By Richard W. Allmendinger, Nestor Cardozo, and Donald M. Fisher (2012). Cambridge University Press, paperback 302 pages. ISBN: 978-1107401389.

This book presents the mathematics behind structural geology and, by implication and example, tectonics—the difference being a matter of scale. The essence of structural geology is to understand the transition of an initial geometry (such as a layered sedimentary sequence) to a different final geometry (such as a fold and thrust belt). A huge part of that task involves characterizing the shape changes and their evolution, a prerequisite for understanding the driving forces behind such changes. That is a geometric and kinematic problem and the mathematical techniques of vector and tensor manipulation are ideally suited to the job. Some textbooks and lecture courses in structural geology tend to avoid or play down this mathematical framework, but this book does not: it puts the mathematics center stage and backs up the theoretical development with a suite of Matlab subroutines, which are given in the text and are available online.

The book opens with a chapter on projections such as those used in constructing stereonets, then Chapters 2–5 introduce the …

Structural geology algorithms: vectors and tensors

Streamflow in high-mountain Asia is influenced by meltwater from snow and glaciers, and determining impacts of climate change on the region’s cryosphere is essential to understand future water supply. Past and future changes in seasonal snow are of particular interest, as specifics at the scale of the full region are largely unknown. Here we combine models with observations to show that regional snowmelt is a more important contributor to streamflow than glacier melt, that snowmelt magnitude and timing changed considerably during 1979–2019 and that future snow meltwater supply may decrease drastically. The expected changes are strongly dependent on the degree of climate change, however, and large variations exist among river basins. The projected response of snowmelt to climate change indicates that to sustain the important seasonal buffering role of the snowpacks in high-mountain Asia, it is imperative to limit future climate change. High-mountain Asia streamflow is strongly impacted by snow and glacier melt. A regional model, combined with observations and climate projections, shows snowmelt decreased during 1979–2019 and was more dominant than glacier melt, and projections suggest declines that vary by river basin.

Climate change decisive for Asia’s snow meltwater supply

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high quality data of three different periods, 1973-1981, 1984-1994, and 1995-2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of _2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

Probabilistic earthquake relocation in three-dimensional velocity models for the Yellowstone National Park region, Wyoming

New fault trace mapping and structural survey of the active faults outcropping within the epicentral area of the Campania-Lucania 1980 normal fault earthquake (Mw 6.9) are integrated with a revision of pre-existing earthquake data and with an updated interpretation of the CROP 04 near-vertical seismic profile to reconstruct the surface and depth geometry, the kinematics and stress tensor of the seismogenic fault pattern. Three main fault alignments, organized in high-angle en-echelon segments of several kilometers in length, are identified and characterized. The inner and intermediate ones, i.e. Inner Irpinia (InIF) and Irpinia Faults (IF), dip eastward; the outer Antithetic Fault (AFA) dips westward. Both the InIF and the IF strike NW-SE along the northern and central segments and rotate to W-E along the southern segments for at least 16 km. We provide evidence of surface coseismic faulting (up to 1m) not recognised before along the E-W segments and document coseismic ruptures with maximum vertical displacement up to ~1m where already surveyed from other investigators 40 years ago. Fault/slip data from surface data and a new compilation of focal mechanisms (1980-2018) were used for strain and stress analyses to show a coherent NNE-directed least principal stress over time and at different crustal depths, with a crustal-scale deviation from the classic SW-NE tensional direction across the Apennines of Italy. The continuation at depth of the outcropping faults is analysed along the trace of the CROP-04 profile and with available hypocentral distributions. Integrating all information, a 3D seismotectonic model, extrapolated to the base of the seismogenic layer, is built. It outlines a graben-like structure with a southern E-W bend developed at depth shallower than 10-12 km, at the hanging wall of an extensional NE- to E-dipping extensional basal detachment. In our interpretation, such a configuration implies a control in the stress transfer during the 1980 earthquake ruptures and provides a new interpretation of the second sub-event, occurred at 20 seconds. Our reconstruction suggests that the latter ruptured a hanging-wall NNE-dipping splay of the E-W striking main fault segment and possibly also an antithetic SSW-dipping splay, in two in sequence episodes.

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Fault pattern and seismotectonic style of the Campania – Lucania 1980 earthquake (Mw 6.9, Southern Italy): new multidisciplinary constraints

Here I compare estimates of tectonic strain rates from dense Global Positioning System measurements with the seismicity released in the last ~500 years in the Apennines (Italy). The rates of seismic moment accumulation from geodesy and of historical seismic release by earthquakes agree within the uncertainties, ruling out significant aseismic deformation. Within the considered 400 km long section of the Apennines, this balance yields an average recurrence interval of 30–75 years for MW≥6.5 events without requiring a future earthquake larger than those observed historically (MW~7). A minimum estimate of unreleased strain allows MW≥6.5 and MW≥6.9 events to be released in ~35% and ~10% of the central-southern Apennines, respectively. The definition of the seismic potential for smaller events is more uncertain, and their occurrence remains a significant threat throughout the Apennines.

Complete Seismic Release of Tectonic Strain and Earthquake Recurrence in the Apennines (Italy)

We present high-resolution mapping and surface faulting measurements along the Lost River fault (Idaho-USA), a normal fault activated in the 1983 (Mw 6.9) earthquake. The earthquake ruptured ~35 km of the fault with a maximum throw of ~3 m. From new 5 to 30 cm-pixel resolution topography collected by an Unmanned Aerial Vehicle, we produce the most comprehensive dataset of systematically measured vertical separations from ~37 km of fault length activated by the 1983 and prehistoric earthquakes. We provide Digital Elevation Models, orthophotographs, and three tables of: (i) 757 surface rupture traces, (ii) 1295 serial topographic profiles spaced 25 m apart that indicate rupture zone width and (iii) 2053 vertical separation measurements, each with additional textual and numerical fields. Our novel dataset supports advancing scientific knowledge about this fault system, refining scaling laws of intra-continental faults, comparing to other earthquakes to better understand faulting processes, and contributing to global probabilistic hazard approaches. Our methodology can be applied to other fault zones with high-resolution topographic data. Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.13198259

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High-resolution surface faulting from the 1983 Idaho Lost River Fault Mw 6.9 earthquake and previous events

Studying faults capable of releasing moderate-to-strong earthquakes is fundamental for seismic hazard studies, especially in a territory that was subject to the strongest peninsular Italy earthquake (1857, Mw 7.1) and hosting the largest European oil field on-land. Fieldwork-based observations in the Campania-Lucania area highlight a SSW-dipping ~ 65 km-long normal-oblique-segmented fault, showing evidence of recent activity and possibly responsible for the 1857 earthquake. It crosses the Maddalena ridge, linking separate Quaternary basins. Two seismic reflection profiles cross the fault trace where it is buried beneath the Val d'Agri Quaternary deposits. Similarities between fault-controlled small basins in the highest portion of the massifs in the study area and the neighboring 1980 Irpinia area (1980 earthquake, Mw 6.9) are interpreted as evidence of trans-ridge fault activity. Kinematic analyses and the stress field inversion provide a N032-trending near-horizontal s3-axis, the same computed in literature for the Irpinia area, highlighting a deviation from the ~N045-axis which characterizes most of the Apennines. This study demonstrates how detailed fieldwork, supported by geophysics and innovative data analysis techniques, can unravel unknown faults while giving a novel interpretation of the trans-ridge faults' style in controlling strong earthquakes, moving away from classical interpretations, and providing a helpful approach in similar contexts worldwide. 

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Complex trans-ridge normal faults controlling large earthquakes


 Following observations made in a survey campaign along the Lost River Fault (Idaho, USA) in 2019, we integrate both original and previously published data to obtain a detailed segmentation of the fault sections that failed in the 1983 Borah Peak earthquake (Mw 6.9). The earthquake ruptured the topographic surface with an oblique-normal faulting mechanism, activating two SW-dipping fault segments (Thousand Springs and Warm Springs) and a branching SSW-dipping fault (Arentson Gulch Fault) and producing coseismic surface ruptures with up to 3 m of vertical separation. We augment the 1983 earthquake description by interpreting high-resolution topography and fault mapping. We use quality vertical separation data, rupture zone width measurements, and fault slip data to analyze major and minor structural-geometric complexities, highlighting a partition of the deformation and a fault segmentation into four detail levels (i.e., segments, sections, subsections, and sectors). Our work provides new details of the 1983 Borah Peak earthquake, constraints for paleoseismic and seismotectonic studies, and a methodological approach applicable in other areas of the world. Our fault-slip data show variations along fault-strike that we interpret as kinematic partitioning. In 1983, the main southern segment had a large rupture zone width, while the northern segment localized the deformation. The distributed ruptures accommodate a large portion of the rupture length (~19.5 km versus 31 km for the main rupture) and displacement (~66%). 83% of the surface faulting and 80% of the displacement are located at the hanging wall of the main rupture. There is a strong correlation between vertical separation, rupture zone width, rupture position (footwall or hanging wall), and fault geometry. We highlight the control of the obliquity and kinematic partitioning in the surface expression of the earthquake propagation. We interpret the coseismic (i.e., 1983) and long-term (i.e., Quaternary) behavior, showing that the two activated segments had similar cumulated behaviors in distributing the deformation between synthetic and antithetic ruptures, despite the different geometries. Our results have implications for fault rupture behavior with application to rupture hazard.

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High-Detail Fault Segmentation: Deep Insight into the Anatomy of the 1983 Borah Peak Earthquake Rupture Zone (Mw 6.9, Idaho, USA)

The study of seismic activity triggered by transient stress changes associated with hydrocarbon reservoir exploitation and deep fluid injection is especially important, in a seismic risk perspective, in regions, where relevant natural seismicity is coupled with widespread and regionally distributed petroleum fields. Hydrocarbon occurrences in Italy derive from a variety of petroleum systems characterized by source/reservoirs distributed in age from Mesozoic to Quaternary. Several of them are located along and in the proximity of the Apennine Outer Thrust System which started to develop in Late Pliocene times and is still active, as well as seismogenic, along most of its extent (especially northern-central Italy and Sicily). Other occurrences are sited along the front of the more internal Pede-Apennine compressional belt, which was mainly active in Early Pliocene times. Still, a large percentage of hydrocarbon fields are observable within the Adriatic-Pelagian foreland, whereas a small number of fields, highly relevant in terms of oil and/or gas extraction, are sited within the intermountain Apennine active extensional domain. In this paper, after grouping the Italian oil and gas fields in stratigraphically and kinematically homogenous Hydrocarbon Field Assemblages (HFAs), we build a regional seismotectonic zonation of the HFAs, taking into account their surface and depth location with respect to the 3D boundary of regional seismogenic provinces. The provinces differ in kinematics (extensional, compressional, strike-slip) and in characterizing seismogenic thickness and depth, which may coincide or not with the depth range of the hydrocarbon recovery, as well as with the depth of fluid withdrawal and injection activities. The HFAs zonation is a new conceptual product intended as a regional-scale tool that contains base 3D geometric-kinematic information on the active and potentially seismogenic deformation in correspondence of all large Italian hydrocarbon fields. In case of a seismic sequence ongoing near an HFA (epicentral distance <~10 km), it could provide information on the Quaternary fault pattern and structural style characterizing the area, as well as on the associated crustal state of stress, as input constraints to the discussion on the likelihood of a direct or indirect link, controlled by tectonics, between the hypocentral area and the hydrocarbon field.

Simone Bello

Papers

Fault pattern and seismotectonic style of the Campania – Lucania 1980 earthquake (Mw 6.9, Southern Italy): new multidisciplinary constraints

High-resolution surface faulting from the 1983 Idaho Lost River Fault Mw 6.9 earthquake and previous events

Complex trans-ridge normal faults controlling large earthquakes

High-Detail Fault Segmentation: Deep Insight into the Anatomy of the 1983 Borah Peak Earthquake Rupture Zone (Mw 6.9, Idaho, USA)

Regional Seismotectonic Zonation of Hydrocarbon Fields in Active Thrust Belts: A Case Study from Italy