Lecture 2: Magnitude, frequency and long term view Flashcards
Magnitude and Frequency of natural hazards
Affects
- Perception of people’s risk
- Insurance risk
- Societies response to events
How often an event of a certain magnitude occurs is
often expressed as:
- The probability of such an event, or
2. The recurrence interval (return period)
The concept of Recurrence Intervals occur in a variety of natural hazard research including:
earthquake hazards
volcanic eruption frequency
severe weather and storms
meteor impacts
When natural hazards are random it is helpful to represent their frequency as
An average time between past events, a “recurrence interval” also known as a “return time”.
Recurrence intervals refer to
past occurrence of random events.
Forecasting refers to
future likelihood of random events.
Frequency analysis uses
time series of a hazard to estimate the probability of a hazard of different sizes occurring.
Empirical distribution function
Rank all the data and plot each event against the proportion of events that are greater than it.
Frequency and magnitude analysis based on probability and recurrence intervals assume stationarity of the time series of these events.
In other words
the mean and variance are assumed not to vary over time.
However trends, persistence and intermittency in the behaviour of natural systems makes these assumptions insecure!
Q1: Have the frequencies of natural hazards increased?
To answer that we need to take a longer term view of the physical events and processes that generate the disasters…but how?
Climate independent events (Volcanoes)
Key Steps in reconstruction of volcanic records from Ice Cores.
Establish Ice Core Chronology
Separate Volcanic Sulphates from other sulphate sources.
Quantify volume of sulphate flux
Account for uncertainty in Chronology and separation method.
Test against known eruption events (fingerprint eruption using tephra)
Tephrochronology
Determine Geochemistry of Tephra
Match to Database of Eruptions Geochemistry
Identify known dated eruption
What do we learn?
Climate independent events (Volcanoes)
Many (larger) Volcanic events in the past.
No real trends in natural hazard time series.
Large events are infrequent but random.
We can identify the location and frequency of very large eruptions and use this to define risk IF we can identify their origin.
What do we learn?
Climate dependent events (Coastal Storms)
Many Cyclone landfall events in the past.
Considerable variability and uncertainty but..
Phases of increase Cyclone landfall broadly correlating with climate changes.
Recent increase in tropical cyclone activity.
People
The demography of populations is a function of peoples ability to choose.
Choice in crude terms is driven by access to wealth, political structures (borders), and other social factors.
The key is the movement of people into areas or a change in socio-economic status that makes them more VULNERABLE to natural hazards.
