Introduction

Radioactivity was discovered in 1896 by Henri Becquerel. The new understanding of the atom that came from the work of Rutherford, Soddy, Boltwood and others over the following decades had a major impact on geology. Before this work, the age of the Earth was unknown. In the nineteenth century, Lord Kelvin (William Thompson) attempted to calculate the age of the Earth by assuming that the planet was a hot body cooling by conduction. He obtained a young age that conflicted with the observations of geologists, who had concluded that the Earth must be at least several hundred million years old. The geological reasoning was based on rather qualitative evidence such as the observation of sedimentary deposition rates, calculations about the amount of salt in the sea and guesses of evolutionary rates. Not unnaturally, Kelvin's apparently more rigorous and quantitative physical calculation was regarded as much sounder by most scientists. In 1904, while at McGill University in Montreal, Ernest Rutherford realized that radioactive heat could account for some of the apparent discrepancy. Kelvin was sceptical to the extent that he bet the younger Rayleigh (Hon. R. J. Strutt) five shillings on the matter, but later he paid up. Rutherford gave a lecture at the Royal Institution in London in 1904 about which he wrote the following:

Geophysics is a diverse science. At its best it has the rigour of physics and the vigour of geology. Its subject is the Earth. How does the Earth work? What is its composition? How has it changed? Thirty years ago many of the answers to these questions were uncertain. We knew the gross structure of our planet and that earthquakes occurred, volcanoes erupted and high mountains existed, but we did not understand why. Today we have a general knowledge of the workings of the planet, although there is still much to be discovered.

My aim in writing this book was to convey in a fairly elementary way what we know of the structure and dynamics of the solid Earth. The fabric of geophysics has changed dramatically in the decades since the discovery of plate tectonics. The book places a strong emphasis on geophysical research since the initial formulation of plate theory, and the discussion centres on the crust and upper mantle. It also outlines the recent increases in our knowledge of the planet's deeper interior.

To whom is this book addressed? It is designed to serve as an introduction to geophysics for senior undergraduates in geology or physics and for graduate students in either subject who need to learn the elements of geophysics. My hope is that the book will give them a fairly comprehensive basis on which to build an understanding of the solid Earth.

Introduction

Complex continents

We have seen something of the general simplicity of the Earth's internal structure and the detailed complexity of the motions of tectonic plates and convective systems. The clues to this simplicity and complexity come from the oceans, the study of whose structures has led to an understanding of the plates, of the mantle beneath and, to some extent, of the core, via its magnetic properties.

Although complex details must be sorted out and theories may change slightly, we can now be reasonably confident that the oceans are understood in their broad structure. In contrast, the continents are not understood at all well. Yet we need to understand the continents because in their geological record lies most of the history of the Earth and its tectonic plates, from the time that continental material first formed over 4400 Ma ago (see Section 6.10). The oldest oceanic crust is only about 160 Ma old, so the oceanic regions can yield no earlier information.

In the broadest terms, the continents are built around ancient crystalline crust, flanked by younger material representing many events of mountain building, collision, rifting and plate convergence and subsidence. Figure 3.30 shows the recent motions of the plates, illustrating how continents have collided and been torn asunder.

A major problem in the geological and geophysical study of continents is that we can observe only what is exposed at or near the surface.

INTRODUCTION

Climatic hazards originate with the processes that move air across the Earth's surface due to differential heating and cooling. Surprisingly, examination of these processes has focused upon heating at the tropics and downplayed the role of cold air masses moving out of polar regions due to deficits in the radiation balance in these latter regions. Fluctuations – in the intensity of pulses of cold air moving out of polar regions or of heating at the equator – and the location of the interaction between these cold and warm air masses, are crucial factors in determining the magnitude, frequency, and location of mid-latitude storm systems. While most of these factors are dictated by internal factors in the Earth–atmosphere system, modulation by 11-year geomagnetic cycles linked to solar activity and by the 18.6 year MN lunar tide also occurs. This chapter examines these processes and mechanisms. The responses in terms of centers of storm activity will be examined in the following chapter.

MODELS OF ATMOSPHERIC CIRCULATION AND CHANGE

(Bryson & Murray, 1977; Lamb, 1982)

How air moves

Barometric pressure represents the weight of air above a location on the Earth's surface. When the weight of air over an area is greater than over adjacent areas, it is termed ‘high pressure’. When the weight of air is lower, it is termed ‘low pressure’.

INTRODUCTION

Of all natural hazards, the most insidious is drought. However, for some countries such as Australia and United States, droughts have not led to starvation, but to spectacular fires as tinder-dry forests ignite, grasslands burn and eucalyptus bushland erupts in flame. Of all single natural hazard events in Australia, bushfires are the most feared. Here, the litany of disasters over the past 150 years reads like the membership list of some satanic cult: ‘Black Thursday’, ‘Black Friday’, and ‘Ash Wednesday’ (Figure 7.1). For firefighters, state emergency personnel and victims, each name will invoke stories of an inferno unlike any other. North America has suffered just as badly in the past from fires, as have Europe and the former Soviet Union. Approximately 143 × 106 km2 of the Earth's surface is covered by vegetation, of which 0.17 per cent burns on average each year. Even tropical rainforests can dry out and burn. For example, the U Minh forest in Vietnam (see Figure 7.2 for the location of major placenames mentioned in this chapter) ignited naturally in 1968. Large fires have also burnt through tropical vegetation in the Brazilian highlands and the Amazon Basin, especially in association with land clearing. Many urban dwellers would consider that, following wide-scale deforestation, there is no forest fire threat near cities. Fires are perceived to be a hazard only in virgin forest or, perhaps in southern California or southern France, a hazard where urban expansion has encroached upon vegetated mountains.

CHANGING HAZARD REGIMES

(Houghton et al., 1995, 1996; Bryant, 2001; Nott, 2003)

Throughout this book the commonness of natural hazards has been continually emphasized. For example, the eruption of Krakatau in 1883 was not an unusual event. It was matched by earlier eruptions of similar magnitude and will be witnessed again. This argument smacks of uniformitarianism, but it is not meant to support this concept. Rather, it acknowledges that our existing realms of natural hazards fit a magnitude–frequency distribution that can be described, and from which the probability of occurrence of future extremes can be predicted. This is not necessarily how natural hazards behave over time. Existing hazard regimes are not immutable. They can change. My present research into mega-tsunami illustrates this point. In the historic record, tsunami in Australia have not exceeded 1.07 m on tide gauges, did not have run-ups of 4 m above sea level nor penetrate more than 1 km inland. Over the past fifteen years, evidence has been found along Australia's New South Wales coastline for large tsunami that have gone inland up to 10 km, transported boulders the size of boxcars up 30 m high cliffs (Figure 14.1) and swept over headlands 130 m high. Nor is the evidence restricted to the New South Wales coastline. In north-western Australia, one event penetrated 35 km inland in the Great Sandy Desert.

INTRODUCTION

The mitigation and survival of any natural hazard ultimately depends upon the individual, family, or community. An individual has the choice to heed warnings, to prepare for an impending disaster, and to respond to that event. In the end, the individual, family unit or local community endures the most of a disaster in the form of property loss, injury, loss of friends or relatives, or personal death. If these small social units could recognize the potential for hazards in their environment, and respond to their occurrence before they happened, then there would be minimal loss of life and property. Unfortunately, not everyone heeds advice or recognizes warning signs, not necessarily because of stupidity, but for very important personal and socio-economic reasons. The reaction of individuals, families, and community groups after a disaster also determines their ability to survive physically and mentally. Some individuals and families can overcome all bureaucratic obstacles in rebuilding. Others end up with shattered lives despite all the support available to them. This chapter will describe and account for some of these personal and group reactions.

BEFORE THE EVENT

(Burton et al., 1978)

Warnings and evacuation

People react differently to their perception of a hazard. While some may not know that an earthquake is about to occur until they hear the rumbling and feel the shaking of the ground, most will know that they live in an earthquake-prone area.

TYPES OF SHOCK WAVES

(Hodgson, 1964; Holmes, 1965; Whittow, 1980)

Earthquakes are shock waves that are transmitted from an epicenter, which can extend from the surface to 700 km beneath the Earth's crust. Earthquakes generate a number of types of waves, illustrated in Figure 10.1. A primary or P-wave is a compressional wave that spreads out from the center of the earthquake. It consists of alternating compression and dilation, similar to waves produced by sound traveling through air. These waves can pass through gases, liquids and solids, and undergo refraction effects at boundaries between fluids and solids. P-waves can thus travel through the center of the earth; however, at the core–mantle boundary they are refracted producing two shadow zones, each 3000 km wide, without any detectable P-waves on the opposite side of the globe.

The second type of wave is a shear or S-wave, which behaves very much like the propagation of a wave down a skipping rope that has been shaken up and down. These waves travel 0.6 times slower than primary waves. While the velocity of a primary wave through Earth depends upon rock density and compressibility, the rate of travel of a shear wave depends upon rock density and rigidity. Shear waves will travel through the mantle, but not through the Earth's rigid core. Thus, there is a shadow zone on the opposite side of the Earth that does not record S-waves.

INTRODUCTION

One of the most widespread natural hazards is the unexpected and sometimes unpredictable movement of unconsolidated weathered material (regolith) or weathered rock layers near the Earth's surface. Landslides and avalanches, while historically not renowned for causing as large a death toll as other natural disasters such as tropical cyclones or earthquakes, have had just as dramatic an impact on property and lives. The sudden movement of slope material is as instantaneous as any earthquake event but it is a more widespread problem. In any moderate- to high-relief region subject to periods of high rainfall, slippage of part or all of the regolith downslope is probably the most common hazard. Nowhere is this problem more prevalent than in cold regions underlain by permafrost or ground ice. Of a slower nature, and just as widespread a hazard, is land subsidence. While much of a land surface may be stable or even flat, there is a wide range of natural processes that can generate ground collapse. Another important aspect of land instability is the multitude of factors that can trigger ground movement. Almost all of the hazards presented in this book can generate secondary land instability problems. In many cases, associated landslides have contributed significantly to the large death toll from earthquakes and cyclones. Even droughts can exacerbate ground instability through the process of repeated drying and wetting of expansive clays.

INTRODUCTION

Of all natural hazards, earthquakes and volcanoes release the most energy in the shortest time. In the past 40 years, scientists have realized that the distribution of earthquakes and volcanoes is not random across the Earth's surface, but tends to follow crustal plate boundaries. In the past 20 years, research has been dedicated to monitoring these regions of crustal activity with the intention of predicting – several days or months in advance – major and possibly destructive events. At the same time, planetary studies have led to speculation that the clustering of earthquake or volcanic events over time is not random, but tends to be cyclic. This knowledge could lead to prediction of these hazards decades in advance. Before examining these aspects, it is essential to define how earthquake intensity is measured, because earthquakes are always characterized by their magnitude. This aspect will be examined first, followed by a description of the distribution of earthquakes and volcanoes over the Earth's surface and some of the common causes of these natural disasters. The chapter concludes with a discussion on the long- and short-term methods for forecasting earthquake and volcano occurrence.

SCALES FOR MEASURING EARTHQUAKE INTENSITY

(Holmes, 1965; Wood, 1986; Bolt, 1993; National Earthquake Information Center, 2002)

Seismic studies were first undertaken as early as 132 AD in China, where crude instruments were made to detect the occurrence and location of earthquakes.

INTRODUCTION

(Bolt et al., 1975; Tazieff & Sabroux, 1983)

Of all natural hazards, volcanoes are the most complex. Whereas a tropical cyclone has a predictable structure, or a drought can generate a predictable sequence of events in rural communities, such predictions cannot be made in respect of volcanoes. There is a multitude of volcanic forms, and each event appears unique in the way that it behaves, and the physical and human consequences it produces. This chapter will examine the different types of volcanoes and the secondary phenomena associated with their occurrence. It will conclude with a detailed description of some of the more spectacular volcanic disasters that have occurred in recorded history.

Volcanoes are conduits in the Earth's crust through which gas-enriched, molten silicate rock magma reaches the surface from beneath the crust. The origin of magma is still debated, but it is generally believed from seismic evidence that the mantle is partially liquefied 75–300 km below the Earth's surface. There are two types of magma. The first type consists of silica-poor material from the mantle, and forms basaltic volcanoes. The second type consists of silicarich material originating from either the melting of the crust in subduction zones or the partial differentiation of liquefied mantle material. This second type forms ‘andesitic’ volcanoes, the largest group of which rings the western Pacific Ocean, where the Pacific Plate is subducting beneath the Eurasian Plate (Figure 9.1).