The Structure of Koolau Volcano from Seismic Refraction Studies

Volume: 19Start Page: 306End Page: 31

Geophysical Exploration on the Structure of Volcanoes: Two Case Histories

Geophysical methods of exploration were used to determine the internal structure of Koolau Volcano in Hawaii and of Rabaul Volcano in New Guinea. By use of gravity and seismic data the central vent or plug of Koolau Volcano was outlined. Magnetic data seem to indicate that the central plug is still above the Curie Point. If so, the amount of heat energy available is tremendous. As for Rabaul Volcano, it is located in a region characterized by numerous block faulting. The volcano is only a part of a large block that has subsided. Possible geothermal areas exist near the volcano but better potential areas may exist away from the volcano

Geophysical Evidence for the Availability of Geothermal Energy in New Britian

This paper combines some of the results and interpretations of geological mapping, seismic refraction, marine seismic, and gravity surveys to show that large tracts of New Britain could be favorable targets for geothermal power development. It is shown that the fractured and faulted lithosphere is associated with grabens and rifts in which mantle material has risen to within 10 to 15 km from the surface. The grabens and rifts are marked by volcanism in which the dominant volcanic rocks are olivine--and tholeiitic--basalts, with a sprinkling of more acid volcanics ranging from dacite to andesite. Following A. Rittman the basalts are believed to have originated in the asthenosphere when the lithosphere was broken up under a tensional stress regime; the acid volcanics were formed by magmatic differentiation within the crust. it was argued that ideal geothermal reservoirs are capped with altered ash deposits or other nonpermeable volcanics. To feed such reservoirs conduits are required which are naturally located on fault or shear zones. The two areas selected as favorable for future geothermal power development are located between Talasea and Lolobau Is., say around Hoskins; and near Rabaul, between Matupi Harbor and Matupi. As a type area, the rift between the Gazelle Peninsula and New Ireland resembles the Afar triangle, at the northern end of the Great Valley Rift system of Africa

Slow Slip Events and Time-Dependent Variations in Locking Beneath Lower Cook Inlet of the Alaska-Aleutian Subduction Zone

We identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface deformations inferred from GPS for pre-2004 and 2010–2011 are similar to each other, as are 2004–2010 and post-2011. We estimate the slip distribution on the Alaska-Aleutian subduction plate interface accounting for upper plate block rotations and interpret this toggling between two deformation patterns as caused by transient slip. We find that by allowing negative slip deficit rates (i.e., creep rates in excess of relative plate motion), the data in Lower Cook Inlet are fit significantly better during pre-2004 and 2010–2011, suggesting the occurrence of slow slip events (SSEs) there during those time periods. The earlier SSE lasted at least 9 years (observations in that area began in 1995) with Mw ~7.8. The latter SSE had almost the same area as the earlier one and a duration of ~2 years with Mw ~7.2. During 2004–2010 and post-2011, the inversions result in only positive slip deficit rates (i.e., locking) in Lower Cook Inlet. Slip rates are nearly constant during the Lower Cook Inlet SSEs, and the events start and stop abruptly. Both of these properties contrast with observations of SSEs in Upper Cook Inlet and elsewhere. The Lower Cook Inlet SSEs are consistent with previously proposed duration-magnitude scaling laws and demonstrate that slow slip events can last as long as a decade