A seismotectonic model for western Hawaii based on stress tensor inversion from fault plane solutions

Abstract

For 57 fault plane solutions of earthquakes with magnitude ML≥3.0 located west of the southwest rift zone of Mauna Loa volcano in Hawaii, which occurred between 1972 and 1988, the dominant focal mechanisms (44 events) are decollement type with one nodal plane nearly horizontal (dip ≤ 30°). The average slip vector of the upper crust on the decollement plane points in an azimuth of 260° toward the ocean, away from Mauna Loa's southwest rift zone at an angle of about 150° with respect to the NE‐SW oriented rift. Two other types of focal mechanisms are present: normal faults (four events) and strike‐slip faults with normal component (nine events). The orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism. After subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west Hawaii. The orientation for the stress tensor is best resolved in the area located between latitudes 19.27°N and 19.4°N and longitudes 155.7° and 155.9° W where the greatest principal stress directions within the 95% confidence limits are nearly vertical with some spreading to the west. Their plunge varies between 53° and 85°. The intermediate and least principal stresses are mostly horizontal and have similar magnitudes. Their plunge varies between 1° and 36°. The area between latitudes 19.4° and 19.6°N and longitudes 155.7° and 156°W is characterized partly by near‐vertical and partly by east‐west oriented greatest principal stresses. The least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses. In the two areas where the stress tensor was well resolved, the principal strain directions differed from the principal stress directions by 30° in the plunge, suggesting that the faulting in western Hawaii takes place on a weak plane, but this result could not be established at the 95% confidence level. The tectonic model proposed for west Hawaii is similar to the model for the Kalapana region. The strain is accumulated through magmatic intrusions in the southwest rift zone of Mauna Loa, and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth. The west flank of Mauna Loa slips in the direction away from the rift.