Mainshock-Aftershock Sequences v. Swarms: A Systematic Study Based on Energy

The distinction between mainshock-aftershock (MS-AS) sequences and swarms has generally been ambiguous; a magnitude difference of 0.5 to 1 between the mainshock and largest aftershock is usually the discriminator. Seismologists have assumed that the mainshock carries most of the energy, but this is true only if it is sufficiently large compared to the size and number of all the aftershocks. Energy, therefore, is a logical, physical quantity on which to draw the distinction between MS-AS sequences and swarms. b-value represents the relative number of small to large events, thus providing a convenient way to estimate energy. When plotting the minimum b-value at which the energy of the sequence (excluding the mainshock or largest event) exceeds the energy of the mainshock, versus the magnitude separation (M mainshock - M largest aftershock), a linear relationship emerges. Pairings of b-value and magnitude separation that plot above this line represent swarms (i.e., E-sequence > E-mainshock) and, conversely, pairs plotting below this line represent MS-AS sequences (i.e., E-mainshock < E-sequence). This study employs this quantitative approach to investigate the fundamental properties of MS-AS sequences and swarms for standard shear fracture earthquakes (i.e., low-frequency or long-period events are excluded). We assembled a data set of 65 seismic sequences from the literature and from a systematic search of Alaskan earthquake catalogs. 43 of these sequences occur within tectonic and 22 within volcanic provinces. By plotting the b-value against the magnitude separation, no absolute distinction between sequences occurring within these provinces arises. Systematic differences, however, do exist. The tectonic sequences have broader ranges in magnitude and magnitude separations. Though a handful of volcanic sequences overlap the tectonic sequences, tectonic sequences generally have greater magnitudes and magnitude separations than volcanic ones. Tectonic and volcanic sequences vary in magnitude from 3.3 to 9.3 and 1.4 to 5.6, and magnitude separations from 0.1 to 2.8 and 0 to 1.8, respectively. The data sets share similar distributions in b-value. With 2 exceptions, the b-values of tectonic and volcanic sequences are coincident and range from 0.6 to 1.78. With respect to our scheme, every tectonic sequence, irrespective of magnitude, and those volcanic sequences with magnitudes > 5 plot in the MS-AS field. Those sequences that occurred in volcanic provinces with magnitudes < 5, on the other hand, either straddle the dividing-line or lie well within the swarm field. Not all of the activity previously classified as 'swarms' were in fact swarms, but rather some were MS-AS sequences. Of the 22 volcanic sequences, 11 are re-classified as MS-AS sequences, 6 as swarms, and 5 cases are ambiguous. As suggested by the 2008 Kasatochi MS-AS sequence, which culminated in an eruption, those MS-AS sequences occurring within volcanic regimes may be associated with igneous activity and need not be strictly tectonic in origin. When used in concert with our scheme, the post-mainshock seismic decay pattern seems to be a robust parameter in distinguishing MS-AS sequences from swarms.