Paleomagnetic constraints on fault motion in the Hilina Fault System, south flank of Kilauea Volcano, Hawaii

Abstract

Movement of the south flank of Kilauea Volcano in Hawaii has been associated with catastrophic landslide events. The surface expression of this former movement is the Hilina Fault System with fault scarps as high as 500 m. Paleomagnetic directions for lava flows exposed in the Hilina Fault scarps at Puu Kapukapu and Keana Bihopa on the Hilina Pali are used to determine the average rate of movement along faults (slip surfaces) separating the two sections. This paper reports results from two independent paleomagnetic studies within the Hilina Pali area. Twenty-one paleomagnetic sites (143 cores) were sampled by the Michigan Technological University group from lava flows between the Mo'o Ash and Middle Pohakaa Ash at Keana Bihopa in the footwall block of the 500-m-high Hilina Pali fault scarp. Thirty paleomagnetic sites (152 cores) were collected by the California Institute of Technology group from lava flows between the Mo'o Ash and Middle Pohakaa Ash in the 300-m-high Puu Kapukapu section (the hanging-wall block). A comparison of site-mean directions show that lava flows in the lower part of the Puu Kapukapu section have been tilted more than lava flows in the upper part with respect to the Keana Bihopa section. The systematic steepening of remanent directions downsection at Puu Kapukapu indicates that slippage of this block occurs along listric normal faults. The average amount of backward tilt of the Puu Kapukapu block, based on a comparison of mean directions from the two sections, is 7.8°±7.7°. Using slope stability methods, the average rate of movement of the Puu Kapukapu block since deposition of the Middle Pohakaa Ash is 1.7–2.4 cm/yr, and the average displacement (subsidence) is 680–740 m. Assuming that the average displacement resulted from a series of earthquakes producing subsidence equal to that observed in the 1975 Kalapana earthquake (3 m of subsidence along a 40-km segment of coastline on the south flank of Kilauea Volcano), one Kalapana-size earthquake occurring every 200 years would account for this displacement. Lastly, overall mean directions for the two sections indicate that Puu Kapukapu has rotated counterclockwise with respect to the Hilina Pali by 14.8°±8.5° about a nearby vertical axis. This also suggests that slippage between the two blocks occurs along listric normal faults.