I am sure by now you have all heard about the destructive earthquake that hit Canterbury and Christchurch, New Zealand, yesterday.  When I heard the news on the radio my first reaction was that it was a strong aftershock of the 7.0 magnitude earthquake that hit that area last September, one month before the General Assembly of the Presbyterian Church of Aotearoa New Zealand was held in Christchurch.  (The PCANZ message on the new earthquake.) That opinion quickly changed as the damage reports started to appear in the media.  So here is a brief summary of what I know now.

The earthquake struck at 12:51 PM local time and was located just to the south of Christchurch.  While the 6.3 magnitude is significantly less than the 7.1 magnitude earthquake that occurred in September, the earlier event was some distance away in rural areas where the lower population density and single-family residential construction significantly reduced the cultural impact.  At the present time there are reports and pictures of significant damage in downtown Christchurch.  The death toll is currently at 65 but that is likely to rise as rescue efforts continue.  There are even reports of the shaking producing breaks and ice chunks to separate from a near-by glacier.

Besides the size and location, there is one other difference in the two earthquakes which may have increased the damage in this one.  While not as important as the close proximity to the urban center, yesterday’s earthquake was compressional, moving the ground upward.  The September quake was strike-slip, moving the ground sideways.  We will have to wait for the survey data to know how much the ground moved upward, but my initial calculation is about six to eight inches near the fault.  Looking at some of the pictures I think I see some of this compression but without the full context and exact location of the picture I can not be sure.  There is also discussion about this being a shallower earthquake than the September event and therefore more destructive.  Since that depth is generally only a measure of the start point of the earthquake I am not sure this is really a significant difference.  The September quake may have started deeper, but because of its size broke all the way to the surface.  Until we have detailed source-time functions for this event it will be difficult to really know the impact of the depth.

The question is asked if the two earthquakes are related.  My response is very likely yes, but exactly how I am not certain yet.  The most likely linkage is that September’s quake made this new one more likely through stress triggering.  A quick back of the envelope calculation last night indicated a good possibility, but I need to get better numbers today for the geometry of both quakes to be certain since it initially appears to fall right on a dividing line.  The second guess is triggering related to visco-elastic relaxation.  I’ll leave it at that.

It is interesting how the media grabs on to these stories and runs with them.  Without negating the impact of what is now being called one of New Zealand’s greatest natural disasters, it is important to keep this in perspective with other earthquakes.  I have heard news reports call this earthquake “huge.”  Is it huge?  Consider in the last 13 months we have had the Haiti earthquake – 7.0 magnitude with an uncertain death toll between 100,000 and 300,000.  We have also had the Chile earthquake one year ago – with an 8.8 magnitude it was one of the ten largest earthquakes of the last century.

These two earthquakes in New Zealand are a remarkable parallel to a couple we had here in SoCal and that I use in my class as a comparison of risk and hazard, and the different ways we measure the “size/impact” of an earthquake.  Back in 1992 the Landers earthquake hit an rural area (we call it desert) outside Los Angeles. While it was a significant earthquake at 7.3 magnitude and widely felt in the LA area, because of its location damage was fairly limited and there were only three deaths.  Two years later in 1994 the Northridge earthquake struck within the Los Angeles metropolitan area (“The Valley”).  It was a major, but smaller, earthquake at 6.7 magnitude but because it was in the heavily developed area the casualties were higher at 60 (higher by some counts) and the cost of damage was about 200 times higher than Landers.  (In the billions rather than millions of US$).  Location, location, location!

The parallel actually goes a bit further.  While they occurred further apart in space and time than the New Zealand quakes and are not linked in the same way, the Landers was a strike-slip earthquake like the September quake, and the Northridge was a compressional quake like yesterday’s.

Again, I in no way want to trivialize the death and destruction of yesterday’s quake and the continuing aftershocks.  In putting this in a seismological perspective I encourage prayers for all those affected and the rescue workers who are putting in hard and extended hours to help them.  I also ask for prayers for all others around the world who are impacted by natural disasters in their countries.