Soft soils do not dampen ground vibrations. They amplify them. Buildings on soft soil suffer much worse earthquake damage than those on bedrock.
Wave amplitude may double in passing from rock to soil.
Dominant natural period of the shock waves also increases, from about 0·3 seconds in solid rock, to 1–4 seconds on soils. The natural period further increases with soil depth, and with distance from the epicentre.
Buildings have a natural period of about N/10 seconds (N ⫽ number of stories). Maximum damage is due to resonance, when periods of building and soil match.
Deep soft soils have long periods that match those of high-rise buildings susceptible to more catastrophic damage – as in the Mexico City earthquake in 1985.
Soft soils cause damage that is typically 1–3 intensities higher than on adjacent bedrock.
Secondary earthquake phenomena
Subsidence due to liquefaction of low-density sands.
Landslides and slope failures of all sizes and speeds.
Tsunamis – oceanic seismic waves (section 17).
Seiches – oscillating waves on lakes.
Explosive volcanoes all lie on the convergent plate boundaries (e.g. Krakatoa, St Helens), where magma is generated by subduction melting. Viscous magma, of andesite or rhyolite, makes gas pressures build up.
Eruptions produce high ash clouds, explosive blasts and very dangerous pyroclastic flows (of hot gas and ash) which turn into lahars (mud flows) lower down valleys;
lava flows are minor and short.
Flank collapses can cause massive lateral blasts.
These eruptions are dangerous, largely unpredictable and totally uncontrollable; they must be avoided.
11 Rocks of Britain
Britain covers an area small enough to have its geology viewed as a single sequence of processes encompassing the whole country. With a single history, the geology of Britain is sensibly divisible by rock age, and with few exceptions the older rocks are stronger and more deformed than the younger. For such a small area there is amazing diversity within the geology, and all ages are represented within the rocks.
The tectonic framework of Britain has evolved over two successive convergent plate boundaries and then a divergent boundary; these have annealed fragments of continental crust to create the complexity of Europe, followed by the western breakaway of the Atlantic opening and the tensile thinning of the North Sea crust.
This evolution has created major contrasts in ground conditions across the country.
The old rocks of the northwest:
• huge thicknesses of rock crumpled on convergent plate boundary 400 million years old;
• have formed land subject to erosion ever since;
• now strong metamorphic rocks, intensely folded;
• accept high bearing pressures;
• yield valuable stone and aggregate resources.
The young rocks of the southeast:
• thin sediment sequences formed on the edge of subsiding North Sea basin ⬍200 million years old;
• were mostly covered by sea until 25 million years ago;
• now weak sedimentary rocks, gently folded;
• can take only low foundation loading;
• have no good aggregate resources.
The Carboniferous rocks of the middle:
• thick sediment sequences formed on wedge of plate between two boundary disturbance zones;
• include the Coal Measures of Britain’s industrial heartland, formed in equatorial delta;
• now strong sedimentary rocks, well folded;
• have very varied ground conditions;
• yield valuable rock resources of all types.
Geological evolution of Britain can be seen in time sequence of changing patterns of plate boundary pro cesses and sedimentary environments.
The map divides Britain into geological environments, largely related to age but primarily distinguished by the rock types and structures, which are the main concern of the ground engineer. The marked coalfields include concealed parts beneath Permian and Triassic cover.
Ireland represents a western continuation of the geology of Scotland and northern England; it is dominated by Carboniferous and older rocks, with the Antrim basalt plateau covering them in the north.
MAJOR ROCK UNITS OF BRITAIN Tectonics and environment Quaternary
Unconsolidated sand and clay, alluvium and till.
Tertiary
Soft sediments of London and Hampshire basins.
Poorly consolidated sands and clays, with London Clay 200 m thick.
Also basalt lavas, volcanic centres and intrusives of western Scotland.
Jurassic and Cretaceous
Weak sedimentary rocks forming most of southern and eastern England.
Chalk – 200 m thick, soft limestone with flint horizons forming Downs, Chilterns.
Clays and sandstones of Weald and Midlands, with thick Oxford Clay beneath Fenlands, unstable Gault Clay, and Portland limestones in south.
Sandstones and ironstones of Midlands and North Yorkshire Moors.
Oolitic limestones (oolites) from Cotswolds to Lincoln Ridge.
Lias blue-grey clays with thin limestones in Midland lowlands.
Permian and Triassic
Red sandstones and mudstones of Midlands lowlands.
Red mudstones with beds of salt and gypsum, including Mercia Mudstone.
Yellow and red sandstones with conglomerates, including Sherwood Sandstone.
Magnesian Limestone – impure, sandy or dolomitic limestone east of Pennines.
Granites of Devon and Cornwall with associated mineralization.
Carboniferous
Strong sedimentary rocks forming most of the high ground of Northern England, South Wales and Central Scotland, including all coalfield industrial areas.
Clyde Valley basalt lavas, Edinburgh volcanics, Whin Sill dolerite of Pennines.
Coal Measures – 2000 m of repetitive cyclic sequences, of sandstones, siltstones, mudstones, dark shales and thin coal seams (up to 2 m thick).
Millstone Grit Series – alternating sandstones (grits), shales with flagstones.
Limestone – massive limestones with cherts in S. Pennines, Wales and Mendips;
thin bedded impure limestones and shales in Northern England and Scotland.
Slates and grits in Cornwall and Devon.
Devonian
Slates, grits and limestones in Cornwall and Devon.
Brown sandstones and basalt lavas of Tayside and Ochils.
Red and brown sandstones and mudstones of Brecon Beacons and Orkney.
Highland granites west of Aberdeen.
Lower Paleozoic
Mountains of Wales, Lake District and Southern Uplands; repeated greywackes and slates 12 km thick; include Ffestiniog slate, Wenlock limestone,
rhyolite and andesite lavas and tuffs of Snowdon and Borrowdale Volcanics.
Dalradian schists, gneisses and marbles in southern part of Scottish Highlands.
Precambrian
Moine schists and gneisses forming most of Scottish Highlands.
Torridonian Sandstone of northwest Scotland.
Buried basement of England and Wales, only exposed in small inliers, including Charnwood, Birmingham, Long Mynd and Anglesey.
Lewisian basement gneisses of northwest Scotland and Outer Hebrides.
Coastline as now, but land bridge to Europe in low sea levels of Ice Ages.
Gentle Alpine folding of all rocks.
Deltaic sediments in bays of North Sea.
Volcanoes on Atlantic divergence.
Uplift forms land almost as today.
Thin sediment accumulation in shallow seas over England.
Sea forms shelf, marginal to subsiding North Sea basin.
Scotland and Wales form islands, with no deposition.
Atlantic opening starts.
Submergence under sea.
Desert sediment accumulation:
salt playas in low relief;
alluvial fans around mountains.
Marine incursion from east.
Magma from orogenic core.
Hercynian folding includes
Pennine anticline and coal basins;
more intense, with metamorphism, towards boundary in south.
Marginal plate boundary disturbance.
Intermittent swamp forests
esta blished on subsiding delta flats.
Massive delta expanding from north.
Shallow shelf seas and basins.
Convergent plate boundary in south.
Sediments in subducting ocean zone.
Marine sediments in southern ocean.
Red beds in desert basins surrounded by new mountains.
Melting in orogenic core.
Strong metamorphism of all old rocks.
Caledonian folding at plate collision.
Subduction of lapetus Ocean plate, beneath convergent boundaries, with local belts of volcanics.
Northwestern boundary more active.
Active convergent plate boundaries.
Sand deposited on continental block.
Old continental blocks:
SE block now largely buried;
fragment of NW block exposed.
Diagrammatic section of the geology across Britain, from Anglesey to the North Sea.
To follow a time sequence, this table should be read from bottom to top.
Sequence of rocks is in stratigraphic order, with youngest at top.
12 Rocks of the United States
The USA spans an entire continent and every type of geological environment. Unlike Britain its geology cannot be viewed in a simple time sequence: at any one time, plate boundary activity on one side could be distant enough to leave the other side unaffected.
Over such a large area, age alone is meaningless for describing ground conditions; igneous and metamorphic rocks in the west are far younger than weak, barely folded, sedimentary rocks in the east.
It is sensible to divide the USA into geological provinces, each with its own character and geological history, distinguished from its neighbours by both the types and structures of its rocks.