RAPID: Recording Fault-Zone Trapped Waves from Aftershocks of the M6.3 Christchurch Earthquake Sequence in New Zealand to Document the Subsurface Damage Zones

University Of Southern California, Los Angeles CA

Investigators

Abstract

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.