3.5 5.c - Changing tectonic risk, resilience and recovery

How tectonic risk changes over time

A rise in recorded hazard events is not automatically a rise in the underlying geological rate. More instruments, improved sensitivity, satellite observations and faster reporting reveal smaller or remote events that earlier records missed. Comparing a modern comprehensive catalogue with a historical list of only destructive events can therefore create a misleading trend.

The USGS account of changing earthquake records explains why more recorded small earthquakes need not show global tectonic acceleration. Real rates also fluctuate, aftershock clusters occur, and some local seismicity can change. The correct response is to compare consistent magnitude thresholds, coverage, time windows and geological settings, rather than assume either every increase is real or no genuine change is possible. Volcanic records similarly depend on observations and preservation, especially for old or submarine events.

The USGS worldwide statistics for 2000–2021 provide a dated comparison of frequency and impact. Adding its magnitude 7–7.9 and 8.0-or-greater categories gives the magnitude-7-or-greater counts below. Death estimates concern earthquakes of all magnitudes and associated effects, so their scope differs from the frequency column.

Across the complete 2000–2021 series, annual magnitude-7-or-greater counts fluctuated between 7 and 24. These selected years show why impact cannot be inferred from frequency: 2004 had fewer large events than 2010 but a higher recorded death toll. Exceptional disasters, including the 2004 Indian Ocean tsunami and Haiti's 2010 earthquake, interact with where people live and their vulnerability. The low 2019 estimate therefore does not establish permanent safety or a continuing fall in losses. The table measures dated variation, while consistent longer records and exposure evidence are needed to explain a trend.

Risk can rise without more events. Urban growth and investment place more people and valuable infrastructure in hazard-prone areas. Settlement on unstable slopes, reclaimed ground or low coasts may increase exposure to particular processes. Dense interconnected networks can spread disruption: damage to one port, power system or bridge can interrupt activity elsewhere. Japan's 2011 supply and energy consequences illustrate the importance of linked systems beyond the initial rupture zone.

Vulnerability and capacity also change. Better construction, accessible warnings, health services and savings can reduce losses, while poorly enforced urban growth or inequality can increase them. Resilience concerns the ability to withstand, adapt to and recover from disturbance while maintaining essential functions; it includes institutions, relationships and knowledge as well as engineering. The UNDRR capacity definition recognises these combined strengths.

Recorded economic losses require interpretation too. Inflation, higher asset values and changing reporting can raise nominal totals. Deaths, displaced population, livelihood interruption and recovery time capture different dimensions. One unusually severe event can dominate a short series. A defensible trend claim states the measure, denominator and period and separates changing hazard from changing consequences.

Probability and the disaster risk equation

Disaster risk concerns potential losses over a specified period, not certainty that a damaging event will occur tomorrow. A useful conceptual equation links hazard, exposure, vulnerability and capacity. It is a way to organise causal reasoning, not a universally calibrated formula predicting deaths from arbitrary scores.

The UNDRR disaster-risk definition recognises these interacting factors. Zoning can reduce exposure, building improvement can reduce vulnerability, and accessible relief and services can increase capacity. These measures can change risk without changing the tectonic source. In a real quantitative assessment, hazard probabilities are combined with spatial exposure and vulnerability functions for particular loss types; uncertainty and dependencies matter.

Worked example 1: capacity in a conceptual index

Illustrative index: define $R=HEV/C$, with $H=4$, $E=5$, $V=3$ and $C=2$.

Doubling capacity halves this index, with other factors constant. The units are arbitrary; doubling a public budget would not necessarily double effective capacity.

Worked example 2: reducing exposure and vulnerability

Another scenario uses $H=2$, $E=12$, $V=4$, $C=3$. Relocation halves exposure and safer construction halves vulnerability.

The index falls to one quarter of its initial value because two multiplicative factors halve. This organises causal reasoning; the scores do not establish a real city's disaster probability or death toll.

Probability statements require a time window and threshold. A 2% annual chance of a specified event means a chance in each year under the model, not an event scheduled every fiftieth year. It can happen in consecutive years or fail to occur over a much longer interval. Real tectonic probabilities may use complex assumptions, and an estimated return interval is not a countdown.

Worked probability comparisons

For a constant, independent annual probability, first calculate the chance of no event throughout the period, then subtract from one. These are illustrative probability models.

Example 1: a 2% annual chance over ten years:

Example 2: a 3% annual chance over five years:

Neither result schedules an event. Dependencies, changing rates or a different hazard threshold would require a different model.

Even a relatively unlikely severe event can warrant planning where consequences are large. Conversely, an event can be common but create little risk if exposure is minimal. Risk decisions combine probability, plausible consequences, uncertainty and acceptable costs; they cannot be settled by magnitude or frequency alone.

Worked example: placemaking and tectonic risk

Question: Examine how placemaking can influence the risks associated with tectonic hazards.

Placemaking changes the built environment, activities and meanings of a location. The same investment that brings housing, employment and services can alter exposure and vulnerability. A successful regeneration strategy may attract more people and valuable assets into an active region, increasing potential losses even if tectonic event probabilities remain unchanged. The disaster-risk equation therefore requires both the benefits of investment and the location of its users to be considered.

Planning can instead reduce exposure through site choice. Japan's post-2011 reconstruction included relocation towards higher ground and land-use restructuring, reducing the overlap between housing and tsunami inundation. This is a material change in place, rather than merely promoting an image of resilience. It also changes everyday access to fishing, transport and community networks. A safer location that makes livelihoods inaccessible may encourage return or impose unequal costs, so the implementation of placemaking influences whether reduced mapped exposure becomes sustained protection.

Building design and infrastructure investment address vulnerability and capacity. Connected structural elements, suitable foundations and dependable routes to hospitals can reduce collapse or support response. Nepal's owner-driven reconstruction combined grants, technical support and checks to improve housing after 2015. The ability to apply those measures depended on finance, materials, builders and access to remote places. Placemaking therefore operates through institutions and delivery as well as architecture.

Contestation also matters. Landowners, renters, businesses and government may value different versions of a rebuilt place. Rising costs or exclusionary relocation can shift poorer residents towards unstable slopes or insecure housing, moving risk between groups. Community participation helps identify practical needs that an aggregate investment total conceals.

Overall, placemaking can increase risk by concentrating exposure or reduce it through safer sites, construction and services. It does not remove plate motion. Its influence comes from the social and spatial decisions that connect a hazard with people, so a defensible assessment examines who can remain, use safer alternatives and sustain livelihoods.

Future strategies and their limits

Future risk reduction combines better observation with safer decisions and sufficient capacity to act. Technology is valuable when it changes exposure, vulnerability or response, not simply because an instrument produces more data.

Volcano monitoring can combine seismicity, deformation, gas, thermal observations and the geological record. Several changing signals can support assessments of unrest and possible eruption scenarios, as at Grindavík in 2023–2024 and Mayon in 2018. But magma can intrude without erupting, vent location can be uncertain, and conditions can change rapidly. Probabilistic communication should explain scenarios and uncertainty rather than promise an exact harmless or dangerous outcome.