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Jökull - 01.01.2017, Qupperneq 12

Jökull - 01.01.2017, Qupperneq 12
Microearthquakes on the eastern flank of Katla volcano, S-Iceland Figure 4. Time evolution of the seismic activity on the eastern flank of Katla. Top: Registered events in the IMO catalog between January 2001 and August 2013. Bottom: Detected events using cross correlation between June 2011 and August 2013. Two swarms are observed, one in November 2010 (top panel), the other during De- cember 2011 (lower panel). – Tímaþróun virkninnar austan í Kötlu. Efri myndin sýnir þróunina samkvæmt skráningum varanlegs nets Veðurstofu Íslands frá janúar 2001 til ágúst 2013. Neðri myndin sýnir þróunina samkvæmt öllum skjálftum sem fundust með fylgnileit innan tímabundna netsins frá júní 2011 til ágúst 2013. Tvær hrinur koma skýrt fram, ein í nóvember 2010 (efri mynd), hin í desember 2011 (neðri mynd). the S wave (A), assuming a relationship between am- plitude and magnitude of the form ML = c+ log(A). The resulting magnitudes are mostly between -0.5 and 0.5 ML. The relation between magnitudes and cumula- tive number of events (Figure 5) follows the general Gutenberg-Richter law (log(N) = a − bM). We es- timated the magnitude of completeness (MC) using the non-cumulative distribution of magnitude, where the MC corresponds to the maximum of the distribu- tion (see Roberts et al., 2015). MC was estimated to be -0.4 (dashed line in Figure 5). We used the max- imum likelihood method (Aki, 1965) to evaluate b = 1.6±0.1. HYPOCENTRAL LOCATIONS We were not able to locate the seismic events that were recorded on the eastern flank before Septem- ber 2011 with good accuracy, because the seismic network was not dense enough and the three clos- est and most critical seismic stations (RJU, LOD and KKE) were not all in operation until Septem- ber 2011. Therefore, we estimated both absolute and relative locations only for the events detected with cross-correlation between September 2011 and Au- gust 2013. The absolute locations were obtained using a probabilistic, non-linear grid-search method (Lo- max et al., 2000), while we used a simple approach of relative location to locate events in a local one dimen- sional (1-D) velocity model with respect to a master event (see Sgattoni et al., 2016b). Absolute and Relative Time Measurements In order to improve the absolute and relative estima- tion of P- and S-phase arrival times, we used a cross- correlation procedure based on the correlation of the template event with all other events in each family. We identified P and S phases of the template event of the two families at each station (six in total) and se- JÖKULL No. 67, 2017 7
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