Yann Gavillot | Lidar landslides and faults in Jefferson and Deer Lodge Counties, Montana
- MBMG has been working on a geohazards database of active faults, earthquakes and landslides.
- Work has largely followed where Lidar is available, which is currently limited, but progress is being made.
- Where Lidar is present, it is leveraged for locating unidentified active faults and landslides, and helps focus site specific hazards studies
- Mapping using Lidar is used to complete grant-funded geohazards studies (USGS,NEHRP, FEMA-MTDES).
- Lidar makes it easier to identify features that can even be difficult to observe on the ground, and of course allows you to cover a lot of ground quickly.
- Mapping using Lidar is essential to be able to characterize hazardous faults and landslides and produce hazards maps.
- Review of hazardous faults and landslides in Jefferson County (311 landslides) and Deer Lodge County (583+ landslides) symbolized by age & activity.
- New faults were identified using Lidar.
- The longer the fault, the more hazardous it can become.
- Any faults larger than 7km should be in the seismic registry for potential earthquake hazards.
- Some of these are present in NW Montana and the Bitterroots, but more Lidar coverage is needed.
- Slope maps can be created from hillshades and can be used to identify rockslides and rock falls.
- Preview of MT Quaternary Fault & Landslide Databases–Online Map Viewer
- Shows all features that have been confirmed with the Lidar.
- Shows locations of features as well as more detailed attribute table information.
Sackung vs. landslides–Questionable landslides were not included in the Landslide inventory/database.
Fault geometry–There is limited information about fault geometry at depth. MBMG is focusing on fault and geological mapping, and structural cross sections to constrain fault geometry and whether these are low- or high-angle faults. More mapping is needed. The geometry of course affects the hazard (low-angle
faults have more seismogenic width at depth, which would equate to more earthquake hazard potential).
Confidence values–Boundary confidence for faults and landslides are assigned based on mapping certainty and geomorphic signatures. Fieldwork and detailed geological mapping are needed to identify old landslides that do not exhibit activity during the Quaternary and are not well resolved in the Lidar.
Lidar mapping is better geared at resolving young and dormant landslides that are potentially more hazardous.
John Sanford | Geohazards Database
- MBMG is building an extensive hazards database with faults, earthquakes, and landslides.
- Users can download data, create elevation profiles of landslides and view statistics, and visualize correlations between faults and earthquake activity (which might identify dormant structures).
- How best to communicate this to the public?
- As the database matures, the statistics and analyses will also mature.
- Hazard classifications in the database were based on national standard USGS practices. The youngest features are typically the most concerning, so quaternary features are the focus.
Data export–It was mentioned that there is a desire to export xy data for elevation profiles from the database.
National datasets–Info from the geohazards database is plugged into USGS national datasets. These will be going into the update of the national seismic hazard model. The next update is planned for 2023
Creating the database dashboard–The database interface is using standard ArcGIS Online tools to create the dashboard–not custom script.
Yann Gavillot | Earthquake hazards study of the Bitterroot fault, Western Montana
- The Bitterroot fault is a high priority research area since it’s a long and active fault (~100km) within a highly populated area. The hazard and risk are high. Lidar mapping provides the first
documentation of fault scarps which justified multiple site-specific studies.
- Research is ongoing in this area
- Currently dating glacial moraines cut by the fault to estimate the slip rates
- Two paleoseismic trenches were dug to constrain earthquake recurrence associated with earthquake surface ruptures. More trenches are planned this summer.
- Working on more mapping, coring lake beds, & paleoclimate reconstruction
Mike Stickney | Earthquakes and Seismic Hazards in Montana
- History of earthquakes in Montana–most occur within the belt of seismicity.
- 1920s Three Forks (6.6)–lots of building damage
- 1935 Helena (6.3)–Felt earthquakes for days afterwards
- 4 people died from falling building debris. Temporary classrooms were set up in train cars. A tent city was erected for earthquake refugees. Even the State Armory (for emergency relief) collapsed.
- 1959 Hebgen Lake (7.3)–The largest earthquake in Montana
- There were four 6M aftershocks.
- The earthquake left a 21ft high fault scarp and triggered the massive Madison landslide. Displacement along the fault tipped the entire lake basin. This created waves that caused flooding.
- The Hebgen Dam was damaged, but it survived.
- 2005 Dillon earthquake
- Mostly minor damage–chimney damage, ground cracking
- 2017 Lincoln earthquake
- In summary, building codes matter.
- There is a need for more adequate seismic coverage
- They’re using the Raspberry shake network and MBMG just purchased more to add tothis network. They’re looking for host sites for this equipment.
Hosting raspberry shakes–Raspberry shakes cost about $500. Montana DOT was interested in putting them in some of their gatehouses, though noise will likely be an issue there.
Hillary Martens | 2017 Lincoln earthquake mainshock–aftershock sequence
- The Lincoln earthquake (5.8M) was the largest event in 60 years, felt 800km away.
- UM reestablished a seismic network in Montana (2017-2022). This is complimentary to the regional seismic network run by MBMG.
- Using this network, they were able to map the aftershock sequence. There were thousands of aftershocks within the first few months of the mainshock (110 days).
- Their goal was to identify the mode and mechanism of the fault structure. It was unclear from main shock alone.
- Two years of aftershock monitoring was conducted. They initially expected this to create a right-lateral EW fault, but as more data was collected it was identified as a left-lateral NS fault plane. This is almost perpendicular to the structural fault plane. Stress was
being accommodated by bookshelf faulting. This is right-lateral shear that produces multiple faults rupturing perpendicular to the shear (left-lateral).
- There was some variability in the fault orientation, but this is likely due to pre-existing weaknesses in the rock.
- In summary, the mainshock likely ruptured on a north trending LL-strike slip fault, oblique to the LCL.
Faults & seismicity–There is a complicated relationship between faults and seismicity. Quaternary faults may not be tied to the earthquake, but tertiary faults may be. However, we generally think of quaternary faults as being higher risks.
Fault type and evaluating hazard risk–Are strike-slip faults just harder to see than normal faults? Maybe it’s a deformation zone with smaller faults and not just large obvious faults. This is important to consider for evaluating seismic hazards in the future.