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Soil amplification factors in terms of period derived from regressions using Costa Rican data set
Abstract
Amplification factors for two soil types and 23 periods were obtained, using accelerographic records and their correlation with seismic parameters and local geology related to the site for Costa Rica. The factors were obtained from regressions between PSA (pseudo spectral acceleration) for 5% damping as the dependent variable and three independent variables: magnitude, hypocentral distance and soil type at each site, defined as S II (hard soil) and S III (medium to soft soil). It was assumed that the condition S I (rock) does not amplify seismic waves in the range of periods defined. Factors obtained for S II shows an almost constant value throughout the range of periods for the three different data sets considered (subduction, crustal or crustal + subduction combined) and compared with amplifications obtained by other authors, especially for Japan. For S III, amplification factors obtained in this investigation for the entire data set (subduction + crustal origin) are clearly higher than those proposed by other authors for Japan, mainlyabove period of 0.4 s.
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We developed equations to predict pseudoacceleration response spectra (5% damping), peak ground acceleration, and peak ground velocity at free-field rock sites for intermediate-depth, normal-faulting inslab earthquakes of Central Mexico. The data set comprises 16 earthquakes (5.2 <= M-w <= 7.4; 35 <= H <= 138 km) recorded at local and regional distances (R <= 400 km). It represents a homogeneous catalog with respect to tectonic regime, fault mechanism, and soil class. Our results show larger amplitudes in the epicentral area from inslab events than from interplate events, a consequence of higher stress drops during the former type of earthquakes. Peak ground accelerations from moderate to large (M-w > 6.0) inslab events significantly exceed those from interplate events with similar magnitude, reaching almost three times for the largest events. The ground motion due to inslab events, however, decays faster than for the thrust events. Our results are in reasonable agreement with other studies based on Japanese and the worldwide data.
1] The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located $300 events that occurred following the M W 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19° that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.
A new computational method for implementing Brillinger and Preisler's one-stage maximum-likelihood analysis of strong-motion data is introduced. Analysis by Monte Carlo methods shows that both one-stage and two-stage methods, properly applied, are unbiased and that they have comparable uncertainties. Both give the same correct results when applied to the data that Fukushima and Tanaka (1990) have shown cannot be satisfactorily analysed by ordinary least squares. The two-stage method is more efficient computationally, but for typical problems neither method requires enough time to make efficiency important. Of the two methods, only the two-stage method can readily be used with the techniques described by Toro (1981) and MaLaughlin (1991) for overcoming the bias due to instruments that do not trigger. -from Authors
We investigate the effect on Gutenberg and Richter's parameter b of the saturation of moment-magnitude relationships caused by source finiteness. Any conventional magnitude scale measured at a constant period features a saturation which results in a stepwise increase in the slope of the log10 moment-magnitude relationship. We predict that this leads to an increase in b value with earthquake size. This is in addition to the effect of the physical saturation of the transverse dimension of the fault, previously described in the literature. A number of scaling models are used to predict the behavior of b with increasing magnitude, in the case of both the 20 s surface-wave magnitude Ms, and the 1 s body-wave magnitude mb. We show in particular that a b value of unity can be expected only in a range of earthquake size where the relevant magnitude has already started to saturate: it should be the exception, not the rule, and cannot be extended to a wide range of magnitudes, except at the cost of significant curvature in the frequency-magnitude curve. The widely reported b = 1 stems from the common practice of using a heterogeneous magnitude scale, e.g. Ms for large events and m1 for smaller ones.
We have taken advantage of the recent increase in strong-motion data at close distances to derive new attenuation relations for peak horizontal acceleration and velocity. This new analysis uses a magnitude-independent shape, based on geometrical spreading and anelastic attenuation, for the attenuation curve. An innovation in technique is introduced that decouples the determination of the distance dependence of the data from the magnitude dependence. The resulting equations are
log A = − 1.02 + 0.249 M − log r − 0.00255 r + 0.26 P r = ( d 2 + 7.3 2 ) 1 / 2 5.0 ≦ M ≦ 7.7 log V = − 0.67 + 0.489 M − log r − 0.00256 r + 0.17 S + 0.22 P r = ( d 2 + 4.0 2 ) 1 / 2 5.3 ≦ M ≦ 7.4
where A is peak horizontal acceleration in g, V is peak horizontal velocity in cm/ sec, M is moment magnitude, d is the closest distance to the surface projection of the fault rupture in km, S takes on the value of zero at rock sites and one at soil sites, and P is zero for 50 percentile values and one for 84 percentile values.
We considered a magnitude-dependent shape, but we find no basis for it in the data; we have adopted the magnitude-independent shape because it requires fewer parameters.
In this paper we summarize our recently-published work on estimating horizontal response spectra and peak acceleration for shallow earthquakes in western North America. Although none of the sets of coefficients given here for the equations are new, for the convenience of the reader and in keeping with the style of this special issue, we provide tables for estimating random horizontal-component peak acceleration and 5 percent damped pseudo-acceleration response spectra in terms of the natural, rather than common, logarithm of the ground-motion parameter. The equations give ground motion in terms of moment magnitude, distance, and site conditions for strike-slip, reverse-slip, or unspecified faulting mechanisms. Site conditions are represented by the shear velocity averaged over the upper 30 m, and recommended values of average shear velocity are given for typical rock and soil sites and for site categories used in the National Earthquake Hazards Reduction Program's recommended seismic code provisions. In addition, we stipulate more restrictive ranges of magnitude and distance for the use of our equations than in our previous publications. Finally, we provide tables of input parameters that include a few corrections to site classifications and earthquake magnitude (the corrections made a small enough difference in the ground-motion predictions that we chose not to change the coefficients of the prediction equations).
Having a reliable site classification scheme is vital for the development of robust strong-motion attenuation models. We discuss
a promising empirical site- classification scheme based on strong-motion data from Japan. We assigned site classes, according
to the site classification defined in engineering design codes in Japan, for those K-net sites where boreholes reached either
to rock or to stiff soils with shear-wave velocity of 600 m/sec or larger, using four site classes defined by dominant site
period. The average response spectral ratios of the horizontal and vertical components (h/v) of earthquake records for all site classes were found not to be strongly affected by Japan Meteorological Agency (jma) magnitude, hypocentral distance, and focal depth for all site classes. We used h/v ratios for records from the classified K-net sites to establish a site classification index using the mean spectral ratios
over a wide range of spectral period. Using the index, we were able to classify both K-net stations with soil layers thicker
than 20 m and other strong-motion stations in Japan. The peak period of the h/v spectral ratio can also be used to identify soft soil sites. The site amplification factors calculated from the site class
terms based on the new site classification are consistent with the period bands defined for these site classes.
Intermediate-depth earthquakes are often attributed to dehydration
embrittlement reactivating preexisting weak zones. The orientation of
presubduction faults is particularly well known offshore of Middle
America, where seismic reflection profiles show outer rise faults
dipping toward the trench and extending >20 km into the lithosphere.
If water is transported along these faults and incorporated into hydrous
minerals, the faults may be reactivated later when the minerals
dehydrate. In this case, the fault plane orientations should be the same
in the outer rise and at depth, after accounting for the angle of
subduction. To test this hypothesis, we analyze the directivity of 54
large (MW ≥ 5.7) earthquakes between 35 and 220 km depth
in the Middle America Trench. For 12 of these earthquakes, the
directivity vector allows us to identify the fault plane of the focal
mechanism. Between 35 and 85 km depth, we observe both subhorizontal and
subvertical fault planes. The subvertical fault planes are consistent
with the reactivation of outer rise faults, whereas the subhorizontal
fault planes suggest the formation of new faults. Deeper than 85 km, we
only observe subhorizontal faults, indicating that the outer rise faults
are no longer being reactivated. The similarity with previous results
from the colder Tonga-Kermadec subduction zone suggests that the
mechanism generating these earthquakes, and controlling fault plane
orientations, depends on pressure rather than temperature or other
tectonic parameters and that the observed rupture characteristics
constitute a basic feature of intermediate-depth seismicity. Exclusively
subhorizontal faults may result from isobaric rupture propagation or the
hindrance of seismic slip on preexisting weak subvertical planes.
The Costarican isthmus is located at the western limit of the Caribbean oceanic plateau, where the Cocos plate subducts along the Middle American Trench. This plate shows strong lateral variations in its morphology and structure. The main objective of this study is to investigate the effects of subduction-related magmatism as a function of morphology and structure of the subducting plate, by performing a simultaneous inversion of the 3-D crustal velocity field and hypocenter locations from local earthquakes. For that, we used the traveltimes of P-waves first arrivals from more than 5000 events, recorded at the Costarican seismic networks between 1991 and 1998. In order to prevent data and modeling inaccuracies, we followed an inversion scheme consisting of four steps. (1) Selection of an adequate data subset for tomographic inversion, (2) estimation of the best reference 1-D model, (3) determination of the finest parameterization and (4) evaluation of resolution. The results show that the uppermost levels of the crust (0–6 km) are consistent with geology, since the velocity anomalies reflect the most meaningful geological features observed in the surface. Below these levels (6–20 km deep), we found two different zones separated by a SW–NE seismic alignment. The northern part is characterized by a highly heterogeneous velocity field and is seismically active, while the southern part is much more homogeneous and practically inactive. Moreover, the northern part shows a certain accumulation of low velocity material within the upper mantle. We suggest and illustrate that the structural differences between the northern and southern zones can be a consequence of the differences on the geometry and structure of the subducting slab.