G.T. Hancox, G. Dellow, M. McSaveney, B. Scott & P. Villamor
The Rotoehu earthquake swarm began on 18 July 2004 (NZ Standard Time) with the largest event being a 5 km deep, magnitude 5.4 earthquake centred near Lake Rotoehu ~30 km northeast of Rotorua. From 18-25 July 2004 at least 40 earthquakes occurred in the same area, including another two of M 5, seven of M 4 - M 4.9, and many other smaller events. During the largest (M 5.4 and M 5.0) earthquakes on 18 July 2004 significant landsliding and ground damage occurred over a ~70-300 kmarea in and around Lake Rotoehu and Lake Rotoma, with at least 100 landslides triggered. Most of these failures are very small (<10 m to small (10-10 m superficial disrupted soil slides and falls of unconsolidated pyroclastics and tephra deposits around steeper parts of the lake shorelines, and on road cuts greater than 3 m high. No large or deep-seated landslides in rhyolitic lava bedrock occurred during the earthquakes.
The largest landslides observed were extensive but shallow soil slumps and falls of up to ~3000-5000 m on steep (45-75°) bush-covered headlands on the western and northern sides of both lakes, within ~10 km of the epicentre of the M 5.4 earthquake. The many landslides that fell into Lake Rotoehu may have caused or at least contributed to the ~600 mm 'seiche' (wave) observed around the lake shore. Apart from damaging and temporarily closing some roads in the area, including SH 30, the landslides and cracking and collapses of road edge fills did little significant damage. However, landslides that undermined two power pylons located unnecessarily close to a steep lake-edge cliff at Lake Rotoehu had the potential to disrupt electricity distribution in the area. Only minor liquefaction effects occurred in a few places in highly susceptible fmc-grained sediments (saturated fine sands) around the shores of Lake Rotoehu and Lake Rotoma. Observed liquefaction features include a collapsed sand spit in Lake Rotoehu, and an underwater sand flow and sand boils, and ground cracking on the edge of Lake Rotoma. There was also an eye-witness report of a 'water spout' on Lake Rotoehu during theM 5.4 earthquake on 18 July 2004.
The area affected by landsliding during the 2004 Rotoehu earthquake fits well against the area/magnitude mean regression line for worldwide earthquake data, but is slightly above that for historical earthquakes in New Zealand. This is probably because slopes in the area were saturated when the 2004 earthquakes occurred, as the same slopes did not fail when shaken at least as strongly during the 1987 Edgecumbe earthquake. That earthquake occurred in summer when the slopes would have been drier and less susceptible to failure. Based on the type, size and number of landslides, and the minor soil liquefaction effects that occurred, the maximum Modified Mercalli (MM) felt intensity in the epicentral area during the two largest (M 5.4 and M 5.0) earthquakes of the swarm is estimated to have been about MM 7. This is generally consistent with the many felt intensity reports of MM 7 and few of MM 8 in the Lake Rotoehu and Lake Rotoma area.