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

Movement of the south flank of Kilauea Volcano in Hawaii has been associate d with catastrophic landslide events. The surface expression of this former movement is the Hilina Fault System with fault setups as high as 500 m. Pa leomagnetic directions for lava flows exposed in the Hilina Fault scarps at Puu Kapukapu and Keana Bihopa on the Hilina Pall are used to determine the average rate of movement along faults (slip surfaces) separating the two s ections. This paper reports results from two independent paleomagnetic stud ies within the Hilina Pall area, Twenty-one paleomagnetic sites (143 cores) were sampled by the Michigan Technological University group from lava flow s between the Mo'o Ash and Middle Pohakaa Ash at Keana Bihopa in the footwa ll block of the 500-m-high Hilina Pall fault scarp. Thirty paleomagnetic si tes (152 cores) were collected by the California Institute of Technology gr oup 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 respec t to the Keana Bihopa section. The systematic steepening of remanent direct ions downsection at Puu Kapukapu indicates that slippage of this block occu rs 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 s ections, is 7.8 degrees +/- 7.7 degrees. 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 (subside nce) is 680-740 m, Assuming that the average displacement resulted from a s eries of earthquakes producing subsidence equal to that observed in the 197 5 Kalapana earthquake (3 m of subsidence along a 40-km segment of coastline on the south flank of Kilauea Volcano), one Kalapana-size earthquake occur ring every 200 years would account for this displacement. Lastly, overall m ean directions for the two sections indicate that Puu Kapukapu has rotated counterclockwise with respect to the Hilina Pall by 14.8 degrees +/- 8.5 de grees about a nearby vertical axis. This also suggests that slippage betwee n the two blocks occurs along listric normal faults. (C) 1999 Elsevier Scie nce B.V. All rights reserved.