Gravity and Magnetic Modeling of Central Italy: Insights Into the Depth Extent of the Seismogenic Layer

2019 ◽  
Vol 20 (4) ◽  
pp. 2157-2172 ◽  
Author(s):  
P. Mancinelli ◽  
M. Porreca ◽  
C. Pauselli ◽  
G. Minelli ◽  
M.R. Barchi ◽  
...  
Keyword(s):  
Central Italy ◽  
Seismogenic Layer ◽  
Magnetic Modeling ◽  
Gravity And Magnetic ◽  
Depth Extent

scholarly journals From fault creep to slow and fast earthquakes in carbonates

Geology ◽  
2019 ◽  
Vol 47 (8) ◽  
pp. 744-748 ◽  
Author(s):  
Franҫois X. Passelègue ◽  
Jérôme Aubry ◽  
Aurélien Nicolas ◽  
Michele Fondriest ◽  
Damien Deldicque ◽  
...  
Keyword(s):  
Seismic Waves ◽  
Central Italy ◽  
Confining Pressure ◽  
Mediterranean Area ◽  
Rupture Propagation ◽  
Fault Creep ◽  
Seismogenic Layer ◽  
Elastic Loading ◽  
High Frequency Seismic Waves ◽  
Hypocentral Location

Abstract A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3–5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.

Contribution of magnetic modeling to the discovery of a hidden massive sulfide body at Hajar, Morocco

Geophysics ◽  
1991 ◽  
Vol 56 (7) ◽  
pp. 983-991 ◽  
Author(s):  
A. Bellott ◽  
J. Corpel ◽  
R. Millon
Keyword(s):  
Gravity Data ◽  
Massive Sulfide ◽  
Magnetic Survey ◽  
Massive Sulfides ◽  
Magnetic Modeling ◽  
Gravity And Magnetic ◽  
Gravity Response ◽  
Modeling Techniques ◽  
Magnetic Model ◽  
D Model

The Hajar prospect is located in the Guemassa Paleozoic massif, about 30 km southwest of Marrakesh, Morocco. Visean volcano‐sedimentary formations are present in this massif and in the Jebilets massif north of Marrakesh. In these formations, syngenetic massive sulfides occur, and one of these bodies, Kettara in the Jebilets, has been partially mined. Using the Kettara magnetic anomaly to establish the magnetization parameters, we performed a preliminary interpretation of the Hajar anomaly. Our 2.5-D model determined the depth of the structure to be about 150 to 200 m below the surface. This relatively shallow depth was a decisive factor in siting a reconnaissance drillhole, which encountered massive sulfides between 158 and 276 m. After completing a systematic gravity and magnetic survey, 3-D magnetic modeling was attempted, constrained by the results of four drillholes. Now that more than 20 holes have been drilled, this magnetic model still conforms to the newly revealed geology. Unfortunately, it was not possible to extract useful information concerning the orebody from modeling and interpreting the gravity data. The gravity response is masked by disturbances such as faults and variations in depth and nature of the Visean basement. The discovery of the Hajar deposit shows that magnetic investigations, improved by pertinent modeling techniques, can be used at various stages of exploration to help recognize and define massive sulfide bodies.

scholarly journals Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

Geosphere ◽  
2021 ◽  
Author(s):  
V.E. Langenheim ◽  
J.A. Vazquez ◽  
K.M. Schmidt ◽  
G. Guglielmo ◽  
D.S. Sweetkind
Keyword(s):  
Sierra Nevada ◽  
Drainage Density ◽  
Magnetic Data ◽  
Surface Uplift ◽  
Western Cordillera ◽  
Gravity And Magnetic ◽  
Landscape Morphology ◽  
Depth Extent ◽  
Gravity And Magnetic Data ◽  
Existing Structure

In much of the western Cordillera of North America, the geologic frame­work of crustal structure generated in the Mesozoic leaves an imprint on later plutonic emplacement, subsequent structural setting, and present landscape morphology. The Merrimac plutons in the northern Sierra Nevada (California, USA) are a good example of the influence of pre-existing structure at a larger scale. This paper updates and refines earlier studies of the Merrimac plutons, with the addition of analysis of gravity and magnetic data and new 206Pb/238U zircon dates. The gravity and magnetic data not only confirm the presence of two different neighboring plutons, but also (1) support the presence of a third pluton, (2) refine the nature of the contact between the Merrimac plutons as being structurally controlled, and (3) estimate the depth extent of the plutons to be ~4–5 km. The zircon 206Pb/238U dates indicate that the two main plutons have statistically different crystallization ages nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief, roughness and drainage density and generation of chi plots, indicate that the two main plutons are characterized by different elevations with large longitudinal channel knickpoints that we speculatively attribute to possible reactivation of pre-existing structure in addition to lithologic variations influencing relative erosion susceptibility in response to prior accelerated surface uplift.

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