Surface rupture earthquakes over the last ∼1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard

Abstract

The Wellington region is cut by five active right‐lateral strike‐slip faults: Wairarapa, Wellington, Ohariu, Shepherds Gully/Pukerua, and Wairau faults that have average recurrence intervals of meter‐scale surface rupture that range from ∼500 years to 5000 years, and lateral slip rates that range from 1 to 10 mm/yr. Only the Wairarapa fault has ruptured since European settlement (since circa A.D. 1840). Paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past ∼1000 years in the Wellington region. Within this time period, there does not appear to be any temporal clustering of surface rupture events on adjacent faults. TheM8 A.D. 1855 Wairarapa earthquake did not trigger rupture on any other fault in the region. The most recent surface‐faulting event on the Wellington fault (290–440 cal years B. P.) (cal years are calendar years before A.D. 1950) does not coincide with rupture of any other onland fault, and over 300 years separate the timing of the second most recent rupture on the Wellington fault (660–720 cal years B.P.) and the most recent rupture of the Ohariu fault (1060–1140 cal years B.P.). The most recent rupture of the Shepherds Gully/Pukerua fault is probably older than that of the Ohariu fault. The apparent nonclustering of surface rupture earthquakes in the Wellington region has been documented only for the on‐land strike‐slip faults. There are other possible seismogenic sources in the region, and thus important issues remain to be addressed regarding the history of large earthquakes in the Wellington region: (1) the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath Wellington is not known; (2) the timing of rupture events on the offshore portion of the Wairau fault is not known; and (3) paleoseismic data are not available for the section of the Wellington fault north of the Wellington‐Hutt Valley segment. Estimates of earthquake hazard in the Wellington region, for all return times greater than 50 years, that incorporate paleoseismicity data are between one and two Modified Mercalli (MM) intensity units higher than the hazard based solely on the historical seismicity catalog, and the hazard is spatially more variable. Using a deterministic attenuation model, the level of shaking hazard approaches near maximum values within a return time of ∼500 years, largely reflecting the recurrence interval (500–770 years) of surface rupture earthquakes on the Wellington fault. Inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of MM intensity in Wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years.