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Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region
268A
GEOLOGY:EARTHQUAKES
tor of safety increases this hazard zone to 23 m. -Authors
Earthquake mechanisms and effects 946038 The October 12, 1992, Dahshur, Egypt, earthquake P.C. Thenhaus, M. Celebi & R. V. Sharp, Earthquakes & Volcanoes (USGS), 24(1), 1993, pp 27-41. Cairo and northeastern Egypt experienced a rare, damaging earthquake on October 12, 1992. The earthquake, which measured 5.9 on the Richter magnitude scale, was centered near the village of Dahshur, about 18 km south of Cairo. The computed hypocentral depth of the earthquake, about 25 kin, is consistent with the fact that fault rupture associated with the earthquake did not reach the surface. Despite its relatively moderate size, the earthquake caused many casualties and heavy damage. -from Authors 946039 A multidisciplinary assessment of postglacial seismic disturbance: Miramichi area, New Brunswick, Canada B. E. Broster, P. Allen & K. B. S. Burke, Quaternary International, 20, 1993, pp 153-161. Examination of deformed alluvial sediments, archaeological projectile points and historical seismicity formed the basis for an assessment of paleoseismicity at the Oxbow site in the Miramichi region of New Brunswick, Canada. Deformed masses of layered sediment, containing discontinuous and folded layering, were interpreted to have been caused by seismically-induced liquefaction of unfrozen sediment. Artifacts and radiocarbon dating provided a chronological record extending over 2500 years and enabled relative dating of deformation of the sediments to between the 1700s and early 1900s. Results supported the inference that the region is subject to repeated ground disturbance from periodic earthquakes of magnitude 5 or greater. -Authors 946040 Coseismic fault-related folding during the South Golbaf earthquake of November 20, 1989, in southeast Iran M. Berberian & M. Qorashi, Geology, 22(6), 1994, pp 531-534. The South Golbaf earthquake of November 20, 1989 ( m 5.7, m b 5.6, I VII), in southeast Iran, was associated witti coseismlc surface faulting and folding. Surface faults 11 km (west-dipping) and 8 km (east-dipping) long with oblique reverse mechanisms developed on both sides of a small Holocene playa. Apparently, repeated coseismic fault-related folding was responsible for the Holocene 20" tilting of the playa deposits above the active fault tip at the ground surface. This study provided the first evidence of earthquake folding of Holocene plays deposits in Iran. -Authors 946041 The 1953 earthquake in Cephalonia (western Hellenic Arc): coastal uplift and halotectonic faulting S. C. Stiros, P. A. Pirazzoli, J. Laborel & F. LaborelDeguen, Geophysical Journal International, 117(3), 1994, pp 834-849. Geomorphological, marine biological and radiometric data in combination with earlier reports reveal that the M = 7.2, 1953 Cephalonia earth.quake was associated with a ~}.3-0.7 m quasi-rigid-body uplift and westward tilting of the central part of the island. Another palaeoseismic event, around 1500 yr BP, associated with coastal uplifts was also identified. Structural data indicate that the 1953 uplift is bounded by two subparallel, east-dipping major reverse faults and corresponds to a piston-like motion. The 1953 seismic surface deformation mimics long-term halotectonic patterns, but is not directly indicative of the regional stress-field, for it reflects uplift-induced stresses only. -from Authors 946042 The 1957 great Aleutian earthquake J. M. Johnson, Y. Tanioka, L. J. Ruff, K. Satake, H. Kanamori & L. R. Sykes, Pure & Applied Geophysics, 142(1), 1994, pp 3-28. The 9 March 1957 Aleutian earthquake has been estimated
as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded-1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthqluake have been poorly determined. This study examined 1 the available waveform data to determine the seismic moment, rapture area, and slip distribution. Using body waves, the duration of significant moment release was estimated as 4 min. From surface wave analysis, it was determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50-100 x 1020 Nm. Using the tsunami waveforms, the source area of the 1957 tsunami was estimated by backward propagation. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. -from Authors 946043 The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake D. H. Christensen & S. L. Beck, Pure & Applied Geophysics, 142(1), 1994, pp 29-53. The rupture history of the March 28, 1964, Prince William Sound earthquake (Mw = 9.2) has been determined from long-period WWSSN P-wave seismograms. Source time functions determined from the long-period P waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smallerpulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. The 1964 Prince William Sound earthquake is interpreted as a multiple asperity rupture with a very large dominant asperity m the epicentral region and a second major, but smaller, asperity in the Kodiak Island region. The zone that rupturedin the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behave independently during previous events. -from Authors 946044 Coseismic slip in the 1964 Prince William Sound earthquake: a new geodetic inversion S.R. Holdahl & J. Sauber, Pure & Applied Geophysics, 142(1), 1994, pp 55-82. The 1964 Prince William Sound earthquake (March 28, 1964; Mw = 9.2) caused crustal deformation over an area of approximately 140 000 km 2 in south central Alaska. In this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface. Previous seismologic, geologic, and geodetic studies of this region were used to constrain the geometry of the fault surface. The results suggest a variable dip-slip component with local slip maximums occurring near Montague Island (up to - 3 0 m), further to the east near Kayak Island (up to -14 m), and trenchward of the northeast segment of Kodiak Island (up to -17 m). -from Authors 946045 Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region C. G. Bufe, S. P. Nishenko & D. I. Varnes, Pure & Applied Geophysics, 142(1), 1994, pp 83-99. The high likelihood of a gajp-filling thrust earthquake in the Alaska subduction zone within this decade is indicated by two independent methods: analysis of historic earthquake recurrence data and time-to-failure analysis applied to recent decades of instrumental data. Recent (May 1993) earthquake activity in the Shumagin Islands gap is consistent with previous projections of increases in seismic release, indicating that this segment, along with the Alaska Peninsula segment, is approaching failure. Based on this pattern of accelerating seismic release, the study projects the occurrence of one or more M > 7.3 earthquakes in the Shumagin-Alaska Peninsula region during 1994-1996. Areas currently showing accelerating release axe the Shumagin, Alaska Peninsula, Delarof, and Kommandorski segments. Time-to-failure analysis suggests that the large earth(luakes could occur in these latter zones within the next tew years. -from Authors
GEOLOGY:EARTHQUAKES
tor of safety increases this hazard zone to 23 m. -Authors
Earthquake mechanisms and effects 946038 The October 12, 1992, Dahshur, Egypt, earthquake P.C. Thenhaus, M. Celebi & R. V. Sharp, Earthquakes & Volcanoes (USGS), 24(1), 1993, pp 27-41. Cairo and northeastern Egypt experienced a rare, damaging earthquake on October 12, 1992. The earthquake, which measured 5.9 on the Richter magnitude scale, was centered near the village of Dahshur, about 18 km south of Cairo. The computed hypocentral depth of the earthquake, about 25 kin, is consistent with the fact that fault rupture associated with the earthquake did not reach the surface. Despite its relatively moderate size, the earthquake caused many casualties and heavy damage. -from Authors 946039 A multidisciplinary assessment of postglacial seismic disturbance: Miramichi area, New Brunswick, Canada B. E. Broster, P. Allen & K. B. S. Burke, Quaternary International, 20, 1993, pp 153-161. Examination of deformed alluvial sediments, archaeological projectile points and historical seismicity formed the basis for an assessment of paleoseismicity at the Oxbow site in the Miramichi region of New Brunswick, Canada. Deformed masses of layered sediment, containing discontinuous and folded layering, were interpreted to have been caused by seismically-induced liquefaction of unfrozen sediment. Artifacts and radiocarbon dating provided a chronological record extending over 2500 years and enabled relative dating of deformation of the sediments to between the 1700s and early 1900s. Results supported the inference that the region is subject to repeated ground disturbance from periodic earthquakes of magnitude 5 or greater. -Authors 946040 Coseismic fault-related folding during the South Golbaf earthquake of November 20, 1989, in southeast Iran M. Berberian & M. Qorashi, Geology, 22(6), 1994, pp 531-534. The South Golbaf earthquake of November 20, 1989 ( m 5.7, m b 5.6, I VII), in southeast Iran, was associated witti coseismlc surface faulting and folding. Surface faults 11 km (west-dipping) and 8 km (east-dipping) long with oblique reverse mechanisms developed on both sides of a small Holocene playa. Apparently, repeated coseismic fault-related folding was responsible for the Holocene 20" tilting of the playa deposits above the active fault tip at the ground surface. This study provided the first evidence of earthquake folding of Holocene plays deposits in Iran. -Authors 946041 The 1953 earthquake in Cephalonia (western Hellenic Arc): coastal uplift and halotectonic faulting S. C. Stiros, P. A. Pirazzoli, J. Laborel & F. LaborelDeguen, Geophysical Journal International, 117(3), 1994, pp 834-849. Geomorphological, marine biological and radiometric data in combination with earlier reports reveal that the M = 7.2, 1953 Cephalonia earth.quake was associated with a ~}.3-0.7 m quasi-rigid-body uplift and westward tilting of the central part of the island. Another palaeoseismic event, around 1500 yr BP, associated with coastal uplifts was also identified. Structural data indicate that the 1953 uplift is bounded by two subparallel, east-dipping major reverse faults and corresponds to a piston-like motion. The 1953 seismic surface deformation mimics long-term halotectonic patterns, but is not directly indicative of the regional stress-field, for it reflects uplift-induced stresses only. -from Authors 946042 The 1957 great Aleutian earthquake J. M. Johnson, Y. Tanioka, L. J. Ruff, K. Satake, H. Kanamori & L. R. Sykes, Pure & Applied Geophysics, 142(1), 1994, pp 3-28. The 9 March 1957 Aleutian earthquake has been estimated
as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded-1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthqluake have been poorly determined. This study examined 1 the available waveform data to determine the seismic moment, rapture area, and slip distribution. Using body waves, the duration of significant moment release was estimated as 4 min. From surface wave analysis, it was determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50-100 x 1020 Nm. Using the tsunami waveforms, the source area of the 1957 tsunami was estimated by backward propagation. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. -from Authors 946043 The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake D. H. Christensen & S. L. Beck, Pure & Applied Geophysics, 142(1), 1994, pp 29-53. The rupture history of the March 28, 1964, Prince William Sound earthquake (Mw = 9.2) has been determined from long-period WWSSN P-wave seismograms. Source time functions determined from the long-period P waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smallerpulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. The 1964 Prince William Sound earthquake is interpreted as a multiple asperity rupture with a very large dominant asperity m the epicentral region and a second major, but smaller, asperity in the Kodiak Island region. The zone that rupturedin the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behave independently during previous events. -from Authors 946044 Coseismic slip in the 1964 Prince William Sound earthquake: a new geodetic inversion S.R. Holdahl & J. Sauber, Pure & Applied Geophysics, 142(1), 1994, pp 55-82. The 1964 Prince William Sound earthquake (March 28, 1964; Mw = 9.2) caused crustal deformation over an area of approximately 140 000 km 2 in south central Alaska. In this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface. Previous seismologic, geologic, and geodetic studies of this region were used to constrain the geometry of the fault surface. The results suggest a variable dip-slip component with local slip maximums occurring near Montague Island (up to - 3 0 m), further to the east near Kayak Island (up to -14 m), and trenchward of the northeast segment of Kodiak Island (up to -17 m). -from Authors 946045 Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region C. G. Bufe, S. P. Nishenko & D. I. Varnes, Pure & Applied Geophysics, 142(1), 1994, pp 83-99. The high likelihood of a gajp-filling thrust earthquake in the Alaska subduction zone within this decade is indicated by two independent methods: analysis of historic earthquake recurrence data and time-to-failure analysis applied to recent decades of instrumental data. Recent (May 1993) earthquake activity in the Shumagin Islands gap is consistent with previous projections of increases in seismic release, indicating that this segment, along with the Alaska Peninsula segment, is approaching failure. Based on this pattern of accelerating seismic release, the study projects the occurrence of one or more M > 7.3 earthquakes in the Shumagin-Alaska Peninsula region during 1994-1996. Areas currently showing accelerating release axe the Shumagin, Alaska Peninsula, Delarof, and Kommandorski segments. Time-to-failure analysis suggests that the large earth(luakes could occur in these latter zones within the next tew years. -from Authors