I am just willing to know is there any relationship exist between peak ground acceleration and Earthquake Magnitude. If we have local PGA value ,then what will be the effect of future earthquake. How to calculate or predict the future earthquake response?
Hi Shishir,
Acceleration (a) varies with Magnitude (M) and distance (r) from EQ according to the following equation.
a (M,r) = b10cMr -d
b,c,d - constants that depend on
1 Geology of the area
2. EQ depth and faulty geometry
3. Frequency of ground motion
Thus, acceleration (a) increases with M and decreases rapidly with distance at a rate dependent on the rock type.
Regarding future earthquakes you will have to keep monitoring the seismicity. If you have records of PGA during the last 30-40 years then you can expect an acceleration similar to the maximum that has been recorded if the magnitude is the same. If there are no records monitoring will be the only answer.
LRK
Interesting question. This is one of most popular topic of engineering seismology from the beginning of magnitude term use. It could be related with attenuation law of spreaded seismic energy on Earth surface, which first was estimated in terms of damage to different structures, according to several intensity seismic scales builded up at the end of 19 century being modified up to the end of 20 century.
Building up an analitical relationship between PGA and magnitude, also keeping in mind geology of Local conditions, frequency and source fault mechanism with distance and depth, looks very heavy task to achieve for such a try. It is because of the very simplistic method used for describe whole process of wave generated and their propagation through the Earth structure. More complex analysis of this , in particular of getting synthetic acceleration record with peak values on it are Deterministic Methods well developed at the end of classic period in the seismology (from 1970). This step on building the corelation is a serious up to date from simple analytic relationship formulae to complex form of dependence for soil ground motion parameters a, v, d, of the seismic source radiation pattern. Please consider deterministic methods, and your results of investigation will have weight on precision, which is important
Dear Shishir,
There is no clear relationship between Peak Ground Acceleration (PGA) and earthquake magnitude. Note that any earthquake would produce a wide range of acceleration values measured at the surface, depending on the distance to the source and the local geology of the measurement site. Of course, for any given source location, big earthquakes would produce higher PGAs than small earthquakes, and this allowed the establishment of empirical relationships between maximum acceleration at the epicentral area and earthquake magnitude (e.g. classical papers by Gutenberg and Richter in the 1940s). But note that these relationships should be established for earthquakes within a short range of focal depths, since the same value of maximum acceleration at the epicentral area may by caused by a relatively small, shallow earthquake or by a stronger event (larger magnitude) at greater depths.
Predicting future earthquakes and site responses is a difficult task that requires a complete seismic hazard assessment, including compiling as much information as possible about recent (instrumental) seismicity, historical seismicity (through written chronicles), paleoseismological studies in trenches, geodetic studies (to document slip rates), etc. Extending the study to historical and pre-historical times is important because the time interval covered by instrumental studies is usually too short to record the return period and magnitude of big (potentially dangerous) earthquakes.
Best regards,
David
Dear Shishir,
There are many EMPIRICAL relationship between Peak Ground Acceleration (PGA) and earthquake magnitude, focal distance (or distance from causative fault) and site geology situation.
Dear Shishir,
Peak Ground Acceleration (PGA) in any location is a function of earthquake magnitude, focal distance (or distance from causative fault) and site geology situation.
There many EMPIRICAL relationship between Peak Ground Acceleration (PGA) and earthquake magnitude, focal distance (or distance from causative fault) and site geology situation.
No definite answer to your question in the case of Romanian Vrancea intermediate depth earthquakes recorded at surface at different distances from epicenter.
I have tried to show that in Bucharest , in a very limited area, the recorded PGA values are changing from an event to the other, regardless the magnitude of the events. This is the case also for the whole distance from the epicenter situated in Vrancea zone and to Bucharest in the south-west and to Iasi , in the north-east.
There are many things involved, not only the site effects. There are also the wave path , controlled by the depth of the earthquake hypocentre, and also the mechanism of the earthquake. In my opinion the influence of the 2 factors are greater in our case than the site effects upon the PGA's recorded on the city surface. I have discovered a great paper in which the author is mentioning that the deceleration in the process of stopping the rupture along the fault is also of great influence upon the energy released in case of an earthquake. This last process is very hard to model for an earthquake taking place at 100 km depth as it is in our case. Nevertheless this means that we might have earthquakes with the same computed magnitudes, but which might have different amounts of energy radiated from the focal point to the surface.
So the distribution of the PGA's recorded at surface are depending on many things , not only on events magnitude. By consequence the empirical relations derived so far between the earthquake magnitude, focal distance and site effects are not yet convincing, because there are other physical processes that are neglected. I have tried to mention at least some of them.
In general ground motion increases with magnitude because larger magnitude mean higher energy and the energy is related to the amplitudes integrated over time (let's say rupture time). The question is what frequency is more suitable for what magnitude and this is related to the corner frequency and signal to noise ratio. Magnitude is calculated from frequencies below the corner frequency because amplitudes are correlated with seismic moment. For very small magnitudes, higher frequencies can be used as a proxy for magnitude calculations if they are below the corner frequency and the amplitudes are above the noise level. For the events that are important with regard to human perception and damage, frequency at which PGA occurs is certainly above the corner frequency so it is controlled by both the seismic moment (from which we get the magnitude) and the stress parameter.
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