Periodic slow earthquakes on the flank of Kīlauea volcano, Hawaiʻi Benjamin A. Brooks a, ⁎ , James H. Foster a , Michael Bevis b , L. Neil Frazer a , Cecily J. Wolfe a , Mark Behn c a School of Ocean and Earth Science and Technology, University of Hawaiʻi, 1680 East–West Rd, Honolulu, HI 96816, USA b Geodetic Science, Ohio State University, 2036 Neil Ave. Mall, Columbus, OH 43210, USA c Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 21 December 2005; received in revised form 16 March 2006; accepted 20 March 2006 Available online 22 May 2006 Editor: S. King Abstract We analyze 8 years of continuous GPS data from the Hilina slump (HS) on Kīlauea volcano's south flank and identify 3 new slow earthquake (SE) events. The new SEs are very similar to the previously identified one from November 2000, suggesting they share a common source. The series of SEs are separated by regular periods of 774 (± 7) days. None of the newly identified events are associated with increased rainfall rates, precluding rainfall as a necessary SE trigger. All of the SEs are followed by increased microseismicity in a ∼NW–SE trending band in the southern HS, suggesting that the SEs trigger seismicity. SE location and source parameters are not well-constrained by the CGPS network, although moment can be constrained adequately for the January 2005 event. Using this as a reference, we calculate for the four events equivalent moment magnitude values of 5.6, 5.7, 5.5, and 5.8, in their order of occurrence. © 2006 Elsevier B.V. All rights reserved. Keywords: slow earthquakes; deformation; GPS geodesy; landslide; tsunami 1. Introduction Following the initial detection of ‘silent’ or slow earthquakes (SE) using strainmeters [1–3], continuous GPS (CGPS) observations have led to the discovery of silent after-slip events after conventional subduction zone earthquakes in Japan [4], a SE in the Cascadia Subduction Zone unassociated with a conventional earthquake [5], and recognition that Cascadia SEs are periodic [6] and accompanied by seismic tremor [7]. The first SE detected in an intraplate setting occurred in November 2000 following intense rainfall at the Hilina slump (HS) on Kīlauea volcano's south flank [8]. Because of their increasingly widespread detection [9– 12] much recent work has focused on developing a mechanical explanation for subduction zone SEs. Some workers have argued that the tremor associated with the episodic slip (now termed ETS –‘Episodic Tremor and Slip’) is closely related to ‘harmonic tremor’, a seismic phenomenon frequently observed in the early stages of an eruption and which is thought to be driven by forced fluid flow [13]. It has been suggested that ETS is driven by water released during metamorphic phase changes at the interface between a subducting and overriding plate [14–16]. It is unclear, however, whether ETS is applicable in other types of actively deforming regions where Earth and Planetary Science Letters 246 (2006) 207 – 216 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: bbrooks@soest.hawaii.edu (B.A. Brooks). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.035