S41B-1838
Upgrading the Southern California Seismic Network Along the Southern San Andreas Fault for Near-Source Ground Motions and Earthquake Early Warning
The Southern San Andreas Fault (SoSAF) has the highest probability of producing a significant earthquake ( M 6.7) of any fault in California (Field, et al, 2007). Potentially affecting more than 20 million inhabitants, this fault presents the greatest urban earthquake risk in the United States. The Southern California Seismic Network (SCSN) has embarked on a multi-year project to enhance its ability to monitor the SoSAF by installing 11 new and upgrading 6 existing seismic stations along the fault trace from Bombay Beach to Wrightwood. This is made possible by funding from the USGS's Multihazards Demonstration Project. The plan is to achieve a station spacing of less than 20km along strike within 5km of the fault trace. The stations are state-of-the-art and include broad-band and strong ground motion sensors and "smart" data loggers. Continuous, real-time data from these stations will be integrated with the current SCSN operations in Pasadena, CA. SCSN is operated collaboratively by USGS and Caltech. It is part of the California Integrated Seismic Network (CISN) which is a region of the Advanced National Seismic System (ANSS). These new stations will provide several scientific, engineering and practical benefits. They will produce better and faster recording of background seismicity and strong ground motions in the region. When a large SoSAF event does occur they will record on-scale, near-field ground motions; such records are rare and will be of immense scientific and engineering value. Finally, these near-fault stations will pave the way for an earthquake early warning system in southern California. The USGS and ANSS have stated their intent to pursue earthquake early warning (EEW) in California. Currently they are supporting the development and testing of several EEW algorithms in California (Allan, et al, 2008). In addition to robust algorithms there are two main technical requirements for an operational EEW system in California: sufficient station density in source areas and small data delays. The SoSAF project addresses both requirements. First, the 20km target station spacing has been demonstrated to be sufficient for EEW in algorithm tests. Second, the new and upgraded stations will include ruggedized, low power computers capable of running EEW algorithms in the field, thus eliminating whole-waveform transmission delays. We believe this project provides a model for how existing regional seismic networks can evolve into multipurpose, operational EEW systems.
S41B-1839
PG&E's Seismic Network Goes Digital With Strong Motion: Successes and Challenges
Pacific Gas and Electric Company (PG&E) is in year 3 of a 5-year project to upgrade the Central Coast Seismic Network (CCSN) from analog to digital. Located along the south-central California coast, the CCSN began operation in 1987, with 20 analog stations; 15 vertical component and 5 dual gain 3-component S-13 sensors. The analog signals travel over FM radio telemetry links and voice channels via PG&E's microwave network to our facility in San Francisco (SF), where the A/D conversion is performed on a computer running Earthworm v7.1, which also transmits the data to the USGS in Menlo Park. At the conversion point the dynamic ranges of the vertical and dual-gain sensors are 40-50dB and 60-70dB, respectively. Dynamic range exceedance (data clipping) generally occurs for a M2.5 or greater event within about 40 km of a station. The motivations to upgrade the seismic network were the need for higher dynamic range and to retire obsolete analog transmission equipment. The upgraded digital stations consist of the existing velocity sensors, a 131A-02/3 accelerometer and a Reftek 130-01 Broadband Seismic Recorder for digital data recording and transmission to SF. Vertical only stations have one component of velocity and 3 components of acceleration. Dual gain sites have 3 components of velocity and 3 of acceleration. To date we have successfully upgraded 6 sites; 3 more will be installed by the end of 2008. Some of the advantages of going digital are 1) data is recorded at each site and in SF, 2) substantially increased dynamic range of the velocity sensors to 120dB, as observed by on scale, close-by recordings from a M3.9 San Simeon aftershock on 04/29/2008, 3) accelerometers for on scale recording of large earthquakes, and 4) ability to contribute our strong motion data to USGS ShakeMaps. A significant challenge has been consistent radio communications. To resolve this issue we are installing point-to-multipoint Motorola Canopy spread spectrum radios at the stations and communication towers.
S41B-1840
National Project for Intense Observations and Research in the High Strain Rate Zone of Japan
The mechanisms that control the occurrence of large intraplate earthquakes are still not well understood. Although physical models to explain their occurrence exist (e.g., Iio et al., 2004), more detailed geophysical observations are necessary to substantiate these models. The dense Global Positioning System (GPS) array developed by the Geographical Survey Institute of Japan (GEONET) has revealed the presence of a high strain rate zone northwest of central Honshu (Sagiya et al., 2000). Numerous large intraplate earthquakes have occurred in this region (e.g., the 1995 Kobe earthquake, Mw6.9). Thus, a detailed geophysical survey in the area could help to better understand the physical processes responsible for the occurrence of intraplate earthquakes. For this purpose, we are going to install 300 three-component seismometers in the region where two large earthquakes occurred recently (the 2004 Chuetsu, Mw6.6, earthquake and the 2007 Offshore Chuetsu, Mw6.6, earthquake). This temporary network, with an average spacing between stations of about 10 km, but significantly denser in the aftershock regions of the two large events (spacing of about 3 to 5 km) will complement the high-sensitivity seismograph network (Hi-net) of NIED, with a station spacing of about 20 km. The P- and S-wave arrival times of the earthquakes recorded by the combined temporary network and Hi-Net will be used in 3D tomographic inversions to obtain highly accurate hypocenter locations and a high-resolution velocity structure. Waveform cross-correlations will be used to improve the accuracy of arrival time picking. Moreover, we are planning to determine the focal mechanism solutions for the earthquakes recorded in the region and perform stress inversions to estimate the crustal stress field. At the meeting we will present a review of previous studies concerning the high-strain rate zone of Japan and an overview of this project.
S41B-1841
Evidences of Sediment Nonlinearity From Explosion Data in the Mississippi Embayment
Strong- and weak- motion data from the Mississippi Embayment Seismic Excitation Experiment (ESEE) were analyzed for signatures of nonlinear site responses. This experiment was performed jointly by the University of Memphis and US Geological Survey in October 2002, by detonating two explosions of 2500 and 5000lbs. Intrinsic and scattering Q estimates (QI and QS) from the coda of the strong motion data were found to be very low compared to previously determined Q values of P- and Rayleigh-waves of weak motions data from the same explosions. The QI estimates from P-wave late coda of the strong motion data are less by more than a 100 at 3Hz and by more than 200 at 10Hz compared to the P-wave Q values determined from the weak motion data by Langston et al (2005). Also, QI determined from the late coda of strong motion Rayleigh-wave data is less by more than 200 at 0.5Hz and by more than 50 at 3.0Hz compared to Q values determined from Rayleigh-wave weak motion data. A resonance peak spectral amplitude of the early part of a strong motion seismogram is shifted to lower frequencies compared to that from a later part of the same seismogram. Spectral amplitude ratios between transverse and vertical components of the strong motion data are degraded between frequencies 2 and 10Hz for P-waves, and less than 4Hz for Rayleigh-waves compared to the weak motion transverse to vertical spectral ratio. All these are signatures of nonlinear site responses during strong ground motion. This study proves the non-transportability of weak motion attenuation results to estimate ground motion from a future large earthquake that may happen in areas like the New Madrid Seismic zone.
S41B-1842
Nonlinear Responses of High-rise Buildings in Seattle for Simulated Ground Motions From Giant Cascadia Subduction Earthquakes (Mw 9.2)
With the exception of the 2003 Tokachi-oki earthquake, strong ground recordings from large subduction earthquakes (Mw > 8.0) are meager. Furthermore there are no strong motion recordings of giant earthquakes. However, there is a growing set of high-quality broadband teleseismic recordings of large and giant earthquakes. In this poster, we use recordings from the 2003 Tokachi-oki (Mw 8.3) earthquake as empirical Green's functions to simulate the rock and soil ground motions from a scenario Mw 9.2 subduction earthquake on Cascadia subduction zone in the frequency band of interest to flexible and large- scale buildings (0.075 to 1 Hz). The effect of amplification by the Seattle basin is considered by using a basin response Green's function which is derived from deconvolving the teleseismic waves recorded at rock sites from soil sites at the SHIP02 experiment. These strong ground motions are used to excite simulation of the fully nonlinear seismic responses of 20-story and 6-story steel moment-frame buildings designed according to both the U.S. 1994 UBC and also the Japanese building code published in 1987. We consider several realizations of the hypothetical subduction earthquake; the down-dip limit of rupture is of particular importance to the simulated ground motions in Seattle. If slip is assumed to be limited to offshore regions, then the building simulations indicate that the building responses are mostly in the linear range. However, our simulation shows that buildings with brittle welds would collapse for rupture models where rupture extends beneath the Olympic Mountains. The ground motions all have very long durations (more than 4 minutes), and our building simulations should be considered as a minimum estimate since we have used a very simple model of degradation of the structure.
S41B-1843
Evidence that scattering due to nonlinear elasticity contributes to coda waves.
Different factors might affect the propagation of seismic waves producing scattering, including heterogeneities and nonlinear elasticity. A key difference between these two factors is the dependence of the strength of the scattered waves on the strength of the incident wave, being linear for the former and nonlinear for the latter. A detailed study of these dependences using TIPTEQ data, where more than hundred explosions were recorded on 180 three-compomnent stations, most of them in the distance range of approximately 0-18 km (and a few far-offset explosions up to 100 km distance) shows that this dependence is nonlinear. Data were analysed in the following way: (i) the envelope of a bandpass filter between 10 and 40 Hz was obtained for a large number of stations from different ranges and charges of shots, (ii) for these distances we modeled the envelope considering the nonlinear elasticity. The shapes of the theoretical and observed envelopes were in general very similar, and a scale factor for each case was obtained considering the best fit of its complete envelope, (iii) since this scale factor depends mainly on the size of the explosion, we computed the ratio (R) of the scale factor (A) for different size explosions at fixed distances, for distances varying between 0 and 50 km. (iv) we computed the power (p) of the dependence of the ratio (R) on the ratio of charges. (R=(A1/ A2)=(/charge1/charge2/)p). We observe in general that p>1 and for distances between 14 and 18 km, and charges of 75 and 150 kg, the value of p=1.8 ± 0.4. This shows clearly that nonlinear elasticity is an important factor contributing to seismic wave scattering.
S41B-1844
Is Directivity Still Effective in a PSHA Framework?
Source rupture parameters, like directivity, modulate the energy release causing variations in the radiated signal amplitude. Thus they affect the empirical predictive equations and as a consequence the seismic hazard assessment. Classical probabilistic hazard evaluations, e.g. Cornell (1968), use very simple predictive equations only based on magnitude and distance which do not account for variables concerning the rupture process. However nowadays, a few predictive equations (e.g. Somerville 1997, Spudich and Chiou 2008) take into account for rupture directivity. Also few implementations have been made in a PSHA framework (e.g. Convertito et al. 2006, Rowshandel 2006). In practice, these new empirical predictive models incorporate quantitatively the rupture propagation effects through the introduction of variables like rake, azimuth, rupture velocity and laterality. The contribution of all these variables is summarized in corrective factors derived from measuring differences between the real data and the predicted ones Therefore, it's possible to keep the older computation, making use of a simple predictive model, and besides, to incorporate the directivity effect through the corrective factors. Any single supplementary variable meaning a new integral in the parametric space. However the difficulty consists of the constraints on parameter distribution functions. We present the preliminary result for ad hoc distributions (Gaussian, uniform distributions) in order to test the impact of incorporating directivity into PSHA models. We demonstrate that incorporating directivity in PSHA by means of the new predictive equations may lead to strong percentage variations in the hazard assessment.
S41B-1845
Source Process of the 2003 Bam, Iran, Earthquake: Subsurface Rupture that Generated Extreme Ground Motion
The Bam earthquake occurred on December 26, 2003 in southeast Iran. This moderate size event (Mw 6.5) caused heavy damage in the city of Bam and killed about 26,000 people. According to previous studies of geodetic data (e.g., Talebian et al., 2004; Wang et al., 2004) and aftershock distribution (Nakamura et al., 2005), the earthquake was caused by a rupture on a previously unknown strike-slip fault. The strong-motion station located inside the heavily damaged area of the city of Bam in vicinity of the fault recorded a PGA of 988 gal in the UD component and two pulses with a dominant frequency of 1 Hz in the horizontal components. This large PGA and the proportion of damage due to this event might be explained by the combination of source directivity effect and large speed of the rupture front over the fault (Bouchon et al., 2005). To estimate the slip pattern in the source rupture area, precise hypocentral depth, and rupture velocity along the fault, we applied the moment tensor analysis as well as the source inversion method developed by Kikuchi and Kanamori (1991) and Kikuchi et al. (2003) to the IRIS broadband teleseismic data. The result of the source inversion shows the slip distribution that confirms a single asperity, as suggested by Yamanaka (2003), with the rupture propagating in S-N direction along an almost vertical strike-slip fault with dimensions of 25 km in length by 20 km in width. The hypocentral depth for the best fit model is estimated to be 8 km. The maximum slip occurred around the hypocenter at depths of 4-10 km; no slip is associated at a shallower depth. This agrees with the result of subsurface rupture and 'shallow slip deficit' obtained from geodetic data by Fialko et al. (2005) and might explain the extreme ground motion observed at the Bam station as being the result of the subsurface faulting on the immature fault. We also determined the rupture velocity that minimizes the residuals between observed and synthetic waveforms to be 2.8 km/s, though teleseismic waveforms have been proved to have low sensibility to the variation of the assumed rupture velocity. Both teleseimic and geodetic data inversions (Fialko et al., 2005, Talebian et al., 2004) show that the whole source process can be explained well by a single asperity. The analyses of near-field strong motion data by BHRC and regional broadband records from IRIS database show the presence of double pulses in horizontal components of the stations in both forward and backward directions of the fault. We expect that the second pulse might be explained by the northward branch of the fault that extends beneath the city of Bam. However, our source model, due to the low spatial resolution of the teleseimic data, cannot explain the origin of the second directivity pulse. Detailed source process inversion using strong motion and geodetic data will provide more information on the source characteristics and fault geometry of this event.
S41B-1846
An Automatic Scheme for Baseline Correction of Strong-Motion Records in Coseismic Deformation Determination
Coseismic deformation can be determined from strong-motion records of large earthquakes. Iwan et al. (1985) showed that baseline corrections are often required to obtain reliable coseismic deformation because baseline offsets lead to unrealistic permanent displacements. Boore (2001) demonstrated that different choices of time points for baseline correction can yield realistically-looking displacements, but with variable amplitudes. The baseline correction procedures of Wu and Wu (2007) improved upon Iwan et al. (1985) and achieved stable results. However, their time points for baseline correction were chosen by a recursive process with an artificial criterion. In this study, we follow the procedure of Wu and Wu (2007) but use the ratio of energy distribution in acceleragrams as the criterion to determine the time points of baseline correction automatically. This avoids the artificial selection of time points and speeds up the estimation of coseismic deformations. We use the 2003 Chengkung and 2006 Taitung earthquakes in eastern Taiwan to demonstrate this new approach. Comparison between the results from this and previous studies shows that our new procedure is suitable for quick and reliable determination of coseismic deformation from strong motion records.