Additional case studies - natural events

EQs natural event punchline

• Natural tectonic processes are the primary cause of most EQs

• Human activities can and do play a significant role in triggering seismic events

• Specific contexts - mining, reservoir induced seismicity and hydraulic fracturing

Natural causes

Plate tectonics

• Tohoku - subduction of pacific plate under north American plate

• Intraplate

  • Gardonio et al., 2018

  • 2017 Mw6.5 Botswana EQ

  • Location with a lack of deformation and recent tectonic activity

  • Instead of localized stress or strain accumulation

  • reservoir of elastic stress which can be released episodically

  • triggered by deep fluid migration

  • causes stress perturbation

Volcanic EQs

• Magma movement

• Mt St Helens 1980

  • deformation in the rock surrounding the magma chamber

Reservoir induces seismicity

• Mechanism: Filling large reservoirs increases pore pressure in underlying rocks and can lubricate faults.

• Case Study: Koyna Dam, India – M6.3 earthquake in 1967 linked to the filling of the reservoir (Gupta, 2002).

  • most significant site of artificial water reservoir triggered seismicity

  • correlation between frequency of EQ and reservoir level

  • intersecting faults near Koyna = stress build up from plate tectonics

  • annual reservoir loading cycle - changes the ground water table - perturbs the stress build up

    • influx of pore pressure in underlying rocks

    • failure then occurs in response to very small changes in stress

Mining induced seismicity

• Mechanism: Removal of material from underground can change stress fields and cause collapse or fault activation.

• Called induced EQs rather than triggered - as the stress is preexisting it is released by human activities

• mining unloads prestressed rocks and leads to failure conditions

• Case Study: South African gold mines – Frequent small- to medium-magnitude earthquakes linked to deep mining (Gibowicz & Kijko, 1994).

Fracking

• Mechanism: High-pressure fluid injection to fracture shale rocks can activate faults.

• Case Study: Oklahoma, USA – Dramatic increase in earthquakes 40x more from 2008 to 2013, largely linked to wastewater injection from oil and gas activities (Keranen et al., 2014).

• Again linked to pre existing stresses

• pore pressure change critical threshold of 0.07 MPa above which EQs are triggered

  • but where the faults are near failure in the ambient stress field

  • 0.01-0.1 MPa

Tsunami natural event

• primarily natural geophysical processes

• growing recognition of human activities contributing to generation in some cases

  • Underwater mining

  • Nuclear testing

  • Reservoir triggered landslide

Natural

Undersea EQs (most common cause)

• Tohoku - Movement along thrust faults at subduction zones displaces water.

Volcanic eruptions

• Explosive eruptions, caldera collapse, or pyroclastic flows entering the sea

• 1883 Krakatoa Eruption (Indonesia) – Massive eruption and caldera collapse triggered tsunamis up to 40 m high, killing over 36,000 people (Simkin & Fiske, 1983)

Submarine and coastal landslides

• Mass movements displace water locally

• 1998 Papua New Guinea Tsunami – Submarine landslide triggered by M7.1 earthquake; waves reached 15 m and killed ~2,200 people (Tappin et al., 2001)

Comment on human

• This essay will use example which are not specifically tsunamis

• But demonstrate how human activity can contribute to the processes which trigger tsunamis

Coastal and underwater mining

• Mechanism: Large-scale seabed mining could trigger slope failure

• Speculative risk: Increasing concern with growth of deep-sea mining operations (Kusaka et al., 2020)

Nuclear testing

• Mechanism: Large underwater explosions can displace water

• (Mader, 1999)

• EQ → landslide → tsunami

• Case Study: 1958 Lituya Bay Tsunami (Alaska) – Though caused by a landslide, similar effects could theoretically result from massive man-made blasts

• Note: Nuclear tests (e.g. Bikini Atoll) caused large waves, but these are small compared to tectonic tsunamis and rare

Catastrophic landslide with human cause

• Case Study: 1963 Vajont Dam Disaster (Italy) (Kilburn & Petley, 2003)

• Dam filled

• Warning sides of movement in the slopes around the reservoir

• Kept increasing, with water level reduction to reduce pore water stress on the slope

• Despite water level reduction slope movements accelerated to more than 20 cm a day

• on the 9th Oct the mountainside collapsed

• 270 million m3 of rock slid 500m

• the landslide travel 140m up the other side of the valley

• took 45 seconds

• 115 million m3 of water

• wave of water crashed onto surrounding villages

• 2000 people died within minutes

Landslide with human trigger

• Creation of an artificial slope

• Spoil heaps

• 1966 Aberfan disaster in South Wales

• Collapse of overloaded spoil heap causing 144 deaths (Siddle et al., 1996)

• modified FoS through increasing the height and modifying gradient

• Natural element - spoil heap constructed on a known natural spring

ENSO drought

• Australian Millennium Drought (2001-2009)

• (Van Dijk et al., 2013)

• Linked to prolonged ENSO oscillation which disrupted usual rainfall

• El Nino conditioned explain 2/3 of the rainfall deficit in E Australia

• Natural processes changed the timing and magnitude of soil moisture streamflow and groundwater deficits

seasonal drought

• Med has seasonal dry period

• Sun saharan Africa is prone to variable rainfall and frequent droughts due to its climatic setting

• American midwest has seen cyclical drought before modern industrial activities (Woodhouse and Overpeck, 1998)

Human drought

• Mechanism: Vegetation loss reduces local humidity, disrupts precipitation, and increases runoff, reducing soil moisture.

• Amazon Basin – Deforestation alters regional rainfall and contributes to recurring droughts (Spracklen et al., 2015)

  • vegetation controls land-atmosphere moisture exchange

  • reduced ET, increased surface temps (1-3K) and changing boundary layer circulation which reduce circulation

  • Mechanism: Overuse of groundwater, inefficient irrigation, and diversion of rivers worsen water scarcity.

• Aral Sea Disaster (Central Asia) – Over-extraction for cotton irrigation led to severe water stress and ecological drought (Glantz, 2007).

  • fertilizers, hericides and pesticides used excessively

  • cotton is and incredibly water intensive cro

  • Sea has dried up and is now made up of smaller seasonal bodies of water

  • was one the fourth largest lake

• Sahel region: Overgrazing and poor land management contributed to desertification and recurrent drought conditions (UNCCD, 2017).

Flooding

• human activities - urbanisation, deforestation and land use change play substantial role in triggering and intensifying flood events

Urbanisation

• concrete and tarmac reduce infiltration and increase surface runoff

(Feng et al., 2020)

• Urbanisation impacts flood risk

• impermeable surfaces reduce hydrologic response

• Toronto Canada

• uses land use maps to track land use change and generate land use scenarios to model flooding

• urbanization creates higher surface runoff and discharge rates

• flash flooding more likely

Deforestation

• Mechanism: Reduces canopy interception and root absorption, increasing runoff and erosion.

• Bradshaw et al., 2007

  • data from 1990 to 2000

  • from 56 countries

  • flood frequency negatively correlated with the amount of remaining natural forest area

  • Flood frequency is positively correlated with natural forest area loss

• Model accounted for 65% of the variation in flood frequency with 14% of this due to forest cover variables

Climate change

• Mechanism: Warmer atmosphere holds more moisture, increasing extreme rainfall events and sea level rise.

• IPCC Finding: Climate change is “unequivocally” linked to more frequent and intense heavy precipitation events (IPCC, 2021).