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Brittle failure occurs within “seismogenic zone” defined by fault properties

Where do earthquakes occur?. Brittle failure occurs within “seismogenic zone” defined by fault properties Typically 15 km for vertical strike slip faults ~30-50 km for subduction zone faults (depending on dip). Aftershocks from the 1995 Mw 8.1 Antofagasta, Chile earthquake

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Brittle failure occurs within “seismogenic zone” defined by fault properties

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  1. Where do earthquakes occur? • Brittle failure occurs within “seismogenic zone” defined by fault properties • Typically 15 km for vertical strike slip faults • ~30-50 km for subduction zone faults (depending on dip) Aftershocks from the 1995 Mw 8.1 Antofagasta, Chile earthquake From: Patzig et al., 2002 Aftershocks from the Loma Prieta earthquake (1989 in map view (top) and side view (bottom) From: USGS

  2. Earthquake basics: What is magnitude? • Ms Surface wave magnitude • Amplitude of waves with periods of 20 seconds • Mb body wave magnitude • Amplitude of P wave (1 second) • Problems with these methods: • Based on narrow frequency bands of seismic energy • Saturate • above some level earthquakes get bigger, but M doesn’t • Mw Moment magnitude: • physical properties of fault instead of just amplitude of seismic wave (although seismic waves can be used to estimate the seismic moment) Lay and Wallace, 1995

  3. What is magnitude? • ML Richter magnitude • Also known as local magnitude • Defined in Southern California • for a specific type of seismometer Bolt, 1978 Lay and Wallace, 1995

  4. Seismic moment tensor and earthquakes Single Force: Rarely seen except Mt. St. Helens, landslides Force couple: Vector dipole: explosion Need for double couple to conserve angular momentum Moment tensor is a set of these force couples: It is symmetric Related to fault parameters (but ambiguity between auxiliary planes) Relation of double couple to first motion From: Christian Grosse Both from: Christian Grosse

  5. Determining a focal mechanism with waveform fitting From:University of Nevada, Reno

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