The Solid Facts on Christchurch
Total Page:16
File Type:pdf, Size:1020Kb
DOES LIQUEFACTION HAPPEN IN ALL EARTHQUAKES? No. Strong ground shaking is required for liquefaction to occur. will move and un-reinforced masonry buildings will crack. NEW ZEALAND’S TECTONIC SETTING EARTHQUAKE MAGNITUDE Auckland Liquefaction may occur at Modified Mercalli Intensity 7 (MM7). Liquefaction is common in areas where there is MM8 and greater Kaikoura The MMI scale (1 to 10) is a measure of how strong ground shaking. At MM8 alarm may approach panic, steering of vehicles District New Zealand is on the boundary of two of the earth’s plates, the The Alpine Fault can produce magnitude 8 earthquakes and does Earthquake magnitude is a measure of the actual energy KAIKOURA shaking is at a specific location. At MM7 there is general alarm, is affected, substantial damage to buildings occurs, the ground Australian Plate to the west and the Pacific Plate to the east. so about every 300 to 350 years. The last earthquake on the Distance Time MagnItude Amplitude released at the focus of the earthquake and was developed Australian Plate (kilometres) (seconds) (mm) it is difficult to remain standing, large bells will ring, furniture cracks and road cuttings collapse. These plates are moving against each other and, because of this, Alpine Fault was in 1717. Ground shaking intensities in by Charles Richter for comparing the size of earthquakes 500 33 in California, USA. The magnitude of an earthquake is Wellington New Zealand is a highly-active earthquake zone. Christchurch during an Alpine Fault earthquake will be high 50 24 Hurunui 400 24 5.1 100 MODIFIED MERCALLI INTENSITY SCALE District enough to cause liquefaction. The Alpine Fault is not the only 40 6 50 determined by measuring the largest amplitude of either 300 Christchurch 20 a primary (P) or secondary (S) wave recorded at a The earthquake hazard is even more extreme for Canterbury source of earthquakes for Canterbury, with about one hundred 30 5 200 10 seismographic station, and the time lapse between each of Pacific Plate because of the Alpine Fault. It is the “on-land” boundary of the other sources having been identified. Some of these are also 20 The Modified Mercalli Intensity scale (MM scale) is not the same encloses areas of equal intensity of shaking, but actual intensity 4 5 these waves. A straight line is drawn between the time and 100 as magnitude. The MM scale is based on observations of the recorded at any particular location may vary depending on the Australian and Pacific Plates, and is New Zealand’s largest active capable of producing strong ground shaking and liquefaction in 10 2 60 8 3 amplitude scales on the chart to determine the magnitude effects on people, objects (such as furniture and crockery), Alpine Fault fault, running under the Southern Alps for over 500km. Christchurch. 6 1 topographic, geologic and soil conditions. On the right are 40 and the distance of that particular seismographic station THE SOLID FACTS ON 4 2 0.5 buildings and other structures, and the physical environment, isoseismal maps for two historic earthquakes in Canterbury and from the earthquake epicentre. Waimakariri RANGIORA 1 0.2 including lakes, sea, and slopes. The effects vary with distance one predicted earthquake scenario. KAIAPOI 20 District 2 0.1 from the earthquake’s epicentre, with the greatest damage usually Christchurch 0-5 0 City Amplitude 33 mm CHRISTCHURCH occuring closest to the epicentre. A line called an isoseismal An abbreviated version of the MM scale is given below. FAULTS Amplitude MT COOK Selwyn District Generally felt outside. Most sleepers awakened. Small unstable objects move, Banks Peninsula 24 sec District EARTHQUAKE some glassware and crockery broken. Ashburton P-wave S-wave LIQUEFACTION 5 District EPICENTRES ASHBURTON Felt by all. Difficulty in walking steadily. Objects fall from shelves. Slight damage (1840 to 1998) Because of the huge range in earthquake sizes, the magnitude to badly-constructed buildings. scale, based on ground displacement, is logarithmic. 6 GERALDINE WHAT IS LIQUEFACTION? For each step in magnitude, the energy released goes up Mackenzie MAGNITUDE 5 General alarm. Difficulty in standing. Some damage to buildings not designed District Timaru TEMUKA 32 times. Therefore, a magnitude 7.5 is 2.8 times larger District to withstand earthquakes. Furniture moves. Un-reinforced chimneys, roofing Waitaki than a magnitude 7.2 earthquake, and a magnitude 8 Liquefaction (pronounced “lick-wi-fack-shin”) happens during earthquakes. MAGNITUDE 6 7 District TIMARU earthquake releases almost one million times more energy The ground shaking that occurs during an earthquake can cause some soils to liquefy. tiles and water tanks break. than a magnitude 4 earthquake. This means that during an earthquake these soils will behave Alarm may approach panic. Steering of motorcars greatly affected. Some damage MAGNITUDE 7 Waimate more like a liquid than a solid... 8 to earthquake-resistant buildings. Serious damage to less well-designed building District types. Monuments and elevated tanks brought down. 4 June 1869 Christchurch earthquake 1 September 1888 North Canterbury The most probable ground shaking magnitude 5.0. This isoseismal pattern is earthquake magnitude 7.0-7.3. This intensities for a magnitude 8 earthquake Heavy damage to buildings and bridges. Houses not secured to foundations typical of a magnitude 5 to 6 earthquake that isoseismal pattern is typical of a large on the Alpine Fault, which is the largest shifted off. Landslides widespread on steep slopes. Cracking of ground could occur anywhere in Canterbury. shallow magnitude 6 to 7 earthquake that earthquake likely to occur on faults close 9 conspicuous. could occur on any of the known faults in to Canterbury. north, west and southwest Canterbury. 10 Severe damage to many buildings and bridges, even those of most recent design. Source: Ansell, R. and Taber, J. (1999). Caught in the Crunch. HOW DOES LIQUEFACTION HAPPEN? When the ground shakes during an earthquake the soil particles Sand volcanoes or sand boils, water fountains and associated LOW GROUNDWATER TABLE LIQUEFACTION POTENTIAL HAZARD MAP Brooklands are rearranged and the soil mass compacts and decreases in ground surface cracking are evidence that liquefaction volume. This decrease in volume causes water to be ejected to has occurred. Brooklands HIGH GROUNDWATER TABLE LIQUEFACTION POTENTIAL HAZARD MAP LIQUEFACTION POTENTIAL the ground surface. FORMATION OF WATER FOUNTAINS HOW DO WE ASSESS THE CHANCE OF LIQUEFACTION HAPPENING? 1 THE LIQUEFACTION PROCESS sand boil AND SAND BOILS The liquefaction potential rating is based on soil type and strength, Moderate Liquefaction Potential soil particle settlement of ground settlement River kariri R water and sand of ground imak groundwater level, and the location and strength of earthquakes. There are five main factors used to assess the likelihood that The water table level varies across Christchurch, and also varies Wai Areas of Christchurch rated as having “moderate liquefaction Belfast There are four classes of liquefaction potential – High, Moderate, soils will liquefy. These are the strength of ground shaking, considerably during the year and from year to year. The high LEGEND potential” have soil types and strengths in between the High and dense soil dense soil Low and Liquefaction Not Predicted. dense soil Low classes. Two earthquake types could cause liquefaction in duration of shaking, depth to the water table, soil properties groundwater table liquefaction potential hazard map (B) represents 1 (grain size and density), and confining pressures. The liquefaction average to high groundwater conditions. The low groundwater HIGH LIQUEFACTION POTENTIAL High Liquefaction Potential these areas – a foothills’ earthquake and an Alpine Fault loose sand densified sand densified sand potential in any part of Christchurch is very dependent on the table liquefaction potential hazard map (A) is based on January 1 earthquake. before during after before during after Parklands Areas of Christchurch rated as having “high liquefaction potential” height of the groundwater table at the time of the earthquake. averages for the last ten years. A foothills’ earthquake (magnitude 7.2) and an Alpine Fault earthquake earthquake earthquake earthquake earthquake earthquake earthquake MODERATE LIQUEFACTION POTENTIAL have soil types and strengths that are the most prone to (magnitude 8) have return periods (the average time between events) of River liquefaction. Three types of earthquakes could cause liquefaction about 2000 years and 400 years respectively. WHAT ARE THE EFFECTS OF LIQUEFACTION? riri Papanui imaka in these areas. These are a local earthquake, a foothills’ earthquake Wa LOW LIQUEFACTION POTENTIAL and an Alpine Fault earthquake. Low Liquefaction Potential Liquefaction causes damage to the ground. Because the soil 1 Belfast Wainoni A local earthquake (magnitude 6) is likely to have a return period (the Areas of Christchurch rated as having “low liquefaction potential” mass decreases in volume as a result of liquefaction, the LIQUEFACTION NOT PREDICTED average time between events) of about 200 years. A foothills’ earthquake have soil types and strengths that are the least prone to ground surface may subside. Uniform subsidence over large (magnitude 7.2) and an Alpine Fault earthquake (magnitude 8) have return liquefaction. Only an Alpine Fault earthquake could cause 73 periods of about 2000 years and 400 years respectively. areas may go unnoticed. Differential subsidence, particularly 1 liquefaction