Plotting earthquake data

January 2, 2012

(This article was first published on Quantum Forest » rblogs, and kindly contributed to R-bloggers)

Since 4th September 2010 we have had over 2,800 quakes (considering only magnitude 3+) in Christchurch. Quakes come in swarms, with one or few strong shocks, followed by numerous smaller ones and then the ocasional shock, creating an interesting data visualization problem. In our case, we have had swarms in September 2010, December 2010, February 2011, June 2011 and December 2011.

Geonet provides the basic information and there have been several attempts at displaying the full set of shocks. For example, Christchurch Quake Map uses animation, while Canterbury Quake Live uses four panels showing quakes for last 24 hours, last week, last month and since September 2010. While both alternatives are informative, it is hard to see long-term trends due to overplotting, particularly when we move beyond one week during a swarm.

Geonet allows data extraction through forms and queries. Rough limits for the Christchurch earthquakes are: Southern Latitude (-43.90), Northern Latitude (-43.15), Western Longitude (171.75) and Eastern Longitude (173.35). We can limit the lower magnitude to 3, as shocks are hard to feel below that value.

Graph presented by Canterbury Quake Live, notice how difficult is to read the bottom graph.

The file earthquakes.csv contains 2,802 shocks, starting on 2010-09-04. The file can be read using the following code:


# Reading file and manipulating dates
qk <- read.csv('earthquakes.csv', header=TRUE)

# Joins parts of date, converts it to UTC and then
# expresses it in NZST
qk$DATEtxt = with(qk, paste(ORI_YEAR,'-',ORI_MONTH,'-',ORI_DAY,' ',
                  ORI_HOUR,':',ORI_MINUTE,':',ORI_SECOND, sep=''))
qk$DATEutc = as.POSIXct(qk$DATEtxt, tz='UTC')
qk$DATEnz = as.POSIXct(format(qk$DATEutc, tz='Pacific/Auckland'))

The following code produces a plot that, in my opinion, presents a clearer idea of the swarms but that I still feel does not make justice to the problem.

png('earthquakesELE.png',height=600, width=1200)
ele = ggplot(qk, aes(DATEnz, MAG, ymin=3, ymax=MAG))
ele + geom_linerange(color='grey') + geom_point(color='red', size=1) +
scale_y_continuous(name='Magnitude', limits=c(2.5,7.2)) +
scale_x_datetime(name='Date',major='month') + theme_bw() +
opts(panel.grid.major = theme_line(size=0))

Graph displaying quake swarms, but still far from perfect. Looking forward to your ideas.

Please let me know if you have a better idea for the plot.

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