Oborn, L.E. 1979 Tongonan geothermal power development. Engineering geology : seismotectonic hazards of site B. Report of visit, October-November 1978. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 323 23 p. 6 sheets.
Abstract: This report presents opinions found after a second visit to Tongonan in October and November of 1978, and should be read in conjunction with a previous report, EG 314. It is based on field observations, and on the results of a seismic survey and a 24 hole drilling programme. The emphasis is on seismotectonic hazards. The most apparent inference that must be drawn from the available data, is that Site B and the separator station sites are in a region of high seismotectonic risk. The data does not support the contention that either the Philippine Fault or its branches, or the Sambaloran Fault, crosses Site B. From this it is inferred that the risk of a large surface rupture is low. There is a possibility, however, that small-scale secondary movements could be induced along any sub-parallel aones of weakness that may have been produced by past faulting. There is no way of assessing this risk with the information available. The very close proximity of major faults to these sites would ensure that any local fault movement along them, or local earthquakes generated by them, would induce severe ground motions in the foundations and structures. The largest earthquake that could affect the site (the Maximum Credible Earthquake) has been estimated, from Philippine Fault data and world-wide correlations of fault parameters and earthquake magnitude, to be in the range M8.2 to M8.7. This estimate, of course, carries no conotation or probability of occurrence. Future investigations should include the logging of all new rock exposures, including construction excavations, with the aim of determining whether past fault movements have induced zones of weakness sub-parallel tot he principle faults. Continuing effort should be made to assemblr information relevant to the evaluation of past faulting and seismotectonic risk.
