The 1989 Loma Prieta earthquake imaged from inversion of geodetic data

Abstract

We invert geodetic measurements of coseismic deformation from the 1989 MS7.1 Loma Prieta earthquake to determine the geometry of the fault and the distribution of slip on the fault plane. The data include electronic distance measurements, Global Positioning System and very long baseline interferometry vectors, and elevation changes derived from spirit leveling. The fault is modeled as a rectangular dislocation surface in a homogeneous, elastic half‐space. First, we assume that the slip on the fault is uniform and estimate the position, orientation, and size of the fault plane using a nonlinear, quasi‐Newton algorithm. The best fitting dislocation strikes N48°±4°W and dips 76°±9°SW, consistent with the trend of the aftershock zone and moment tensor solutions. Bootstrap resampling of the data is used to graphically illustrate the uncertainty in the location of the rupture plane. The 95% confidence envelope overlaps the aftershock zone, arguing that there is not a significant discrepancy between the geodetic data and the aftershock locations. Second, we estimate the slip distribution using the best fitting uniform slip fault orientation but increase the fault length to 40 km and the downdip width to 18 km. The fault is divided into 162 subfaults, 18 along strike and 9 along dip. Each subfault is allowed to have constant right‐lateral and reverse components of slip. We then solve for the slip on each subfault that minimizes a linear combination of the norm of the weighted data residual and the roughness of the slip distribution. The smoothing parameter, which determines the relative weight put on fitting the data versus smoothing the slip distribution, is chosen by cross validation. Simulations indicate that cross‐validation estimates of the smoothing parameter are nearly optimal. The preferred slip distribution is very heterogeneous, with maximum strike slip and dip slip of about 5 and 8 m, respectively, located roughly 10 km north of the hypocenter. There is insignificant dip slip in the southeastern most part of the fault, causing the rake to vary from nearly pure right‐lateral in the southeast to oblique right‐reverse in the northwest. The change in rake is consistent with a uniform stress field if the fault dip increases by about 10° toward the southeast, as indicated by the aftershock locations. There was little slip above 4 km depth, consistent with the observation that there was little, if any, surface rupture.

Joint estimation of afterslip rate and postseismic relaxation following the 1989 Loma Prieta earthquake

“…From Table 2 The coseismic slip distributions of both Beroza [1991] and Arnadottir and Segall [1994] are shown in Figure 8. We wish to compare our distributed afterslip model with the coseismic slip distribution, and we choose to compare it directly with the slip distribution determined by Arnadottir and Segall [1994] in Figure 8b simply because the slip distributions in that paper and the present paper were derived from geodetic data sources. The distributed afterslip rate for model A is superimposed for both the strike-slip and reverse-slip components of plane 1.…”

“…The M7.1 Loma Prieta earthquake is one of the best studied earthquakes [e.g., Lisowski [Biirgmann et al, 1997; hereafter referred to as paper 1) reveals that continued afterslip on the coseismic rupture plane and its extensions dominates the regional crustal deformation observed in the 5 years since 1989 as obtained by GPS measurements and leveling. Using the constrained nonlinear optimization algorithm of Arnadottir and Segall [1994], their analysis shows that uniform afterslip (predominantly reverse) on two distinct planes at rates of 1-3 cm/yr is sufficient to explain the primary features in the data. Our goal in this paper is to examine the same data set in greater detail by further considering: (1) distributed slip on the two afterslip planes determined by paper I and (2) viscoelastic relaxation of a lower crustal and mantle asthenosphere underlying the northern California upper crust.…”

Landslides triggered by the 12 January 2010 Port-au-Prince, Haiti, <i>M</i><sub>w</sub> = 7.0 earthquake: visual interpretation, inventory compiling, and spatial distribution statistical analysis

“…For example, during the 2013 Lushan, China, M w = 6.6 earthquake, only brittle compressive cracking in the cement-covered pavements can be observed, but continuous fault surface ruptures can not be found (Xu et al, 2013b;Xu and Xu, 2014a). The 18 October 1989 Loma Prieta, California, earthquake (M w = 6.9) was a similar-size, shallow, oblique-slip earthquake that occurred close to a major strike-slip fault but was not accompanied by surface ruptures (Prentice and Schwartz, 1991;Árnadóttir and Segall, 1994;Prentice et al, 2010), though several researchers (e.g., Harp, 1998) also suggest that cracking on the Summit Ridge is fault rupture spread across a wide zone.…”

Combination of Precise Leveling and InSAR Data to Constrain Source Parameters of the Mw = 6.5, 26 December 2003 Bam Earthquake

“…To determine the penalty factor j, we follow the lead of previous studies (Bu¨RGMANN et al, 2002;ARNADOTTIR and SEGALL, 1994) and plot a trade-off curve to find out the relative weight between the fit to the data and the model complexity. Figure 6 shows WRSS as a function of model roughness r…”