Issues

INTRODUCTION

An Mw 5.1 earthquake occurred at 1410 UTC on 12 August 1998 on the San Andreas Fault (SAF), 12 km south-southeast of San Juan Bautista (SJB), at a depth of 9.2 km. This is the largest earthquake to have occurred in the northern creeping-to-locked transition zone of the SAF in the past fifty years. It is of interest because it occurred at the northwestern edge of a gap in the seismicity, it is the largest earthquake in the San Francisco Bay area to have exercised the U.C. Berkeley (UCB) automated Rapid Earthquake Data Integration system (REDI, Gee et al., 1996a), and it was large enough to have produced a detectable signal at the nearest station of the continuously recording GPS BARD network (Bay Area Regional Deformation Network).

SEISMOTECTONIC SETTING

The SJB region extends from the northern end of the creeping section of the SAF to the southern end of...

INTRODUCTION

An ergodic process is a random process in which the distribution of a random variable in space is the same as the distribution of that same random variable at a single point when sampled as a function of time. An ergodic assumption is commonly made in probabilistic seismic hazard analysis (PSHA). A regression analysis is used to obtain a mean curve to predict ground motion as a function of magnitude and distance (and sometimes other parameters). The standard deviation of this ground-motion regression is determined mainly by the misfit between observations and the corresponding predicted ground motions at multiple stations for a small number of well recorded earthquakes. Thus, the standard deviation of the ground-motion regression is dominantly related to the statistics of the spatial variability of the ground motions. An ergodic assumption is made when PSHA treats that spatial uncertainty of ground motions as an uncertainty over time...

It has been suggested that groups of precariously balanced rocks are effectively earthquake strong ground-motion seismoscopes that have been operating on solid rock outcrops for thousands of years, thus providing a constraint on the maximum ground motion that could have occurred during that time (Brune, 1996; Brune et al., I996; Shi et al., 1996; Bell et al., 1998). Modern instrumental strong ground-motion recorders have only provided data for a few decades, have been very costly, and have not recorded data from any great earthquakes along the San Andreas Fault (great earthquakes occurred in 1852 and 1906, before modern instrumentation was available). Nevertheless, there has been a concerted effort to produce probabilistic seismic hazard maps for the U.S. and California (Wesnousky, 1986; WGCEP95, 1995; Ward, 1994; Frankel et al., 1997), stimulated in part by the potential losses in great earthquakes and the high cost of earthquake-resistant seismic design.

Preliminary results from...

INTRODUCTION

The study of paleoliquefaction features for seismic analysis is a new and increasingly utilized technique. Developed only within the past fifteen years, the method of systematically searching for paleoliquefaction features over large geographic areas has been used to interpret the paleoseismic record at numerous locales. These locales lie chiefly in the southeastern, central, and northwestern U.S., where the physical settings and seismotectonic situations differ greatly from one another. Yet despite extensive reliance on paleoliquefaction studies in parts of the U.S., the scope of their capabilities is not widely appreciated. For example, it is not well known that geologic field observations alone can yield important clues about the severity of earthquake shaking and, in many settings, the probable location of the tectonic source zone. Nor is it well known that geotechnical procedures often can be very useful for making reasonable estimates of prehistoric accelerations and magnitudes, even though some of...

INTRODUCTION

A large amount of work has been done in recent years to estimate the distribution of slip on the fault surface during earthquakes. Generally, these slip models are derived from longer period ground motions: strong-motion velocity and displacement, and teleseismic velocity seismograms. At these longer periods, ground motions are predominantly deterministic and their waveforms can in general be accurately modeled using simple descriptions of the source and crustal structure. The opposite situation exists for the prediction of high-frequency strong ground motions. Ground motions at high frequencies are predominantly stochastic, and their waveforms in general cannot be accurately modeled using simple descriptions of the source and crustal structure. However, preliminary evidence (e.g., Hartzell et al., 1996; Kamae and Irikura, 1998; Somerville, 1993; Somerville et al., 1996; Wald et al., 1988) suggests that variable slip models derived from longer-period ground-motion recordings are relevant for the prediction of higher-frequency ground motions. For...

FOREWORD

Anyone who reads any of the computer trade rags, or even the Wall Street Journal, has seen all sorts of blather regarding Java, the computer language, platform, virtual machine, do-all and be-all for everything. Depending on who the author is and from what industry, company, or persuasion, Java is either “an interesting little development” or the “second coming.” Big deals about Java are that it is machine-independent, it can run on everything from super computers to toasters, it is “object-oriented” (OO—which is oh so much better than “structured”), and it contains and embraces inheritance, polymorphism, clever exception handling, and even garbage collection. If you care about these, don't know what these are, or just wonder if Bill Gates really does consider Java a “scary thing” then read on.

The Electronic Seismologist (ES) has talked a couple of young turk seismologists into giving us the inside seismological perspective on all...

Computer and communication technology is at the heart of seismology. Equipped with a computer and a link to the Internet, the world becomes the laboratory for the typical seismologist. New frontiers for collecting, analyzing, viewing, and presenting seismic data are continually pushed as technology advances. Too often, though, our ability to teach on these same frontiers cannot keep pace. The technology we use in our research does not transfer easily to the undergraduate classroom. It is often too advanced for the typical undergraduate student to explore new scientific developments or fundamental theories and share in the excitement of scientific discovery.

One of the greatest obstacles to using real seismic data in teaching at the undergraduate level is that most of the software used in seismological research is UNIX-based. Limited access to computers with the UNIX operating system and the effort required to learn the operating system create a tremendous road...

GURALP DIGITIZERS NOW COMPATIBLE WITH EARTHWORM TRACE BUFFER AND ANTELOPE RING BUFFER

Scream2ew and scream2orb are new modules which interface Guralp Compressed Format (GCF) data from a Guralp digitizer and rebroadcast by SCREAM! to Earthworm and Antelope (BRTT), respectively. Scream2ew runs along with SCREAM! on a PC under NT. Scream2orb runs on a SUN workstation networked to a PC running SCREAM! under NT. SCREAM! acquires serial port data from Guralp digitizer(s), displays data, controls the seismometer and digitizer, and rebroadcasts data packets. Scream2ew and Scream2orb decompress the GCF data packets and place the data into a trace or ring buffer in near real-time. Scream2ew is part of the downloadable Earthworm software package. Scream2orb is included with the Antelope software package.

GURALP CMG-3TB BROADBAND BOREHOLE SEISMOMETER TEST RESULTS

The USGS Albuquerque Seismological Laboratory has released the results of CMG-3TB borehole seismometer tests. The instruments tested were ordered by IRIS for the...

Fifty-three seismologists, geologists, sociologists, emergency managers, and earthquake educators gathered in the heart of Pennsylvania Dutch country for the 70th annual meeting of the Eastern Section of SSA, October 18-20, 1998. Activities began with sixteen hearty souls setting off to examine geologic structure in the vicinity of the Lancaster Seismic Zone in the Pennsylvania Piedmont. This field trip was planned and led by Donald U. Wise, research associate at Franklin and Marshal College in Lancaster, Pennsylvania, and Rodger T. Faill, Assistant Division Chief of the Geologic Mapping Division of the Pennsylvania Geological Survey. I have been informed by those who went that it was an excellent trip, though perhaps the most strenuous ever conducted as part of an Eastern Section meeting. Thank you, Don and Rodger.

The traditional icebreaker reception took place Sunday evening in the Board Room of the Biemesderfer Executive Center on the Millersville University campus. Because of...

The Eastern Section of the Seismological Society of America requests nominations for the fifth annual Jesuit Seismological Association Award for Contributions to Observational Seismology. The award honors outstanding contributions to observational seismology and may be given for one specific contribution or for contributions through the years. There are no restrictions on the recipients or achievements that can be nominated but preference will be given to:

• Members of the Eastern Section

• Members of the Seismological Society of America

• Contributions to the understanding of seismology and/or seismotectonics in eastern North America

This report includes hypocenters for seismic events that occurred March through June 1997 as published in the Preliminary Determination of Epicenters (PDE) Monthly Listing, U.S. Geological Survey, National Earthquake Information Center. It is limited to three types of events: (1) earthquakes with a magnitude of 6.0 or greater, (2) earthquakes causing substantial damage, and (3) events of special interest.

Time is given in Coordinated Universal Time (UTC), unless otherwise specified. Latitude and longitude are given to hundredths of a degree for all locations except for events at the Nevada Test Site, which are given to hundredths of a second of arc. Depths constrained to “normal” (33 km) or other assigned depths are indicated by “constrained.” Depths determined from depth phases are marked “depth phases.” Reports followed by (GS) are taken from the PDE Monthly Listing. All reported intensities correspond to the Modified Mercalli Intensity Scale or other closely related 12-point...