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Focal depth determination of central Aleutian earthquakes

NATIONAL EARTHQUAKE INFORMATION CENTER U.S. GEOLOGICAL SURVEY DENVER FEDERAL CENTER, BOX 25046, MS 967, DENVER, COLORADO 80225
COOPERATIVE INSTITUTE FOR RESEARCH IN ENVIRONMENTAL SCIENCES, CAMPUS BOX 449
UNIVERSITY OF COLORADO/NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, BOULDER, COLORADO 80309

Abstract

Focal depths of suboceanic subduction zone earthquakes best determined by the careful identification and analysis of teleseismic depth phases. On short-period vertical-component seismograms, arc crustal structure can cause water-surface reflections (pwP and swP) and sediment-basement reflections (defined here as [pP] and [sP]) with amplitudes comparable to those of water-sediment reflections (pP and sP). However, changes in source mechanism and local variations in arc structure make the observation of these short-period depth phases highly unpredictable.

Digital and analog vertical-component seismograms were examined from nearly 400 teleseismically recorded central Aleutian earthquakes that occurred during the period 1964 to 1984. A data set consisting of the travel times relative to P of short-period depth phases was assembled for 54 shallow-depth earthquakes and 33 earthquakes deeper than 50 km across the arc. In addition, long-period waveforms for the larger events were examined. Magnitude m_b 4.8 was approximately the lower limit for useful depth phase arrivals.

Short-period reflected phases from shallow-depth earthquakes were analyzed by matching observed travel times relative to P to theoretical values computed for a two-dimensional model of arc crustal structure. Depths resolved in this fashion provide a sharper depth profile of the shallow seismic zone in the central Aleutians than previously available. Outer rise and trench earthquakes occur over a narrow range of subcrustal depths. North of the trench, the plate dips gently beneath the terrace and appears to descend gradually into the mantle. The thickness of the interplate thrust-earthquake zone is no more than 10 to 15 km. Earthquakes near the volcanic arc and above the plate interface occur at depths beneath the sediment-basement interface. Estimation of the focal depths of earthquakes deeper than 50 km poses a special problem because an accurate specification of slab structure is needed to correct for path effects.

The phases pwP and swP are ordinarily only weakly recorded on long-period vertical-component seismograms. However, long-period waves reflected from boundaries with sharp long-period impedance contrasts, such as the sea bottom and the sediment-basement interface, are strongly recorded. Moment tensor depths for shallow earthquakes based on the inversion of long-period waveforms using a standard earth model, once transformed into true depths within a model of arc crustal structure for the central Aleutians, agree with depths determined from short-period reflected phases alone to within a few kilometers.

Reported depth phases are routinely used for depth determination by the Preliminary Determination of Epicenters (PDE) and the International Seismological Centre (ISC) services. However, the phases pP and sP are often obscured in the P coda and for suboceanic earthquakes, e.g., pwP is misidentified and reported as pP. This often results in the overestimation of the focal depths of routinely determined hypocenters reported by these services.

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