The 1999 Mw 7.1 Hector Mine, California, Earthquake Sequence: Complex Conjugate Strike-Slip Faulting

Abstract

The 1999 M w 7.1 Hector Mine mainshock showed right-lateral strikeslip faulting, with an initial strike of N6ЊW and vertical dip. The mainshock was preceded within 20 hours by 18 recorded foreshocks of 1.5 Յ M Յ 3.8 within a few kilometers distance of the mainshock hypocenter. The aftershocks delineate how the Hector Mine earthquake ruptured with strike N6ЊW to the south for a distance of 15 km, and possibly to the north for a distance of several kilometers. The two largest aftershocks of M 5.9 and M 5.7 occurred near the north and south ends of the first mainshock rupture segment. The second segment of rupture, starting 15 km to the south away from the mainshock hypocenter, delineated by strike-slip and thrustfaulting aftershocks, extends 10 km farther away with a strike of S140ЊE along the Bullion fault. The aftershocks also outline an unusual third rupture segment, extending from about 5 km south of the hypocenter with a strike of N30ЊW to N35ЊW for a distance of 20 km. Approximately 10 to 25 km farther to the north and west of the mainshock epicenter, several clusters form a complex aftershock distribution. Threedimensional Vp and Vp/Vs models of the region exhibit only small regional changes, as is typical for the Mojave region. Nonetheless, the mainshock rupture started within a region of rapidly varying Vp, and at least three regions of low Vp/Vs are imaged within the aftershock zone. The rate of decay for the Hector Mine earthquake sequence has been slightly above the mean for both p-values and b-values in southern California. The focal mechanisms of the aftershocks and the state of stress are consistent with strike-slip faulting, including a component of normal faulting most prominent to the north. The orientation of the regional maximum horizontal stress, the variation in orientation of the mainshock fault segments by 30Њ, and scattered distribution of aftershocks suggest that the mainshock and aftershock deformation field exhibit volumetric shear deformation accommodated by complex conjugate sets of strike-slip faults.

Coseismic Deformation from the 1999 Mw 7.1 Hector Mine, California, Earthquake as Inferred from InSAR and GPS Observations

“…Our estimates of fault dip are generally consistent with the aftershock distribution as shown by the cross sections in Figure 13, and positions of our major fault segments inferred from our inversion (Table 2). Aftershock positions from Hauksson et al (2002) are determined from a relocation of catalog events using a 3D velocity model of the area.…”

“…A later field investigation, using the coherence map from IP1 as a guide, discovered about 60 cm of rightlateral offset on this previously unreported surface rupture (Treiman et al, 2002). The central portion of the surface rupture is characterized by a localized linear zone of low coherence as the rupture crosses into the Bullion Mountains (Hauksson et al, 2002) (bottom left), and the azimuth offsets from IP1 (bottom right). (Fig.…”

“…We infer the earthquake rupture geometry using the interferometric images and relocated postearthquake seismicity (Hauksson et al, 2002) (Fig. 6).…”

“…Such a close temporal and spatial proximity has raised the question of a possible causal relationship between these earthquakes (Parsons and Dreger, 2000; Scientists of the USGS et al, 2000;Freed and Lin, 2001). Both the Landers and Hector Mine earthquakes initiated robust aftershock sequences, as well as triggering seismicity as far as several hundred kilometers from the rupture plane (Hauksson et al, 2002). Before addressing the relationship between these two earthquakes and the nature of postseismic deformation from the Hector Mine event (e.g., Pollitz et al, 2001;Hudnut et al, 2002;Pollitz and Sacks, 2002;Masterlark and Wang, 2002;Owen et al, 2002), we (Treiman et al, 2002).…”

Spatial Analysis of b-value Variability in Armutlu Peninsula (NW Turkey)

“…Also, variations are also due to different crustal structures, thickness, rock types, physical properties and ultimately its degree of heterogeneity. Examples of studies that document this class of b-variations include [64][65][66][67][68]. [69] have suggested that the increase in b-value is caused by the presence of uids at a particular depth in the Koyna -Warna region.…”