GNS Science is involved in monitoring, modeling, and community resilience for earthquakes; and has intensified its monitoring of seismic activity in Canterbury since the September earthquake. Neither the September quake nor the 6.3 magnitude earthquake on February 22 was on the main south island "alpine fault", occurring instead on previously unmapped subsidiary faults.
GNS Science increases understanding of earthquakes by identifying faults and measuring tectonic-induced movement in the earth's crust. It also describes earthquake locations and investigates the behaviour of New Zealand's many on-land faults. Permanent earthquake monitoring networks include both seismographs and strong-motion sensors. Seismographs measure the magnitude, location and characteristics of earthquakes. Strong-motion sensors monitor buildings, bridges and infrastructure to ascertain how structures perform in earthquakes. A strong-motion sensor measures ground acceleration, and is not as sensitive to ground movements as are normal seismic instruments. However, they remain operating during the strongest seismic shaking.
In addition, GNS Science operates a national network of GPS stations that measure slow micro-movement in the landscape due to tectonic forces. These instruments help to pinpoint where strain is building up or being released in the Earth's crust.
Modeling work predicts the likely effects of large earthquakes on the community. This underpins their work in community resilience, "developing design requirements and engineering solutions to protect buildings and infrastructure, and improve survival rates".
John Callan, spokesperson for GNS Science, says the Canterbury monitoring network consisted of about 30 strong-motion sensors and 12 seismographs prior to the September earthquake. An additional 20 to 30 temporary seismometers were added after the first earthquake. "And after the magnitude 6.3 quake of 22 February, we installed a dozen more instruments around Christchurch and the Port Hills," Callan stated. Denser instrument networks provide more precise information on the location, depth, and size of aftershocks.
GNS Science also provides the background seismology data that is incorporated in the New Zealand building code. At this stage, however, "it is too early to say" whether building codes will be reviewed and upgraded in the wake of the Christchurch earthquakes.
Mapping of South Island faults hindered by complex systems and geological complexity:
The alluvial river sediment overlying the tectonic plates in much of the South Island - particularly in the Canterbury region – hinders seismic mapping of the unknown faults, observes Dr Barry Brennan of University of Auckland Geophysics department. “There is a network of ‘blind faults’ hidden under the younger alluvial (river-carried) material”. Brennan further states that earthquakes behave as complex systems, much like the global climate does. ‘Complex’ systems exhibit “non-linearity” - large outputs for tiny inputs - the main consequence being hindering their predictability. For example a tiny shift of a plate boundary could have largely amplified consequences in a far-flung location. An implication of this, Brennan admits, is that there could be not just practical but also theoretical limits to the science of earthquake prediction. He agrees that a denser monitoring network will aid in prediction, however.
Christcurch company Roam3 is developing an early-warning system using the faster, non-destructive, P-waves from quakes to trigger mobile phone warnings 3 seconds prior to imminent activity. (As reported in Herald, March 10) Civil Defence rescue teams are currently trialing a prototype of the system, which they claim could ultimately give people several minutes warning of a rupture. Brennan agreed there was a benefit to be had from early monitoring systems.
No comments:
Post a Comment