Course:EOSC311/2020/The Process of a Bone Break in Relation to an Earthquake.

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Introduction

Earthquakes are natural disasters that can cause massive destructions to city infrastructure and result in tsunamis, fires and slope failure[1]. Similarly, bone breaks result in devastating injuries that is influenced by external forces. The characteristics of stress applied to rocks are similar to that of bone breaks in terms of elasticity and plasticity. Both occurrences contain a similar process where a certain level of stress can be handled in which a bone break occurs or energy releases in an earthquake. The elastic region and plastic region are the characteristics that arise when different magnitudes of force is applied externally[2]. On the other hand, rocks experience the same attributes when faced with different magnitudes of force. These characteristics include elastic deformation ductile deformation and fracture [3][4]. Analysis of pre-existing conditions between bone ruptures and earthquake formation are similar in that it influences further damage done to bone and fault lines. The purpose of this analysis is to relate bone fractures to earthquakes and how these processes can assist in understanding the two occurrences.

Applied Stress to Bone

Understanding Bone Anatomy

Overview of Long Bone Anatomy

To further understand the formation of a bone break, it is important to go over the basics of bone anatomy and physiology. There are many different types of bone our body consists of. This includes, long bones that are shaped like a cylinder, short bones that represent a cube-like structure, flat bones that are thin, irregular bones that do not have a specific shape and sesamoid bones that posses a small and round shape[5]. A long bone contains several aspects that aid in bone growth, protection, and mechanical movement[6]. These include[6]:

  • Diaphysis: the shaft of the bone that is cylindrical in nature.
  • Epiphysis: ends of the long bone; proximal and distal ends.
  • Metaphysis: found between diaphysis and epiphysis. Here, the epiphyseal plate made out of hyaline cartilage is found where bone growth occurs. Once bone reaches it maximum length, the hyaline cartilage is replaced by bone and renamed the epiphyseal line.
  • Articular Cartilage: a layer of hyaline cartilage covers the epiphysis of the bone which protects it from friction caused by articulating bones. Here, there is a lack of blood cells which inhibit the repair caused by any damage.
  • Periosteum: surrounds the bone with connective tissue and is associated with the supply of blood.
  • Medullary Cavity: a hollow cylindrical space is found within the diaphysis which aids in minimizing the weight of bone where weight is not needed. Bone marrow and blood vessels are found here.
  • Endosteum: lines the medullary cavity with a thin membrane.

The physiology of bone allows us to understand the different cells and tissue that create stability in the bone. Bones contain collagen fibres which allows inorganic salt crystals to attach at the surface [5]. The result of this attachment is the creation of hydroxyapatite which provides bones with strength and flexibility through collagen fibres[5]. There are 2 different types of bone found in the body - compact and spongy bone. Compact bone is found in the periosteum and the diaphysis of long bone[5]. They are considered stronger than spongy bone as they are organized in rings of calcified matrix[5]. On the other hand, spongy bone is organized in lattice-like structures to reduce weight in the bone for easy maneuvering[5].

Common bone fractures: open fracture (b), comminuted fracture (e), greenstick fracture (g).

Fractures vs Stress Fractures

There are two different types of breakage in bone that affect the way we move - stress fractures and fractures. Fractures are clean breakage in the bone that is identified by a physician as to the severity of the fracture[6]. Common fractures include open, comminuted and greenstick[6]. A clean break through the diaphysis of the bone and cause it to protrude through the skin is classified as an open fracture[6]. Comminuted fractures occur when small fragments of bone are created from external force[6]. Lastly, greenstick fractures are partial fractures that occur on one side of the bone causing the opposite side to bend[6]. However, a stress fracture normally does not show any signs of clear rupture in the bone[6]. Instead, heavy physical activity influences microscopic tear tears that occur in the bone which leads to injury, pain and the possibility of impeding bone calcification

Stress Application on Bones

Stress-Strain Curve

The result of massive external forces can cause bones to break or fracture. There are some characteristics that influence bone breaks such as surface discontinuities and bone make-up[7]. By examining the anatomy of a long bone, it is evident that the epiphysis is slightly textured while the diaphysis is smooth. Majority of the stress we put on our bones is targeted towards the epiphysis of the bone which is why they are textured in nature[7]. Therefore, it is important for the diaphysis of the bone to remain smooth to eliminate discontinuities that may affect the stability of the bone causing it to break[7]. Another important factor that contributes to bone breakage is the make-up of the bone internally. Different areas of the bone are contain elastic strength that reduce stress concentration - apatite and collagen[7].

There are 2 specific characteristics bones undergo when external force is applied - elastic region and plastic region[2]. The different magnitude in external force influences whether or not the bone will act elastic or plastic on the stress-strain curve[2]. When low magnitudes of external force is applied, the bone is capable of storing and returning the applied stress[2]. This displays elasticity in the bone to prevent fractures and extreme strain to the bone[2]. As external forces increase on bone, it unable to return the applied stress and instead deforms the bone causing it to break exhibiting the plastic region of the stress-strain curve.[2]

Applied Stress to Rock

Understanding Plate Tectonics

Examples of Different Plate Boundaries

It is important to understand the basics of plate tectonics as it plays a major role in earthquake formation. The 4 layers of the earth include the inner core, outer core, mantle and crust[8]. Tectonic activity occurs in the lithosphere-asthenosphere boundary found in between the crust and mantle which allows plates to move easily due to the weak nature of the asthenosphere [8][9]. The asthenosphere possess it's weak nature due to melted rock distributed from the lithosphere above[10]. There are 3 different tectonic plate interaction that influence movement of continents, earthquake formation and volcano formation[9]. Convergent boundaries occur when two plates approach each other and create possible subduction[9]. Subduction occurs when sediment accumulates causes the oceanic and continental lithosphere to divide[9]. Lithospheric mantle and crust form tectonics plates which explains why a single plate is capable of possessing oceanic and continental crust[9]. Divergent boundaries occur when two plates travel in opposite directions which forms a new crust[9]. And finally, transform boundaries occur when two plates slide past one another[9]. Understanding plate tectonics is significant because earthquakes normally occur near or at the plate boundaries.

Stress Application on Rocks

Earthquakes are a result of stress applied onto rock, which in turn causes the rock to rupture. When this occurs, rocks on the fault line causes movement which influence the rocks surrounding that area[4]. As motion occurs on the fault line, it is the result of rock rupture and energy released from rocks that created friction that causes earthquakes felt at the surface[4][8]. The hypocenter is beneath the earth where the earthquake begins and at the surface is considered the epicenter[8].

Top: Visual representation of elastic deformation and elastic rebound. Bottom: The affects of stress acting along asperities.

Rocks under external stress are capable of three characteristics - elastic deformation, ductile deformation and fracture[3][4]. Elastic deformation occurs when stress is administered to a rock and causes it to stretch[4]. The intensity of stress does not have the magnitude to deform the rock[4] Instead, the rock stretches and snaps back to its original shape. Ductile deformation is the result of a rock reaching its ability to receive applied stress that results in irreversible strain[3][4]. With increased intensity, the rock can rupture and snap back to place which cause vibrations[4]. When elastic rebound occurs in rocks, the vibration felt from its results causes an earthquake.

Pre-existing faults also influence quakes felt at the surface. Asperities between the faults are created which are surface discontinuities along the fault[4]. These discontinuities bind the faults together and prevent them from moving together[4]. Asperities play a key role in determining seismic activity as breakage cause large amount of energy to be released[11]. Because asperities create irregularities on the fault lines, they are prone to higher stresses than areas without asperities[11]. When stress from tectonic plate activity occurs while the plates are held together, elastic deformation occurs[4]. However, when increased stress is created from tectonic plate activity, it can causes the asperities to break resulting in elastic rebound[4].

Aftershocks and Foreshocks

Foreshocks normally begin as a series of smaller earthquakes and that influence the magnitude of mainshock[4]. When foreshocks occur at a specific area, its stress concentrations decrease and transfer over to another place on the fault[4]. As stress concentration increases and disperses along the fault due to small earthquakes, it leads to the mainshock. Similarly, aftershocks are the same idea as the big earthquake influence the different stress concentrations spread out along the fault[4].

How Bone Breaks Help Us Understand Earthquakes

It is intriguing to see how geology and kinesiology relate closely together when it comes to earthquakes and bone breakage. Rocks and bone encounter a similar process that greatly influence how they react when they undergo stress. As bone receives low external force, strain is produced that result in elasticity where it is capable of absorbing the stress and restoring the applied force[2]. When rocks experience low stress, elastic deformation occurs where it is able to receive the stress but is capable of maintaining its original shape[4]. Here it is seen that low magnitudes of force that act upon bone and rock experience the same result in terms of elasticity. They are able to keep their shape and form during low magnitudes of stress without rupturing its composition. However if the received force is greater than that of elasticity in the bone, it can deform and cause it to break resulting in plasticity[2]. Similarly, this is seen in rocks where ductile deformation and fractures occur due to high magnitudes of stress which causes the rock to reverse the effects of the strain and eventually cause the rock to rupture [11]. It is seen here that high levels of stress that act upon bone and rock display a similar result as they are unable to maintain their composition.

As stress decreases in a specific area due to an aftershock or foreshock, stress increases in the surrounding area and spreads along the fault.

When analyzing surface discontinuities of the bone and asperities along fault lines, a similar event takes place that influence earthquake formation and bone breakage. Surface discontinuities in the diaphysis of the bone affect its ability to be stable during high impacts of force[7]. To increase stability in our bones, they must be free of irregularities that may impact the bones stability[7]. This similar characteristic is seen in fault lines where asperities exist and aid in binding fault lines together. Asperities are not smooth in nature like the diaphysis of a bone. Instead, they possess different surface irregularities to secure fault lines[4]. The bumpy nature of asperities increase surface area for the stress to act upon which exhibit increased amount of stress compared to other regions of tectonic plates[11]. With enough force from tectonic plates, the asperities between fault lines can break and cause the fault lines to move relative to one another

[4].

Another factor to consider is the pre-existing conditions faults and bone experience before rupturing. Through vigorous and repeated exercise, it can lead to trauma in the bones and cause stress fractures that eventually lead to pain and injury[6]. This similar process is seen in foreshocks, mainshocks and aftershocks. Foreshocks influence the magnitude of the mainshock as stress concentrations in the fault lines decrease in a specific area once a foreshock occurs[4]. By decreasing stress in a specific area, it is then transferred over to different parts of the fault until it can no longer bear the stress which results in rock breakage[4]. Aftershocks are similar to foreshocks however, they normally occur in smaller magnitudes[4]. It is clear that pre-existing conditions influence the magnitude of big earthquakes.

References

  1. Panchuk, K. (2017). "The Impacts of Earthquakes". Physical Geology. University of Saskatchewan.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Hart, N., Nimphius, S., Rantalainen, T., Ireland, A., Siafarikas, A., & Newton, R. (2017). Mechanical basis of bone strength: Influence of bone material, bone structure and muscle action. Journal of Musculoskeletal & Neuronal Interactions, 17(3), 114-139. https://www-ncbi-nlm-nih-gov.ezproxy.library.ubc.ca/pmc/articles/PMC5601257/?tool=pmcentrez&report=abstract
  3. 3.0 3.1 3.2 Nelson, S.A. (2015). "Deformation of Rock". Tulane University.
  4. 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 Panchuk, K. (2017). "What is an Earthquake?". Physical Geology. University of Saskatchewan.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 J. Gordon Betts, Tyler Junior College, Eddie Johnson, Central Oregon Community College, James A. Wise, Hampton University, Kelly A. Young, California State University, Long Beach; et al. (2013). Anatomy and Physiology. OpenStax. Explicit use of et al. in: |last= (help)CS1 maint: multiple names: authors list (link)
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Tortora, G.J., Derrickson, B. (2014). Functions of bone and the skeletal system. Principles of Anatomy & Physiology (14th ed., p 170) Danvers, MA: Wiley
  7. 7.0 7.1 7.2 7.3 7.4 7.5 Currey, J.D. (1962). Stress concentrations in bone. Journal of Cell Science 103, 111-133. https://jcs.biologists.org/content/joces/s3-103/61/111.full.pdf
  8. 8.0 8.1 8.2 8.3 Wald, Lisa. "The Science of Earthquakes". USGS. Retrieved June 16, 2020.
  9. 9.0 9.1 9.2 9.3 9.4 9.5 9.6 Panchuk, K. (2017). "Plates, Plate Motions, and Plate-Boundary Processes". Physical Geology. University of Saskatchewan.
  10. Panchuk, K. (2017). "Earth’s Layers: Crust, Mantle, and Core". Physical Geology. University of Saskatchewan.
  11. 11.0 11.1 11.2 11.3 Lay, T. (1995). "Seismic Waveform Modeling". International Geophysics. 58: 397–433. doi:10.1016/S0074-6142(05)80011-7 – via Elsevier Science Direct.