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The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue by Michael J

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The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue by Michael J View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors - Main Bulletin of the Seismological Society of America, Vol. 92, No. 4, pp. 1147–1153, May 2002 The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue by Michael J. Rymer, Victoria E. Langenheim, and Egill Hauksson The Hector Mine, California, earthquake (Mw 7.1) The earthquake allowed in-depth analysis of the limitations struck the Mojave Desert at 09:46 UTC, 16 October 1999. and advantages of the InSAR method. The earthquake occurred approximately 55 km northwest of Like the Landers earthquake, the Hector Mine earth- the town of Twentynine Palms, California, and about 200 quake ruptured multiple faults, some of which had not been km east-northeast of Los Angeles (Fig. 1). The shock was previously mapped. The Hector Mine earthquake produced widely felt throughout southern California, southern Ne- predominantly right-lateral slip (Fig. 2), consistent with its vada, western Arizona, and northernmost Baja California, location within the ECSZ. Also like Landers, the Hector Mexico. The Hector Mine earthquake, like the Mw 7.3 Land- Mine earthquake triggered seismicity at distances greater ers earthquake seven years earlier, was associated with fault than the rupture length. However, unlike Landers, which rupture in the eastern California shear zone (ECSZ) (Fig. 1), triggered seismicity primarily to the north in the direction of which is an approximately 80-km-wide zone of deformation its rupture, the Hector Mine earthquake triggered seismicity that accommodates about 24% of the relative Pacific–North south of the rupture, in the Salton Trough region (Figs. 1 American plate motion (Sauber et al., 1986, 1994; Dokka and 3). and Travis, 1990; Savage et al., 1990, 2001; Gan et al., 2000; We start this special issue with two articles that individ- Miller et al., 2001). A block diagram highlighting some of ually discuss the Hector Mine earthquake and associated sur- the basic aspects of the Hector Mine earthquake is presented face faulting. Hauksson et al. describe foreshocks, the main- in Figure 2. A preliminary summary of the Hector Mine shock, and aftershocks of the Hector Mine earthquake. They earthquake, its effects, and the response of the geoscience demonstrate that temporal clusters of earthquakes in 1992 community is presented by Scientists from the U.S. Geolog- and 1996 occurred a few kilometers away from the eventual ical Survey; Southern California Earthquake Center, and Hector Mine mainshock and migrated southeastward, toward California Division of Mines and Geology (USGS, SCEC, the future epicenter. A cluster of foreshocks likewise pre- and CDMG, 2000). ceded the Hector Mine earthquake, but within the immediate This special issue of the Bulletin contains 35 seismo- hypocentral volume. The Hector Mine mainshock ruptured logic, geologic, geophysical, and engineering studies related to the north, northwest, and south as shear deformation char- to the 1999 Hector Mine earthquake, its effects, and the re- acterized by conjugate strike-slip faulting. Treiman et al. gional geologic and tectonic setting. These articles not only show that the Hector Mine earthquake was associated with build and expand upon the preliminary results of USGS, 48 km of dextral surface rupture. Complex northward rup- SCEC, and CDMG (2000), but they also describe and analyze ture began on two branches of the Lavic Lake fault; rupture crustal deformation before, during, and after the earthquake also propagated southward onto the Bullion fault. Lesser as determined by global positioning system (GPS) and in- amounts of rupture occurred across two right steps to the ;m 0.5 ע terferometric synthetic aperture radar (InSAR) data. These south. Maximum dextral displacement was 5.5 articles also more fully describe the geologic setting and the displacement averaged about 2.5 m along the main rupture regional effects of the earthquake. Figure 3 shows the geo- zone. They conclude that most of the faults that ruptured in graphic distribution of studies presented in this issue. Be- 1999 had prior late Quaternary displacement, and only lim- cause of its proximity to the 1992 Landers event (epicenters ited sections of the rupture showed evidence of prior Holo- about 45 km apart, ruptures as close as 20 km), the Hector cene surface slip. Mine earthquake offers a rare opportunity to study the in- Ji et al. (I) use a new procedure to model rupture com- teractions of two large earthquakes with these data sets. plexity by using static and seismic data that allows for mul- New advances in instrumentation and event processing tiple fault segments, variable local slip, rake angle, rise time, provided hypocentral and magnitude determinations for and rupture velocity. This approach combines a wavelet thousands of aftershocks in near real time. These advances transform approach with a nonlinear algorithm. The result- led to assessment of ground shaking within minutes of the ing model, presented by Ji et al. (II), leads to a seismic mo- N m, distributed along three 1019 ן mainshock and to assessment of aftershock hazards within ment estimate of 6.28 days of the mainshock. New techniques to measure crustal rupture segments. These segments, from north to south, ac- deformation, such as continuously recording GPS networks count for 37%, 41%, and 22% of the total seismic moment, and raster-laser imaging, were used to study this earthquake. respectively. They calculate a rise time that appears roughly 1147 1148 Introduction to the Special Issue Monterey Bay San Joaquin Valley Sierra Nevada ia San Andreas fault Owens Valleyn fault ifor Garlock fault al e C zon Los n Panamint Valley f Angeles r r MOJAVE DESERTste a a he Death V f e e s PACIFIC n e o OCEAN n z o t l z Landers u t rupture l a f u a o f t n Hectorrupture Mine e i r TwentyninePalms c o a n J i s n l a E S Brawley do River seismic zone lora SALTON TROUGH Co fault Imperial PACIFIC OCEAN Cerro Prieto fault CALIFORNIA GULF OF CALIFORNIA CALIFORNIABAJA ARIZONA SONORA GULF OF CALIFORNIA Figure 1. Oblique satellite view of northern Baja California, Mexico, westernmost Arizona, and all of southern California. Select cities, geographic features, and generalized traces of major faults drawn for reference. Hector Mine rupture (thick black line) and 1992 Landers rupture shown within eastern California shear zone. View to the northwest. Pho- tograph from National Aeronautics and Space Administration (NASA) Johnson Space Cen- ter, Space Shuttle mission STS103, roll 701, frame 39, December 1999. proportional to the slip and an average rupture velocity that et al. use a revised spectral technique for estimating radiated is relatively slow compared to the Landers rupture velocity. energy. Their estimates of regional and teleseismic radiated dyne 1022 ן dyne cm and 3.2 1022 ן 0.7 ע Wiemer et al. examine in great detail the Hector Mine energy are 3.4 aftershock sequence, particularly the spatial and temporal cm, respectively. They find little evidence of directivity in seismicity parameters, and compare it with the Landers se- corrected velocity spectra. Venkataraman et al. also use re- quence. Asymmetrical b-values and hazard patterns for both gional data to estimate radiated energy. They remove path earthquakes lead to the hypothesis that mainshock rupture and station attenuation effects with an empirical Green’s directivity and slip distribution influenced aftershock haz- function deconvolution; the radiated energy is then com- ards. They compute probabilistic aftershock-hazard maps for puted using calculated source spectra. They determine a re- .J 1015 ן 0.9 ע the Hector Mine earthquake and suggest that greater hazards gional estimate of radiated energy of 3.0 were in the northern part of aftershock zone. Stress field They also use two different techniques to estimate teleseis- .J 1015 ן J and 2.0 1015 ן heterogeneity was unusually high near the Landers and Hec- mic energy of 1.8 tor Mine hypocenters, particularly near patches of large slip. Kaverina et al. discuss inversion of broadband regional Two articles estimate the amount of energy radiated and local waveforms using GPS and InSAR measurements. from the Hector Mine earthquake. Although these two arti- They determine a peak slip of 5.5 m and a seismic moment N m. The majority of slip was on the western 1019 ן cles use different methods, regional and teleseismic esti- 6.8 mates of radiated energy are in close agreement. Boatwright branch of the Lavic Lake fault, but overall rupture was bi- Introduction to the Special Issue 1149 Figure 2. Schematic block diagram with vertical cut along 1999 Hector Mine rupture plane (Simons et al., 2002, this issue) and summarizing basic features and observations of the Hector Mine earthquake. View to the northeast. Features shown in this figure are gen- eralized; for details see appropriate article in this issue. Magnitude, Mw 7.1; Time, 9h 46m km (Hauksson et al., 2002); seismic 4 ע UTC, 16 October 1999; hypocentral depth, 5 ;(N m (Ji et al., 2002); average stress drop, 25 bars (Ji et al., 2002 1019 ן moment, 6.28 length of surface rupture, ϳ48 km (Treiman et al., 2002); maximum right lateral displace- m; average along main rupture, ϳ2.5 m (Treiman et al., 2002); faults 0.5 ע ment, 5.5 involved: Lavic Lake, Bullion, East Bullion, West Bullion, Mesquite Lake, and unnamed faults (Treiman et al., 2002). lateral. Average slip ranged from 1.6 to 2.7 m on the Lavic valley. Local earthquakes were recorded beginning 2 hr after Lake and Bullion faults. They calculate a rupture velocity of arrival of the Hector Mine seismic waves; local seismicity 1.8 km/sec with a delay of several seconds before the onset continued for about 2 weeks.
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