Long Period Ground Motion from a Great Earthquake (original) (raw)
Related papers
Strong ground motion of the San Fernando, California, earthquake: Ground displacements
Bulletin of the Seismological Society of America, 1975
Two hundred and thirty-four components of ground displacement are the basis of an investigation of long-period strong ground motion in southern California arising from the San Fernando, California, earthquake. The displacement data are obtained from the double integration of strong-motion accelerograms via the base-line adjustment and filtering operations routinely performed in the series “Strong Motion Earthquake Accelerograms”. These procedures can recover long-period data from strong-motion accelerograms with considerable accuracy. Many-station comparisons of displacement data for which the station spacing is small compared to the wavelengths of interest reveal that uncertainties in displacement are less than 1 cm in the period range 5 to 8 sec, 1 to 2 cm at periods near 10 sec, and 2 to 4 cm in the period range 10 to 15 sec, for a data sensitivity of approximately 7.6 cm/g. For limited variations in epicentral distance (R) and source-station azimuth (ϕ), ground displacements sho...
Soil Dynamics and Earthquake Engineering, 2013
Variations of average and maximum power of strong earthquake ground motion during the 1994 Northridge, California earthquake were gradual and smooth over distances as large as tens of kilometers. Correlation of the contours of recorded power with the depth of sediments and vertical offsets of the basement rocks along the faults in the Los Angeles basin implies a horizontal flow of earthquake wave energy through the deep waveguides of this basin. If the fault-to-station distances were to be measured along the three-dimensional wave paths through these sedimentary waveguides, rather than along straight lines emanating from the source, as is common in empirical studies of strong motion amplitudes, the accuracy of empirical-scaling equations for the prediction of the power of strong shaking could improve significantly.
Duration of strong ground motion during Northridge, California, earthquake of January 17, 1994
2012
Contours of spatial variations of the duration of strong earthquake ground motion during the 1994 Northridge, California earthquake are smooth over distances as large as tens of kilometers. Visual comparison of those contours with the depth of sediments and with vertical offsets of the basement rocks along the faults in the Los Angeles basin are in excellent qualitative agreement with the trends predicted by the previously published empirical scaling equations of strong-motion duration. It is argued that if the source-to-station distances were measured along the three-dimensional wave paths through the sedimentary wave-guides, rather than along straight lines as is common at present, the accuracy of empirical scaling equations could be improved significantly.
Soil Dynamics and Earthquake Engineering, 1997
Plots of smoothed contours of peak amplitudes and of areas with the same peak sign are presented for the radial, transverse and vertical components of acceleration, velocity and displacement. These were drawn by hand based on strong motion recordings, and represent direct observational evidence of the nature of the attenuation of strong motion with distance at high, intermediate and low frequencies. The contours of peak amplitudes indicate that, close to the source, those are affected by the source radiation pattern, and away from the source, by the geological structure. Slower attenuation of peak amplitudes with distance IS observed for waves travelling through the sediments of the Los Angeles basin. Large areas with consistent peak polarity are observed, often tens of kilometers in size, indicating that the sign of the peak is not random. The time of the peak amplitude, relative to first arrivals of S-waves, was also calculated; the areas where this time was greater than 7s were contoured and shaded, indicating peak occurrence later than the direct arrivals from the source. These plots show that, at distances larger than about 20.--30 km, for acceleration, the largest peak occurs mostly before, and, for displacement, mostly after the arrival of surface waves. This indicates that the attenuation of strong ground motion is governed by body waves at short periods and by surface waves at long periods. The presented plots will be useful in refinement of attenuation laws for ground motion peak amplitudes, and for frequency dependent response spectrum ordinates. ({3 1997 Elsevier Science Limited.
A study of the strong ground motion of the Borrego Mountain, California, earthquake
1977
Several synthetic models are constructed to fit the first 40 sec of the transversely polarized displacement, as recorded at El Centro, of the April 9, 1968 Borrego Mountain earthquake. The modeling is done in the time domain using the response computed for a distributed set of point shear dislocations embedded in a layered half-space. The beginning 10 sec of the observed record is used to model the spatial and temporal distribution of faulting whereas the remaining portion is used to determine the upper crustal structure based on surface-wave periodicity. A natural depth criterion was provided by comparing the amplitude of the direct arrival with the surface-wave excitations. Trade-offs are found to exist between source models and velocity structure models. Within the framework of a layer over a half-space model, faulting of finite vertical extent is required, whereas the horizontal dimensions of faulting are not resolvable. A model which is also consistent with the teleseismic results of Burdick and Mellman indicates massive faulting near a depth of 9 km with a fast rise time producing a 10-cm displacement pulse of 1 sec duration at Et Centro. The faulting appears to slow down approaching the surface. The moment is calculated to be approximately 7 X 1025 dyne-cm which is somewhat smaller than the moment found by Burdick and Mellman (1976).
Earthquake dynamics and the prediction of strong ground motion
2006
A large number of earthquakes have been modelled in detail using seismological, geological and geodetic information. Several common traits have been found for earthquakes kinematics at periods longer than 3s. At these frequencies, all large earthquakes (M>7) appear complex with highly variable slip, and propagate with rupture velocities close to about 80 % of the shear wave speed. Starting from these kinematic inversions, it is possible to use numerical wave propagation models in order to estimate the complete radiated field including near and far field effects. Radiation can be separated into two main components: a near field term responsible for the socalled fling steps due to permanent, geodetic offsets; and the far field that produces pulse like motions. Using seismological scaling relations it is possible to explain the main features of displacement spectra using classical seismological models at long periods. Seismic simulations may now be extended to the frequencies up to a few Hz by means of dynamic rupture propagation, where rupture is simulated starting from the kinematic models. In this talk I will review the main results obtained so far and the new avenues of research that have been opened thanks to new near field earthquake data and the ability to simulate increasingly complex and realistic seismic ruptures in a computer.
Simulations of strong ground motion for earthquakes in the Mexicali?Imperial Valley region
Geophysical Journal International, 1984
In this paper computer modelling is used t o test simple approximations for simulating strong ground motions for moderate and large earthquakes in the Mexicali-Imperial Valley region. Initially, we represent an earthquake rupture process as a series of many independent small earthquakes distributed in a somewhat random manner in both space and time along the rupture surface. By summing real seismograms for small earthquakes (used as empirical Green's functions), strong ground motions at specific sites near a fault are calculated. Alternatively, theoretical Green's functions that include frequencies up to 20 Hz are used in essentially similar simulations. The model uses random numbers to emulate some of the non-deterministic irregularities associated with real earthquakes, due either t o complexities in the rupture process itself and/or strong variations in the material properties o f the medium. Simulations of the 1980 June 9 Victoria, Baja California earthquake ( M L = 6.1) approximately agree with the duration of shaking, the maximum ground acceleration, and the frequency content of strong ground motion records obtained a t distances of u p t o 35 km for this moderate earthquake. In the initial stages of modelling we do not introduce any scaling of spectral shape with magnitude, in order to see at what stage the data require it. Surprisingly, such scaling is not critical in going from M = 4-5 events t o the M = 6.1 Victoria earthquake. However, it is clearly required by the El Centro accelerogram for the Imperial Valley 1940 earthquake, which had a much higher moment (Ms -7). We derive the spectral modification function for this event. The resulting model for this magnitude -7 earthquake is then used t o predict the ground motions at short distances from the fault. Predicted peak horizontal accelerations for the M -7 event are about 25-50 per cent higher than those observed for the M = 6.1 Victoria event.
: We evaluated spectral amplification factors of long-period ground motions (3 to 10 s) in the Los Angeles (LA) basin with respect to its surrounding reference hard-rock sites from the Mw7.2 April 4, 2010 El Mayor-Cucapah earthquake records and presented period-specific maps of amplification factors for long periods. This earthquake was the first event providing many (236) high-quality recordings to study spatial variation of long-period amplification in the LA basin. We also tried numerical wave propagation simulations for one of the recent 3D seismic-velocity models for south California: CVM-H 6.2. From comparison of the observed amplification factors with the simulated ones, the CVM-H 6.2 is considered to be almost enable to account for the observed amplification factors with periods from 8 to 10 s in the LA basin, and it leaves more to be improved so that the observed shorter-period (4 to 6 s) amplification factors can be better simulated.
A Study on the Duration of Strong Earthquake Ground Motion
Bulletin of the Seismological Society of America, 1975
A simple definition of the duration of strong earthquake ground motion based on the mean-square integral of motion has been presented. It is closely related to that part of the strong motion which contributes significantly to the seismic energy as recorded at a point and to the related spectral amplitudes. Correlations have been established between the duration of strong-motion acceleration, velocity, and displacement and Modified Mercalli intensity, earthquake magnitude, the type of recording site geology, and epicentral distance. Simple relations have been presented that predict the average trend of the duration and other related parameters as a function of Modified Mercalli intensity, earthquake magnitude, site geology and epicentral distance.
Bulletin of The Seismological Society of America, 2007
The 2003 San Simeon, California, earthquake (M 6.5) generated a set of colocated and closely spaced high-rate (1-sample-per-second) Global Positioning System (GPS) positions and ground motions from digital accelerographs in the Parkfield region (at epicentral distances of 50 to 70 km). The waveforms of displacements derived from the 13 GPS receivers in the region have dominant periods between about 7 and 18 sec. The waveforms are similar in shape, with a systematic change in waveform as a function of distance from the source. The GPS motions are smaller than the accelerograph motions for periods less than about 2 sec. From this we conclude that the 1-sample-per-sec GPS receivers provide a good representation of ground motion at periods longer than about 2 sec. Perhaps more important for earthquake engineering is that the accelerograph data are similar to the GPS data for periods as long as 30 sec, if not longer. This means that data from digital accelerographs can provide reliable relative-displacement response spectra at the periods needed in the design of large structures, at least for earthquakes with magnitudes of 6.5 or above at distances within 70 km. We combine the colocated or very closely spaced GPS and accelerograph data sets in the frequency domain to obtain a single broadband time series of the ground motion at each accelerograph station. These broadband ground motions may be useful to seismologists in unraveling the dynamic process of fault rupture and to engineers for designing large structures with verylong-period response.