Stress drops and radiated energies of aftershocks of the 1994 Northridge, California, earthquake

Abstract

[1] We study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern California earthquakes. We use empirical Green functions to obtain source time functions for 47 of the larger (M ! 4.0) aftershocks of the 1994 Northridge, California earthquake (M6.7). We estimate static and dynamic stress drops from the source time functions and compare them to well-calibrated estimates of the radiated energy. Our measurements of radiated energy are relatively lowcompared to the static stress drops, indicating that the static and dynamic stress drops are of similar magnitude. This is confirmed by our direct estimates of the dynamic stress drops. Combining our results for the Northridge aftershocks with data from other southern California earthquakes appears to show an increase in the ratio of radiated energy to moment, with increasing moment. There is no corresponding increase in the static stress drop. This systematic change in earthquake scaling from smaller to larger (M3 to M7) earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults.

Seismic scaling relations and aftershock activity from the sequences of the 2004 mid Niigata and the 2005 west off Fukuoka earthquakes (MW 6.6) in Japan

“…According to Ide and Beroza [2001], Ide [2003] and Yamada et al [2005Yamada et al [ , 2007, E R /M 0 is almost constant between 10 À6 and 10 À4 over a wide moment range from micro earthquakes in gold mines (M 0 $ 10 4 Nm) to great earthquakes in subduction zones (M 0 $ 10 21 Nm). On the other hand, Kanamori et al [1993], Abercrombie [1995], Mayeda and Walter [1996], Izutani and Kanamori [2001], Mori et al [2003], Mayeda et al [2005] and Izutani [2005] pointed out that E R /M 0 is scale dependent for earthquakes in southern California or in Japan. E R /M 0 obtained in the present study falls within the scatter summarized by Ide and Beroza [2001].…”

“…The variation of radiated seismic energy from the small events may also be related with tectonic stress redistribution in a complex fault system during the earthquake sequence. If the fracture energy (E G ) [see Mori et al, 2003;Venkatarman and Kanamori, 2004] is large in the complex structure, the event could end in a small event without much energy left for radiation. It leads us to debates regarding the radiated portion of energy (E R ) in the total energy budget of an earthquake.…”

“…[2] The seismic scaling laws of moment (M 0 ) versus corner frequency ( f c ) and radiated seismic energy (E R ) have been studied by a number of researchers using recent high quality waveform data [e.g., Abercrombie, 1995;Mayeda and Walter, 1996;Ide and Beroza, 2001;Mori et al, 2003;Venkatarman and Kanamori, 2004]. The published scaling relationships show variation of M 0 / f c…”

Source parameters and stress release of seismic sequences occurred in the Friuli-Venezia Giulia region (Northeastern Italy) and in Western Slovenia

“…The apparent stress increase as a function of M 0 can also be described in terms of a larger ratio of radiated energy to seismic moment for larger earthquakes. As a possible explanation of this, Mori et al (2003) claimed that the larger events tend to be more complex and characterized by greater high-frequency content, therefore radiating more energy.…”

“…Most of the observational studies in the seismological literature investigated scaling relationships characterizing different seismic areas and different seismotectonic environments or some particular pattern of seismicity (Cocco and Rovelli, 1989;Somerville et al, 1987). An interesting application of source parameters estimation uses seismic sequences to investigate the stress levels, the radiated energy and the mechanism of stress release (Archuleta et al, 1982;Fletcher and Boatwright, 1991;Mori et al, 2003).…”