INTERMEDIATE-TERM EARTHQUAKE PREDICTION USING PRECURSORY EVENTS IN THE NEW MADRID SEISMIC ZONE

Earthquake prediction may be possible for some mainshock events. The t ime-to-failure method described by Varnes (1989) and Bufe and Varnes ( 1990) uses precursory events (foreshocks) to define an accelerated ene rgy release curve. By fitting an equation to the data, a predicted tim e of failure and magnitude can be calculated. Until recently, this met hod has been used in only a few studies in tectonically active areas, and for moderate-to large-magnitude mainshock events. Using the microe arthquake network data set from the New Madrid Seismic Zone (NMSZ), wh ich qis reasonably complete for earthquakes of magnitude greater than or equal to 1.5 in the area of interest, the method has yielded predic ted values of past events as small as m(b) = 3.5. The network data set used in this evaluation covers the time interval from 29 June 1974 to 20 July 1995 for the NMSZ. There have been 36 earthquakes of magnitud e greater than or equal to 3.5 over the 21-yr period in which the netw ork has been operating. Because precursory events are required for the application of the time-to-failure method, mainshocks that occurred b efore 1980 do not have enough data to adequately define the accelerate d energy release curve. Therefore, we utilized the 26 earthquakes that occurred after 1980 and that had a magnitude greater than or equal to 3.5. Sixteen of the 26 mainshock events were modeled. In most cases, the precursory sequences yielded predicted times of failure and magnit udes that were reasonably close to the actual mainshock values. The re maining mainshocks, including those occurring before 1980, could not b e modeled due to either (1) not enough events to adequately define the precursory sequence or (2) interfering events that disrupt the accele rated energy release curve. In addition, two events were modeled from the Nuttli catalog (Nuttli, 1979) along with one that used a combinati on of both catalogs. Nineteen earthquakes with magnitudes greater than or equal to 3.5 were evaluated using the time-to-failure method. The first calculation using the time-to-failure method gave predicted resu lts with large error bounds, essentially no upper bound on the predict ed magnitude. An empirical relationship between parameters has helped to constrain the range of the predicted magnitude and, to a lesser ext ent, the estimated time of failure. This relationship modifies the tim e-to-failure equation and yields predicted values for magnitudes that have an upper limit. Another empirical relationship suggests that the logarithm of the moment of the mainshock increases Linearly with the l ogarithm of the size of the precursory event search diameter. The rela tive seismicity of the region also influences the optimum search diame ter used to find precursory events. In addition to the evaluation of t he acceleration sequences associated with the mainshocks, an analysis of the occurrence of false-positive acceleration sequences (accelerati on sequences that do not end in a mainshock) was conducted. The prelim inary false-positive analysis was conducted by randomly selecting pote ntial mainshock locations. The results yielded a false-positive accele ration sequence occurrence rate of 2%. With the incorporation of the e mpirical relationships into the time-to-failure method, the potential for future intermediate-term earthquake predictions for the NMSZ is en couraging.