October 24, 2014
As Northern Californians picked up the pieces and cooled their nerves on the afternoon of August 24th, just hours after being jostled or lurched from bed by the 3:20am magnitude 6.0 South Napa quake,
a satellite an aircraft whizzing by overhead snapped a shot of the scene. Check out some of these remarkable scenes within it that show damage, response, and recovery.
The earthquake was caused by 15 km of rupture along a north-south trending West Napa Fault, which runs through vineyards, suburbs, and infrastructure. Fault slip was on the order of 25 cm or less–scarcely a foot–but the right-lateral offset shows up clearly in the sharp paved and painted lines of Highway 12. (Also a testament to the resolution of this aerial image.)
Caltrans worked fast that day to get the region up and running. Even for such a modest quake they had a lot of bridges to inspect and a lot of road offsets to patch. Farther south we see two big asphalt-filled dump trucks patching up another site on their days-long tour of broken roads, the beginning of months and $5.3 million of work:
Other rapid responders included the scientists that mobilized from nearby institutions and agencies to go find answers to everyone’s questions. Extent of damaged region, causative fault, risk of afterslip, risk of aftershocks, risk of more and bigger earthquakes… these are all pressing questions that of course in hindsight we have answers to. On the day of the earthquake, the extent of the rupture was just being discovered, and field scientists were beginning to spot the dramatic effects of ongoing “afterslip.” In this early afternoon aerial pass, we can spot my own officemates out with the research group field vehicle, measuring offsets visible of the road, fence line, driveway pavers, and vine rows.
But utility crews and scientists weren’t the only ones dealing all day with the effects of the quake. Violent shaking unsettled the citizens of Napa, and the image captures a more poignant scene of a neighborhood grappling with the quake’s effects. This portion of the scene is striking to me; there’s a lot going on. The fault rupture runs through the center, delineated by arrows at the top and bottom of the screenshot. Its effects on infrastructure are clear, where roads and sidewalks have been buckled and subterranean pipes ruptured, spewing wet sand across the road. Utility trucks abound, addressing these issues and surveying the homes that straddle the fault line (PG&E trucks are light blue). Meanwhile the residents of one torqued house unload all their belongings into a trailer, filling the front yard with furniture with the help of their neighbors in order to vacate the structure before it’s deemed uninhabitable.
Of course even more famous damage was captured from the sky, for example the flopped-over carports at a suburban apartment complex:
And last but not least a different perspective of the iconic damage from this earthquake: the collapsed turrets of the Alexandria Square office building in downtown.
From this vantage a couple things stand out: the streets have already been cordoned off–that happened early on upon discovery of this damage–and a press and emergency crew camp set up across from the damaged structure. In addition, you can see just how much traffic is moving around the city, crossing a bridge no less! This is the reality of an earthquake in which there’s a great deal of damage but very little loss of life: people will be moving around, whether it’s to get supplies for themselves (for clean-up OR survival), to check on family and friends, or just to lookie-loo. This is a big management challenge for emergency response operations that requires truly rapid assessment of damaged and undamaged structures. It also makes clear how important it is to check on lifelines to a city and have alternatives if, for example, that bridge had suffered more damage than it did. (Check out the CA Earthquake Clearinghouse’s online map interface to see photos from structural surveys and inspections following the quake.)
Satellite Aerial imaging provides a spectacular resource to earthquake responders of all stripes. It is fortunate to have such a timely acquisition, which is often difficult with existing satellite platforms. Napa’s climate also avoided one of the other major limitations of airborne imaging data: cloud cover.
The extremely high resolution imagery* says © Google which suggests that it isn’t licensed DigitalGlobe imagery from Worldview or GeoEye satellites–futhermore those platforms don’t appear to have an acquisition on this date. Perhaps this is imagery from Skybox satellites, which Google just acquired… although it appears perhaps too high resolution for that. I’m curious if anyone can track down more detail about the specific satellite source of this imagery.
*Update 10-24-14 16:02 UTC: I’ve learned from Tim Dawson at the CGS that this Google orthoimagery was actually captured from an airplane after discussions between the Google crisis response team and the USGS & CGS. Testament to the power Google has for rapid mobilization, and commendable cooperation among relevant agencies. No wonder they got around the obstacles of satellite data.