1. 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) Where do earthquakes occur? Aftershocks from the Loma Prieta earthquake (1989 in map view (top) and side view (bottom) From: USGS Aftershocks from the 1995 Mw 8.1 Antofagasta, Chile earthquake From: Patzig et al., 2002

2. Earthquake basics: What is magnitude? Ms Surface wave magnitude Amplitude of waves with periods of 20 seconds Lay and Wallace, 1995 Mb body wave magnitude Amplitude of P wave (1 second) 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) Problems with these methods: Based on narrow frequency bands of seismic energy Saturate above some level earthquakes get bigger, but M doesn’t

3. What is magnitude? M L 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) Both from: Christian Grosse Relation of double couple to first motion From: Christian Grosse

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