## 3 Results and Discussion
### Forecasts Preceding Grimsvotn's 2011 Eruption  
The VEFA was used to assess the probability of volcanic activity at Grimsvotn preceding its 2011 eruption. Since raw GPS data from GFUM is not available in this situation, only seismic data was used for unrest detection. The boxplots shown in Figure 12 represent the distributions of the number of earthquakes per day and the magnitude of all earthquakes between 2005 and 2011. The thresholds used to defined unrest are 8.0 events per day and a magnitude (\(M_{l}\)) of 2.27. Monitoring was initiated on 24 November 2010 and triggered on an anomalously large seismic event (\(M_{L}\) 3.54) on 25 November 2010. Unrest severity and trigger state vectors are shown in Figure 13 A and B. The rapidly fluctuating unrest severity is caused by elevated periods of anomalous seismicity throughout the episode (See Figure 14 A and B). On algorithm day 134, positive modeling results were injected into the algorithm. This injection was done to simulate potential delays associated with manual source modeling exercises that may be initiated upon the detection of unrest. Figure 15A shows a time series of intrusion, eruption, and intensity forecasts for each stage of the algorithm. Initially, activity forecasts fluctuate between 0.60-0.80, 0.18-0.20, and 0.04-0.09 for the intrusion, eruption, and intensity estimates. The large variation in forecasts are due to the rapidly changing seismic conditions at the site. Forecast probabilities increase dramatically upon the injection of positive modeling results into the algorithm. Revised forecasts suggest this event has a 0.91 probability of being caused by a magmatic intrusion, a 0.68 probability of culminating to an eruption, and a 0.24 probability of exceeding an intensity of 1.0. Figure 12: Boxplots highlighting the distribution of seismicity beneath the Grimsvötn caldera between 2005 and 2011, where the events per day and magnitude whiskers are set to 1.5 time the interquartile range. Monitoring thresholds are 8.0 events per day and a \(m_{l}\) of 2.27. **Figure 13**. : Algorithm state as a function of processing day. (**A**) Unrest severity estimates per day; (**B**) Trigger state per day. The color code declarations shown in Figure 15 B change dramatically over the course of the episode. Initial results place the hazard level as green. Upon the determination that the event is the result of a magmatic intrusion, the hazard level immediately transitions to red. This declaration suggests that an eruption of VEI greater than 1.0 may be imminent. Figure 14: VEFA input parameters. (**A**) Count and average (\(X_{NE}\)) number of earthquakes per episode day (\(X_{DAYS}\)), shown in red and blue; (**B**) Raw and average (\(X_{CSM}\)) normalized seismic moment per episode day (\(X_{DAYS}\)), shown in red and blue. A time series of selected spatial PDFs highlighting probable volcanic vent locations at Grimsv\(\ddot{\text{o}}\)tn are shown in Figure 16. Here the spatial resolution of the geographic grid is 1 arc-second. Since the average epicenter uncertainty for the Icelandic seismic network is unknown, a value of 2.3 km is assumed for demonstration purposes. Thus, all \(X_{seis}^{j}\) samples within 1.15 km of each epicenter are set to 1. Due to the absence of modeling information, maps for days 1 and 37 show probable vent locations scattered over the Vatnajokull icecap. Initially, the search area spans 2979 km\({}^{2}\), \(J\) = 7,094,495, and all \(X_{def}^{j}\) = 0. Modeling results shown in Figure 11 suggest that the deformation source is located approximately 3.33 km below the caldera center and has an isotropic radiation pattern with a 17 km radius. This information provides  
Figure 15: Forecasts of volcanic activity preceding Grimsv\(\ddot{\text{o}}\)tn 2011 eruption, where the intrusion, eruption, and intensity probabilities and thresholds are shown by the red, black, and blue, solid and dotted lines. Introduction of positive modeling results occurs on episode day 134.