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Will This Earthquake Cause Damage? – Part 2

Tips to Think Like a Seismologist!

Maiclaire Bolton    |    Natural Hazard Risk

In a previous blog, we touched on understanding big earthquakes and the recent earthquakes that have caused impact around the globe. Today we will dive into tips on how to think like a seismologist in efforts to turn your panic in to preparedness, next time an earthquake is on your radar. To get a quick understanding of the possible extent of the damage, here are three quick tips on how to think like a seismologist. There’s more to consider than just magnitude!

1. Location - Is the epicenter, or rupture area, close to major exposure?

Just like in real estate – location can make all the difference. The strongest ground motions are generally recorded within 50km of the epicenter or rupture area, depending on the underlying soil conditions. An example of this is the 2010-2011 earthquakes in New Zealand. The February 2011, M6.1 earthquake in Christchurch was a direct hit on the city. Compare this to the larger September 2010, M7.0 event about 40km west of the city, which was far less damaging.

A large earthquake closer to the area of exposure will yield a much more damaging event. Compare the level of shaking intensity in the city of Christchurch in the following images. In the September 2010 event (left) the more severe ground shaking is away from the city, compared to the February 2011 event, where the severe ground shaking intersected with the city.

2. Depth - How deep is this earthquake?

If the earthquake is shallow (i.e. less than 30 km), be concerned, especially if it’s near exposure. (Don’t forget tip #1 – Location!). If it’s an intermediate depth (i.e. 50-150 km) it will likely cause less damage, but the footprint could be larger because of the way seismic waves travel. If it’s deeper than 200km, there’s no need to worry. The event is too deep to cause much damage.

For example, the M8.3 earthquake off Russia’s Kamchatka Peninsula mentioned previously – is much too deep to cause damage. An earthquake at a depth of 600km – that’s like being 600km away from the epicenter at the epicenter. As such, only a moderate level of shaking occurred from this M8.3 event.

3. Construction Practices / Building Codes – what is the construction quality like?

In addition to location and depth, you must consider the construction quality in the location of the earthquake:

  • Is this an area that typically sees earthquake damage, especially from moderate magnitude earthquakes?
  •  Is the country known for having a strong building code?

An earthquake with the same magnitude and depth could cause drastically different damage depending on the vulnerability of the exposure. For example, a M6 earthquake in California or Japan could cause very little damage due to the stringent seismic provisions in the building codes. But in other countries with very old construction (especially unreinforced masonry or adobe structures), a M6 earthquake could be extremely damaging in countries as experienced in Italy or Greece, or very poor construction practices like experienced in Haiti.

Conclusion 

To fully understand if an earthquake has the potential to cause damage, there are many other aspects to consider other than magnitude. Yet a perspective of location, depth, construction practices and magnitude, are just a few quick starting points to consider when evaluating your potential need for concern.

In general, a good rule to follow is that earthquakes with a large magnitude, near an urban center should cause concern. In countries with high quality construction practices it generally takes an earthquake M6.0 or greater to cause structural damage. Slight damage can occur in the M5.5 to 6.0 range, and minor damage for magnitudes lower than M5.5. Major earthquakes with magnitudes of M7.0 and greater near urban centers, especially those with shallow depths are of major concern as they can cause significant damage and devastation1.

[1]http://www.geo.mtu.edu/UPSeis/magnitude.html

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