Short- and long-period subevents of the 4 February 1965 Rat Islands earthquake

1984 ◽  
Vol 74 (4) ◽  
pp. 1331-1347
Author(s):  
Jim Mori
Keyword(s):  
Wave Velocity ◽  
Fault Plane ◽  
High Stress ◽  
P Wave ◽  
Long Period ◽  
P Wave Velocity ◽  
Short Period ◽  
Main Fault ◽  
The Times ◽  
Stress Drops

Abstract Short- and long-period records of the P wave of the 1965 Rat Islands earthquake were analyzed to locate subevents within the main rupture. Four subevents were identified on the short-period records in the first 100 sec and on the two long-period records in the first 30 sec. The short-period subevents cluster in an area 100 km south of the initial epicenter which appears to be off of the main fault plane, an area in which two larger aftershocks have relatively high stress drops. The long-period subevents are located 90 km west of the initial epicenter. The times and locations of the first short- and long-period subevents indicate they were triggered by a front moving near the P-wave velocity.

scholarly journals Broadband time domain modeling of earthquakes from Friuli, Italy

1981 ◽  
Vol 71 (4) ◽  
pp. 1215-1231
Author(s):  
John Cipar
Keyword(s):  
Time Domain ◽  
Main Shock ◽  
Waveform Inversion ◽  
High Stress ◽  
Point Sources ◽  
Domain Modeling ◽  
Long Period ◽  
Short Period ◽  
Stress Drops ◽  
Period Instruments

abstract Short-period (SP) and long-period (LP) seismograms written by the main shock and two principal aftershocks of the 1976 Friuli, Italy, earthquake sequence are modeled in the time domain using synthetic seismograms. The main shock occurred on 6 May 1976 (20h 00m, Ms = 6.5) and both aftershocks on 15 September 1976 (03h 15m, Ms = 6.0 and 09h 21m, Ms = 5.9). Source models were determined initially by trial and error and then refined using a waveform inversion program. Two point sources of radiation are required to adequately model the aftershock short-period records. For the 09h 21m aftershock, the model derived from short-period records also produces good fits to the long-period data. The seismic moment of this earthquake is found to be 0.8 to 1.0 × 1025 dyne-cm. The SP model for the 03h 15m aftershock, on the other hand, predicts long-period synthetics which do not agree with the observations. In particular, the SP moment (0.37 × 1025 dyne-cm) is about 212 times smaller than the LP moment (1 × 1025 dyne-cm). Adding a long-period component to the SP model considerably improves LP waveform and moment agreement. In the case of the main shock, a reasonable fit to the observed SP data is obtained using three point sources of radiation. However, LP synthetics computed using this model do not agree with the observations, and the SP moment (0.65 × 1025 dyne-cm) is a small fraction of the LP moment (3 to 5 × 1025 dyne-cm). Time function durations indicate that the individual events inferred from the SP records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars. In comparison, stress drops estimated from LP data are found to be 12 bars (main shock) and 24 bars (09h 21m aftershock). These observations suggest that the short-period instruments are sensitive to the high-frequency radiation emitted from small, high-stress drop areas on the fault plane whereas the long-period instruments record the overall motion during the earthquake.

scholarly journals Comparing ray-theoretical and finite-frequency teleseismic traveltimes: implications for constraining the ratio of S-wave to P-wave velocity variations in the lower mantle

2020 ◽  
Vol 224 (3) ◽  
pp. 1540-1552
Author(s):  
Carlos A M Chaves ◽  
Jeroen Ritsema ◽  
Paula Koelemeijer
Keyword(s):  
North America ◽  
Wave Velocity ◽  
Lower Mantle ◽  
P Wave ◽  
Finite Frequency ◽  
S Wave ◽  
P Waves ◽  
Long Period ◽  
P Wave Velocity ◽  
The Pacific

SUMMARY A number of seismological studies have indicated that the ratio R of S-wave and P-wave velocity perturbations increases to 3–4 in the lower mantle with the highest values in the large low-velocity provinces (LLVPs) beneath Africa and the central Pacific. Traveltime constraints on R are based primarily on ray-theoretical modelling of delay times of P waves (ΔTP) and S waves (ΔTS), even for measurements derived from long-period waveforms and core-diffracted waves for which ray theory (RT) is deemed inaccurate. Along with a published set of traveltime delays, we compare predicted values of ΔTP, ΔTS, and the ΔTS/ΔTP ratio for RT and finite-frequency (FF) theory to determine the resolvability of R in the lower mantle. We determine the FF predictions of ΔTP and ΔTS using cross-correlation methods applied to spectral-element method waveforms, analogous to the analysis of recorded waveforms, and by integration using FF sensitivity kernels. Our calculations indicate that RT and FF predict a similar variation of the ΔTS/ΔTP ratio when R increases linearly with depth in the mantle. However, variations of R in relatively thin layers (< 400 km) are poorly resolved using long-period data (T > 20 s). This is because FF predicts that ΔTP and ΔTS vary smoothly with epicentral distance even when vertical P-wave and S-wave gradients change abruptly. Our waveform simulations also show that the estimate of R for the Pacific LLVP is strongly affected by velocity structure shallower in the mantle. If R increases with depth in the mantle, which appears to be a robust inference, the acceleration of P waves in the lithosphere beneath eastern North America and the high-velocity Farallon anomaly negates the P-wave deceleration in the LLVP. This results in a ΔTP of about 0, whereas ΔTS is positive. Consequently, the recorded high ΔTS/ΔTP for events in the southwest Pacific and stations in North America may be misinterpreted as an anomalously high R for the Pacific LLVP.

scholarly journals P-wave complexity and fault asperities: The Borrego Mountain, California, earthquake of 1968

1982 ◽  
Vol 72 (2) ◽  
pp. 413-437 ◽  
Author(s):  
John E. Ebel ◽  
Donald V. Helmberger
Keyword(s):  
Strong Motion ◽  
Source Model ◽  
P Wave ◽  
Aftershock Activity ◽  
P Waves ◽  
Deconvolution Analysis ◽  
Long Period ◽  
Short Period ◽  
Forward And Inverse Modeling ◽  
Stress Drops

Abstract Results from a synthetic seismogram analysis of the short-period P waves from the Borrego Mountain earthquake of 9 April 1968 (ML = 6.4) are used to model the strong-motion recording at El Centro. A short-period-long-period deconvolution analysis of the teleseismic P waves suggested that a two-source model would fit the data much better than the single-source model presented by Burdick and Mellman (1976). Forward and inverse modeling of the data demonstrated that two sources, each of less than 2-sec duration, the second occurring 2.2 sec after the first and both being at about 8-km depth, best fit the short-period waveforms. From this model, long-period synthetics were generated which were found to be quite compatible with the data. This source model was also used to synthesize the strong-motion SH displacement, velocity, and acceleration records from El Centro, California. The close match of synthetics and data is used to argue that short-period waveforms contain much information about asperities which play a crucial role in the near-source strong motions from an earthquake. The Borrego Mountain event probably began with the failure of a fault asperity. The evidence for this is the several-hundred-bars stress drops of the two short-period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault.

scholarly journals Geophysical and analytical determination of overstressed zones in exploited coal seam: a case study

2021 ◽  
Author(s):  
Dariusz Chlebowski ◽  
Zbigniew Burtan
Keyword(s):  
Coal Seam ◽  
Wave Velocity ◽  
Seismic Tomography ◽  
Analytical Modeling ◽  
Coal Mines ◽  
Vertical Stress ◽  
P Wave ◽  
Rockburst Hazard ◽  
State Of Stress ◽  
P Wave Velocity

AbstractA variety of geophysical methods and analytical modeling are applied to determine the rockburst hazard in Polish coal mines. In particularly unfavorable local conditions, seismic profiling, active/passive seismic tomography, as well as analytical state of stress calculating methods are recommended. They are helpful in verifying the reliability of rockburst hazard forecasts. In the article, the combined analysis of the state of stress determined by active seismic tomography and analytical modeling was conducted taking into account the relationship between the location of stress concentration zones and the level of rockburst hazard. A longwall panel in the coal seam 501 at a depth of ca.700 m in one of the hard coal mines operating in the Upper Silesian Coal Basin was a subject of the analysis. The seismic tomography was applied for the reconstruction of P-wave velocity fields. The analytical modeling was used to calculate the vertical stress states basing on classical solutions offered by rock mechanics. The variability of the P-wave velocity field and location of seismic anomaly in the coal seam in relation to the calculated vertical stress field arising in the mined coal seam served to assess of rockburst hazard. The applied methods partially proved their adequacy in practical applications, providing valuable information on the design and performance of mining operations.

Crustal P-wave velocity structure and earthquake distribution in the Jiaodong Peninsula, China

2021 ◽  
pp. 228973
Author(s):  
Junhao Qu ◽  
Stephen S. Gao ◽  
Changzai Wang ◽  
Kelly H. Liu ◽  
Shaohui Zhou ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Velocity Structure ◽  
P Wave ◽  
Jiaodong Peninsula ◽  
P Wave Velocity ◽  
Earthquake Distribution ◽  
P Wave Velocity Structure
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