A year ago GG posted on the Kaikoura M7.8 earthquake with the title “Single quake slip partitioning”. With a year past, it seems a quick look at the literature that has appeared is in order. Was this diagnosis correct? In some work, it seems the answer is yes; in others, it seems no.
The most comprehensive overview is probably a paper by Kaiser et al. in Seismological Research Letters. This paper summarizes geologic, seismologic, geodetic, and engineering observations from this quake. They note that 13 separate mapped faults all ruptured together, more than was anticipated prior to the quake. It took about two minutes for things to unwind from south to north along this collection of faults, with substantial step-overs was one strand to the next. Most of the energy released came in two distinct jumps, one 20 seconds into the quake, the next about 70 seconds in.
But as to GG’s hypothesis of slip-partitioning during the quake, the interpretation of the slip history from high-frequency seismic data is no; the faulting was dominantly strike-slip to oblique-slip on land, though the authors do note a period during the rupture when they don’t really locate the source of seismic energy very well.
A second paper comes at this from a different angle. Read More…
Last November, a M5.0 quake caused some damage in Cushing, Oklahoma. A number of folks at the time were relieved that there wasn’t any noticeable damage to the nation’s largest oil storage facility. This was only a few months after the Oklahoma Corporation Commission ordered a cutback in disposal of wastewater in injection wells. Since then, seismicity has mostly quieted down, but it seems that recognition of the scale of the hazard has been seeping into the awareness of a broader part of the media, leading to a lengthy piece in Politico Magazine on the potential disaster lurking in Cushing from facilities not really designed to survive an earthquake. While most of the stories of earthquake hazards in Oklahoma have been more focused on falling chimneys and old brick buildings, this piece exposes a pretty critical flaw in Oklahoma’s infrastructure.
So we will hope that there isn’t another unrecognized fault slowly being lubed up under Cushing. But remember, the largest events from the infamous Rocky Mountain Arsenal injection adventure in the 1960s came more than a year after injection was totally stopped. Oklahoma hasn’t stopped injecting fluids, and the volume of water injected so far dwarfs anything that happened in Denver in the 1960s…
For those of us in earth science, this past week has highlighted an awful lot of potential “told you so” moments. Like how warming climate and a warming ocean will lead to higher precipitation events. Like how you really do need to plan for floods. And we just missed hearing more about the barrier island/marsh protection talking point. And almost at the same time we’ve been greeted with ever more evidence that the Trump administration has little or no use for scientific input–not even choosing to ignore it, they seem more eager to simply not have any scientific input at all. Just as it is ever clearer that we are facing real decisions in trying to prepare for a warmer world, we seem the have a government yelling “la la LA LA” with its fingers in its ears.
But that isn’t the point here today.
One aspect of the tragedy in Houston is that the absence of any sensible planning has led to more flooding (the worst example might well be letting houses be built within the basin and below the spillway elevation of flood control dams); this is exacerbated by the combination of government subsidized flood insurance and the out-of-date or inadequate flood zone maps. Of course some now point to the zoning-free and laissez-faire approach to building in Texas as the bargain they made with the devil, implying that other places where strict zoning has been enforced will be safer.
If GG has noticed one thing about strict zoning (and Boulder has a pretty heavy hand on building), it is that it is rarely used to prevent building in stupid places–it is mainly used to keep people from building on land other people enjoy as it is. Some years ago when Colorado Springs was approached by a developer who wanted to build houses on an active landslide, the city council had to look away from the evidence they were given in order to approve this ongoing disaster. You can find similar stories elsewhere. Yes, fear of flooding is brought up when a new development is proposed…but mainly as part of the larger arsenal serving Fort NIMBY (sometimes there is a legitimate fear, but sometimes it is greatly exaggerated). California has the Alquist-Priolo act to prevent construction near active faults, but it only moves buildings 50 feet from an active fault. Direct destruction of a building by a fault being directly under it is one of the least likely modes of destruction (even some dams do OK on faults: the Upper Crystal Springs dam survived having several feet of offset in the 1906 earthquake). Earthquakes do most damage by shaking weak soils: recall the Marina District in San Francisco, far from surface faulting, where shaking from the Loma Prieta earthquake damaged dozens of structures. What strict zoning clearly does is raise housing prices.
The main exceptions to non-use of zoning as a disaster preventative is in the wake of disasters. Even then, the most common refrain after a disaster is “we’re going to rebuild and bring it back better than before.” After a tornado, this makes sense. After a flood, whether storm surge or heavy rain? Not so much. The harder statement? “We learned a lesson and we aren’t going to make that mistake again.” It is very hard to say, but if we are going to avoid paying to rebuild over and over again in increasingly vulnerable places, risking the lives of inhabitants in the meantime, it’s time to start saying it and then walking the walk.
Its been awhile since Oklahoma earthquakes made news and so it seems timely to look in on the Sooner State to see how things are going.
Last we looked in, numbers of earthquakes were down but the moment release was still pretty high. Predictions from Stanford late last year were that the decreased injection of wastewater would lead to a decrease in earthquakes over the succeeding five years. The USGS, in contrast, has continued to note that the decrease in the number of events does not mean a decrease in damaging earthquakes.
So far, the news is good. Not too surprisingly, the number of earthquakes continues to drop:
So that continues the trend from 2016. What about moment release? Well, given the absence of news reports, you’d guess there is a decline there, too, and you’d be right:
And, indeed, 2017 has been dead quiet moment-wise as well.
Does this mean that the seismic risk is now gone? Well, no. That M5.7 earthquake in late summer last year was on an unrecognized fault. The fluids migrating in the basement could encounter another critically stressed fault and trigger a significant earthquake. But for now, this is good news for Oklahoma residents.
With 2016 coming to a close, GG thought we might want to see just how things are shaking out in Oklahoma, home of the great induced earthquake experiment. And there is something for everybody, depending on how you want to look at it.
For the optimists hoping that Oklahoma’s actions to slow wastewater injection will end the plague of induced earthquakes, we have plot number one: number of quakes with a magnitude of 3.0 or higher by month:
The rate of such earthquakes dropped from nearly 4 a day in January to about one a day this month. And you could hope that this had something to do with this:
The 25% reduction during the year in the rate of injection in the area where triggered seismicity has been observed might be responsible for this. But there are other things to watch as well. First, these are still pretty high volumes of water going back into the Arbuckle, and all the water that went down earlier is still making its way through the subsurface. Second, presumably a lot of produced water is going to other wells, either in the Arbuckle outside the area of interest or into other formations. Third, a decrease in the number of M3+ events is not the same thing as a decline in seismic moment:
The 9/3/2016 M5.8 Pawnee, Oklahoma earthquake put a big damper on any celebration of a decrease in seismicity. The overall moment release of 7.8 x 1024 dyne cm is the largest single year moment release in Oklahoma history. As we noted before, this isn’t unexpected: the Rocky Mountain Arsenal sequence in the 1960s produced its largest quakes after the injection ended.
So we enter 2017 on a note of caution. If you bought earthquake insurance in Oklahoma, don’t let it lapse just yet. You might get shaken a bit less often, but when you do get a quake, it might still be pretty big.
P.S.: there are a couple of nice visualizations out there. Tulsa World put together an interactive map a year ago showing how produced water injection was varying over time and by county. The Oklahoma state government has an interactive figure with recent earthquakes and disposal well locations.
UPDATE 2 11/22: GNS has assembled quite a lot of information, and the puzzlement deepens. It appears from the satellite and ground analysis that the bulk of the motion–up to 11 m of slip–was more nearly strike-slip and not the thrusting that appears in the focal mechanism (below). But the uplift of some areas of the coast by 6 meters (!) seems to suggest there is something more.
UPDATE 11/18: A considerable amount of information was put in an article on stuff.co.nz. This includes a map from GNS showing where the faults are that ruptured, a good deal of geodetic information.
Yesterday’s M7.5/7.8 Kaikoura earthquake in New Zealand is one of the more bizarre large earthquakes we have seen in some time. On the face of it, this appears to mostly be rupture of a subduction zone under northeasternmost part of the South Island of New Zealand. But there is a lot of other stuff going on….
First, the main focal mechanism as reported by the USGS:
Now this beachball would suggest a fault dipping to the NW while paralleling the coast. But the appearance that a toddler was not coloring in the lines tells you that there is something more here.
Some of that became apparent when the New Zealand’s GNS Science group went looking to see if there was any slip on earthquake faults. This is what they found:
Rapid field reconnaissance indicates that multiple faults have ruptured:
- Kekerengu Fault at the coast – appears to have had up to 10m of slip
- Newly identified fault at Waipapa Bay
- Hope Fault – seaward segment – minor movement
- Hundalee Fault
I’ve tried to sketch these out from my copies of geologic maps of New Zealand:
(The base map is from Google).
This is where the other shoe drops. The Hope and Kekerengu faults are mapped as strike-slip. Now minor slip on the Hope Fault might not mean much, but 10m on the Kekerengu means there was a lot of slip (I’ve assumed above it is strike-slip, but perhaps there is a thrust component). Plus, the epicenter of the quake–where it started–is somewhere between Cheviot and Rotherham, well to the south (this is why initially this was called the Cheviot earthquake). Toss in a very odd slip history (the moment release was low for a minute and then things really broke) and you get the impression that a relatively small earthquake on an unnamed fault southeast of Rotherham started tripping things off to the north, which eventually tripped off a big rupture.
That big rupture probably is not on the map. It is likely offshore, in the very southern end of the Hikurangi Trench (which is in part responsible for the whale watching that is so popular at Kaikoura). This is the northeast trending thrust fault that the focal mechanism captured and is responsible for the large slip amounts found on the finite-fault map the USGS shares. This is probably also the reason for the ~1m uplift of the seashore at Kaikoura, which led to many photos of paua and crawfish out of the ocean (though uplift at the southwest end of the big strike-slip fault is also possible).
Presumably the large strike-slip faulting on the Hope and Kekerengu faults is what has contaminated the focal mechanism, making it a composite of complex motions instead of the clean double-couple. (Pure strike-slip faulting is seen in many aftershocks.) As such, it seems this earthquake might well have captured both major thrust motion on the subduction zone and strike-slip on the upper plate faults, a form of slip-partitioning in a single event that is quite striking.
It will be interesting to see how the seismological and geological analysis continues; the main seismological slip appears north of these faults and so there could well be more to be found. But rain is in the forecast, which tends to ruin the easiest of signals to see.
Back in September, Oklahoma had a M5.6. Some of you might recall the difference in opinion between USGS scientist Dan McNamara, who expected continued seismicity, and Oklahoma Geological Survey director Jeremy Boak, who said “I’d be surprised if we had another 5.0 this year.”
Well, Director Boak hopefully was in the vicinity to be surprised in person by the M5.0 today that damaged buildings in Cushing, OK, site of the largest oil storage facility in the country (which at least apparently escaped any damage). Yeah, once more wishful thinking trumped by actual scientific examination….increasingly it seems the branch McNamara has climbed out on is the real stout one while the hopes of the Oklahoma injection operators rest on thin reeds.
At least nobody has died, but when you are evacuating a senior housing facility in the night and cancelling school, you know you are playing with fire.
And hey, we aren’t even done with 2016 yet.