scholarly journals Body force equivalents for stress-drop seismic sources

1976 ◽  
Vol 66 (6) ◽  
pp. 1801-1804
Author(s):  
Robert J. Geller
Keyword(s):  
Principal Stress ◽  
Stress Drop ◽  
Body Force ◽  
Seismic Source ◽  
Seismic Sources ◽  
Body Forces ◽  
Independent Functions ◽  
Double Couple ◽  
Stress Drops

abstract The equivalent body forces for a stress-drop seismic source are found. When the isotropic stress drop and one of the three principal stress drops are zero, then the equivalent body forces are the same double couple without moment which would result from a shear dislocation. In general however, all six stress-drop components must be specified as independent functions of time.

Strong ground motion of mine tremors: Some implications for near-source ground motion parameters

1981 ◽  
Vol 71 (1) ◽  
pp. 295-319
Author(s):  
A. McGarr ◽  
R. W. E. Green ◽  
S. M. Spottiswoode
Keyword(s):  
Ground Motion ◽  
Source Parameters ◽  
Seismic Source ◽  
Shear Velocity ◽  
Recording System ◽  
Ground Acceleration ◽  
Hypocentral Distance ◽  
Earthquake Size ◽  
Mine Tremors ◽  
Stress Drops

abstract Ground acceleration was recorded at a depth of about 3 km in the East Rand Proprietary Mines, South Africa, for tremors with −1 ≦ ML ≦ 2.6 in the hypocentral distance range 50 m < R ≦ 1.6 km. The accelerograms typically had predominant frequencies of several hundred Hertz and peak accelerations, a, as high as 12 g. The peak accelerations show a dependence on magnitude, especially when expressed as dynamic shear-stress differences, defined as σ˜ = ρRa, where ρ is density. For the mine tremors, σ˜ varies from 2 to 500 bars and depends on magnitude according to log σ˜ = 1.40 + 0.38 · ML. Accelerograms for 12 events were digitized and then processed to determine velocity and, for seven events with especially good S/N, displacement and seismic source parameters. Peak ground velocities v ranged up to 6 cm/sec and show a well-defined dependence one earthquake size as measured by ML or by seismic moment, Mo. On the basis of regression fits to the mine data, with −0.76 ≦ ML ≦ 1.45, log Rv = 3.95 + 0.57 ML, where Rv is in cm2/sec, and log Rv = −4.68 + 0.49 log Mo. These regression lines agree excellently with the corresponding data for earthquakes of ML up to 6.4 or Mo to 1.4 × 1026 dyne-cm. At a given value of ML or Mo, a, at fixed R, shows considerably greater variation than v and appears to depend on the bandwidth of the recording system. The peak acceleration at small hypocentral distances is broadly consistent with ρRa = 1.14 Δτrofs/β, where Δτ is stress drop, ro is the source radius, β is shear velocity, and fs is the bandwidth of the recording system. The peak velocity data agree well with Rv = 0.57 βΔτro/μ, where μ is the modulus of rigidity; both expressions follow from Brune's model of the seismic source and were compared with data for events in the size range 5 × 1016 ≦ Mo ≦ 1.4 × 1026 dyne-cm. Measurements of the source parameters indicated that, as for earthquakes, the stress drops for the tremors range from 1 to 100 bars and show no consistent dependence on Mo down to Mo = 5 × 1016 dyne-cm.

Source dimensions and stress drops of small earthquakes near Parkfield, California

1984 ◽  
Vol 74 (1) ◽  
pp. 27-40
Author(s):  
M. E. O'Neill
Keyword(s):  
Stress Drop ◽  
Small Region ◽  
Static Stress ◽  
Depth Range ◽  
Zero Crossing ◽  
Source Dimensions ◽  
Duration Magnitude ◽  
Short Period ◽  
Static Stress Drop ◽  
Stress Drops

Abstract Source dimensions and stress drops of 30 small Parkfield, California, earthquakes with coda duration magnitudes between 1.2 and 3.9 have been estimated from measurements on short-period velocity-transducer seismograms. Times from the initial onset to the first zero crossing, corrected for attenuation and instrument response, have been interpreted in terms of a circular source model in which rupture expands radially outward from a point until it stops abruptly at radius a. For each earthquake, duration magnitude MD gave an estimate of seismic moment MO and MO and a together gave an estimate of static stress drop. All 30 earthquakes are located on a 6-km-long segment of the San Andreas fault at a depth range of about 8 to 13 km. Source radius systemically increases with magnitude from about 70 m for events near MD 1.4 to about 600 m for an event of MD 3.9. Static stress drop ranges from about 2 to 30 bars and is not strongly correlated with magnitude. Static stress drop does appear to be spatially dependent; the earthquakes with stress drops greater than 20 bars are concentrated in a small region close to the hypocenter of the magnitude 512 1966 Parkfield earthquake.

Rotation of the principal stress directions due to earthquake faulting and its seismological implications

1995 ◽  
Vol 85 (5) ◽  
pp. 1513-1517
Author(s):  
Z.-M. Yin ◽  
G. C. Rogers
Keyword(s):  
Shear Stress ◽  
Normal Stress ◽  
Principal Stress ◽  
Stress Drop ◽  
Rotation Angle ◽  
Overburden Pressure ◽  
Rupture Area ◽  
Earthquake Faulting ◽  
Stress Directions ◽  
Before And After

Abstract Earthquake faulting results in stress drop over the rupture area. Because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault, the principal stress directions must rotate to adapt such a change of the state of stress. Using two constraints, i.e., the normal stress on the fault and the vertical stress (the overburden pressure), which do not change before and after the earthquake, we derive simple expressions for the rotation angle in the σ1 axis. For a dip-slip earthquake, the rotation angle is only a function of the stress-drop ratio (defined as the ratio of the stress drop to the initial shear stress) and the angle between the σ1 axis and the fault plane, but for a strike-slip earthquake the rotation angle is also a function of the stress ratio. Depending on the faulting regimes, the σ1 axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal. The rotation of the stress field has several important seismological implications. It may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks. The rotation angle can be used to estimate the stress-drop ratio, which has been a long-lasting topic of debate in seismology.

Large Uncertainties in Earthquake Stress-Drop Estimates and Their Tectonic Consequences

2020 ◽  
Vol 91 (4) ◽  
pp. 2320-2329 ◽  
Author(s):  
James S. Neely ◽  
Seth Stein ◽  
Bruce D. Spencer
Keyword(s):  
Seismic Hazard ◽  
Stress Drop ◽  
Hazard Analysis ◽  
Earthquake Hazards ◽  
Teleseismic Data ◽  
Tectonic Environment ◽  
Zero Correlation ◽  
Environment Stress ◽  
Stress Drops ◽  
Erroneous Inferences

Abstract Earthquake stress drop, the stress change on a fault due to an earthquake, is important for seismic hazard analysis because it controls the level of high-frequency ground motions that damage structures. Numerous studies report that stress drops vary by tectonic environment, providing insight into a region’s seismic hazard. Here, we show that teleseismic stress-drop estimates have large uncertainties that make it challenging to distinguish differences between the stress drops of different earthquakes. We compared stress drops for ∼900 earthquakes derived from two independent studies using teleseismic data and found practically zero correlation. Estimates for the same earthquake can differ by orders of magnitude. Therefore, reported stress-drop differences between earthquakes may not reflect true differences. As a result of these larger uncertainties, some tectonic environment stress-drop patterns that appear in one study do not appear in the other analysis of the same earthquakes. These large uncertainties in teleseismic estimates might lead to erroneous inferences about earthquake hazards. In many applications, it may be more appropriate to assume that earthquakes in different regions have approximately the same average stress drop.

Exploiting wind-turbine noise for seismic imaging and monitoring

2020 ◽  
Author(s):  
Elmer Ruigrok ◽  
Lisanne Jagt ◽  
Britt van der Vleut
Keyword(s):  
Wind Turbine ◽  
Seismic Noise ◽  
Seismic Source ◽  
Seismic Network ◽  
Seismic Sources ◽  
The Novel ◽  
Wind Turbine Noise ◽  
Cost Reductions ◽  
Cost Efficient ◽  
Subsurface Monitoring

<p>Wind turbines (WTs) have proven to be an increasingly cost-efficient source of sustainable energy. With further cost reductions and growth of environmental awareness, the amount and size of WTs will further expand. In the seismic literature, WTs have mainly been considered a threat rather than an opportunity. WTs act as infrasound and seismic sources, whose wavefield might overwhelm signal from earthquakes. Rather than focusing on the detrimental effects, we embrace the WT revolution and focus on the novel possibilities of the WT seismic source. We show detailed characteristics of this source using recordings over the Groningen seismic network. We further show examples of using the WT seismic noise for extracting medium parameters. Moreover, we exploit the repeatable nature of the source for subsurface monitoring.</p>

Stress Drop Mapping in the Northern Chilean Subduction Zone

2020 ◽  
Author(s):  
Jonas Folesky ◽  
Joern Kummerow ◽  
Serge A. Shapiro
Keyword(s):  
Subduction Zone ◽  
Stress Drop ◽  
Observation Interval ◽  
Spectral Ratio ◽  
Temporal Variations ◽  
Data Set ◽  
Systematic Analysis ◽  
Megathrust Earthquakes ◽  
Time Period ◽  
Stress Drops

<p>The Northern Chilean subduction zone has been monitored by the IPOC network for more than ten years. During this time period two very large earthquakes occurred, the 2007 M<sub>W</sub>7.7  Tocopilla earthquake and the 2014 M<sub>W</sub>8.1 Iquique earthquake. Over the entire subduction zone a vast amount of seismic activity has been recorded and a huge catalog was compiled including over 100000 events (Sippl et al. 2018). With this exceptional data base we attempt a systematic analysis of the stress drops of as many events from the catalog as possible. We apply different estimation techniques, namely the spectral ratio type, the spectral stacking approach, and the lower bound method. A goal of our research is a comparison and possibly a combination of the techniques to obtain reliable and well constrained results.</p><p>The data set covers events at the interface, within the subducting plate, crustal events, and intermediate depth events. It therefore bears a great potential to better understand the stress drop distribution within a subduction zone. Also, the long observation interval allows to analyze temporal variations according to pre-, inter-, and post-seismic phases of megathrust earthquakes.   </p><p>We present preliminary results where a subset of 730 events with a magnitude range of M<sub>L</sub>2.7 - M<sub>L</sub>4.8  was used for analysis with the spectral ratio technique. For these events we show maps of spatial stress drop variation, and we analyze the time dependent stress drop variance. </p>

Seismic source inversion using Hamiltonian Monte Carlo and a 3-D Earth model in the Japanese Islands

2020 ◽  
Author(s):  
Saulė Simutė ◽  
Lion Krischer ◽  
Christian Boehm ◽  
Martin Vallée ◽  
Andreas Fichtner
Keyword(s):  
Monte Carlo ◽  
Moment Tensor ◽  
Seismic Source ◽  
Monte Carlo Algorithm ◽  
Earth Model ◽  
Model Parameters ◽  
Hamiltonian Monte Carlo ◽  
Double Couple ◽  
Full Moment Tensor ◽  
Japanese Islands

<p>We present a proof-of-concept catalogue of full-waveform seismic source solutions for the Japanese Islands area. Our method is based on the Bayesian inference of source parameters and a tomographically derived heterogeneous Earth model, used to compute Green’s strain tensors. We infer the full moment tensor, location and centroid time of the seismic events in the study area.</p><p>To compute spatial derivatives of Green’s functions, we use a previously derived regional Earth model (Simutė et al., 2016). The model is radially anisotropic, visco-elastic, and fully heterogeneous. It was constructed using full waveforms in the period band of 15–80 s.</p><p>Green’s strains are computed numerically with the spectral-element solver SES3D (Gokhberg & Fichtner, 2016). We exploit reciprocity, and by treating seismic stations as virtual sources we compute and store the wavefield across the domain. This gives us a strain database for all potential source-receiver pairs. We store the wavefield for more than 50 F-net broadband stations (www.fnet.bosai.go.jp). By assuming an impulse response as the source time function, the displacements are then promptly obtained by linear combination of the pre-computed strains scaled by the moment tensor elements.</p><p>With a feasible number of model parameters and the fast forward problem we infer the unknowns in a Bayesian framework. The fully probabilistic approach allows us to obtain uncertainty information as well as inter-parameter trade-offs. The sampling is performed with a variant of the Hamiltonian Monte Carlo algorithm, which we developed previously (Fichtner and Simutė, 2017). We apply an L2 misfit on waveform data, and we work in the period band of 15–80 s.</p><p>We jointly infer three location parameters, timing and moment tensor components. We present two sets of source solutions: 1) full moment tensor solutions, where the trace is free to vary away from zero, and 2) moment tensor solutions with the isotropic part constrained to be zero. In particular, we study events with significant non-double-couple component. Preliminary results of ~Mw 5 shallow to intermediate depth events indicate that proper incorporation of 3-D Earth structure results in solutions becoming more double-couple like. We also find that improving the Global CMT solutions in terms of waveform fit requires a very good 3-D Earth model and is not trivial.</p>

Theoretical far‐field radiation from a low‐frequency horizontal acoustic point force in a vertical borehole

Geophysics ◽  
1986 ◽  
Vol 51 (4) ◽  
pp. 930-939 ◽  
Author(s):  
Andrew L. Kurkjian
Keyword(s):  
Body Force ◽  
Low Frequency ◽  
Vibration Sensor ◽  
Point Force ◽  
Small Radius ◽  
Seismic Sources ◽  
Low Frequencies ◽  
Field Radiation ◽  
Point Body ◽  
Vertical Borehole

A subsurface point body force can be realized using a body force on the axis of a small‐radius borehole drilled perpendicular to the force. This observation, originally made by Kitsunezaki (1980), is having a significant impact in shear‐wave logging, and may potentially affect downhole seismic sources. Applying the principle of reciprocity to this finding, a horizontal vibration sensor on the axis of a vertical hole will be unaffected by the existence of the hole at low frequencies. Numerical methods determine both the frequency below which, and the offset beyond which, borehole‐related effects are negligible. If the shear wavelength is greater than ten times the diameter of the hole and if the measurement is made at least one shear wavelength from the point force, then the borehole effects will be minimal.

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