earthquakes

Faulting and Earthquakes

Movement of the earth's crust:

strong deformation forces on the earth's crust strata may result in:
- folds - crumples & wrinkles called monoclinal folds or monoclines in a similar way to those in a tablecloth pushed on a table-top
- joints - cracking of rocks - esp. in granites & in lava flows where there is shrinkage due to cooling
- faults - displacement of rock masses
deep-seated changes in the earth which result in folding, jointing or faults are collectively known as diastrophic movements.
types of movements include:
- orogenic movements:
  - movements that produce folds that result in mountain chains forming are called orogenic movements.
  - it is a feature of orogeny that the fold mountain chains originating during any one period are formed in very long & relatively narrow orogenic belts which are then gradually eroded over time
    - examples:
      - Tertiary orogenic belts are the youngest and form the highest mountain chains such as the European Alps & the Himalayas.
      - Palaeozoic & Mesozoic belts are represented by less continuous mountain zones of lower elevation such as the Appalachians of eastern USA, the Urals and the Great Dividing Range in Australia
      - the older Precambrian folds have largely been eroded to extensive areas very low relief, either rocky lowlands or plateaux, form as it were the cores of continents, these are known as shields and including the Canadian, Russian, Brazilian & Afro-Arabian shields.
- epeirogenic movements:
  - simple uplift, depression, tilting or broad warping affecting wide regions rather than narrow belts
  - examples:
    - depression of the Murray Basin
- eustatic movements:
  - changes in the level of the sea result in global effects on shorelines, etc.
  - examples:
    - a major withdrawal of the sea from all coasts during the last glacial stage of the Pleistocene to a level 150m lower than present.
    - a rise in average sea water temperature of 1decC globally would result in sea levels rising by 2m due to expansion of water, and if the ice caps melted, a rise of 90m would occur.

Folding:

those folds forming ridges are called anticlines whilst those forming troughs are called synclines.
folds both small (eg a centimetre) and large (eg. kilometres) as well as faults result mainly from squeezing (lateral compression) of the land mass.
rocks of Palaeozoic age in Victoria are strongly folded, as may be seen in road & railway cuttings in the Melbourne district (esp. at Studley Park) & also Castlemaine, Bendigo & Daylesford.

Faulting:

a fault is a fracture in the earth, on which the blocks adjoining the fault on either side are displaced relative to each other by slip along the fault
whilst we are mainly aware of faults in the earth's crust, deep-seated faults affect the mantel down to depths of 750km and which are concerned in the origin of deep-focus earthquakes.
in many instances, faults occur in groups, so that the crust is divided into a number of fault blocks.
- where the fault blocks come to rest at different elevations, they are each bounded by steep fault scarps, these being the exposed portions of the actual fractures or fault planes.
- blocks depressed between opposed scarps form fault troughs, which if very long compared to their width, are called grabens
- raised blocks are called horsts, and those that are markedly tilted, tilt blocks
  - the surface of a tilt block sloping away from the scarp is called the back slope
  - the natural depression between the scarp of one block and the back slope f the next adjacent is a fault angle depression
faults may result in either:
- vertical slip down a sloping fault
- lateral movement - a "strike-slip fault" or "wrench fault" eg. San Andreas Fault, California and the Alpine Fault in NZ.
major faults occur at junctions of the main tectonic plates:
- junction of the Indo-Australian Plate & the Pacific Plate - Alpine Fault NZ
- junction of the Indo-Australian Plate & the Fiji Plate

Earthquakes:

when a true earthquake of tectonic origin occurs, the rocks, which are in a state of elastic strain due to internal stresses in the earth, suddenly yield. As fracturing occurs, the strained rock springs back, and it is this sudden impact of the rebounding rock mass that generates the earthquake shock
along a fault plane, movement goes on intermittently involving alternate periods of adherence during which elastic strain builds up, and displacement when an earthquake occurs
the sub-surface location of the seat of an earthquake is called its focus, and it is from this that seismic waves travel away (at ~14km/sec) due to the elastic vibrations set up. The focus can be as deep as 700 kilometres. Earthquakes do not occur deeper than this because rocks are no longer rigid at very high pressures and temperatures - they can't store stress because they behave plastically. Most intra-plate quakes have shallow focus of < 20km;
the small point of land above the focus is called the epicentre of the earthquake, and it is there that maximum disturbance is experienced
the magnitude of an earthquake is a single calculated figure based on interpretation of standard seismographic records located 100km or more from the epicentre
- the Richter scale is logarithmic measure of the seismic amplitude measured in microns
  - the smallest recordable earthquake of 1 micron = zero on the Richter scale (ie. = log1)
  - the most severe earthquake analysed is 8.9
  - humans can detect those greater than 1.5
  - less than 3.4 are recorded only by seismographs although 2.0 to 3.4 may be felt within a few kilometres of the epicentre.
  - 3.5-4.2 are felt by some people who are indoors
  - 4.3-4.8 are felt by many people and windows rattle
  - 4.9-5.4 are felt by everyone, while dishes break and doors swing
  - 5.5-6.1 cause slight building damage with plaster cracking, and bricks falling
  - 6.2-6.9 cause much building damage and houses move on their foundations
  - 7.0-7.3 cause serious damage with bridges twisting, walls fracturing, and many masonry buildings collapsing
  - 7.4-7.9 cause great damage and most buildings collapse, M 7.5 is the lower limit of shocks described as "major earthquakes"
  - more than 8.0 cause total damage with waves seen on the ground surface and objects are thrown in the air, and if under-sea, generate mega-tsunamis
in severe earthquakes, the ground may be thrown into waves, deep fissures open & may close again, and in soft rocks such as river alluvium, jets of water charged with mud and sand spurt up under pressure, the damage to buildings being generally greater in such soft formations than on hard rocks. Displacements of up to several metres may result.
on average, each year the earth experiences:
- one earthquake of magnitude 8
- ten earthquakes of magnitude 7
- one hundred earthquakes of magnitude 6
reservoir-triggered earthquakes:
- large new reservoirs can trigger earthquakes. This is due to either:
  • change in stress because of the weight of water, or more commonly by
  • increased groundwater pore pressure decreasing the effective strength of the rock under the reservoir.
- for triggered earthquakes to occur, both mechanisms require that the area is already under considerable tectonic stress.
- reservoir triggered earthquakes are often referred to as reservoir induced seismicity (RIS), but use of the term "induced" is now becoming unfashionable. To many people it implies that the reservoir caused the earthquake. The energy released in a reservoir triggered earthquake is normal tectonic strain energy that has been prematurely released because of the reservoir.
- groundwater pore pressure:
  - ground water plays a large part in earthquake activity. Fluid injection into wells in USA, Japan and elsewhere has triggered small earthquakes.
  - water pore pressure reduces the normal stress within a rock while not changing the shear stress. Under any circumstances, an increase in water pore pressure means that a failure is more likely. The critical value of shearing stress may be made arbitrarily low by increasing the pore pressure.
  - pore pressure can increase in two ways:
    • due to the decrease in pore volume caused by compaction under the weight of the reservoir. This occurs while the reservoir is being filled.
    • due to diffusion of reservoir water through permeable rock under the reservoir. The rate of flow depends on the permeability of the rock, so this effect is not instantaneous. The increase in pore pressure takes more time depending on the distance from the reservoir. It may take years for the pore pressure to increase at depths of kilometres beneath a reservoir.
- Reservoir induced seismicity is a transitory phenomenon which will occur either immediately after filling of the reservoir, or after a delay of a few years. If there is a delay, this depends on the permeability of the rock beneath the reservoir.
- Once stress and pore pressure fields have stabilised at new values, reservoir induced seismicity will cease. Earthquake hazard will then revert to similar levels that would have existed if the reservoir had not been filled.
- Even for those reservoirs that show a correlation between earthquake activity and water level, reservoir induced seismicity does not continue indefinitely as it is limited by the available tectonic energy.
- examples of RIS:
  - World:
    - Koyna, India, 1967, M 6.7
    - Xinfengjiang, China, 1962, M 6.2
  - Australia
    - Warragamba, NSW, 1973, ML 5.5
    - Thomson, Victoria, 1996, ML 5.1

Global earthquakes:

biggest since 1900:
- Chile 1960 Richter 9.5 (5,000 killed)
- Sumatra 2004 R 8.9 (15,000 killed by tsunamis)
- Macquarie Island - Southern Ocean south-east of Tasmania Dec 2004 R 8.1 (no casualties)
- Alaska, Canada 1964 R 8.3
- Alaska 1957 R 8.1
deadliest since 1900:
- Tangshan, China 1976 (240,000 killed)
- Tokyo-Yokohama, Japan 1923 (>140,000 killed)
- Quetta, India (50,000 killed)
- Northwest Iran (50,000 killed)
- southeastern Turkey 2003 (>41,000 killed)
- Erzincan province, Turkey 1939 (33,000 killed)
- Chilan, Chile 1939 (28,000 killed)
- NW Armenia 1988 (25,000 killed)
- NE iran 1978 (25,000 killed)
- Guatemela 1976 (22,778 killed)

Earthquakes of south-eastern Australia:

vulcanicity and earthquakes tend to occur primarily at the junctions of the crustal plates which are mainly:
- the edges of the Pacific Ocean (including Japan, Phillipines, Papua New Guinea, NZ, western coast of South America and North America - the San Andreas Fault) - these areas account for 80% of the world's earthquakes!
- north-eastern coast of Mediterranean through Turkey, Iran
- southern aspect of Indonesia
fortunately, the Australian continent is quite distant from the edges of the Australian Plate and thus vulcanicity is extinct and major earthquakes rare. Intra-plate earthquakes still do occur and tend to be shallow (<20km deep).
the edge of the Australian Plate includes NZ (forming the Alpine Fault), the Pacific Islands, Papua New Guinea & the southern aspect of Indonesia, mid-Indian Ocean and mid-Southern Ocean
there are on average 200 earthquakes of magnitude 3 or more in Australia each year. Earthquakes with a magnitude of 5.5, like that at Newcastle, occur on average once every 2 years. Every 5 years or so there is an earthquake of magnitude ML 6.0 or more, a potentially disastrous earthquake.
the largest known earthquake in or close to Australia occurred in 1906 off the coast of Central Western Australia, its magnitude 7.2.
the largest known onshore earthquake was in 1941 near Meeberrie, 600 km north of Perth, WA. Its magnitude was 6.9, the same as the January 1995 Kobe earthquake in Japan.
very few earthquakes since white settlement have caused significant damage with the main ones being:
- Dec 28, 1989, Newcastle M5.6 - 13 killed & $1.5b damage
- May 22, 1961, Bowral NSW M5.6 - $0.5m damage
- Mar 1, 1954, Adelaide M5.4 - $8.8m in 1954dollars damage to buildings, esp. along the alluvial plains along the coast
- Nov 3, 1944, Eildon, extending to Bendigo, Geelong & Gippsland
- Apr 29, 1941, Meeberrie, WA - M7.0 - Australia's biggest onshore event
- Sept 3, 1932, Mornington extending across Westernport
- April 10, 1922, Bass Strait extending from Warrnambool to Ballarat, Dandenongs and Wilson's Prom.
- April 7 & July 14, 1903, Bass Strait, extending from Portland to Ballarat and Geelong, damaged buildings in Warrnambool
- July 10, 1903, Maryborough
- Sept 19, 1902, north-west of Grampians
- May 27, 1900, western Gippsland extending to south & eastern suburbs
- 1897, Stawell
- May 10, 1897, Kingston SA
- Jan 27, 1892, Bass Strait extending from Wilson's Prom to Eden to Tasmania M6.9
- Oct 8, 1885: Bright to Wodonga and eastwards
the main areas of seismicity over the past century have been:
- Canberra region
- Wilson's Promontory/Foster region
- Otway's region (centred in Bass Strait)
- Flinder's Island (north-east of Tasmania)
- Port Augusta region
- Eyre Peninsula
- Adelaide
major fault lines near Melbourne:
- Selwyn's Fault and the parallel Tyabb Fault - Mornington Peninsula
- Beaumaris Monocline transected by the perpendicular Melbourne Warp under the south-eastern suburbs
- multiple fault lines running from Gippsland to the eastern aspect of Westernport Bay
- Rowsley Fault - north-west of Geelong running northwards to Bacchus Marsh
- Barrabool Fault - running west from Geelong to Colac
- a fault on Bellarine Peninsula parallel to Selwyn's Fault
- Torquay Fault - running along coastline

earthquake risk:

World:

Australia