed in 1986, most observations have been made in September or October. The two tilt stations east of Mýrdalsjökull were observed in May 1994 in addition to observations in October 1993 and October 1994. Both stations show tilt down toward a westerly direction between October 1993 and May 1994, a period of increasing glacier load, and both stations show tilt up towards west from May to October 1994 (Figure 3), a period of decreasing glacier load. A model study of the Mýrdalsjökull ice field (Tryggvason 1973) suggested that annual variation in the glacier load is equivalent to about 2 m of water over the entire area of the ice field, about 700 km2. Observed tilt near the edge of the ice field indicate that the amplitude of the annual tilt oscillation is about 6 //rad. A model of elastic lithosphere of uniform thickness floating on viscous íluid, using the approx- imation of Hertz (1884) suggest that the amplitude of the vertical ground oscillation at the center of the ice JÖKULL No. 48 5

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