Strong motion seismology

The study of earthquakes or the imaging of the Earth using seismic arrivals often requires detecting arrivals above background noise. The noise level is considered a very important criterion for choosing sites for seismic stations. The noise level at a site depends both on the quality of the installation and on the natural sources of noise close to and far from the site. This chapter discusses the impact of borehole installation on signal-to-noise ratios for seismic arrivals both on land and under the sea. The perception of the seismic background depends on the plotting of the noise spectra. These spectra are generally plotted in acceleration, minimizing the apparent range. Microseism describes either any small background disturbance observed on a seismogram or more usually the noise near 6 sec period. Undersea measurements are usually much noisier than land observations. The primary motivation for installing stations on the sea floor is to improve the global ray path coverage for tomographic studies of the Earth and for regional studies of seismicity and structure. A composite spectrum of short- and long-period vertical component data from the sea floor south of Hawaii shows noise levels considerably higher than at most continental sites at all frequencies.

19 - Seismic Noise on Land and on the Sea Floor

Teleseismic receiver function analysis and published refraction studies are used to infer the existence of first-order discontinuities within the crust and mantle of the Ozark uplift at the Incorporated Research Institutions in Seismology (IRIS) station Cathedral Cave, Missouri (CCM). This information is then used to study the nature of regional wave propagation from the New Madrid seismic zone to CCM across the Ozark Dome to interpret broadband seismograms written by small events. Receiver function inversion indicates a crust 40 km thick characterized by smooth-velocity gradients down to the Moho. Secondary Ps conversions late in the receiver functions show the existence of a high-velocity mantle layer 10 to 15 km thick at approximately 60 km depth with P -wave velocity of about 8.5 km/sec. The data resolve both the top and bottom of the layer. Such high velocities suggest rocks with eclogite or dunite compositions. Regional data from the 26 September 1990 Cape Girardeau earthquake ( M 1 = 4.7) are used to model average crustal velocities and the nature of the velocity gradient near the Moho through inversion of relative arrival times of observed phases. Regional P and S waveforms are exceptionally simple at ranges of 180 km showing the mantle head waves Pn and Sn with a triplicated Pg and Sg arriving within a few seconds, respectively. sP is quite prominent and is an accurate indicator of source depth for the Cape Girardeau event, which occurred between 14 and 16 km depth. There is no evidence for other major phases, except Rg , after the Sg arrival confirming the observation that the crust is dominated by smooth-velocity gradients. A grid search technique is used to model relative phase amplitudes to obtain the focal mechanism for the Cape Girardeau event. Allowable focal mechanisms are dominated by thrust faults striking NS or NE-SW. The inferred earth structure is used to model the 4 May 1991 Risco event ( M 1 = 4.6), but observed differences in relative arrival times between P, S , and Rayleigh phases suggest that crustal velocities are slightly lower within the Mississippi Embayment and that the uppermost mantle has a higher positive gradient than further northwest. Nevertheless, sP constrains the source depth between 7 and 9 km, and P, SV , and SH wave amplitudes yield mechanisms consistent with that obtained by the Portable Array for Numerical Data Acquisition (PANDA) deployment. Waveforms from two small events with poor signal-to-noise ratios are compared with waveforms of the larger events to infer source depths. Modeling broadband waveforms from small local and regional events shows great promise in determining source parameters and inferring details of crustal and upper mantle structure. However, lateral heterogeneity in velocity structure will remain one of the principal obstacles to overcome.

An integrated study of crustal structure and regional wave propagation for southeastern Missouri

This historical review traces the origins of the current national seismic system in the United States, a cooperative effort that unifies national, regional, and local‐scale seismic monitoring within the structure of the Advanced National Seismic System (ANSS). The review covers (1) the history and technological evolution of U.S. seismic networks leading up to the 1990s, (2) factors that made the 1960s and 1970s a watershed period for national attention to seismology, earthquake hazards, and seismic monitoring, (3) genesis of the vision of a national seismic system during 1980–1983, (4) obstacles and breakthroughs during 1984–1989, (5) consensus building and convergence during 1990–1992, and finally (6) the two‐step realization of a national system during 1993–2000. Particular importance is placed on developments during the period between 1980 and 1993 that culminated in the adoption of a charter for the Council of the National Seismic System (CNSS)—the foundation for the later ANSS. Central to this story is how many individuals worked together toward a common goal of a more rational and sustainable approach to national earthquake monitoring in the United States. The review ends with the emergence of ANSS during 1999 and 2000 and its statutory authorization by Congress in November 2000.

Origins of a National Seismic System in the United States

FINAL TECHNICAL REPORT Integrated Regional and Urban Seismic Monitoring—Wasatch Front Area, Utah, and Adjacent Intermountain Seismic Belt

Recent developments in digital communication and seismometry
are allowing seismologists to propose revolutionary
new ways to reduce vulnerability from earthquakes, volcanoes,
and tsunamis, and to better understand these
phenomena as well as the basic structure and dynamics of the
Earth. This document provides a brief description of some of
the critical new problems that can be addressed using modem
digital seismic networks. It also provides an overview of existing
seismic networks and suggests ways to integrate these
together into a National Seismic System.
A National Seismic System will consist of a number of
interconnected regional networks (such as southern California,
central and northern California, northeastern United
States, northwestern United States, and so on) that are jointly
operated by Federal, State, and private seismological research
institutions. Regional networks will provide vital information
concerning the hazards of specific regions. Parts of these networks
will be linked to provide uniform rapid response on a
national level (the National Seismic Network).
A National Seismic System promises to significantly
reduce societal risk to earthquake losses and to open new areas
of fundamental basic research. The following is a list of some
of the uses of a National Seismic System.

https://pubs.usgs.gov/circ/1989/1031/report.pdf

National Seismic System science plan

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National Seismic System science plan