Collection and analysis of strong motion data is critical for seismic hazard assessment and risk reduction. Strong motion studies at the Alaska Earthquake Information Center range from data processing to research on various topics. The important aspects of these studies include data collection, archiving, processing and dissemination, real-time representation of ground shaking, development of predictive models of ground motion for scenario earthquakes, and seismic hazard assessment.
Several organizations operate strong motion stations in Alaska, including the Geophysical Institute of the University of Alaska Fairbanks (GI UAF), the Advanced National Seismic System (ANSS), the National Strong Motion Program (NSMP), and the Alyeska Pipeline Service Company. In total, there are more than 80 digital, 3-component free-field stations operating statewide, 21 of which are collocated with broadband stations. However, the majority are concentrated in urban areas, including about 40 instruments in the Anchorage metropolitan area, 10 in the Fairbanks area, and three in each Valdez and Juneau. The ANSS also operates a building-downhole strong motion array in Anchorage.
The stations are equipped with Kinemetrics K2 (or Episensor) and Guralp CMG-5TD accelerometers, which combine triaxial strong motion sensors with 24-bit digitizer modules. The accelerometers have 4.0 g full-scale sensitivity and a flat frequency response from DC to 50-100 Hz. A significant number of stations operate in continuous mode, sending data to AEIC via real-time telemetry. Other stations (mostly in Anchorage) operate in trigger mode sending the waveform recordings to the NSMP data processing center via dial-up connections. A majority of the stations are maintained by the AEIC.
At AEIC the strong motion data are acquired, processed, and archived using the Antelope Real-Time System (ARTS), a software package for seismic data management. These data are distributed to the users and the general public in various ways. One of the products is the Shakemap, which automatically calculates and posts distribution of ground shaking amplitudes within minutes after significant earthquakes. Furthermore, the records with relatively large peak accelerations are separately processed and baseline corrected at the NSMP Data Center. An important strong motion dataset was produced by the Mw7.9, 2002 Denali Fault earthquake, which was recorded by more than 55 stations statewide at distances up to 280 km from the fault rupture (Martirosyan et al., 2004). These and other strong motion records and parametric data are available to the scientific and engineering communities for research purposes.
Intensity of the ground motions depends on earthquake source characteristics, propagation path, and local soil conditions. The latter is especially important since the wave characteristics are significantly modified within the uppermost soil layers. An extensive study of local site effects in the Anchorage metropolitan area was carried out over the last 10 years using the Anchorage strong motion data (Biswas et al., 2004). It was particularly shown that there is a prominent amplification effect at low-frequencies in the central and northern parts of the city, largely related to the thick, soft layers of Quaternary deposits called the Bootlegger Cove Formation. Similar site effect studies are underway for the Fairbanks area. Other related studies include the seismic site class map of Anchorage, development of 1D seismic soil models for Anchorage, and site-specific hazard investigations.
Biswas, N., Martirosyan, A., Dutta, U., Dravinski, M. and Papageorgiou, A. (2004). Investigation for seismic zonation of Anchorage, Alaska, in: Y.T. Chen and G.F. Panza and Z.L.Wu, Earthquake Hazard, Risk, and Strong Ground Motion, Seismological Press, Beijing, China, 286 p.
Martirosyan, A., Hansen, R., and Ratchkovski, N. (2004). Strong-Motion Records of the 2002 Denali Fault, Alaska, Earthquake, Earthquake Spectra, 20, 579-596.
Text by A.Martirosyan
Updated: October 2006