#thrust fault

decided to have some fun. my little halloween thing is turning my transformers ocs into magical girls.ta-da. sorry it's late. i've always wanted to turn my characters into magical girls so here we are. it took a while for me to make designs i was happy with. they were harder than they look. so anyway like with my Magical Humanformers, each girl has an element and virtue they protect. so, starting from the left we have

Eureka, Animals and Justice

Thrust Fault, Metal and Passion

Nitrogen, Digital Technologies and Intelligence

Sorellus, Light and Love

Neurotoxin, Radioactivity and courage

Loco Motion, Flowers and Faith

Twin Drills, Crystals and Ingenuity

Jolt Drive, Stars and Creativity

Speedsmoke, Storms and Compassion

Diamond Claw, Carbon and Morality

i decided to give them all crowns. there's some design notes i want to put here but i'm like super tired.maybe later. but yeah.i hope you enjoy them.

okay magic weapons

Eureka has a spear

Thrust Fault has a rapier.

Nitrogen has an energy sword 

Sorellus has a parasol

Neurotoxin has a club

Loco Motion has a scythe

Twin Drills has a lance

Jolt Drive has a wand

Speedsmoke has a crossbow

and Diamond Claw has a pendulum

And in the unavoidable sequel: millions of Americans die along the west coast of the United States when the tsunami hits.

This movie unavoidably describes a thrust fault event that sinks the entire island. A thrust fault quake of this magnitude pushes down such huge amounts of land that it is unavoidable that a gargantuan tsunami no less than a thousand feet high would hit California.

Crude calcs yes but ask a geologist I will bet you that this is what happens in that scenario.

Domenica scorsa mi trovavo in un bosco a Sud Est del lago artificiale di Redona e stavo correndo tranquillamente in salita... vabbè camminavo tranquillamente... ok, arrancavo con fatica... comunque il punto è che ad un certo punto, poco dopo l’ottavo chilometro, noto che lungo il lato Sud del sentiero, a poche decine di metri da esso, spuntava una grossa parete grigiastra di roccia:

La panoramica non rendere l’idea di quanto imponente e quasi perfettamente rettilinea fosse la parete, per cui ho scatato un eloquente anche un dettaglio:

Ok, ad ogni modo, in quel momento ho pensato: caspita, dev’essere una faglia! Una tal parete che si staglia tanto nettamente nel paesaggio non può essere frutto di semplice erosione, deve per forza emergere per via di una forza che l’ha spinta verso il sentiero!

Così, tornato a casa, dopo una doccia e un pranzo con spaghetti panna e salmone, ho provato a confrontare la geolocalizzazione delle foto con la traccia GPX del mio cardio-gps da polso:

La geolocalizzazione delle foto sovraimpressa ad una immagine aerea mostra anche l’ombra proettata dalla parete rocciosa, particamente sempre presente dal momento che è esposta a nord.

Il piccolo puntino nero corrisponde alla posizione degli scatti (a sinistra si vede il lago di Redona) in una foto area in scala minore con sovraimposta parte della traccia GPX.

Non mi restava che andare sull’amatissimo sito di Cartografia Geologica del Friuli Venezia Giulia per verificare l’eventuale presenza di faglie:

Ed ecco che si trova, quasi perfettamente orientata nella direzione Est Ovest, una linea strutturale di sovrascorrimento secondario... ovvero la scaglia tettonica a Sud vuole ricoprire degli strati rocciosi a Nord a causa di forze compressive. In quest’altra mappa vengono colorate le unità litostratigrafiche:

Si vede come le rocce calcaree a Sud (verde spento), risalenti ad un intervallo di tempo compreso tra Giurassico superiore e il Cretaceo inferiore (da 164 a 100 milioni di anni fa), sono state spinte verso Nord scontrandosi con le rocce calcaree (verde brillante) del Cretaceo superiore - Paleocene superiore (da 100 a 66 milioni di anni fa) e hanno generato una cosiddetta faglia inversa:

Se le spinte dovessero continuare a lungo, come già successo in altre zone, si creerebbe una piega coricata, ovvero lo strato a Sud coprirebbe parzialmente gli strati a Nord. Questo fenomeno, che è alla base della formazione stessa delle catene montuose, ha la particolarità di violare il principio di sovrapposizione ovvero fossili più antichi si trovano sopra fossili più giovani e quindi la sovrapposizione degli strati diventerebbe opposta alla norma, con gli strati più antichi sopra quelli più recenti: 

Tenendo conto di tutto ciò e del fatto che non scopro ora che il Friuli sia una zona sismica, vorrei porre la seguente domanda: perché hanno costruito la bellissima diga sul fiume Meduna che crea il lago di Redona?

How do we know what kind of fault an earthquake happens on?

This image shows data from the earthquake in Nepal a few years ago. This quake happened on a thrust fault - a shallowly dipping fault in the crust, where the rocks are being squeezed together. From reading data in a plot like this one after basically every major earthquake geologists around the world can describe the type of fault that caused this earthquake and explain how it fits with the other tectonic features in the area. How do we actually do that? How does this image tell me it was a thrust fault rather than a normal fault? This post will explain it.

We can tell what kind of fault hosted an earthquake and even the angle of the fault using a plot like this, called a moment tensor plot, a focal mechanism plot, or a “beachball plot” for obvious reasons.

At the instant an earthquake occurs, there is a pressure change; the rocks that were squeezed together in the fault suddenly feel less pressure while the surrounding rocks suddenly feel more. You can even simulate this effect using your hands: if you squeeze them together and slide them past each other, simulating an earthquake, when your hands move some areas move forward and would be put into compression, other areas feel a pressure release. If you change the angle of the fault by twisting your hands, you change the pattern of which rocks at the surface feel compression and which feel extension.

This pattern of rocks that are squeezed or decompressed can actually be recorded. Seismometers near the quake will detect whether the first wave to reach them shows compression or extension and that data is enough to reconstruct the fault motion.

This beachball plot shows that data in a 360-degree projection where each curve represents an angle relative to the surface – curves near the edge represent planes at shallow angles. The area shown in blue represents seismometers that saw compression in this event, the area shown in white represents areas that extended. When we see a pattern like this, with compression at the center surrounded by extension on the sides and a low angle between the boundaries and the surface, we can tell that the quake happened on a thrust fault.

If we see instead a beach ball that is split into 4 quadrants, like a pie chart, that’s a signal of a strike slip fault. Finally, if the area that is compressed is at the edges of the plot, we can tell that’s a normal fault, formed in an extensional environment.

Beachball plots are one of geology’s many tools for interpreting earthquake data; a handful of seismic stations feeding data into a system can be enough to reconstruct the strike and dip of a fault in addition to how the fault moved in a quake within only a couple hours. The fault that broke in Nepal is one of the low-angle thrust faults produced by the collision of India and Eurasia, in this case a fault with a dip of 11° relative to Earth’s surface.

-JBB

Image credit: USGS http://on.doi.gov/1PB5h9Z

Read more: http://bit.ly/1DKMT6i

http://www1.gly.bris.ac.uk/~george/focmec.html

Tiny Thrust

A really cool, small-scale example of geologic processes. This is a layered beach sand found along the California coastline near San Francisco. The different layers of sand probably have different abundances of organic material, creating alternating light and dark layers. Those layers are cut by a tiny thrust fault. A small landslide outside of the photographed area created a compressive stress pushing against the beach sand. The layers were strong enough to hold together and instead of deforming or folding actually broke, forming a small thrust fault that relieved some of the stress. The landscape on the surface then leveled out by continued movement of sand into the newly-created topography.

-JBB