## 5 Discussion
### Lava Flow Emplacement and Subsidence  
The rate of subsidence on the 2010 flow varied both spatially and temporally, with the maximum subsidence rate initially at \(\sim\)0.1 cm/year after emplacement, reducing to \(\sim\)0.02 cm/year between 2013 and 2014. This subsidence rate is low compared to other measurements made using InSAR, such as the 10 cm of subsidence measured in 44 days at Okmok [28]. This is likely due to differences in lava flow thickness estimates; the Okmok flow is estimated to be 20 m thick [40], while the 2010 flow at Pacaya is estimated to be 1-4 m [20]. Incoherence within the flow boundaries of ALOS-1 interferogram immediately after the eruptions, spanning from 31 May through 16 July (Figure 5a), shows that the flow surface was changing enough to destroy coherence during the first \(\sim\)1.5 months after emplacement, presumably as a result of the mechanical movement of unstable blocks. This fits field reports by CONRED, which state that the flow ceased on 30 June. The thickest area of the flow becomes coherent 78 days after initiation (Figure 5c), with the majority of the flow becoming coherent only 32 days after initiation (Figure 5b). Due to the temporal resolution of ALOS-1 data (46 days), these dates cannot be further constrained. However, this timeframe falls on the lower end of the range of other InSAR studies; K\(\tilde{l}\)laeuea flows reached coherence 13 to 903 days after emplacement [41], while Okmok flows reached full coherence \(\sim\)3 years after emplacement [28]. At Pacaya, fast coherence rates are likely due to the fast emplacement and relative thinness (1-4 m) of the 2010 flow. Post-emplacement lava flow deformation has been attributed to several processes, including thermal contraction and consolidation [42], substrate compaction [38], and clast repacking or gravity-driven compaction [43]. It is difficult to determine the exact cause of subsidence for the 2010 lava flow without more frequent temporal sampling of subsidence and temperature measurements, but the measured deformation is likely related to both thermal and mechanical processes shortly after the emplacement and then primarily cooling of lava flows afterwards. Given the full spatial coverage of the flow provided by InSAR, thickness estimates of the flow, which currently range from 1 m to 4 m [21], could be constrained (_i.e._, [43]), ultimately leading to remote monitoring of lava flow extrusion rates. Given the high effusive activity since 1961 [20], this could lead to knowledge of effusive eruption dynamics over time. Mapping of discontinuities along the ancestral collapse scarp has shown that Pacaya is subjected to both an E-W extensional regime of the Guatem City graben to the north and the NW-striking Jalpatagua shear zone, resulting in a transtenal local stress regime with an ENE-WSW \(\sigma_{3}\) component [23]. Regional stress regimes have been known to favor vertical magma migration by propagation of dikes through pre-existing cracks or faults, which reduce overpressure [44]. This could help explain the dike intrusion elongated NNW-SSE (Figure 5a), which stretches over the ancestral collapse scarp following the edge of the slope displacement. This tells us that propagation of the dike was controlled by both the local transtenal stress regime and the reduction of overpressure as a result of southwest flank displacement. This also suggests that the topographic anomaly of the ancestral collapse scarp is not the primary mechanism controlling vent emplacement. A NNW drive system is further evidenced by cracking along this theoretical weakness zone as seen after the May 2010 eruptions and continued emplacement of vents aligned in a NNW-orientation over time (Figure 7a). Thus, injection of magma along this orientation during the 2010 eruptions could have caused the force needed to induce the perpendicularly-oriented slope movement. The 2010 collapse could have been a result of either the intrusion or the slope movement.