Rapid Mapping of Surface Rupture from the South Napa Earthquake

Rapid documentation (<1 day) of co-seismic surface rupture location and slip is essential for scientific and emergency response. We demonstrate how social media (text messaging and Twitter) and the emerging 3D data collection technique known as Structure from Motion (SfM), used in conjunction with traditional field reconnaissance, enabled us to rapidly locate and document surface ruptures from the Mw 6.0 South Napa earthquake. On the morning of the event, our field team used information available on social media to identify locations with potential surface rupture. Preliminary observations of surface rupture (measurements and geo-tagged photographs) were texted to the office-based team member who created digital maps of the rupture trace and shared them online via Twitter in near-real time. We documented many ephemeral features (such as offset roads, curbs, and driveways) along the rupture trace within 12 hours of the event, before these features were destroyed by road and infrastructure repair. We were able to return to most sites again within several days, allowing us to document continuing slip and create time-series datasets of offset features. After the collection and re-collection of data at selected sites, we made detailed measurements remotely using 3D models constructed with SfM. The ability to quantitatively project features into the fault plane using these models allows for accurate measurements of small features often difficult to observe and quantify in the field.

Traditionally, even preliminary maps of rupture extent and offset magnitudes are not available for several days after an event because office-based processing and compilation is required. Because we were able to compile our data in real time, we distributed our results while they were still valuable for ongoing scientific response. Our work helped other science teams efficiently target fieldwork and instrument deployment; for example, one geodetic survey team used our surface rupture map to adjust their field deployment plans in an effort to capture rapidly-decaying postseismic movement. With social media and rapid, inexpensive data collection methods like SfM in mind, scientific response to future events has the potential to be more efficient and coordinated than ever before.