Gisborne 2007 earthquake tectonics and strong-motion records. In the Pacific plate subducting beneath the North Island

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1 Gisborne 2007 earthquake tectonics and strong-motion records Graeme McVerry and GNS Hazards Group and GeoNet 20 December 2007 Gisborne earthquake Occurred at 8:55 pm (NZ local time) Centred offshore at 44 km depth Epicentral distance 47 km, hypocentral distance 64 km from Gisborne Magnitude M L 6.9 Moment magnitude M W December Earthquake epicentre In the Pacific plate subducting beneath the North Island Gisborne Tension from bending of subducting plate, rupture towards interface (& Gisborne) Comparatively small number of aftershocks Few aftershocks is consistent with rupture of subducted Pacific plate Largest aftershock M4.8 on December 22nd Stephen Bannister,

2 Aftershock zone defines probable rupture zone Aftershock zone was in the down-going Pacific Plate Top edge at 13 km horizontal distance and 28 km depth from Gisborne = 31 km shortest distance (hypocentral distance 64 km) Martin Reyners, TSUNAMI THE RULE IS If you feel strong earthquake shaking in a low-lying coastal area like Gisborne Assumed to represent fault plane of main shock Felt quite punchy in Gisborne because the direction of fault rupture was towards Gisborne Aftershocks direction of rupture SELF-EVACUATE Stephen Bannister, BECAUSE. FELT REPORTS Tsunami may arrive before the earthquake can be located and a warning issued 3245 felt reports submitted to the GeoNet website Help provide a detailed picture of the shaking Allow us to anticipate the level of shaking from similar earthquakes in the future Maximum intensity MMVIII =MM 8 (heavily damaging) =MM 7 (damaging) =MM 6 (slightly damaging) =MM 5 (strong) =MM 4 (largely observed) =MM 3 (weak)

3 Variation across three intensity units around Gisborne MMV1-VIII E D B C Gisborne =MM 8 (heavily damaging) =MM 7 (damaging) =MM 6 (slightly damaging) =MM 5 (strong) =MM 4 (largely observed) =MM 3 (weak) NZS1170 Site Classes B Rock C Shallow Soil D Deep or Soft Soil E Very Soft Soil MICROZONE EFFECTS MAY HAVE AMPLIFIED MOTIONS 67 pga > 1000 mm/s/s 66 pga mm/s/s TVZ 65 pga < 100 mm/s/s no accelerogram 64 epicentre 63 TVZ boundary km km km Peak ground accelerations Squares > 0.1g Circles g Triangles < 0.01g Dots no records Much more data from GeoNet than from earlier networks all digital Much stronger pgas to SW than NW, no triggerings beyond about 250km to NW - effect of high attenuation in TVZ? Deep or Soft Soil pgas fit model reasonably well Peak Ground Acceleration (mm/s/s) 10,000 1, D (Deep or Soft Soil) E (Very Soft Soil) D (Deep or Soft Soil) (TVZ path) D (Deep or Soft Soil) (model) Source Distance (km) Note lower values for TVZ paths Other site classes and spectral periods give similar comparisons Data compared with median pga from NZ attenuation model (McVerry et al. 2006)

4 Recorded pgas in Gisborne in g range for third time since March Mw 5.64 plate-interface earthquake 0.3g GISBORNE PEAK HORIZONTAL ACCELERATION 0.28g 11 peaks reached 0.2g Synthesized from acceleration history recorded at GISS, Gisborne 2ZG N 0.3g 10 August 1993 Mw 6.19 subducting-slab earthquake 0.26g 20 December 2007 Mw 6.6 subducting-slab earthquake 0.28g North-South Acceleration (mm/s/s) g 0.2g More peaks > ~0.2g this time (mid MMVII range) Gisborne Earthquake East-West Acceleration (mm/s/s) Gisborne 1966 and Ormond 1993 earthquakes Displacements emphasize stronger 2007 motions 3000 N 0.3g 30 N 30 N g North-South Acceleration (mm/s/s) g North-South Displacement (mm) North-South Displacement (mm) Ormond Earthquake East-West Acceleration (mm/s/s) Gisborne earthquake1966 PGA 0.3g 8 peaks around 0.2g or greater Ormond earthquake 1993 PGA 0.26g Only 3 peaks around 0.2g or greater Ormond Earthquake 1993 Gisborne Earthquake East-West Displacement (mm) East-West Displacement (mm)

5 Recorded versus NZS1170:2004 Deep Soil Spectra 1 Hz spectral peak in earlier earthquakes Recorded vs NZS1170 Deep Soil Spectra SA(T( (g) yr ULS design-level 100 yr 0.45s peak 2ZG N51W 2ZG S39W 2ZG Larger Component NZS1170 Deep Soil Z=0.36 R=1 RP=500yr ULS NZS1170 Deep Soil Z=0.36 R=0.5 RP=100yr NZS1170 Deep Soil Z=0.36 R=0.25 RP=25yr SLS yr serviceability-level Period T(s) 1/3 code retrofit level exceeded across broad period range Comparison with current 2004 design motions depends on building period Peak around 2 Hz in December Hz peak of this earthquake Summary of tectonics and strong motions Source-related? Pervasive in this earthquake at many locations Occurs in vertical component too Likely to have been amplified in Gisborne, and by many soil sites Around the period of many of the larger buildings in NZ towns i.e. similar to those badly damaged in Gisborne Typical of earthquake type, or a peculiarity of this event? Frequency higher than handled by usual source modelling (< Hz) Mw 6.6 subducting slab earthquake, 44 km depth Gisborne at 64 km hypocentral distance 31 km closest distance from aftershock zone 3245 felt reports to GeoNet, max intensity MMVIII Gisborne pgas of g for third time since 1966 More peaks > 0.2g this time NZS1170 spectrum exceeded in narrow band around 0.45s period Spectral peak at s, soil sites likely to amplify

6 Significance of slow-slip events Gisborne 2007 earthquake New technologies continuous GPS and InSAR They usually occur at the transition zone between coupled and creeping portions of the interface Moderate to high coupling most slip at subduction interface between Pacific and Australian plates is likely to occur in earthquakes Low coupling most slip is likely to occur smoothly in earthquakes Sergey Samsonov, GNS Hazards Group and GeoNet 7 SSE observed so far Slow-slip events relieve portion of a total stress on subduction interface; therefore, reducing the size (magnitude) of possible earthquake that can occur in a coupled part of the interface. Continuous GPS sites in Gisborne region Currently in New Zealand 114 continuous GPS sites and 5 are currently under construction. The recent quake was within the subducting Pacific Plate The earthquake was due to stresses that have built-up in the Pacific Plate as it buckles on its way down into the mantle The aftershocks of the earthquake define a fault plane cutting through the Pacific Plate The event felt quite punchy in Gisborne because the direction of fault rupture was upward towards Gisborne Martin Reyners, Aftershocks direction of rupture Stephen Bannister,

7 CGPS time series - Gisborne earthquake, 20 Dec 2007 Main shock in slab triggers slow-slip on interface Coseismic Triggered slow slip Main shock is a normal fault in the subducting plate - aftershocks show a clear plane dipping SE Triggered slow-slip is on the subduction interface. Available ALOS PALSAR data InSAR of Gisborne earthquake So far we have 3 images for each track, acquired on: (right image) and (left image) Coseismic displacements based on CGPS and seismic data projected on the satellite line-of-sight Actual ALOS PALSAR interferogram. Some signal is observed in the south part. More data has been requested.

8 Recent research results on interplate coupling from GPS - North Island SLOW-SLIP AND COUPLING Wallace et al., 2004 Red & white - m oderate to high coupling -m ostslip atsubduction interface between Pacific and Australian plates is likely to occurin earthquakes Blue - low coupling -m ostslip is likely to occursm oothly,withoutsignificant earthquakes

9 PREVIOUS SLAB EARTHQUAKES Previous earthquakes in New Zealand that were also within the subducted plate: 1993 Ormond Magnitude 6.3 event 1921 Hawke s Bay Magnitude 6.8 event 1 August 1942 Wairarapa Magnitude 7.0 event So, this is a reasonably common type of large earthquake to have in the North Island Tsunami- and seiching-related descriptions on the web (from William Power and Gaye Downes) TSUNAMI While fishing at Wharewaka Point, Lake Taupo earthquake was felt while sitting in car. A large was suddenly developed at the point and surged into the bay washing up the beach eight metres. My husband had to retreat quickly from the waters edge. we watched the wave curl around the bay until we lost sight of it in the darkness. seiching? About thirty minutes after the quake there was several waves making loud slapping noises as they struck the beach just north of Napier (Whirinaki beach). They where heard inside houses fronting the beach. [?? Napier tide gauge shows only very small fluctuations at this time]

10 1947 Gisborne earthquakes (relocated) and tsunami Tsunami Earthquakes Note large differences between magnitude measures Max height 10m March 6m May Gaye Downes, Tsunamis and slab and interface earthquakes SLAB EARTHQUAKES Unlikely to cause direct deformation of the seafloor BUT Could cause tsunami from landslide or massive seafloor slumps SUBDUCTION INTERFACE EARTHQUAKES Sudden seafloor deformation possible tsunami The 1947 (March and May) and 1966 Gisborne earthquakes were subduction interface quakes THE TYPE MAY NOT BE KNOWN IMMEDIATELY MMVI MM INTENSITIES MM 6People Felt by all. People and animals alarmed. Many run outside. Difficulty experienced in walking steadily. Fittings Objects fall from shelves. Pictures fall from walls. Some furniture moved on smooth floors, some unsecured free-standing fireplaces moved. Glassware and crockery broken. Very unstable furniture overturned. Small church and school bells ring. Appliances move on bench or table tops. Filing cabinets or "easy glide" drawers may open (or shut). Structures Slight damage to Buildings Type I. Some stucco or cement plaster falls. Windows Type I broken. Damage to a few weak domestic chimneys, some may fall. Environment Trees and bushes shake, or are heard to rustle. Loose material may be dislodged from sloping ground, e.g. existing slides, talus slopes, shingle slides.

11 MMVII MMVIII MM 7People General alarm. Difficulty experienced in standing. Noticed by motorcar drivers who may stop. Fittings Large bells ring. Furniture moves on smooth floors, may move on carpeted floors. Substantial damage to fragile contents of buildings. Structures Unreinforced stone and brick walls cracked. Buildings Type I cracked with some minor masonry falls. A few instances of damage to Buildings Type II. Unbraced parapets, unbraced brick gables, and architectural ornaments fall. Roofing tiles, especially ridge tiles may be dislodged. Many unreinforced domestic chimneys damaged, often falling from roof-line. Water tanks Type I burst. A few instances of damage to brick veneers and plaster or cementbased linings. Unrestrained water cylinders (water tanks Type II) may move and leak. Some windows Type II cracked. Suspended ceilings damaged. Environment Water made turbid by stirred up mud. Small slides such as falls of sand and gravel banks, and small rock-falls from steep slopes and cuttings. Instances of settlement of unconsolidated or wet, or weak soils. Some fine cracks appear in sloping ground. A few instances of liquefaction (i.e. small water and sand ejections). MM 8People Alarm may approach panic. Steering of motorcars greatly affected. Structures Buildings Type I heavily damaged, some collapse. Buildings Type II damaged, some with partial collapse. Buildings Type III damaged in some cases. A few instances of damage to Structures Type IV. Monuments and pre-1976 elevated tanks and factory stacks twisted or brought down. Some pre-1965 infill masonry panels damaged. A few post-1980 brick veneers damaged. Decayed timber piles of houses damaged. Houses not secured to foundations may move. Most unreinforced domestic chimneys damaged, some below roof-line, many brought down. Environment Cracks appear on steep slopes and in wet ground. Small to moderate slides in roadside cuttings and unsupported excavations. Small water and sand ejections and localised lateral spreading adjacent to streams, canals, lakes, etc. MMIX MM 9Structures Many Buildings Type I destroyed. Buildings Type II heavily damaged, some collapse. Buildings Type III damaged, some with partial collapse. Structures Type IV damaged in some cases, some with flexible frames seriously damaged. Damage or permanent distortion to some Structures Type V. Houses not secured to foundations shifted off. Brick veneers fall and expose frames. Environment Cracking of ground conspicuous. Landsliding general on steep slopes. Liquefaction effects intensified and more widespread, with large lateral spreading and flow sliding adjacent to streams, canals, lakes, etc.