From: cowen@JADE.UCDAVIS.EDU
Newsgroups: sci.geo.geology
Subject: Cairo Quake revisited
Date: 6 Nov 92 21:38:29 GMT
Reply-To: cowen@JADE.UCDAVIS.EDU
Organization: Geology Department, University of California, Davis

A couple of weeks ago I posted a suggestion that the Cairo earthquake 
might be human-induced, from the extra irrigation made possible by 
the Aswan High Dam after about 1960, and the extra leakage from the 
water and sewage systems of the metropolis of Cairo, whose population 
has increased enormously in recent years. After the responses both 
on and off the net (thank you all), here is where I think we are.  
For those who do not want to read much more, the bottom lines are

1. Cairo has had enough historic earthquakes that the suggestion of 
human induction is not needed at this time to explain the data 
available. Since it is unnecessary, forget it as an explanatory 
hypothesis, unless and until we get more data that does not fit the 
historical pattern.

2. Human induction of the sort I suggested is nevertheless a 
possible mechanism for triggering earthquakes. One can make 
reasonable predictions for Cairo earthquakes over the next decade 
if in fact human induction is beginning to kick in: anomalously 
frequent (but smaller) quakes would begin to affect the area.

Here are the details, for those who care. Cairo seems to get 
damaging earthquakes every 200 years or so (+ or - 100). For example, 
there were big (damaging) earthquakes in 885, 950, 1212, 1262, 1303, 
1588, and 1754. The damage is greater than the shocks would really 
warrant because of the prevalent brick construction, and because at 
any time you care to name there are a lot of old buildings in the city.

Induction of earthquakes is well established for dams, and for deep 
injection and withdrawal of fluids, as under Denver and in the Rangeley 
oil field in Colorado. But what about simple flooding? 

ItUs possible. The Rangely earthquakes were associated with a 
pressure change of about 35 atmospheres, but earthquakes were induced 
at Matsushiro, Japan, at only 14 atmospheres pressure change (= about 
40 meters of water pressure). More exciting, however, is the story of
earthquakes generated by the Aswan High Dam.

The first anomaly was that there was such a long time lag between 
completion of the dam and the first earthquakes (17 years). However, 
the earthquakes were induced along a pre-existing fault structure 
well to the west side of the reservoir. The earthquake area did not 
get any seepage until the lake was close to full, and at that critical 
point the reservoir rose over the narrow confined of the Nile ravine, 
and extended several miles to the west, along a shallow depression 
that it filled to only 10 meters water depth. And *then* the 
earthquakes began. Groundwater levels had been about 90 meters 
below the surface in the area before it was flooded.

You can find this discussed in a fairly new book by H. K. Gupta 
called Reservoir-Induced Earthquakes, published by Elsevier. It 
is catalogued in our library as TA705 D48, volume 64, and the 
Aswan phenomenon is discussed in Chapter 12. It is Gupta who makes 
the point that it is not the heavy stress by water loading that 
started the earthquakes here.

OK, then, we have a case in which rather shallow flooding of a 
previously desert area may be involved in starting an earthquake 
sequence. Could it apply to Cairo? Here are some facts. 
1. The natural rainfall at Cairo is 1 inch, and the potential 
evaporation is about 50 inches. That means there is no natural 
recharge (and little acid rain!). The pristine groundwater level 
has to be below the current one.
2. The irrigation is now year-round, rather than seasonal. Earlier 
Egyptian irrigation schemes were based around the *flood* peak of 
the Nile. That is why the Egyptians did not have a schistosomiasis 
problem until modern times, because the parasite did not have 
year-round standing water to live in. Year-round irrigation along 
unlined canals has to mean a greater amount of water seeping into 
the local sediments.
3. Both the water-supply and the sewage system of Cairo are 
leaky. The water supply leaks at perhaps 35%, and no-one knows how 
much the sewage system leaks. Furthermore, the sewage system was 
built in the 1920s to serve about a million people, and would be 
seriously overloaded now by the current multi-million population, 
even if the system were leak-free. All this means an increasing 
seepage into the ground.

I think there is still a strong possibility that the October 
earthquake was induced (or accelerated) by this geologically new 
supply of water. But how would one *test* the idea, in view of the 
significant seismic background in Cairo? Here is a simple prediction: 
we should see from now on an anomalous level of seismicity. It should 
be concentrated around the Greater Cairo area, and although we will 
not see major earthquakes, we will see many more smaller ones, 
comparable to the experience of Denver. It will take a while to 
measure the increase in frequency and to determine that indeed it 
is different from the historical background of widely spaced larger 
quakes.

Dont hold your breath.

Finally, are there any more places we could look for such an effect? 
Not many, because we should be looking for areas that were formerly 
bone-dry in order for this large-volume, low-pressure flooding idea 
to work. The Imperial Valley of California is one: a bone-dry 
RValley of the DeadS in 1896, it was renamed and irrigated from the 
turn of the century. The problem here is that there is no historical 
record from the area, which in any case sits on the San Andreas Fault. 
Phoenix is rain-poor, but has had the Salt River flooding it and 
recharging its aquifers until recently. So really, Cairo seems to me 
to be the best place for testing the idea.

And yes, I know there was a 4.9 at or near Cairo yesterday, November 5. 
But that was an aftershock....
wasnUt it?

Again, thanks for your input: load up the napalm and fire away.

Richard Cowen, Department of Geology, University of California, Davis