Tensional intraplate seismicity in the Eastcentral Pacific

Abstract

We identify a region of enhanced intraplate seismicity west of the Mathematicians Ridge in the Eastcentral Pacific. Recent activity includes the December 2, 1984 event ( M s = 6.2) which was accompanied by a swarm of seismic activity, including 45 recorded shocks in the 3 months preceding the earthquake, and 10 aftershocks extending into 1986. Modeling of the main shock P and SH waveforms and a grid search fit to the body wave amplitudes indicates normal faulting along a NE-SW plane. Inversion of WWSSN P waveforms suggests a depth of 14 km and a source time function duration of 7 s; such figures preclude the anomalous low stress drops observed for documented volcanic earthquakes. We obtain similar focal mechanisms for the October 5 foreshock ( m b = 5.5) and the May 28, 1986 aftershock ( m b = 5.5). Relocation shows that the large June 30, 1945 earthquake (with Gutenberg magnitude 6 3 4 ) was an intraplate shock which occurred 400 km south of the 1984 epicentral region. Body waveform modeling and moment variance analysis of P, SH, Love, and Rayleigh waves suggest a normal faulting mechanism with east-west striking fault planes at a depth similar to the 1984 event. Small earthquakes also occurred near the 1945 epicenter in 1973, 1978, and 1985. The seismicity occurs in tectonically complex lithosphere formed just before a re-orientation of spreading on the Mathematicians Ridge associated with the elimination of offset along the Clarion Fracture Zone. SEASAT data indicate a significant gravity anomaly in the region of the 1945 event, which may represent a fracture zone of substantial offset, but no such features are found near the 1984 epicenter. The earthquakes suggest that concentrations of normal faulting intraplate earthquakes, found previously in the Indian Ocean, occur elsewhere in the oceanic lithosphere.