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Regional map indicating surface rupture (solid black) of the Hector Mine (M7.1) earthquake (epicenter indicated by star). Regional faults are shown by the dashed grey lines. Labeled bars crossing the surface rupture indicate 1999 and 2000 array locations.
Source publication
We are measuring shear wave anisotropy along the surface rupture of the
Hector Mine fault zone. Portable seismic arrays deployed following the
October 16, 1999 M7.1 Hector Mine earthquake recorded numerous
aftershocks in 1999 and 2000. We are analyzing data from two arrays
(northern and southern) deployed in 1999 and 2000 in approximately the
same...
Contexts in source publication
Context 1
... main shock ruptured two faults: the Bullion fault and the previously unnamed Lavic Lake fault. The rupture was bilateral, with approximately 10 km of surface rupture to the north and 33 km to the south (Figure 1). ...
Context 2
... To determine a suite of splitting directions, we examine data from each seismic array deployed after the main shock. Three main deployments took place in the days, months, and years following the main shock (Figure 1). ...
Context 3
... delays suggest no temporal evolution of the crack field. Table 3 and Figure 10 give average splitting parameters for each bin, not separated by year. Using these averages we can examine in more detail the spatial characteristics of the crack field. ...
Context 4
... Figure 11 illustrates crack rotation along the fault in relation to Hector Mine rupture strike, regional maximum compressive horizontal stress direction (s h ), and regional fault strike. It is evident that measured crack orientation is controlled by the recent rupture and s h . ...
Context 5
... Owing to the scatter inherent in delay times determined by shear wave splitting, it is often difficult to constrain the depth extent of anisotropy [e.g., Gamar and Bernard, 1997;Savage et al., 1989;Zhang and Schwartz, 1994]. Figure 12 plots delay time versus hypocentral distance and versus depth for the 1999 and 2000 NA and SEA arrays. There is not a clear increase in delays with distance or depth. ...
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Citations
... Due to preferential alignment of fractures and parallel layering, fault zones are generally considered to be anisotropic Li et al. 1987;Cochran & Vidale 2001). In many cases, this anisotropy can be approximated by assuming transverse-isotropy, which means that rays travelling parallel to the principal fault plane have the same velocity Li et al. 1987). ...
... Fault zones are expected to be anisotropic due to preferential alignment of fractures and fault-parallel layering of rock beds Li et al. 1987;Cochran & Vidale 2001). Many fault zones can be assumed to be transversely isotropic if the fractures and/or bedding are aligned parallel to the fault plane Here, we have assumed very low spatial attenuation of Q α = Q β = 1000. ...
Synthetic fault-zone trapped wave (FZTW) dispersion curves and amplitude responses for FL (Love) and FR (Rayleigh) type phases are analysed in transversely isotropic 1-D elastic models. We explore the effects of velocity gradients, anisotropy, source location and mechanism. These experiments suggest: (i) A smooth exponentially decaying velocity model produces a significantly different dispersion curve to that of a three-layer model, with the main difference being that Airy phases are not produced. (ii) The FZTW dispersion and amplitude information of a waveguide with transverse-isotropy depends mostly on the Shear wave velocities in the direction parallel with the fault, particularly if the fault zone to country-rock velocity contrast is small. In this low velocity contrast situation, fully isotropic approximations to a transversely isotropic velocity model can be made. (iii) Fault-aligned fractures and/or bedding in the fault zone that cause transverse-isotropy enhance the amplitude and wave-train length of the FR type FZTW. (iv) Moving the source and/or receiver away from the fault zone removes the higher frequencies first, similar to attenuation. (v) In most physically realistic cases, the radial component of the FR type FZTW is significantly smaller in amplitude than the transverse.
... It is also expected that many fault zones have bulk anisotropic elastic properties Li et al. 1987;Cochran & Vidale 2001). This anisotropy could be caused by parallel layering of fault rocks or juxtaposed geology, preferential alignment of fractures or other intrinsic rock properties such as crystal orientation and type. ...
We develop a computationally efficient approach to compute the waveforms and the dispersion curves for fault-zone trapped waves guided by arbitrary transversely isotropic across-fault velocity models. The approach is based on a Green's function type representation for FL and FR type fault-zone trapped waves. The model can be used for simulation of the waveforms generated by both infinite line sources (2-D) and point sources (3-D). The numerical scheme is based on a high order finite element approximation and, to increase computational efficiency, we make use of absorbing boundary conditions and mass lumping of finite element matrices.
... Also, the strain observed around the faults was a step function in time 9 , whereas the velocity changes that we report were more protracted. In addition, we have not observed obvious changes in the S-wave splitting directions or amplitudes in surveys after the Landers and Hector earthquakes as healing progressed, as might be expected from systematic changes in the crack population 11 . ...
... Also, the strain observed around the faults was a step function in time 9 , whereas the velocity changes that we report were more protracted. In addition, we have not observed obvious changes in the S-wave splitting directions or amplitudes in surveys after the Landers and Hector earthquakes as healing progressed, as might be expected from systematic changes in the crack population 11 . ...
Crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes, which allows for the relative motion between adjacent crustal blocks. Although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present-day crustal stresses and fault motions to geological structures formed in the past, fault-zone damage and healing have been documented quantitatively in only a few cases. We have been monitoring the healing of damage on the shallow Johnson Valley fault after its rupture in the 1992 magnitude-7.3 Landers earthquake, and here we report that this healing was interrupted in 1999 by the magnitude-7.1 Hector Mine earthquake rupture, which occurred 20-30 km away. The Hector Mine earthquake both strongly shook and permanently strained the Johnson Valley fault, adding damage discernible as a temporary reversal of the healing process. The fault has since resumed the trend of strength recovery that it showed after the Landers earthquake. These observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking, and may be involved in friction reduction during faulting.
Agnathan fish hold a key position in vertebrate evolution, especially regarding the origin of the head and neural-crest-derived tissue. In contrast to amphioxus, lampreys and other vertebrates possess a complex brain and placodes that contribute to well-developed eyes, as well as auditory and olfactory systems. These sensory sytems were arguably a trigger to subsequent vertebrate diversifications. However, although they are known from skeletal impressions in younger Palaeozoic agnathans, information about the earliest records of these systems has been largely wanting. Here we report numerous specimens of the Lower Cambrian vertebrate Haikouichthys ercaicunensis, until now only known from the holotype. Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs, as well as what may be otic capsules. A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates. Although Haikouichthys somewhat resembles the ammocoete larva of modern lampreys, this is because of shared general craniate characters; adult lampreys and hagfishes (the cyclostomes if monophyletic) are probably derived in many respects.
