Week_4 Part3

Earthquake Mechanics

Rubber Band Analogy

• The stretching of a rubber band serves as a simple illustration of how stress accumulates in rocks during tectonic activity.

• When stress is gradually applied to the rubber band:

  • The band stretches and can return to its original shape once the stress is removed, demonstrating elastic deformation.

  • However, if the band is overstretched beyond its elastic limit, it leads to permanent deformation or breakage, resulting in an energetic 'snap'—a metaphor for how rocks behave under extreme stress.

Earthquakes Along Faults

• The process of earthquakes can be likened to the behavior of a rubber band under stress.

• Tectonic stresses cause rocks to deform elastically until a threshold or critical point is reached, leading to a sudden fault slip.

• This slip releases accumulated energy in a phenomenon known as elastic rebound, which is the primary cause of earthquakes.

• Notably, rocks along a fault line do not move immediately under increasing stress; rather, they accumulate energy until the stress exceeds their capacity, resulting in a rupture.

• The transition from stress accumulation (points a to b) to the rapid energy release (points b to c) occurs in a matter of seconds.

• Post-rupture, the rocks may either rebound to their original positions or settle into new configurations, reflecting the strain they have undergone. Increased strain equates to more elastic energy stored in the rocks, heightening the potential for future earthquakes.

Earthquake Cycle

Hypothesis Post-1906 San Francisco Earthquake

• The earthquake cycle consists of four distinct stages:

  • Stage 1: Inactive Period

    • During this phase, strain accumulates without any significant seismic events, allowing tension to build silently in the Earth's crust.

  • Stage 2: Small Earthquakes Occur

    • Small quakes, often referred to as microearthquakes, are triggered by the accumulated strain and can indicate stress release along the fault line.

  • Stage 3: Foreshocks

    • These are small earthquakes that occur hours or days prior to a major quake, serving as potential warning signs, although their occurrence is not guaranteed.

  • Stage 4: Main Shock

    • This stage features the major earthquake which typically follows the foreshocks, resulting in significant ground shaking and potential destruction.

• Aftershocks

  • Following the main shock, a series of smaller earthquakes known as aftershocks can occur. These can last days to months and may reset the earthquake cycle, leading to renewed stress accumulation along the fault line.

Measuring Earthquakes

Magnitude vs. Intensity

• Magnitude: Refers to the quantifiable energy released by an earthquake, typically measured on scales such as the Richter Scale or Moment Magnitude Scale.

• Intensity: This measures the degree of shaking experienced at various locations based on the damage that has occurred in those specific areas.

Richter Scale

• Developed by Charles Richter in 1935, the Richter Scale is one of the most recognizable magnitude scales used to assess earthquakes.

• Properties of the Richter Scale:

  • It is a logarithmic scale, meaning each whole number increase on the scale corresponds to a tenfold increase in measured amplitude and approximately 31.6 times more energy release.

  • Its formula allows for rapid calculations post-earthquake, making it valuable for immediate assessments, though it is less suited for large-scale global comparisons.

Estimating Earthquake Magnitude

• Nomograph Method:

  • This method utilizes the differences in arrival times of seismic waves (S and P waves) recorded on seismograms. For example, if the S-P wave arrival time is recorded as 25 seconds and the amplitude of the S wave is 15, it can yield an approximation of the earthquake's magnitude of about 5.

• Moment Magnitude Scale:

  • Developed to provide accurate measurements for larger earthquakes, the Moment Magnitude Scale considers not just the amplitude of the waves, but also other critical factors such as the area of fault rupture and the amount of fault slip. While this scale takes longer to calculate (days to weeks), it is regarded as more precise for significant seismic events.

Intensity Scales

• Intensity scales assess the effects of an earthquake at specific locations. Various scales exist, with the Modified Mercalli Intensity Scale being widely used in the United States.

• Characteristics of the Modified Mercalli Intensity Scale:

  • It ranges from imperceptible shaking (I) to catastrophic destruction (XII), indicating varying degrees of damage and human perception.

  • This scale is represented in Roman numerals and is more descriptive and qualitative rather than mathematical.

Shake Maps

• Shake Maps are critical tools in assessing damage immediately following an earthquake.

• They leverage data from high-quality seismograph networks to outline ground motion patterns.

• Utilizing instrumental intensity measurements, Shake Maps can quickly illustrate areas of severe shaking and potential damage, aiding in disaster response and planning.