The onset of Kīlauea's summit eruption in 2008 was not preceded by a dramatic change in the numbers of discrete earthquakes, nor by the usual inflationary ground deformation16, but SO 2 emissions (Fig. 3b) and seismic tremor (Fig. 3a) began to increase several months before the 19 March explosion15. SO 2 emissions increased from a background of ~150 tonnes per day in late 2007 to over 1500 tonnes per day before the eruption17, providing the most obvious indication of volcanic unrest and indicating a heightened rate of degassing of other magmatic volatiles, such as water. SO 2 emissions and seismic tremor have remained elevated throughout the course of the eruption, but both dropped when major vent collapses triggered week-long pauses in the eruption in December 2008 and again in mid 2009 (Fig. 3a, solid grey bars)14.

Figure 3: Time series from January 2007 to December 2010. (a) RSAM recorded at seismic station RIM. (b) SO 2 emission rates at Kīlauea's summit17; vertical bars represent the s.d. for all traverses on a single day. (c) GPS line length between stations UWEV and AHUP. (d–i) SWS ϕ results and V p /V s at seismic stations in the north caldera. Grey points are individual measurements with error bars indicating 95% confidence interval, and black points are 30-point moving averages with error bars indicating the s.d. of the average. Solid blue vertical bar represents the Father's Day 2007 eruption, red dashed vertical bar marks the beginning of the summit eruption and solid grey vertical bars indicate pauses in the eruption. Full size image

Variations in seismic anisotropy at Kīlauea's summit in 2008 may reflect changes associated with the formation of the summit eruptive vent between March and December 2008. The beginning of the shift in ϕ can be identified in the time series late 2007, coincident with the increase in both SO 2 emissions and tremor. Although deflating, pressure beneath the caldera remained high owing to the engorged nature of the magma system16, causing the maximum horizontal compressive stress to be radial. The increased gas pressure in shallow pore spaces opened more cracks parallel to the maximum horizontal compressive stress, changing the observed anisotropy to be radial to the caldera centre from late 2007 through 2008. This effect must be limited to the shallow subsurface because stations ~1 km apart display different ϕ. Because ϕ displays the direction of anisotropy from the last anisotropic media that the wave travelled through, it is reasonable that the anisotropy is acting at depths <1 km. Stations outside the caldera wall did not display the variation that those on the caldera floor exhibited, probably because the caldera floor is comprised of porous recent lava flows18,19. The state of stress inside the caldera, particularly that affected by the shallow (~1–2 km deep) magma reservoir, may be partially decoupled from outside the caldera due to the ring-fault system, which may also contribute to the lack of changes observed at stations on the caldera rim.

The end of anomalous local SWS occurred in December 2008, at about the same time as a pause in the summit eruption (solid grey bar in Fig. 3). By this time, the new eruptive vent had stabilized and become the focal point of summit gas emissions (which were still high) as a direct pathway between degassing magma and the surface20. Gas was no longer fed into pore space in the shallow subsurface, causing pore pressure to return to background and SWS to return to its pre-eruptive, structurally governed state.

Increased gas content in pores and cracks has the effect of lowering P-wave speed due to the higher fluid compressibility, but not significantly affecting the shear modulus, and hence S-wave speed. The ratio between P-wave velocity (V p ) and S-wave velocity (V s ), V p /V s , is therefore useful for characterizing pore-fluid content and supporting the SWS evidence. Laboratory measurements21 show that crack opening induced by increasing pressure in gas-enriched pores leads to decreased V p /V s . Gas-enriched pore space has been reported to affect V p /V s above magmatic intrusions22,23, and several studies combine both SWS parameters and V p /V s to characterize pore content13,24. V p /V s was anomalously low (1.46±0.04) at Kīlauea in 2008 (Fig. 3), after which it returned to a typical crustal value of 1.71±0.03 (ref. 25), supporting the hypothesis that the pore-gas content at shallow levels in the caldera was high in 2008. Use of catalogue origin times to calculate V p /V s will yield larger uncertainties (see the Methods section); however, if we take a V p /V s of 1.46, we can calculate a Poisson's ratio of ν=0.06, which is considerably lower than values previously observed (ν=0.25–0.32)26. V p /V s after 2008 corresponds to ν=0.24, closely agreeing with previous investigators. V p /V s is sensitive to the entire ray path, and this is probably why all of the summit stations display a change in V p /V s rather than only the stations inside the caldera (the majority of the waves probably spend some time in the affected region).

A small but similar deviation from background occurred in July 2007 (Fig. 3). Although not a statistically significant change in ϕ or V p /V s , the presence of fluctuations in all of the time series and concurrence with the ‘Father's Day’ intrusion and eruption16 suggests that the changes were driven by the same mechanism—degassing due to decompression of shallow magma16, increasing pore pressures and changing the seismic properties of the caldera. The Father's Day 2007 event was short-lived, allowing the system to quickly return to a background state and preventing strong SWS signals. Conversely, the onset of the summit eruption in 2008 and its continued activity was either prolonged enough or of a large enough magnitude to be reflected in the SWS signal, which returned to the background state only after the vent had become a well-established pathway for gas escape.