The violence of Earthquakes

Much has been written about Christchurch earthquakes and the massive impact it has had on peoples lives. For those outside New Zealand, many do not understand why these earthquakes, which seem relatively low on the Richter and Moment Magnitude scales, have caused such widespread destruction. Are New Zealand's building standards poor? Was something amiss in a relatively wealthy western nation?

My view is that this is not the case and that the Christchurch earthquakes have been some of the most violent in the world. The Richter and Moment Magnitude scales measure the total energy of an earthquake across the entire fault. Christchurch's earthquakes had relatively short faults, but they occurred directly under the city and surrounding areas, and hence the energy was concentrated.

A different way of looking at ground movement is to look at Peak Ground Acceleration (PGA). This is the ground acceleration in a localised area.

Here is a table reproduced from Wikipedia on the top 10 PGA events:

PGA single direction (max recorded)	PGA vector sum (H1, H2, V) (max recorded)	Mag	Depth	Fatalities	Earthquake
2.7g[8]	2.99 g[9][10]	9.0	30 km [11]	>15000 [12]	2011 Tōhoku earthquake and tsunami
2.2g[13][14]		6.3[13]	5 km	181	February 2011 Christchurch earthquake
2.13g[13][15]		6.3	6 km	1	June 2011 Christchurch earthquake
	4.36g[16]	6.9/7.2	8 km	12	2008 Iwate-Miyagi Nairiku earthquake
1.7g[17]		6.7	19 km	57	1994 California earthquake
	1.47g [18]	7.1	42km[19]	4	April 2011 Miyagi earthquake
1.26g[20][21]		7.1	10 km	0	2010 Canterbury earthquake
1.01g[22]		6.6	10 km	11	2007 Chūetsu offshore earthquake
1.01g[23]		7.3	8 km	2,415	1999 Jiji earthquake
0.8g		6.8	16 km	6,434	1995 Kobe earthquake
0.78g[24]		8.8	23 km [25]	521	2010 Chile earthquake
0.6g[26]		6.0	10 km	143	1999 Athens earthquake
0.51g[27]		6.4		612	2005 Zarand earthquake
0.5g[17]		7.0	13 km	92,000-316,000	2010 Haiti earthquake

As you can see, out of the top 7 events, 3 of those are from Christchurch including #2 and #3.

Not too surprisingly that there was so much damage when buildings are being accelerated at 2.2g.