What Makes an Earthquake ‘Significant’?

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If you google “What is the definition of a significant earthquake?” you are met with Michigan Tech’s* response; Major – magnitude 7-7.9. However when looking at ‘significant’ earthquakes on the USGS** web page there are ones as low as magnitude 3.3. So to different people (and/or institutions) how we classify earthquakes seems to vary greatly and this occurs from top seismologists right through to media reporting and how we perceive the threat.

 

At 11.23 UTC on May 30th a Mg 7.8 earthquake struck off the coast of Japan. This is the same magnitude as that of Nepal’s April 25th quake but one managed to devastate an entire region and the other barely shook a few skyscrapers. Unless, like my self you concern your selves with the rumblings of our planet, or you live in Japan or the surrounding area you probably did not ever know last Saturdays earthquake even happened.

1. Aftermath of Nepal earthquake April 25th 2015.

The main difference between the two is the location of their foci. The focus of an earthquake (sometimes called the hyprocenter) is often confused with the epicentre, however the epicentre is the surface area directly over where the earthquake takes place, whereas the focus is the actual point at depth where the snap of energy takes place. With the Nepal earthquake the focus was just 15 km under a heavily populated region. The buildings on the surface were poorly built and unable to with stand the violent shaking, bringing them crumbling to the ground killing over 8000 people.

2. Displacement by Japan’s March 11th 2011 earthquake.

The Japan earthquake in contrast occurred off the coast, below the Pacific Ocean, although the some shaking was felt onshore. Many may assume this is safer than an earthquake under an urban area but several of the most deadly earthquakes occur at sea as they can induce tsunamis like that of March 11th 2011 which killed nearly 30,000 in Japan or the infamous Boxing Day Tsunami which killed as many as 230,000! Luckily on Saturday no tsunami alert was even issued, as the biggest difference between these two 7.8 earth quakes is depth.

Occurring at 677 km beneath the surface, this deep-focus (below 300 km) earthquake happened so deep its distance from focus to surface is only slightly shorter than travelling from London to Berlin (690 km)!!! As seismic waves travel they dissipate, loosing energy so are never as intense as what they are closer to the source.

3. Diagram of an earthquake, highlighting its focus and Epicenter.the waves lighten in colour with distance from the focus to show their loss of strength.

 

So so far we have magnitude, depth and location which impact on the devastation potential, but is there any thing else? Well we can expand on the last, location, to highlight other potential threats posed by an earthquake. A moderate sized earthquake in the heart of Los Angeles or Tokyo may stop the subway and send food flying off shop shelves but casualties should be low. The same earthquake in a country like Nepal or Haiti can kill thousands. Earthquakes don’t kill people per say, I have never heard of some on being shaken to death by a quake.What kills people is poorly constructed buildings collapsing, bridges failing, gas mains bursting causing fires. After past disasters such as San Fransisco’s great earthquake of 1906, wealthy countries which sit along active fault lines have put in place strict building codes and pumped millions in to disaster management programs and construction.   Obviously earthquake-proof is not always a possibility by earthquake-resistant definitely is and has saved the lives of many of the past few decades. Sadly not all at risk areas have that luxury of these safe guards at the expense of hundreds of lives.

4. Damage and fires caused by the Greath San Fransisco earthquake in 1906.

Seismology is a tricky business. With so much to take in to consideration when classify earthquakes, it is easy to see where there is often conflicting statements. Things are complicated further by the multitude of scales actually used to quantify them. When asked what scale is used, I can guarantee most will say the Richter scale (or local magnitude, ML), that is even what I was taught in school. Charles Richter first put his scale to use in 1935 to give a more scientific quantification for earthquakes than the previously used Mercalli scale which was solely based of human perspective and building damage (this is still used today but not as often). The Richter scale was limited in many ways being primed for nearby, mid-sized earthquakes (M 3-7). Seismologist Beno Gutenberg expanded on Richter’s work greatly enabling the scale to factor in greater distances and separated scales for surface waves (MS) and body waves (Mb).The revised scales still had difficulties and were particularly ineffective when looking at earthquakes which spanned great lengths of fault lines such the Aleutian Fox Island quake of 1952. The Richter scale was finally replaced by the Moment magnitude scale (MW) back in 1979 and this is the scale used by most institutes today including USGS.

Moment magnitude was born from elastic dislocation theory put forward in 1972 which suggests that energy release from a quake is proportional to the surface area that breaks free, the average distance that the fault is displaced, and the rigidity of the material adjacent to the fault. It is based on a similar logarithmic scale to the Richter scale with each step equating to an increase in the amount of energy released 101.5 ≈ 32 more than the previous. Earthquakes usually have similar Richter and moment magnitude numbers but rarely exactly the same and this can be one way one earthquake can be reported at different levels across the media if their sources used different scales. Another way which causes different figures  is precision; the more seismic stations used to calculate magnitude the more precise the result. When an earthquake is first recorded institutes are likely to only use their own data but as soon as they have access to the global seismic network they can give a more accurate classification. This happen with Japan’s earthquake on May 30th, initial reports put it over a magnitude 8 but this was quickly downgraded to 7.8.

As you can see an earthquakes significance is a matter for debate and in many cases personal opinion. Magnitude and location (not just geographically but also politically) are the main factors but it tends to vary earthquake to earthquake.

Station VRI seismogram

5. Example of a seismograph.

 

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