Abstract There were significant changes in seismicity preceding a large earthquake, the Urakawa-Oki earthquake of 21 March 1982. This event, a M JMA = 7.1 thrust-fault, crustal earthquake, is the largest from a data set of 35,000 earthquakes beneath the southeastern corner of Hokkaido Island (Japan) during the period July 1976 through December 1986. There was a quiescence for the 2 years preceding the main shock, surrounding, but not including, the immediate epicenter, and only for magnitudes ≧ 3. During the same time span, there was an increase in the number of small (magnitudes ≦ 2.4) events restricted to the general focal region of the main shock. The magnitude dependence of the precursory seismicity patterns is a departure from self-similarity, one of the currently prevalent assumptions in earthquake source models. These seismicity anomalies can be seen using conventional methods such as cumulative number versus time for selected magnitudes in a bounded geographical region. However, such displays require a priori knowledge or assumptions about the prospective target region and magnitude range. For this anomaly, there was no reason to target the epicentral region in advance, and the usual assumption that quiescence should be most evident at small magnitudes was not appropriate here. We have developed an efficient, objective procedure for alerting one to the possible existence of a developing anomaly and then analyzing its spatial bounds and its magnitude dependence. This procedure has potential value as a general monitoring tool.

Epicenter

Anomaly (physics)

Thrust fault

Source

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Citations (18)

10Be in island-arc volcanoes and implications for subduction

Nature (1982)

Louis Brown J. Klein R. Middleton I. Selwyn Sacks F. Tera

Island arc

Flood basalt

10.1038/299718a0

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Citations (140)

Erratum: Slow earthquakes triggered by typhoons

Nature (2009)

ChiChing Liu A. T. Linde I. Selwyn Sacks

Typhoon

10.1038/nature08202

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Citations (36)

Continuous borehole strain observations at italian volcanoes

EGUGA (2009)

R. Scarpa A. T. Linde I. Selwyn Sacks Pierdomenico Romano

Strain (injury)

Source

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Stress in the contorted Nazca Plate beneath southern Peru from local earthquakes

Journal of Geophysical Research Atmospheres (1987)

John F. Schneider I. Selwyn Sacks

We study earthquake focal mechanisms in a region of highly contorted subducting lithosphere to identify dominant sources of stress in the subduction process. We observe a stress pattern in the contorted Nazca plate beneath southern Peru from an analysis of hypocentral trend and focal mechanisms of intermediate‐depth earthquakes. Expanding on previous studies, we examine the hypocentral trend using 1673 of 2178 well‐located local events from the nine‐station Arequipa network. The dip of the plate beneath southern Peru averages 25°–30° from 25‐ to 100‐km depth. Below this depth there is an 80‐ to 100‐km‐wide contortion between a zone of increasing dip (convex) to the southeast and a flat lying (concave) zone to the northwest. Using more than 6000 P wave first motions of events deeper than 50 km, we derive stress orientations from a moving average of composite focal mechanisms across a 200 by 350 km region including the contortion. The in‐plate distribution of tension ( T ) and compression ( P ) axes reveals a coherent stress pattern. The trend is most clear beneath south‐central Peru (NW section) and below 100‐ km depth in southernmost Peru (SE section). Both T and P axes tend to be dominantly in plate, especially below 100‐km depth. T axes orient toward the contortion in a fan‐shaped trend, which suggests that the deepest part of the seismic zone, within the convex SE section, is sinking and pulling the more buoyant NW section. We conclude that from 50‐ to 200‐km depth, slab‐pull forces are dominant in the observed stress. Our results suggest that a significant amount of plate extension occurs in this region of intermediate‐depth subduction.

Tension (geology)

Clockwise

Slab

Pacific Plate

10.1029/jb092ib13p13887

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Citations (43)