Seismogenic Rupture Research Articles

Seismogenic structure and rupture characteristics of the M S=4.8 Jingyang earthquake

Introduction An earthquake of MS=4.8 occurred near Yongle town, Jingyang county, Shaanxi Province on January 5, 1998. Its epicenter is 34.5N, 108.9E, origin time is 09h36min (Beijing time), focal depth is 14 km, intensity is >VI and its perceptibility is great in Xin city. The eastern part of Guanzhong in Shaanxi Province is the key earthquake monitoring area in China, and many great earthquakes occurred here in history. This earthquake is the greatest in the past forty years in Shaanxi Province, therefore great attentions have been given by the seismologists in Shaanxi Province and other parts in China.

Subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake

The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century, with a recurrence time of 100 to 200 years. The 1946 Nankaido earthquake was unusual, with a rupture zone estimated from long-period geodetic data that was more than twice as large as that derived from shorter period seismic data. In the center of this earthquake rupture zone, we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers. We propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture.

Modeling of strong ground motions from the 16 September 1978 Tabas, Iran, earthquake

AbstractWe have examined the adequacy of the published KS3 and L1 slip distribution models developed for the MS = 7.4 16 September 1978 Tabas earthquake (Hartzell and Mendoza, 1991) by modeling the high-frequency accelerograms recorded at Dayhook, Boshrooyeh, and Tabas stations. The agreement in peak ground acceleration (PGA) and duration between data and simulated accelerograms was chosen as the criterion for the model adequacy. The fault was specified with a seismogenic rupture area 95 km long and 45 km wide. We used a semi-empirical simulation method in which accelerograms from an aftershock from the 1979 Imperial Valley earthquake were used to represent the radiation pattern of P, SV, or SH waves for a source-receiver geometry. The fault surface is divided into many subfaults. The contributions from these subfaults, weighted by the slip amounts, were lagged and summed to simulate the accelerograms of the main event. We computed accelerograms assuming constant rake and variable rake on the fault plane. The former was simulated using the SUM slip model (i.e., the slip model representing the vector sum of the slip components at every point on the fault), and the latter was simulated by summing the accelerograms generated by the slip models of the dip-slip and strike-slip faults. The influence of the rise time of the main event was investigated. The KS3 asperity model produces results marginally better than the L1 asperity model. The duration is predicted, consistent with the data. The peak amplitudes of the simulated ground motions remain within a factor of 1.5 and 3 of the recorded data at Dayhook and Boshrooyeh, respectively, for the variable-rake and within a factor for 2.5 at both stations for the fixed-rake angle (KS3 model). The large PGA recorded on the Tabas accelerograms could not be reproduced, which warrants additional study. A notable agreement was obtained in the response spectra at Dayhook. This study demonstrates that accelerograms generated by an earthquake in one tectonic region can be transported to another tectonic region so that they can be used to represent the empirical radiation pattern of the subfaults in simulating ground motions for a large event.

Estimated ground motions in Los Angeles due to MW = 7 earthquake on the Elysian thrust fault

Abstract The Elysian thrust fault has been identified as a blind thrust fault (Davis et al., 1989; Hauksson and Jones, 1989) representing a potentially serious seismic hazard to the metropolitan Los Angeles and its neighboring areas. We have simulated time histories, peak ground accelerations and their uncertainties using a semi-empirical method for a M W = 7.0 earthquake on the Elysian thrust fault using a flat-layered crustal structure. The accelerograms from the 4 October 1987 Whittier-Narrows aftershock (10h:50m; M L = 5.3) are used to represent the source functions of each subfault on the fault surface. To account for the velocity variation in the surface sediments, we compared simulated ground motions using two separate shearwave velocities, 0.6 km/sec and 0.9 km/sec, respectively, in the top layer of the crustal model. The use of such a simple crustal model is validated by modeling the accelerograms recorded during the 1 October 1987 Whittier-Narrows mainshock ( M L = 6.0). The duration and the relative frequency content observed on these accelerograms are successfully modeled. At some stations, the simulated peak accelerations agree well with the observed values; however, at some sites, the simulated and observed values differ by a factor of 2 to 3. The variation is attributed to a laterally varying crustal structure that is significant for small earthquakes. Additional site specific study is needed for an improved prediction of the observed peak accelerations. For large magnitude earthquakes, the near-source peak ground acceleration appears to be controlled by the source-receiver geometry relative to the fault, as well as the location of asperities on the fault surface. This is validated by simulating the peak acceleration data of the 1989 Loma Prieta earthquake recorded within 30 km of the source. For our simulations on the Elysian thrust fault, an additional component is added to the uncertainty by analyzing several asperity models. Finally, an analytical representation is given to the simulated average peak horizontal ground accelerations, Y ( R ), which is expressed by ln ( Y ( R ) ) = ( 5.38 ± 0.085 ) + ( − 2.09 ± 0.0268 ) ln ( R + 8.0 ) ± 0.343 in the range of 7.5 ≤ R ≤ 35 km ( R is the closest distance to the seismogenic rupture zone). This functional form predicts a rapid fall-off rate for the simulated ground motions compared with the fall-off rate predicted by the other published empirical attenuation relations. We use the finite-difference method to investigate the effects of irregular structure on ground motions resulting from point sources on the Elysian Park fault. The response computed at several depths beneath the basin suggests that the response is dominated by the initial direct arrivals for sources located interior to the basin. When the receivers are located at one end of the basin, and the seismic sources are located at the opposite edge, the characteristic features on the seismograms are the long durations caused by the trapped energy within the basin. The level of trapped energy decreases as the depth of the source increases. Thus, the fault model becomes important in determining the level of peak ground motions and shaking duration.

Analogy in precursors of dynamic events at different scales

This paper presents a study of the time-space behaviour of certain physical parameters, namely, the number of pulses of acoustic and seismic emission, the density of seismogenic ruptures, and the b- value in laboratory, mine and field experiments during the preparation and development of dynamic events at different scales. The preparation stage before failure in rock specimens, mine shocks and large earthquakes was studied. It was found that, in all cases, the fault density parameter varies from 3 to 8; before a dynamic event the variance of the number of elastic pulses of acoustic and seismic emission as a rule increases, and the b- value undergoes changes. It is shown that the behaviour of the parameters under study does not qualitatively depend on the linear dimensions of the object being loaded. The results obtained strengthen the physical basis for the further development of studies on the prediction of dynamic events.

TECTONIC ASPECTS OF THE 1983 KUM-DAG EARTHQUAKE, WEST TURKMENIA

This paper treats the tectonic aspects of the Kum-Dag earthquake of 1983 in western Turkmenia, particularly the morphology and kinematics of its surface expression, as well as its geodynamic features, its setting in the Late Quaternary structure and in the current tectonic pattern. A seismogenic rupture occurred, by which is meant emergence of the focal rupture on the surface, which was examined in detail in 1983. The authors are with the Institute of Geologic Sciences. Features of the surface trace nicely match the character of the fault zone as indicated by regional geology and by seismologic data. Kum-Dag is in the Nebit-Dag oil producing district. The seismogenic rupture, dynamic parameters of the main earthquake focus, and the structural setting of the earthquake focus are discussed. 28 references, 5 figures.