Tomoo Echigo

Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India

Coastal stratigraphy near Port Blair, Andaman Islands, where the A.D. 2004 Sumatra-Andaman earthquake was accompanied by ∼1 m of subsidence, provides evidence for two prior earthquakes, perhaps both from the past 400 yr. The first of these (event I) is marked by an abrupt mud-over-peat contact best explained by subsidence similar to that in 2004. Event II is evidenced by an overlying chaotic layer composed of mud clasts in a sandy matrix that is connected with feeder dikes. These mud clasts, probably produced by liquefaction, are capped by laminated sand and mud that we ascribe to an event II tsunami. Radiocarbon ages of plant remains in the peat give discordant ages in the range 100 B.C. to A.D. 1950. Event I probably resembled the 2004 Sumatra-Andaman earthquake in that it was accompanied by subsidence (as much as 1 m) but not by strong shaking near Port Blair. If event II was the A.D. 1762 Arakan earthquake, the laminated sand and mud provide the first evidence that this earthquake was associated with a tsunami.

Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

The Mid-Niigata Prefecture earthquake in 2004 (MJMA 6.8) generated surface ruptures along the eastern rim of the Uonuma Hills. To elucidate the structural linkage between the surface ruptures and the source fault at depth, the high-resolution seismic reflection profile across the surface ruptures and nearby active faults, and the data of aftershock distribution are examined. The 5.2-km-long, high-resolution, depth-converted seismic section reveals an emergent thrust beneath the surface ruptures. A two-dimensional model of the fault geometry has been constructed based on the aftershock distribution and the shallow reflection profile. The development of the main geologic structure are well explained by forward modeling using a balanced cross-section method. In detail, the fault system generated the main shock dips at a steep angle (60°) below 5 km depth and more shallowly (30°) near the surface.

Coastal deformation associated with the 2007 Noto Hanto earthquake, central Japan, estimated from uplifted and subsided intertidal organisms

The March 25, 2007 Noto Hanto earthquake (Mj = 6.9, Mw = 6.7) generated vertical crustal movement along the northwestern coast of the Noto Peninsula, central Japan. Soon after the event, we estimated the pattern and amount of coseismic coastal movement based on uplifted and subsided intertidal sessile organisms. Our observations reveal a broad 20-km-wide asymmetric zone of surficial deformation above and across the south-dipping source fault, with a steep north-facing frontal limb and a gentle south-facing back limb. The maximum coseismic uplift was approximately 40 cm at the crest of the zone of deformation. The result of forward modeling suggests that the top of the south-dipping source fault is buried at a depth of approximately 2 km, and that 1.2 m of slip on the fault provides the best fit to our surface observations. Our results demonstrate that traditional field investigations should be combined with modern instrumental observations such as GPS and InSAR to obtain the most effective and reliable spatio-temporal estimates of crustal movement associated with large earthquakes.

Evidence for coseismic and aseismic uplift in the last 1000 years in the focal area of a shallow thrust earthquake on the Noto Peninsula, west‐central Japan

[1] Raised shorelines on the Noto Peninsula on the western coast of central Japan suggest a millennial history of recurrent uplift. Emergence of the coastline by up to 50 cm was recently recorded in association with a moderate earthquake (M w 6.7) on 25 March 2007. The shoreline uplift is indicated by the displacement of sessile organisms such as calcareous tubeworms, which can also be examined as indicators of past shifts in shoreline level. Three levels of pre-2007 paleoshorelines are thus identified on this stretch of the Japan Sea coast. The lowest of these, dated at AD 1720-1950, tilts northward away from the likely source of a pair of ca. M 6.4 earthquakes in 1892, which occurred in a different area to the 2007 earthquake. The middle paleo-shoreline, dated at AD 1430-1655, records a 30-40 cm emergence that may be aseismic. The high paleo-shoreline, dated at AD 1025 1235, records a coseismic uplift of 50 cm and is limited to the area raised coseismically in 2007. The most recent event preceding the 2007 earthquake thus appears to have occurred approximately 1000 years ago.