Fig. 2
(Abstract and text for paper to be presented at the Fall Meeting of the American Geophysical Union December 12, 1997)
We have estimated seismic hazard throughout New Mexico based on 34 years of instrumental data collected from 1962 through 1995. After tests for completeness, our data set contained 865 earthquakes with MD >= 2.0; 290 within an ~6000 km2 area located in the central Rio Grande rift. This small region, designated the Socorro Seismic Anomaly (SSA), is one of the two source zones considered in the hazard analysis; the other is the remainder of the state (RNM). Dependent events were removed from the earthquake catalog using time and space windows of 7 days and 4 km for the SSA and 7 days and 25 km for the RNM. The remaining 477 independent earthquakes with MD >= 2.0 (133 within the SSA) were used to obtain a universal b for the entire state. For this computation, we assumed a Poisson distribution with upper and lower bound magnitudes of 6.5 and 2.0, and a magnitude bin size of 0.1.
In order to estimate spatial variations in seismic hazard, the state was subdivided into 20 km x 20 km areas. Computational errors that arise when a gridded zone contains no events were avoided by assigning a level of background seismicity for the SSA and RNM equal to 25% of the average observed in each of these two source zones. Therefore, for computation of seismic hazard, the seismicity in each 400 km2 is 75% of the observed level plus 25% of the average level for the source zone (SSA or RNM) in which it is located.
Seismic hazard estimates were obtained by combining the temporal probability of occurrence with the spatial probability of occurrence and a relation between ground acceleration and magnitude (Joyner and Fumal, 1985). Presented below is a map of our estimates of seismic hazard in New Mexico in terms of maximum horizontal ground accelerations with a 10% probability of exceedance in a 50 year period.
Fig. 1 The first plot is the earthquake epicenters for the New
Mexico area for events with magnitudes >= 1.3 for the time period
1962 through 1995. Note that earthquakes have occurred throughout
this region and that on the basis of the seismicity boundaries
between the major physiographic provinces are not defined. Among the
recorded 1606 events, 572 fall inside the boundary of the red area
which we called the Socorro Seismic Anomaly or the SSA. The data for
the map come from a catalog in which all magnitudes are based on a
single duration magnitude scale for the region. Our duration
magnitude scale is tied to the local magnitude scale which Hanks and
Kanamori have demonstrated is equivalent to moment magnitude scale.
The complete catalog for the 34 year period was the initial raw data
for our seismic hazard analysis. The final data for the seismic
hazard analysis was obtained after tests for completeness and removal
of dependent events. Our tests indicate that a cut-off magnitude of
2.0 assures completeness of data throughout the region with a
substantial margin of safety. Dependent events in the catalog were
identified and removed using moving time and space windows of 7 days
and 4 km for the SSA and 7 days and 25 km for the events in the
remainder of the region.
Fig. 2 shows the geographical distribution of earthquakes in the final data set adopted for the hazard analysis. All shocks with magnitude less than 2.0 and dependent events have been removed. The final catalog contained 477 events, 131 were located inside the SSA. For the hazard analysis, we subjectively divided the region into two source zones, the SSA and the remainder of the state (RNM).
Fig. 3 For modelling recurrence relationships for the two source zones, we used a truncated exponential recurrence model. We assumed a Poisson distribution with upper and lower bound magnitudes of 6.5 and 2.0, and a magnitude bin size of 0.1. For estimating slope b, the uncertainty in the measurement of magnitudes was taken into account by using Bender's equation for fitting b using magnitude grouped data. This plot shows the maximum likehood slope b for both the SSA and the RNM. It is clear that the two source zones have about the same slope b, the SSA is 0.6927 and the RNM 0.6568. However, the annual seismic density per square km for the SSA is about 15 times higher than the RNM. To simplify the computation process, we used a universal b of 0.6675 for the whole area, which is about the mean b for the two source zones.
Fig. 4 We divided the region into small blocks of 20 x 20 km2 and evaluated seismic hazards on the basis of blocks. The size of block was set so that it was large enough to accommodate the maximum horizontal epicentral error for nearly all recorded earthquakes. Each block had its own recurrence relationship and during the hazard analysis interacts with the other blocks. The cumulative number of events N in the recurrence model for each block is the combination of 75% of the events that occurred within the block and 25% background seismicity. For each block, probabilities of occurrence were calculated for ground accelerations ranging from 0.05g to 0.4g at 0.05g interval. Desired values of probability of ground acceleration were then interpolated directly from the curve. In this study, a total number of 1330 probability-ground acceleration curves were evaluated.
Fig. 5 We present the probabilistic seismic hazard map in the format of maximum horizontal ground accelerations at 10% probability of exceedence in a 50 year period. In general, seismic hazards for the area are considered from moderate to low. The highest ground acceleration is 0.21g and the lowest is near 0g. Like the distribution of seismicity, the physiographic provinces are not identifiable from the seismic hazard map. The area inside the SSA has the highest level of seismic hazard, 0.21g. Along the major population corridor of the state from Albuquerque to Santa Fe, the maximum ground acceleration is ~0.1g, which is equivalent to Modified Mercalli intensity VI-VII effects.
Fig. 6 We compared our hazard estimates with the 1996 seismic hazard maps published by the USGS. In this plot, the figure on the left is the reproduction of the USGS hazard map covering the state of New Mexico. The figure on the right is the hazard map I have just shown. Our map is clearly more complex than the USGS map. However, these two maps generally agree on the areas with highest seismic hazards. I will discuss two of the most important subjective parameters which dictate differences in these maps, the level of smoothing and the choices of seismic source zones. The level of smoothing of the USGS map is substantially greater than on the New Mexico Tech map. This produces lower upper bounds and higher lower bounds on the USGS seismic hazard map. The differences between these two maps are ~0.07 g on the high end and ~0.02g on the low end.
Fig. 7 The other major factor that has affected the seismic hazard estimates is the choice of seismic source zones. In our hazard map, we selected seismic source zones based on the distribution of seismicity. Therefore only two zones appeared justified for our analysis, the SSA and the RNM. The next plot shows the source zones selected by the USGS. New Mexico was divided into three source zones that generally follow boundaries of the Colorado Plateau, the Basin and Range including the Rio Grande rift, and the Great Plains. The selection of different seismic source zones is the main reason for the major contrasts in the two maps. Perhaps the difference is greatest for the eastern one-third of the state because of the north-south boundary between the Basin and Range and Great Plains seismic source zones for the USGS map.
Fig. 8 On the USGS map there is a rapid eastward dropoff in seismic hazard along this boundary because the background seismicity for the Great Plains is much lower than the Basin and Range. Our map does not show this uniform dropoff in seismic hazard because we have only one source zone for the entire area except for the small SSA. As a result, our map has a seismic hazard ~0.06g higher than the USGS map along the northeastern border of New Mexico.
In summary, we have generated a seismic hazard map for the New Mexico based on a short 34 year catalog of instrumentally located earthquakes. Because our analysis does not incorporate some strong earthquakes prior to 1962, there is a possibility it may underestimate the hazard, almost certainly in the Socorro area.
The earthquake catalog upon which our seismic hazard analysis is based was collated at New Mexico Tech (NMT). Locations and origin times are primarily from NMT (78%) but significant contributions came from Los Alamos National Laboratory (LANL, 13%), U.S. Geological Survey (USGS, 7%) and others (2%). The magnitudes from LANL, USGS, and other organizations were adjusted to be consistent with NMT duration magnitudes.
