Mw 7.1 Canterbury, New Zealand Earthquake Michael Bunds Department of Earth Science Utah Valley University and Laura Benninger U.S.
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Mw 7.1 Canterbury, New Zealand Earthquake Michael Bunds Department of Earth Science Utah Valley University and Laura Benninger U.S. Bureau of Reclamation Copyright 2010, Michael P. Bunds, all rights reserved This material may be used for educational purposes only. Users agree to acknowledge the original author and the Department of Earth Science, Utah Valley University when using any original portion of this material. What is an Earthquake? Ground shaking caused by a sudden release of energy within Earth. Most result from slip on a fault. from Marshak, 2009 from Marshak, 2009 epicenter hypocenter fault from Tarbuck & Lutgens Note: in large earthquakes, slip on the fault initiates at the hypocenter and then propagates along the fault from Marshak, 2009 Normal fault; Wasatch fault is an example from Marshak, 2009 Thrust faults; common at convergent plate boundaries Types of Faults from Marshak, 2009 Strike-slip faults; San Andreas fault Types of Seismic Waves P-waves: Fastest, higher frequency. from Marshak, 2009 S-waves: 2nd fastest. Potentially damaging. from Marshak, 2009 Surface waves: Slowest. Damaging to structures. from Marshak, 2009 from Tarbuck & Lutgens Seismogram P-waves arrive first, followed by S then surface waves Delay between arrival of different wave types increases with distance from the earthquake New Zealand For those of you accustomed to looking at the Earth upside-down New Zealand Auckland Wellington Christchurch Major New Zealand Metropolitan Areas and Volcanoes Auckland North Island fault system 5.4 cm/yr Wellington Australian plate Alpine fault Pacific plate Marlborough fault system Christchurch M 7.1 9/4/10 3.1 cm/yr New Zealand Plate Tectonic Setting New Zealand South Island Seismic Hazard From USGS The Earthquake Our room © Michael Bunds Damage along roof line in our hotel © Michael Bunds Interior wall cracking in our hotel We learned later that the hotel had been reinforced for earthquake safety in 2004 © Michael Bunds So What the • Just Happened? Was it the Alpine fault? – Might be able to generate the shaking, but should have been more rolling, longer lasting • Marlborough fault system? Maybe? Faults too small + distant? • ???? Something else? • Solution: – Cell network was still up! (but $25/mb, eeegads) – Danny Horns had already emailed me 23 minutes after the earthquake! Australian plate North Island fault system Pacific plate Alpine fault Marlborough fault system Christchurch So I called Danny, and he had answers! (more on what the answers were later) Damage in Christchurch • Major damage mostly restricted to unreinforced masonry – Some roof collapses – Collapsed walls – Collapsed facades – Chimneys – Damaged buildings: aftershock hazard • Liquefaction Extensively Damaged Buildings © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds Source of bricks shown in previous slide © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds Damaged Buildings at Risk from Aftershocks © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds © Michael Bunds Liquefaction • Water-saturated sediment is liquified by shaking • Sand blows (also called sand volcanoes) • Lateral spread • Substantial damage to structures, sewers, storm drains, roadways sand blow © Michael Bunds © Michael Bunds sand blow © Michael Bunds sand blow © Michael Bunds sand blow © Michael Bunds deposits from sand blows © Michael Bunds sand blows © Michael Bunds sand blow Lateral Spread © Michael Bunds © Michael Bunds Lateral Spread Lateral Spread © Michael Bunds foundation damage Sand blow © Michael Bunds © Michael Bunds Sand blow & foundation damage © Michael Bunds structural damage lateral spread © Michael Bunds structural damage Geology & Geophysics of the Earthquake • • • • • Seismicity Surface rupture Aftershocks Shaking intensities Comparison to other earthquakes aftershocks main event from New Zealand Geonet Seismogram from day of event recorded at station near Christchurch from New Zealand Geonet Seismogram of Event Complex – multiple pulses of energy? T P modified from USGS Focal Mechanism Right lateral strike-slip on E-W fault or left lateral on N-S fault Rt. Lat. On E-W was more likely based on regional geology Christchurch USGS GeoNet USGS & GeoNet Epicenters Used for Surface Rupture Search © Michael Bunds Location 2: 4 m right-lateral slip; negligible north down dip slip Surface Rupture • Distributed en echelon shears • Riedel shears • right-lateral slip + minor north-side down • Probably extends to at least 10 – 12 km depth Christchurch 6 5 4 32 1 Surface Rupture Trace, Visited Locations, USGS and NZ Geonet Epicenters Circles mark photo locations; Mapped rupture trace in black (from GeoNet) © Michael Bunds Location 2: 4 m right-lateral slip; negligible north down dip slip © Michael Bunds Location 2: 4 m right lateral slip; negligible north down dip slip © Michael Bunds Location 4: Pressure ridge, 4 m right lateral slip © Michael Bunds Location 4: Extended fence; pressure ridge; Reidel shears; 4 m right lateral slip Aerial view of location 4 from GeoNet © Michael Bunds Location 5: 4 m right lateral slip © Michael Bunds Location 5: 4 m right lateral slip Location 5 4 m right lateral slip © Michael Bunds © Michael Bunds Location 6: ~2.5 m right lateral slip; approaching western limit of surface rupture Aerial view from GeoNet Riedel Shears Aerial view from GeoNet Earthquake: unfaulted sediment & soil over bedrock R Experiment: clay cake over cut wood P R’ Aerial view from GeoNet Aerial view from GeoNet from New Zealand Geonet Aftershock Locations Aftershocks 9/4 – 9/7 Aftershocks 9/7 – present From New Zealand Geonet Aftershocks: Several > Mw 5 Classic sequence From New Zealand Geonet Shaking Intensities • Measured as Mercalli Magnitude and/or peak ground acceleration (pga) • Christchurch generally MM VI to VIII (strong to severe; pga 0.2 to 0.4 g) • Up to MM IX, 1.2 g pga near fault rupture • Good strong motion data collected approximate surface rupture trace From New Zealand Geonet From New Zealand Geonet Comparison to Other Earthquakes • Haiti • Landers / Hector Mine From USGS Shaking Intensity and damage from Haiti Earthquake 3.5 million people exposed to MM VII – IX shaking Many buildings vulnerable to earthquake damage Port au Prince Comparison to Haiti Earthquake • Both earthquakes had similar magnitudes, proximities to cities • Huge loss of life (~230,000) vs. no lives loss – – – – Higher population density in Haiti; greater shaking intensity Much more resistant buildings in Christchurch Time of day (4:53 pm vs. 4:35 am) Good building codes and retrofitting buildings saves lives • Haiti earthquake on or near recognized fault, Canterbury earthquake on previously unknown fault – We are good at identifying hazardous faults, and there is lots of work to do Comparison to Landers Earthquake • Landers: Mw 7.3, 1992, remote So. Cal. desert • Landers & Canterbury earthquakes were on little-known faults with very long recurrence interval (10,000 + years) • Both were complex, (probably) resulting from several shorter fault segments rupturing in rapid succession Hector Mine event (Mw 7.1) – Raises concern of future earthquakes in the area From USGS • Landers was followed 7 years later by Aftershocks 9/4 – 9/7 Aftershocks 9/7 – present from King, Stein & Lin, 1994 Regional stress changes caused by slip on a fault. From New Zealand Geonet Red indicates increased stress for right lateral faulting from King, Stein & Lin, 1994 from New Zealand Geonet Aftershocks and areas likely to be under increased stress for right-lateral E-W faulting Conclusions and Lessons • We are good at identifying hazardous faults, but lots of work needs to be done • Preparations – Proper building construction and retrofitting works – Good community preparation counts (infrastructure, insurance, responders) • During and immediately after an earthquake – Don’t run outside – duck and cover – Leave building as soon as you can – Remain aware of surroundings after the event – don’t stand next to buildings, especially brick buildings – aftershocks happen!