Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.s33c1473s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #S33C-1473
Other
7223 Earthquake Interaction, Forecasting, And Prediction (1217, 1242), 7230 Seismicity And Tectonics (1207, 1217, 1240, 1242), 7299 General Or Miscellaneous, 7536 Solar Activity Cycle (2162)
Scientific paper
An updated catalog of instrumentally recorded great shallow earthquakes has been assembled from standard sources. This compilation is based on the catalog of Pacheco & Sykes (BSSA 82, 1306, 1989) for the years 1900-1975, and on the Harvard CMT catalog for the years 1976-present. The updated catalog includes 78 great earthquakes with seismic moment >=10e21 Nm (moment magnitude Mw>=7.93). Annual and decadal totals of event frequency and seismic moment for earthquakes with Ms>= 7.0 have also been compiled to provide context and allow comparisons. We are at present experiencing a remarkable surge of great earthquake activity. Nine great shallow earthquakes have occurred within the past 4 yr, a rate that is unprecedented in the catalog of instrumentally recorded events since 1900. In terms of worldwide seismic moment and energy release, the current "burst" of great earthquakes is the third largest in the record. This is an expected result, as the giant earthquakes of 1960 and 1964 each dominate burst episodes similar to the present one, and both of these earthquakes were larger than the Mw=9 Northern Sumatra event of 2004. We find that great earthquakes contribute 84% of the total moment for the period Jan 1900-Aug 2007, which is also an expected result. However, great earthquakes did not dominate total seismic moment and energy release for the decades of the 1980s and 1990s, when great events were less frequent; the percentages for these decades were 37% and 40% respectively. The updated catalog reveals a strong tendency for clustering of great earthquakes in time. This has been remarked upon by many investigators and has led to suggestions that the largest earthquakes may somehow be coupled on a planetary scale. To examine this tendency, we employ a simple descriptive method to discriminate between burst intervals and periods of quiescence (or "gaps"). Gaps are: 1) intervals of >3 yr that include no more than 1 great earthquake, or 2) intervals of >2 yr with 0 great earthquakes. All other periods were characterized by the occurrence of multiple closely spaced great earthquakes, termed bursts. Burst intervals span <39 of the 107+ years of the updated catalog, while including 91% of the great earthquakes. The event frequency during bursts is 1.84 yr-1, while the corresponding rate for gap intervals is 0.10 yr-1. The current burst duration of >4 yr exceeds the mean burst duration of 2.7 yr but is considerably shorter than that for the longest burst (11 yr). While the physical origins of this temporal clustering remain mysterious, I report here the discovery of a remarkable and surprising correlation: The distribution of great earthquake occurrence times with respect to the 11-yr solar sunspot cycle is nonrandom at the 99.4% level. A bimodal distribution of phases is found, with increased event frequency found for the years approaching sunspot minima (as now), and with a second population found associated with sunspot maxima. A pronounced deficit of events is found for the intervening times, when solar activity is most rapidly rising, and then most rapidly falling. We will discuss four possible interpretations. This work was supported by the private resources of the author.
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