6
$\begingroup$

I want to know if large earthquakes can happen twice in the same region? The reason is that the recent earthquake in Nepal was followed by many after shocks of considerable magnitude.

I wanted to know if there are chances of earthquake of this magnitude in the same region again. Can we expect another earthquake in the same region, lets say in a month or few months?

$\endgroup$
6
$\begingroup$

Aftershocks after a major earthquake can occur for a period of weeks after the initial major earthquake, but the magnitude of the aftershocks is always less than that of the initial major quake. The paper History of Earthquakes of India & the Himalayas discusses the each of the major earthquakes in the region for the last 200 years. Information on earthquakes before 1800 is patchy. The earthquakes discussed are the ones that occurred in: 1803, 1833, 1869, 1897 & 1905.

Nothing was mentioned about a second or third major earthquake happening shortly after a major earthquake; even for the devastating 1934 earthquake.

Due to the different tectonic settings and causes for earthquakes in different parts of the world it is best not to look at the history of one part of the world and assume the same will occur elsewhere. The expectancy of earthquakes and their patterns in the Himalayas should not be compared with earthquakes in Japan, New Zealand, western USA, Turkey or elsewhere.

$\endgroup$
  • 5
    $\begingroup$ There can also be fore-shocks. Which means the main shock can sometimes be only identified after the fact. Some large events also act more like swarms - eg. the New Madrid earthquakes in 1811-2 which had a number of 'main shocks'. earthquake.usgs.gov/earthquakes/states/events/1811-1812.php $\endgroup$ – winwaed Apr 29 '15 at 14:26
3
$\begingroup$

It can happen but not like you may guess.

I believe you have seen rupture maps for large earthquakes, such as the 2011 Japan earthquake. In that earthquake, a foreshock of 7.3 took place at the perimeter of the area of largest slip during the main earthquake. This could be interpreted like this (using the asperity model): the foreshock hit at the "slopes" of the asperity but failed to "climb" up immediately; aftershocks were slowly migrating towards the "top" of the asperity, then the main shock happened. Furthermore, once the whole fault was activated, slip at the shallowest parts went beyond charts (40, 50, even 80m of slip were reported locally).

The main (deep) asperity had the fewest aftershocks (it slipped completely and without restraints), but there were enough of them in the shallowest parts of the fault. This is probably because the shallower slip was not due to an asperity but rather due to diffused slip of unconsolidated sediments (the "bookshelf" model was used, plus there were aftershocks that appeared to actually... pull back the pieces closer to their original locations). Anyways let's focus on the deeper asperity.

An earthquake can take place at the "slopes" of a large asperity but fail to rupture towards the "top" It can however rupture towards the perimeter and achieve respectable sizes. Then another earthquake can take place nearby, but still low at the "slopes". Finally an earthquake can climb over the top and force even the previously ruptured areas to re-rupture. All 3 earthquakes can very well have epicenters pretty close to one another (such as the two eastern Nepal aftershocks recently). It seems that the 1960 huge Valdivia earthquake demonstrated this behavior but I have yet to find a good, detailed, open-access study on it.

Concerning Fred's answer, unfortunately Bilham (he mentions Jones and Molnar, 1986) (search for "preceded") says that 10% of strong earthquakes in the history of Himalaya have been preceded by strong shocks. Other sources support Fred's answer, but I would choose to believe Bilham.

Concerning the "lots of aftershocks" notice that the author of the question pointed, for a given earthquake with a normal aftershock sequence it is expected that its strongest aftershock is 1.2 magnitude smaller than the mainshock. For this case, this would yield 6.6. In fact, the strongest ones were the 7.3, an 6.7 and a 6.6, so yes this is anomalous, but the empirical 1.2 value I mentioned is a bit smaller for Himalayas. Still, an 7.3 is anomalously large but before that time we had no reason to suspect an anomalous aftershock sequence (to my knowledge of course, I searched for b-values but did not find anything during the first days of aftershock activity).

$\endgroup$
2
$\begingroup$

You will only have aftershocks, which are always smaller than the original earthquake. Aftershocks are the accomodation of the surface to its new position after the big movement of the original earthquake.

But you might have another big earthquake near the original one, because since big earthaquakes means than a whole line of earth moves (not a single point, but a line where plates touch), you might have another masive earthquake next to the original.

That happened on 1960 on Chile. On May 21 an earthquake 7.4 Ms hit a zone north to the city of Concepción, and the next day another earthquake, this time 9.5 Ms hit the zone next to the first one. Actually the first earthquake was the sum of three earthquakes, and the second was the sum of 37 earthquakes distributed on 1000km.

https://en.wikipedia.org/wiki/1960_Concepci%C3%B3n_earthquakes

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.