Jökull


Jökull - 01.01.2017, Page 15

Jökull - 01.01.2017, Page 15
Zeinab Jeddi et al. Figure 7. Non-linear absolute locations of the eastern events presented as the combined probability density of all 132 events normalized by its maximum. Black contours correspond to 0.9 – 0.1 with 0.1 spacing. Gray con- tours are at 0.05, 0.03, 0.01. – Dreifing staðsetninga skjálftanna austan í Kötlu sýnd sem heildardreififall þeirra (summa dreififalla allra 132 skjálfta). Svartar jafnhæðarlínur svara til hlutfallslegrar stærðar dreififallsins við 0.9 - 0.1 med millibilinu 0.1. Gráar hæðarlínur eru teiknaðar við hlutfallslegar stærðir 0.05, 0.03 og 0.01. egy described in Sgattoni et al. (2016b), we solved a weighted least squares problem where the relative weights are the relative uncertainty estimates of the differential times. We solved this explicitly (not with an iterative equation solver). This allowed us to prop- agate the data uncertainty through the calculation and estimate the covariance matrix of relative locations. If the data were appropriately explained (within their uncertainty), we would expect the misfit scaled by the data covariance to equal the number of degrees of freedom in the residual data (i.e., the number of data minus the number of independent location pa- rameters). We do not know the absolute errors, but we have estimated their relative sizes based on the corre- lation coefficients. We scale the error estimates such that the normalized misfit becomes its expectation, i.e., unity, and thereby estimate data errors in absolute terms. Thus, the initial normalized misfit is around 280 and it decreases to 1 after one iteration, i.e., the normalized misfit is reduced by 99.7%. The inversion converges in one iteration suggesting that the problem is effectively linear, indicating that the distribution of the sources is small. Figure 8 shows the relative location results for all selected events of family 1 (Figure 8b) and family 2 (Figure 8c) together with absolute location of master events using non-linear location method (Figure 8a). The horizontal distribution of the 132 events of family 1 is concentrated in a small area, about 400 m wide, with an average horizontal uncertainty of ∼25 m. Their depth distribution is concentrated around the master event over an area ∼400 m deep except for a few events, which are 500–700 m deeper than the template. The average depth uncertainty is ∼80 m. The events in family 2 are also located in a very small area, less than 150 m wide, around the template, with uncertainties of ∼10 and ∼30 m in the horizontal and vertical directions, respectively. DISCUSSION AND CONCLUSIONS We have analyzed and described a seismic cluster lo- cated on the eastern flank of Katla volcano near the tip of Sandfellsjökull glacier. We were able to de- tect these small events due to the dense seismic net- work that was operating on and around Katla between 2011 and 2013. A few events were registered by the IMO monitoring network before 2011, but the area has not previously been identified as a persistent seis- mic source at Katla. We can now add the eastern seismic source area to the better known caldera and Goðabunga seismic source areas, and the recently ac- tive south flank area. 10 JÖKULL No. 67, 2017

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