Quantifying the incompleteness of New Zealand's prehistoric earthquake record
Authors: Professor A Nicol, R J Van Dissen, Professor M W Stirling, Dr M C Gerstenberger
Paper number: 49 (EQC 14/668)
Abstract
Many damaging earthquakes in New Zealand's historical period have occurred on faults that were previously unknown, or not known to be capable of generating earthquakes. Historical moderate to great magnitude earthquakes (since 1845; Mw 6-8.2) and on-land active faults have been analysed to estimate how many earthquake sources we could be missing in the National Seismic Hazard Model of New Zealand. Based on today’s understanding we estimate that about half of the active faults capable of generating future earthquakes of greater than or equal to magnitude 7.0 may not be explicitly identified in the hazard model. The majority of historical earthquakes on faults not previously identified as capable of generating future large earthquakes were less than magnitude 7.3, although historical events up to at least magnitude 7.7 have occurred on faults we did not know about. Our record of prehistorical events is not complete because some earthquakes did not rupture the ground surface (and remain undetected by surface investigations), while other earthquakes that did break the ground surface are often difficult to identify because their traces are eroded or buried. Outside of the Taupo-Whakatane region in the central North Island, no historical earthquakes with magnitudes less than 7.0 produced detectable rupture of the ground surface. Therefore, throughout much of onshore New Zealand the prehistoric earthquake record primarily provides information for large earthquakes of magnitude 7.0 and above. Removal or burial of ground-surface displacement features produced during earthquakes is most likely to occur on mountain ranges and young alluvial plains where the rates of erosion and sedimentation are comparatively high. The active-fault earthquake sources most likely to be missing from the present National Seismic Hazard Model have long recurrence intervals or repeat times between earthquakes of ≥10 000 yr. In onshore New Zealand we estimate that at least 140 active-fault earthquake sources with recurrence intervals of ≥10 000 yr are needed to be added to the National Seismic Hazard Model to account for the observed number of historical ground rupturing earthquakes. These ‘missing’ active faults will, in many cases, be located in areas with low rates of earthquake activity and have been accounted for by the background seismicity model of the National Seismic Hazard Model. Ongoing work is required to ensure that faults that cannot be identified through conventional geological investigations are adequately accounted for in our seismic hazard analysis.
Technical Abstract
Recent damaging earthquakes in New Zealand ruptured faults that were previously unknown, or not known to be earthquake sources. Historical moderate to great magnitude earthquakes since 1845 (Mw 6-8.2) and terrestrial active faults have been analysed to estimate the level of completeness of earthquake fault sources in the National Seismic Hazard Model of New Zealand. About half of Mw ≥ 7.0 earthquakes rupture faults that, based on today’s state of knowledge of active fault locations, would not have been identified as active prior to the event. The majority of historical events on faults previously not identified as active were Mw <7.3, however, events up to at least Mw 7.7 are possible. Incompleteness of the active-fault record arises because not all earthquakes rupture the ground surface, and surface-ruptures for low slip rate faults (e.g., <1 mm/yr) with longer recurrence intervals (e.g. > 10 000 yr) can be eroded or buried. Outside of the Taupo Rift in the central North Island, no historical events of Mw <7.0 produced resolvable rupture of the ground surface. Therefore, throughout much of onshore New Zealand, paleoearthquake data from active fault traces are primarily providing information about Mw ≥ 7.0 events. Removal or burial of active fault scarps is most likely in areas where the rates of erosion or burial exceed fault-slip rates (e.g., in mountain ranges and young/active alluvial plains). Incompleteness of active fault sources in the present National Seismic Hazard Model is greatest for earthquake fault sources with long recurrence intervals of ≥10 000 yr. Onshore in New Zealand many (≥140) additional unidentified active faults capable of generating Mw ≥ 7 earthquakes and with recurrence intervals of ≥10 000 yr need to be added to the National Seismic Hazard Model in order to reconcile with the historical earthquake catalogue These inferred unidentified active faults will, in many cases, be located in low strain rate areas where they may make an important contribution to seismic hazard. Our observations emphasise the importance of identifying seismogenic active faults in key parts of New Zealand (e.g., within or close to major urban areas) and of continuing efforts to develop good background seismicity models which complement paleoseismic datasets at magnitudes of Mw >7.
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