Recognizing Reflected Tremor Phases: Guidance from Synthetic Seismograms Amanda Klaus ESS 522 – Geophysical Data Analysis Final Project June 10, 2010

Outline 1.Background: Tremor – What it is – Where it happens 2.My tremor-related research 3.Why care about tremor? 4.My ESS 522 project – Synthetic seismograms – Data processing and seismogram analysis

Earthquake: July 15, 2004, 12:06, M 5.9 (Vancouver Island) - Station: YOUB Tremor: July 25, 2004 Station YOUB Background Tremor: an emergent, low-amplitude seismic signal, persistent in time and space

Where Tremor Happens: Map View 5 years of tremor location data (courtesy of Aaron Wech) Cascadia subduction zone schematic (Source: pnsn.org, after Hyndman and Wang 1995)

Where Tremor Happens: Depth Due to large uncertainties in vertical locations, tremor depth is unknown! Two possibilities: Option 1: Tremor is distributed throughout the lower crust, and maybe the upper subducting slab. Tremor is likely caused by movement of fluids. Option 2: Tremor is located at the interface between the continental and oceanic plates. Tremor is a manifestation of plates sliding past each other.

Tremor Depth: Why It Matters 1. If tremor is at the plate interface, tremor means plates are sliding 2. This sliding downdip of the locked zone increases stress in the locked zone 3. Locked zone ruptures in megathrust earthquakes after enough stress builds up

My research Look for signal of tremor bouncing off the Moho (seismic velocity discontinuity) Hope to see consistent delay times that indicate bouncing off of an interface ca. 7 km below source Moho

ESS 522 Project Part 1: Generation of synthetic seismograms Goal: investigate feasibility of seeing bounces of low-amplitude seismic energy off of the Moho

Synthetics - Method Did a literature search for best estimates of seismic velocities and densities Assembled these into a simple 1D model of flat, horizontally infinite layers, over a half- space representing the upper mantle Used Lu-pei Zhu’s FK code for generating synthetics – Earthquake source parameters: 7° dip, reverse fault, magnitude 2

Input Model Simple 1D velocity model (no dipping layers) Source: M 2 earthquake – dip 7° east - thrust fault. Source is at km depth, just above interface.

Project Part 2: Data Processing Many things to consider: Filtering: what frequencies are we concerned with? Instrument response and how it affects data – Not included today Signal-to-noise ratio – Tremor data: very low signal-to-noise ratio We’ll look at some of these things in the demo…

Demonstration Step 1: load data Step 2: plot data: plotFKjdf( eastVert, 0 )

Demonstration Use reduced time to shift seismograms: plotFKjdf(eastVert, 6)

Demonstration: Adding Noise Data_out = addrand(data_in, noise_amp) Noise_amp is % of largest data point – default is 10%

With noise added…

Filtering & Effect on Noise Use command coralFilter  coralFilter(data, cutoff_freq, type, order, phase)

Filtering Example 4 th Order Butterworth Filter 16 th Order Butterworth Filter

An illustration of stacking 10 copies of the same seismogram, with different random noise added to each Vertical channel – station east of source, 40 km away

Noisy data, stacked

Analogous for stacking arrays 5 miles