GEOL102-Lecture-25 Measuring Earthquakes
Introduction to Measuring Earthquakes
In this lecture, we begin the discussion on measuring earthquakes, aiming to link the scientific understanding of earthquakes to their impacts on people and infrastructure. This understanding will be important for future exercises, particularly one involving the Alpine Fault.
Recap of Key Concepts from Previous Lecture
In the previous week, we discussed the concept of elastic strain in relation to earthquakes, wherein stress builds up until the strength threshold of the fault is surpassed, leading to an earthquake. We also differentiated between key terminologies such as:
Hypocenter: The point within the Earth where an earthquake rupture starts, characterized by its depth.
Epicenter: The point on the Earth's surface located directly above the hypocenter.
The lecture also covered the rupture process, where the fault movement propagates outward, and the slip generated during this rupture radiates seismic energy.
Types of Seismic Waves
We discussed seismic waves, including:
Body Waves: These waves travel through the interior of the Earth.
Surface Waves: These waves travel along the Earth's surface and typically have greater amplitudes than body waves.
Understanding these wave types is critical for analyzing the potential for damage and risk.
Measuring Earthquake Magnitudes
Richter Scale vs Moment Magnitude Scale
The discussion transitions to how we measure earthquake magnitudes. A common media reference to earthquakes is a number on the Richter scale (e.g., "7 on the Richter scale"). However, in geology, this is often misleading as geologists do not use the Richter scale for current measurements anymore. Instead, they utilize:
Moment Magnitude (M_w): A more accurate measure of the energy released by an earthquake.
Local Magnitude (M_L): Originally defined by Charles Richter, this scale is empirical and tied to the amplitude of seismic waves.
Definition and Concept of Magnitude
Moment Magnitude (Mw) is based on the seismic moment ($M0$) of the earthquake, defined by the equation:
where the force corresponds to the shear modulus ($ ext{μ}$), slip, and area of the fault surface.
The important aspect is that the moment magnitude scale is logarithmic, meaning that:
A magnitude 7 earthquake releases ten times more energy than a magnitude 6 earthquake, and 100 times more than a magnitude 5 earthquake.
The logarithmic nature of this scale reflects a significant increase in energy release with small increases in magnitude.
Comparisons with Historical Earthquakes
The Darfield earthquake in New Zealand illustrated an example with a slip of about 4 meters.
In contrast, the 2004 Boxing Day earthquake had a rupture area similar to the size of South Island, significantly larger in magnitude.
The lecture also humorously notes energy comparisons to food, e.g., comparing the energy of an earthquake to an equivalent number of Moro bars, illustrating how vast earthquake energies can be conceptualized.
Modified Mercalli Intensity Scale (MMI)
In addition to moment magnitude, the Modified Mercalli Intensity (MMI) scale is introduced as it mixes hazard and risk assessment:
Low MMI values reflect perceptible shaking (e.g., how many people feel the earthquake).
Higher MMI values indicate the level of damage to the structures.
MMI varies based on the locality and characteristics of the ground and buildings near the epicenter. For instance, a 7.1 magnitude earthquake in New Zealand produced an MMI of up to 5 in Christchurch, while a similarly magnitudinous earthquake in Haiti registered MMI values upwards of 8, showing that local geology and infrastructure significantly influence damage outcomes.
Important Factors Affecting Ground Shaking
Understanding ground shaking involves several factors:
Distance from Epicenter: As seismic waves spread out from the source, they lose energy (attenuation).
Local Geology: Soft sediments can amplify seismic waves; for instance, moving from hard rock into softer sediments can increase ground motion and shaking intensity. This leads to phenomena like topographic amplification where hills may exacerbate shaking.
Topography and its Effects
In the Darfield earthquake, higher amplitudes were observed in areas like the Port Hills due to topographic effects causing ground vibrations to amplify. These dynamics lead to hazards such as rockfalls and other geological disturbances. The lecture includes examples from real scenarios post-earthquake, including video evidence from observing rock movements post the Geraldine earthquake.
Duration of Shaking
The duration of shaking during an earthquake includes:
The fault length and whether the rupture is bidirectional or unilateral.
Basin effects: Seismic waves can be trapped in geological basins, prolonging shaking effects.
The frequency content of seismic waves correlates directly with the infrastructure's resonance frequency and determines the structural vulnerability of buildings.
Resonance and Its Implications
The discussion highlights that buildings can resonate with certain frequencies of shaking, especially when the earthquake's frequencies match the natural frequencies of structures, increasing risks of collapse. The logic extends to various building types, demonstrating how stiffer, shorter buildings react differently to seismic vibrations than taller, flexible structures.
Specific Earthquake Case Studies
In contextualizing these concepts with real events, we examine the Christchurch earthquake in 2011 which was catastrophic due to its proximity to the city and the direction of the rupture. Factors include:
Rupture Directivity towards the CBD, causing amplified shaking effects and damage.
Basin effects where soft sediment increases shaking duration and intensity.
Special Phenomena: Slap Down or Trampoline Effect
A unique phenomenon discussed is the slap down effect, reminiscent of a trampoline, where two layers of earth (soils and bedrock) respond differently to seismic waves, leading to amplified vertical accelerations. In Christchurch, accelerations exceeding gravity were recorded, indicating a significant risk factor for buildings not designed to withstand such motions.
Conclusion
The moment magnitude alone cannot encapsulate the risks and effects of earthquakes. A comprehensive understanding requires integrating multiple factors contributing to ground motion and damage potential, linking back to societal implications and safety structures. The key takeaway is that when planning for the effects of earthquakes, a multifaceted approach is essential, acknowledging the complex interplay between geology and engineering.
Future workshops will expand on practical applications of these principles, particularly concerning the Alpine Fault.