Download Larry J. Ruff - Seismological Research Letters

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There are a few phces around the world where seismologists operate a dense network o f seismographs on top of
shallow seismicity. In these places, it is technically possible to
determine a more precise depth from observations of P and
S arrival times; in detail, seismologists hope to have a seismograph located within ten kilometers or so of the epicenter to
get a precise depth. (Educational Web page activities related
to this style of earthquake location can be found at the
USGS site at http://quake.wr.usgs.gov/more/eqtocat~on/
eql0c.htmt and the Virtual Earthquake pages at http://
E
Larry J. Ruff
E-mail: [email protected] 9
Dept. of Geological Sciences
University of Michigan
Ann Arbor, MI 48109
Telephone: (734) 763-9301
Fax: (734) 763-4690
vearthquake.caLstateLa.edu/edesktop/VirtApps/
VirtuatEarthQuake/VQuakeIntro.html). However, most global
EarthquakeDepth
Some of the toughest questions from students are about
earthquake depth. A typical one goes something like this:
"You told us that earthquakes at midocean ridges occur no
deeper than 15 km, but I just saw one on the NEIC Web
page that was at 33 k m - - i s this a major discovery?" To help
teachers respond to questions such as these, this EduQuakes
column is focused on earthquake depth, and the EduPhase
section treats _pP. Although not a comprehensive survey of
hypocentral depth, at least it provides a starting point for
students to learn about the nagging problems with hypocentral depths in global seismicity catalogs.
Earthquakes occur within the Earth, and the depth of
rupture initiation is the hypocentral depth. Most earthquakes are "shallow", except in subduction zones, where
earthquakes can be as deep as 700 km. There are a few funny
spots around the world with deep earthquakes and no subduction zone, but most seismologists think that geologically
recent subduction explains these events.
H o w shallow is "shallow" for global seismic@? There are
two answers to this question. The scientific answer is that it
depends on the tectonic environment. In regions of the world
that can be characterized as spreading, rifting, volcanic, and
transform faulting, detailed seismological studies show that
most earthquakes have a hypocentral depth of 15 km or less.
In subduction zones, most seismicity occurs along the plate
boundary down to a depth of about 40 km. Thus from a scientific perspective, "shallow" is 40 km or less in subduction
zones, and 15 km or less nearly everywhere else.
The technical answer to the above question is that "shallow" means somewhere above about 70 km or so. Given the
technical difficulty in determining hypocentral depth more
precisely than this, the depth is usually assigned the default
shallow d e p t h - - 3 3 km is the most common depth assigned
by the USGS NEIC. If you search through a global seismicity catalog, you might think there is something magical happening at a depth of 33 kin, but alas, it is just the numerical
equivalent of"somewhere between 0 and 70 km."
seismicity occurs beneath the ocean in remote parts of the
world; this is why most shallow earthquakes in the global
seismicity catalog are assigned the default depth. Since there
is no comparable default depth for deeper (i.e., depth more
than 70 km) earthquakes in the catalog, does this mean that
deeper depths are better determined? If so, how is that possible? Is there any hope to improve hypocentral depths for
shallow global seismicity?
EduPhase: pP
pP is the best answer to the above questions. The best way to
determine depth is to look in the seismogram just after the P
wave arrival to find its reflective cousin: the pP arrival. As it
name implies, pP initially goes up from the earthquake
source, reflects off the Earth's surface, and then follows
closely behind the P wave to arrive at the seismograph. H o w
soon after the P wave does pP arrive? Well, that depends
directly on the hypocentral depth---hence the great value of
p ~ Take the simple ray geometry o f a P w a v e that leaves the
source nearly straight down to travel to a faraway station.
Then the additional travel time for ?P is simply twice the
vertical travel time from hypocenter to the surface. We can
write the extra travel time as ( p P - P)=2d./v, where ( p P - P) is
the travel time difference, dis hypocentral depth, and v is the
average P wave velocity above the source. In the EduPhase
Web pages (start at http://www.seismosoc.org, or go directly
to http://www.geo.tsa.umich.edu/MichSeis/eduPhaselO, the
expression for the ( p P - P) time is shown for the more general case of the P wave leaving at some angle to the vertical.
Even for the more general case, there is a direct linear dependence between ( p P - P) time and hypocentral depth. Thus,
just use a standard value for the P wave velocity--and our
velocity models are usually correct to within 1 0 % - - a n d we
can make a fairly precise estimate of hypocentral depth from
observations of ( p P - P) time. Indeed, the best way to determine hypocentral depth for global earthquakes, shallow and
deep, is to use pP. Another indication of the importance of
pP is that seismologists commonly refer to it as a "depth
phase." Yes, there is another "depth phase": sP goes up from
the earthquake as an S wave and converts to a P wave upon
reflection at the Earth's surface, and then follows closely
behind the pP phase.
IfpP is so proficient at yielding hypocentral depth, why
are most shallow earthquakes still assigned the default depth
Seismological ResearchLetters Volume70, Number4 July/August1999 435
by the USGS NEIC? The EduPhase Web pages will illustrate
why with example seismograms, but here I will just say that
it can be quite tricky to pick the pP arrival correctly. There
are many bumps and wiggles on the seismogram after the P
wave. Which one is pP? When you look at just one seismogram, it can be very difficult to identify pP properly. The
USGS NEIC and ISC earthquake location services do collect and report thepP picks, but they have learned to be careful in using these picks for their standard earthquake
locations. Inclusion of an incorrect depth phase pick can
cause huge errors in the depth that are not apparent in the
formal uncertainties of the hypocentral parameters. Dr. R.
Engdahl at the USGS and colleagues have gone back
through the seismicity catalog with careful consideration of
depth phases to relocate many earthquakes. This work is producing detailed pictures of the deep Wadati-Benioff zone
seismicity.
If you have a global collection of seismograms for an
earthquake, you can exploit the systematic behavior of depth
phases to ensure an accurate depth determination. Unfortunately, for hypocentral depths less than 35 km, pP arrives
within ten seconds or so after the P wave, and the seismogram can appear to be just a tangled mess from the P arrival
past the depth phases. For these shallowest events, seismologists need to use fancy techniques like "waveform inversion"
to determine a good depth. These problems with shallow
earthquakes are quite evident in the examples given on the
pP Web pages. Thus, we end with the surprising result that
the deepest earthquakes have the best-determined hypocentral depths, while the exact depths of most "shallow" earthquakes are still unknown! El
SRL encourages guest columnists to contribute to
"EduQuakes." Please contact Larry Ruffwith your ideas. His
e-mail address is [email protected].
436 SeismologicalResearchLetters Volume70, Number4 July/August1999