Geology topics

J. Luetgert

Publications and source records attributed to J. Luetgert.

3 recordsLinked to original sources

USGS2003

The dependence of PGA and PGV on distance and magnitude inferred from Northern California ShakeMap data

We analyze peak ground velocity (PGV) and peak ground acceleration (PGA) data from 95 moderate (3.5 ??? M < 5.5) and 9 large (5.5 ??? M ??? 7.1) earthquakes in northern California. The 95 moderate earthquakes occurred from August 1998 through December 2002, and their peak motions were compiled and mapped by ShakeMap. The nine large earthquakes include the M 6.2 Morgan Hill earthquake in 1984, the M 6.9 Loma Prieta earthquake in 1989, and the M 7.1 Petrolia earthquake in 1992. For r > 100 km, the peak motions attenuate more rapidly than a simple power law (that is, r-??) can fit. Instead, we use an attenuation function that combines a fixed power law (r-0.7) with a fitted exponential dependence on distance, which is estimated as expt(-0.0063r) and exp(-0.0073r) for PGV and PGA, respectively, for moderate earthquakes. We regress log(PGV) and log(PGA) as functions of distance and magnitude. We assume that the scaling of log(PGV) and log(PGA) with magnitude can differ for moderate and large earthquakes, but must be continuous. Because the frequencies that carry PGV and PGA can vary with earthquake size for large earthquakes, the regression for large earthquakes incorporates a magnitude dependence in the exponential attenuation function. We fix the scaling break between moderate and large earthquakes at M 5.5; log(PGV) and log(PGA) scale as 1.06M and 1.00M, respectively, for moderate earthquakes and 0.58M and 0.31M for large earthquakes.

California↗

USGS1996

Origin of high mountains in the continents: The Southern Sierra Nevada

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

California↗

USGS1994

LITHOPROBE East onshore-offshore seismic refraction survey -constraints on interpretation of reflection data in the Newfoundland Appalachians

Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.

Newfoundland Appalachians↗