Hazard mapping with GIS

Chih-Hung Lee, William Manegold, Stuart Nishenko, Joseph Sun and Kent Ferre, Pacific Gas & Electric Company, San Francisco, California

Geographic information systems (GIS) technology enables sophisticated, numerical-based mapping of earthquake hazards, including liquefaction and landslide susceptibility, on a regional basis for pipeline systems.

Recently, existing earthquake hazard mapping was integrated with interpretation of topographic, geologic, hydrologic, and geotechnical data to update an earthquake hazard database for Pacific Gas & Electric Company’s California Gas Transmission (CGT), as part of the CGT Pipeline System Integrity program.

The regionally consistent, map-based database covering CGT’s pipeline system in northern California allows for modeling of possible pipeline impacts for various earthquake scenarios and immediately following moderate to large earthquakes. GIS-based modeling that incorporates the hazard mapping is a powerful tool for planning and emergency response purposes.

Specifically, real-time models of possible pipeline damage locations can be derived from internet-based groundshaking records (USGS ShakeMap) and integrated with internet-based decision support tools (PG&E’s Map Server) to prioritize emergency response activities. Used as a screening tool, this information helps to rapidly identify potential gas transmission problem areas prior to the receipt of initial damage reports from the field. The database also provides direct input for evaluation of long-term pipe integrity, prioritization of maintenance activities, and ongoing management of the transmission pipeline system.

Groundshaking hazard

In Northern California, numerous major active faults are capable of generating large (M>6.7) earthquakes, and thus pose significant groundshaking hazard to pipelines and other facilities. As part of a regional assessment of seismic hazards for the entire Pacific Gas & Electric Company (PG&E) gas distribution system in northern California (Figure 1), we have updated existing PG&E regional (1:24,000 to 1:250,000 scale) hazard mapping of the California Gas Transmission (CGT) system by integrating digital hazard data to evaluate liquefaction and slope failure hazards associated with large earthquakes.

We have incorporated topographic, geologic, hydrologic, and geotechnical data to develop liquefaction and landslide hazard GIS databases. For our update of the liquefaction hazard map layer, we incorporated unpublished digital groundwater data from the California Department of Water Resources and new 1:24,000-scale liquefaction hazard mapping of the nine-County San Francisco Bay Area prepared by the U.S. Geological Survey (USGS) and William Lettis & Associates, Inc. (WLA), in cooperation with the California Geological Survey.

The goal here is to present our approach to mapping earthquake hazard on a regional scale for the CGT gas system. Specifically, we describe the construction of regionally consistent, multi-category maps that display the relative susceptibility of surficial deposits and slopes based on the threshold peak ground acceleration (PGA) values produced by earthquakes that are required to initiate ground failure. Our hazard maps allow for integration of multiple earthquake scenarios as well as near real-time earthquake information; and provide sufficient detail for emergency response, regional hazard assessment and local mitigation planning.

Liquefaction hazard

Liquefaction is an earthquake ground failure mechanism that occurs in loose, saturated granular sediments (principally sand and silty sand), and has caused extensive damage to water systems and pipelines during past earthquakes. Damage to pipelines and support structures in liquefied sediments can occur as a result of differential settlement, bearing capacity failure of foundations, flotation of buried structures, lateral translation (lateral spreading) and slope failure, ground cracking, and ground oscillation.

Liquefaction does not occur randomly, but rather is typically limited to areas underlain by saturated recent sandy geologic deposits (Holocene alluvium), and poorly compacted granular fill deposits. Liquefaction has occurred within Holocene sediments and artificial soil during several past historic earthquakes in California, including the 1906 San Francisco and 1989 Loma Prieta earthquakes. Older geologic sediments (Pleistocene and older) lose their susceptibility to liquefaction by long-term aging effects that include compaction, dewatering, intergranular rearrangement/packing, and cementation.

Our method of mapping susceptibility to liquefaction therefore includes correlation of the age and type of geologic unit with depth to groundwater and the threshold triggering PGA required for initiation of liquefaction. Our revision of liquefaction hazard for PG&E’s service area emphasizes the integration of digital groundwater data and geologic mapping, including newly developed nine-county Bay Area geologic and liquefaction hazard mapping completed for the USGS by WLA at 1:24,000-scale, in cooperation with the California Geological Survey. We use surficial geologic mapping as a means to improve correlations among subsurface data and increase confidence in the distribution of susceptibility units.

WLA’s liquefaction susceptibility mapping process involved five steps: (1) creating detailed Quaternary geologic maps delineating deposits of various ages, depositional environments, and texture; (2) evaluating Quaternary deposit thickness and depth to groundwater; (3) performing initial evaluation of relative liquefaction susceptibility using a decision tree process; (4) evaluating liquefaction triggering thresholds using geotechnical borehole data and the Seed and Idriss “Simplified Procedure”; and, (5) identifying units of similar susceptibility and grouping them to form liquefaction susceptibility zones (Table 1). The additional detail provided by incorporation of the 1:24,000-scale mapping allows for more focused evaluation of liquefaction hazards to the CGT system (Figure 1).

Outside of the nine-county Bay Area, we incorporated available groundwater information provided by the Department of Water Resources (DWR) to refine the liquefaction hazard. Groundwater levels provided by DWR were obtained from monitoring wells and boring logs as part of a groundwater database developed by DWR. Our previous study, and PG&E existing hazard map layers, did not include interpretation of depth to groundwater. Incorporation of regional groundwater data substantially reduces the area of liquefaction hazard identified along the pipeline routes. The final maps provide detailed hazard for multiple pipeline corridors (Figure 2).

For the final CGT hazard map database, areas within moderate-to-high liquefaction hazard zones likely would experience moderate-to-severe liquefaction under strong earthquake shaking, and moderate liquefaction under moderate levels of earthquake shaking (Table 1). In contrast, zones ranked as low hazard would either experience no significant liquefaction, or isolated and relatively minor liquefaction even under very strong earthquake shaking.

Landslide hazard

Landslides involving bedrock and colluvium pose the primary slope stability hazard along pipeline corridors. Those types of slides include shallow earth flow and slumps, which typically contain colluvium and weathered bedrock. Deep rotational and translational landslides are less common, and typically involve underlying bedrock. These slides typically are associated with steep slopes greater than 30° and typically occur where bedding dips in the same direction as the slope.

Earthquakes can produce several types of slope failure over broad areas of mountainous terrain. Earthquakes with magnitudes greater than about M4 can trigger landslides on susceptible slopes, and greater than about M6 can generate widespread and large landslides. The types of slope failure that are most commonly triggered by earthquake shaking are rock falls and topples in jointed hard rock, debris and rock slides in weathered bedrock, and translational/rotational slumping and sliding of colluvial slopes and stream banks. Slopes exhibiting marginal static stability, and areas of past slope failure, are usually the most susceptible to earthquake-induced slope failure. Oversteepened slopes in previous landslide headscarps and disrupted slides masses are particularly prone to earthquake-induced failure.

Polygons of map units derived from existing non-digital WLA hazard maps prepared for PG&E’s Seismic Vulnerability Assessment of the Gas Supply Business Unit System (1992-1997) were digitized for the buffer zone strip map by either tracing the contacts by hand onto mylar, and registering scanned and digitized contacts to GIS coordinates for digitization, or by heads-up on-screen digitization by a geologist. Existing digital landslide data also was incorporated where available.

Map layers were intersected with a buffer of one mile along the pipeline centerline, such that a boundary “strip” map was delineated. The buffer distance was chosen to minimize digitizing input and computational requirements, while maximizing hazard information relevant to the pipeline routes. The strip map layer was populated with additional information derived from interpretation of existing non-digital mapping and slope information derived from digital elevation models (DEMs).

The evaluation of hazard for the landslide layer involved the following steps: (1) develop a percent slope map from existing U.S. Geological Survey 30-meter resolution digital elevation models; (2) evaluate correlations between bedrock/soil characteristics from regional geologic mapping and slope inclination to develop a landslide susceptibility criteria matrix; (3) assign a point score system for integration of map units, and (4) development of the derivative landslide susceptibility map layer (Figure 3).

Areas within the moderate-to-high susceptibility zones are expected to experience widespread slope failure from large earthquakes, and moderate distributed slope failure from smaller events. Slope displacements could range between inches to several feet, depending on PGA levels. We note that the susceptibility zones are quite broad and do not factor site-specific conditions that could affect the slope susceptibility rating at a microscale.

Areas with moderate susceptibility zones could experience localized slope failure during large earthquakes, and minor slope movements during smaller events. Areas in the low-to-moderate zone areas should not experience significant occurrences of slope failure or movements even under very strong ground shaking from the largest scenario events.

Discussion

Our method of keying liquefaction susceptibility ratings to estimated threshold triggering ground motion values allows potential for liquefaction and landsliding from various earthquake scenarios, and near real time earthquakes, to be assessed across the entire CGT service area. The potential for liquefaction and landslides to occur depends upon the opportunity for ground motions to exceed the threshold level required for initiation of liquefaction or slope failure. By overlaying and querying groundmotion data with the CGT liquefaction and landslide susceptibility databases within GIS, PG&E can identify areas along the pipeline system where ground motions may exceed the threshold levels required for ground failure, and possible impacts to buried pipelines. This information can be used in advance of an earthquake to prioritize mitigation of pipelines and after an earthquake to direct repair efforts.

Because our existing liquefaction and landslide susceptibility map databases incorporate the estimated threshold ground motion values required to initiate liquefaction or trigger landsliding (e.g., PGA triggering values), the calculation of liquefaction or landslide potential for a scenario earthquake is fairly straightforward. In the scenario-based approach, a specific earthquake can be selected (i.e., with a particular magnitude and location) and ground motions computed using applicable attenuation relations. Anticipated ground motions calculated for a scenario earthquake are directly compared to the threshold, or “triggering,” values associated with liquefaction or landslide susceptibility to calculate liquefaction or slope failure potential. The resultant deterministic liquefaction potential map depicts the areas of possible liquefaction and slope failure for a large earthquake.

In addition, the CGT GIS database incorporates real-time earthquake data in northern California from ShakeMap. ShakeMap is a product of the U.S. Geological Survey Earthquake Hazards Program, in conjunction with PG&E and other regional seismic network operators. The USGS ShakeMaps are internet-based maps derived from instrumental recordings of strong ground motion during an earthquake. Integration of this internet-based dataset with the CGT GIS database allows for rapid identification and characterization of possible locations and severity of earthquake-related ground failure along the CGT system, resulting in immediate input for prioritization of field inspections and other emergency response decisions.

Once constructed, GIS databases can incorporate ground photographs and monitoring of active landslides (Figure 4) that may impact the CGT pipeline system and allow for regular monitoring of landslide movement from the ground and aerial reconnaissance. In addition, incorporation of field observations allow for calibration and updating of the hazard map database.

Conclusions

The final products from this study include a fully digital, seamless earthquake hazard map database for use by PG&E California Gas Transmission (CGT), as part of the CGT Pipeline System Integrity program. The regionally consistent, map-based database covers CGT’s entire pipeline system in northern California and allows for modeling of possible pipeline impacts from moderate to large earthquakes. Near real-time models of possible pipeline damage locations can be derived from scenario or internet-based ground shaking records (USGS ShakeMap) produced after earthquakes.

Acknowledgment

Based on a paper presented at the ASME’s 6th International Pipeline Conference held in Calgary, Alberta, Canada.