Definition of Earthquake:
- An earthquake is the shaking caused by the rupture (breaking) and subsequent displacement of rocks beneath Earth’s surface.
- It involves one body of rock moving with respect to another.
Deformation of Rock:
- When a body of rock is under stress, it becomes deformed.
- The deformation continues until the rock can no longer withstand it.
- Upon reaching the deformation limit, the rock breaks and the two sides slide past each other.
Location of Earthquakes:
- Most earthquakes occur near tectonic plate boundaries but not exclusively on them or existing faults.
Elastic Deformation Principle:
- This engineering principle helps understand how earthquakes function (illustrated in Figure 11.2).
- Stress applied to rock leads to strain or deformation.
- Most rocks can withstand significant deformation without breaking, but every rock has a deformation limit.
Rupture and Displacement:
- Upon breaking, there is displacement along the rupture surface.
- The size of the earthquake's magnitude is contingent upon:
- Extent of the area that breaks (area of the rupture surface).
- Average amount of displacement (sliding) that occurs.
11.2 The Concept of the Rupture Surface
Definition of a Rupture Surface:
- An earthquake happens over an area within a plane – not at a single point.
- Within this area, displacement varies and decreases to zero at the edges of the rupture surface because the surrounding rock remains undisturbed.
Factors Affecting Rupture Surface:
- Determined by:
- Type and strength of the rock.
- Degree of stress the rock underwent before rupture.
Visual Representation:
- Figure 11.3 shows a dark pink rupture surface on a steeply dipping fault plane.
- The diagram can encompass lengths of tens or hundreds of kilometers.
- The near side of the fault moves to the left; arrows represent varying amounts of displacement.
11.3 Propagation of Earthquake Rupture
Initiation and Propagation:
- Rupture initiation occurs at a single point and spreads rapidly from there.
- Propagation typically occurs within seconds to several tens of seconds, depending on the rupture surface extent.
- The starting point may not be at the center; it can be close to ends, top, or bottom of the rupture surface.
Figures Illustrating Propagation:
- Figure 11.4 illustrates the process of rupture commencement and spreading, showing directional propagation indicated with arrows.
11.4 Aftershocks
Definition of Aftershock:
- Aftershocks are earthquakes triggered by the stress transfer from a preceding main earthquake.
- They can occur within a few tens of seconds of the main event or may be delayed for extended periods (hours, days, weeks, or years).
Magnitude of Aftershocks:
- Aftershocks can be of any magnitude; most are smaller than the triggering earthquake, but some may exceed it.
Example - Loma Prieta Earthquake:
- Following the Loma Prieta earthquake, an area approximately 60 km long and 15 km wide experienced a cascade of aftershocks as illustrated in Figure 11.5.
- Each red dot in the figure indicates a recorded aftershock by seismometers, with a highlighted hexagon representing the location of the main shock.
Stress Distribution After the Main Shock:
- The main earthquake ruptured and displaced rock, altering stress across the fault.
- As stress was relieved in the area of the rupture (depicted in blue), it increased at adjacent parts of the fault (shown in red and yellow, see Figure 11.6).
11.5 Stress Transfer and Fault Interaction
Understanding Stress Transfer:
- Stress transfer can affect surrounding rocks and faults beyond the earthquake's immediate fault zone.
- The added stress from one area may not immediately lead to rupture but accumulates until it exceeds the fault’s strengths.
11.6 Episodic Tremor and Slip (ETS)
Definition of ETS:
- Episodic tremor and slip is periodic slow sliding along a section of a subduction boundary, resulting in rapid seismic vibrations without recognizable earthquakes.
- First identified on Vancouver Island's Cascadia subduction zone.
Behavior of Subduction Boundary:
- The boundary between the Juan de Fuca Plate and the North American Plate is divided into three segments:
- Cold Upper Segment: Locked and does not move until significant earthquakes (~every 500 years).
- Warm Lower Segment: Continuously sliding due to weaker rock.
- Central Segment: Intermittent slipping every 14 months for about 2 weeks, moving a few centimeters each time.
Implications of ETS:
- While ETS seems to relieve tension, it may actually increase the risk of large earthquakes in the locked segment, enhancing stress along the subduction zone.
Observations of ETS:
- Since 2003, ETS processes have also been recorded in subduction zones in Mexico and Japan.