Reconnaissance of Western end of Surface Rupture at Hororata and GreenDale

Wednesday 9-20

It’s hard to believe but this is Day 4 of the reconnaissance. The weather has turned a bit as we have some winds coming out of the west along with a bit of moisture. We can only hope that it will hold out a bit so that we can stay in the field and, more importantly, so that these fragile features are preserved for awhile.

Today we met with my good friend Mark Stirling, GNS Science, to head to the west end of the surface rupture in the vicinity of the Selwyn River and community of Hororata. This portion of the fault had large accelerations during the earthquake. 

We first visited several locations along the fault were roads and fence lines have been offset. Due to the enormous number of roads, fences, hedgerows and plow lines, there are tremendous opportunities to measure the amount of surface displacement along the rupture. The fault did not have uniform displacement along its length, but, instead had portions with large slip (> 4.5 m) and others that had along a fraction of that. We wanted to observe this range of slip if possible.

At Highfield, Kivers and Telegraph Roads the slip ranges from 3 to 4 m in a right lateral sense. Additionally, there appears to be vertical displacement on the order of up to 1 m. Near Hororata and the Selwyn River, horizontal displacements appear to be less than 1 to 2 m and vertical slip appeared negligible.

We visited the St. John’s Anglican church in Hororata. The church, built in 1910, was a beautiful, large cut-stone structure with a wooden frame ceiling and slate roof. Large accelerations from the earthquake toppled the bell tower into the alter portion of the church. We have heard it said many times while on South Island that it was good fortune that the earthquake didn’t occur during services because tons of stone crashed through the ceiling and destroyed the church. 

We then travelled to Greendale where the highest accelerations (that means the ground accelerating, much like a car suddenly moving from a dead stop) were recorded for the event. The vertical accelerations have been reported at 1.25 g (that is 1.25 times gravity). If the ground moves upward at 1.25 g then it becomes airborne which is a frightening thought. It should be noted that this only the highest REPORTED acceleration. The accelerometer is located in a power station that was displaced by the fault. To the credit of the engineers and builders of this power station it suffered little damage and was providing electricity not long after the earthquake.

We finally travelled eastward closer to the area of the fault where we GNS Science has been conducting the detailed LIDAR surveys. Near Courteney Road we observed a road that has about 4.5 m of displacement. A landowner invited us to look at the deformation on her farm and stated that they have observed the fault still moving after almost three weeks. We found that the paddock had a large (1 to 1.5 m-high linear mound on it with numerous fractures and faults that trended oblique to the mound. The landowner said that the cracks on the mound have been expanding fairly continuously. It appears that this may be the result of after slip, a phenomenon where the surface portion of the fault didn’t slip as much as the fault at depth but is slowly catching up. 

Aftershocks are a still a common occurrence. M 3 + earthquakes have been occurring several times a day since we’ve been here.

Tonight geodesists from GNS Science including John Beaven and Susan Ellis, bearing take-out Indian food and beer came to our motel room for dinner. They are reoccupying monuments all across the fault zone to try to measure its post-earthquake motion.

Tomorrow we will head into the area of Kaiapoi where significant liquefaction and lateral spreads from shaking-induced soil failures occurred. We also plan to travel into downtown Christchurch to observe the structural damage to buildings.