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Abstract:
We analyze and interpret the stress features from a c. 1.5 km deep fully cored borehole (PTA2) on the Big Island
of Hawai0i within the Humu0ula saddle region, between the Mauna Kea and Mauna Loa volcanoes. The Big Island
of Hawaii comprises the largest and youngest island of the Hawaiian–Emperor seamount chain and is volumetrically
dominated by shield stage tholeiitic volcanic rocks. Mauna Kea is dormant whereas Mauna Loa is still active.
There are also a series of normal faults on Mauna Loa’s northern and western slopes, between its two major rift
zones, that are believed to be the result of combined circumferential tension from the two rift zones and from added
pressure due to the westward growth of the neighboring K¯ılauea volcano.
The PTA2 borehole was drilled in 2013 into lava dominated formation (Pahoehoe and A¯a) as part of the Humu0ula
Groundwater Research Project (HGPR) with the purpose of characterizing the groundwater resource potential in
this area. In 2016 two downhole logging campaigns were performed by the Operational Support Group of the International
Continental Scientific Drilling Program (ICDP) to acquire a set of geophysical data as part of the Volcanic
Margin Petroleum Prospectivity (VMAPP) project. The main objective of the logging campaign was to obtain high
quality wireline log data to enable a detailed core-log integration of the volcanic sequence and to improve understanding
of the subsurface expression of volcanic rocks. We identify stress features (e.g. borehole breakouts) and
volcanic structures (e.g. flow boundaries, vesicles and jointing) at depth using borehole images acquired with an
ABI43 acoustic borehole televiewer. We analyzed and interpreted the stress indicators and compared their orientation
with the regional stress pattern.We identified a set of stress indicators along the hole dominantly concentrated
within the lower logged interval of the PTA2 borehole. Two primary horizontal stress indicators have been taken
into account: borehole breakouts (bidirectional enlargements) (BB) and drilling induced tensile fractures (DIF).
BB and DIF occur when the stresses around the borehole exceed the compressive and tensile yield stress of the
borehole wall rock respectively causing failure. A breakout is caused by the development of intersecting conjugate
shear planes that cause pieces of the borehole wall to spall off. For a breakout to develop, the stress concentration
around a vertical borehole is largest in the direction of the minimum horizontal stress. Hence, BB develops approximately
parallel to the orientation of the minimum horizontal stress. For the DIF, the stress concentration around a
vertical borehole is at a minimum in the maximum horizontal stress direction. Hence, DIF develop approximately
parallel to the orientation of the maximum horizontal stress. Based on theWorld Stress Map, the present-day stress
in this area is defined only by focal mechanism solutions. These data give a unique opportunity to characterize the
orientation of the present-day stress field between two large volume shield volcanoes on an active volcanic island
using a different approach and stress indicators.
