Paleoseismology

James P. McCalpin

• Publisher: Academic Press
• Year: 2009
• Print ISBN: 9780123735768
• eBook ISBN: 9780080919980
• Edition: 2
• Topic: Naturwissenschaften
• Subtopic: Geologie & Geowissenschaften

Paleoseismology has become an important component of seismic risk analysis, which is mandated for nuclear power plants, dams, waste repositories, and other critical structures. This book is the first in the English language to be devoted solely to paleoseismology. It summarizes the development of the field from the 1960s to the present, encompassing material that is currently widely dispersed in journal articles. - Includes a comprehensive review of the techniques currently used in paleoseismology - Emphasizes practical methods of data collection and field studies - Covers interpretation of field data based on current theory concerning fault segmentation and recurrence cycles - Contains more than 170 line drawings and 50 photographs of paleoseismic phenomena

Earthquake Magnitude Scales

James P. McCalpin (email: [email protected])

GEO-HAZ Consulting. Inc., Crestone, Colorado 81131, USA

Several magnitude scales are widely used and each is based on measuring of a specific type of seismic wave, in a specified frequency range, with a certain instrument. The scales commonly used in western countries, in chronological order of development, are local (or Richter) magnitude (M_L), surface‐wave magnitude (M_s), body‐wave magnitude (m_b for short period, m_B for long period), and moment magnitude (M_w or M). Reviews of these magnitude scales are given by Bath (1981), Kanamori (1983), and dePolo and Slemmons (1990); their interrelations are shown in Figure A.1.1. Throughout this book we prefer to cite magnitudes as moment magnitudes (M_w). If the type magnitude is unknown or unspecified, it it cited as M_w.

Figure A.1.1 Graph showing the relationship of various magnitudes to moment magnitude (Mw). Relation for mbLg is from Atkinson and Boore (1987). For Ms and ML the relations come from fitting a quadratic to the data compiled by Ekström (1987) and Hanks and Boore (1984), respectively. From Boore and Joyner (1994); reprinted with permission of the Applied Technology Council.

A.1.1. Local (Richter) Magnitude (ML)

Richter magnitude was the first widely used instrumental magnitude scale to be applied in the USA (Richter, 1935). The scale is based on the amplitude (in mm) of the largest seismogram wave trace on a Wood–Anderson seismograph (free period 0.8 s), normalized to a standard epicentral distance of 100 km. Richter defined his magnitude 0 earthquake as that which produced a maximum amplitude of 0.001 mm at a distance of 100 km. Each successively larger magnitude was defined as a 10‐fold increase in amplitude beyond the base level. Thus, a maximum seismogram amplitude (at a distance of 100 km) of 0.01 mm represents M_L 1.0, 0.1 mm equals M_L 2.0, 1 mm equals M_L 3.0, and so on. Richter (1935) devised a nomograph to normalize the amplitudes for earthquakes closer or farther away than 100 km, based on the attenuation of seismic energy in California.

The Richter magnitude scale accurately reflects the amount of seismic energy released by an earthquake up to about M_L 6.5, but for increasingly larger earthquakes, the Richter scale progressively underestimates the actual energy release. The scale has been said to “saturate” above M_L 6.5, from a combination of instrument characteristics and reliance on measuring only a single, short‐period peak height (see details in Kanamori, 1983).

A.1.2. Surface‐Wave Magnitude (MS)

The surface‐wave magnitude scale was developed to solve the “saturation” problem of Richter magnitude above M_L 6.5. The measurement procedure is similar to measuring the Richter magnitude, except that the peak wave amplitude is measured for surface waves that have periods of 20 s, from long‐period seismographs at teleseismic distances (Gutenberg, 1945). The surface‐wave magnitude calculation does not require a seismograph record within 100 km (or nearby) of the epicenter, so the teleseismic records of many large‐to‐moderate magnitude earthquakes worldwide have been assigned surface‐wave magnitudes. Because of this large data set, M_s is the typical magnitude used in empirical comparisons of magnitude versus earthquake rupture length or displacement (e.g. Bonilla et al., 1984). However, the surface‐wave magnitude scale also saturates, at about M_s > 8.

A.1.3. Body‐Wave Magnitude (MbLg)

The short‐period body‐wave magnitude (m_bLg) is the principal magnitude used in the tectonically “stable” eastern part of North America and Canada. This magnitude is measured from peak motions recorded at distances up to 1000 km on instruments with a passband in the range 1–10 Hz. Peak motions usually correspond to the Lg wave. This magnitude scale is little used in paleoseismology because it saturates at magnitude levels below that of M_s. However, it is possible to convert m_bLg values to other magnitude scales, and vice versa (Kanamori, 1983).

A.1.4. Moment Magnitude (MW OR M)

The moment magnitude scale is the most recent scale (Kanamori, 1977; Hanks and Kanamori, 1979) and is fundamentally different from the earlier scales. Rather than relying on measured seismogram peaks, the M_w scale is tied to the seismic moment (M₀) of an earthquake. The seismic moment is defined as

(A.1.1)

where D is the average displacement over the entire fault surface, A is the area of the fault surface, and μ is the average shear rigidity of the faulted rocks. The value of D is estimated from observed surface displacements or from displacements on the fault plane reconstructed from instrumental or geodetic modeling. A is derived from the length multiplied by the estimated depth of the ruptured fault plane, as revealed by surface rupture, aftershock patterns, or geodetic data. The method thus assumes that the rupture area is rectangular. The shear rigidity of typical crustal rocks is assumed to be about 3.0–3.5 × 10¹¹ dyne/cm² (Aki, 1966; dePolo and Slemmons, 1990).

The seismic moment thus more directly represents the amount of energy released at the source, rather than relying on the effects of that energy on one or more seismographs at some distance from the source. Moment magnitude is calculated from seismic moment using the relation of Hanks and Kanamori (1979) for southern California

(A.1.2)

where M_w is the moment magnitude and M₀ is the seismic moment.

The seismic moment scale was developed to circumvent the problem of saturation in other magnitude scales, and is typically used to describe great earthquakes (i.e., M_s > 8). Kanamori (1983) composed a graph relating M_w to M_L, M_s, m_b, and m_B (Fig. A.1.1). In the interest of standardization, paleoearthquake magnitude should be estimated on the M_w scale; if not, then the magnitude scale used should be clearly noted.

Radiocarbon Sampling Techniques

James P. McCalpin (email: [email protected])

GEO-HAZ Consulting, Inc., Crestone, Colorado 81131, USA

A.2.1. How much to Sample

For conventional radiocarbon dating, suggested minimum and optimum dry weights are given in Table A.2.1. For AMS, dating samples containing as little as a few milligrams of carbon can be dated.

Ta...

Table of contents

• Cover Image
• Table of Contents
• Preface to the Second Edition
• Earthquake Magnitude Scales
• Radiocarbon Sampling Techniques
• Field Evaluation of Liquefaction Susceptibility of Soils with High Fines Content
• Chapter 1 Introduction to Paleoseismology
• Chapter 2A Field Techniques in Paleoseismology—Terrestrial Environments
• Chapter 2B Sub‐Aqueous Paleoseismology
• Chapter 3 Paleoseismology in Extensional Tectonic Environments
• Chapter 4 Paleoseismology of Volcanic Environments
• Chapter 5 Paleoseismology of Compressional Tectonic Environments
• Chapter 6 Paleoseismology of Strike‐Slip Tectonic Environments
• Chapter 7 Using Liquefaction‐Induced and Other Soft‐Sediment Features for Paleoseismic Analysis
• Chapter 8 Using Landslides for Paleoseismic Analysis
• Chapter 9 Application of Paleoseismic Data to Seismic Hazard Assessment and Neotectonic Research
• References