Linda M. Warren

Investigating the frequency dependence of mantle Q by stacking P and PP spectra

Using seismograms from globally distributed, shallow earthquakes between 1988 and 1998, we compute spectra for P arrivals from epicentral distances of 40° to 80° and PP arrivals from 80° to 160°. Selecting records with estimated signal-to-noise ratios greater than 2, we find 17,836 P and 14,721 PP spectra. We correct each spectrum for the known instrument response and for an ω−2 source model that accounts for varying event sizes. Next, we stack the logarithms of the P and PP spectra in bins of similar source-receiver range. The stacked log spectra, denoted as log(DP′) and log(DPP′), appear stable between about 0.16 and 0.86 Hz, with noise and/or bias affecting the results at higher frequencies. Assuming that source spectral differences are randomly distributed, then for shallow events, when the PP range is twice the P range, the average residual source spectrum may be estimated as 2 log(DP′)–log(DPP′), and the average P wave attenuation spectrum may be estimated as log(DPP′) – log(DP′). The residual source spectral estimates exhibit a smooth additional falloff as ω−0.15±0.05 between 0.16 and 0.86 Hz, indicating that ω−2.15±0.05 is an appropriate average source model for shallow events. The attenuation spectra show little distance dependence over this band and have a P wave t¯* value of ∼0.5 s. We use t¯* measurements from individual P and PP spectra to invert for a frequency-independent Q model and find that the upper mantle is nearly 5 times as attenuating as the lower mantle. Frequency dependence in Qα is difficult to resolve directly in these data but, as previous researchers have noted, is required to reconcile these values with long-period Q estimates. Using Q model QL6 [Durek and Ekstrom, 1996] as a long-period constraint, we experiment with fitting our stacked log spectra with an absorption band model. We find that the upper corner frequency f2 in the absorption band must be depth-dependent to account for the lack of a strong distance dependence in our observed t¯* values. In particular, our results indicate that f2 is higher in the top 220 km of the mantle than at greater depths; the lower layer is about twice as attenuating at 1 Hz than at 0.1 Hz, whereas the upper mantle attenuation is relatively constant across this band.

Using the Effects of Depth Phases on P-wave Spectra to Determine Earthquake Depths

For shallow earthquakes, the surface-reflected depth phases ( pP and sP ) arrive shortly after the primary arrival, and the time separation among the three phases can be used to determine the origin depth of the earthquake. To model the relative arrival times and amplitudes of these phases, and the core reflections and water-column reverberations for a given earthquake, we construct stick seismograms using the IASPEI91 velocity model and the Harvard CMT focal mechanisms at the distances and azimuths of the recording seismometers. While the differing arrival times and amplitudes are features observable in the time series, they also affect the spectrum, and we compute the spectrum for a time window that includes the P wave and subsequent arrivals. We quantify the effects of variations in these properties over the focal sphere in terms of differences in the slope of the log spectrum at different stations. To determine the depth of an earthquake, we compare our observed spectral variations with the predicted spectral variations for earthquakes originating at depths within 30 km of the pde depth and identify the depth with the smallest L1 misfit as the true earthquake depth. We demonstrate the effectiveness of this method by applying it to a group of 35 thrust earthquakes in the Aleutian arc near the Andreanof Islands, but we also describe some complications introduced by strongly directive ruptures, as illustrated for the 1995 Jalisco, Mexico, event. Online material: Observed and predicted variations in pulse width for Aleutian Island earthquakes.

Dominant fault plane orientations of intermediate‐depth earthquakes beneath South America

The South American subduction zone exhibits considerable variation: the subduction angle alternates between flat and steep; the subducting plate has complex structures; and arc volcanism in the overlying plate has gaps. I investigate the effect of these differences in incoming plate structure and slab geometry on intermediate-depth earthquakes, specifically their fault orientations and rupture characteristics, and find that slab geometry has the largest impact on fault orientation. I use rupture directivity to estimate rupture direction and rupture velocity and to distinguish the fault plane from the auxiliary plane of the focal mechanism. From analysis of 163 large (Mw≥5.7) intermediate-depth (60–360 km depth) earthquakes from along the length of South America, estimated rupture azimuths and plunges show no trends, appearing to be randomly distributed on the determined population of fault plane orientations, and a majority of earthquakes are made up of multiple subevents. As seen in other subduction zones, subduction segments descending at normal angles have predominantly subhorizontal faults. Flat slab segments also have a dominant fault orientation, but those earthquakes slip along the conjugate nodal plane of the focal mechanism. In strongly curved slab segments, such as at the downdip edge of flat segments where the slab resubducts, earthquakes may slip along either nodal plane orientation. While both fault orientations could be consistent with the reactivation of fossil outer rise faults, the fault orientations are also consistent with expectations for newly created faults in agreement with the ambient stress field. Fault reactivation alone does not explain why different fault orientations are active in segments with different geometries, so the preferred explanation for having regionally consistent fault orientations is that they minimize the total work of the system. The previously observed predominance of subhorizontal faults appears to be a consequence of slab geometry.

Search for Tectonic Tremor on the Central North Anatolian Fault, Turkey

Abstract Tectonic tremor has previously been observed along two major transform faults: the San Andreas fault (SAF) and the Alpine fault (AF); and we extend the search for tectonic tremor to another transform fault, the North Anatolian fault (NAF). We investigate a two‐year‐long temporary broadband seismic deployment on an ∼400  km long NAF segment in central Turkey. The central NAF network has stations within a few kilometers of the fault. Although the station spacing is larger than for the SAF and AF networks, we still expect to be able to observe coherent tremor at several stations, if it occurs in similar amounts as elsewhere. We search for tremor triggered by the surface waves of regional and teleseismic earthquakes but do not observe any tremor associated with the passage of surface waves. Next, we search for ambient tremor and observe occasional tremor‐like signals, but the signals are not coherent between stations. Thus, we cannot identify any unambiguous tremor episodes and suggest that if tremor occurs along the central NAF, it occurs at a lower amplitude and/or rate than along the SAF or AF.

Precise Locations for Intermediate‐Depth Earthquakes in the Cauca Cluster, ColombiaPrecise Locations for Intermediate‐Depth Earthquakes in the Cauca Cluster, Colombia

In subducting slabs, a high seismicity rate in a concentrated volume (an earthquake nest) is often associated with geometric complexities such as slab detachment, tearing, or contortions. In Colombia, the Cauca cluster has a high rate of intermediate-depth earthquakes between 3.5°–5.5° N and 77.0°–75.3° W. From January 2010 to March 2014, the Colombian National Seismic Network reports 433 earthquakes in the cluster at depths of 50–200 km with local magnitudes ranging from ML 2.0 to 4.7. We determine precise relative locations of the intermediate-depth earthquakes in the cluster and investigate the cause of the cluster by estimating its geometry from earthquake relocations. Earthquake relocations show a continuous 20-km-thick intraslab seismic zone dipping at 33°–43°, with the dip angle increasing to the south. In addition, earthquakes locate in two isolated fingers that extend 30–40 km normal to and above the slab. The depth and vertical separation of the earthquakes in the two fingers indicate that the two fingers do not belong to the intraslab seismic zone or the overriding crustal seismic zone and instead are in the mantle wedge. Changes in the velocity model, starting earthquake locations, or the precision of the arrival picks do not lead to significant changes in the relative locations. The Cauca cluster, with earthquakes located in and above a continuous slab, appears to have a different mechanism than previously studied earthquake nests. The high seismicity rate in the cluster may, instead, correspond to high volumes of dehydrated fluid. Electronic Supplement: Tables with earthquake locations and slab contours, and figures supporting the earthquake relocation tests.