Reassessing the New Madrid Seismic Zone

Atkinson et al., Eos Trans. AGU, v. 81, 2000 Reassessing the New Madrid Seismic Zone Atkinson, G., B. Bakun, P. Bodin, D. Boore, C. Cramer, A. Frankel, P. Gasperini, J. Gomberg, T. Hanks, B. Herrmann, S. Hough, A. Johnston, S. Kenner, C. Langston, M. Linker, P. Mayne, M. Petersen, C. Powell, W. Prescott, E. Schweig, P. Segall, S. Stein, B. Stuart, M. Tuttle, R. VanArsdale Introduction For many, the central enigma of the large mid-continent seismic region known as New Madrid Seismic Zone (NMSZ, Fig. 1) involves understanding the mechanisms that operate to permit recurrent great earthquakes remote from plate boundaries. Underlying this question is the more fundamental one of whether great earthquakes have, in fact, recurred there. Given the lack of significant topographic relief that is the hallmark of tectonic activity in most actively deforming regions, most of us feel a need to “pinch ourselves and see if we are dreaming” when confronted with evidence that, at some risk levels, the zone might represent a hazard locally as high as areas near the San Andreas fault. There certainly is room for argument on the subject. Direct physical examination of the active faults is not possible, because they are buried by up to a kilometer of Mississippi Embayment unconsolidated sediments (Fig. 1). Flowing across the surface, the Mississippi River tends to erase surface evidence of faulting. Current microseismicity reveals a pattern of active crustal faults, yet even collectively the three major segments it illuminates appear too short to support the assignment of M~8 for the three earthquakes in the 1811-1812 sequence (Johnston, 1996). Modern seismic networks have not been in place long enough to constrain larger earthquake recurrence rates or magnitudes. We must, therefore, rely on historical records of earthquake effects (intensities), paleoseismic traces of strong shaking (mostly sandblows), scant geomorphological evidence, and geodetic observations to provide data to constrain models of recurrence. Applying interpretive models, and seismic source theory (i.e. what we think we know about earthquake mechanics), to turn these basic observations into useful constraints, we must also draw on evidence from outside of the central US. Additional uncertainty arises from not being sure how applicable these external constraints. 1 Figure 1. Schematic map of the New Madrid seismic zone showing major tectonic features (see text), modern seismicity (pluses), state boundaries and major rivers. The shaded oval approximately covers the area of mapped liquefaction features created during the 1811-1812 earthquake sequence. Illustrating some of the difficulties, Figure 2 compares isoseismals from the 1994 Northridge, California, earthquake and the 1895 Charleston, Missouri, earthquake, thought to be of similar magnitudes. There are no recordings of ground motion from 1895, but intensity‡Mo relations “trained” on instrumentally recorded eastern US earthquakes yield a well-constrained estimate. The larger eastern US isoseismals may represent the combined effects of lower intrinsic attenuation, systematically higher eastern US stress-drops, and stronger site amplification. The effect of site amplification of ground motions in river sediments can be seen in the damage (inner) isoseismals. Figure 2. Although earthquakes in the central and eastern United States are less frequent than in the western United States, they affect much larger areas. This is shown by two areas affected by earthquakes of similar magnitude- the 1895 Charleston, Missouri, earthquake in the New Madrid seismic zone and the 1994 Northridge, California, earthquake. Darker shading indicates 2 minor to major damage to buildings and their contents. Outer, lighter shading indicates shaking felt, but little or no damage to objects, such as dishes. A workshop, sponsored by the US Geological Survey and the Mid-America Earthquake Center, was held recently at the University of Memphis to discuss these issues, to come to some consensus on our understanding of them. To continue this consensus building process we herein summarize the workshop findings, and seek input from the wider community (see our website at: http://cordova.ceri.memphis.edu/~meeting). We review the various classes of observations, starting with the fundamental data and then moving to the interpretive data. Instrumental and Historical Seismicity Synoptic seismic network coverage of New Madrid began in the mid-1970s, and modern broadband recording only began about one year ago. The pattern of microearthqakes reveals a zigzag pattern of planar faults, surrounded by a "halo" of earthquakes not clearly associated with any known structure(s) (Fig. 1). Almost all the focal depths are above ~15 km with the exception of a few at ~25 km just outside the NMSZ in southern Illinois. Within the NMSZ only two Mw>5 have occurred this century. The most recent M~5 earthquake occurred near Marked Tree, AR in 1976. Moment-magnitudes and intensity information exists for all M>4.5 events since the 1960s. The bottom line seems to be that the instrumental seismic catalog is insufficient to assess the question of recurrence of large earthquakes. However, the uncertainty of extrapolating the occurrence statistics to large magnitude almost certainly overwhelms the uncertainties in producing the instrumental catalog (see later discussion). Nevertheless, microseismicity is useful for highlighting active structures. That said, there is a widespread feeling that structures other than those highlighted by microearthquakes may represent potential earthquake sources. Other potentially significant faults include the Reelfoot Rift boundaries, the Commerce Geophysical Lineament, the Crittendon County fault zone, and the Bootheel Lineament (Fig. 1). The latter is delineated by flower structures that cut Quaternary strata, identified in reflection profiles. Approximate lengths of faults delineated by microearthquakes are ~150 km on lower SW/NE seismicity trend, ~50 km on upper SW/NE seismicity trend, and ~70km on central reverse seismicity trend or the Reelfoot fault. The Blytheville Arch underlying the longer SW/NE seismicity trend has been imaged in reflection data and is consistent with a fault-deformed zone. To better constrain the seismicity rate at higher magnitudes than the instrumental catalog reaches, it is necessary to augment it with seismic intensity reports (with some events constrained by sparse instrumental recordings in this century). A number of authoritative catalogs exist and a discussion of these may be found in Mueller et al. (1997), which reports completeness levels of M >~3 since 1924, M>~4 since 1860, and M>~5 since 1700. The rates of occurrence of historic earthquakes are generally consistent with those expected based on the rather brief instrumental catalog and a stationary "b-value". However to estimate magnitudes of the historic earthquakes precisely is difficult and the accuracy and precision of such estimates remain controversial. As will be discussed later, this is especially problematic for the 1811-1812 earthquakes. 3 The most widely used estimates of the 1811-1812 earthquake magnitudes are based on intensity reports compiled by Nuttli (1973), Street (1982, 1984) and Street and Nuttli (1984). To estimate moment magnitudes from the intensity data, an empirically derived relationship must be employed. Because of low intraplate seismicity rates, Johnston (1996) used a global dataset to constrain isoseismal area‡ Mw regressions. The uncertainties Johnston reported only account for the scatter in the data and thus do not represent any systematic effects. Examples of relevant systematic effects might include whether the 1811-1812 earthquakes were significantly deeper than, or had significantly higher stress drops than, the regressed examples. Residual isoseismal areas (observed minus regression estimates) plotted against published stress drops showed a clear systematic correlation – positive residuals corresponding to larger stress drop events (Fig.12, Johnston, 1996). Another sort of systematic error might appear in intensity observations if “site effects” were treated differently for events on which the regressions were based and events to which they were applied. In the case of 1811-1812 the isoseismals reflect the population distribution and its concentration in areas of probable site-amplification (i.e. in alluviated valleys). Depending on how such a systematic bias was treated, it could result in estimates of Mw as much as one unit smaller (~M7; Hough et al, 1999). However, additional constraints on a lower bound for the 1811-1812 earthquakes’ magnitudes include comparing their effects (intensities) with other eastern North America M~7 earthquakes that were either widely felt (i.e. 1886 Charleston) or recorded instrumentally (e.g. 1929 Grand Banks, 1933 Baffin Bay). Intensities for Charleston and New Madrid events reported in New Madrid and Charleston, respectively, indicate that New Madrid was more strongly felt in Charleston than the reverse. Liquefaction was much more severe for New Madrid than Charleston, even though the materials seem to be less susceptible in New Madrid (Casey et al., 1999), and accounting for the fact that there were three New Madrid events. Of course if systematic biases bedevil the intensity‡Mo estimates, then the magnitude estimated for Charleston might be subject to similar systematic biases as NMSZ earthquakes. A promising method to evaluate the size of historical earthquakes eschews the use of felt areas

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