Precise measurements help gauge Pacific Northwest\u27s Earthquake potential

Except for the recent rumblings of a few moderate earthquakes and the eruption of Mt. St. Helen\u27s, all has been relatively quiet on the Pacific Northwestern front. The Cascades region in the Pacific Northwest, a sporadically active earthquake and volcanic zone, still has great seismic potential [Atwater, 1987], as comparisons with other subduction zones around the world have shown [Heaton and Kanamori, 1984]. Recent tsunami propagation models [Satake, 1996] and tree ring studies suggest that the last great Cascadia earthquake occurred in the winter of 1700 A.D. and had a magnitude of −8.9. The North Cascades or Wenatchee earthquake followed in 1872. With an estimated magnitude greater than 7, it was the largest earthquake in the written history of Washington and Oregon

GPS-determination of along-strike variation in Cascadia margin kinematics: Implications for relative plate motion, Subduction zone coupling, and permanent deformation

High‐precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along‐strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades‐long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3–7 mm/yr) to northern and central Oregon (∼9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin‐parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north‐westward velocities reflect the influence of Pacific‐North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin

Late Neogene horizontal and vertical displacement rates during simultaneous contraction and extension in the Southern Apennines orogen, Italy

Assessment of vertical and horizontal displacements and displacement rates within the western Adriatic orogens, where contractional and extensional deformation coexists since the Miocene (PATACCA et alii, 1990; DOGLIONI, 1991), has the potential to supply vital insight into crustal and lithospheric processes operating during continental collision. Using the tight age control on contractional motion provided by synorogenic sequences preserved in outcrop and in petroleum exploration wells, the constraints on extensional motion provided by the crustal structure of the extended hinterland, and the differential elevation of uplifted markers of ancient base level, we establish a regional pattern of vertical and horizontal motion in the Southern Apennines for the last ∼6 Ma, which points to an intricate interplay between lithospheric delamination and crustal structure (FERRANTI & OLDOW, 2005a; 2005b). During latest Miocene to Early Pleistocene, the frontal thrust of the orogen migrated toward the foreland rapidly (∼16 mm/yr) and was accompanied by subsidence with the frontal thrust belt and foredeep remaining at or below sea level. In contrast, the orogenic hinterland experienced extension, which was accompanied by uplift at -0.3 mm/yr along the eastern transition to the contractional belt but net subsidence and formation of the Tyrrhenian basin farther west. Through time, the extensional belt progressively widened toward the northeast at the same rate as the encroachment of the thrust front on the Adriatic foreland. Following a mid-Pleistocene reduction in horizontal displacement rate associated with impingement of the thrust belt on thick crust of the Adriatic interior, the frontal thrust belt and foreland experienced uplift at ∼0.5 mm/yr as contraction stepped to deeper structural levels. Uplift of the eastern margin of the extensional hinterland continued at ∼0.3 mm/yr and is followed by tectonic subsidence along the Tyrrhenian coast of southern Italy. Today, the pattern of mid-Pleistocene displacements continues, as suggested by seismicity and GPS velocities (OLDOW & FERRANTI, 2005). The similarity in migration rates of contractional and extensional fronts across southern Italy over the last 6 million years supports models of crustal delamination and roll-back of the subducted Adriatic slab (ROYDEN et alii, 1987; DOGLIONI, 1991) as a fundamental driving mechanism for deformation along the western margin of Adria. Temporal changes in the vertical and horizontal rates of deformation, however, probably reflect differences in crustal structure and are not directly related to lithospheric processes. The reduction in the horizontal displacement rate associated with the onset of rapid foreland and frontal thrust belt uplift during the Early Pleistocene corresponds to a change from thin - to thick-skinned contraction initiated with the involvement of thick continental crust in regional shortening. Unlike segments of the Apenninic chain in central Italy (LAVECCHIA et alii, 1994; CAVINATO & DE CELLES, 1999), uplift and formation of the Southern Apennine mountain chain was not primarily a response to contractional deformation. Much of the orogenic elevation, at least before the mid-Pleistocene onset of uplift in the frontal thrust belt and foreland, was accrued during the initial stages of extension related to crustal delamination

Pre-Quaternary orogen-parallel extension in the Southern Apennine belt, Italy

The Southern Apennine fold and thrust belt differs from other parts of the peri-Tyrrhenian orogen. In most of the peri-Tyrrhenian belt, hinterland extension is oriented at a high-angle to the orogen axis and appears to be related to rifting and formation of oceanic crust within the Tyrrhenian basin. The Southern Apennines share the late-stage development of normal faults related to the opening of the Tyrrhenian Sea, but also experienced an episode of extension parallel to the strike of the tectonic belt. The orogen-parallel extension was apparently formed in response to the increase in length of the deformed belt during arcuation. Arcuation ostensibly was related to asymmetrical rifting in the hinterland, which was greater in the Southern Tyrrhenian Sea than in areas to the north, and proportionately greater shortening in the frontal parts of the southern belt as compared to regions in the north. During arcuation, extension was spatially concentrated within structural domains and was accomplished by displacement on low-angle detachment faults cutting through a previously imbricated thrust stack. During the Miocene-Pliocene, NNW-SSE extension in the interior of the Southern Apennine belt formed coevally with ENE-WSW shortening in the foreland. Longitudinal extension ceased in the Pleistocene, when younger high-angle normal faults formed in response to the easterly migration of Tyrrhenian Sea rifting and NE-SW extension associated with lithospheric stretching