ACTIVE STRUCTURES ON THE WESTERN SOUTH FLANK OF KILAUEA VOLCANO, HAWAII DELINEATED BY RELOCATED EARTHQUAKES

Approximately 8,000 earthquakes from the Southwest Rift Zone and western south flank of Kilauea Volcano were relocated by cross-correlation and double-difference methods. Relocation of these earthquakes produced sharpened images of active structures within Kilauea. Structures include the Southwest Rift Zone magma pathway, a decollement (boundary between the old ocean crust and the volcano above), several seaward-dipping normal faults on the south flank, and a mantle fault zone.

Earthquakes related directly to magma movement occurred from 1981-1983, when a number of intrusions invaded the Southwest Rift Zone. Soon after larger intrusions migrated downrift, earthquakes occurred on the decollement seaward of the pooled magma. Timing of these events indicates a strong relationship between the magmatic and tectonic systems of Kilauea.

Tectonic earthquakes (those not directly related to magma movement) occurred throughout the study period, 1981-2001. They were located on normal faults between 0 and 11km depth, on a sub-horizontal decollement between 7 and 11km depth, and within a mantle fault zone between 23 and 60km depth. The decollement dips inland toward Kilauea’s summit, and focal mechanisms indicate thrust motion along the decollement. Portions of Kilauea that are seaward of the rift are moving up-dip along this plane.

Mantle earthquakes included in this study are not related to magma movement. Waveforms show impulsive-onset, short-period events atypical of magma migration. These events also yield high-confidence double-couple focal mechanisms, which would not occur with tensional fracture during magma migration.