1. Earthquake Hazard Session 1 Mr. James Daniell Risk Analysis
Earthquake Risk Analysis 1 1

2. Learning Objectives
Develop an understanding of basic earthquake processes
From the source, via a path, to the site
Explore seismic principles such as waves and other concepts in terms of hazard
Know about ground motion, site effects
Know how seismic hazard is integrated into loss models
Know the difference between probabilistic and deterministic models
Apply hazard assessment to a real situation
The learning objectives for this presentation are to a) develop an understanding of basic earthquake processes including the what, where, how and why of earthquakes, from their beginnings at the source, the production of waves travelling through the earth, to the site on the earth’s surface where damage occurs, b) Explore seismic principles such as waves and other concepts in terms of hazard, c) Know what ground motion is and the impact of site effects on ground motion, d) Know how seismic hazard is integrated into loss models, e) Know the difference between deterministic and probabilistic hazard assessment and then f) to apply hazard assessment to a real location such as your home country.

3. Plate Tectonics
Tectonic plates are moving relative to each other via a process called plate tectonics
250 million years ago all current land masses were a part of Pangaea before splitting into Laurasia and Gondwanaland. India broke from Gondwanaland to join Asia and the continents reformed to today’s location. There are about 30 crustal plates which float on the aesthenosphere which is a liquid part of the mantle. The aesthenosphere supports the upper layers of solid material (the lithosphere or crust) which move under large convection currents.
The plates are moving relative to each other via a process called plate tectonics. In the picture above, only the main plates are shown. This movement of plates is only in the order of mm-cm/year but over a geologic time scale much stress and strain is built up and released.
This constant movement of plates against one another, under compression, tension and shear, can cause a brittle fracture stress release. This stress release causes major earthquake and volcanic activity at such boundaries and within continents. Plate boundary earthquakes are generally more common than intraplate earthquakes.
The following diagram shows the process of production of new crust from the cooling of the mantle via seafloor spreading forming mid-ocean ridges, and the subduction of an old crustal plate down into the mantle underneath another crustal plate with trenches and volcanic activity.
Plate Tectonics, mantle, crust, subduction 3

4. Seismic Hazards
Most earthquakes occur on faults – areas of broken and displaced rocks.
There are 3 main types of faults: Strike-Slip, Reverse (compression) and Normal (tension).
Earthquakes occur in both oceanic and continental crust at varying depths.
Strike-Slip
Reverse
Normal
Most earthquakes occur on faults which are areas of broken and displaced rocks within the crust. There are two types of crust: oceanic and continental, and earthquakes occur in both. Where the old oceanic crust moves under the continental crust, this is called subduction, a special form of thrust faulting. Otherwise earthquakes occur in standard settings. In terms of the standard fault types as seen in the diagrams, there are 3 main types of faults that occur within the crust.
Strike-Slip or transform faulting occurs by two vertical faults moving side-by-side relative to one another in a shearing motion. The two sides of a non-vertical fault are called the hanging wall and foot wall, coming from old mining terminology of where the miners’ feet were or where the hanging lamp was. The hanging wall occurs above the fault, and the foot wall below.
Reverse or thrust faulting occurs when the hanging wall moves above the foot wall, and is caused by compression.
Normal faulting occurs due to tension with the hanging wall moving down relative to the foot wall. These concepts will be further explained through animations in the Earthquake Learning Animations presentation.
Blind thrust faults can also occur where the fault is completely under the surface and is therefore not recognized in the surface topography of the earth and oblique-slip faults which also occur are a combination of strike-slip movement with a vertical component of movement.
85% of the world’s earthquake energy release occurs near the earth’s surface in the top 0-60km. Intermediate (60-300km) and deep ( km) earthquakes also occur deep in the crust or within subducting plates.
Faults, oceanic and continental crust, reverse, normal, strike-slip 4

5. How do earthquakes occur?
Built up energy, stress and strain release on plate boundaries and faults, via deformed rock vibrating.
Initial earthquake rupture occurs on the fault at the hypocentre (also known as focus).
Movement (slip) occurs along the whole fault plane with rapid energy release in all directions.
Earthquakes form as a result of built up energy, stress and strain which can occur on plate boundaries as well as along other weakened fault zones. The plates slip slowly over a long time period, and then, to realign the plates, a rapid release of energy occurs – the earthquake. This process is called elastic rebound.
The initial earthquake rupture occurs at a source on the fault called a hypocentre (otherwise known as a focus) with the epicentre defining the point on the earth’s surface directly vertical above the hypocentre. Movement or what geologists define as slip occurs along the whole fault plane and as this occurs, the deformed rock vibrates in order to move back to its original shape. Energy is rapidly released as a result of the slip in all directions in the form of waves.
Hypocentre, epicentre, slip, fault plane, rupture, elastic rebound 5

6. What happens when the fault slips?
This energy release sends seismic waves into the earth.
These waves have a certain wavelength (called period) and a certain amplitude.
From the source, the waves travel along the path, and arrive at the earth’s surface (i.e. at the site). The amplitude and period of these waves are measured and called the ground motion. This is measured by a seismometer.
An energy release occurs along the area of slip (from metres to 100s of kilometres), sending out seismic waves into the earth as well as converting energy to friction and sound. These waves have a certain wavelength (called period) and certain amplitude. This amplitude generally decreases as the waves move away from the source along what is called the path.
When these waves arrive at the earth’s surface (i.e. at the site), the amplitude and period of these waves are measured in what is called the ground motion. Here we see the wavefronts from the source, moving through the earth, to the site where the ground motion is measured via a seismometer or accelerometer. It measures the ground motion of the Earth relative to a stationary mass on a rotating drum to record a digital or analog record of the horizontal and vertical motion.
Seismic waves, period, amplitude, path, site, ground motion, seismometer 6

7. Seismic Waves There are 2 types of waves – body and surface.
Body waves travel through the earth (primary waves=P waves with a push-pull action, and secondary waves=S waves with an up-down action).
Surface waves have both horizontal and vertical motion, have a high amplitude and cause the most damage close to the epicenter.
Waves are faster in rock and slower in loose soils. The amplitude of the wave is increased in loose soils.
L. Braile, 2005.
There are 2 main types of seismic waves – body and surface. Body waves travel through the earth’s interior. Primary waves, otherwise known as P waves or longitudinal waves use a push-pull action of compression and expansion. They can travel through solids, liquids and gases. Secondary waves, otherwise known as S wave or transverse waves, on the other hand, can only travel through solids. They move with an up-down action, for example with the shaking motion at right angles to their travel direction. P waves generally travel about 1.7 times faster than S waves in solids such as the crust.
Surface waves travel along the top part of the earth and have complex methods of transport including elliptical and horizontal shear, including both horizontal and vertical motion. They have longer periods than body waves and the largest amplitude and slower velocities than P and S waves. The surface waves cause the most destruction in most cases.
Waves are faster in rock and slower in unconsolidated soils. The amplitude of the wave is increased in unconsolidated soils and is less in rock. This is a function of velocity, depth and frequency.
P and S waves = body waves. Love and Rayleigh waves = surface waves. The following diagram shows the relative speeds and destructive power of these wave types.
SDSU, 2009.
Body waves, P and S waves, surface waves. 7

8. Duration and EQ Records
The longer the duration of earthquake shaking, in general the greater the damage caused by an earthquake.
Measurement can occur at seismic stations by printing out the movement (ground motion) at a certain location – called an earthquake record or an accelerogram.
In general, the longer the duration of earthquake shaking, the greater the damage caused by an earthquake. Measurement can occur at seismic stations by printing out the movement (ground motion) at certain locations around the world. Below is one such earthquake record or accelerogram with the P wave, S wave and surface wave arrivals marked. The epicentre is pinpointed by looking at the time difference between P and S wave arrivals, at 3 different locations, in order to triangulate the point.
Duration, earthquake record, accelerogram. 8

9. Size and Distance of Earthquakes
To measure an earthquake, we use magnitude or intensity.
Intensity
Magnitude
Measured from human damage reports.
Simple and easy measurement of earthquake effects.
Not good for physical size of earthquake – more useful for risk.
Measured from wave amplitude or energy release.
Logarithmic; for a 1.0 increase: 10x ground motion & 32x energy release.
Does not always correlate with damage and many different methods.
The distance away from an earthquake source reduces the ground motion/shaking called attenuation.
In areas where many earthquakes occur, this attenuation is stronger than in areas where few earthquakes occur.
There are two main scales which are used to define the size of an earthquake:-magnitude-based and intensity-based.
Intensity-based methods rely on destruction reports from humans, which are useful for risk but are not a good method of calculating physical size of an earthquake. They are easy to measure, and simple to understand.
There are many different magnitude scales which are used to define the size of an earthquake from its source. However, differences in magnitude occur due to sensitivity of the seismographs being used to measure earthquakes and the calculation method used – which can be confusing.
One main magnitude scale is Local or Richter scale (ML) which is calculated via the amplitude of S waves. This is generally recorded by the first seismometers to record the signal. Similar scales have been developed for surface waves (Ms). Another measure is Moment Magnitude (Mw) which is calculated by measuring the energy release (via seismic moment in Newton-Meters) of an earthquake. This is related to the displacement that occurs on the fault.
Magnitudes are logarithmic i.e. based on powers of ten. For those of you who cannot remember logarithms, it is simply the exponent required to produce a given number. In this case 1 = 10, 2 = 10X10 = 100, 3=1000 and so on.
A magnitude increase from 6.0 to 7.0 i.e. by 1 point, generally means a ground motion increase of 10, and an energy release increase of 32 times. This is because energy release is a function of fault size, and some other spatial factors. It is really 10 to the power of 1.5.
The distance away from an earthquake source reduces the ground motion/shaking, as energy of the waves is transferred to other forms. This is called attenuation. If a magnitude 9.5 earthquake occurred in the middle of the southern Indian Ocean, the first settlement to feel the earthquake could record it as an intensity V (5) earthquake (i.e. cause minor damage). However, if this same earthquake occurred directly under a town, this would most likely be recorded as intensity XII (12). Intensity-magnitude relationships have been created; however, this is a downside to using intensity measures.
This attenuation is stronger in areas where many earthquakes occur than in areas where few earthquakes occur. Thus, in locations where few earthquakes occur, there could be higher damage further away from the earthquake source than for a location which frequently has earthquakes. This occurs because there is a lot more cracking (rock fractures) in an area where more earthquakes have occurred, and therefore as the wave energy travels through, more energy is dissipated to friction, heat, and other energy transfers.
Magnitude, logarithm, intensity, attenuation 9

10. How do we measure ground motion?
Ground motion is measured using accelerometers/seismometers (spectra-based) or as human impact indices (damage-based).
Damage-based indices include MMI which is a 12 class system ranging from no damage to complete destruction based on qualitative measurement of people’s perception of damage at a location. Other similar scales include MSK (Russia), JMA (Japan), EMS (Europe) and Ross-Forel.
To measure ground motion at a certain location, measurement using an accelerometer OR by human impact indices, can be used. Like calculating the size of an earthquake, there are also two main types of ground motion measurement – damage-based and spectra-based.
Damage-based indices include MMI (Modified Mercalli Intensity) which is a 12 class system ranging from no damage to complete destruction, based on qualitative measurement of people’s perception of damage at a location. Other similar scales include MSK (Russia), JMA (Japan), EMS (Europe) and Ross-Forel. They are not all with 12 damage levels.
This shows the 1949 British Columbia earthquake and the relative shaking as measured by MMI. Although MMI cannot be directly compared to magnitude scales, on a local scale this conversion is a good estimate.
damage-based scale, MMI, MSK, JMA, EMS, Ross-Forel. 10

11. How do we measure ground motion?
Spectra-based indices include measurements of acceleration, velocity and displacement.
PGA (Peak Ground Acceleration) is the maximum amplitude of ground acceleration (measured in m/s2)
Spectral Acceleration (Sa) is the ground motion as measured at different periods to find the peaks in ground motion. Waves travel at both short and long periods.
Spectra-based indices include PGA (Peak Ground Acceleration), PGV (Peak Ground Velocity), PGD (Peak Ground Displacement) and Spectral Acceleration, Acceleration is the rate of change of velocity measured in m/s2. Velocity is the rate of change of displacement measured in m/s. Displacement is simply the difference between the initial position of a reference point and any later position.
PGA is the maximum amplitude of ground acceleration.
(The peak ground acceleration (PGA) is the maximum amplitude of the ground acceleration time-history. In terms of structural response, it corresponds to the peak value of the absolute acceleration of a single degree of freedom (SDOF) system with infinite stiffness, i.e. with a natural period of vibration equal to zero. This parameter does not necessarily provide an exact representation of the severity of the earthquake, in terms of its potential to induce structural damage as this is needed via the period of the structure vs. the shaking for the particular period of resonance.)
Spectral Acceleration is the ground motion as measured at different periods to find the peaks in ground motion, which can then be correlated to the infrastructure better. Wave energy is contained at short and long periods. For example this may be measured at period = 0 (PGA), 0.3, 0.5, 0.7, 1, 2 and 4 secs at a certain location. The ground motion will be different for each of these periods.
PGA, Spectral Acceleration 11

12. Some other effects on Ground Motion
Site soil structure impacts on ground motion. For soft soils, clays etc., increase the shaking of the ground (ground motion amplification) during an earthquake.
Topography also impacts on the ground motion; valleys with deep soil layers increase ground motions.
Direction of fault rupture can also focus earthquake energy, resulting in greater shaking at a certain location (directivity).
Shakemaps show differences in ground motion on maps.
The ground motion at the site of interest is impacted on by soil structure. With very soft soils, clays etc., a ground motion amplification can occur.
With increasing hardness of soils, i.e. dense soils, less amplification occurs due to a faster S wave velocity. A scale called the NEHRP scale is generally used which classifies sites by classes A-E, where A is hard rock > 1500m/s S-wave velocity as measured in the top 30 metres of the earth and E is very soft soil i.e. < 180m/s S wave velocity as measured in the top 30 metres of the earth. This can be measured by geotechnical investigation.
Topography also impacts on the ground motion. Valleys with deep loose soil or clay layers increase ground motions. Alternatively in very hard rock, ground motions can significantly reduce. Also in valleys, constructive interference of waves can occur due to rebounding.
Direction of fault rupture or directivity can also focus earthquake energy, resulting in greater shaking at a certain location. This is a complex process, but can add to or reduce the expected ground motion at a site.
Shakemaps show these differences in ground motion on maps over the affected areas using a number of different ground motion parameters, combining data from many stations. They can be spectra- or intensity-based.
Amplification, topography, directivity, Shakemaps 12

13. Secondary Hazards
There are 5 main sources of secondary hazard due to earthquakes:
Fire
Liquefaction
Landslides
Earthquake shaking is the primary cause of earthquake related hazard, as shaking occurs over an entire area. However, earthquakes can cause secondary hazards of many different forms.
There are 5 main sources of secondary hazard due to earthquakes which include the following:-
Fire is caused as a result of earthquake shaking influencing electricity, gas or fire sources to ignite in and around infrastructure that is in the shaking area.
Liquefaction occurs where soil (usually sand) layers are turned from solid to liquid, causing rapid failure. This generally only occurs in earthquakes with higher ground motion.
Landslides induced by earthquakes where slopes are loosened by shaking, causing a sliding of the land surface. This can be accentuated by rainfall and vegetation and mainly occurs in mountainous or steep sloping regions.
Fault Rupture is simply the visible displacement along the fault which causes surface cracks or surface slip to appear, sometimes causing catastrophic damage such as the Chile 1960 or Alaska 1964 earthquakes.
Tsunamis occur where fault movement from an undersea earthquake causes a large volume of water to be displaced due to an undersea landslide. The water volume travels at about 800km/hr in open seas with wavelength ocean waves travelling large distances, with a very small amplitude. Eventually the water waves travel from deeper waters to shallow waters at the coast line, slowing the wave, increasing the amplitude and resulting in large, destructive waves.
Fault Rupture
Tsunami 13

14. Earthquake Sequences
An earthquake can be a single event or can contain a foreshock or aftershock sequences if the fault needs to release more energy.
Note: tree is only to indicate ground level. NOT TO SCALE.
An earthquake may just occur as a single event or can contain a foreshock or aftershock sequences if the fault needs to release more energy to return to a state of low stress.
Foreshocks are earthquakes which occur prior to a larger magnitude earthquake (the mainshock) in the same location. Foreshocks do not always occur, and thus a larger earthquake is not usually predictable.
The mainshock is the largest earthquake in a short time sequence and has the highest magnitude.
Aftershocks are earthquakes that follow the mainshock close to the fault. This is generally within 1 to 2 lengths of the mainshock fault rupture. Aftershocks occur with a smaller magnitude than the mainshock, decreasing over a period of time (days, weeks, months, years). In general, the larger in magnitude the earthquake, the longer the sequence of aftershocks. Generally the highest magnitude of an aftershock is M1.0 less than the mainshock (Bath’s law), but there are exceptions.
In geologic time (millions of years), earthquakes occur periodically on a fault to release energy. Some faults will release energy every few years, whereas some may have earthquakes every million years, depending on their activity rate. As modern seismometers have only been in place worldwide for approximately 100 years, much information is still unknown as to the return periods of earthquakes on certain faults.
In geologic time (millions of years), earthquakes occur periodically on a fault to release energy. Some faults will have a major energy release every few hundred years on a certain rupture length, whereas some may have earthquakes every million years.
Foreshock, mainshock, aftershock, sequence, geologic time. 14

15. Earthquake Catalogues
Earthquakes appear to follow a pattern through time in terms of no. of earthquakes vs. magnitude.
More smaller magnitude earthquakes occur than larger magnitude earthquakes worldwide.
At any location, predictive earthquake catalogues can be produced to give people an idea of how often a certain magnitude earthquake or ground motion will be exceeded (rate of exceedance).
Earthquakes appear to follow a pattern through time in terms of no. of earthquakes vs. magnitude. This is called the Gutenberg-Richter criterion. Other relationships have also been created such as Youngs Coppersmith, Delta Functions and other such probability-based magnitude-frequency calculations. Thus, even though many earthquakes throughout history have not been recorded, these earthquakes can be predicted to have occurred using these relationships. In a certain location, this can therefore be used to fill in gaps.
Now is a good time to introduce the terms ‘rate of exceedance’ and ‘return period’. Return period in earthquakes is the average time span between events for a certain hazard (generally on a certain fault). Rate of exceedance, is the chance of the x-axis parameter is exceeded. Thus, Annual rate of exceedance, refers to 1/return period. In the following diagram, there is a % ( ) chance in that year or one earthquake every years, a PGA of 0.43g will be exceeded. Similarly, there is a (0.1%) Annual Rate of Exceedance that 0.1g will be exceeded.
Thus for each point on the earth, predictive hazard catalogues can be produced to give people an idea of how often a certain magnitude earthquake occurs on a fault. Of course, this requires another tool if we are to calculate the ground motion that will be exceeded at any point on the earth. Here we see a graph of the annual rate of exceedance vs. PGA.
magnitude-frequency, earthquake catalogue, rate of exceedance 15

16. How is hazard shown in Building Codes?
Generally, buildings should be designed for a certain additional earthquake loading based on this hazard catalogue.
Under low probability events, the infrastructure should not collapse and under frequent events, minor damage.
Zone V – 0.36g (Intensity IX)
Zone IV – 0.24g (Int. VIII)
Zone III – 0.16g (Int. VII)
Zone II – 0.10g (Int. VI or less)
Generally, buildings should be designed for a certain additional earthquake loading based on this hazard catalogue. Zones are given to regions depending on the relative hazard, and then building codes are employed based on the ground motions that this hazard is applied for. Under low probability events, the infrastructure should not collapse and under frequent events, minor damage should only occur.
For India, these are presented, where a ground motion is approximately tied to an intensity to withstand.
Earthquake zone, building code 16

17. Ground Motion Prediction Equations (GMPEs)
GMPEs or attenuation relations predict the ground motion at sites of interest for a scenario earthquake using existing or simulated earthquake data.
A GMPE is of the following form:
log[Sa(T)] = median fn(M,R,T,V) + uncertainties,
where the spectral acceleration (Sa) can be calculated for given periods (T)
Many models exist around the world
When choosing a GMPE, check for:
1) Tectonic regime (stable or active)
2) Fault type (normal, subduction etc.)
3) Distance
4) Magnitude
5) Geology/Geotechnical Information
6) Location
7) Ground Motion Parameter.
Using previous earthquake ground motion record catalogues, ground motion prediction equations have been produced to predict the ground motion spectral intensity at certain locations, given the site conditions, magnitude, location and fault mechanism of a scenario earthquake. This means that it is possible to calculate the ground motion that could occur given a M7.0 earthquake on a fault 100km away, at this very location.
A Ground Motion Prediction Equation or GMPE is also known by the name of attenuation relationship. They are generally of the following form:- log[Sa(T)] = median fn(M,R,T,V) + uncertainties, where the logarithm of the median spectral acceleration (Sa) can be calculated for given periods (T) using magnitudes (M=7.0), distances (R=100km) and shear wave velocities (V=500m/s) at this site.
The output is the median spectral acceleration and a standard deviation of the uncertainties, as there are many uncertainties associated with such a result. It is always important to know the following things when choosing a GMPE:
1) Tectonic regime (Is it stable or active? Do many earthquakes occur there?)
2) Fault type (Is it normal faulting, thrust, subduction zone, strike-slip, or a volcanic region?)
3) Distance (GMPEs are generally designed for a distance range up to 200km, but this differs - thus you need to choose one where your site is within the range). Similarly, there are different distance measures used in GMPEs. Some go from the hypocentre to the site, some from the epicentre to the site, and some use other forms (Joyner-Boore etc.). Care should be taken.
4) Magnitude (Similarly for magnitude, the GMPEs are designed for a range of magnitudes)
5) Geology/Geotechnical Information (Site classes and site effects. Is the GMPE designed for rock or soil?)
6) Location (In which countries is the GMPE applicable? – It may only be based on data from a few countries and may not be applicable worldwide)
7) Ground Motion Parameter (Is the ground motion parameter that the GMPE calculates that which you want? Is it MCS, MMI, PGA, PHA?).
This is what a comparison of median PSA ground motion prediction equations looks like. Here we see the NGA attenuation relationships. These are 5 different GMPEs from 5 different authors. In GMPEs, it is always important to include the variability, as there is generally a large scatter between models, but also within the models themselves.
NGA (Next Generation Attenuation relationships) are some of the most advanced of these GMPEs around the world but certain regional equations have also been produced, and are included in OpenSHA and can be found. A good overview of models is included in the GEM-1 (Global Earthquake Model) hazard module for certain regions at
GMPE, attenuation relation, tectonic region, uncertainty 17

18. Deterministic Seismic Hazard Assessment (DSHA)
A DSHA is undertaken to calculate the probability of ground motions for a single scenario earthquake, (historical, worst case or otherwise).
Useful for emergency planning, seismic risk awareness, simple assessment or high-risk facilities.
There are three main steps:
Define all the possible sources to cause significant hazard at a site using historic data.
Choose a fixed distance, fixed magnitude earthquake and place it on the closest position to the site on each source.
Estimate ground motions via GMPEs to determine the ground motions at the site in terms of PGA, MMI, Sa or other measures.
Variability in ground motions can be modeled within a DSHA but not extensively.
You now have all the tools necessary to undertake a hazard assessment at a site. There are some complexities, but those are the basics. I will now explain about the two different types of hazard assessments that are done, given a certain earthquake scenario.
A Deterministic Seismic Hazard Assessment or DSHA is undertaken to calculate the probability of ground motions for a single scenario earthquake. This is generally used for a historical earthquake scenario, i.e. the Kangra Earthquake of It could also be calculated for a worst case scenario earthquake on a fault (generally the maximum historical earthquake magnitude units), or the Maximum Credible Earthquake. It could also be calculated for any earthquake size, to look at the possible hazard associated with different magnitude earthquakes.
This type of assessment can be useful for emergency planning, seismic risk awareness or simple assessment as well as lifelines and critical loss facilities. Critical loss facilities such as nuclear power plants generally rely on a deterministic worst case scenario for their hazard assessment.
There are 3 main steps:-
1. Define all the possible sources to cause significant hazard at a site from historic tectonic, geologic or geotechnical data – i.e. faults and not yet discovered faults.
2. Choose a fixed distance, fixed magnitude earthquake such as the ones discussed before and place it on the closest position to the site on each source
3. Estimate ground motions via GMPEs to determine the ground motions at the site in terms of spectral ordinates (PGA, MMI, Sa, or otherwise).
Variability can also be modelled for the ground motions within a DSHA using the GMPE formulas, and also rerunning the process several times. For DSHA this ground motion and its uncertainty is specific to a single case for a certain event. The standard deviation of the GMPE can be applied however.
This ground motion hazard can then be used for design and decision-making purposes.
Deterministic Seismic Hazard Assessment 18

19. Probabilistic Seismic Hazard Assessment (PSHA)
A PSHA calculates the probability of exceeding all levels of ground shaking at a certain location (all different earthquake scenarios)
This is useful for reinsurance/insurance purposes (annual premiums), design code exceedance and government hazard.
It consists of the following steps:
1) Collect data on tectonics & geology
2) Compile an earthquake catalogue for the region
3) Define seismic sources zones
4) Determine magnitude-frequency relationships
5) Select an appropriate set of GMPEs
6) Calculate probability of each level of acceleration
7) Construct hazard curves and maps
Depending on the complexity and use wanted for your hazard assessment, a second type of assessment called a Probabilistic Seismic Hazard Assessment or PSHA exists. A PSHA calculates the probability of exceeding all levels of ground shaking (multiple earthquake scenarios – i.e. all magnitudes, all distances, all effects) at a certain location, rather than just a single event. This is useful for setting annual premiums for reinsurance/insurance purposes as the institutions want to know the holistic hazard. It can also be used for looking at whether building design codes in the region have the correct level of hazard (demand) and also for government-based studies to know where the hazard is in the country.
It traditionally consists of the following steps: Firstly data on tectonics & geology should be collected, including what type of fault and tectonic regimes exist and also the soil characteristics. Secondly, an earthquake catalogue for the region should be sourced or compiled. All possible seismic sources for a certain location should then be examined. For each of these seismic sources, a magnitude-frequency relationship should be determined and then an appropriate set of GMPEs which relate to that location should be chosen. i.e. if it is not a volcanic region then do not use a volcanic GMPE. Using this information, the probability of each level of acceleration can be calculated for the location and then the construction of hazard curves (Step 7) and maps can be undertaken for use in design codes etc. Thus, the ground motion has been defined for all possible earthquake scenarios.
Probabilistic Seismic Hazard Assessment 19