Earthquakes triggered by silent slip events on Kı̄lauea volcano, Hawaii

Slow-slip events, or ‘silent earthquakes’, have recently been discovered in a number of subduction zones including the Nankai trough in Japan, Cascadia, and Guerrero in Mexico, but the depths of these events have been difficult to determine from surface deformation measurements. Although it is assumed that these silent earthquakes are located along the plate megathrust, this has not been proved. Slow slip in some subduction zones is associated with non-volcanic tremor, but tremor is difficult to locate and may be distributed over a broad depth range. Except for some events on the San Andreas fault, slow-slip events have not yet been associated with high-frequency earthquakes, which are easily located. Here we report on swarms of high-frequency earthquakes that accompany otherwise silent slips on Kı̄lauea volcano, Hawaii. For the most energetic event, in January 2005, the slow slip began before the increase in seismicity. The temporal evolution of earthquakes is well explained by increased stressing caused by slow slip, implying that the earthquakes are triggered. The earthquakes, located at depths of 7–8 km, constrain the slow slip to be at comparable depths, because they must fall in zones of positive Coulomb stress change. Triggered earthquakes accompanying slow-slip events elsewhere might go undetected if background seismicity rates are low. Detection of such events would help constrain the depth of slow slip, and could lead to a method for quantifying the increased hazard during slow-slip events, because triggered events have the potential to grow into destructive earthquakes. A silent earthquake beneath the south flank of Kı̄lauea volcano on 10–11 November 2000 displaced Global Positioning System (GPS) stations as much as 1.5 cm over about 36 hours. The depth of the subhorizontal fault was not well constrained, but inversions favoured depths of 4–5 km, considerably shallower than the decollement thought to occur at the base of the volcano. We now recognize similar events on 20–21 September 1998, 3–4 July 2003, and 26–27 January 2005 (ref. 12 and additional events have been reported). All four events have similar durations and displacement patterns (Figs 1 and 2). Inversions assuming uniform slip dislocations place the four sources in virtually the same location (Fig. 1). Whereas the November 2000 slow slip was preceded by extreme rainfall, the other events were not. All four slow-slip events were associated with heightened levels of microseismicity (Fig. 2). The cumulative magnitude of the microearthquakes is far too small to explain the observed displacements. For example, the cumulative moment of the 2005 earthquake swarm is,1.8 £ 10Nm, far less than that of the slow slip, 6.8 £ 10Nm. The microearthquakes, concentrated adjacent to the landward edge of the dislocation (Fig. 1), are thus not the source of the deformation. The association of high-frequency earthquakes with slow slip could be explained by either (1) the earthquakes unpinning the fault, allowing slow slip to occur, or (2) the slow slip stressing the adjacent fault, thereby increasing the seismicity rate. To constrain the onset and duration of fault slip relative to the microearthquakes, we invert the GPS observables during the 2005 slow event directly for fault slip as a function of time. The slow slip started early on 26 January 2005, well before the dramatic increase in seismicity, and LETTERS