It means that the earthquake has been detected by our instruments, but it occurred far away. These locations cannot be considered reliable. Very poor locations are not posted to the website, but can still be extracted by Quake Search.
Then the event is very likely to have been an earthquake, but this has not been confirmed.
It means the earthquake has been reviewed by a seismologist. These locations are the most reliable. Currently our staff review all earthquakes of magnitude three or greater, as well as any smaller earthquakes that have been reported felt.
GeoNet currently uses SeisComP software for earthquake analysis but this wasn't always the case.
| Years | Location Algorithm | Software |
|---|---|---|
| 2012 - now | LocSAT and NonLinLoc | SeisComP |
| 1987 - 2011 | GROPE | CUSP and others |
| 1942 - 1986 | LOCAL | LOCAL and MICRO |
Earthquakes in GeoNet's catalogue prior to the twentieth century are not based on instrumental readings. These have been collated from Maori history and accounts in newspapers, books and diaries. By comparing shaking and damage reports from different towns and settlements, likely epicentres and magnitudes have been assigned. Geological evidence, such as the movement of surface faults or landsliding, can also improve the magnitude estimate, but only the very largest earthquakes leave a fault trace behind.
Even once instruments started to be installed in New Zealand after 1884, there were too few of them to comprise a network that allowed earthquakes to be located. It was not until the 1930s that the locations of stations started to form a network, and not until 1942 that it was complete enough to locate some earthquakes with any confidence.
Computers were first used in New Zealand to locate earthquake origins and determine magnitudes in 1964. Most earthquakes from 1942 to 1986 were reviewed using the LOCAL and MICRO software packages, described in Smith, W.D. (1979), Documentation for earthquake location programs LOCAL and MICRO, and supporting software. Technical note / Geophysics Division 80. Earthquakes from 1987 to 2011 used a modified version of the CUSP (Caltech-USGS Seismic Processing) system. Both of these packages relied on a seismologist to review each and every earthquake. Since 2012 GeoNet has used SeisComP, which fully automates the location of earthquakes and produces answers in minutes.
Origin times are given in Co-ordinated Universal Time (UTC, which is atomically-kept time, adjusted when necessary by one second steps ("leap seconds") to agree with the astronomically determined time known as UT1). For most purposes this may be regarded as Greenwich Mean Time.
GeoNet seismographs are equipped with GPS receivers for timing purposes. A temperature-compensated quartz crystal clock is synchronised regularly with GPS time. The GPS time is extremely precise (nanosecond accuracy) but the crystal clock may drift between synchronisations at a rate of a few milliseconds per day. Prior to the availability of GPS satellites for timing, New Zealand seismographs relied upon radio time-signals transmitted by Radio New Zealand.
What was the time in New Zealand when an earthquake occurred? The time zones used in New Zealand (except the Chatham Islands) are New Zealand Standard Time (NZST), and New Zealand Daylight Time (NZDT), which are defined in the Time Act, 1974. New Zealand Standard Time is 12 hours, and New Zealand Daylight Time 13 hours, ahead of UTC. Currently Daylight Time is in effect from the beginning of the year until 02:00 NZST on the first Sunday in April, and from 02:00 NZST on the last Sunday in September until the end of the year. If you want to know the local time for historical earthquakes, you can read a history of how the definition of New Zealand's time has evolved. The GeoNet web page for each quake shows the appropriate local time in force at the time of the earthquake.
The time observed in the Chatham Islands (CHAST and CHADT) is 45 minutes in advance of that currently in use in New Zealand (NZST and NZDT respectively), whilst all other New Zealand offshore islands and Scott Base (Antarctica) observe current New Zealand time.