Earthquake questions for Pat Abbott — and the geologist’s responses

Readers sent us great questions for Pat Abbott after we announced our upcoming day tour with the local geologist, in light of a recent inewsource story about an active downtown earthquake fault.

They wanted to know about UCSD’s Shake Table, how faults affect both coastal communities and those in the South Bay, what steps the city and county have taken to retrofit buildings, and more.

Abbott answered all of them while hopscotching between downtown locations, such as the San Diego County Administration Center and the John D. Spreckels building on Broadway, during our earthquake safety tour – which you can read about here. Here are the questions and answers:

How can you be sure you’re not buying a house on top of an earthquake fault?

ABBOTT: First off, how can you be sure you’re not buying a house that sits on a fault?

Let’s say this, the faults that are known are available. They’re available on your website courtesy of the San Diego Association of Geologists. If you just Google “SDAG” and scroll down a little bit, you’ll see an interactive map, in color, that lists the various faults, shows their locations, and it’s a great resource for somebody to look at. It’s the most up to date resource that somebody could see. So please consult that. Just Google “SDAG” and look at their interactive fault map.

Another question that’s interesting is – they say for buildings on the ground here, you have to set back 50 feet from the fault. And the question from Bill is, “Well why only 50 feet? That doesn’t seem like it’s far enough.”

Well actually it is. This is called the Alquist Priolo act. And what they were trying to do was prevent having building foundations literally ruptured by the fault movement. So 50 feet is enough to save the building from being directly ruptured. It’s still shaken, but it’s not physically ruptured.

But let me also state, let’s go back to the Northridge Earthquake in 1994 or Loma Prieta in 1989. Each of those, the fault movement occurred 11 miles deep. Eleven miles deep. When you’re coming from that deep up to the surface – whether you’re 50 feet away from the fault or a couple miles away from the fault, fundamentally that’s about the same thing. The same distance of travel for the seismic waves, about the same amount and level of shaking. So the 50 feet number is actually a good one. It saves the building from being ruptured, but of course protection from the shaking, it’s really going to be the same for everybody when the Rose Canyon fault moves here in town. All buildings will be shaken and will be tested.

What’s the point of UCSD’s “Shake Table”?

ABBOTT: We have a question about, basically largely about the results of the UC San Diego, their giant shaker table out there at Miramar. They literally construct whole buildings to scale or even to actual size, and with seismograms – the records of past earthquakes – you can literally shake these structures to see how they would behave in actual historical earthquakes. So this is very expensive, time consuming work, but it’s extremely valuable work.

And let me tell you why that’s so valuable. In terms of preparing for earthquakes, one of our biggest problems is – this is in a sense of planning and understanding – we don’t have enough earthquakes. So in other words, we don’t get to test things. We don’t know how – ‘What about if we make this change to a building, will that help or not?’ The shake table provides answers to questions like that. Instead of just waiting 50 years for the next earthquake, we can actually tell some of the things that are going to happen, learn from it, improve our buildings. So the UCSD shaker table and the work they’re doing out there on testing buildings is extremely valuable work and information that’s useful to society as a whole.

Will most survive the Big One? Are our public agencies ready for it?

ABBOTT: A question from Lois Dorn. She asked, ‘Will most people survive the big one when it hits San Diego? Also, how long will it take for the city to get back to some sort of normal after it happens? Finally, our public agencies ready for this?”

Well, first off, will most people survive the big one when hit San Diego? Absolutely, yes. No question about it. We’ve done lots of good work with building codes of buildings over the years. The problem here is not going to be anything like you see in the news like what happens in Haiti or Mexico City or things of that sort. I’m not trying to minimize the threat or the hint of destruction and deaths, but I just want to emphasize that it’s not going to be widespread. It’s not going to be total or anything like that. There’ll be a few isolated buildings, a few isolated areas that are hit hard, but for most people, most of it, it’s not going to be nearly as bad as as certainly Hollywood makes us think.

And how long will it take the city to get back to normal? We have excellent city and state staffs. They’ll be on things right away. And of course as we see from things like, well recently here, we can see Hurricane Harvey for example. Look how the aid pours in from other states, even other countries. So we’re not alone when something happens, we will have loads of support. So how long does it take to get back to some kind of normal? One, it depends on the size of the earthquake. You say the big one, I’d say within a week there ought to be some sense of normal and really for a lot of people, probably even most people, some sense of normal will come back within a day. And by that I mean most homes will be able to handle the earthquake. A good one story, two story wood frame house should be able to handle it. And if you have your supplies on hand and you don’t have things that are loose and flying around inside of your house, order should be able to be restored significantly. Bottom line on these things is, recognize it’s coming, prepare for it in advance, think about it, but for the most part, the vast majority of people will not suffer serious problems.

Any comments on the inactive(?) La Nacion fault line?

ABBOTT: Vickie Church asks about comments on the inactive (question mark) La Nacione fault that follows part of 54th street that runs south. It runs south almost down to the Mexican border. This is a fault zone that doesn’t get talked about as much. First off, she puts paranthetically (inactive?). I personally consider it to be an active fault. It’s not a primary fault. The Rose Canyon fault is pulling the land apart. That’s why we have San Diego Bay. The land has been pulled apart, so as the land drops down to make San Diego Bay, it has to rupture and separate from the rest of the San Diego, Chula Vista area. And that’s the La Nacione fault, what we call down to the basin faults. San Diego Bay is dropping down and the bad news is a lot of homes in that I’ll say greater Chula Vista, National City area, a lot of them have been built over the faults strands, which should not have happened, but it has happened.

So I’d say, treat that fault with respect. Do not build on top of it. I don’t know if you’re legally allowed to now. You were not — it was legal even a short time ago, but it definitely is an active fault. It’s not going to make an earthquake as big as we get on the Rose Canyon fault or Elsinore. Maybe something in the magnitude fives, but that’s going to do damage to a home built on top of it.

What steps have the city and county taken to retrofit buildings, bridges, and other infrastructure?

ABBOTT: We have a question here on Twitter. It states that the bay area has had lots of earthquake retrofits after Loma Prieta. That’s the 1989 earthquake. What steps have the city and county of San Diego taken to retrofit its buildings, bridges, and other infrastructure?

Well, it’s not only the city and county, but also the state. You mentioned bridges. Let’s start with that one. You may have seen some of those things here as Caltrans has gone through and done retrofits on bridges. You can see the retrofits very distinctive, for example, the Interstate 805 bridge and onramps going across Mission Valley. Significant about of retrofits done there. We also have gone to things like passing laws, like for example, URMs – unreinforced, masonry buildings. Bricks and all. The state required each city to do an inventory of these buildings which are out of code, which are things that are liable to fail in an earthquake and to take measures to make those buildings be retrofitted.

Now, since they’re owned by private individuals, there’s a limit to how much you can insist on, but there’s been a lot of pressure. The inventory of these URM buildings has gone down markedly in the last 15 years or so, and we still have a ways to go yet. But progress definitely is being made and very frankly, I’m pleased with a lot of the progress I see. Not to say we’re done. There’s more I’d like to see done, but I’m very happy with the way things have gone in recent years in terms of fixing things up.

How do faults affect coastal communities like Encinitas?

ABBOTT: A question from Beth Herman here about the faults, how they affect coastal communities like Encinitas.

We know from Encinitas, we have the Rose Canyon fault sits just off shore to the west, but let’s move farther out offshore. When you look off shore from Encinitas and you see the Coronado Islands and the San Clemente Island, all those islands exist because the land has been lifted up by active faults. When you look at those islands, they came to be only because of a lot of earthquakes. And the earthquakes that happened there in the past, they will happen again in the future. So there are significant earthquake faults running just off shore from Encinitas, directly to the east of the Elsinore fault, and then directly off shore with things like the offshore faults.

Related:

We’ll let you know when big things happen.
Email address: