Relocation study of Virginia earthquakes (1959-1981) using the Joint Hypocenter Determination and Joint Epicenter Determination methods

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Virginia Polytechnic Institute and State University

A relocation study is made for 50 Virginia earthquakes (1959-1981) using a variation (JED/SE77) of the Joint Hypocenter Determination (JHD) method. Joint Epicenter Determinations (JED) were used in conjunction with a companion single event location program (SE77) to relocate hypocenters from data sets in the Giles County seismic zone (GCSZ) in southwestern Virginia and the central Virginia seismic zone (CVSZ).

In the GCSZ six earthquakes, recorded both regionally and teleseismically and previously relocated jointly by Dewey and Gordon (1980), and nine earthquakes, recorded by a local network and individually located using the HYPOELLIPSE program (Lahr, 1979), were relocated. The JED/SE77 relocated hypocenters virtually all fall within the 70% confidence ellipses of the corresponding original hypocenters.

In the CVSZ 25 events, originally located individually using HYPOELLIPSE, were relocated. The JED/SE77 relocations produce, for the most part, only small changes in epicenters (<5 km average displacement) and comparably small changes in focal depths. Geometrically, they can be grouped into three planes and one cluster of events. The majority (17/25) of the events are contained in two of the planes, both of which have an ENE trend and a 45° southerly dip.

The JED/SE77 results: 1) indicate that the original HYPOELLIPSE hypocenters are quite accurate and should constitute the preferred hypocenters; 2) give added confidence to the instrumental definition of the GCSZ and the CVSZ; and 3) show by the depth distribution of foci that the entire upper portion of the crust is involved in the release of seismic energy.