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Gregory C. Beroza - One of the best experts on this subject based on the ideXlab platform.
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evidence for widespread nonlinear strong ground motion in the mw 6 9 Loma Prieta Earthquake
Bulletin of the Seismological Society of America, 2004Co-Authors: Justin L Rubinstein, Gregory C. BerozaAbstract:We exploit 55 repeating microEarthquake sequences on the San Andreas Fault, just south of the rupture zone of the 1989 M W 6.9 Loma Prieta Earthquake, to search for time-dependent properties of the Earth's crust. Using moving window waveform cross correlation, we identify clear and systematic delays as large as 20 msec for the direct S wave and exceeding 50 msec in the early S -wave coda following the Loma Prieta mainshock. Others have also identified phase delays (velocity reductions) associated with damaging Earthquakes and they have suggested a myriad of possible causal mechanisms. Here, we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an Earthquake. A strong correlation between the spatial patterns of S delays and the intensity of strong ground motion in the Loma Prieta Earthquake suggests that physical damage, the formation or growth of cracks during strong ground motion, to the Earth's shallow crust is responsible for the observed velocity reductions. The strong spatial variability in S delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver. The effect is stronger at stations on young, soft rocks than at stations on old, hard rock. Disproportionately larger S coda delays than P coda delays suggest that the cracks formed by the strong shaking are fluid filled. In the 10 years after Loma Prieta, the initial increase in travel times reduces logarithmically with respect to time, often back to the premainshock levels. We attribute this behavior to the same “slow dynamic” healing observed in laboratory studies of the recovery of materials from transient nonlinear strain. Manuscript received 14 January 2004.
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Stability of coda wave attenuation during the Loma Prieta, California, Earthquake sequence
Journal of Geophysical Research: Solid Earth, 1995Co-Authors: Gregory C. Beroza, Alex Cole, William L. EllsworthAbstract:The Loma Prieta, California, Earthquake occurred in a densely instrumented region with a history of microEarthquake recording beginning more than a decade before the October 1989 mainshock. This affords an unprecedented opportunity to detect changes in seismic wave propagation in the Earth's crust associated with a major Earthquake. In this study we use pairs of nearly identical Earthquakes (doublets) to search for temporal changes of coda attenuation in the vicinity of the Loma Prieta Earthquake. We analyze 21 Earthquake doublets recorded from 1978 to 1991 that span the preseismic, coseismic, and postseismic intervals and measure the change in coda Q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 Hz. This method provides an estimate of changes in coda Q that is insensitive to other factors that influence coda amplitudes. Our observations place an upper bound of about 5% on preseismic, coseismic, and postseismic changes of coda Q in the epicentral region of the Loma Prieta Earthquake. Even at this low level, the changes are neither spatially coherent nor correlated between adjacent frequency bands. The only hint of a signal is in the preseismic data where there is a possible precursory increase in coda Q of approximately 5% in the two years before the mainshock. The stability of coda Q throughout the Loma Prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda Q for other Earthquakes.
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Comparison of ultra-low frequency electromagnetic signals with aftershock activity during the 1989 Loma Prieta Earthquake sequence
Bulletin of the Seismological Society of America, 1993Co-Authors: M. A. Fenoglio, Gregory C. Beroza, Antony C. Fraser-smith, M. J. S. JohnstonAbstract:Abstract Ultra-low frequency (0.01 to 10.0 Hz) magnetic field fluctuations near the epicenter of the 1989 Loma Prieta Earthquake rose sharply immediately before the Earthquake following indications of increased disturbance during the previous 12 days. The magnetic activity remained much higher than the pre-Earthquake background level for 6 weeks following the mainshock. These observations suggest a causal relationship between the Earthquake failure process and the magnetic signals. A search for similar precursory electromagnetic signals associated with aftershocks of this Earthquake yields negative results. Specifically, no correlation appears to exist between the amplitude of the electromagnetic activity and the frequency or magnitude of aftershocks following the mainshock. Either a “threshold” Earthquake magnitude larger, in this case, than M L 5.5, may be necessary to generate precursory electromagnetic signals or the continued generation of magnetic signals related to the mainshock may have masked signals generated by the larger aftershocks.
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evidence for near frictionless faulting in the 1989 m 6 9 Loma Prieta california Earthquake and its aftershocks
Geology, 1993Co-Authors: Mark D Zoback, Gregory C. BerozaAbstract:Analysis of the unusually diverse right-lateral, left-lateral, reverse, and normal faulting aftershocks of the Loma Prieta Earthquake reveals a systematic relation between fault-plane orientation and slip direction; right-lateral aftershock planes strike slightly more north-ward (10°-15°) than the main shock, left-lateral planes strike slightly more westward, reverse planes dip slightly less steeply, and normal planes dip slightly more steeply than the main shock. We demonstrate that these slip patterns are consistent with an approximately uniaxial stress field acting nearly perpendicular to the main-shock fault plane. These results imply an extremely weak fault zone, possibly resulting from near-lithostatic pore pressure.
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mechanism diversity of the Loma Prieta aftershocks and the mechanics of mainshock aftershock interaction
Science, 1993Co-Authors: Gregory C. Beroza, Mark D ZobackAbstract:The diverse aftershock sequence of the 1989 Loma Prieta Earthquake is inconsistent with conventional models of mainshock-aftershock interaction because the aftershocks do not accommodate mainshock-induced stress changes. Instead, the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane. This orientation implies that (i) stress drop in the mainshock was nearly complete, (ii) mainshock-induced decreases of fault strength helped were important in controlling the occurrence of after-shocks, and (iii) mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip.
A Bernardi - One of the best experts on this subject based on the ideXlab platform.
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results of ulf magnetic field measurements near the epicenters of the spitak ms 6 9 and Loma Prieta ms 7 1 Earthquakes comparative analysis
Geophysical Research Letters, 1992Co-Authors: O A Molchanov, Yu A Kopytenko, P M Voronov, E A Kopytenko, T G Matiashvili, A C Frasersmith, A BernardiAbstract:The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 Earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 7.1 Earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak Earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta Earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock; (3) ULF activity remained high for about two weeks after the Spitak Earthquake, and for several months after the Loma Prieta Earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the after-shocks. A major difference in the ULF activity preceding the two Earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.
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ulf magnetic field measurements near the epicenter of the ms 7 1 Loma Prieta Earthquake
Physics of the Earth and Planetary Interiors, 1991Co-Authors: A Bernardi, A C Frasersmith, P R Mcgill, O G VillardAbstract:Abstract Anomalous measurements of the ultra-low frequency (ULF) magnetic field fluctuations prior to the magnitude 7.1 Loma Prieta Earthquake of October 17, 1989, have been studied. For the past few years we have been monitoring fluctuations of the magnetic field of the Earth in the ULF range at Corralitos, California; our instruments were located 7 km from the epicenter of the Earthquake. We have observed four anomalies in our data which may turn out to be precursors to the Earthquake. First we observe narrowband noise fluctuations centered at 0.1 Hz and estimated to have a bandwidth of 0.00143–0.00167 Hz, i.e. a Q value of 60–70. The narrowband fluctuations appear to have a maximum equivalent amplitude of over 1400 pT/√Hz which is roughly 31 dB higher than the typical quiet average amplitude background levels. These fluctuations begin around September 12 and last until October 5. Next we observe the appearance of additive wideband noise fluctuations beginning around October 5 and continuing until the occurrence of the Earthquake. These wideband fluctuations, which cover almost the entire 0.01–10 Hz frequency range of the system, have an average amplitude that is approximately 19 dB larger than typical levels in the 0.01–0.02 Hz band. Thirdly, we observe an atypical decrease in noise levels in the 0.2–5 Hz band throughout the day prior to the Earthquake. The fourth anomaly is a jump in the power of magnetic field fluctuations, mostly in the 0.01–0.5 Hz band, in the three hours preceding the Earthquake. This activity reached its highest level in the lowest 0.01–0.02 Hz band, and had a magnitude of roughly 60000 pT/√Hz, which is about 40 dB larger than typical background noise levels in the band. Our anomalous measurements do not appear to be the result of any magnetic field fluctuations generated in the upper atmosphere or to movement of our sensor caused by shocks preceding the quake. In describing these anomalous magnetic field fluctuations, both electrokinetic and piezoelectric theories of the generation of magnetic field fluctuations prior to Earthquakes suggest wideband fluctuations. In our measurements, we have observed both wideband and narrowband fluctuations, so that our narrowband measurements do not seem to be easily explicable by the above theories. However, the wideband fluctuations could turn out to be attributable to the above mechanisms, since at the monitored frequencies, the skin depths of electromagnetic waves are comparable with the distances from our sensors to the Earthquake focus, and therefore our measurements would not be unexpected.
Qinya Liu - One of the best experts on this subject based on the ideXlab platform.
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time evolving seismic tomography the method and its application to the 1989 Loma Prieta and 2014 south napa Earthquake area california
Geophysical Research Letters, 2017Co-Authors: Ping Tong, Dinghui Yang, Qinya LiuAbstract:We propose a time-evolving approach to conduct traveltime seismic tomography in the 1989 Mw 6.9 Loma Prieta Earthquake and 2014 Mw 6.0 South Napa Earthquake area, California. The recording period of the chosen seismic data between January 1, 1967 and the day before the 2014 South Napa Earthquake is divided into two time windows, separated by the 1989 Loma Prieta Earthquake. In each time window the subsurface velocity structure is iteratively updated. Starting from the final model of the first time window, the velocity model has been successively improved throughout iterations in the second time window, indicating that the traveltime data of later time windows have provided extra information to refine the subsurface images. Strong heterogeneities are observed in the final P-wave velocity model. Both of the two large Earthquakes occurred at transition zones in between high Vp and low Vp anomalies. In all, this study shows the effectiveness of the time-evolving seismic tomography method.
Ping Tong - One of the best experts on this subject based on the ideXlab platform.
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time evolving seismic tomography the method and its application to the 1989 Loma Prieta and 2014 south napa Earthquake area california
Geophysical Research Letters, 2017Co-Authors: Ping Tong, Dinghui Yang, Qinya LiuAbstract:We propose a time-evolving approach to conduct traveltime seismic tomography in the 1989 Mw 6.9 Loma Prieta Earthquake and 2014 Mw 6.0 South Napa Earthquake area, California. The recording period of the chosen seismic data between January 1, 1967 and the day before the 2014 South Napa Earthquake is divided into two time windows, separated by the 1989 Loma Prieta Earthquake. In each time window the subsurface velocity structure is iteratively updated. Starting from the final model of the first time window, the velocity model has been successively improved throughout iterations in the second time window, indicating that the traveltime data of later time windows have provided extra information to refine the subsurface images. Strong heterogeneities are observed in the final P-wave velocity model. Both of the two large Earthquakes occurred at transition zones in between high Vp and low Vp anomalies. In all, this study shows the effectiveness of the time-evolving seismic tomography method.
A C Frasersmith - One of the best experts on this subject based on the ideXlab platform.
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results of ulf magnetic field measurements near the epicenters of the spitak ms 6 9 and Loma Prieta ms 7 1 Earthquakes comparative analysis
Geophysical Research Letters, 1992Co-Authors: O A Molchanov, Yu A Kopytenko, P M Voronov, E A Kopytenko, T G Matiashvili, A C Frasersmith, A BernardiAbstract:The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 Earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 7.1 Earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak Earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta Earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock; (3) ULF activity remained high for about two weeks after the Spitak Earthquake, and for several months after the Loma Prieta Earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the after-shocks. A major difference in the ULF activity preceding the two Earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.
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ulf magnetic field measurements near the epicenter of the ms 7 1 Loma Prieta Earthquake
Physics of the Earth and Planetary Interiors, 1991Co-Authors: A Bernardi, A C Frasersmith, P R Mcgill, O G VillardAbstract:Abstract Anomalous measurements of the ultra-low frequency (ULF) magnetic field fluctuations prior to the magnitude 7.1 Loma Prieta Earthquake of October 17, 1989, have been studied. For the past few years we have been monitoring fluctuations of the magnetic field of the Earth in the ULF range at Corralitos, California; our instruments were located 7 km from the epicenter of the Earthquake. We have observed four anomalies in our data which may turn out to be precursors to the Earthquake. First we observe narrowband noise fluctuations centered at 0.1 Hz and estimated to have a bandwidth of 0.00143–0.00167 Hz, i.e. a Q value of 60–70. The narrowband fluctuations appear to have a maximum equivalent amplitude of over 1400 pT/√Hz which is roughly 31 dB higher than the typical quiet average amplitude background levels. These fluctuations begin around September 12 and last until October 5. Next we observe the appearance of additive wideband noise fluctuations beginning around October 5 and continuing until the occurrence of the Earthquake. These wideband fluctuations, which cover almost the entire 0.01–10 Hz frequency range of the system, have an average amplitude that is approximately 19 dB larger than typical levels in the 0.01–0.02 Hz band. Thirdly, we observe an atypical decrease in noise levels in the 0.2–5 Hz band throughout the day prior to the Earthquake. The fourth anomaly is a jump in the power of magnetic field fluctuations, mostly in the 0.01–0.5 Hz band, in the three hours preceding the Earthquake. This activity reached its highest level in the lowest 0.01–0.02 Hz band, and had a magnitude of roughly 60000 pT/√Hz, which is about 40 dB larger than typical background noise levels in the band. Our anomalous measurements do not appear to be the result of any magnetic field fluctuations generated in the upper atmosphere or to movement of our sensor caused by shocks preceding the quake. In describing these anomalous magnetic field fluctuations, both electrokinetic and piezoelectric theories of the generation of magnetic field fluctuations prior to Earthquakes suggest wideband fluctuations. In our measurements, we have observed both wideband and narrowband fluctuations, so that our narrowband measurements do not seem to be easily explicable by the above theories. However, the wideband fluctuations could turn out to be attributable to the above mechanisms, since at the monitored frequencies, the skin depths of electromagnetic waves are comparable with the distances from our sensors to the Earthquake focus, and therefore our measurements would not be unexpected.
