ALBERT

Description: The Pegasus Bay aftershock sequence is the most recent aftershock sequence of the 2010 September 3 UTC moment magnitude ( M w ) 7.1 Darfield earthquake in the Canterbury region of New Zealand. The Pegasus Bay aftershock sequence began on 2011 December 23 UTC with three events of M w 5.4–5.9 located in the offshore region of Pegasus Bay, east of Christchurch city. We present a summary of key aspects of the sequence derived using various geophysical methods. Relocations carried out using double-difference tomography show a well-defined NNE–SSW to NE–SW series of aftershocks with most of the activity occurring at depths 〉5 km and an average depth of ~10 km. Regional moment tensor solutions calculated for the Pegasus Bay sequence indicate that the vast majority (45 of 53 events) are reverse-faulting events with an average P -axis azimuth of 125°. Strong-motion data inversion favours a SE-dipping fault plane for the largest event ( M w 5.9) with a slip patch of 18 km  x 15 km and a maximum slip of 0.8 m at 3.5 km depth. Peak ground accelerations ranging up to 0.98 g on the vertical component were recorded during the sequence, and the largest event produced horizontal accelerations of 0.2–0.4 g in the Christchurch central business district. Apparent stress estimates for the two largest events are 1.1 MPa ( M w 5.9) and 0.2 MPa ( M w 5.8), which are compatible with global averages, although lower than other large events in the Canterbury aftershock sequence. Coulomb stress analysis indicates that previous large earthquakes in the Canterbury sequence generate Coulomb stress increases for the two events only at relatively shallow depths (3–5 km). At greater depths, Coulomb stress decreases are predicted at the locations of the two events. The trend of the aftershocks is similar to mapped reverse faults north of Christchurch, and the high number of reverse-faulting mechanisms suggests that similar reverse-faulting structures are present in the offshore region east of Christchurch.

Description: In this study, a new local magnitude ( M L ) scale is developed for New Zealand and adjacent offshore regions. SeisComP3 (SC3) has been in use for earthquake analysis in New Zealand since September 2012 with the original Richter (1935) log A 0 attenuation relationship for calculating M L . The attenuation characteristics of New Zealand differ significantly from southern California, where M L was originally defined, and therefore result in M L that is consistently high when compared with moment magnitude ( M w ). Using M w from 528 regional moment tensor solutions along with peak observed amplitudes, a new log A 0 curve is derived, along with station correction factors that define a revised M L scale for New Zealand earthquakes that is more consistent with M w . The new log A 0 curve is similar to the original Richter (1935) definition at hypocentral distances of ~100–200 km but differs significantly at closer and farther distances. The new M L is more consistent with M w across New Zealand, including crustal earthquakes and earthquakes below the crust. The California Institute of Technology (Caltech)–U.S. Geological Survey seismic processer (CUSP) system was used for earthquake analysis prior to SC3, and previous studies have derived regression relationships relating CUSP M L with M w . After applying the regression relationships to CUSP M L , we found very good agreement between CUSP M L and the new SC3 M L , which is important for developing a consistent M L between different catalogs. Online Material: Table of station corrections for the New Zealand seismograph network.

Description: New Zealand is one of the more seismically active countries in the world with over 15,000 earthquakes each year in New Zealand and the adjacent offshore region. Routine regional moment tensor (RMT) analysis was implemented by GeoNet in 2007, and nearly 1300 RMT solutions have been calculated for the New Zealand region from August 2003 through early September 2012. The New Zealand RMT catalog contains events with moment magnitude ( M w ) 3.2–7.3 and centroid depth 2–375 km, and includes RMT solutions from aftershock sequences for two major earthquakes: 15 July 2009 M w  7.8 Dusky Sound and 3 September 2010 M w  7.1 Darfield. The RMT solutions provide important constraints on the active tectonics of New Zealand including stress and strain, and the RMT catalog, along with 155 Global Centroid Moment Tensor Project solutions, is used to examine the relationship between M w and local magnitude ( M L ). For shallow focus (≤33 km depth) the M L / M w relationship is found to be similar across New Zealand except for the Fiordland region. For all of New Zealand except Fiordland, M L =(0.93±0.06) M w +(0.54±0.24), and for Fiordland, M L =(0.83±0.04) M w +(1.09±0.13). For deep-focus earthquakes (〉33 km depth) M L is consistently larger than M w by more than a full magnitude unit for some events. The discrepancy between M L and M w is found to be depth dependent with ( M L – M w )=–0.94exp(– h /75.37)+0.80, where h is focal depth in km.

Description: In this study, a new local magnitude ( M L ) scale is developed for New Zealand and adjacent offshore regions. SeisComP3 (SC3) has been in use for earthquake analysis in New Zealand since September 2012 with the original Richter (1935) log A 0 attenuation relationship for calculating M L . The attenuation characteristics of New Zealand differ significantly from southern California, where M L was originally defined, and therefore result in M L that is consistently high when compared with moment magnitude ( M w ). Using M w from 528 regional moment tensor solutions along with peak observed amplitudes, a new log A 0 curve is derived, along with station correction factors that define a revised M L scale for New Zealand earthquakes that is more consistent with M w . The new log A 0 curve is similar to the original Richter (1935) definition at hypocentral distances of ~100–200 km but differs significantly at closer and farther distances. The new M L is more consistent with M w across New Zealand, including crustal earthquakes and earthquakes below the crust. The California Institute of Technology (Caltech)–U.S. Geological Survey seismic processer (CUSP) system was used for earthquake analysis prior to SC3, and previous studies have derived regression relationships relating CUSP M L with M w . After applying the regression relationships to CUSP M L , we found very good agreement between CUSP M L and the new SC3 M L , which is important for developing a consistent M L between different catalogs. Online Material: Table of station corrections for the New Zealand seismograph network.

Description: We calculate stress drops for 176 earthquakes (M2.6–M6.6) from four sequences of earthquakes in New Zealand. Two sequences are within the subducting Pacific plate (2014 Eketahuna and 2005 Upper Hutt), one in the over-riding plate (2013 Cook Strait) and one involved reverse faulting at the subduction interface (2015 Pongaroa). We focus on obtaining precise and accurate measurements of corner frequency and stress drop for the best-recorded earthquakes. We use an empirical Green's function (EGF) approach, and require the EGF earthquakes to be highly correlated (cross-correlation ≥ 0.8) to their respective main shocks. In order to improve the quality, we also stack the spectral ratios and source time functions obtained from the best EGF. We perform a grid search for each individual ratio, and each stacked ratio to obtain quantitative uncertainty measurements, and restrict our analysis to the well-constrained corner frequency measurements. We are able to analyse both P and S waves independently and the high correlation between these measurements strengthens the reliability of our results. We find that there is significant real variability in corner frequency, and hence stress drop, within each sequence; the range of almost 2 orders of magnitude is larger than the uncertainties. The four sequences have overlapping stress drop ranges, and the variability within a sequence is larger than any between different sequences. There is no clear systematic difference in the populations analysed here with tectonic setting. We see no dependence of the stress drop values on depth, time, or magnitude after taking the frequency bandwidth limitations into consideration. Small-scale heterogeneity must therefore exert a more primary influence on earthquake stress drop than these larger scale factors. We confirm that when fitting individual spectral ratios, a corner frequency within a factor of three of the maximum signal frequency is likely to be underestimated. Stacked ratios are smoother and more reliable near the frequency limits. We find that only corner frequencies within about a factor of two of the maximum signal frequency are likely to be underestimated.

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.

Description: Local magnitude (M (sub L) ) values of earthquakes off Canada's west coast are known to be underestimated by at least 0.5 magnitude units compared with other magnitude scales. Moment magnitude (M (sub W) ), derived from moment tensor analysis, provides the most robust estimate of the magnitude of earthquakes. Moment tensor analysis of regional seismic data in western Canada is now possible due to the installation of more than 40 three-component broadband stations in western Canada, the U.S. Pacific Northwest, and southeast Alaska. Moment tensor solutions are now possible down to M approximately 4.0. More than 230 regional moment tensor solutions have been calculated off Canada's west coast at the Geological Survey of Canada for 1995-2002. These solutions, along with 14 previous solutions by Oregon State University in 1994-1995 and 13 Harvard solutions for 1984-1993, allow a systematic M (sub W) -M (sub L) calibration for earthquakes in this region. The study area extends from the Queen Charlotte Islands region in the north to the area off the west coast of southern Vancouver Island. At the northern end of the study area, where there is little oceanic crust in the source-receiver travel path, M (sub W) is systematically larger than M (sub L) by 0.28+ or -0.08 magnitude units. At the southern end of the study area, where there is a significant amount of ocean crust in the source-receiver travel path, M (sub W) is systematically larger than M (sub L) by 0.62+ or -0.08 magnitude units. Calibration of M (sub L) with M (sub W) will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.