scholarly journals Determination of rupture duration and stress drop for earthquakes in southern California

1983 ◽  
Vol 73 (6A) ◽  
pp. 1527-1551
Author(s):  
Arthur Frankel ◽  
Hiroo Kanamori
Keyword(s):  
Stress Drop ◽  
Pulse Width ◽  
Southern California ◽  
Main Shock ◽  
P Wave ◽  
Fault System ◽  
Zero Crossing ◽  
Main Shocks ◽  
Seismic Networks ◽  
Rupture Duration

Abstract A simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3.5 and 4.0 using the time between the P-wave onset and the first zero crossing (τ1/2) on seismograms from local seismic networks. This method is applied to 10 main shocks in southern California to investigate regional variations in stress drop. The initial pulse widths of 65 foreshocks or aftershocks of these events were measured. Values of τ1/2 for small earthquakes below about magnitude 2.2 are generally observed to remain constant with decreasing magnitude in four sequences studied. The relative pulse width of a particular main shock (M ≧ 3.5) at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station. These observations are interpreted to signify that the waveforms of these small events (M ≦ 2.2) are essentially the impulse response of the path between the source and receiver. Values of τ1/2 determined from small foreshocks and aftershocks are, therefore, subtracted (in effect deconvolved) from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects. Significant variations in rupture duration and stress drop are observed for the main shocks studied. Aftershock locations and azimuthal variations in τ1/2 both indicate that the rupture zone of one earthquake expanded unilaterally. A factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the San Jacinto fault system. The first event in this pair had the highest stress drop of the events studied (860 bars) and was followed within 8 months by a magnitude 5.5 earthquake 2 km away.

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

1976 ◽  
Vol 66 (6) ◽  
pp. 1931-1952
Author(s):  
Donald J. Stierman ◽  
William L. Ellsworth
Keyword(s):  
Fault Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Body Wave ◽  
P Wave ◽  
Fault System ◽  
Wave Radiation ◽  
Fault Plane Solutions ◽  
Complex Deformation ◽  
Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location

1996 ◽  
Vol 86 (2) ◽  
pp. 470-476 ◽  
Author(s):  
Cheng-Horng Lin ◽  
S. W. Roecker
Keyword(s):  
Southern California ◽  
Seismic Array ◽  
P Wave ◽  
Small Aperture ◽  
Data Set ◽  
Single Interface ◽  
Seismic Networks ◽  
Dense Seismic Array ◽  
Beamforming Technique

Abstract Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

P-wave spectra for ML 5 foreshocks, aftershocks, and isolated earthquakes near Parkfield, California

1981 ◽  
Vol 71 (2) ◽  
pp. 423-436
Author(s):  
Willian H. Bakun ◽  
Thomas V. McEvilly
Keyword(s):  
Stress Drop ◽  
High Stress ◽  
P Wave ◽  
Wave Radiation ◽  
Focal Region ◽  
Wave Spectra ◽  
Rupture Length ◽  
Main Shocks ◽  
Relative Stress ◽  
Fault Trace

abstract Wood-Anderson seismograms recorded at Mount Hamilton (MHC, 185 km, 327°), Santa Barbara (SBC, 180 km, 158°), and Tinemaha (TIN, 240 km, 56°) provide data for comparing P-wave spectra for two immediate (17-min) foreshocks, one early (55-hr) foreshock, two aftershocks, and two “isolated” Parkfield earthquakes. All are ML 5.0 shocks with epicenters within 7 km of the common epicenter of the 1934 and 1966 Parkfield main shocks. The set of events is well suited for testing the hypothesis that foreshocks are high-stress-drop sources. Calculated stress drops are controlled by source directivity at azimuths aligned with the fault break (at MHC and SBC). P-wave radiation from the three foreshocks is focused along one fault trace azimuth, suggesting that foreshock sources are characterized by pronounced unilateral rupture expansion. At TIN, broadside to the fault where directivity has minimum effect on calculated relative stress drop, the two immediate foreshocks are higher stress-drop sources. The early foreshock is a low-to-average stress-drop source, indicating the possibility that stress concentration is a rapidly occurring phenomenon in rupture nucleation. Alternatively, the stress field is highly variable on the scale of 2 to 3 km in the focal region of an impending earthquake with a rupture length of 20 to 30 km.

Seismotectonics of the northern Longitudinal Valley, Taiwan, inferred from aftershock sequences of 2018 Mw6.4 and 2019 Mw6.2 Hualien earthquakes

2020 ◽  
Author(s):  
Wei-Fang Sun ◽  
Hao Kuo-Chen ◽  
Zhuo-Kang Guan ◽  
Wen-Yen Chang
Keyword(s):  
Main Shock ◽  
Surface Deformation ◽  
Plate Boundary ◽  
Aftershock Sequence ◽  
Tectonic Structures ◽  
Seismicity Rate ◽  
Transitional Area ◽  
Main Shocks ◽  
Aftershock Sequences ◽  
Seismic Networks

<p>In the Hualien area, two Mw6.4 and Mw6.2 earthquakes, 20 km apart, occurred in February 2018 and April 2019 respectively. The former to the northeast, located offshore to ​​the Liwu river, triggered several earthquake clusters along the Milun fault and southward to the Longitudinal Valley, the suture of the Eurasian and the Philippine Sea plates; the latter to the southwest, located in the Central Range, also triggered several seismic swarms in the Central Range,  along the Liwu river to the northeast and at Ji'an to the southeast. Except for the Milun fault, neither GPS nor InSAR observations detects significant surface deformation after the occurrence of these two main shocks, indicating that the earthquake ruptures mainly developed within the crust. Therefore, seismic observation becomes an efficient tool for revealing the seismotectonics of the two earthquake sequences. For monitoring the aftershock sequences, two days after the main shocks, we deployed two geophone arrays, 70 Z-component RefTek 125A TEXANs for two weeks in 2018 and 47 three-component Fairfield Nodal Z-Lands for one month in 2019, with 1-5 km station spacing around the Hualien City. These earthquake swarms were well recorded and analyzed through the dense seismic networks. The numbers of aftershock sequences manually identified are two-fold more than that issued by the Central Weather Bureau, Taiwan. The seismicity of the 2018 aftershock sequence, to depths of between 5-15 km, was significantly reduced within 10 days after the main shock. however, the seismicity of the 2019 aftershock sequence, to depths of between 2-50 km, was still above background seismicity rate 30 days after the main shock. The spatial distribution of the 2018 aftershock sequence could be related to a fault zone of the plate boundary, but that of the 2019 and the relocated 1986 aftershock sequences show a conjugate thrust fault pair beneath the eastern Central Range. Our results clearly depict several local tectonic structures that have not been observed at the northern tip of the Longitudinal Valley, not only a suture but also a transitional area from collision to subduction.</p>

Source dynamics of the Dasht-e Bayāz earthquake of August 31, 1968

1969 ◽  
Vol 59 (5) ◽  
pp. 1843-1861
Author(s):  
Mansour Niazi
Keyword(s):  
Stress Drop ◽  
Main Shock ◽  
Mean Value ◽  
The Body ◽  
P Wave ◽  
Polarization Angle ◽  
S Wave ◽  
Long Duration ◽  
First Motion ◽  
Complicated Pattern

abstract Radiation patterns of the P-wave first motion and S-wave polarization angle of the Dasht-e Bayāz earthquake of August 31, 1968, as well as its principal aftershock which occurred about 20 hours after the main shock are studied. The main shock data are consistent with the observed left-lateral strike-slip fault which accompanied it. The radiation pattern of the aftershock differs somewhat from that of the main shock and agrees with the directions of the secondary faulting in the area. Several lines of evidence pointing to a multiple source for the main shock are presented. They include complexity of the body phases, low value of the rupture speed as studied from the analysis of the surface wave spectra, reported long duration of shaking and complicated pattern of striations produced by faulting. Energy, moment and stress drop associated with the main shock are estimated. The resulting mean value of stress drop over the faulted surface has a range of 40-100 bars. Based on the age of some well-built structures in the area, it is proposed that no earthquake as severe as the recent one has occurred near the location of the August 31, 1968 earthquake during the last 800 years.

Empirical Green's functions: A comparison between pulse width measurements and deconvolution by spectral division

1999 ◽  
Vol 89 (1) ◽  
pp. 178-189
Author(s):  
Nicholas Deichmann
Keyword(s):  
Initial Phase ◽  
Pulse Width ◽  
Circular Crack ◽  
P Wave ◽  
Far Field ◽  
Velocity Pulse ◽  
Zero Crossing ◽  
Northern Switzerland ◽  
Field Displacement ◽  
Width Measurements

Abstract Data from a microearthquake cluster in northern Switzerland and synthetic seismograms simulating the observed signals are used to compare two different techniques of obtaining information about earthquake source-time functions. Comparisons between the observed P-wave velocity pulse widths and the rise times of far-field displacement pulses obtained from empirical Green's function (EGF) deconvolutions show significant discrepancies. Whereas the observed velocity pulse widths of the larger events scale with seismic moment over a broad range, this scaling is practically lost in the deconvolutions. The reason is that velocity pulse widths are usually measured at high trace magnifications from the first break to the first zero crossing. At lower magnifications, these pulse widths are seen to include an emergent onset, which can be attributed to an initial phase of gradual rupture acceleration and whose duration scales with moment. Synthetic simulations, based on a source model of a circular crack with constant stress drop and rupture propagating outward from the center with a gradually increasing velocity, correctly reproduce these emergent onsets. Deconvolutions using the synthetic signals show that the slow initial phase is masked by the noise amplification and stabilizing measures inherent in the deconvolution. Therefore, despite the uncertainties in the necessary corrections for attenuation and scattering along the path, relative pulse width measurements are more reliable and provide better resolution for small earthquakes than rise-time measurements on far-field displacement pulses obtained from EGF deconvolutions by spectral division.

High-frequency spectral scaling of a main shock/ aftershock sequence near the Norwegian coast

1988 ◽  
Vol 78 (2) ◽  
pp. 561-570
Author(s):  
Eric P. Chael ◽  
Richard P. Kromer
Keyword(s):  
Stress Drop ◽  
High Frequency ◽  
Background Noise ◽  
Main Shock ◽  
Source Model ◽  
Aftershock Sequence ◽  
Constant Stress ◽  
P Wave ◽  
Western Coast ◽  
Seismic Signals

Abstract A high-frequency seismic element was recently added to the NORESS regional array in Norway. This system can monitor seismic signals at frequencies up to 50 Hz. In February 1986, the high-frequency seismic element recorded an mbLg 4.7 main shock and several aftershocks which occurred 420 km northwest of NORESS, off Norway's western coast. These events produced high-frequency signals which were well above the background noise at the station. P-wave spectra of these events scale in a manner consistent with the ω-square, constant-stress-drop source model. The data do not require any change in this scaling to magnitudes (mbLg) below 2, in contrast to previous reports that constant-stress-drop scaling breaks down at smaller magnitudes.

scholarly journals Seismic Damage Accumulation in Highway Bridges in Earthquake-Prone Regions

2015 ◽  
Vol 31 (1) ◽  
pp. 115-135 ◽  
Author(s):  
Jayadipta Ghosh ◽  
Jamie E. Padgett ◽  
Mauricio Sánchez-Silva
Keyword(s):  
Service Life ◽  
Damage Accumulation ◽  
Main Shock ◽  
Prediction Models ◽  
Time Window ◽  
Damage Index ◽  
Active Regions ◽  
Highway Bridges ◽  
Main Shocks ◽  
Aftershock Sequences

Civil infrastructures, such as highway bridges, located in seismically active regions are often subjected to multiple earthquakes, including multiple main shocks during their service life or main shock–aftershock sequences. Repeated seismic events result in reduced structural capacity and may lead to bridge collapse, causing disruption in the normal functioning of transportation networks. This study proposes a framework to predict damage accumulation in structures subjected to multiple shock scenarios after developing damage index prediction models and accounting for the probabilistic nature of the hazard. The versatility of the proposed framework is demonstrated on a case-study highway bridge located in California for two distinct hazard scenarios: (1) multiple main shocks during the service life and (2) multiple aftershock earthquake occurrences following a single main shock. Results reveal that in both cases there is a significant increase in damage index exceedance probabilities due to repeated shocks within the time window of interest.

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