We use a database of more than 80 finite-source rupture models for more than 50 earthquakes (M w 4.1-8.1) with different faulting styles occurring in both tectonic and subduction environments to analyze the location of the hypocenter within the fault and to consider the correlation between hypocenter location and regions of large slip. Rupture in strike-slip and crustal dip-slip earthquakes tends to nucleate in the deeper sections of the fault; subduction earthquakes do not show this tendency. Ratios of the hypocentral slip to either the average or the maximum slip show that rupture can nucleate at locations with any level of relative displacement. Rupture nucleates in regions of very large slip (D Ն 2/3 D max ) in only 16% of the events, in regions of large slip (1/3 D max Ͻ D Ͻ 2/3 D max ) in 35% of the events, and in regions of low slip (D Յ 1/3 D max ) in 48% of the events. These percentages significantly exceed the percentages of fault area with very large (ϳ7%) and large (ϳ28%) slip. Ruptures that nucleate in regions of low slip, however, tend to nucleate close to regions of large slip and encounter a zone of very large slip within half the total rupture length. Applying several statistical tests we conclude that hypocenters are not randomly located on a fault but are located either within or close to regions of large slip.

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We analyze the Fourier spectra of S+Lg+surface wave groups from the horizontal and vertical components of broadband and accelerogram recordings of 120 small and moderate (2< Mw <6) earthquakes recorded by Canadian and American stations sited on rock at distances from 3 to 600 kilometers. There are seven Mw 4.0-4.5, six Mw 4.5-5.0, and three Mw ≥5 earthquakes in this event set. We test the regional spectral analysis by comparing the moment magnitudes with the moment magnitudes from the earthquake moment tensors determined by Bob Herrmann (St. Louis University) for 27 events, obtaining dMw=0.004±0.074. We determine the Lg attenuation in seven regions within northeastern North America: Charlevoix, lower St. Lawrence, Maine, Northern New York, lower Great Lakes, Ontario, and Nunavut. These attenuation estimates yield an average attenuation Q=(368±13)f (0.54±0.02) for the Appalachian region, a stronger attenuation Q=(317±16)f (0.54±0.03) for the Appalachian lowlands, and a weaker attenuation Q=(455±20)f (0.51±0.02) for Ontario and western Quebec. For events in Nunavut and northernmost Quebec, we estimate a similar attenuation for r <450 km, but a weaker attenuation Q=(773±70)f (0.27±0.06) for Lg propagation for 450< r <1700 kilometers. This farregional attenuation allows us to analyze recordings of the 1989 Ungava and Payne Bay earthquakes obtained in Ontario and southern Quebec. We use these regional attenuations to determine the corner frequencies, stress drops, and radiated energies of the 120 earthquakes.

Accelerations and acceleration envelopes of S waves from a small (ML = 3.6) aftershock of the 1975 Oroville, California, earthquake have been interpreted using a dynamic model of the failure of a single, isolated asperity on a fault plane. It is shown that the distinctive characteristics of the asperity radiation are a prolonged, emergent onset followed by a two-sided acceleration pulse. This energetic pulse is radiated at the completion of failure of the asperity. Our purpose is to demonstrate the characteristics of asperity radiation using a specific event as an example rather than a trial and error modeling of this event.