Q&A: Revealing the threat that lies beneath

Linesmen prepare to repair damaged power lines along a cracked-open road at Darfield following the 7.1 quake that struck near there in September 2010. Photo: File

The web of active faults beneath us pose an ever-present threat to lives and property across the country. A team of GNS Science researchers are constantly updating a database that now contains more than 300 known faults, in what's designed as a user-friendly tool for planners, policymakers and the property sector. Science reporter Jamie Morton discussed the project with GNS earthquake geologist Dr Rob Langridge.

Q. Can faults be found in most regions of the country, or just those immediately near the boundary of the Australian and Pacific plates?

A. Active faults have been mapped in nearly every region of New Zealand.

Using our definition an active fault is one that has ruptured the ground surface during the past 125,000 years which is of course more time than most of us think about on a daily basis. Northland is the only region that has no mapped active faults and this is because it is too far away from the tectonic plate boundary.

Q. What is the active faults database, why was it originally created and how is it regularly updated?

A. The New Zealand Active Faults database was originally created to catalogue all of the active faults throughout New Zealand.

The ground surface deformation that occurs when a large earthquake ruptures an active fault is recognised as a natural geological hazard that can damage buildings and infrastructure.

The database is typically updated when new fault mapping comes to light.

Q. Why was the database recently relaunched?

A. GNS Science relaunched a version of the database at a uniform map scale of 1:250,000 to be compatible with other databases and to provide a consistent summary version that is publicly available.

Q. Before the database was started, how was information on new and known faults collected and kept?

A. Prior to the advent of digital data in GIS, active fault information was archived on paper maps.

GNS still holds those original maps, though in many cases the fault linework has been more recently updated.

The New Zealand Active Faults Database was designed as a handy tool for planners and policymakers. Photo: GNS Science

Q. How is the relaunched database proving useful to planners and policymakers? How can ordinary people use it?

A. The 1:250,000 scale version of the NZ Active Faults database can now be freely downloaded from the GNS website making it available to everyone worldwide.

The scale of the database means that councils, planners, scientists and landowners can get an immediate idea of what active faults occur in their region or area and what is known about specific faults, but is not suitable for use for site-specific purposes.

Q. How are new faults identified? Does a large quake have to happen for them to reveal themselves?

A. Most of the significantly active faults, for example those that have ruptured during the past 10,000 years, have been mapped already.

This is in part because there are many landscapes in New Zealand that have formed during or since the last ice age.

Therefore, if a fault has moved at least once since that time it will have left an active fault trace or scarp that can be mapped from aerial photos or in the field.

Peter Fitzgerald, left, and his brother Rex, stand in a crack where Highfield Rd was lifted and wrenched sideways by the 7.1 earthquake near Darfield, Canterbury, on September 4, 2010. Photo: File

Q. The database contains information on around 300 known on-land faults. How many more are there that we don't know about - and if there's a much larger proportion that haven't been mapped yet, does this affect the reliability of the database?

A. Our knowledge or completeness in terms of active faults is improving all of the time.

There are certainly other active faults that are out there in the landscape, though they may not have been mapped yet if they are associated with smaller earthquakes - typically less than magnitude 7 - or if they have a repeat time for ruptures of more than 10,000 years, as the Greendale Fault (magnitude 7.1 Darfield earthquake) has, or if their fault traces have been eroded away, such as in high erosion areas such as the Southern Alps.

Q. Are there any efforts to include more off-shore faults?

A. More offshore faults could be included, but one of main purposes of the NZ Active Faults database is to identify the hazard of ground deformation to built structures.

Therefore, in the case of offshore faults there is little hazard in offshore areas.

Offshore faults are built in to other databases such as the National Seismic Hazard Model which accounts for the seismic shaking from all faults in the New Zealand setting.

Q. With future advances in technology, can we expect our ability to find new faults will be boosted?

A. Yes, for example, the advent of airborne LiDAR (Light Detection and Ranging) has allowed us to identify new faults in the landscape and to map existing faults more accurately.

LiDAR combined with GIS tools provide a huge step-up in topographic accuracy from traditional techniques that used aerial photographs and hard copy maps.