An Investigation of Seismic Amplification of the Los Angeles Basin

Transcription

1 An Investigation of Seismic Amplification of the Los Angeles Basin 12/22/14 A thesis proposal submitted to the faculty of the Geological Sciences Department, California State Polytechnic University, Pomona in partial fulfillment of the requirements for the degree of Master of Science. Advisor: Dr. Jascha Polet Proposal by Raymond Ng

2 Introduction Earthquake hazard is a growing concern especially now that society is residing in and closer to dense populated large sky-rise cities built on basins. It is well documented and known that basin sediments amplify earthquake energy. Amplification due to basin sediments raises many questions regarding areas such as the Los Angeles basin. Further investigation of the seismic amplification in the Los Angeles basin will help improve earthquake engineering standards and hazard assessments. In the 1985 M8.1 Michoacan earthquake, Mexico City highrise buildings were heavily affected by a large intensity of shaking while smaller buildings were unaffected indicating a lower level of shaking. This implies that fundamental frequency and duration caused the substantial damage (Wald, et al. 1999). My study involves the examination of amplification and the fundamental period of the Los Angeles basin using two different approaches, applied to data from a temporary seismic network. The Los Angeles Syncline Seismic Interferometry Experiment (LASSIE) is a collaborative experiment that involves a high density linear array of broadband seismometers across the Los Angeles Basin, shown in Figure 1. The collaborative efforts of LASSIE are from academia (USGS, Caltech, UCLA, and Cal Poly Pomona) and industry (NodalSeismic, Signal Hill Petroleum, and Occidental Petroleum). 73 broadband seismometers were installed and had data collected continuously for a month. The retrieved data will be used to further our understanding of the basin structure and improve on existing velocity models.

3 Figure 1 Broadband survery across the Los Angeles Basin. The red dots are the locations of the Los Angeles Syncline Seismic Interferometry Experiment (LASSIE) broadband survery. The stations are nominallly spaced 1km apart. The black open circles are the existing Southern California Seismic Network (SCSN) stations, and the blue dots are the LABSPE survery of 1997 with short-period sensors. Figure 2 Amplification map during ground motion derived from surface geology. (Field et al. 2001)

4 Motivation The Los Angeles basin is subject to many studies because the basin amplifies earthquake energy. Research in basin amplified seismic energy is essential for the protection of the dense population and high economic value of the communities residing within the Los Angeles basin. Further understanding of how the basin structure, characteristics, and sediments interact with earthquake energy will help further improve earthquake engineering guidelines, earthquake hazard assessments, and building codes. Observed damage in Mexico City from the 1985 Michoacan earthquake shows that fundamental frequency plays a very important role in earthquake engineering and seismic design. Site conditions influence the distribution of earthquake energy and damage. From LASSIE we expect a more detailed image of the basin across the linear array, an improvement to the existing velocity models of the Los Angeles basin, and enhancement in our understanding of seismic amplification due to large local and regional earthquakes. Oil companies involved with this project are interested in imaging the basin structure for oil and gas exploration. Figure 3 Cross-section modified from Wright (1991). The LASSIE array is slightly west of this cross section, but still includes the main features shown here. The red mark shows the approximate location of the Cal Poly portion of the installation.

5 The goal of this research is to determine the amplification and the fundamental frequency at the seismometer locations, relating these findings to previous observations of amplification and damage patterns, and try to extrapolate my results to the rest of the basin. One of the products of study is an amplification map along the LASSIE line and extrapolated to the rest of the basin using the seismic data gathered similar to Figure 2. Figure 2 is derived from a map of surface geology. Although the surface geology does play a role in amplification it is only one factor in determining amplification. The basin structure, depth, and shape have very strong impacts on seismic amplification (Wald and Graves, 1998). A general cross section in Figure 4 Google Earth image of the seismometer locations at the edge of the Los Angeles basin. Cal Poly Pomona seismometers are installed at sites

6 Figure 3 represents the major features the LASSIE array transects. Equipment and Installation The Cal Poly Pomona portion of the LASSIE array, Figure 4, used the Guralp Seismometer CMG-6TD, shown in Figure 5. This unit is a three axis seismometer that measures north/south, east/west, and vertical components. The Figure 5 Guralp Seismometer CMG-6TD shown above with cables and GPS unit. seismometer is capable of recording a frequency range of Hertz as long as the base of the seismometer is within 3 degrees of being level. Installation of the Guralp Seismometer CMG-6TD requires two eight cubic foot holes, shown in Figure 6, to be dug for proper burial of the seismometer, wires, Figure 6 Site 144 Terry C. and Dandan Z. using pick axes to dig installation holes for the battery and seismometer in very warm weather. and battery. Obstacles to

7 overcome were maintaining the level of the seismometer, potential water damage to the equipment, temperature fluctuations, and potential ground disturbance caused by human influence and wildlife. A concrete step stone is used with potting sand to provide a level base and adequate drainage from water for the seismometer. Although the CMG-6TD is a waterproof unit, the unit and a 12V marine battery are secured and sealed within large plastic bags to protect against dust and water damage. Temperature variability causes thermal expansion and contraction of the high precision and sensitive components within the CMG-6TD unit itself. The excavated depth of allowed for the surrounding soil to insulate the unit from the radical changes in ground and air temperatures. The split wire conduit was used to help protect the cables from possible wildlife and human intervention. Chicken wire was buried approximately 1 below the surface to prevent accidental excavation of the unit. Cross section of the installation is shown in Figure 7. Figure 7 Side profile of the seismometer installation. Not shown in this diagram: the plastic bags used as a moisture barrier for the equipment.

8 Post installation, service visits were scheduled in three week intervals. During each service visit data collection and routine battery change was performed, Figure 8. Available data from the seismometers was tracked by a data availability chart. Figure 8 Site 138 (right to left) Raymond N., Terry C., and Mikey H. uninstalling the seismometer unit after being buried for approximately two months. Data is transferred to the orange LaCie hard drive prior to removal. Anomalies in the available data were investigated with this data availability chart, see Figure 9. Seismic data from the entire LASSIE array will be available for all parties involved with the experiment in the late winter. My research will analyze all of the data from all of the stations in the LASSIE array. Figure 9 Data availability chart. The green squares mark the installation date. The light blue squares mark standard service visit dates. The deep blue bars show the amount of available data collected from the seismometers. The light blue squares with a red outline show the uninstall dates. The light blue with black stripes boxes show services visits where equipment issues were observed.

9 Methods The first step in the analysis is to gather the data from the installed seismometers using the Guralp software Scream! and GFC extract. Data that is available will be visualized, analyzed, and interpreted using computer software such as Geopsy, SAC, and GMT. H/V spectral ratio or Nakamura method uses ambient noise and empirically derives a fundamental frequency. This method is used in seismic microzonation due to its low cost in Figure 10 H/V spectral ratio of site 137 for 9/25/14 produced by Geopsy. The gray vertical bar represents the peak denoting the fundamental frequency and amplitude of the spectral ratio peak. The solid line shows the average H/V curve while the dashed line above and below are the standard deviations of the H/V curve.

10 survey and analysis (Acerra et al., 2004). An example of the H/V ratio is represented in Figure 10. Data from site 137 was used to determine the fundamental frequency of 5.43Hz and a minimal amplification of The amplitude of the spectral ratio does not show the true amplification, however, it can be considered as the minimum value for amplification (Acerra et al., 2004). This method is not a stand-alone method. Additional studies are often used to supplement the use of the H/V ratio or Nakamura method such as investigating teleseismic P- waves for relative amplification between stations. Figure 11 Teleseismic P-wave of the El Salvador Earthquake on 10/14/14 for 6 stations. 5 second period waves show a small difference in peak amplitudes for the different stations. Horizontal components shows time in seconds and vertical component shows velocity of ground motion in counts. Teleseismic P-waves data from teleseismic earthquakes defined as at a distance of at least 3000km or 30 degrees will be investigated and used as a supplement to the H/V spectral

11 ratio study. Longer period earthquake data will be interpreted since it is more sensitive to deeper basin depths. Comparing the amplitudes of the same earthquake over different seismometers will allow us to examine local amplification. Measurements from teleseismic waves will have a slightly longer period than the amplification measured by the spectral method. A total of 9 teleseismic earthquakes larger than 6.5M W occurred from September 13, 2014 November 22, 2014 and will be used in the teleseismic investigation. In Figure 11, the teleseismic event of the 7.3M W El Salvador earthquake is shown across 6 seismometers. A difference in amplification is seen in the initial P-wave arrival. Timeline Late Summer 2014 Learn and understand the Guralp CMG-6TD Install 8 seismometers for LASSIE Fall Quarter 2014 Maintain and service the seismometers installed by Cal Poly Pomona Uninstall seismometers (Figure 8) and organize data Submit and present research proposal Winter Quarter 2015 Acquire the rest of the LASSIE seismometer data

12 Use Geopsy to analyze data to determine fundamental frequency of sites Learn GMT. Use GMT/ GIS to plot maps of preliminary results of data analysis Submit abstract to annual conference of the Seismological Society of America (SSA) in Pasadena Spring Quarter 2015 Results & interpretation Present preliminary findings at SSA Summer 2015 Continue results, interpretation Submit abstracts to Southern California Earthquake Center (SCEC) annual meeting in Palm Springs and Fall meeting of the American Geophysical Union in San Francisco Present at SCEC Fall 2015 Finish interpretations and begin writing Present findings at American Geophysical Union (AGU) Winter 2016

13 Thesis writing Spring 2016 Thesis defense

14 References Acerra, C., Aguacil, G., Anastasiadis, A., Atakan, K., Azzara, R., Bard, P. Y.,... & Moreno, B. (2004). Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations measurements, processing and interpretation. Field, N., Jones, L., Jordan, T., Benthien, M., Wald, L., (2001), Earthquake Shaking Finding the Hotspots, USGS Fact Sheet Guillier, B., Chatelain, J. L., Bonnefoy-Claudet, S., & Haghshenas, E. (2007). Use of ambient noise: From spectral amplitude variability to H/V stability.journal of Earthquake Engineering, 11(6), Hatayama, K., Kalkan, E. (2010). Characteristics of Long-Period (3 to 10 s) Strong Ground Motions Observed in and around the Los Angeles Basin during the Mw7. 2 El Mayor-Cucapah Earthquake of April 4, InAGU Fall Meeting Abstracts 1, 2. Nakamura, Y.(2000). Clear Identification of Fundamental Idea of Nakamura s Technique and its Applications, 12WCEE. Siddiqqi, J., Atkinson, G. (2002). Ground Motion Amplification at Rock Sites across Canada as Determined from the Horizontal to Vertical Component Ratio., Bull. Seism. Soc. Am. 92, Wald, D. J., Graves, R. W., (1998). The Seismic Response of the Los Angeles Basin, California., Bull. Seism. Soc. Am. 88,

15 Wald, D. J., Quitoriano, V., Heaton, T. H., Kanamori, H., Scrivner, C. W., Worden, C. B., (1999). TriNet ShakeMaps : Rapid Generation of Peak Ground Motion and Intensity Maps for Earthquakes in Southern California., Earthquake Spectra. 15(3). Wright, T., (1991), Structural Geology and Tectonic Evolution of the Los Angeles Basin, in Active Margin Basins, AAPG Memoir. 52.