There is considerable variety in the ways in which events at regional distances are treated in different parts of the world. Most seismically active regions have a well distributed set of stations across the zone where earthquakes are most common, and the tendency in recent years has been for higher concentrations of instrumentation as, e.g., in Southern California and Japan. Where there are a significant number of seismic stations an indication of the likely position of an event is provided by the station recording the first indications of ground motion. This helps to start the process of location which is usually accomplished by a linearised inversion scheme or using some grid search algorithm (see Section 29.1). With relatively close stations it is usually possible to get quite good constraints of the depth of the event by exploiting both P and S information, using a velocity model designed to fit local conditions. The major difficulties in tectonically active areas come with the presence of substantial 3-D structure which introduces systematic errors into simple location procedures. It is frequently difficult to be confident of the association of smaller events with mapped faults.

For the San Andreas fault in central California, the marked contrast in seismic properties between the two sides of the transcurrent fault makes precise location difficult. There appears to be an offset between events at depth and the surface trace of the fault (Thurber et al., 2000) and this has been one facet of efforts to set up an observatory system with drilling through the fault zone at depth (the SAFOD project). Seismometers down the drill hole are planned to allow close observations of small magnitude events and so pin down the region where slip is occurring on this major fault.

In less seismically active areas the networks of stations tend to be rather sparse, so that there is both poorer coverage of events and poorer location capability. Also there are frequently more man-made events, e.g., from quarrying or mining which can be picked up using the networks. Even in such areas the majority of analysis is based on using short-period records for single stations.

ground is large and there is a strong interaction with man-made structures. The first instrument specifically designed to record strong ground motion close to an earthquake was installed in Tokyo at the end of the nineteenth century (Sekiya & Omori, 1894). This instrument was built to withstand severe shaking and employed mechanical recording with no amplification. The record displayed by Sekiya & Omori, for an event that caused some damage in Tokyo, was recorded with 3 separate orthogonal components on a circular plate. The largest amplitude of motion is about 45 mm on the NE/SW component with a period close to 2 s for the main pulse but also a long duration of coda. The large amplitudes and extended shaking reflect the influence of the Kanto sedimentary basin on which Tokyo is built.

Strong ground motion recording has been developed in a variety of ways since these pioneering studies, and now many classes of structures have their own instrumentation, notably on dams. In urban areas, recordings are commonly made with accelerometers which are triggered by movement of their surroundings (e.g. in buildings); in the past, this has often meant that only the S waves have been well recorded. The advent of digital recordings means that the new generation of strong ground motion recorders have sufficient memory that the P wave motion can be preserved for stations out to 100–200 km from the event. The emplacement of many such recorders in buildings, which is important for engineering seismology, means that the ground motion is modulated by the interaction with the man-made structure.

Nature of the local wavefield

For larger events where the area of faulting is comparable to the distance of stations fromthe fault, the details of the source process have a strong influence on the nature of the ground motion. The rupture of a major fault occurs progressively and is modified by the presence of stronger portions of the fault. Such ‘asperities’ often serve as nuclei for the distribution of aftershocks.

The nature of the local conditions can also have a profound influence on the recordings of ground motion, both through amplification and reverberation in soft surface sediment but also through interaction with 3-D structure in the vicinity of the fault.

Studies of the uppermost part of the mantle have largely been undertaken with man-made sources. Long-range profiles using conventional explosive sources rarely achieve data coverage beyond 1000 km. In consequence, much of our knowledge of the structures below 100 km depth in the deeper mantle lithosphere and mantle transition zone come from studies using earthquake sources, with composite results from many events. Nuclear explosions can provide sufficient seismic energy to yield records spanning the complex returns from the upper mantle discontinuities. Nuclear tests in the United States of America formed the basis of the work by Helmberger & Wiggins (1971), Wiggins & Helmberger (1973) which used synthetic seismograms to control the nature of the wavespeed profile in the upper mantle. In the former Soviet Union an extensive campaign of ultra-long-range profiles was undertaken using peaceful nuclear explosions, often specifically detonated for the experiments. Seismometers were deployed out to 3000 km from the source with multiple coverage along a number of profiles (see Section 22.2.1).

The uppermost mantle

The times of arrivals for stations from 200 to 1000 km away from the source show little variation in apparent slowness for P waves. A typical phase slowness for Pn is around 0.124 s/km, corresponding to a phase velocity of 8.15 km/s. However, once the opportunity arose to examine the nature of the seismograms in detail, it became apparent that this relatively uniform slowness for the first arrivals did not correspond to a single phase branch but rather to a sequence of en echelon P contributions. Such behaviour has emerged in all cases where there is sufficient data density to follow the details of the arrivals returned from the uppermost mantle.

For example, in the 1971 long-range profile across France (Hirn et al., 1973), a Pd arrival was correlated from 150 to 250 km and is then superseded by a set of further phases PI from 300 to 500 km and PII from 450 to 600 km; a further PIII phase was correlated by Kind (1974). On the basis of the amplitude behaviour (Hirn et al., 1973; Kind, 1974) and systematic travel time inversions (Kennett, 1976), the retrograde

In the previous chapter we have seen how the global seismic wavefield evolves with time after source initiation, and the way in which the complex pattern of internal wavefronts is reflected in the nature of seismograms at surface receivers. The increase in seismic wavespeed with depth through the mantle plays an important role in determining the character of seismograms. The refraction of body waves back to the surface yields the distinctive features of teleseisms through the P and S arrivals. Multiple surface reflections of the body wave arrivals carry phases such as PP, PPP to great distances. The high wavespeed gradients in the upper part of the mantle create a waveguide that traps shear wave energy between the surface and reflection from the increase in wavespeeds leading to complex sets of multiple S reflections grading into the surface wave trains. Surface reflections with conversion, such as PS, have rather asymmetric propagation paths, but can play an important role at greater distances.

For SH waves, the whole mantle acts as a waveguide because these waves cannot penetrate into the core. The boundary conditions on the SH wavefield is thus a requirement of vanishing traction at both the Earth's surface and the core mantle boundary, with complete reflection of SH waves at each surface. The core reflection ScS is therefore both more prominent for SH waves than SV waves and its multiple reflections (ScSH)n continue for a long time. In Section 25.3 we will show how the long term reverberations can be used to investigate the presence of discontinuities in mantle structure.

Mantle phases

The P and S body waves become simpler in character beyond 30°, once their turning points lie in the lower mantle and so no longer feel the influence of the upper mantle discontinuities that we have discussed in Chapter 22. The surface multiples PP, SS etc. turn at shallower depths and so extend the influence of the transition zone to 60° and beyond for higher order multiples (see Section 21.3). The core reflections cut across other phases and so there are limited intervals in which they appear as distinct arrivals.

When we come to study a specific region of the Earth we can exploit both seismic events within and surrounding the region, and those at teleseismic distances, by looking at different aspects of the wavefield.

Where waveform inversion is used for fundamental and higher mode surface waves, the results of global studies of the mantle can provide a useful starting point for the characteristics of large-scale heterogeneity. Particularly where temporary stations have been deployed to enhance coverage it is possible in a regional survey to obtain much higher horizontal resolution as has been demonstrated, e.g., by the Skippy experiment in Australia (van der Hilst, Kennett & Shibutani, 1998).

For body wave studies, we can use the waves refracted back from the upper mantle within the region if we can disentangle the triplications induced by the presence of the mantle discontinuities. Such regional arrivals travel at shallow angles through the structure. We can also use the arrivals from distant events for which the ray paths travel relatively steeply through the upper part of the mantle and so provide a different class of information. Where we have local seismicity, we can also exploit the observations of these events at distant stations with a knowledge of the global 3-D structure outside the region of interest.

Regional surface-wave tomography

In order to obtain high resolution images of the shallower part of the Earth we need to use seismological probes whose sensitivity is greatest in this region. Fortunately we are able to exploit the surface wave portion of the shear wavefield which commonly represents the largest amplitude portion of broad-band seismograms.

The fundamental mode Love waves and Rayleigh waves are preceded by higher mode surface waves which represent the superposition of multiply reflected S waves interacting with the free surface (cf. § I:16.2). The fundamental mode energy is concentrated near the surface, but the penetration increases as the frequency is reduced because the S wavelength is longer. The higher modes provide both deeper penetration at comparable frequency and complementary information on shallow structure because the pattern of sampling is rather different.

The influence of Earth structure on the surface waves appears through the frequency dispersion of the different surface wave modes so that the character and frequency content changes along the wavetrain as it is recorded at an individual station.

The propagation of seismic phases at regional distances has been a topic of continuous interest since the work of Mohorovičić (1909) on the Kulpatal earthquake in Croatia, from which he proposed the presence of a discontinuity in seismic wavespeeds at a depth near 50 km. The study of crustal structure was pushed forward, mostly using information from natural sources, by many authors including Jeffreys and Conrad. But, it was only when a significant number of records were available from a single source that the full nature of the crustal wavefield could be appreciated.

The first major experiment designed to exploit the refracted waves through the continental crust and mantle was the Heligoland explosion in 1947 (Willmore, 1949) recorded out to 350 km from the source on portable stations, and beyond 700 km at permanent stations. The analysis of crustal and mantle properties using explosive sources to tackle structural problems became a major tool during the 1960s. Initial analysis was based on the interpretation of travel times of the various phases. To try to resolve ambiguities in the models, amplitude information began to be exploited and this lead to the development of both the computation of theoretical seismograms for realistic models (Fuchs & Müller, 1971) and more sophisticated ray-tracing tools (see, e.g., Červený, Pšenčí k & Molotkov, 1977).

Such studies of seismic structure are based on an analysis of the details of seismic arrivals, with emphasis on the correlations between successive seismograms in a record section display. The use of multiple seismograms, with a close spacing in distance, is very helpful in enabling specific types of arrival to be tracked across the record section, so that the influence of noise on individual seismograms can be reduced. The notation used to mark arrivals in such structural studies is designed to provide a detailed correlation between the classes of arrivals and the structural features which give rise to them.

A further impetus to understand the detailed characteristics of the regional wavefield has come from efforts to monitor underground explosions of low yield and to discriminate them from earthquakes (see, e.g., Pomeroy, Best & McEvilly, 1982). In work directed towards source characterisation, the available data commonly consists of a set of seismograms from widely separated arrays or three-component stations.

As we have seen in the previous chapter, it is nowpossible to undertake investigations of the nature of seismic wave propagation in 3-D models. We now turn to the way in which information about such 3-D structures is extracted from seismic observations that provide direct coverage of the region of interest with multiple crossing propagation paths. This process has come to be known as “seismic tomography”. The original meaning of the term tomography implies reconstruction from multiple sections through the feature of interest. Such a configuration occurs in medical applications, where equipment is also designed to provide rather uniform sampling. In studies of the Earth the sampling is much more irregular, being dictated by the location of available sources and receivers. Further, our sensors sit at, or close to, the surface of the Earth so that it is rare to achieve all-round coverage of individual features. As a result we face a situation with somewhat limited sampling and hence significant differences in the potential resolution of different parts of the 3-D structure. The majority of studies have employed fixed grids or basis functions, but in the future we are likely to see increased emphasis on adaptive inversion schemes designed to exploit the characteristics of individual data sets.

Elements of seismic tomography

Seismic tomography depends on the presence of contrast in seismic properties. Such differences in three-dimensional structure are reflected directly in the times of arrival of seismic phases or through the shape and amplitude of seismic waveforms.

The normal procedure is to examine the departures of the observed properties of the seismic wavefield from the predictions for a reference model. In most cases the significant quantity is a time shift in the arrival of a phase, which can be measured directly for a short-period body wave, by correlation techniques for long-period body waves and surface waves, or by waveform inversion where the record contains a number of arrivals as, e.g., the shear wavefield containing both body wave and surface wave components. The travel-time residuals for body waves or modified dispersion characteristics for surface waves can then be used as the basis for recovering a three-dimensional model.

In the discussion of the nature of global wave propagation in Chapter 24 we have seen the complexity of the wavefront patterns in the Earth's core associated with the many different propagation processes in the core, with internal multiples from the core-mantle boundary, strong refraction of P waves from the gradients in the outer core, and conversions between P and SV at the core-mantle boundary.

In this chapter we will focus first on the major core phases and their multiples. We illustrate the travel time behaviour and the relation to the associated ray patterns through the Earth. The nature of the seismograms will be illustrated with record sections covering the significant distance ranges in different frequency bands.

The seismic velocity structure in the core is constrained by both the properties of the Earth's normal modes and the observations of the travel times of seismic phases. Because the core itself is a low velocity zone for P waves relative to the mantle, but not relative to S in the mantle, direct sampling of the outermost part of the core comes from the SKS phases. There is thus a very close tie between the velocity structures adopted in the mantle and the core.

The inner core can only be studied by the way in which it interacts with different classes of seismic waves and so we are probing with waves that can be influenced by the heterogeneity of the mantle, particularly near the core-mantle boundary. Nevertheless there are indications of anisotropy in seismic properties, and some degree of heterogeneity, in this body at the centre of the Earth.

The strong heterogeneity near the core-mantle boundary has been invoked as one of the causes of scattered waves associated with core phases which are, e.g., prominent as precursors to PKP near 140° epicentral distance.

Representation of core propagation processes

In Chapter I:14 we have made a representation of the propagation processes affecting a seismogram and have shown how we can separate off the influence of the shallow part of the structure, including near-source and near-receiver effects, and concentrate attention on the return from structure at depth.

The analysis of seismic records depends on being able to disentangle the influence of the source and the structure of the Earth. Effective source characterisation thus becomes an important prerequisite for gaining new information on earth structure. Conversely, knowledge of the structure of the Earth is essential for providing estimates of the location of seismic events and the quantitative assessment of source mechanisms.

The need to know both the structure of the Earth and the properties of the source with only the information available from seismograms has dictated the development of seismology. Improved source locations have led to better Earth models, which in turn have been used to extract more information on all aspects of the source.

Seismic source estimation

The major part of the information used in the location of a seismic event is a set of arrival times of seismic phases which has been measured from displays of seismic waveforms by experienced analysts. For seismic arrays it is possible to provide additional information such as the apparent azimuth of the arriving energy and its velocity across the array.

For each seismic station the object is to provide arrival times (and array information) for a number of different seismic phases which have followed different propagation paths through the earth, together with an indication of the likely character of the arrival. Particular effort is made to try to extract information which is sensitive to the depth of the event, e.g., by recognising the phases pP, sP which are reflected back from the surface above the source.

The initial identifications of phase character may need to be modified when information from many stations are combined. One of the most difficult tasks has then to be faced. From the assemblage of arrival time picks, azimuth, and apparent velocity, those readings fromdifferent stations which should be grouped together have to be recognised and a preliminary location assigned. This location will then be refined by matching the patterns of observed and predicted arrival times.

Around the globe, many different seismic events can occur in a given time period so there is no guarantee that all readings correspond to the same event.

Much of the impact made in recent years by seismic tomography has come from global studies in which three-dimensional structure has been revealed throughout themantle, and even in the inner core.

Two very different styles of approach have been employed, based both on the nature of the data sets which have been used and also on the style of representation of heterogeneity. For studies in which seismic waveforms are calculated using modal summation or dispersion is used, spherical harmonics are a natural choice for the basis of representation of heterogeneity because they are closely related to the mode representation itself. On the other hand for ray information, although it is possible to project rays onto a spherical harmonic basis there is not the same direct relation as in modal methods.

Where information from modal and body wave times is combined, then a single representation in terms of spherical harmonics is commonly adopted (see, e.g., Su & Dziewonski, 1997). However, studies using just travel times have frequently used cellular representations (e.g., Grand, 1994; van der Hilst et al., 1997), but spherical harmonics can be useful where summary properties are desired as in the work of Robertson & Woodhouse (1995, 1996) on the relationship of P and S wavespeed heterogeneity.

A very important aspect of global studies arises from the distribution of earthquake sources across the globe (figure 32.1). As is well known, the occurrence of earthquakes is concentrated into zones associated with subduction processes. There is a modest level of activity along the mid-ocean ridges and scattered, infrequent events in stable continental and oceanic regions. The distribution of high quality seismic stations is also uneven because they also tend to concentrate in areas of significant seismicity. The efforts of Geoscope and IRIS in establishing global networks of broad-band seismometers have made a major difference to coverage, but still such regions as Africa have a very limited number of stations. The other major gap is represented by the world's oceans and this has been partly infilled by island stations notably by IRIS and the Ocean Hemisphere Project. Ocean bottom geophysical observatories, either attached to disused telephone cables or free-standing are planned, but will be expensive to establish and maintain.

As noted in volume I, the inspiration for this book came from the remarkable set of articles by Beno Gutenberg in the Handbuch der Geophysik, particularly vol. 4, 1932. Gutenburg provided a comprehensive summary of the current theory and accompanied this with a discussion of the nature of seismic observations.

This second volume of The Seismic Wavefield is primarily devoted to the interpretation of observed seismograms in terms of the physical processes which control their properties, with a strong link to the theoretical development in the first volume. References to sections in the first volume are indicated using a section marker (e.g., § I:3.1.2) and for equations from the earlier volume the volume number is represented explicitly as in (I:14.2.1). The emphasis throughout the book is on body waves and surface waves with frequencies above 10 mHz, rather than phenomena which are best treated through a normalmode development for the whole Earth, which are well covered in Theoretical Global Seismology by Dahlen & Tromp (1998).

The treatment starts with a discussion of near-source effects and strong ground motion. Then attention is directed to the wavefield as seen at progressively larger distances. For regional ranges, out to 1000 km from the source, the properties of the crust and uppermost mantle play an important role. In the far-regional range, for epicentral distances from 1000 km to 3000 km, the complex interactions with upper mantle discontinuities dictate the character of P and S arrivals. For larger epicentral distances a global perspective is appropriate, but surface multiples still carry relatively shallowly propagating waves to substantial distances.

The introduction to the discussion of seismic wave propagation across the globe was strongly influenced by the 2 m long wall chart From Earthquake to Seismogram prepared in 1962 under the direction of the late Professor S. Warren Carey at the University of Tasmania. In this chart the wavefronts of body waves and surface waves are carefully mapped through the Earth and across its surface, and these wavefronts are related to the features of a seismogram from an earthquake in the Kuriles recorded at the Hobart Observatory in Tasmania.