| Title | Using Earthquake Clusters to Identify Fracture Zones at Puna Geothermal Field, Hawaii |
|---|---|
| Authors | A Lucas, C Kenedi, E Shalev, P Malin |
| Year | 2011 |
| Conference | Asian Geothermal Symposium |
| Keywords | microseismicity, shear wave splitting, fracture zones, focal mechanisms |
| Abstract | The actively producing Puna geothermal system (PGS) is located on the Kilauea East Rift Zone (ERZ), Hawaii. In the Puna area the rift trend is identified as NE-SW from surface expressions of normal faulting, at PGS the rift offsets in a left step. The spread of earthquake locations follows the rift trend. Double difference (DD) relocation of all earthquakes identified one large cluster but could not conclusively identify trends within the cluster. For five stations out of the six, where shear wave splitting was observed, the dominant polarization direction was rift parallel. Two of the five stations also showed a smaller rift perpendicular signal. The shear wave splitting time delays indicate that fracture density is higher at the PGS compared to the surrounding ERZ. Correlation co-efficient clustering was used to identify clusters based on similar earthquake waveforms. In total, 40 localized clusters containing ten or more events were identified. Most of the clusters had linear features when their locations were plotted and were further enhanced by DD relocation. The concentration of individual linear features was higher in the PGS than the surrounding ERZ. Mapping of the linear features showed that a number of the larger features ran rift parallel. However a large number of rift perpendicular features were also identified. We assume that events occurring on the same fracture zone have similar source mechanisms and thus similar waveforms. It is concluded that the linear features identified by earthquake clustering are fracture zones. The orientation and concentration of the fracture zones is consistent with that of the shear wave splitting polarizations. |