How one utility uses earthquake data for earthquake mitigation and rapid emergency response
Figure 1 shows how we use the MapServer. PG&E facilities and earthquake locations
are shown on the map for the southern San Francisco Bay Area and represent a particular
point in time. All of the available layers of information can be found by scrolling along
the left side of the map. Layers are turned on or off simply by clicking a check in the box
next to the layer name. Commonly used buttons above the map allow for zooming in or
out, rescaling, selecting or printing. Dropdown menus above the buttons allow options
such as viewing distances using point-click-drag distance measuring and links to other
GIS (MapServer) applications that are specific to a particular group, such as to hydro or
electric transmission. Our specialized Quake Tools menu includes an option to refresh
the map to get the latest earthquake locations and an archive feature for plotting past
earthquakes over designated magnitude and time ranges. This feature is particularly
helpful for keeping track of seismicity patterns.
With the MapServer we have been able to combine layers into one view for rapid
assessments. Placing the cursor on any of the symbols on the map produces a flag of
information; for example, time, date, location and magnitude information from a
particular earthquake, peak ground motion values in units of g from a ShakeMap contour,
or facility information from a building.
In Figure 1 we shows how we used the MapServer to compare the location of PG&E’s
facilities with a magnitude 4.9 earthquake that occurred on 13 May 2002, near Gilroy,
CA. We were able to see immediately how close the earthquake epicenter was to nearby
115 KV and 500 KV electric lines and, perhaps more importantly, that the earthquake did
not occur beneath the lines or associated towers. Also shown are the peak ground motion
contours from the ShakeMap. In this case the largest ground motion estimate (inner-most
contour) was .2 g.
The MapServer is available to all of the various groups at PG&E, and all levels of
management. Double-clicking on an earthquake symbol produces a separate page of
information about the earthquake, including a list of all facilities within 50 miles of the
earthquake. Fifty miles was chosen as the default radius because the California
Department of Safety of Dams requires PG&E to walk-down any hydro facility within
fifty miles of a magnitude 5 earthquake. The facilities are listed by distance from the
event and alphabetically by facility type (e.g. dams, substations, buildings, etc.). This
allows PG&E personnel quickly to see which facilities they need to look at first. The list
can be printed out and immediately available to field crews or emergency response
personnel.
PG&E Strong Motion Instrumentation Program
The second aspect of PG&E’s earthquake preparation is the installation of strong motion
seismic instruments for direct site-specific damage assessments following a significant
earthquake. These instruments are part of the PG&E Strong motion Instrumentation
Program, consisting of 50+ strong motion accelerometers. The majority of the
instruments are quasi-free field accelerometers, located in one-story cement slab critical
buildings and substation control rooms. Five hydro dams are instrumented on the crest
and abutments. The instruments are from various manufacturers and vintages, dating
back to about 1991. All are trigger-only systems. Generally data are manually
downloaded via phone lines; some of the instruments have the capability to, upon
triggering, push a small ascii file of key information to a PG&E computer.
Both the manually downloaded and automatic data are used to assess facility performance
following an earthquake. This assessment includes comparing the recorded data to the
building design specifications. Such information can be used to decide whether or not a
building is safe to re-occupy. The strong motion data from PG&E instruments are also
compared to the initial ShakeMaps, and added to subsequent ShakeMap revisions. A
future enhancement of the MapServer will be to link these ground motion data to the
MapServer, so that the values will show as labels when the cursor is placed over the
instrument locations.
Conclusions
Our goal has been to find the most efficient and secure way to integrate as much
information as is available, via the Internet and Intranet after a significant earthquake.
Our web-based GIS tool and strong ground motion instruments at critical facilities
provides us means to achieve this goal. By utilizing data products from the USGS, in
addition to our own data, we have been able to more effectively direct our emergency
response efforts, assess damage, and restore gas and power to customers.
Figure 1. A sample view of the PG&E MapServer. Shown are earthquake locations (yellow or blue
colored squares and diamonds), ShakeMap peak ground motion contours (yellow), transmission lines (see
legend in figure) and critical buildings (stars and checkered circles). The large blue triangle is the
Magnitude 4.9 earthquake of 13 May 2002. Green and black checkered circles are locations of PG&Eowned
strong motion instruments at critical facilities.
