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The 2016 Kaikōura (New Zealand) earthquake generated large ground motions and resulted in multiple onshore and offshore fault ruptures, a profusion of triggered landslides, and a regional tsunami. Here we examine the rupture evolution using two kinematic modeling techniques based on analysis of local strong‐motion and high‐rate GPS data. Our kinematic models capture a complex pattern of slowly (Vr < 2 km/s) propagating rupture from south to north, with over half of the moment release occurring in the northern source region, mostly on the Kekerengu fault, 60 s after the origin time. Both models indicate rupture reactivation on the Kekerengu fault with the time separation of ~11 s between the start of the original failure and start of the subsequent one. We further conclude that most near‐source waveforms can be explained by slip on the crustal faults, with little (<8%) or no contribution from the subduction interface.

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A moment magnitude (M w) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealand's second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 M w 7.8 Hawkes Bay earthquake and the most expensive in New Zealand's recorded history. The effects of the earthquake on the region's population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the M w 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeastÁsouthwest-striking fault dipping southeast at c. 698, with maximum slip at 3Á4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

S U M M A R YUsing the boundary integral method to simulate SH waves numerically in 2-D homogeneous full-or half-space media with randomly distributed cavities, we compare the amplitude attenuation of direct waves with the temporal decay of the coda. The boundary integral method includes the effect of any degree of multiply scattered waves for a wide frequency range, up to wavelengths smaller than the size of the cavities. We consider seismograms on the free surface so that heterogeneities exist only on one side of the receivers, a situation that resembles actual seismic observations. Seismograms are computed for a vertically incident plane wave and for an isotropic line source. In both cases, the value of Q-' as a function of kd, where k is the wavenumber and d is the cavity diameter, peaks around kd = 2 for the direct wave, which is consistent with some single-scattering models. Coda Q-' determined by the temporal decay of the coda envelope agrees well with Q-' for the direct wave for models with a root-meansquare fluctuation of velocity, Q, of about 10 per cent in a half-space. On the other hand, the coda Q-' is systematically larger than the direct wave Q-' in full-space models, that is, without the inclusion of the reflection at the free surface. When the cavity density is doubled (a>20 per cent), the coda energy increases rapidly and its temporal decay decreases, so that coda Q-' becomes smaller than the direct wave Q-', even for full-space models. With a smaller value of cr (about 5 per cent), the coda decays rapidly and the relation between the two types of Q-' is reversed: the coda Q-' becomes larger than the direct wave Q-'. By comparing results from seismograms composed only of singly scattered waves with those that include multiply scattered waves, we can compare the relative contribution of each singly and multiply scattered wavefield to the two measures of Q. Single scattering mainly determines both the direct wave Q-' and the coda Q-' for the smallest value of a, while the values of both kinds of attenuation, particularly the direct wave Q-', are strongly affected by multiple scattering when a is large. Our results imply that a reasonable estimate of scattering attenuation can be obtained by measuring the temporal decay of the coda, if the scattering character of the Earth is similar to our models with a Q of around 10 per cent, where the single scattering is found to be dominant.