Monday, December 20, 2010

2010 Earthquakes

The following is a set of questions that I answered by email to Mark Weisenmiller, who contacted me representing "the international Chinese news wire agency Xinhua". The responses are of broader interest.



1) Why does it seem that 2010 has had more recorded eathquakes than in previous years? Is it possible that more earthquakes have happened in more populated areas this year, than in previous years, thus giving the impression that 2010 has had more earthquakes than in previous years?,
According to the US Geological Survey's web site (http://earthquake.usgs.gov/earthquakes/eqarchives/year/graphs.php), the annual average number of earthquakes since 1900 is the following. The numbers in parentheses are the range of values, 1980-2009.
6.0-6.9: 134 (121-178)
7.0-7.9: 15 (5-18)
8.0-9.9: 1 (0-4)

So far in 2010, the same web page gives the following numbers:
6.0-6.9: 143
7.0-7.9: 19
8.0-9.9: 1

Thus so far the year 2010 has been a rather ordinary year in terms of the total number of earthquakes with magnitudes of 6.0 or larger.

So you are correct, that the impression of this being a more active year is caused by the earthquakes that have happened to occur in urban areas - especially the earthquake in Haiti, that struck close to the capital city of a very poorly prepared country. The Haiti earthquake was not a scientific surprise, in the sense that it occurred on a fault system that was known to cause earthquakes of this size and has done so in the past.





2) Does the possibility also exist that since more and more people everyday are online and/or part of a Internet social network (Facebook, Twitter, etc.) that these people are reporting earthquakes to their respective local government or science agencies. Also, how do said agencies verify such claims?,
The earthquakes reported by the USGS are detected and measured by instruments.
The USGS operates a great page for people to report earthquakes that they have felt:
Reports from those pages are correlated with measurements of earthquakes.



3) What seems to be, in your professional analysis and estimation, seems to be causing this increase in seismic activity and energy (i.e., hence, earthquakes)? Could it be anything due primarily from human beings or is it completely the actions of plate tectonics?,
Since there is not a significant increase in seismic activity, I disagree with the premise of the question. Whatever it is that is causing you to ask this question is based on inaccurate perceptions.



4) In regards to plate tectonics, do we know yet precisely what causes this action. Is it still believed to be, as had been the case for decades for many scientists and seismologists, convection in the earth's mantle?,
There is no doubt but that plate tectonics is driven by the heat that is inside the earth, which in turn causes some sort of convection in the mantle.



5) Granted, I know that it is very difficult to predict earthquakes but are such massive earthquakes like the kind that shook up the island of Hispanola and virtually destroyed Haiti this past January predictable at all by seismologists?,
No. Also, yes, but that is if you define prediction carefully and in a way that is different from how I think you mean it.
No: There is no reliable way to create a short-term prediction of earthquake (i.e. within the next few days). Many very smart seismologists all over the world are doing their best to find a way, though. By short-term prediction, I mean that the prediction must identify the location, magnitude range, and time interval during which a future earthquake will occur. It is accepted that when we observe an increase in the numbers of small earthquakes in a region, then the probability of a large earthquake is increased, but not to a very high value (e.g. >10%).
If you meant long-term prediction, the story would be a little different. We can measure the accumulation of strain on active faults, or recognize past earthquakes in trenches across the faults, and if the data is sufficient we can determine the average time interval between large earthquakes on those faults. Then we can recognize which faults ought to be getting "close" to rupture. Expressed as probabilistic statement, we might conclude that the probability of a large earthquake on a particular fault is high (e.g. >50%) over a time interval of a few decades.



6) What, to te best of your knowledge, is the casulty and fatality totals due to earquakes in 2010?,
I don't have a handy source of information to answer this question completely. The USGS web site I identified above (http://earthquake.usgs.gov/earthquakes/eqarchives/year/graphs.php) gives statistics of the numbers of deaths since 1980.



7) Is there anything else besides engineers designing earthquake "friendly" buildings that humans can do to protect themselves from earthquakes?,
By "earthquake friendly" buildings, I presume you mean buildings that are strong enough to withstand the shaking of a large nearby earthquake. That is an essential first step. In the United States, our building code has the premise that all structures should protect life safety for levels of shaking that have an annual probability of occurring of 10% in the next 50 years (corresponding roughly to shaking that occurs on average once every 2500 years). Besides that, people can do the following:
a. Protect themselves and their children (Secure or move items that could fall and hurt a person in an earthquake. Protect your valuables, such as your house, by strapping heavy furniture, water heaters, putting precious china inside a cabinet with latches on the doors that will not open in an earthquake, etc. )
b. Prepare to recover (Have food that won't spoil, first aid supplies, water, a little cash, and other items that will help you recover after the earthquake happens.)
c. Prepare to help their neighbors (Because your neighbor may not be as prepared as you are, and helping your neighbor is the right thing to do.)





8) Do you believe that the number of significant earthquakes will increase in 2011 and the upcoming years since, frequently, earthquakes happen at plate boundries and more and more people are living at or near plate boundries? Why or why not?,
I expect that the annual number of earthquakes will go up and down, within or close to the rates given in response to your first question.
I also expect that the number of disasters caused by earthquakes will tend to increase over time because of global population growth, and considering that a significant fraction of that growth takes place in areas that are susceptible to earthquakes.



9) Is it true that some animals can detect earthquakes before they actually occur (I have read articles that state that cows, and especially toads, have this ability)?
It is true that there have been many claims that animals can detect earthquakes before they occur. However, so far as I know, there has never been a rigorous study meeting scientific standards that confirms this belief. My personal opinion is that after people have felt a strong earthquake, if they have noticed any "unusual" animal behavior beforehand they are inclined to report it, but the same "unusual" behavior would be quickly forgotten otherwise.



Thursday, March 18, 2010

More About Japan's Hazard

These are some figures from the Japan Meteorological Agency showing more about the earthquake hazard for southern Japan. This figure shows the sources of earthquakes back to 1605.

Image source:

Japan Meteorological Agency (2009). Earthquakes and Tsunamis Disaster Prevention, http:/www.jma.go.jp/jma/en/Activities/brochure_earthquake_and_tsunami200903.pdf, accessed March 2010, 28 pages.

This is also a very good brochure to look at for a summary of information that the Japan Meteorological Agency provides to the general public.




Friday, November 13, 2009

Small earthquake during President Obama's visit to Japan

We had a small earthquake here in Tokyo this morning. Actually very small, M=4.1 (JMA), 50 km deep with epicenter in Tokyo Bay, at 4:23 AM JST. It was strong enough to awaken my wife Gisela, but I slept through it. President Obama is here in Tokyo, and dominating the news. He might have felt it.

The following are images of the intensity map from the Japan Meteorological Agency web site, http://www.jma.go.jp/en/quake/. These show the instrumental intensities with three different scales.


According to the JMA web site as of 9:00 AM local time, the details are:

"Earthquake Information (Information about Seismic Intensity at each site)
Issued at 04:33 JST 14 Nov 2009

Occurred at (JST) Latitude
(degree)
Longitude
(degree)
Depth Magnitude Region Name
04:23 JST 14 Nov 2009 35.4N 139.8E 50km 4.1 Tokyo-wan



Thursday, September 24, 2009

More About the Hazard for Tokyo & Vicinity

My post on August 12 did not tell the whole story about the hazard for Tokyo. It suggests that shaking like what was felt on August 9 occurs every two years. But the real concern is the hazard from earthquakes that are much less frequent, but cause much larger levels of shaking.

One of the big concerns for Tokyo is what will happen when the Tokai seismic gap breaks. A seismic gap is, to put it simply, a part of an active fault that has not had a large earthquake for a long time.

I found the figure below on the web site prepared by Prof. Terry Tullis for an introductory class in Earth sciences at Brown University. This shows the parts of the subduction zone in southern Japan that have ruptured in earthquakes over the past 300 years. The entire zone ruptured in 1707. It ruptured completely again in two earthquakes on two days in 1854. Most of it ruptured again in earthquakes in 1944 and 1946. However, the 1944 earthquake left what looks on this map to be a short segment at its northern end unbroken.


What looks like a short segment on this map, labeled the Tokai Gap, is actually 100 to 150 km long. This is easily a big enough fault to cause an earthquake with magnitude close to 8.

Another web site that describes the Tokai Gap is:
http://cais.gsi.go.jp/Virtual_GSI/Seismology/Tokai_Slow/1t.html

Wednesday, August 12, 2009

Our Apartment


Earthquake preparedness starts with the quality of the structures. So I found it interesting to look at the web site for our apartment. Part of one web page is reproduced above. One noticeable feature is the entry in the table: "Structure. Steel frame and reinforced concrete structure". That should perform ok in an earthquake. I don't know if this information is placed on the web by popular demand, or by some requirement. But it is valuable information. It should be this easy for everyone to find out about how their homes and offices are constructed.

How often do you feel an earthquake in Tokyo?

There is a rigorous way to answer this question. The plot below gives one answer.

This plot is based on a station in the K-net array of strong motion stations operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). From their web pages (www.k-net.bosai.go.jp), one can obtain records of ground motion from any of their national network of over 1000 stations installed to record strong earthquake motions. The nearest station to our apartment is station TKY022. This station has been operating since May 2002. From the list of all the records at that station, and assuming that the station didn’t miss any earthquakes, I counted the number of times that different levels of ground motions have been exceeded. This plot shows the results. A station that has observed longer would give better statistics, and different stations feel different levels of shaking on average. But this plot is ok for an approximate answer to the question.

We have felt two earthquakes since we arrived, and they are included in this plot. The first one was on Sunday evening, August 9. It was 311 km from the station and 340 km deep with magnitude 6.9 (JMA). The second one was Tuesday morning, August 11. It was smaller (magnitude 6.6), closer (154 km), and shallower (20 km deep). The motions on August 11 were not as strong as on August 9. What this plot shows is that earthquakes stronger than the August 9 event have happened about once every two years, and that earthquakes stronger than the August 11 event have happened just under twice a year, on average. Shaking of 10 cm/s2 is usually strong enough to feel, however, and that happens on average about 6 times per year. So we expect to feel some more earthquakes.

Tuesday, July 7, 2009

Preparedness and Resilience

We often urge people to be prepared for earthquakes.  That usually means having your home ready and supplies on hand, and knowing what to do before, during, and after the earthquake. There are many resources on the web that can help.  Here is one useful site, maintained by the US Geological Survey:  
http://earthquakes.usgs.gov/learning/preparedness.php 

Resilience goes beyond preparedness.  Resilience is measured by how quickly after an earthquake you are able to resume normal activities.  

Resilience is vital for business and communities.  Imagine a hardware store that is not resilient - not able to reopen right after an earthquake, at the very time that many people in the community need parts to repair earthquake damage.  Their lack of resilience not only hurts themselves, but also hurts the entire community that depends on them as they try to make repairs and get back to normal.    So we urge individuals, businesses, and communities to aim for a high level of resilience to earthquakes and other disasters.

The US Geological Survey web site has some links that emphasize steps that go beyond preparedness and help to be more resilient.