Automatic Extraction of Permanent Ground Offset from Near-Field Accelerograms: Algorithm, Validation, and Application to the 2004 Parkfield Earthquake

Permanent ground offsets, constituting a prime dataset for constraining final fault-slip distributions, may not be recovered straightforwardly by double integration of near-field accelerograms due to tilt and other distorting effects. Clearly, if a way could be found to recover permanent ground offsets from acceleration records, then static datasets would be enlarged, and thus the resolution of fault-slip inversions would be enhanced. Here, we introduce a new approach for extracting permanent offsets from near-field strong-motion accelerograms. The main advantage of the new approach with respect to previous ones is that it corrects for source time functions of any level of complexity. Its main novelty is the addition of a constraint on the slope of the ground velocity spectra at long periods. We validated the new scheme using collocated accelerograms and Global Navigation Satellite Systems (GNSS) records of the 2011 Mw 9 Tohoku-Oki earthquake. We find a good agreement between accelerogram-based and GNSS-based ground offsets over a range of 0.1–5 m. To improve the spatial coverage of permanent ground offsets associated with the 2004 Parkfield earthquake, near-field accelerograms were baseline corrected using the new scheme. Static slip inversion of the combined GNSS-based and accelerogram-based ground displacements indicates appreciable seismic moment release south of the epicenter, about 5 km into the Cholame section of the San Andreas fault. We conclude that the strong shaking observed to the south of the epicenter is directly related to the slip in that area and is not the result of local amplification. KEY POINTS • WA new technique for extracting permanent ground offset from near-field accelerograms is introduced. • The new scheme is validated using collocated strong motion and Global Positioning System (GPS) records of the 2011 Tohoku-Oki earthquake. • A new 2004 Parkfield slip model is presented that uses the combined strong motion and GPS dataset. INTRODUCTION In addition to a transient component, the ground motion near strong earthquakes includes a geodetically observable component of permanent offset, the contribution of which rises with decreasing distance from the earthquake focus (Fig. 1). These transient and permanent ground motions are commonly referred to as the far-field and the near-field components of the radiated elastic wavefield, respectively (Aki and Richards, 2002). Because permanent ground offsets are far less sensitive to the subsurface structure than transient motions are, and are more directly related to the final deformation at depth, they constitute a prime dataset for constraining the final fault-slip distributions (Segall, 2010). To date, however, near-field permanent offsets cannot be recovered straightforwardly through double integration of near-field accelerograms as these are often distorted by baseline shifts (e.g., Iwan et al., 1985; Boore, 2001; Wu andWu, 2007; Melgar et al., 2013). A common cause for acceleration baseline shift is ground tilt, the effect of which is to shift the horizontal and vertical baselines according to 1. Department of Geophysics, Tel Aviv University, Tel Aviv, Israel *Corresponding author: asafinbal@tauex.tau. ac.il Cite this article as Inbal, A. , and A. Ziv (2020). Automatic Extraction of Permanent Ground Offset from Near-Field Accelerograms: Algorithm, Validation, and Application to the 2004 Parkfield Earthquake, Bull. Seismol. Soc. Am. XX, 1–9, doi: 10.1785/ 0120200040 © Seismological Society of America Volume XX Number XX – 2020 www.bssaonline. org Bulletin of the Seismological Society of America • 1 Downloaded from http://pubs.geoscienceworld. org/ssa/bssa/article-pdf/doi/10.1785/0120200040/5091806/bssa-2020040.1. pdf by Tel Aviv Univ user on 13 October 2020 EQ-TARGET;temp:intralink-;df1;53;419Δghor g sin θ and Δgver g 1 − cos θ ; 1 with g being the reference gravitational acceleration, θ being the tilt angle, and the superscripts hor and ver standing for the horizontal and vertical components, respectively. For very small tilts, when θ → 0, the horizontal baseline shift is proportional to θ, and the vertical shift is proportional to 0:5θ2. Consequently, even a very small tilt, say 10−5 radians, can cause about 20 cm of drift in the horizontal static offset within only 1 min from the tilt event. Such drifts are easily recognized by visual inspection; instead of flattening after transient motion has ceased, near-field double-integrated accelerograms vary (increase or decrease) as a quadratic function of time. To obtain accurate static offsets, the acceleration time series must be corrected before it is integrated. It is worth noting that additional causes for acceleration baseline offset include the transducer’s response to strong shaking and/or problems in the AD converter (Boore, 2001). Clearly, if a way could be found to recover permanent ground offsets from accelerograms, then static datasets would be enlarged and their spatial coverage improved, and thus the resolution of fault-slip inversions would be enhanced. This would be most advantageous in cases in which Interferometric Synthetic Aperture Radar (InSAR) and optical imagery contain a considerable amount of postseismic deformation and/or the fault orientation is unfavorable for InSAR observations. It would also be advantageous in seismically active regions withcontinuous geodetic coverage that is sparse. In addition to finite fault-slip applications, rapid permanent offset extraction may also be used for earthquake rapid response and on-land local tsunami alert systems. These lines of thinking have motivated several researchers to seek ways to extract permanent ground offset from near-fault accelerograms experiencing baseline shifts. This article is organized as follows: first, available approaches for correcting near-field accelerograms are described along with their shortcomings. Second, a new scheme for extracting the permanent ground offsets from strongmotion (SM) accelerograms is introduced. The new scheme is then validated using collocated SM and Global Navigation Satellite Systems (GNSS) records of the 2011 Mw 9 TohokuOki earthquake. Third, the new approach is used for augmenting and improving the spatial coverage of the permanent ground offsets associated with the 2004 Parkfield earthquake. Finally, a new 2004 Parkfield slip model is presented that uses this combined dataset. PREVIOUS STUDIES If continuous displacement data are available from collocated high-rate GNSS receivers, then these data may be used to correct the SM accelerograms (e.g., Nikolaidis et al., 2001; Emore et al., 2007; Bock et al., 2011; Melgar et al., 2013). Yet, only a few modern seismic networks operate both SM and high-rate GNSS instruments, and even in these so called seismogeodetic networks, only a small subset of the GNSS monuments are collocated with accelerometers. It is therefore useful to provide a correction scheme that does not rely on accurate long-period measurements from a nearby GNSS receiver. In the following, commonly used approaches for correcting distorted SM data are described. Iwan et al. (1985) introduced a scheme for obtaining permanent offsets through double integration of SM accelerograms. According to this approach, corrections are applied to two consecutive intervals of the acceleration time series. The earliest data interval between t1 and t2 begins at the onset of strong ground motion and ends when SM has resided, and the subsequent interval starts at t2 and ends at the end of the record. The correction applied to times greater than t2, with a 0 1 2 3