P wave travel times: Stability and change within the source volume of A M = 7. 2 earthquake
The dense seismograph network on the south flank of Kilauea afforded a unique opportunity to study travel time changes as a function of space and time within the source volume of the Hawaii earthquake (M/sub S/ = 7.2) of 1975. Careful analysis of more than 600 teleseismic P arrivals from deep Fiji-Tonga earthquakes revealed the following: out of eight stations studied, six showed constant travel times to within +- 0.03 s for up to 10 years with the exception of some small changes of about 0.03 s at the time of the main shock. One station, AHU, showed a highly significant and unique travel time decrease by 0.13 s during 1971/1972. At the beginning of this velocity anomaly a southwest rift intrusion caused closure of surface cracks associated with normal faults located near the station AHU. Also, geodetic measurements revealed that between August and October 1971 compressive strain of 4 x 10/sup -5/ was accumulated perpendicular to the southwest rift in the area of AHU. We conclude that these data show for the first time that in situ velocity increases occur due to the closure of cracks by tectonic forces. The AHU residuals returned to normal approximately at the time of a major earthquake swarm on the fault zone near AHU, during which surface cracks were observed to have opened again.We have interpreted the teleseismic residual change at AHU as due to a P velocity increase of about 10% in the top 3.5-1.5-km of the crust. The only station, WHA, which showed a large (0.2 s) and extended (1972 to 1975) travel time increase was located only 4-km from the main shock epicenter. We interpret this velocity decrease as a precursor to the 1975 main shock, and we hypopthesize that a process reverse from that at AHU caused this anomaly by first opening and then closing cracks in the crust below WHA.
- Research Organization:
- CIRES/Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309
- OSTI ID:
- 6087228
- Journal Information:
- J. Geophys. Res.; (United States), Vol. 87:B8
- Country of Publication:
- United States
- Language:
- English
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