Quakes – Bleak City

Don’t be scared, be prepared

I like science and I’m always interested in reading about some study revealing more about the history of the Alpine Fault or showing in detail what happened during the Kaikoura quake. These studies are interesting to science geeks like me, but they aren’t just for entertainment, they’re about assessing risks and preparing for them.

Six years after Christchurch’s most damaging quake and four months into Kaikoura’s post-quake phase, New Zealanders are a bit more aware of the need to prepare for all the natural disasters we face. There’s the Get Thru website, the Long or strong, get gone message being put out following the widespread confusion following the Kaikoura quake, and reports on what would happen if a volcano erupted in Auckland. Yes, scary stuff.

Sometimes people don’t want to hear about these things because they feel there’s no point, that natural disasters are unpredictable and there’s nothing you can do about them. But scientists and authorities aren’t trying to scare us, they’re trying to prepare us. Although we can’t avoid the natural disaster (except by moving somewhere where there are none), we can prepare for them. That’s what anchoring stuff to walls and emergency kits are all about: being prepared. If the worst doesn’t happen, that’s great, but if it does, isn’t it best to have what you and your family need right there, ready for you?

Another thing we can do is to document your property, so you have before and after pics to use when making your insurance claim. More on that in Is it historic? Or is it quake damage?

Is it historic? Or is it quake damage?

I have a relative whose house repair has been botched. Floors are uneven, doors have been shaved into non-rectangular shapes to make them fit into doorframes and there is still cracking in the brickwork. This is the family home, the one she raised her kids in. She’s a grandmother now, so that will give you some idea of how long she’s been in this house. She knows what it was like before the quakes and her repair hasn’t brought it even close to that pre-earthquake state, much less to the as new condition stipulated in her insurance policy. She has complained to EQC, but the response has been that her damage is all “historic”.

I’ve heard that word too much in the last 6+ years. The EQC and insurers are big fans of it. What’s historic and what’s not is difficult to determine in older houses, but too often the homeowner is left with earthquake damage that they can’t get repaired without getting a lawyer involved. EQC and insurers just dig their toes in, insisting that you, the homeowner, prove that it was quake damage. But how do you do that if your only evidence is your recall of how the house was? Dig through all the family photos and hope someone caught a shot of the living room door showing that it didn’t have a bowed lower edge?

My husband and I fell foul of this with the cracked slab of our garage. It was put down as historic in the report the insurer’s engineer submitted, but our own engineer said it was quake damage, going by floor level readings and continuity with an acknowledged quake damage crack outside the garage. We didn’t want to get into the battle of the engineers, but we had no evidence to show that this crack wasn’t there before the quakes. In the course of picking our battles, we decided to let this one go.

But we learned from that experience, and whenever we move into a new property, we’ll be doing a thorough photographic record of the state of the place, floor to ceiling, inside and out. When my in-laws moved into a new place a couple of years ago, we had them take photos, not just a photo per room, but detailed photos that showed the condition of the interior and exterior, the foundations, walls, skirting boards, ceilings. If I ever have to go through this post-earthquake claim process again, at least I’ll know I’ve done all I can do avoid hearing the dreaded “historic”.

The 22nd

This time of year creeps up on me. One moment I’m enjoying the hot, dry days of summer, the next it’s triggering memories and disturbing thoughts. In early February 2011, my aunt and uncle arrived from the US, and so those weeks before the quake were a family time. The day my aunt and uncle arrived, my mother, my husband and I took them into the city. We parked on Cambridge Terrace by the PGC Building that wouldn’t be standing at the end of that month and walked along the river to Victoria Square. It was natural to talk about The Quake, they were curious. The city was quiet that day, there weren’t many people around. I remember the hot, dry emptiness of Victoria Square and thinking about The Quake that could have been so much worse.

We spent the weekend before the quake in Murchison. My uncle, who spent his life in Texas and Arizona, couldn’t understand how it could be so hot in the mountains. That weekend coincided with the local A&P show. We ate venison, watched sheep shearing and fed the eels in a nearby creek.

All those memories. And then the bad ones.

The 22nd of February. That day, my mum, auntie and uncle did the tourist thing, going around the city together. I was working.

Where I still live (and work) is north of the epicentre of the February quake, and between September 2010 and February 2011, I became used to the regular rumbles, the house shaking, those small quakes passing through. The morning of the 22nd, there was a little jink at around 9.30 or 10 o’clock. Background noise. Soon after, I received a text message from my mother: they were going to do the cathedral, then have lunch. I’d meet them later in the city.

By 12.30, I had managed to get a big chunk of work out of the way. I’d had a shower and lunch and settled back at my desk for another couple of hours before going into the city. The quake started quickly and was more violent than any quake I had experienced before. I had no time to do anything except be afraid. When the worst of the shaking eased, I thought about the city and where my family might be. All those months of thinking how lucky Christchurch had been with the 7.1 were replaced with the horror over what was likely happening in the city. Mum had said they were going into the cathedral, a vulnerable stone building. They could be dead, I might never see them again. I started to shake and cry while at the same time trying to get past the debris between my office and my cellphone charging in the adjacent room. My phone rang. I just managed to get to it and have a few words with my husband, who worked out by the airport. We were both okay, he would come home, but it might take a while. I was going into the city, I said, to get Mum and my aunt and uncle. Later he said he thought of trying to talk me out of it but could tell from my voice that that wasn’t going to happen.

Just after 1pm, I received a scrambled text message from Mum. Enough to know she was alive and ok, enough for me to head in to the city and pick them up. I was lucky. I heard quickly. Some waited the rest of the day, and far too many never heard back at all.

I left my car in Moorhouse Ave, there was just too much traffic to try and drive in to the city. I walked in to the city and found them just after 3 o’clock in the Botanic Gardens. I’ve never seen these people who loomed so large in my life look so small and frail. We walked back out to Moorhouse Ave, crossing the river at the Antigua boatsheds, the water roiling with the silt stirred up by the liquefaction process. We quickly reached the car and finally reached home over an hour after that, following what would normally be a ten minute drive up Brougham Street. After my husband arrived home, we packed up the cars and left the city, staying at a cousin’s house north of the Waimak for the next couple of nights.

I’ve never been as terrified as I was that day, not just of the force of the quake but at the thought I might have lost the people I love. The memory of that day is sharp for me, but I’ve been able to move on, my family were okay. But there are 185 families that will never be the same again, and it’s them I think of every February, as the weather heats up and dries out, reminding me of those hot summer blue-sky days before the world changed for them.

How to tell ghost quakes from real ones

Back in the days of the Canterbury earthquake sequence, Cantabrians had to wait up to half an hour to learn the magnitude of a quake. These days, GeoNet Rapid tells us within a couple of minutes, or at least gives us a good estimate. But this rapid report can occasionally produce ghost quakes, alerts for quakes that didn’t really happen. So in the immediate aftermath of a quake report, how do you tell a real quake from a ghost quake?

Drums.

Geonet has a network of seismographs that keeps track of what’s going on seismically in our shaky little country. You can get a rough idea of where a quake is located by looking at the snapshot of the whole seismograph network. This the snapshot for the morning of 4 September 2010, 80 minutes after the quake that started it all for us here in Christchurch:

If you look closely, you can see that the Canterbury seismographs show the quake before the others throughout the country.

More recently (this morning), a 5.2 quake near Culverden resulted in a ghost quake measuring 6.3 near Opotiki:

Immediately after the Opotiki quake alert, the seismograph snapshot looked like this:

There’s just a single line for a single large quake sending seismic waves up and down the country. Again, the leading edge of the curve is centred on South Island sensors. That’s the first hint that the other quake reported might be a ghost quake.

For the second hint, we look at the individual seismographs. The seismograph nearest a quake picks it up before those far away, and the one that is near a quake shows it differently than a drum further away does. The trace on a seismograph for a nearby quake tends to be a bit cleaner than one from faraway. Nearby sensors may show part of the event as a red rectangle. This is where the sensor is indicating full scale deflection of the measurement at the displayed sensitivity range – the needle is literally off the chart. Although in this case there isn’t a literal needle, as these are electronic sensors.

This is the 5.2 quake that occurred near Culverden this morning (12th of February 2017), shown on the nearest drum:

Now here’s the trace from a drum a fair bit further away, and much closer to the supposed 6.3 Opotiki quake, the Urewera drum near the Bay of Plenty:

A bit messier, yeah? This is the Culverden quake we see on the Urewera trace shown above, even though the Urewera sensor is over 600 km away. There’s no big local quake showing up on the Urewera trace (although there might be a small 3.2 that occurred around the same time as the Culverden one that could be partly the reason the ghost quake got spawned; at time of writing, that’s not yet clear).

Drums on GeoNet are updated every five minutes, so looking at these drums is a good way of quickly determining whether a quake is a real one or a ghost one. Of course, once the duty officer has a look at the data, the information on the quake will be updated and its size and location will be revised. You can see this process by looking at the event summary for individual quakes. This is for a quake that was picked up by the automated system, analysed and revised by that system and then checked by the duty officer:

At 1 minute and 49 seconds, the GeoNet Rapid system published an initial magnitude and location when the data was received from the equipment closest to the location. Over the next ten minutes, the automated system continued to update magnitude and location as more information was received from more distant sensors. Then, at 12 minutes and 31 seconds after the quake, the duty officer posted their review of the information the system gathered.

This is what those statuses mean:

Equipment further from the location picks up the seismic waves as they travel away from the epicentre. The sensors are sharing their data and the system matches movements at each sensor to identify which are ‘the same’ quake, which will slow up slightly later and smaller on a more distant sensor when compared to one closer to the epicentre. This automatic matching is not perfect. If the system mismatches a quake with a smaller, later shake on another sensor, it might extrapolate that into a larger quake in the opposite direction, and a ghost quake is born. Once reviewed by the duty seismologist, these quakes are removed from the actual quakes list.

See this story of a ghost quake that showed up in Central Otago after a quake west of the Macquarie Islands.

See Dilemma of deep, distant earthquakes for more information about why ghost quakes occur.

See GeoNet Rapid is here! for a good overview of how GeoNet Rapid works.