The M6.3 Christchurch earthquake struck the Canterbury region in New Zealand's South Island on 22 February 2011, causing widespread damage and multiple fatalities. The earthquake occurred at 5 km depth, 10 km south-east of Christchurch, New Zealand's 2nd-most populous city. It followed ~6 months after the Sept. 4, 2010 M7.1 Darfield earthquake in the same region. The Christchurch earthquake was part of a series of earthquakes and aftershocks in the region following the 2010 M7.1 Darfield. New Zealand's GNS Science describe it as "technically an aftershock" of the earlier event while other seismologists from USA and Australia consider it a separate event, given its location on a separate fault system. Seismologically, this M6.3 quake is classed as an aftershock because of its relationship to the ongoing activity since September last year. However, it has generated a significant series of its own aftershocks, many of which are considered big for a M6.3 earthquake. Over 200 aftershocks including a M5.7 were experienced in the first week. It did not occur on the Greendale Fault, on which the 2010 M7.1 Darfield quake occurred, but on a previously unknown blind fault line running 17 km east-west south of Christchurch, at depths of 3?12 km. On the contrary, precise aftershock relocations suggest that at least two north-east/south-west trending faults lie between the two and that there is no evidence from the earthquake data of an extension of the Greendale Fault. However, there are still many aftershocks of the 2010 M7.1 Darfield earthquake spread along the fault line of the 2011 M6.3 Christchurch earthquake.
In order to document the complicated subsurface structure of the damage zones caused by the sequence of the 2010 M7.1 Darfield and the 2011 M6.3 Christchurch earthquakes in the Canterbury region of NZ?s South Island, the investigator proposes to record fault-zone trapped waves (FZTWs) generated by aftershocks, and use the FZTWs to image the rupture zones composed by damaged fault rocks at seismogenic depths. Because the amplitude and dispersive feature of FZTWs are sensitive to the geometry and physical properties, and the location of aftershocks (within or outside) of the low-velocity fault-zone waveguide formed by severely damaged rocks in these two earthquakes, observations and numerical modeling of recorded FZTWs allow us to learn more about (1) the width, velocity reduction, Q value and depth extension of damage zones of the 2010 M7.2 and 2011 M6.3 earthquakes, (2) the shape of subsurface rupture zones with the principal slip plans of the mainshocks, and if the two rupture segments are connected at seismogenic depths, (3) the difference in rock damage magnitude caused by these two earthquakes occurring at different depths with different sizes, (4) the fault healing with time after the mainshock.This RAMP experiment will be collaborated by USC/UoA /UoC/GNS in NZ to pursue the value of the work proposed, likewise with the ongoing efforts by the NZ national GEONET seismic network and researchers. GNS will offer to follow the USC/UoA/UoC work and help them keep up with other initiatives and projects concerning the Darfield event.
The M6.3 Christchurch earthquake struck the Canterbury region in New Zealand's South Island on 22 February 2011, following ~6 months after the Sept. 4, 2010 M7.1 Darfield earthquake in the same region. In order to document the complicated subsurface structure of damage zones caused by multiple slips in this earthquake sequence, we deployed two linear arrays of 12 PASSCAL seismometers, respectively, across the surface breaks along the Greendale fault (GF) which ruptured in the Darfield earthquake and the surface projection of aftershock zone along the blind Port Hills fault (PHF) which ruptured in the Christchurch earthquake but did not expose at the ground surface. We recorded the data at our arrays for more than 1,000 aftershocks, including four M5+ quakes between May and September in 2011. We have systematically examined waveform data for 853 aftershocks and identify prominent fault-zone trapped waves (FZTWs) with large amplitude and long wavetrains following S-arrivals at stations of Greendale array within the ~200-300-m-wide rupture zone for on-fault aftershocks occurring along the GF and the PHF. The post-S durations of FZTWs generated by on-fault aftershocks increase but with varying increasing rate as travel distances increase along the fault strike and with depth. Extensive observations of these FZTWs show (1) an effective low-velocity waveguide formed by severely damaged rocks extends total ~55 km along the GF and PHF, but with variations in fault geometry, velocity and rock damage magnitude along multiple rupture segments, particularly in the dilatational step-over where the cumulated energy release in this earthquake sequence is minimum. (2) while the 2010 M7.1 Darfield mainshock initiated on a NNW-SSE trending blind fault, the main rupture ran along the E-W Greendale fault with its surface breaks in a total length of ~30-km and likely penetrated to the depth of at least ~8-10 km. Its easternmost portion bifurcated into two blind segments extending further ~5-8 km eastward beneath the dilatational step-over. (3) The main rupture zone of the 2011 M6.3 Christchurch earthquake is ~15-km long along the blind Port Hills fault dipping to SSE. Its west extension formed by moderated damaged ran beneath the dilatational step-over approaching to the easternmost branch of Darfield rupture zone. (4) The rupture segment of the largest aftershock of M6 is along a NNW-SSE conjugate fault of the east portion of the blind PHF. We tentatively interpret that if a consequent M6+ aftershock happens in Canterbury Plain, it is more chance to occur east of Christchurch out to the sea rather than elsewhere along the GF and PHF which rocks have been severely damaged. We have used a test model for Darfield-Christchurch rupture zones formed by damage rocks to fit observed FZTWs, within which seismic velocities are reduced by ~35-50% in the 300-m-wide low-velocity waveguide containing a 75-m-wide damage core zone with the maximum velocity reduction. The waveguide geometry and velocity reduction (damage magnitude) vary along the fault strikes and with depth. This test model is primarily applicable, but we shall conduct a systematic modeling procedure using FZTWs generated by more aftershocks at different depths and epicentral distances to further refine our model for the complicated damage structure of the 2010-2011 Canterbury earthquake sequence as well as the temporal variations associated with co-seismic damage and post-mainshock heal of fault zone rocks. The spatial extent of fault-zone damage and the loss and recovery of strength across the earthquake cycle are critical ingredients in understanding of fault mechanics and physics. The present project is in a few cases to illuminate the complicated subsurface fault-zone rock damage structure on multiple rupture segments associated with multiple slips in the M7.1 Canterbury - M6.3 Christchurch earthquake sequence. The result from this research is helpful to address the question if the Christchurch quake is an aftershock of the Darfield earthquake or an individual mainshock. Our research provides a potential approach to image the buried part of rupture zone using fault-zone trapped waves recorded at seismic array deployed at the surface-exposed part of rupture zone. This approach is also useful for investigation of the earthquake rupture zone in urban areas where the deployment of seismic array is difficult, but we can record FZTWs at seismic array deployed at the exposed rupture zone beyond the populated areas. Our research is collaborative with other groups working in Canterbury Plain.
