Geophysics and Seismology

Introduction to Geophysics

Geophysics applies physics methods to study Earth's structure, composition, dynamics, and history across all scales—from the nanometre-scale crystal structure of mantle minerals to the planet-scale dynamics of tectonic plate motion and the geodynamo generating Earth's magnetic field. Because direct sampling of Earth's deep interior is impossible (the deepest borehole, Kola Superdeep, reached only 12 km), geophysics relies on remote sensing through natural and artificial signals: seismic waves—elastic vibrations from earthquakes and explosions—propagate through Earth's interior carrying information about density and elastic properties at every depth. Seismology has revealed Earth's layered structure: crust (0-35 km), mantle (35-2900 km), liquid outer core (2900-5100 km), solid inner core (5100-6371 km).

Geophysics also encompasses geodesy (measuring Earth's shape and gravity field), geomagnetism (Earth's magnetic field and its secular variation and reversals), heat flow (constraining internal heat sources from radioactive decay and primordial heat), and electromagnetic methods for subsurface imaging. Applied geophysics exploits these techniques for oil and gas exploration, mineral deposit discovery, groundwater mapping, engineering site characterisation, and environmental contamination monitoring. The global seismic network provides continuous measurement enabling near-real-time earthquake detection, tsunami warning, and nuclear explosion monitoring under the Comprehensive Test Ban Treaty.

Seismic Waves and Earth Structure

Body Waves and Surface Waves

Seismic waves are elastic perturbations propagating through the Earth in two main categories. Body waves: P-waves (primary, compressional—particle motion parallel to propagation direction, propagate through solid and liquid, velocity ~6-14 km/s in mantle); S-waves (secondary, shear—particle motion transverse to propagation, propagate only through solid, velocity ~3.5-7.5 km/s in mantle, zero in liquid outer core). The absence of S-waves through the outer core proved its liquid state. Surface waves travel along Earth's surface: Rayleigh waves (elliptical retrograde particle motion in the vertical plane, dispersive—different frequencies sample different depths); Love waves (horizontal transverse motion, fastest surface waves). Seismic tomography uses travel time differences of thousands of earthquake P and S wave arrivals worldwide to construct 3D velocity models of the mantle—revealing subducting slabs as fast anomalies (cold, dense tectonic plates) and mantle plumes as slow anomalies (hot upwellings beneath Hawaii, Iceland, Afar) below the lithosphere.

Earthquake Mechanics

Earthquakes are sudden releases of elastic strain energy accumulated as tectonic plates interact at faults—shear failure when shear stress exceeds frictional strength on a fault plane. Moment magnitude scale Mw = (log M0 / 1.5) - 10.7 (M0 = seismic moment = mu * A * D, mu=shear modulus, A=fault area, D=average slip)—a physically based measure replacing the older Richter scale. Stick-slip dynamics: rate-and-state friction laws describe fault behaviour—velocity-weakening faults are seismically unstable (earthquakes); velocity-strengthening are stable (aseismic creep). Foreshock-mainshock-aftershock sequences follow Omori's law for aftershock rate decay (n(t) = K/(t+c)^p, p~1). The 2011 Tohoku Mw 9.0 earthquake ruptured ~500 km of the Japan Trench subduction zone with ~50m slip in places—generating a devastating tsunami that overtopped seawalls and caused the Fukushima nuclear disaster. Real-time earthquake early warning systems (ShakeAlert in the US, Japan's J-Alert) detect P-wave arrivals seconds before damaging S-waves allowing seconds to tens-of-seconds warning.

Earth's Interior and Geodynamics

Mantle Convection and Plate Tectonics

Plate tectonics—the lateral movement of lithospheric plates driven by mantle convection—is the overarching geodynamic framework governing earthquakes, volcanism, mountain building, and the rock cycle. The mantle (Rayleigh number Ra ~ 10^7—strongly convecting) circulates on timescales of ~100 million years; subducting slabs carry cold dense lithosphere into the mantle driving plate motion ("slab pull" is the dominant driving force, ~3× stronger than "ridge push" from mid-ocean ridge buoyancy). Seismic tomography images entire slab graveyards in the lower mantle (below 660 km discontinuity) showing subduction history over hundreds of millions of years. Hotspot plumes (narrow upwelling columns of hot mantle material) generate chains of volcanic islands (Hawaiian-Emperor seamount chain—currently beneath Kilauea, was beneath the Emperor Seamounts 80 Mya) as plates move over stationary plumes. Mantle phase transitions at 410 km (olivine→wadsleyite) and 660 km (ringwoodite→bridgmanite+ferropericlase) govern mantle convection style—layered or whole-mantle.

Gravity and Geodesy

Earth's gravity field provides information about interior density distribution. The geoid—the equipotential surface of Earth's gravity field at mean sea level—deviates from a reference ellipsoid by up to ±100 metres due to lateral density variations in the mantle and crust. GRACE and GRACE-FO (Gravity Recovery And Climate Experiment) satellite gravity missions measure monthly gravity field changes at ~300 km resolution from inter-satellite microwave ranging—directly measuring groundwater depletion in aquifers (Ogallala, Central Valley in California, Ganges-Brahmaputra), ice sheet mass loss from Greenland and Antarctica, and seasonal hydrological storage changes. These space gravity measurements provide crucial independent data on climate-related mass redistribution. GPS/GNSS geodesy measures tectonic plate velocities to sub-millimetre per year accuracy—confirming the 2-5 cm/yr rates of major plates and resolving detailed deformation around fault systems.

Examples and Applications

Example 1: Probing Earth's Core

Earth's liquid iron outer core (~5500 K, 135-360 GPa) and solid iron inner core (~6000 K, 360-365 GPa) are inferred from seismic wave behaviour. The outer core is liquid because: S-waves do not traverse it (shear waves cannot propagate in liquids); P-wave velocities drop to ~8 km/s at the core-mantle boundary (CMB) from 13.7 km/s in the mantle base. The inner core was discovered by Inge Lehmann in 1936 from P-wave arrivals in what should be a shadow zone for outer core P-waves—PKIKP phases penetrating the inner core arrive slightly early indicating faster velocities. Inner core seismic anisotropy—P-waves travel ~3% faster parallel to Earth's rotation axis than equatorially—probably reflects aligned crystal texture of iron under core conditions. Inner core oscillations (normal modes) excited by large earthquakes allow measurement of inner core rigidity and the still-debated inner core differential rotation (0.3–3°/year faster than mantle).

Example 2: Earth's Magnetic Field and Geodynamo

Earth's magnetic field—primarily a geocentric axial dipole with moment ~8×10^22 A·m²—is generated by the geodynamo: convective flow of electrically conducting liquid iron in the outer core driven by secular cooling and inner core solidification (releasing latent heat and light elements), interacting with Earth's rotation to produce self-sustaining electromagnetic induction. The geomagnetic field protects Earth's surface from solar wind and cosmic rays. Paleomagnetic record preserved in volcanic rocks and oceanic crust shows ~450 polarity reversals in the past 160 million years (most recently 780,000 years ago—Brunhes-Matuyama reversal), with irregular reversal frequency from ~5/Myr in the late Cenozoic to superchrons of zero reversals. The Southern Atlantic Anomaly—a region of weakened field intensity over southern Atlantic—may indicate an upcoming reversal or excursion over geological timescales. Numerical geodynamo simulations (Glatzmaier-Roberts 1995 first successful simulation) reproduce spontaneous polarity reversals and field morphology.

Example 3: Seismic Reflection and Oil Exploration

Seismic reflection surveys are the primary exploration tool for oil and gas—generating artificial seismic waves (explosive charges, vibroseis source trucks on land; air guns at sea) and recording reflections from subsurface geological interfaces at arrays of receivers (geophones/hydrophones). Two-way travel time of reflections converted to depth using velocity analysis reveals stratigraphy, fault geometry, and potential hydrocarbon trap configurations. 3D seismic surveys—grids of parallel profiles processed into 3D volumes—provide detailed subsurface geometry for reservoir modelling and well placement. 4D seismic (time-lapse 3D surveys repeated over years) monitors reservoir fluid changes during production—tracking water flooding, gas cap expansion, and pressure depletion providing data for optimising extraction. Amplitude versus offset (AVO) analysis of seismic amplitude variations with source-receiver distance distinguishes gas-filled from brine-filled sands at exploration wells—a critical direct hydrocarbon indicator.

Example 4: Tsunami Physics

Tsunamis are long-wavelength shallow-water waves generated by sudden seafloor displacement (earthquakes, submarine landslides, volcanic caldera collapse). In deep ocean (4000m), tsunami wavelength ~200 km, propagation speed c = sqrt(gH) ~200 m/s, wave height ~0.5m—undetectable by ships. As water shallows near coasts, wave energy compresses and height grows (shoaling, h ~ H^(-1/4) approximately), focusing ~×100 amplification into devastating walls of water. Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys—bottom pressure sensors on seafloor transmitting to surface buoys—detect tsunami waves in real-time enabling warnings minutes to hours before coastal impact. Run-up height is determined by coastal geometry: V-shaped bays (Palu Bay 2018, 11m run-up) and funnel topography concentrate energy; coral reefs and mangroves attenuate—empirical demonstrations of the difference protective ecosystems make. The 2004 Indian Ocean tsunami (Sumatra Mw 9.1) killed 228,000 people across 14 countries before tsunami warning systems were deployed in the Indian Ocean.

Example 5: Electrical Methods in Geophysics

Resistivity and induced polarisation methods inject electrical currents into the ground and measure voltage distributions to map subsurface resistivity contrasts—distinguishing conducting clays and water-saturated rocks from resistive bedrock and hydrocarbons. Electrical resistivity tomography (ERT) uses electrode arrays and tomographic inversion to image cross-sections of the subsurface—widely used for contamination plume mapping, karst void detection, and dam safety monitoring. Magnetotellurics (MT) uses naturally occurring electromagnetic fields (generated by lightning and geomagnetic micropulsations at different frequencies) to probe depths from metres to hundreds of kilometres depending on frequency (skin depth delta = sqrt(2 rho / (omega * mu))). MT revealed the partial melt zone beneath the Tibetan Plateau, magma chambers beneath volcanoes, and conductive zones of aqueous fluids in the lower crust. Airborne EM systems mounted on helicopters rapidly survey large areas for mineral exploration, groundwater mapping, and permafrost characterisation in the Arctic.

Example 6: Geodetic Monitoring of Volcanoes

Ground deformation before volcanic eruptions—caused by magma intrusion inflating volcanic edifices—is monitored by GPS, InSAR (Interferometric Synthetic Aperture Radar—measuring centimetre-scale ground deformation from satellite radar phase differences), and tiltmeters. InSAR revealed precursory inflation at Mount St Helens before the 2004-2008 eruption; detected uplift episodes beneath Yellowstone caldera—a supervolcano with frequent resurgent episodes (2004-2006, up to 7 cm/year uplift). Volcanic seismicity—volcano-tectonic (VT) earthquakes from brittle failure, long-period (LP) events from resonating fluid-filled cracks, and harmonic tremor from sustained fluid flow—provides seismic precursors complementary to deformation. Infrasound arrays at volcanic observatories detect explosion bursts from pyroclastic eruptions—distinguishing degassing pulses from explosive events seconds after onset for aviation hazard notification. The 2022 Hunga Tonga-Hunga Ha'apai eruption generated the largest atmospheric pressure wave since Krakatoa, observable in atmospheric pressure records globally.

Example 7: Earthquake Early Warning Systems

Earthquake early warning (EEW) systems exploit the ~8 km/s velocity of P-waves versus ~3.5 km/s S-waves to deliver seconds of warning before damaging shaking begins. ShakeAlert (USGS/West Coast, deployed since 2018) detects P-wave arrivals at seismic networks, estimates magnitude and location in real-time, and sends automated alerts to MyShake phones, school PA systems, and infrastructure operators. Japan's J-Alert system provides nationwide EEW to phones, TV/radio, and trains—automatically halting Shinkansen trains (0 derailments from EEW-triggered stops) and pausing factory lines. Warning times range from seconds (near the epicentre) to ~1 minute (distant cities)—enough to duck-and-cover, stop surgeries, evacuate lifts, or shut gas valves. MEMS-based ShakeAlert phone seismometers (Android Earthquake Alerts using phone accelerometers) crowdsource seismic sensing to millions of devices—providing dense networks in regions without professional seismograph infrastructure.

Example 8: Planetary Seismology

The InSight mission (2018-2022) operated the first seismometer on Mars, detecting over 1,300 marsquakes revealing Mars's interior structure—crust ~24-72 km thick, liquid iron-nickel-sulfur outer core ~1830 km radius, mantle S-wave velocity profile. The Moon has an extensive seismic dataset from Apollo ALSEP seismometers (1969-1977)—moonquakes (deep, shallow, and meteorite impact events) revealed a ~1300 km solid inner core and ~400 km fluid outer core. Seismology on other worlds constrains planetary formation models: Mars has a core too large for a fully rocky planet without sulfur dilution; the Moon's small iron core explains its weak global magnetic field. Future planetary seismic missions: ISSE (Ice Giant Seismology Satellite) proposed for Neptune/Uranus; seismometers on Europa landers would detect ice shell resonances and ocean tides revealing subsurface ocean depth and ice shell structure critical for habitability assessment.

Try it live

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