ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉This article presents modified Mercalli intensity (MMI) data for the 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquake. These data include intensity levels above MMI 8 that have not been assigned previously. Two sources of data have been used in this research: GeoNet’s “Felt Classic” online questionnaires and felt reports gathered during a field study in Christchurch in February 2013. Taken together, these sets of data provided 331 valid (i.e., with all the needed information) felt reports in areas of MMI 8 or above, with 299 (90%) of the reports used to assign MMI levels above 8.This article presents a more detailed picture of the geographical damage distribution of this earthquake than has previously been available. The data differentiate damage in the center of Christchurch, with 8 communities assigned a community MMI (CMMI) of 9, 11 communities a CMMI of 10, and 8 communities a CMMI of 11, which is the maximum possible intensity in the New Zealand MMI scale, and a level of intensity not previously reported in New Zealand (〈a href="https://pubs.geoscienceworld.org/srl#rf6"〉Dowrick 〈span〉et al.〈/span〉, 2008〈/a〉).The geographical damage distribution for Christchurch has been updated for intensities below MMI 8. This was done using a recently developed method that groups intensity data and allows intensities to be aggregated for a community and a single value assigned. Comparisons between MMI and peak ground velocity using the CMMI data and two ground‐motion intensity correlation equations (GMICEs) indicate an underestimation of MMI when using the GMICEs and the need to review New Zealand’s GMICE.〈/span〉

Description: The Canterbury, New Zealand, earthquake sequence, which began in September 2010, occurred in a region of low crustal deformation and previously low seismicity. Because, the ensuing seismicity in the region is likely to remain above previous levels for many years, a hybrid operational earthquake forecasting model for Canterbury was developed to inform decisions on building standards and urban planning for the rebuilding of Christchurch. The model estimates occurrence probabilities for magnitudes M ≥ 5.0 in the Canterbury region for each of the next 50 yr. It combines two short-term, two medium-term and four long-term forecasting models. The weight accorded to each individual model in the operational hybrid was determined by an expert elicitation process. A retrospective test of the operational hybrid model and of an earlier informally developed hybrid model in the whole New Zealand region has been carried out. The individual and hybrid models were installed in the New Zealand Earthquake Forecast Testing Centre and used to make retrospective annual forecasts of earthquakes with magnitude M 〉 4.95 from 1986 on, for time-lags up to 25 yr. All models underpredict the number of earthquakes due to an abnormally large number of earthquakes in the testing period since 2008 compared to those in the learning period. However, the operational hybrid model is more informative than any of the individual time-varying models for nearly all time-lags. Its information gain relative to a reference model of least information decreases as the time-lag increases to become zero at a time-lag of about 20 yr. An optimal hybrid model with the same mathematical form as the operational hybrid model was computed for each time-lag from the 26-yr test period. The time-varying component of the optimal hybrid is dominated by the medium-term models for time-lags up to 12 yr and has hardly any impact on the optimal hybrid model for greater time-lags. The optimal hybrid model is considerably more informative than the operational hybrid model at long time-lags, but less so when the period of the Canterbury earthquakes is excluded from the tests. The results highlight the value of including medium-term models and a range of long-term models in operational forecasting. Based on the tests carried out here, the operational hybrid model is expected to outperform most of the individual models in the next 25 yr.

Description: Potential earthquake sources are revealed by the locations of earthquakes in historical and instrumental catalogs and of geologically mapped faults, including plate boundaries. We derive a set of multiplicative hybrid earthquake likelihood models that combine earthquake and fault data for the New Zealand region. In these models, the cell rates in a spatially uniform baseline model are scaled using selected subsets of five covariates derived from the magnitudes and locations of past earthquakes, the location of the boundary between the Australian and Pacific plates, and the location and slip rate of mapped faults. The hybrid model parameters are optimized for earthquakes of M  5 and greater over the period 1987–2006 and tested on earthquakes from the period 2007–2014. No updating of models is undertaken during the fitting or testing period, but we consider two cases of the earthquake-based covariates in the tests: (1) all data prior to 1987 and (2) all data prior to 2007, respectively. Hybrids containing the earthquake-based covariates perform better in the latter case. The most informative hybrid models in the fitting and testing period are composed of three and four covariates, respectively, including both earthquake- and fault-based variables. Proximity to mapped faults is overall the most informative individual covariate. These results can be used to inform better modeling of long-term earthquake occurrence rates for probabilistic seismic-hazard analysis.

Description: 〈span〉〈div〉ABSTRACT〈/div〉This article presents modified Mercalli intensity (MMI) data for the 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquake. These data include intensity levels above MMI 8 that have not been assigned previously. Two sources of data have been used in this research: GeoNet’s “Felt Classic” online questionnaires and felt reports gathered during a field study in Christchurch in February 2013. Taken together, these sets of data provided 331 valid (i.e., with all the needed information) felt reports in areas of MMI 8 or above, with 299 (90%) of the reports used to assign MMI levels above 8.This article presents a more detailed picture of the geographical damage distribution of this earthquake than has previously been available. The data differentiate damage in the center of Christchurch, with 8 communities assigned a community MMI (CMMI) of 9, 11 communities a CMMI of 10, and 8 communities a CMMI of 11, which is the maximum possible intensity in the New Zealand MMI scale, and a level of intensity not previously reported in New Zealand (〈a href="https://pubs.geoscienceworld.org/srl#rf6"〉Dowrick 〈span〉et al.〈/span〉, 2008〈/a〉).The geographical damage distribution for Christchurch has been updated for intensities below MMI 8. This was done using a recently developed method that groups intensity data and allows intensities to be aggregated for a community and a single value assigned. Comparisons between MMI and peak ground velocity using the CMMI data and two ground‐motion intensity correlation equations (GMICEs) indicate an underestimation of MMI when using the GMICEs and the need to review New Zealand’s GMICE.〈/span〉

Description: Since 2014, research groups from Japan, Taiwan, and New Zealand have been collaborating to share research ideas and expertise about topics relevant to the development of national seismic hazard models (NSHMs) in each region. The trilateral collaboration is centered on a series of workshops designed to bring the researchers together. So far, three such meetings have been held: a kick-off workshop in Taiwan in May 2014, a second workshop in Japan in August 2015, and a third workshop in New Zealand in November 2015. A fourth workshop is planned for late 2016. In this focus section on the joint Japan–Taiwan–New Zealand NSHM, we have collected eight papers that highlight some of the ongoing research that specifically targets improvements in the respective NSHMs.

Description: Earthquake-hazard models are one of the major contributions provided by the seismological community to tangibly support disaster risk reduction policies at a national level. Although the societal impact of hazard analyses can be huge, the development of models remains a scientific activity developed within frameworks that have a strong national emphasis and partial international recognition. However, broad acceptability of hazard models is a key aspect for achieving authoritativeness and a prerequisite for warranting that their construction is completed using well-recognized methodologies. As part of an enduring international collaboration between leading organizations operating in the hazard and risk fields in Japan, New Zealand, Taiwan, and the Global Earthquake Model initiative, we discuss the main characteristic of the earthquake source models—as implemented for the OpenQuake engine—used for the calculation of the most recent national seismic-hazard maps of these three countries. Particular emphasis is placed on comparing the various modeling options adopted in the different tectonic regions, on emphasizing commonalities, and on discussing the most controversial modeling solutions. Despite the many connections from a seismotectonic point of view between the three countries, the comparison highlights different modeling choices for the various tectonic regions, which constitute a spectrum of possible epistemic uncertainties, as well as modeling issues that could be collectively explored in future phases of this international collaboration.

Description: We developed a hybrid time-dependent seismic-hazard model for application to the recovery of Canterbury, New Zealand, following the Canterbury earthquake sequence. We combined earthquake-clustering models of three timescales (short-term, medium-term, and long-term) to develop a model that accounts for the significant epistemic uncertainty in the earthquake rates. For the probabilistic seismic-hazard calculations, these models were coupled with two ground-motion prediction equations. The weights for the construction of the hybrid model and for the logic tree were obtained using a structured expert elicitation process. Forecasts from the model have been used to provide earthquake probabilities to a range of end users on timescales from 1 day to 50 years. Additionally, the 50-year hazard forecast has been used in the revision of the New Zealand building design guidelines and other aspects of the reconstruction of Christchurch.

Description: Recent damaging earthquakes in New Zealand ruptured faults that were not known to be active. We analyzed New Zealand historical moderate-to-great magnitude earthquakes since 1845 ( M w  6–8.2) to estimate the level of completeness of earthquake fault sources in the National Seismic Hazard Model (NSHM) and the paleoseismic record. Our analysis assumes that the historical earthquakes are representative of the paleoseismic record and, due to the small number of events (45 M w ≥6), is qualitative. We find that about half of all historical earthquakes M w ≥7.0 ruptured 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 M w 〈7.3 and either did not displace the ground surface or were located in areas where the rates of erosion or burial exceed fault-slip rates. Incompleteness of active-fault sources in the present NSHM is the greatest for earthquake fault sources with long recurrence intervals of ≥10,000 yr. These inferred unidentified active faults will, in many cases, be located in low strain-rate areas, where they may make an important contribution to the seismic-hazard budget.

Description: The 2016 M w  7.8 Kaikōura earthquake continued a notable decade of damaging earthquake impacts in New Zealand. The effects were wide ranging across the upper South Island, and included two fatalities, tsunami, tens of thousands of landslides, the collapse of one residential building, and damage to numerous structures and infrastructure. We present a preliminary overview focused on the seismological aspects of this earthquake and the corresponding seismological response effort. The earthquake rupture was extremely complex, involving at least 13 separate faults extending over ~150 km, from the epicenter in north Canterbury to near the Cook Strait. We use backprojection and slip inversion methods to derive preliminary insights into the rupture evolution, identifying south-to-north rupture, including at least three distinct southwest-to-northeast propagating phases. The last phase is associated with a strong second pulse of energy release in the northern half of the rupture zone ~70 s after rupture initiation, which we associate with the Kekerengu–Needles faults where some of the largest surface displacements (dextral) were observed. The mechanism of the mainshock was oblique thrust and relocated aftershocks show a range of thrust and strike-slip mechanisms across three dominant spatial clusters. GeoNet datasets collected during the Kaikōura earthquake will be crucial in further unraveling details of the complex earthquake rupture and its implications for seismic hazard. Ground motions during the earthquake exceeded 1 g at both ends of the rupture. Spectral accelerations exceeded 500-year return period design level spectra in numerous towns in the upper South Island, as well as in parts of the capital city of Wellington at critical periods of 1–2 s, influenced by site/basin and directivity effects. Another important part of the response effort has been the provision of earthquake forecasts, as well as consideration of the implications of slow slip on the Hikurangi subduction interface triggered as a result of the Kaikōura earthquake.