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Moment Magnitude

Development of a Moment-Magnitude Based
Earthquake Catalog for the Northeastern U.S.

USGS Award Number 1434-HQ-98-GR-00042

PI John E. Ebel

Weston Observatory
Boston College
Department of Geology and Geophysics
381 Concord Rd.
Weston, MA 03193

Tel: 617-552-8300
Fax: 617-552-8388
Email: ebel@bc.edu
URL: /westonobservatory

Program Element: Evaluating National and Regional Hazard and Risk

Investigations

The goal of this research is to develop a moment-magnitude based earthquake catalog for the northeastern U.S. that is as complete as possible down to M3. Seismic moments from old events will be gathered as known or computed as needed for those earthquakes for which instrumental data or details of the felt areas exist. For routine seismic moment determinations of small earthquakes, we will utilize the coda-wave approach of Biswas and Aki (1984). Biswas and Aki (1984) determined the seismic moments of small events in Alaska using the amplitudes of lower frequency coda waves detected well after the S/Lg wavetrain. They calibrated their coda amplitude moment measurements using the coda amplitudes of two events for which seismic moments had been determined. We will adapt the Biswas and Aki (1984) methodology for application in the northeastern U.S. so that it can be applied routinely for the determination of M for all possible earthquakes, including those recorded on either digital and analog systems. The moment-magnitude based earthquake catalog developed in this study can be updated in the future as seismic sensors and recording systems change.

Preliminary Results

The initial efforts on this project have involved using earthquake data from the region to determine the appropriate modifications to the Biswas and Aki (1984) formula. A generalized form of the Biswas and Aki (1984) formula is

log10(Mo) = log10(A) + X log10 (t) + C

where Mo is the seismic moment of the event, A is the coda wave amplitude at time t after the origin time of the event, and C is a constant. Our research has focused on finding values of X and C appropriate for the Weston Observatory New England Seismic Network stations.

A dataset of recent earthquakes was selected to determine the constants X and C for the broadband digital seismic stations that Weston Observatory operates as its New England Seismic Network (NESN). Figure 1 is a plot of these earthquakes. This dataset was chosen because the earthquakes were widely recorded across the network and because they are large enough that the coda amplitude at a number of different lapse times could be measured for each station.

The constant X in the above equation is found from the decay of the coda amplitudes with time after the origin time of an event. Since all of the New England Seismic Network stations have identical responses, the coda amplitudes from multiple stations can be used to measure the coda decay slope X. Figure 2 shows the coda decays for the events from Figure 1. Overall, there is a fair amount of consistency in the coda amplitude decay slope from one event to another. The average of the slopes from all of these events gives an estimate of the value of X for use in the above formula for seismic moment.

Two of the events of Figures 1 and 2 have seismic moment determinations from independent studies. Those two events can be used to find the calibration constant C for the above formula. Table 1 lists the moment magnitude M values for each of the events in Figure 1. In general, the M values are a few to several tenths of a magnitude unit less than the corresponding MN values.

Further Work

The Weston Observatory archive has earthquake recordings in many different forms and from several different types of seismographs, dating back to the early 1930s. The above determination of the constant C is applicable only to the modern digital NESN stations, and it must be determined separately for each of the different seismographic systems. For each system, the constant C is determined in the same way; the seismic moment for a set of events is found from some independent method and then the constant C in the moment formula given above is calibrated to give the correct seismic moment for that system. Once there is a formula for each system, then the seismic moments for each recorded event will be measured.


Table 1

Events

Known Magnitudes

Mean Mw unique formula made for each individual event that had a known Mo

Mean Mw formula made by taking the average slope from all events and average C from the three events with the known Mo

Lewiston, ME

mb=3.2

Mw=2.98

2.98

3.06

Lisbon, NH

mb=3.8

Mw=3.53

3.54

3.32

PA-OH Border

mbLg=5.2

MN=4.5

4.55

4.53

Southern Ontario

MN=4.2



3.66

SW Plattsburg, NY

MN=3.4



3.3

New Brunswick

MN=4.3



3.8

Quebec

MN=4.4



3.82

NE of Machias, ME

MN=2.6



2.71


Figure 1

Figure 1. Map of the earthquake epicenters used to calibrate the coda-wave seismic moment formula for the modern digital seismic stations of the New England Seismic Network of Weston Observatory of Boston College. (Return to text).

Figure 2

Figure 2. Coda-amplitude decay with time after the origin time of each of the events shown in Figure 1. The equations fit to the datapoints from each event are listed in the upper right hand part of the figure. (Return to text).

Non-Technical Summary

The goal of this research is to develop a moment-magnitude based earthquake catalog for the northeastern U.S. that is as complete as possible down to M3. Earthquake seismograms are analyzed to find the mathematical formula for the computation of the M3 scale for all earthquakes recorded in the region. This research will provide an earthquake catalog that will be more useful than current earthquake catalogs for the computation of earthquake hazards in the region.

Reference

Biswas, N.N. and K. Aki (1984). Characteristics of coda waves: central and southcentral Alaska, Bull. Seism. Soc. Am., 74, 493-507. (Return to text).

Published Abstract

Macherides, A. and J.E. Ebel (1999). The development of a moment-magnitude based earthquake catalog for the northeastern U.S. Seism. Res. Lett., 70(1), 120.