Speculating About Very Shallow Earthquakes, The Greendale Fault, And The Halswell-Lyttelton Area

Andrew writes:

This news report mentions a very shallow quake (1 km!!) within 2 km of my house. I am in Queenstown so don’t know how that would have felt.

Do you know what the implication of such shallow earthquakes would be?

Friends in Diamond Harbour tell me today that there is no water now in the village, so water tankers are providing water. I think I will stay in Queenstown for a while! The office is closed until Wednesday anyway.

Best regards,
Andrew

I reply:

Andrew:

I’m glad you are in Queenstown, and glad you have an office to return to! And it’s probably-best, under the circumstances, to be spared the rigors of your home base.

Since the Diamond Harbour aftershock was fairly-small, there probably won’t be any noticeable damage. Nevertheless, the shock was remarkably shallow! There was another, slightly-more-powerful, very shallow quake at Diamond Harbour on Thursday (3.5M; 07:23, 2 km depth). There have probably been plenty of others too.

This publication states:

Tectonic earthquakes at such shallow depths have rarely been confirmed in California (Fletcher and others, 1987; Frankel and others, 1986) and Nevada. A depth of 5 km is typically the upper limit of the “seismogenic zone,” the depth range where the lithostatic pressure from the weight of overlying rock is high enough, and the coefficient of friction is high enough, to stress the rocks to the level required for stick-slip faulting behavior.

Because the Rock Valley earthquakes are near the upper limit of the seismogenic zone, they are of fundamental interest for earthquake mechanics. At such low lithostatic stress values, the magnitude and orientation of the local tectonic stresses may play a particularly important role in generating shallow earthquakes.

And:

Tectonic earthquakes are rarely observed at depths of less than 3 km. Sanders (1990) suggested that in southern California very shallow earthquakes are confined to relatively stable blocks between major fault zones, and that the major fault zones themselves do not usually include shallow events.

So, presumably such a shallow aftershock is being driven entirely by tectonic stress, not lithostatic pressure at all. Perhaps it also means that the block of rock is relatively-stable. So, relatively-stable rock subjected entirely to tectonic stress, and breaking as a result.


(Speculation)
It is interesting that the Greendale Fault (the surface expression of the tectonic stress) has yet to make an appearance in the Halswell/Lyttelton area. At first, I thought it might be because Halswell seems to be a kind of basin with lots of unconsolidated sediment, and that the fault was just being obscured by the sediment, but maybe it’s that the rock is more-resistant to breakage in the Halswell/Lyttelton area. Maybe because it’s volcanic rock. Like a porcelain plate breaking in slow motion, that Greendale Fault is probably extending eastwards, and will make an appearance at the surface over the next few thousand years in the Lyttelton area. Some of the aftershocks are now offshore, to the east, so the faulting may eventually extend there too.

It would be nice to know more about the geologic history of the Lyttelton Harbour area. I was always casually puzzled as to why Lyttelton Bay exists. It doesn’t seem to be an estuary, or connected to a river system. I thought maybe there were two calderas associated with the Banks Peninsula: first, the main one associated with Akaroa Bay, and a second, smaller one associated with Governor’s Bay, and the entire complex was highly-eroded over time. That’s sort-of how it looks to me. But it bothers me that the Greendale Fault is parallel to the axis of Lyttelton Bay. Is it coincidence, or is the expression of something more significant? I don’t know.

[UPDATE]:

According to Wikipedia:

Banks Peninsula forms the most prominent volcanic feature of the South Island. Geologically, the peninsula comprises the eroded remnants of two large composite shield volcanoes (Lyttelton formed first, then Akaroa). These formed due to intraplate volcanism between approximately eleven and eight million years ago (Miocene) on a continental crust. The peninsula formed as offshore islands, with the volcanoes reaching to about 1,500 m above sea level. Two dominant craters formed Lyttelton and Akaroa Harbours.

Then, there is this regarding September's Darfield quake:

The eastern tip of the fault is also creeping slowly, suggesting it is possible that the subsurface extent of the Greendale Fault extends further to the east then the surface rupture.

Dr. Campbell has written an excellent lay summary:

The pattern of aftershocks following Tuesday's big jolt has revealed yet another previously unidentified active fault. This is the culprit that has ruptured within the earth's crust and which has given rise to the intense seismic shaking in the Christchurch region. However, it may also be thought of as a valve that has enabled pent-up energy to be released. In many ways faults actually focus and channel energy.

It has ruptured over a length of about 17km on a near vertical plane slightly inclined to the south and between 3 and 12km in depth. It is more or less parallel to the E-W trending Greendale Fault that ruptured in the Darfield Earthquake. It may be thought of as an eastern extension but it is clearly dislocated from the trend of the Greendale fault and stepped to the south.

...East-west faults in Canterbury are relatively unfamiliar to geologists. Most active faults in New Zealand are sub-parallel to the plate boundary that is they trend northeast-southwest.

However, if you take away Banks Peninsula (extinct Miocene volcanos that erupted between 10 and 6 million years ago) and the gravels of the Canterbury Plains, the underlying geology is essentially that of the western end of the Chatham Rise. And the Chatham Rise is riddled with old east-west oriented faults. So maybe the current plate motion is exploiting old faults within the earth's crust at depth, causing them to fail.

I was trying to envision the movement along the Greendale Fault, and the best analogy I could come with is a Tilt-A-Whirl.

Picture the Canterbury Plains (which is carrying the City Of Christchurch) as one of the cars rotating counterclockwise here – preferably one of the cars moving to the left, on the far side of the Tilt-A-Whirl (mimicking the movement of the Pacific Plate as it moves westwards to collide with the Australian Plate). The Greendale Fault is the visible portion of the circular base upon which the car sits.

As the car rotates, the circular base of the car slips to the right of the observer.

Similarly, as viewed from Diamond Harbour, the City of Christchurch slips to the right (and vice-versa: as seen from Christchurch, Diamond Harbour seems to slip to the right).