Wild Land

Seismologists are studying data to determine whether yesterday’s Darfield earthquake was actually two or more separate earthquakes.

Sunday 5th September 2010

Yesterday’s seventh magnitude earthquake in Canterbury, now called the Darfield earthquake, may have been the result of more than one earthquake happening in quick succession. Speculation that it was three earthquake events was expressed as early as yesterday afternoon, but much more research is required.

The U.S. Geological Survey has revised its initial report of the earthquake, reporting a foreshock of magnitude 5.3 centred 40 km west of Christchurch followed four seconds later by the main shock of magnitude 7.0 centred 45 km west of Christchurch. Both earthquakes were shallow, the first thought to be 12 km deep, the second at a depth of 5 km.

GeoNet, in its latest update on the earthquake still reports the quake as magnitude 7.1 centred 40 km west of Christchurch at a depth of 10 km. It notes that, “there are several parts to this earthquake occurring within seconds of each other and it will take some time to decipher what the waveforms recorded by our seismographs tell us about the sequence of events.” In a media release made on Saturday morning, the duty seismologist said that a foreshock of about magnitude 5.4 occurred a few seconds before the main event.

The fact that a magnitude five foreshock occurred a few seconds before the main quake has been taken as an explanation of why many members of the public reported waking up and hearing the rumble of the main quake “coming in” before they experienced the shaking. However, earthquakes generate two main waves of energy ”“ a fast moving primary (P) wave followed by a stronger and slower secondary (S) wave. The P and S are also handily used to describe experiencing the affects ”“ the P wave is felt as a jolt or push as it rapidly passes by and the S wave produces the shaking that can last seconds or in the case of large earthquakes, minutes.

People regularly report hearing an earthquake “approaching like a rumbling diesel locomotive” because the jolt of the P wave passing by has woken them while the slower S wave is still radiating out from the earthquake epicentre.

In the case of the Darfield quake, the existence of a four second foreshock may have magnified the experience for some observers, depending on where they were located with respect to the epicentre, waking them in time to remember more about experiencing the passage of the sharp P wave from the main earthquake. The strong shaking from the large earthquake would have followed.

[click for larger view] This snapshot, taken nearly two hours after the earthquake, shows the strong traces which appeared on GeoNet’s seismic network as a result of the Darfield quake and the first dozen or so aftershocks. Seismologists will be wading through the data from individual sites to look for evidence of a foreshock and main shock or a multiple earthquake event.

If the earthquake does turn out to be a multiple event, it may explain why one observer told Wild Land that they experienced “a rolling motion punctuated by sharp jolts.”

The discovery that the Darfield Earthquake was a multiple event would not be unique. Just last month it was revealed that detailed analysis of last year’s earthquake and tsunami that caused damage in Samoa and Tonga found it was actually the result of two earthquakes separated by a few minutes. The earthquakes of magnitude 8.0 and 7.9 were separated by two minutes and located 70 km apart.

Analysis of earthquake data takes place over many weeks and months. A quick assessment is made to assist emergency services plan for civil emergencies and analysis of more data then allows a fine-tuning of the size of the event. This is followed up by field observations, measurement of ground movement, analysis of reports from the public and close examination of data from as many seismic instruments as possible.

Geophysicists have already commenced the detailed study of the Darfield earthquake while GeoNet processes the information about the many aftershocks that have occurred. On Monday, portable Global Positioning Satellite (GPS) instruments will be deployed to known reference points in Canterbury to measure how much the land surface has changed. This will show whether a particular survey point has moved sideways since it was last measured, and whether the land has risen or fallen.

Portable seismographs will also be deployed to study the aftershock activity that is rattling the residents of the region.

Initial ground observation was undertaken on Saturday afternoon, just hours after the earthquake. This work has already discovered the trace of the previously unknown earthquake fault which was hidden by material laid down during the last period of glaciation, about 18,000 years ago.

The fault trace has broken through the soft material of the Canterbury Plains and can now be plainly seen running west to east for a distance of about 13 km. Roads, fences, tracks and irrigation channels have been disrupted along the trace of the earthquake fault, suggesting that the rupture has allowed the two sections of the plain to move to the right with respect to each other by a distance of about three metres.

Some sections of the rupture show a rise of up to a metre where one side of the fault has risen above the other. The fault passes directly below two houses, a farm shed, close to other buildings and under high voltage transmission lines.

The observation team have begun studying the the flooding which occurred when the Hororata River burst its banks, and have found a groundwater bore that is overflowing. The landslides reported in the Rakaia Gorge have also been investigated. These are comparatively small, comprising hundreds of cubic metres, and appear to be limited to steep cliff sections.

Whilst a fault movement of about 3 metres has been detected, the deformation of land further away from the rupture zone will be studied to learn how the shape of the Canterbury Plains has been altered by this event. It has already been found that the McQueen’s Valley GPS site on Banks Peninsula has moved west-north-west by about 135 mm.

GeoNet is maintaining one of its nifty running reports on the Darfield earthquake updating it as they find out more about its effects over the next few days. The report and photographs can be found here.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST; GNS Science media releases, and the US Geological Survey and its contributing agencies.]

This entry was posted on Sunday, September 5th, 2010 at 8:05 pm and is filed under Earthquakes - Observations. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.