A Tale of Two Earthquakes (and their Data)

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In the last day we have seen two earthquakes having magnitudes M>6.  The first was a M6.6 event near Ya'an, China, which, according to news accounts, killed more than 150 persons, and injured more than 5500.  The second earthquake was a M6.1 event that occurred in the Kamchatka region following a M6.9 earthquake on February 28.

These earthquakes, together with the forecast tools available on this website under the Tools tab, illustrate the importance of earthquake data.  For in the Kamchatka region, the data is generally pretty good, the networks detect and store the smaller earthquakes on which the forecasts depend.  On the other hand, data in China has always been rather spotty and unavailable, particularly for small magnitude earthquakes, so the forecast methods don't work as well.  Examples are shown below. 

At the left, we see a screen shot from the earthquake viewer.  It shows the region of Ya'an China where the M6.6 earthquake occurred. 

I have drawn a 150 mile circle around the epicentral region.  Some of the recent earthquake activity can be as the blue google teardrops in the area.  Despite the large size of the main shock, you see very few aftershocks. 

The table at the lower left of the figure, which appears when you click on the circle select box, indicates that on April 1, the chance of a M.6 earthquake as computed by our algorithms,  was about 1.4%. 

We can also use a new tool on the web site, the forecast time series chart, which appears when you click the forecast time series bar.  It shows no evidence of the impending earthquake (note that once you click on the bar, you have to be patient and wait for the plot to appear -- and you also have to allow popups in your browser for the site.  A screen shot of the timeseries is below.   

Here we see the last 5 years of data. The little yellow boxes show the M>6 earthquakes that have occurred in the region. This is a forecast for the M>6 earthquakes that may occur within 1 year in the circular region.

In particular, you can see the M8 Wenchuan earthquake effect that occured in May, 2010.  The chart rises rapidly following that major earthquake and then falls, flattening off at about 1.8%.

Clearly the chance of large aftershocks from the M8 earthquake should probably be larger, indicating that the forecasted probability is very likely too small.

This is an unfortunate effect of poor data quality in the catalog. 

Our forecast method for large earthquakes is based on counting numbers of small earthquakes.  So if the catalog doesn't include all the small earthquakes to count, the forecasted probabilities will be unrealistically small.

We can compare these charts with those for the Kamchatka events.  Shown at left is a similar screen shot for the Kamchatka region, together with the same kind of circle of 100 mile radius. 

You can see all of the smaller earthquakes in that region of the Kuriles and Japan.  The coverage of the region with seismographs is typically very good, so the catalog includes most of the smaller earthquakes. 

On the left below, the table indicates that the chance of a M>6 earthquake on April 1, prior to today's M6.1 earthquake, was about 10.4%. (By the way, if you draw a bigger circle, the computed probabilities will be much higher.  Localizing probabilities with small circles produces smaller probaility numbers as you might expect, since the chance of a large earthquake in an arbitrarily small area will be smaller)

You can also see in the screen shot the many M>5 earthquakes in the Japan region that have occurred in the last 7 days.  These are events associated with potential future large and great earthquakes in the Japan region. 

These possible large earthquakes (M>8) were the subject of an earlier blog, and the subject of a talk I gave at the Seismological Society of America meeting at Salt Lake City this past week.

In the forecast time series chart at right, you can clearly see the run up to the M6.9 earthquake on February 28, which began in mid 2012. 

Following that earthquake, the probability has stayed at the same level, culminating in today's M6.1 earthquake. 

Comparing these charts clearly illustrates the need to keep improving the quality of data in these online catalogs.

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