Temporal changes in <i>P</i> wave attenuation in the Loma Prieta rupture zone

Chun, Kin-Yip; Henderson, Gary; Liu, Jingsong

doi:10.1029/2003jb002498

Cited by 18 publications

(25 citation statements)

References 24 publications

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“…Other studies have reported postseismic changes in t ∗ of similar magnitude (e.g., Chun et al . []). This signal could originate from time‐dependent variations in the source or along the travel path.…”

Section: Analysis and Resultsmentioning

confidence: 99%

Temporal changes in attenuation associated with the 2004 M6.0 Parkfield earthquake

et al. 2013

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Elevated seismic attenuation is often observed in fault zones due to the high degree of fracturing and fluid content. However, temporal changes in attenuation at the time of an earthquake are poorly constrained but can give indications of fracture damage and healing. In this study, spectral ratios between earthquakes within repeating clusters are calculated in an attempt to resolve temporal variations in attenuation at the time of the 2004 M6.0 Parkfield earthquake. A sharp increase in attenuation is observed immediately after the earthquake, which then decays over the next 2 years. Influences of intercluster magnitude variations, time window length and previously reported postseismic velocity changes are investigated. The postseismic decay is fit by a logarithmic function. The timescale of the decay is found to be similar to that in GPS data and ambient seismic noise velocities following the 2004 M6.0 Parkfield earthquake. The amplitude of the attenuation change corresponds to a decrease of approximately 10% in Qp at the time of the earthquake. The greatest changes are recorded on the northeast of the fault trace, consistent with preferential damage in the extensional quadrant behind a north‐westerly propagating rupture tip. Our analysis suggests that significant changes in seismic attenuation and hence fracture dilatancy during coseismic rupture are limited to depths of less than about 5 km.

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Section: Analysis and Resultsmentioning

confidence: 99%

Temporal changes in attenuation associated with the 2004 M6.0 Parkfield earthquake

et al. 2013

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“…They have identical source parameters and produce nearly identical seismograms at the same stations. Repeating events are therefore often used to identify temporal changes in the medium [see, e.g., Beroza et al , 1995; Baisch and Bokelmann , 2001; Chun et al , 2004]. In the Parkfield region these repeating sequences of earthquakes have been found to be quite common [ Nadeau et al , 1995; Nadeau and Johnson , 1998].…”

Section: Stress Drop Resultsmentioning

confidence: 99%

“…Furthermore, we are not taking into account any short‐term temporal changes to the stress drop change imposed by the main shock due to, e.g., healing processes. Fault zone healing has been observed with other seismic methods at several locations in California [e.g., Li et al , 1998, 2003; Chun et al , 2004; Li et al , 2006]. Such changes are difficult to assess with our method since we require a large number of events per grid bin in order to determine spatial variations of source parameters reliably.…”

Section: Discussionmentioning

confidence: 95%

Spatial and temporal stress drop variations in small earthquakes near Parkfield, California

2007

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We estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the Parkfield segment of the San Andreas Fault in central California. We use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed P wave source spectra with a spatially varying empirical Green's function (EGF). Our Brune‐type stress drop estimates vary from 0.1 to over 100 MPa with a median value of 6.75 MPa, which is nearly constant with moment, implying self‐similarity over the ML = 0.5 to 3.0 range of our data. The corner frequency decreases for earthquakes at shallower depths, consistent with slower rupture velocities and reduced shear wave velocities in local velocity models. The estimated median stress drops show significant lateral variations: we find lower stress drops in the Middle Mountain asperity and along the creeping fault section, and higher stress drops in the hypocentral region of the 2004 M6.0 Parkfield earthquake. The main shock did not alter the overall pattern of high and low stress drop regions. However, a statistical test reveals areas with significant changes in computed stress drops after the main shock, which we compare to estimated absolute shear stress changes from a main shock slip model. By calculating Δt* from the spectral EGF ratio, we also identify areas with increased attenuation after the main shock, and we are able to distinguish source effects and near‐source attenuation effects in the spectral analysis. These results are confirmed independently from spectral ratios of repeating microearthquake clusters.

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“…For such reasons, seismic data and velocity field analysis are used on a daily basis in the oil industry to quantify the oil content and the identify fluid properties, i.e., oil or gas. In seismotectonics, a key unanswered question is whether earthquakes and volcanic eruptions can be predicted by quantifying time dependent precursory damage accumulation using elastic wave velocity variations (CHUN et al, 2004;VOLTI and CRAMPIN, 2003;GAO and CRAMPIN, 2004). Thus the understanding and quantification of elastic wave velocity variations is critical to extract information on the physical state of rocks from seismic and seismological data.…”

Section: Introductionmentioning

confidence: 99%

Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities

Schubnel

Benson

Thompson

et al. 2006

Pure appl. geophys.

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Crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive crack effective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic properties. Using this same tool, we show that both the anisotropy and shear-wave splitting of elastic waves can be derived. Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range of validity of the non-interactive effective medium theory model extends to a total crack density of approximately 0.5, considering symmetries up to orthorhombic. In the isotropic case, KACHANOV's (1994) non-interactive effective medium model was used in order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and give coherent results in terms of crack density and aperture evolution. Crack density variations can be interpreted in terms of crack growth and/or changes of the crack surface contact areas as cracks are being closed or opened respectively. More importantly, the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio (and therefore aperture) with pressure, which has broader geophysical implications, in particular on fluid flow. The recovered evolution of aspect ratio is also consistent with current mechanical theories of crack closure. In the anisotropic cases-both transverse isotropic and orthorhombic symmetries were considered-anisotropy and saturation patterns were well reproduced by the modelling, and mean crack fabric orientations we recovered are consistent with in situ geophysical imaging.Our results point out that: (1) It is possible to predict damage, anisotropy and saturation in terms of a crack density tensor and mean crack aspect ratio and orientation; (2) using well constrained wave velocity data, it is possible to extrapolate the contemporaneous evolution of crack density, anisotropy and saturation using wave velocity inversion as a tool; 3) using such an inversion tool opens the door in linking elastic properties, variations to permeability.

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Temporal changes in P wave attenuation in the Loma Prieta rupture zone

Cited by 18 publications

References 24 publications

Temporal changes in attenuation associated with the 2004 M6.0 Parkfield earthquake

Temporal changes in attenuation associated with the 2004 M6.0 Parkfield earthquake

Spatial and temporal stress drop variations in small earthquakes near Parkfield, California

Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities

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