By David Carver, Timothy Plucinski, Arthur Frankel and Erdal Safak
U.S. Geological Survey Open-File Report 03-158
| Following the Mw 7.9 Denali Fault earthquake that occurred on November 3, 2002, a temporary eight-station array of Kinemetrics K2 accelerographs was installed on both sides of the Denali fault near the Richardson Highway and the Alyeska Pipeline. The installation was complete on November 9, 2003 and it recorded aftershocks until November 23, 2002. Sixty-seven aftershocks were recorded at 5 or more array stations. A search of the ANSS earthquake catalog yields a list of aftershock locations recorded by 5 or more of our temporary array instruments. The list includes aftershocks with magnitudes ranging from 2.5 to 5.1. A map shows the aftershocks recorded at 5 or more stations and the station locations. This report describes the field effort and the characteristics of the data that were recorded. | |||||||||
| The primary research goals of the array were to record earthquake ground motion at sites that represent the major types of mapped surficial geologic materials on which the pipeline and highway are built and to record ground motion induced by the pipeline. With this information seismic hazard maps will be produced to show the variability of ground shaking near the pipeline’s crossing of the Denali Fault. Our strategy to achieve the goals includes: | |||||||||
| Sampling ground motion at sites that represent the major geologic materials including hard rock and alluvial surficial materials, Recording high-resolution ground motions from a tripartite array located under and near the pipeline (PIPN, PIPM, PIPS, and GRAN). This sub-array approximated an equilateral triangle about 100 feet on a side. The sub-array was located on a granite hill between the mapped north and south traces of the Denali Fault, Recording ground motion in areas with active stream deposition and older alluvial surficial materials which may have a potential for liquifaction, Recording aftershocks at a site very near the instrument that recorded the main shock at Alyeska Pipeline Pump Station 10. Contribute to aftershock relocations. | |||||||||
| The operational characteristics of the array are summarized in the accompanying instrument configuration table. The following sections of this report correspond to the column headings in the configuration table and describe the information in the information in the table in more detail. | |||||||||
Station Locations |
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| Each K2 has a Global Positioning System (GPS) antenna and an internal GPS receiver that calculates the location of the instrument and provides accurate timing. The GPS system was turned on for a maximum of 20 minutes per hour to conserve power. The instrument averages the results of many of location determinations to increase the accuracy of the location. In the configuration table, we report latitude and longitude determinations that are the average of at least 1500 location determinations by the GPS. Experience has shown us that these averaged locations have error ellipses with a major axis that is less than 5 meters. GPS errors are larger in the vertical dimension so elevation of the recording sites is less reliable. The locations refer to the WGS84 datum. | |||||||||
Sensor Orientation |
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| Sensor orientations are listed for each station in the configuration table using a “left hand rule” convention. For example, a “sensor dip” of 90 and an “Orientation Azimuth” of 180 indicates a horizontal sensor with ground motion south generating a positive voltage. All sensors were originally oriented with respect to magnetic north to minimize confusion in the field. Azimuths contained in the event headers and in the configuration table have been corrected to refer to True North. Magnetic declination in the study area is currently 24 degrees East. | |||||||||
Timing Quality |
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| The K2 GPS system also provides precise Universal Coordinated Time (UTC) to compare against the K2’s internal Real Time Clock (RTC). Each hour, the K2 synchronizes its RTC with UTC it has just received from the GPS. When the RTC drifts more than 3 milliseconds from UTC, the clock is reset. The result is that the K2 timing is always within 3 ms when the GPS has acquired a satellite time signal within one hour of the trigger time. If there is no GPS signal, the RTC drifts without control and there is no way to determine the clock correction between UTC and the RTC. In that case, the timing is probably acceptable for some non-critical uses but the precise RTC correction with respect to UTC is unknowable. Station PIPM failed to receive enough satellites to achieve a GPS lock for triggers between 2002/11/09+22:59 and 2002/11/10+00:35. Fortunately, there were no earthquakes recorded during that time by 5 or more array stations. All other triggers recorded by the array stations were within one hour of the last GPS lock and therefore had timing precision to within 3 milliseconds. | |||||||||
| The K2 sample rate is given in the configuration table as samples per second. Instruments in the closely spaced pipeline sub-array (stations GRAN, PIPN, PIPM, and PIPS) sampled at 200 samples per second. All other stations operated at 100 samples per second. The K2 samples all channels simultaneously. | |||||||||
Instrumentation and Installation |
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| All of the array instruments were Kinemetrics K2 (www.kmi.com) with internal Force Balance Accelerometers (fba-23) or the newer Episensor accelerometers. The instruments at stations PIPN and PIPM had K2s with internal fba-23 sensors. All of the other stations had instruments with internal Episensors. All of the instruments have tri-axial, 2G full scale accelerometers. Channel 1 recorded the vertical sensor, channel 2 was the nominal N-S channel, and E-W is on channel 3, although the user should consult the configuration table or the event file headers for the correct orientation azimuth. | |||||||||
| Sensor-response data for the transducers are given in the headers of the event files, and also, in the poles and zeros file that accompany the data served up by the database at the FTP site (see Data Availability below). | |||||||||
| The K2 cases have locking nuts installed on their leveling feet to make them rigid. The three leveling feet are each epoxy glued to a concrete pad at the site. All of the instruments were checked during each site visit to ensure that they were level and rigidly attached to the concrete pad. Typically, when the instruments are removed, the concrete fails instead of the epoxy and three small divots are left in the concrete. | |||||||||
| Stations PS10 and ADOT were installed inside buildings on concrete pads previously poured on grade. The other instruments were installed on pre-cast concrete paving stones about 38 centimeters square and 12 centimeters thick. A shallow hole about 10 cm deep was dug in the soil, concrete was poured into the hole about 4 cm thick, and the pre-cast concrete paving stone was leveled atop the concrete. The remainder of the concrete was poured around the edges of the pavers to fill between the concrete paving stone and the sides of the hole. This worked well until temperatures caused the concrete to freeze before it set. After temperatures remained below freezing, we used a bed of wet sand under the concrete paving stone and backfilled wet sand around the slab in the hole. Then, water was poured over the sand to freeze the paving stone solidly in place. Temperatures remained cold enough during the experiment to insure that the pads remained frozen. All instruments remained level throughout the experiment. | |||||||||
| The outdoor stations operated on a single external 12 Volt, 108 Amp-hr gel-cel battery for 7 days. Average daily high temperatures were about 20 degrees F and lows were around 5 degrees F during the deployment. | |||||||||
Soil Type |
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| The Soil Type column in the station configuration table gives the surficial geology as indicated for that site by the “Preliminary engineering geologic maps of the proposed Trans-Alaska Pipeline route, Mount Hayes Quadrangle” compiled by Florence R. Weber in 1971 (USGS Open-File Report 71-0318). | |||||||||
| Stations PIPN, PIPM, PIPS, and GRAN were all located on a hill composed of granite with a thin veneer of soil as described in the measurements figure. | |||||||||
Structure Type |
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| Three stations, PIPN, PIPM, and PIPS, were installed on-grade directly beneath the pipeline. These pipeline sites were located between the vertical support members (VSM) that elevate the pipeline above the ground. Measurements were taken of each of the structures and a sketch map was completed showing the relative locations of the pipeline sub-array stations. The three VSMs that were chosen are in a nearly straight line and at the same elevation. Station GRAN was located 94 feet away West of the pipeline to serve as a free-field site beyond the influence of pipeline shaking. | |||||||||
| Two sites were located in pre-existing structures. Station ADOT was attached to the concrete pad in the office of the Alaska Department of Transportation, Trims Camp. Station PS10 was in an unused garage-sized building 160 feet west of the permanently installed Alyeska-owned accelerograph that recorded the main shock. The original goal was to install our instrument at PS10 on the same pad as the Alyeska-owned instrument. However, there was no access to the instrument shelter without the use of heavy equipment to remove the shed ’s roof. | |||||||||
| Most of the array stations were installed as described in the Instrumentation and Installation section above. We constructed these sites out-of-doors so there was no structure. These sites had only a lightweight plastic tub sheltering the instruments. | |||||||||
Data Availability |
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| Our Denali aftershock data is available in the original .EVT format that is recorded by the K2s, ASCII, and SAC formats. The headers have been edited to reflect the true operational characteristics of the instruments in the field including sensor orientation and response. The database also includes poles-and-zeros data for the sensors, the earthquake catalog, and the configuration file. | |||||||||
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| Five or more of our temporary array stations recorded a number of aftershocks not located by the regional network at the time of this report. That data is included in the database. | |||||||||
Acknowledgements |
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| The authors would like to thank Greg Jones, Eldon Johnson, and David Brown of the Alyeska Pipeline Company for quickly arranging for us to install our instruments under the pipeline. Mike Metz was most helpful in providing geotechnical information about the sites and construction details of the pipeline. We would also like to thank the Alyeska Security staff for being very helpful with our installations. | |||||||||
| This earthquake response would not have been nearly as successful without the quick response from the USGS Alaska Volcano Observatory. Tom Murray, Donna Eberhard-Phillips, and John Powers were right there to support us in any way possible. | |||||||||
| The Alaska Earthquake Information Center, Fairbanks provided logistical support and kept us informed about the aftershock sequence, our thanks to them. | |||||||||
| Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS. |
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