7.1 Anatomy of a seismogram
2 min read•august 9, 2024
Seismograms are like fingerprints of earthquakes, revealing crucial details about seismic events. They show different types of waves, each with unique characteristics and arrival times. Understanding these waves helps seismologists decode Earth's inner workings.
Interpreting seismograms involves analyzing wave amplitudes, arrival times, and waveform shapes. This information allows scientists to pinpoint earthquake locations, determine their , and study Earth's structure. Mastering seismogram reading is key to unraveling seismic mysteries.
Seismic Wave Types
Primary and Secondary Waves
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- P-waves travel through solids and liquids as compressional waves
- P-waves move fastest, arriving first on seismograms
- S-waves propagate through solids only as shear waves
- S-waves arrive second on seismograms, slower than P-waves
- Both P-waves and S-waves are classified as body waves, traveling through Earth's interior
Surface Waves and Coda
- Surface waves travel along Earth's surface, arriving after body waves
- Love waves move horizontally, perpendicular to the direction of propagation
- Rayleigh waves exhibit elliptical particle motion, combining vertical and horizontal movement
- Surface waves cause more damage due to their larger amplitudes and longer durations
- Coda waves appear as a gradual decrease in following the main seismic arrivals
- Coda consists of scattered waves from heterogeneities in Earth's crust and upper mantle
Seismogram Components
Amplitude and Time Measurements
- Amplitude represents the strength or of seismic waves
- Vertical axis of seismogram displays amplitude in units of (displacement, velocity, or acceleration)
- Time axis shows the progression of seismic wave arrivals
- Horizontal axis typically uses seconds or minutes as units
- Waveform depicts the oscillating pattern of ground motion over time
- Different seismic phases produce distinct waveform shapes on seismograms
Baseline and Instrument Response
- Baseline serves as the reference line for measuring wave amplitudes
- Zero-line on seismogram represents the undisturbed ground state
- Instrument response affects the recorded waveform
- Seismometers have -dependent sensitivity
- Raw seismograms require instrument correction for accurate ground motion analysis
- Modern digital seismographs provide high-fidelity recordings across a wide frequency range
Seismic Wave Arrival
P-wave Characteristics and Arrival
- P-waves arrive first due to their higher velocity
- Arrival time marks the onset of seismic energy at the recording station
- velocity in typical crustal rocks ranges from 5 to 7 km/s
- First motion of P-waves indicates compressional or dilatational movement
- P-wave arrivals used to determine earthquake origin time and location
- Travel-time curves help predict P-wave arrivals at different distances
S-wave Identification and Analysis
- S-waves arrive after P-waves, creating a distinct S-P time interval
- velocity typically 60% of P-wave velocity in the same medium
- S-P time interval increases with distance from the earthquake source
- S-wave arrivals often have larger amplitudes than P-waves
- Polarization of S-waves helps determine the direction to the earthquake source
- S-wave splitting occurs in anisotropic media, providing information about crustal structure
Key Terms to Review (24)
Aftershock: An aftershock is a smaller seismic event that follows the main shock of an earthquake, typically occurring in the same area. Aftershocks can happen minutes, days, or even years after the main earthquake and are a result of the Earth adjusting to the changes in stress along fault lines. Understanding aftershocks is crucial as they can cause additional damage to structures already weakened by the primary quake and can provide insight into the dynamics of seismic activity.
Amplitude: Amplitude refers to the maximum displacement of a wave from its rest position, essentially measuring how strong or intense the wave is. In seismology, it’s crucial because it helps indicate the energy released during an earthquake and can influence the interpretation of seismic data. Amplitude is not only important for understanding the strength of seismic waves but also plays a role in distinguishing between different types of waves and their behavior as they propagate through various geological structures.
Dispersion: Dispersion refers to the phenomenon where seismic waves travel at different speeds depending on their frequency, causing the waveforms to spread out over time. This is particularly significant in understanding how different seismic wave types and frequencies behave as they propagate through various geological materials, influencing the interpretation of seismograms and the analysis of waveforms.
Epicenter: The epicenter is the point on the Earth's surface directly above the focus of an earthquake, where seismic waves first reach the surface. Understanding the epicenter is crucial for identifying seismic phases, analyzing seismograms, and studying how body waves interact with Earth’s internal structure.
Foreshock: A foreshock is a smaller earthquake that occurs in the same general area as a larger earthquake that follows, often serving as a precursor to the main seismic event. Foreshocks can provide vital information about the impending larger quake, helping seismologists understand the stress accumulation in geological faults and the patterns of seismicity leading up to significant events.
Frequency: Frequency refers to the number of oscillations or cycles that occur in a given time period, typically measured in Hertz (Hz). In seismology, frequency is critical for understanding the characteristics of seismic waves and how they interact with different geological structures, influencing everything from wave behavior to the interpretation of seismic data.
Ground motion: Ground motion refers to the movement of the Earth's surface caused by seismic waves during an earthquake. It is a key factor in understanding how earthquakes affect structures and landscapes, and it can be measured and recorded using specialized instruments. Analyzing ground motion helps in interpreting seismograms, designing effective seismographs, and understanding the propagation of different seismic waves, including Rayleigh waves.
Hypocenter: The hypocenter is the point within the Earth where an earthquake rupture starts. It is often referred to as the focus of the earthquake, and it plays a crucial role in understanding seismic events and their impacts. The depth and location of the hypocenter are vital for identifying seismic phases, analyzing seismograms, and determining how earthquakes can be located using different methods, all of which contribute to managing earthquake data effectively.
Intensity: Intensity refers to the measure of the strength of shaking produced by an earthquake at a specific location. It is a subjective measure that considers various factors, including the earthquake's magnitude, depth, distance from the epicenter, and local geological conditions, to describe how strongly people feel the shaking and the level of damage caused.
Longitudinal wave: A longitudinal wave is a type of mechanical wave where the particle displacement is parallel to the direction of wave propagation. This means that as the wave travels through a medium, the particles of that medium move back and forth along the same direction in which the wave is moving. In the context of seismic waves, understanding longitudinal waves helps in interpreting how these waves travel through the Earth and how they are recorded in seismograms.
Magnitude: Magnitude is a measure of the energy released during an earthquake, commonly represented on a logarithmic scale. This measurement helps in comparing the size of different earthquakes and is crucial for understanding seismic events, their impact, and the geological processes behind them.
Moment Magnitude Scale: The moment magnitude scale is a logarithmic scale used to measure the total energy released by an earthquake, providing a more accurate representation of its size compared to earlier magnitude scales. This scale relates closely to the seismic moment, which incorporates the area of the fault that slipped, the average amount of slip, and the rigidity of the rocks involved. It is crucial in understanding seismic activity, especially for large earthquakes and those occurring in different geological settings.
P-wave: A p-wave, or primary wave, is a type of seismic wave that travels the fastest through the Earth and is the first to be detected by seismographs after an earthquake. These compressional waves move in a back-and-forth motion, causing particles in the Earth's crust to oscillate parallel to the direction of wave propagation. Understanding p-waves is crucial as they provide vital information about the Earth's interior and play an important role in analyzing earthquake sources and geological structures.
Period: In the context of seismology, the period refers to the time it takes for one complete cycle of a seismic wave to pass a given point. This measurement is critical because it helps in understanding the frequency of seismic waves, which is closely related to their energy and the potential impact of an earthquake. The period is inversely related to frequency; as the period increases, the frequency decreases, affecting how the waves interact with geological structures.
Reflection: In seismology, reflection refers to the bouncing back of seismic waves when they encounter a boundary between different types of geological materials. This process is crucial for understanding the internal structure of the Earth, as it helps identify different layers and their properties by analyzing how seismic waves behave at these boundaries.
Refraction: Refraction is the bending of seismic waves as they pass through different layers of the Earth's interior, caused by variations in wave speed due to changes in material properties. This phenomenon is crucial for understanding how seismic waves travel and interact with different geological structures, which aids in identifying seismic phases, analyzing travel time curves, and interpreting seismograms.
Richter Scale: The Richter Scale is a logarithmic scale used to measure the magnitude of seismic events, specifically earthquakes, by quantifying the amplitude of seismic waves recorded on seismographs. This scale helps in comparing the sizes of different earthquakes and provides a standardized way to communicate their intensity.
S-wave: An s-wave, or secondary wave, is a type of seismic wave that moves through the Earth during an earthquake, characterized by its shear motion which causes particles to move perpendicular to the direction of wave travel. S-waves are slower than primary waves and cannot travel through fluids, making them crucial in understanding the Earth's internal structure and behavior during seismic events.
Seismic hazard: Seismic hazard refers to the probability of experiencing harmful seismic events, such as earthquakes, at a specific location over a given timeframe. It incorporates factors like ground shaking, fault lines, and historical seismic activity to assess the potential risk to structures and populations. Understanding seismic hazard is crucial for effective planning and designing earthquake-resistant infrastructure.
Seismograph: A seismograph is an instrument that measures and records the vibrations of the ground caused by seismic waves, such as those generated by earthquakes. It captures the intensity, duration, and frequency of these vibrations, which are crucial for understanding seismic events and the Earth's internal structure.
Seismometer: A seismometer is an instrument that detects and records the motion of the ground caused by seismic waves from earthquakes or other vibrations. It plays a crucial role in understanding seismic activity by capturing the details of seismic waves, enabling scientists to analyze their characteristics and origins.
Surface wave: Surface waves are seismic waves that travel along the Earth's surface, causing most of the shaking felt during an earthquake. They typically arrive after the faster body waves and can result in significant ground displacement, contributing to the damage seen in structures during seismic events. These waves are crucial for understanding earthquake effects and evaluating seismic hazards.
Tectonic plate movement: Tectonic plate movement refers to the shifting and interaction of the Earth's lithospheric plates, which float on the semi-fluid asthenosphere beneath them. These movements can cause various geological phenomena, such as earthquakes, volcanic activity, and the formation of mountain ranges, all of which are recorded on a seismogram. Understanding this movement is crucial for interpreting the patterns and characteristics of seismic waves captured in seismograms.
Transverse wave: A transverse wave is a type of wave where the particle displacement is perpendicular to the direction of wave propagation. This characteristic means that as the wave travels, particles move up and down or side to side while the wave itself moves forward. Transverse waves are essential in understanding how seismic waves travel through the Earth, particularly in how they are recorded in seismograms and their unique properties during propagation.