Fig. 1

a Simplified view of Oahu with Digital Elevation Model (NOAA—Digital Elevation Models: https://www.ngdc.noaa.gov/mgg/coastal/coastal.html) and the two main volcanic structures, the Koolau and Wai’anae volcanic ranges. The sampling sites area over the Wai’anae volcano is indicated, b elevation contour map of the Wai’anae volcano, dike orientations (adapted from Zbinden and Sinton 1988) and site locations of this study