Long-Period (3 to 10 s) Ground Motions in and around the Los Angeles Basins during the 2010 M7.2 El Mayor-Cucapah Earthquake

Ken Hatayama, National Research Institute of Fire and Disaster, Japan, ken.hatayama@dolphin.ocn.ne.jp
Erol Kalkan, U.S. Geological Survey, Menlo Park, ekalkan@usgs.gov


We examined the characteristics of long-period strong ground motions within a period range of 3 to 10 s in and around the Los Angeles (LA) basin during the Mw7.2 El Mayor-Cucapah earthquake. The contour map of the observed peak ground velocity (PGV) values clearly shows that the LA basin significantly amplifies the long-period motions. The largest PGV values observed within the LA basin range from 0.1 to 0.12 m/s, though the basin is around 350 km away from the epicenter. These largest PGV values were recorded at seven stations in the central part, and one station in the western part of the basin. The Fourier acceleration spectra of records from these eight stations are predominant at the periods of 6 to 8 s with the corresponding peak values of 1 to 1.4 m/s. The ratio of Fourier amplitudes with respect to a reference site (STG station of the Southern California Seismic Network, located on hard rock in the southeast edge of the LA basin) show that the spectral amplitudes at these eight stations are 5 to 13 times larger than those at the reference site within a wide period range, 5.5 to 9 s. To understand how the spatial variation of amplification for the long-period motions is related to the basin underground structure, we plotted contours of the amplification factors at 3, 5, 7 and 10 s spectral periods with respect to the reference site (i.e. STG station) onto several different maps, where depths to different S-wave velocities (Vs) are also depicted using the latest SCEC Community Velocity Model (CVM-H 6.2). Comparison of amplification contours with the basin model indicates that for the 10 s spectral period, the largest amplification occurs in the central part of the LA basin where the depth to the Vs of 3.2 km/s reaches to the maximum, 9.5 km. For the 3, 5 and 7 s spectral periods, the amplification becomes larger in almost the same area as the 10 s case (the central part of the LA basin) and also in the San Gabriel valley, where the depth to the Vs of 1.5 km/s reaches to the maximum, 3.7 km. Although the depth to bedrock (Vs > 3.2 km/s) is shallower (3.6 to 4 km) in the San Gabriel valley than the LA basin, the large amplification is due to the thick soft sediments (Vs < 1.5 km/s). This suggests importance of precise seismic velocity profiles at both shallow- and deep-part of sedimentary basins for a more accurate prediction of long-period strong ground motions.

LA Basin

Los Angeles basins and location of strong motion stations

 

Comparison of period-specific amplification factors (contour lines) with depths (colors) to isosurfaces above which the S- wave velocities are less than a given value. The contour lines of the amplification factors for periods of 10, 8, 6 and 4 s are superimposed onto the six maps showing depths to the isosurfaces above which S-wave velocities (Vs) are less than 3.6, 3.2, 2.8, 2.0, 1.5 and 1.0 km/s. The amplification factors are the ratio of the Fourier acceleration spectra to the one averaged among the reference stations. The depths to the isosurfaces are after the CVM-H 6.2.

 

LA_Poster

Printable EGS4 2011 poster (Low Resolution: ~4 Mb) | (High Resolution: ~40 Mb)

Printable EGS4 2011 Symposium Paper

Downloadable AGU2010 Presentation (12.8 Mb)

LA_Poster

Printable SCEC 2010 poster (Low Resolution: ~16 Mb) | (High Resolution: ~100 Mb)

Related Papers and Presentations

1 Hatayama, K., Kalkan, E. (2012). “Spatial Amplification of Long-Period (3 to 16 s) Ground Motions in and around the Los Angeles Basin during the 2010 M7.2 El Mayor-Cucapah Earthquake”, Proc. of the 15th World Conf. on Earthquake Engineering, Lisbon, Portugal.
2. Hatayama, K., Kalkan, E. (2011). “Long-Period (3 to 10 s) Ground Motions in and around the Los Angeles Basin during the Mw7.2 El-Mayor Cucapah Earthquake of April 4, 2010”, Proc. of the 4th IASPEI / IAEE International Symposium, August 23–26, University of California, Santa Barbara.