160 0 20 20 40 60 140 100 80 120 60 40 20 0 20 40 60 80 100 120 140 160 180 40 60 0 20 20 40 60 80 80 40 60 80 80 Canadian Shield Guiana Shield Amazonian Shield Indian Shield Angara Shield Arabian Shield Baltic Shield Australian Shield Chinese Shield Ethiopian Shield V a l l e y Equator Tropic of Cancer Arctic Circle Tropic of Capricorn Antarctic Circle 2 12 Holocene 10,000 BP to present Pleistocene Pliocene 26 37 53 65 135 190 225 280 345 395 430 500 570 4600 0 100 200 300 400 500 1000 2000 4600 Miocene Oligocene Eocene Paleocene Jurassic Cretaceous Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian Tertiary Quaternary CENOZOIC MESOZOIC PALEOZOIC PRE-CAMBRIAN ERA PERIOD EPOCH COPYRIGHT PHILIP’S Pre-Cambrian shields Sedimentary cover on Pre-Cambrian shields Paleozoic (Caledonian and Hercynian) folding Sedimentary cover on Paleozoic folding Mesozoic folding Sedimentary cover on Mesozoic folding Cenozoic (Alpine) folding Sedimentary cover on Cenozoic folding Intensive Mesozoic and Cenozoic vulcanism Principal faults Oceanic marginal troughs Midoceanic ridges Overthrust faults Earthquake magnitude is usually rated according to either the Richter or the Modified Mercalli scale, both devised by seismologists in the 1930s. The Richter scale measures absolute earthquake power with mathematical precision: each step upward represents a tenfold increase in the amplitude of the shockwave. Theoretically, there is no upper limit, but most of the largest earthquakes measured have been rated at between 8.8 and 8.9. The 12-point Mercalli scale, based on observed effects, is often more meaningful, ranging from I (earthquakes noticed only by seismographs) to XII (total destruction); intermediate points include V (people awakened at night; unstable objects overturned), VII (collapse of ordinary buildings; chimneys and monuments fall), and IX (conspicuous cracks in ground; serious damage to reservoirs). Notable Earthquakes Since 1900 Year Location Mag. Deaths 1906 San Francisco, USA 8.3 3,000 1906 Valparaiso, Chile 8.6 22,000 1908 Messina, Italy 7.5 83,000 1915 Avezzano, Italy 7.5 30,000 1920 Gansu (Kansu), China 8.6 180,000 1923 Yokohama, Japan 8.3 143,000 1927 Nan Shan, China 8.3 200,000 1932 Gansu (Kansu), China 7.6 70,000 1933 Sanriku, Japan 8.9 2,990 1934 Bihar, India/Nepal 8.4 10,700 1935 Quetta, India* 7.5 60,000 1939 Chillan, Chile 8.3 28,000 1939 Erzincan, Turkey 7.9 30,000 1960 S. W. Chile 9.5 2,200 1960 Agadir, Morocco 5.8 12,000 1962 Khorasan, Iran 7.1 12,230 1964 Anchorage, USA 9.2 125 1968 N. E. Iran 7.4 12,000 1970 N. Peru 7.8 70,000 1972 Managua, Nicaragua 6.2 5,000 1974 N. Pakistan 6.3 5,200 1976 Guatemala 7.5 22,500 1976 Tangshan, China 8.2 255,000 1978 Tabas, Iran 7.7 25,000 1980 El Asnam, Algeria 7.3 20,000 1980 S. Italy 7.2 4,800 1985 Mexico City, Mexico 8.1 4,200 1988 N.W. Armenia 6.8 55,000 1990 N. Iran 7.7 36,000 1992 Flores, Indonesia 6.8 1,895 1993 Maharashtra, India 6.4 30,000 1994 Los Angeles, USA 6.6 51 1995 Kobe, Japan 7.2 5,000 1995 Sakhalin Is., Russia 7.5 2,000 1996 Yunnan, China 7.0 240 1997 N. E. Iran 7.1 2,400 1998 Takhar, Afghanistan 6.1 4,200 1998 Rostaq, Afghanistan 7.0 5,000 1999 Izmit, Turkey 7.4 15,000 1999 Taipei, Taiwan 7.6 1,700 2001 Gujarat, India 7.7 14,000 2002 Baghlan, Afghanistan 6.1 1,000 2003 Boumerdes, Algeria 6.8 2,200 2003 Bam, Iran 6.6 30,000 2004 Sumatra, Indonesia 9.0 250,000 2005 N. Pakistan 7.6 74,000 2006 Java, Indonesia 6.4 6,200 An earthquake off the coast of Sumatra on December 26, 2004, triggered a deadly tsunami that swept across the Indian Ocean, causing devastation in many countries, in particular Sri Lanka, India, Thailand, and Indonesia, where the loss of life was greatest. * now Pakistan Time, in millions of years before the present, is shown on a sliding scale, greatly compressed in the distant past. Geologists devised their timescale on the basis of relative, not calendar, ages. Accurate dating was impossible and estimates were often bitterly disputed, but the order in which the rocks were formed could be deduced from careful observation. The advent of radioactive dating – culminating in the 1950s with the development of a mass spectrometer capable of accurately measuring tiny quantities of isotopes – appears to have settled the arguments. The Earth is far older than geologists first imagined, but their painstakingly-created structure of geological time has withstood the advent of high technology. The 4.6 billion (4,600 million) years since the formation of the Earth are divided into four great eras, further split into periods and, in the case of the most recent era, epochs. The present era is the Cenozoic (“new life”), extending backward through “middle life” and “ancient life” to the Pre-Cambrian, named after the Latin word for Wales, the location of some of the earliest known fossils. Most of the Earth’s geological history is encompassed by the Pre-Cambrian: though traces of ancient life have since been found, it was largely the proliferation of fossils from the beginning of the Paleozoic era onward, some 570 million years ago, which first allowed precise subdivisions to be made. Like the Cambrian, most are named after regions exemplifying a period’s geology. Others – such as the Carboniferous (“coal-bearing”) or the Cretaceous (“chalk-bearing”) – are more directly descriptive. Anchorage 1964 San Francisco 1906 Managua 1972 Mexico City 1985 N. Peru 1970 S.W. Chile 1960 Chillan 1939 Valparaiso 1906 El Asnam 1980 Agadir 1960 Avezzano 1915 Messina 1908 Erzincan 1939 N. Iran 1990 Tabas 1978 Quetta 1935 Maharashtra 1993 Bihar 1934 Tangshan 1976 Gansu 1920/1932 Nan Shan 1927 Yokohama 1923 Los Angeles 1994 Kobe 1995 Sakhalin 1995 Equator Tropic of Cancer Arctic Circle Tropic of Capricorn Antarctic Circle Flores 1992 Takhar/Rostaq 1998 Izmit 1999 Taipei 1999 Gujarat 2001 Guatemala 1976 Sanriku 1933 N.E. Iran 1968/1997 Yunnan 1996 N.W. Armenia 1988 S. Italy 1980 Khorasan 1962 N. Pakistan 1974 Baghlan 2002 Boumerdes 2003 Bam 2003 Sumatra 2004 N. Pakistan 2005 Java 2006 160 0 20 20 40 60 140 100 80 120 60 40 20 0 20 40 60 80 120 140 160 180 40 60 0 20 20 40 60 40 60 100 Shockwaves reach the surface Epicenter – point on the surface directly above the origin Origin or focus Shockwaves travel outward Mobile land areas Submarine zones of mobile land areas Stable land platforms Submarine extensions of land platforms Midoceanic volcanic ridges Oceanic platforms Principal earthquakes and dates (since 1900) 1976 X Subduction zone Earthquakes are a series of rapid vibrations originating from the slipping or faulting of parts of the Earth’s crust when stresses within build up to breaking point. They usually happen at depths varying from 5 to 20 miles. Severe earthquakes cause extensive damage when they take place in populated areas, destroying structures and severing communications. Most initial loss of life occurs due to secondary causes such as falling masonry, fires, and flooding. EARTHQUAKES GEOLOGICAL TIME GEOLOGY OF THE EARTH 71 US p70-71 AOW HB IWG '06 21/6/06 2:28 pm Page 71