(1)
In this paper we report on a probabilistic seismic hazard assessment of the Socorro, New Mexico, area based solely on instrumental data over the 34 year period from 1962 through 1995. This region is of particular importance because it contributes a disproportionate share of the state's seismicity. The concentration of earthquake activity in the Socorro area shows up clearly on a map of the seismicity of New Mexico (Figure 1). The dense cluster of activity in the west central part of the state is shown in greater detail in Figure 2. Because the concentration of activity near Socorro is so sharply defined and unusual, the name Socorro Seismic Anomaly (SSA) has been proposed for the region (Sanford et al., 1995a). The size of the SSA is 5200 km2 which is only 1.6 percent of the total area of New Mexico yet it accounted for 36 percent of the state's earthquake activity from 1962 through 1995.
The SSA is centered over the Socorro mid-crustal magma body (Figure 3), which is approximately 150 m thick (Ake and Sanford, 1988), near 19 km in depth (Hartse et al., 1992) and covers an area of 3400 km2 (Balch et al., 1996). The magma body is believed to be inflating because analysis of level-line data (Larsen et al., 1986) indicates surface uplift, maximum rate 1.8 mm/year, is geographically coincident with the position of the magma body (Figure 3). The SSA and its surrounding aseismic halo can be explained qualitatively by stresses arising from the stretching of the crust over the inflating magma body (Sanford et al., 1995a).
An earlier probabilistic estimate of earthquake hazard for the SSA was made by Sanford (1994). The evaluation of the seismic hazard we present here is an improvement over the earlier effort in a number of ways. First, a more complete and accurate catalog is used in which nearly all SSA events have been relocated using the computer location program SEISMOS (Hartse, 1991). Second, dependent events have been removed from the data set. Finally, a truncated recurrence model (Reiter, 1990) is adopted to more accurately represent seismological conditions in the Socorro area, at least in the short term (50 years).
A three step procedure was used to obtain the final earthquake catalog used in the seismic hazard analysis. First, we combined all available catalogs, retrieving every earthquake which occurred within the SSA. Second, we sorted the events in the combined catalog into chronological order. Last, we removed duplicate shocks. The final catalog contained 544 earthquakes with duration magnitudes of 1.3 or greater for the 34 year period from 1962-1995. Duration magnitudes were obtained from the relation
(1)
where tD is the duration in seconds (Ake et al., 1983).
Temporal Variation in Activity. The graphs of number of events versus time in Figures 4 and 5 clearly show that the level of activity has not been uniform for the entire 34 year period at magnitude 1.3 or greater or at magnitude 2.0 or greater. Seismicity for the period 1982-1995 is significantly higher than for the previous 20 years. Obviously, the total 34 year period will provide a better estimate of the seismic hazard than either one of the shorter time periods. Note that the activity is episodic as well at shorter time intervals because there have been periods of up to one year without a magnitude 1.3 or greater event and up to two years without a magnitude 2.0 or greater shock.
Completeness of the Data. The completeness of the earthquake data was tested using the recurrence relation (Richter, 1958)
,
(2)
where N is the cumulative number of earthquakes. Data for two time periods, 1962-1981 and 1982-1995, are shown in Figure 6 for two cutoff magnitudes, 1.3 and 2.0. The data were separated into these two time-periods because of a marked improvement in instrumentation of the SSA beginning in 1982. The fall-off in cumulative number of events at magnitudes less than 2.0 for the time period 1962-1981 indicates incompleteness of the earlier part of the data set below that magnitude. Therefore, in order to use the entire 34 years of data, it was necessary to restrict the analysis to the 199 events of magnitude 2.0 or greater.
Removal of Dependent Events. We identified and removed aftershocks and swarm events using a moving time-and-distance window with parameters based on the spatial and temporal clustering of earthquake sequences from 1962-1995. Earthquakes occurring within 7 days and 4 km of each other were removed from the data set leaving an event list of 113 earthquakes with magnitudes of 2.0 or greater. Figures 4 and 5 illustrate how the removal of dependent events effects the temporal variation of seismic activity. Figure 7 shows the annual recurrence relations for the restricted data set for the two time periods 1962-1981 and 1982-1995. Despite the removal of dependent events, the interval between 1982-1995 remains significantly more active than the previous 20 years.
A truncated exponential recurrence relationship (Reiter, 1990) was used for the probabilistic seismic hazard assessment:
. (3)
In equation (3), N(m) is the number of earthquakes of magnitude m or larger, mo and mu are upper and lower bounds for the earthquakes, N(mo) is the number of earthquakes equal to the lower bound or greater and B is equivalent to the B value from the linear recurrence model written using natural rather than base 10 logarithms. The lower bound earthquake was set at magnitude 2.0, the cut-off of the 113 event data set. The upper bound was set at magnitude 6.5, the largest random earthquake likely to occur anywhere within the SSA. The B value is based on a B (0.636) obtained from linear regression of data with dependent events removed (Figure 7). Figure 8 shows the annual recurrence rates for the SSA based on the truncated exponential relationship. Note that the recurrence relationship is asymptotic to the upper bound magnitude such that when m=mu, N(m)=0.
Seismotectonic Province. We have restricted our probabilistic hazard analysis to a single seismotectonic province in New Mexico, the Socorro Seismic Anomaly. There is an identifiable geologic process, inflation of a midcrustal magma body, which is creating this unusual concentration of earthquake activity. The boundaries of the SSA seismotectonic province, determined from the distribution of epicenters in Figure 2, enclose an area of 5200 km2.
Procedure. To generate probabilistic acceleration maps for the SSA for the next 50 years, the following calculations were made:
1. Recurrence intervals. Earthquakes with duration magnitudes of 4.0 or greater are considered capable of contributing to the seismic hazard. Therefore the interval between magnitude 4.0 and 6.5 was separated into 2500 magnitude bins and the expected recurrence intervals for each bin were calculated from
. (4)
where the N(m) values are annual rates of occurrence from the data in Figure 8.
2. Probability of occurrence in 50 years. The probability of occurrence for each magnitude bin was calculated from the Poisson relation which assumes that each earthquake occurs independently of any other earthquake. This time independent probability is calculated from
(5)
where T is the expected recurrence interval and t is 50 years.
3. Ground acceleration footprints. Every earthquake will generate at the surface a roughly circular area within which the ground acceleration exceeds a value determined by the magnitude and depth of an earthquake. We used a two-step procedure to obtain the radii of the circular regions and the accelerations at their perimeters. Three relations established by Toppozada (1975) between magnitude and area enclosed by isoseismals were used to calculate the radii of circular regions within which an earthquake of prescribed magnitude produces Modified Mercalli (MM) intensity VI, VII and VIII effects or greater. For example, Toppozada's relation between magnitude and the area enclosed by the VII isoseismal is
(6)
The radius calculated from the quantity AVII defines the intensity VII or greater circular footprint for a shock of specific magnitude. The accelerations at the perimeters of the circular areas were found from the Trifunac and Brady (1975) relation between ground acceleration and Modified Mercalli intensity
. (7)
Ground acceleration footprints were calculated for three perimeter accelerations, 65 cm/s2 (MMI=VI), 130 cm/s2 (MMI=VII) and 259 cm/s2 (MMI=VIII).
4. Areal probabilities of occurrence. Earthquakes occur throughout the SSA (Figure 1) and although the activity seems somewhat more concentrated towards the center, some of the strongest sequences have occurred towards the margins. For this reason, we have assumed earthquakes of all magnitudes are equally probable throughout the entire SSA. The procedure for determining the areal probability that a prescribed acceleration will be equalled or exceeded at a specific point within or bordering the SSA can best be understood by considering a specific case. Assume we have an earthquake of magnitude 6.0. This earthquake will produce a footprint of 18 km radius with a perimeter acceleration of 130 cm/s2 (MMI=VII). For purposes of calculating areal probabilities, it is important to note that the footprint also defines the region within which any earthquake of magnitude 6.0 can occur and produce accelerations >= 130 cm/s2 at the center of the region. Thus the probability that a magnitude 6.0 earthquake will produce accelerations >=130 cm/s2 at a point can be obtained from the ratio of the area of the footprint falling within the SSA to the total area of the SSA (Figure 9). If the footprint falls totally outside the SSA, the areal probability is 0.0, and if it falls totally within, the areal probability is the ratio of the footprint area (1033 km2) to the total SSA area (5200 km2) or 0.2. At the bottom of Figure 9 is a point just outside the boundary of the SSA. Because only 31 percent of the area of the footprint lies within the boundary of the SSA, the areal probability for this point is 0.062.
In the hazard analysis, we divided the SSA and bordering areas into cells 1 km on a side and calculated areal probabilities for the 2500 magnitude bins for exceedance of accelerations of 65 cm/s2, 130 cm/s2 and 259 cm/s2, respectively.
5. Overall probability of occurrence in 50 years. The overall probability that a prescribed level of acceleration will be equalled or exceeded in a 50 year period at a particular location was found from
, (8)
where P(n) is the product of the areal probability (Pa) and the Poisson probability (Pt) (Equation 5) for each of the 2500 magnitude bins. Because we have considered exceedance of three accelerations in the areal probabilities, we obtained three values of probability for each 1 km2 region of the SSA and bordering areas. Curve fitting was used on the three values at each location to obtain acceleration values for 5, 10, 15, 20 and 25 percent probability of exceedance (Figure 10).
As expected the contours of horizontal ground acceleration in Figure 10 parallel the outline of the SSA and are most closely space along the boundary. The areal extent of the maximum hazard within the SSA is greatest when the probability of exceedance is five percent, and least when it is 25 percent. The largest accelerations range from 0.28g for five percent probability of exceedance to 0.15g for 25 percent probability of exceedance. These levels of horizontal acceleration can be characterized as moderate. A 0.28g ground motion will produce MM VIII intensity effects and 0.15g MM VII intensity effects (Richter, 1958).
The USGS has generated 10 percent probability of exceedance earthquake acceleration maps, 50 year interval, whose acceleration values for the Socorro area can be compared with our results in Figure 10b. The highest level of horizontal acceleration on our seismic hazard map is 0.21g which is only slightly higher than the 1990 USGS probabilistic hazard assessment for the Socorro area (Algermissen et al., 1990). By contrast, the 1996 USGS probabilistic map estimates a horizontal acceleration for the Socorro area of 0.10g (Frankel et al., 1996), less than half of our value. We suspect the recent USGS assessment of hazard is smaller because they have elected to distribute the seismicity of the SSA over a large area, perhaps a good fraction of the Rio Grande rift in New Mexico. We believe that because the SSA is a clearly defined concentration of earthquake activity arising from unique geologic conditions, it should be considered an independent seismotectonic province.
Our assessment of the seismic hazard for the SSA indicates that for a 50 year interval there is a 10 percent probability that horizontal accelerations will be 0.21g or greater within a NNW oriented elliptical area extending from just north of San Antonio to Bernardo (Figure 10b). Listed below are some expected risks from horizontal ground motions on the order of 0.2g:
1. Significant damage to adobe structures and walls; some damage to ordinary masonry structures.
2. Frame houses moved off foundations if not bolted down.
3. Fall of plaster, loose bricks, clay roof tiles, cornices, unbraced parapets, chimneys, etc..
4. Damage to concrete irrigation ditches.
In modern well-designed buildings, structural damage is unlikely from horizontal accelerations on the order of 0.2g. On the other hand, injuries and property loss from non-structural damage can be significant. Examples of non-structural damage at 0.2g that have the potential to produce serious injuries and/or loss of property are:
1. Rupture of gas lines.
2. Fall of suspended room heaters, coolers, fans, lighting fixtures, etc..
3. Breakage of containers of hazardous materials (chemical, medical, etc.).
4. Fall of book shelves, library stacks.
5. Rupture of fire sprinklers and distribution lines.
Magnitudes. Our estimation of seismic hazard could be in error if we systematically overestimated or underestimated magnitudes. A comparison of New Mexico Tech and USGS magnitudes for 49 earthquakes with magnitudes greater than 2.0 yielded a mean difference of -0.03 +-0.20 s.d.. Unless both organizations are in error, magnitudes do not appear to be a problem in the determination of seismic hazard for the SSA.
Truncated Exponential Recurrence Relationship. We used a magnitude 6.5 event for the upper bound in the truncated recurrence relationship. We assumed that the largest random event in the SSA should be stronger than the largest historical earthquakes that are estimated to have had magnitudes on the order of 6.0 based on the extent of the felt areas (Reid, 1911). On the other hand, it should not be so strong as to produce easily identifiable scarps, i. e. not exceed a magnitude of about 6.5 (Wells and Coppersmith, 1994). Aside from the La Jencia fault (Machette, 1986), there are no known Holocene age fault scarps in the SSA.
The 35 km long La Jencia fault falls just inside the western boundary of the SSA. Using the relations of Wells and Coppersmith (1994), we believe the observations by Machette (1986) on the scarps of the La Jencia fault can be explained qualitatively by the movements of five 6.8 moment magnitude earthquakes in the past 33,000 years. A recurrence interval of 6600 years is too great to have an effect on a 50 year probabilistic assessment of seismic hazard.
Temporal Variation in Activity. The estimate of seismic hazard could be in error if the earthquake activity from 1962-1995 was unusually high or low. We can compare the predicted level of activity from our truncated recurrence relationship with the SSA historical activity from 1862-1961. During this 100 year period, 12 shocks with maximum intensity of MM VI or greater (equivalent to magnitude 5 or greater) occurred within the SSA (Sanford et al., 1995b). Although some of these may have been dependent events, the number clearly exceeds the 3.6 shocks of magnitude 5 or greater predicted from our truncated recurrence relation (Figure 8). By excluding the historical activity in the SSA, we may be underestimating the hazard.
Completeness of Data. We have no reason to believe that the data are incomplete at the magnitude 2.0 or greater level. The instrumentation within the SSA was substantially better from 1982-1995 than from 1962-1981. If the data were incomplete, we would expect it to occur in the 20 years of poorest instrumentation and to yield a lower B value than for the data in the 1982-1995 period. The opposite is observed although the difference is probably not significant.
Removal of Dependent Events. The number of dependent events removed from the data set is a function of the space and time windows used. Although 4 km and 7 days appear reasonable, we cannot be certain whether too many or too few events have been removed with these parameters.
Magnitude and Areas Enclosed by Isoseismals. We used the Toppozada (1975) relations between magnitude and the areas enclosed by isoseismals. These relations are based on California data. We suspect they are appropriate for the Rio Grande rift, but we lack data for confirmation, i. e. detailed isoseismal maps of strong events in the rift with instrumental magnitudes.
Uniform Distribution of Seismicity. Although more activity appears to occur along the NS axis of the SSA (Figure 2), some of the strongest earthquakes from 1962-1995 were near the margins. In addition, the very strong 1906-1907 swarm may have originated near the southwest margin of the SSA (Reid, 1911). Therefore the assumption of uniform seismicity seems appropriate.
Intensity and Acceleration. The relation between intensity and acceleration we have used is based on data from the western U.S., particularly California. We cannot be certain it applies to the Rio Grande rift in New Mexico. However, we have little choice because there is not a single strong motion recording of an earthquake in New Mexico.
Our analysis to date has used instrumental data only. We intend to incorporate historical data for the period 1862-1961 into the assessment of seismic hazard and to evaluate the sensitivity of our results to uncertainties in the parameters used. However, at this time we believe our probabilistic hazard map for the Socorro area of New Mexico presents a more realistic estimate than the 1996 USGS map (Frankel et al., 1996). Furthermore, because we have not included historical data, there is a chance we may have underestimated the hazard in the maps presented in this report.
Ake, J.P., A.R. Sanford and S.J. Jarpe (1983). A magnitude scale for central New Mexico based on signal duration, New Mexico Institute of Mining and Technology Geophysics Open File Report 45, 26p.
Ake, J.P., and A.R. Sanford (1988). New evidence for the existence and interanl structure of a thin layer of magma at mid-crustal depths near Socorro, New Mexico, Bull. Seismol. Soc. Amer., 78, 1335-1359.
Algermissen, S.T., D.M. Perkins, P.C. Thenhaus, S.L. Hanson, and B.L. Bender (1990). Probabilistic earthquake acceleration and velocity maps for the United States and Puerto Rico, U.S. Geological Survey, Misc. Field Studies Map MF-2120.
Balch, R.S., H.E. Hartse, A.R. Sanford, and K.W. Lin (1996). A new map of the geophysic extent of the Socorro midcrustal magma body, Bull. Seismol. Soc. Amer.
Frankel, A., C. Mueller, T. Barnhard, D. Perkins, E.V. Leyendecker, N. Dickman, S. Hanson, and M. Hopper (1996). National Seismic Hazard Maps, June 1996, U.S. Geological Survey.
Hartse, H.E., A.R. Sanford, and J.S. Knapp (1992). Incorporating Socorro magma body reflections into the earthquake location process, Bull. Seism. Soc. Am., 82, 2511-2532.
Hartse, H.E. (1991). Simultaneous hypocenter and velocity model estimation using direct and reflected phases from microearthquakes recorded within the central Rio Grande rift, New Mexico, Ph. D. Dissertation , New Mexico Institute of Mining and Technology, 251p.
Larsen, S., R. Reilinger, and L. Brown (1986). Evidence for ongoing crustal deformation related to magmatic activity near Socorro, New Mexico, J. Geophys. Res., 91, 6283-6292.
Machette, M.N. (1986). History of Quaternary offset and paleoseismicity along the La Jencia fault, central Rio Grande rift, New Mexico, Bull. Seism. Soc. Am., 76, 259-272.
Reid, H.F. (1911). Remarkable earthquakes in central New Mexico in 1906 and 1907, Bull. Seism. Soc. Am., 1, 10-16.
Reiter, L (1990). Earthquake Hazard Analysis: Issues and Insights, 254p., Columbia University Press, New York.
Richter, C.F. (1958). Elementary Seismology, 768p., W. H. Freeman, San Francisco, California.
Sanford, A.R. (1994). An estimate of the earthquake hazard in the Socorro area based on instrumental data: July, 1960 - December, 1993, New Mexico Institute of Mining and Technology Geophysics Open-File Report 72, 3p.
Sanford, A.R., R.S. Balch, K.W. Lin (1995a). A seismic anomaly in the Rio Grande rift near Socorro, New Mexico, New Mexico Institute of Mining and Technology Geophysics Open-File Report 78, 18p.
Sanford, A.R., K.W. Lin, I.C. Tsai (1995b). Felt earthquakes in the region 31šN to 38šN and 101šW to 111šW from 1830 through 1961 and their relation to the Socorro Seismic Anomaly and the Socorro Fracture Zone, New Mexico Institute of Mining and Technology Geophysics Open File Report 80, 8p.
Toppozada, T.R. (1975). Earthquake magnitude as a function of intensity data in California and Western Nevada, Bull. Seismol. Soc. Amer., 65, 1223-1238.
Trifunac, M.D. And A.G. Brady (1975). On the correlation of seismic intensity with peaks of recorded strong ground motion. Bull. Seismol. Soc. Amer., 65, 139-162.
Wells, D.L. and K.J. Coppersmith (1994). New empirical relationships among magnitude, rupture length, rupture width, and surface displacement, Bull. Seism. Soc. Am., 84, 974-1002.
If the foregoing report we used Toppozada (1975) relations between magnitude and isoseismal areas in conjunction with the Trifunic and Brady (1975) empirical equation between peak ground acceleration and intensity. An interesting consequence of the combined use of these relations is that peak acceleration is very nearly proportional to the inverse of the square root of the distance (R-1/2). This implies that observations used in assigning intensity may be the result of surface waves whose amplitudes fall off as R-1/2.
It is generally assumed that peak ground acceleration will occur during the arrival of the S phase, a body wave whose amplitude drops off as R-1. For this reason, we have recalculated the seismic hazard for the Socorro area using an equation by Joyner and Fumal (1985) which relates peak horizontal acceleration to magnitude (Mw), hypocentral distance (R), and crustal absorption,
log a = 0.43 + 0.23 (Mw-6) - log R - 0.0027 R,
and R = (Ro2 + h2)1/2.
For our computations, we have assumed that h is the vertical distance to the center of the rupture surface and Ro in the horizontal distance from the recording site to the center of rupture surface. These are not exactly the same parameters as defined by Joyner and Fumal (1985) but the differences are small for earthquakes with magnitudes less than 6.5. The third term on the right side of the equation represents the fall-off in amplitude with distance (R-1) and the fourth term the loss of amplitude because of absorption. Note that even when R is large, e.g. 100 km, the fourth term is only a small fraction of the third term.
In calculating the seismic hazard using the Joyner and Fumal relation, we are justified in assuming that our duration magnitudes are equivalent to Mw because they are tied to ML (Ake et al., 1983) and Hanks and Kanamori (1979) have demonstrated the equivalence of Mw and ML. We have not included the fourth term of the Joyner and Fumal relation in our calculations because the appropriate coefficient for this term in the Central Rio Grande rift is likely to be smaller than the one they have used. The coefficient is proportional to the inverse of crustal Q and the majority of data for the Joyner and Fumal relation come from California where crustal Q is likely to be less than that measured in the Socorro area (Carpenter and Sanford, 1985).
The seismic hazard maps we obtained using the Joyner and Fumal (1985) relation are presented in Figures 11 and 12. Each figure presents two maps, one for 10 percent probability of exceedance (PE), the other for 25 percent PE. In Figure 11, the depth h to the center of the rupture surface is 5 km, and in Figure 12 the depth h is 7 km. The hazard maps in Figures 11 and 12 should be compared with the maps b (10% PE) and e (25% PE) in Figure 10. For a depth of 5 km (Figure 11), the peak horizontal accelerations near the center of the SSA are nearly the same as in the earlier calculations, but because of the R-1 rather than R-1/2 dependence, the fall-off in accelerations is more rapid at the boundary of the SSA in Figure 11 than Figures 10b and 10e. Increasing h to 7 km (Figure 12) reduces the peak acceleration in the center of the SSA by 10 to 15 percent. The base of the seismogenic zone in the SSA is probably variable but not likely to exceed ~10 km. For this reason we feel a depth of 5 km rather than 7 km is the more appropriate depth for estimating the seismic hazard of the SSA.
Carpenter, P.J. and A.R. Sanford (1985). Apparent Q for upper crustal rocks of the central Rio Grande rift, J. Geophys. Res., 90, 8661-8674.
Hanks, T.C. and H. Kanamori (1979). A moment magnitude scale, J. Geophys. Res., 84, 2348 2350.
Joyner, W.B. and T.E. Fumal (1985). Predictive mapping of earthquake ground motions, in J.I. Ziony (ed.) Evaluating Earthquake Hazards in the Los Angeles Region - An Earth-Science Perspective, U.S. Geological Survey Professional Paper 1360, 203-220.
Figure 1. Seismicity of New Mexico from 1962 through 1995 with duration magnitudes greater than or equal to 1.3. The concentration of earthquake activity in the Socorro area accounts for 544 of the total 1322 events in New Mexico.
Figure 2. Seismicity of the Socorro area from 1962 through 1995 with duration magnitude „1.3. The boundary of the SSA is shown with a long dashed line.
Figure 3. Geographic location of Socorro Seismic
Anomaly (SSA) relative to the Socorro Magma Body (SMB) and surface
uplift. Red area indicates the lateral extent of the SMB (Balch et
al., 1996). The SMB falls inside the SSA (dark pink). Dashed contours
are surface uplift in millimeters for the period 1911 through 1980
(Larsen et al., 1986).
Figure 4. Temporal seismicity for the SSA with
magnitude >= 1.3. Individual peaks in the plots represent
cumulated events per month. Original event list contains 544 events
(top). After dependent events are removed, 267 events remain in the
list (bottom). The interval from 1982-1995 has significantly higher
activity than from 1962-1981, even after the removal of dependent
events.
Figure 5. Temporal seismicity for the SSA with magnitude >= 2.0. Individual peaks in the plots represent cumulated events per month. Original event list contains 199 events (top). After dependent events are removed, 113 events remain in the list (bottom).
Figure 6. Annual recurrence relations for the data
sets with different cut-off magnitudes and time periods. The fall-off
in cumulative number of events at magnitudes less than 2.0 for the
period 1962-1981 indicates incompleteness of the earlier part of the
data set below that magnitude.
Figure 7. Annual recurrence relations after removal
of dependent events for the two periods 1962-1981 and 1982-1995. The
period 1982-1995 has higher activity than the previous 20 years
despite the removal of dependent events.
Figure 8. Annual recurrence rates for the truncated
exponential recurrence model. The lower and upper bounds of
magnitudes are set as 2.0 and 6.5, respectively. The recurrence
relation is equivalent to linear recurrence model if upper bound is
set as infinity.
Figure 9. Areal probability (Pa) is defined as the
ratio of the footprint area (1033
km2) falling
within the SSA to the total area of the SSA (5200
km2). (1) The
footprint falls outside the SSA, Pa=0/5200=0. (2) The footprint falls
inside the SSA, Pa=1033/5200=0.2. (3) The footprint is 31% inside the
SSA, Pa=(1033*0.31)/5200=0.062.
Figure 10. Horizontal ground acceleration seismic hazard maps for SSA with a) 5%, b) 10%, c) 15%, d) 20%, and e) 25% probability of exceedance in a 50 year period. Acceleration is given as a fraction of gravitational acceleration.
(c)
(d)
Figure 11. Peak horizontal acceleration seismic hazard maps based on the Joyner and Fumal (1985) attenuation relation with depth h equal to 5 km. Map a) is for 10% probability of exceedance in 50 years; map b) is for 25% probability of exceedance in 50 years.
Figure 12. Peak horizontal acceleration seismic
hazard maps based on the Joyner and Fumal (1985) attenuation relation
with depth h equal to
7 km. Map a) is for 10% probability of exceedance in 50 years; map
b) is for 25% probability of exceedance in 50
years.
(b)
