The figure below shows the occurrence times of earthquakes with different magnit
ID: 298278 • Letter: T
Question
The figure below shows the occurrence times of earthquakes with different magnitudes relative to the M9.0 Tohoku earthquake in Japan in 2011. Larger earthquakes are marked with their focal mechanism, and events are coloured by depth. (a) Assuming the earthquake fault plane dips to the northwest, what type of earthquake mechanism is indicated by the focal mechanism for the Tohoku earthquake? Is this mechanism typical of other events too and. why might this be? [3 marks] (b) How does the rate and magnitude of earthquakes change after the mainshock (largest magnitude event)? (c) What are the events that precede the mainshock? To what extent could these events be used to forewarn of the impending mainshock. and what mitigative measures could be employed?Explanation / Answer
A. The focal mechanism of Tohoku earthquake indicates that is an oblique strike slip fault.
B. the best fitted trend among the earthquakes in logrithmic scale always gives a linear line with negative slope. This infers as b value estimation.
Here also higher manitude earthquakes are fewer and less frequent than the small magnitude earthquakes.
C. These earthquakes are known as forshocks and they are important in calculating the rupture velocity and moment magnitude of the fault plane which may lead the bigger earthquakes in future. The total stress accumulation over the active patches at the fault plane, and converted energy that has been released during a foreshock can help a bit to understand the focal mechanism and behaviour for future assessment.