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).

INTRODUCTION

The previous chapter was mainly concerned with the effects of strong winds generated by secondary features of general air circulation. These winds were associated with the development of low-pressure cells spanning areas of 10,000–100,000 km−2. While tropical cyclones and extra-tropical depressions produce some of the strongest winds and highest amounts of precipitation over the widest areas, they are by no means the only source of high winds or heavy precipitation. These values can be matched by thunderstorms, which cover no more than 500 km2 in area and rarely travel more than 100–200 km before dissipating. They can produce high-magnitude, short-period rainfalls leading to flash flooding. Thunderstorms are also associated with a wide range of climatic phenomena such as lightning, hail and tornadoes that bring death and destruction. Tornadoes generate the highest wind speeds and can produce localized wind damage just as severe as that produced by a tropical cyclone. This chapter will examine first the development and structure of thunderstorms resulting in lightning and hail. This is followed by a description of tornadoes and the major disasters associated with them. The chapter concludes with a discussion of warning and response to the tornado threat, an aspect that has been responsible for decreasing death tolls throughout the twentieth century.

THUNDERSTORMS, LIGHTNING AND HAIL

Thunderstorms

(Whipple, 1982; Eagleman, 1983)

Thunderstorms are a common feature of the Earth's environment. There are about 1800–2000 storms per hour or 44,000 per day.

INTRODUCTION

The earlier chapter on large-scale storms did not consider in detail the effects of flooding associated with tropical cyclones, extra-tropical storms or east-coast lows. Nor was it appropriate to discuss large-scale flooding in that chapter because some of the worst regional flooding in the northern hemisphere has occurred in spring in association with snowmelt. In addition, rainfall associated with thunderstorms is a major cause of flash flooding. This chapter examines flash flooding events and regional floods. Flash flooding refers to intense falls of rain in a relatively short period of time. Usually the spatial effect is localized. Modification of the overall landscape during a single event is minor in most vegetated landscapes; however, in arid, semi-arid, cultivated (where a high portion of the land is fallow) or urban areas, such events can be a major cause of erosion and damage. Steep drainage basins are also particularly prone to modification by flash floods because of their potential to generate the highest maximum probable rainfalls. Large-scale regional flooding represents the response of a major continental drainage basin to high-magnitude, low-frequency events. This regional flooding has been responsible historically for some of the largest death tolls attributable to any hazard. When river systems alter course as a response to flooding, then disruption to transport and agriculture can occur over a large area.

INTRODUCTION

The previous chapter concentrated upon large-scale pressure patterns and their changes across the surface of the Earth. While these patterns regionally control precipitation, large-scale vortices amplify the amounts and add a wind component to produce devastating storms. In addition, moderate-to-strong winds on a regional scale have the capacity to suspend large quantities of dust. This material can be transported thousands of kilometers, a process that represents a significant mechanism for the transport of sediment across the surface of the globe. Dust storms are exacerbated by drought in low rainfall, sparsely vegetated regions of the world. For that reason, they pose a slowly developing but significant hazard in sparsely settled semi-arid regions. Unfortunately, many of these areas have been marginalized by the degrading agricultural practices of humans over thousands of years. However, in some regions where settlement has occurred only in the last 200 years, we are presently witnessing this marginalization process. Dust storms are one of the most prominent and long-lasting signatures of human impact on the landscape.

In this chapter, the process, magnitude, and frequency of tropical cyclones are described first, followed by a description of some of the more familiar cyclone disasters. This section concludes with a comparison of the human response to cyclones in Australia, the United States, and Bangladesh – formerly East Pakistan.

INTRODUCTION

Drought and famine have plagued urban–agricultural societies since civilizations first developed. While many definitions exist, drought can be defined simply as an extended period of rainfall deficit during which agricultural biomass is severely curtailed. In some parts of the world, such as the north-east United States and southern England, a drought may have more of an effect on urban water supplies than on agriculture. The definition of drought, including the period of rainfall deficit prior to the event, varies worldwide. In southern Canada, for instance, a drought is any period where no rain has fallen in 30 days. Lack of rain for this length of time can severely reduce crop yields in an area where crops are sown, grown, and harvested in a period of three to four months. In Australia, such a definition is meaningless, as most of the country receives no rainfall for at least one 30-day period per year. Indeed, in tropical areas subject to monsoons, drought conditions occur each dry season: most of tropical Australia, even in coastal regions, endures a rainless dry season lasting several months. In Australia, drought is usually defined as a calendar year in which rainfall registers in the lower 10 per cent of all the records. Unfortunately, in the southern hemisphere, a calendar year splits the summer growing season in two. A more effective criterion for drought declaration should consider abnormally low rainfall in the summer growing season.

I have reread the preface to the first edition many times: extreme events, dire warnings about Green-house warming, El Niño–Southern Oscillation prediction … Little has changed in the fifteen years since I wrote about them. I am still perplexed because extreme events continue to happen and global warming is no closer to occurring. As Sydney in February 2004 experienced a heat wave of a magnitude unprecedented since at least 1939, I was chasing my favourite research topic – cosmogenically induced mega-tsunami – on Stewart Island, New Zealand some two thousand kilometres away where an unprecedented cold snap was occurring. One event witnessed by four million people got all the publicity; the other played out in a remote cabin in front of half of dozen trekkers got none. Yet both climatic extremes were produced by the same pattern of atmospheric circulation controlled by the same sequence of mobile polar highs. Sydney lay on the equatorial ‘greenhouse’ side of the highs and Stewart Island lay on the poleward ‘Ice Age’ side. This book covers two of the phenomena I experienced in my February of extremes – mobile polar highs and tsunami. As with the first edition, the book does not cover the third phenomena, Green-house warming. This book is about everyday climatic and geological hazards that can be explained, predicted, and alleviated.

RATIONALE

The field of environmental studies is usually introduced to students as one of two themes. The first examines human effects upon the Earth's environment, and is concerned ultimately with the question of whether or not people can irreversibly alter that environment. Such studies include the effect of human impact on climate, of land-use practices on the landscape in prehistoric and recent times, and of nuclear war upon the Earth's environment. The second theme totally disregards this question of human impact on the environment. It assumes that people are specks of dust moving through time subject to the whims of nature. In this sense, calamities are ‘acts of God’, events that make the headlines on the evening news, events you might wish on your worst enemy but would never want to witness yourself.

University and college courses dealing with this latter theme usually treat people as living within a hostile environment over which they have little control. Such courses go by the name of ‘“Natural” Hazards’. The difference between the two themes is aptly summarized by Sidle et al. (2003). Both themes describe hazards. The first theme can be categorized as chronic while the second is episodic or periodic. Chronic hazards would include desertification, soil degradation, and melting of permafrost. The causes could be due to humans or global warming. Periodic hazards are large magnitude events that appear over a short time period.

INTRODUCTION

In Chapter 3, waves were often mentioned as one of the main agents of erosion and destruction by storms. In fact, destructive waves can occur without storm events, and in association with other climatic and oceanographic factors such as heavy rainfall and high sea levels, to produce coastal erosion. In this sense, waves form part of a group of interlinked hazards associated with oceans. This chapter examines these and other oceanographic hazards. Wave mechanics and the process of wave generation will be outlined first followed by a description of wave height distribution worldwide. This section will conclude with an appraisal of the hazards posed by waves in the open ocean.

Wind is the prime mechanism for generating the destructive energy in waves. In cold oceans, seas or lakes, strong winds can produce another hazard – the beaching of drifting sea-ice. Sea-ice is generally advantageous along a coastline that experiences winter storms, because broken sea-ice can completely dampen the height of all but the highest waves. When frozen to the shoreline as shorefast ice, sea-ice can completely protect the shoreline from any storm erosion. However, floating ice is easily moved by winds of very low velocity. These winds might not generate appreciable waves, but they can drive sea-ice ashore and hundreds of metres inland. The force exerted by this ice can destroy almost all structures typically found adjacent to a shoreline.